Steam and Hot-Water Space Heating Boilers

Boilers are used to supply steam or hot water for heating, process­ing, or power purposes. This chapter is primarily concerned with a description of the low-pressure steam and hot-water (hydronic) space heating boilers used in the heating systems of residences and small buildings (Figures 15-1 and 15-2).

The basic construction of both low-pressure steam and hot — water space heating boilers fired by fossil fuels consists of (1) an insulated steel jacket enclosing a lower chamber in which the combustion process takes place; and (2) an upper chamber con­taining cast-iron sections or steel tubes in which water is heated or converted to steam for circulation through the pipes of the heat­ing system.

Steam and Hot Water Boiler Similarities and Differences

Steam and hot-water (hydronic) space heating boilers are very

Similar physically, but there are some important differences.

• Steam boilers operate only about three-fourths full of water, whereas hot-water boilers operate completely filled with water.

• Steam boilers in residential steam heating systems operate at 2 psi pressure or slightly more, whereas residential hot — water boilers operate at approximately six times that pressure.

• Steam boilers are equipped with a low-water cutoff device to protect the appliance from burning out if it should run out of water. Only large hot-water space heating boilers with a capacity exceeding 400,000 Btu/h are presently required by code to be equipped with low — water cutoffs. (Note: Many HVAC contractors who install the smaller residential hot-water boilers strongly recommend the addition of a low-water cutoff device to these appliances to prevent burnout if the boiler loses its water.)

Steam and Hot-Water Space Heating Boilers

• Steam boilers require makeup feed to replace water lost through evaporation and the production of steam during normal opera­tion. Hot-water boilers can operate with little or no need for makeup water under the same normal operating con­ditions.

Figure 15-2 Residential hot water

(hydronic) boiler. (Courtesy Hydrotherm, Inc.)

подпись: 
figure 15-2 residential hot water
(hydronic) boiler. (courtesy hydrotherm, inc.)
The design and construction of the lower chamber depends upon the type of fuel used to fire the boiler. It serves as a combus­tion chamber for coal-fired and oil-fired boilers and as a com­partment for housing the gas burner assembly on gas-fired boilers. These gas burner assem­blies are commonly designed for easy removal so that they can be periodically cleaned or serviced.

Oil burners are externally mounted with the burner nozzle extending into the combustion chamber. This is also true of gas con­version burners. Gas burner assemblies, on the other hand, are located inside the lower chamber of the boiler.

The cast-iron sections or steel tubes in the upper chamber of the boiler contain water that circulates through the pipes in the heating system in the form of either steam or hot water. The heat from the combustion process in the lower chamber of the boiler is trans­ferred through the metal surface of the cast-iron sections or steel tubes to the water contained in them, causing a rise in temperature. The amount of water contained in these passages is one of the ways in which steam boilers and hot-water space heating boilers are dis­tinguished from one another. In hot-water space heating boilers these passages are completely filled with water; whereas in low- pressure steam boilers only the lower two-thirds are filled. In steam boilers the water is heated very rapidly, causing steam to form in

The upper one-third. The steam, under pressure, rises through the supply pipes connected to the top section of the boiler.

A boiler jacket contains a number of different openings for pipe connections and the mounting of accessories. The number and type of openings on a specific boiler jacket depends upon the type of boiler (i. e., steam or hot water). Among the different openings to be found on a boiler jacket are the flue connection, water feed (supply) connection, inspection and cleanout tapping, blowdown tapping, relief valve tapping, control tapping, drain tapping, expansion tank tapping, and return tapping. There are also gas and oil burner con­nections. Figure 15-3 illustrates the arrangement of control tap­pings in a Weil-McLain oil-fired boiler.

Most (but not all) of the controls on low-pressure steam and hot-water space heating boilers fired by the same fuel are similar in design and function, but there are exceptions. For example, a few boiler controls and fittings are designed to be specifically used on steam boilers; others are found only on hot-water space heating boilers. The various boiler controls and fittings are described in the appropriate sections of this chapter.

Location Size Steam Water

On Boiler (Inches) Boilers Boilers

A

6

Supply

Supply

B

4

Return

Return

C

3

Safety

Valve

Relief

Valve

D

Rn

Blow-off

Not

Required

E

1

Water

Feeder

Not

Required

F

3/4

Pressure

Limit

Temperature

Limit

G

3/4

Drain

Drain

H

1/2

Pressure

Gauge

Temperature and altitude gauge

J

1/2

Gauge glass and low water cut-off

Not

Required

K ■

3/e

Try-

Cocks

Not

Required

■ Tappings available on special order only

Boiler Rating Method

The construction of low-pressure steel and cast-iron heating boilers is governed by the requirements of the ASME Boiler and Pressure Vessel Code. This is a nationally recognized code used by boiler manufacturers, and any boiler used in a heating installation should clearly display the ASME stamp. State and local codes are usually patterned after the ASME Code.

The location of the identification symbols used by the ASME is specified by the code and determined by the type of boiler. For example, on a water-tube boiler, it appears on a head of the steam — outlet drum near and above the manhole opening. On vertical fire — tube boilers, the stamp bearing the identification symbol should appear on the shell above the fire door and handhole opening. Other types of boilers (e. g., Scotch marine and superheaters) have their own specified location for the identification symbol stamp.

The ASME Boiler and Pressure Vessel Code applies only to boiler construction, specifically to maximum allowable working pressures, not to its heating capacity. A number of different meth­ods are used to rate the heating or operating capacity of a boiler. The boiler manufacturers have developed their own ratings, but these are generally used along with rating methods available from several professional and trade associations.

The Steel Boilers Institute no longer exists, but its SBI rating is still found on many existing steel boilers. The I=B=R (or IBR) logo was created by the now defunct Institute of Boiler and Radiator Manufacturers to indicate the gross output(s) at 100 percent firing rate for most sectional cast-iron boilers. The I=B=R rating logo is now used by the Hydronics Institute Division of the Gas Appliance Manufacturers Association (GAMA).

The Mechanical Contractors Association of America has devised a method for rating boilers not covered by either the SBI or I=B=R codes. Finally, gas-fired boilers are rated in accordance with meth­ods developed by the American Gas Association.

Other rating logos appearing on boilers and in their installation and operation manuals are the Underwriters Laboratories, Inc. (UL) and the Underwriters’ Laboratories of Canada logos.

In terms of its heating capacity, the rating of a boiler can be expressed in square feet of equivalent direct radiation (EDR) or thousands of Btu/h. Sometimes a boiler horsepower rating is also given, but this has proven to be misleading.

For steam boilers, 1 square foot of equivalent direct radiation (EDR) is equal to the emission of 240 Btu/h. For a water boiler, 1 square foot of EDR is considered equal to the emission of 150 Btu/h.

A boiler horsepower (bhp) is the evaporation of 34.5 lb of water into dry steam from and at 212°F. For rating purposes, 1 bhp is considered as the heat equivalent of 140 ft[4] of steam radiation per hour. In some cases bhp ratings are obtained by dividing steam SBI ratings by 140.

A boiler is rated according to its operating or heating capacity, but this rating will vary in accordance with the type of load used as the basis for the rating. The three types of connected loads used to determine the rating of a boiler are:

1. Net load

2. Design load

3. Gross load

Net load refers to the actual connected load of the heat-emitting units in the steam or hot-water heating system. Design load includes the net-load rating plus an allowance for piping heat loss. Finally, gross load will equal the net load and the piping heat loss, plus an additional allowance for the pickup load.

Boiler Heating Surface

The boiler heating surface (expressed in square feet) is that portion of the surface of the heat transfer apparatus in contact with the fluid being heated on one side and the gas or refractory being cooled on the other side. The direct or radiant heating surface is the surface against which the fire strikes. The surface that comes in contact with the hot gases is called the indirect or convection surface.

The heating capacity of any boiler is influenced by the amount and arrangement of the heating surface and the temperatures on either side. The arrangement of the heating surface refers to the ratio of the diameter of each passage to its length, as well as its con­tour (straight or curved), cross-sectional shape, number of passes, and other design variables.

Boiler Efficiency

The boiler efficiency is the ratio of the heat output to the caloric value of the fuel. Boiler efficiency is determined by various factors including the type of fuel used, the method of firing, and the control settings. For example, oil — and gas-fired boilers have boiler efficiencies ranging from 70 to 80 percent. A hand-fired boiler in which anthracite coal is used will have a boiler efficiency of 60 to 75 percent.

Boiler Energy Efficiency

Two government programs have been created within the last 20 years to rate the energy efficiency of different heating appliances such as furnaces, boilers, water heaters, and heat pumps. These two programs are (1) the annual fuel utiliza­tion capacity (AFUE) program and (2) the Energy Star Certification program.

• Annual Fuel Utilization Capacity (AFUE). The energy effi­ciency of an oil-, gas-, or coal-fired boiler is measured by its annual fuel utilization capacity (AFUE). The AFUE ratings for boilers manufactured today are listed in the boiler man­ufacturer’s literature. Look for the EnerGuide emblem for the efficiency rating of that particular model. The higher the rating, the more efficient the boiler. The government has established a minimum rating for boilers of 78 percent. Mid-efficiency boilers have AFUE ratings ranging from 78 to 82 percent. High-efficiency (condensing) boilers have AFUE ratings ranging from 88 to 97 percent. Conventional (noncondensing) steam and hot-water space heating boilers have AFUE ratings of approximately 60 to 65 percent.

• Energy Star Certification. Energy Star is an energy perfor­mance rating system created in 1992 by the U. S. Environmental Protection Agency (EPA) to identify and certify certain energy-efficient appliances. The goal is to give special recognition to companies who manufacture products that help reduce greenhouse gas emissions. This voluntary labeling program was expanded by 1995 to include furnaces, boilers, heat pumps, and other HVAC equipment. Both the Energy Star label and an AFUE rating are used to identify an energy-efficient appliance.

Types of Boilers

The boilers used in low-pressure steam and hot-water space heating systems can be classified in a number of different ways. Some of the criteria used in classifying them are:

5. Length of travel of the hot gases

6. Type of heating surface

7. Type of fuel used

Most boilers are constructed of either cast iron or steel. A few are constructed from nonferrous materials such as aluminum. Cast — iron boilers generally display a greater resistance to the corrosive effects of water than steel ones do, but the degree of corrosion in steel boilers can be significantly reduced by chemically treating the water.

The heating core of many boilers is formed by joining together a series of cast-iron sections either horizontally (so-called pancake construction) or vertically (Figure 15-4). In the horizontal cast-iron section design, the heating surface of each cast-iron section is exposed at right angles to the rising flue gases (Figure 15-5). The water travels in a zigzag path from section to section in a manner similar to the flow of water in a steel tube boiler (Figure 15-6).

Steel boilers may be classified with respect to the relative posi­tion of water and hot gases in the tubular heating surface. In fire — tube boilers, for example, the hot gases pass within the boiler tubes while the water being heated circulates around them. In water-tube boilers, the reverse is true. Flexible steel tubes are used in some boil­ers for the circulation of the water around the heat rising from the fire (Figure 15-7).

A hot-water (hydronic) copper-fin tube operates on a different principle from the cast-iron and steel boilers. It is designed to transfer heat almost instantly to the water (Figure 15-8). Water flows across the boiler heat exchanger, picks up heat, and then moves through the pipes to the heat convectors, radiators, or panels.

Note

If the water stops flowing while the burner is still running, heat will build up until the water flashes into steam and dam­ages the boiler. This condition is similar to dry firing in cast — iron and steel boilers. It can be avoided by installing a flow switch in the path of the water. The switch turns off the burner when the water stops flowing.

Boilers can also be classified according to the number of passes made by the hot gases (e. g., one pass, two passes, three passes) The length of travel of the hot gases is another method used for classify­ing boilers. The efficiency of a boiler heating surface depends, in

Steam and Hot-Water Space Heating Boilers Steam and Hot-Water Space Heating Boilers

HORIZONTAL CAST-IRON BOILER SECTIONS

подпись: horizontal cast-iron boiler sectionsVERTICAL CAST-IRON BOILER SECTIONS

Figure 15-4 Cast-iron boiler sections.

Part, upon the ratio of the cross-sectional area of the passage to its length.

Among the various fuels used to fire boilers are oil, gas (natu­ral and propane), coal, and coke. Conversion kits for converting a boiler from one gas to another are available from some manu­facturers. Changing from coal (or coke) to oil or gas can be accomplished by using conversion chambers and making certain other modifications. See Chapter 16, “Boiler and Furnace Conversions.”

Steam and Hot-Water Space Heating Boilers

Figure 15-5 Direction of heat travel. (Courtesy Hydrotherm, Inc.)

Steam and Hot-Water Space Heating Boilers

FLUE

OUTLET SLIPPl-Y

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers

DRAFT DIVERTER

RELIEF VALVE

OUTER STEEL JACKET-

FLEXIBLE STEEL TUBES

INSULATION

GAS BURNER PORTS

PRESSURE GAUGE

CONTROL PANEL

GAS BURNER MIXER

TUBES GAS BURNER CONTROLS

Figure 15-7 Steel-tube boiler. (Courtesy Bryan Steam Corp.)

Electricity can also be used to fire boilers. One advantage in using electric-fired boilers is that the draft provisions required by boilers using combustible fuels is not necessary.

Note

Unlike the boilers fired by fossil fuels (oil, gas, coal, etc.), elec­tric boilers do not have an AFUE efficiency rating. They oper­ate at almost 100 percent efficiency.

Compression Tank

Steam and Hot-Water Space Heating Boilers

The classification criteria described above are selective and lim­ited to the more common types in use. Considering the multiplicity of boiler types and designs available, it is extremely difficult to establish a classification system suitable for all of them.

Gas-Fired Boilers

Gas-fired boilers generally consist of several closely placed cast-iron sections or steel tubes with a series of gas burners (i. e., a gas burner assembly) placed beneath them. The flue gases pass upward between the sections or tubes to the flue collector.

Draft losses are kept low in these boilers because the pressure at which the gas is supplied is generally sufficient to draw in the amount of air necessary for combustion. The fact that there is no fuel-bed resistance, as there is with coal-fired boilers, also con­tributes to low draft loss.

The draft in gas-fired boilers is generally nullified by the diverter; consequently, the resistance offered by the boiler passages is not an important variable. When there is a problem of excessive draft, it can be controlled by installing a sheet-metal baffle in the flue con­nection at the boiler [see Control Excessive Draft (Gas-Fired Boilers) in this chapter].

Most of the controls and accessories used to operate gas-fired boilers are described in considerable detail in Chapter 2, “Gas Burners”; Chapter 5, “Gas and Oil Controls”; and Chapter 6, “Other Automatic Controls” of Volume 2. The exact placement of these controls and accessories on the steel boiler jacket may differ slightly from one manufacturer to another, but not significantly. The major difference will be in the types of controls and accessories used to govern the temperature, pressure, and of the heat-conveying medium; and this is determined by whether it is a steam or hot — water space heating boiler (see Steam Boiler Valves, Controls, and Accessories and Hot-Water Boiler Valves, Controls, and Accessories in this chapter). An exploded view of a conventional gas-fired hot-water (hydronic) boiler is shown in Figure 15-9.

OilFired Boilers

An oil-fired boiler contains a heat transfer surface consisting of either cast-iron sections or steel tubes and a special combustion chamber shaped to meet the requirements of an oil burner.

The oil burner is a device designed to mix fuel oil with air under controlled conditions and to deliver the mixture to the combustion chamber for burning. The burner is mounted outside the chamber. The oil burners used in residential heating boilers are usually high — pressure atomizing burners, although other types are used on occasion.

It is possible to convert a coal-fired boiler to oil by redesigning the combustion chamber. However, boilers specifically designed to use oil as a fuel have proven to be more efficient and economical than coal-fired boilers converted to oil.

Read Chapter 1, “Oil Burners”; Chapter 5, “Gas and Oil Controls”; and Chapter 6, “Other Automatic Controls” in Volume 2 for a description of the controls and accessories used to operate

Oil-fired boilers. Also read the appropriate sections in this chapter (Steam Boiler Valves, Controls, and Accessories and Hot-Water Boiler Valves, Controls, and Accessories). An exploded view of a conventional oil-fired boiler is is shown in (Figure 15-10).

Coal-Fired Boilers

A typical coal-fired boiler contains a grate for the fuel bed, located beneath metal heat transfer surfaces such as cast-iron sections (Figure 15-11). The mixture of hot gases resulting from combustion

Steam and Hot-Water Space Heating Boilers

(Courtesy Hydrotherm, Inc.)

Rises through the passages in the sections, transferring its heat to the water contained in them.

The controls and accessories used to operate a coal-fired boiler are described in Chapter 3, “Coal Firing Methods” of Volume 2. The controls and accessories used to govern the temperature, pres­sure, and flow rate of the heat-conveying medium are described in

Boiler jacket Crossover assembly

Steam and Hot-Water Space Heating Boilers

Figure 15-10 Conventional oil-fired hot water (hydronic) boiler.

(Courtesy Hydrotherm, Inc.)

This chapter (see Steam Boiler Valves, Controls, and Accessories and Hot-Water Boiler Valves, Controls, and Accessories).

A coal-fired boiler requires more draft than gas- or oil-fired boil­ers because of the resistance of the fuel bed. Special care must be given to the design of the chamber surrounding the grate to ensure

Steam and Hot-Water Space Heating Boilers

Figure 15-11 Older model of a vertical, round coal-fired boiler.

Sufficient volume for a proper mixture of fuel and air. Provisions must also be made for introducing sufficient air through the grate (and fuel bed) for the combustion process.

Before the introduction of oil burners, coal was the fuel in general use, and it is still largely used, especially for heating plants already built and in cases where the owner cannot afford the expense of converting to oil. Moreover, some cast-iron boilers designed to burn coal are not suited to burn oil, which adds to the expense of a conversion job. Finally, the increasing worldwide shortage of

Oil will find it more and more necessary to find applications for low-polluting uses of coal as a heat source. An important use for coal will be steam and hot-water heating systems equipped with devices to reduce the polluting effect of burning coal.

Coal-burning boilers may be hand-fired or fitted for automatic stoker operation. In many automatic stokers, fuel is carried from the hopper through a feed tube by means of a rotating worm. Intermittent action of the worm agitates the fuel bed, prevents arch­ing of the coal in the retort, and ensures that the incoming air reaches every part of the fire at all times. An auxiliary air connec­tion between the feed tube and the windbox prevents gas accumu­lating in the tube and eliminates any tendency to “smoke back” through the hopper.

An underfeed stoker is one in which the fuel is fed upward from underneath. The action of a screw or worm carries the fuel back through a retort, from which it passes upward as the fuel is con­sumed, the ash being finally deposited on dead plates on either side of the retort, from which it can be removed.

Only part of the fuel being burned is actually burned in the fuel bed. Under the influence of the high temperature created in the fuel bed and lack of sufficient air, unburned gases are released above the retort and tuyeres; and unless these gases are mixed with air and burned inside the combustion chamber, they will leave the boiler unburned, carrying away a large percentage of the heat energy value originally contained in the coal.

Nearer the outside of the fuel bed, the fuel burns less violently, and much more air is passed through the fuel bed than is necessary. It is this excess air that must be mixed with the unburned gases issuing from the central point of the fuel bed if high combustion efficiency is to be realized.

Coke can also be used to fire boilers, but it requires different handling. In order to be completely consumed, coke needs a greater volume of air per pound of fuel than coal and therefore requires a stronger draft, which is increased by the fact that it can burn economically only in a thick bed.

Because less coke is burned per hour per square foot of grate than coal, a larger grate and a deep firepot are required to accom­modate the thick bed of coke.

The quick-flaming combustion that characterizes coal is not pro­duced by coke because the latter fuel contains very little hydrogen; however, a coke fire is more even and regular.

Electric Boilers

Compact wall-mounted electric boilers are used in residential hot — water (hydronic) heating systems (Figure 15-12). Heat is generated by electric heating elements immersed in water housed in a water­proof cast-iron shell. Although small in size, these boilers are capa­ble of generating as high as 90,000 Btu/h, enough heat for the average eight-room house.

Steam and Hot-Water Space Heating Boilers

Figure 15-12 Wall-mounted electric boiler. (Courtesy American Standard)

The basic components of a conventional electric boiler are shown in Figure 15-13. An automatic air vent located above the cast-iron boiler shell is used for bleeding off trapped air from the water system. Safety devices include a water pressure-relief valve for reducing system pressure, an adjustable limit control that allows selection of maximum boiler water temperature for system design, and preset pressure controls to guarantee safe operation within the prescribed pressure range.

The limit control is an immersion device that shuts off the boiler if the temperature exceeds a predetermined setting. The preset pres­sure controls consist of a high-pressure switch and a preset safe-fill switch. The high-pressure switch is designed to deenergize the boiler if the pressure reaches 28 psi. This switch is reinforced by the relief valve, which is preset to relieve system pressure at 30 psi. The preset safe-fill switch prevents the electric heating elements from being energized unless pressure in the system is 4 psi. This

Steam and Hot-Water Space Heating Boilers

Figure 15-13 Basic components of a conventional

Electric boiler. (CourtesyAmerican Standard)

Prevents any chance of the heating elements burning out should an attempt be made to operate the boiler dry.

The circulator, cast-iron boiler shell, and expansion are all enclosed within the outer steel jacket of the boiler. The electric heat­ing elements are mounted inside the ends of the boiler casting. A drain valve mounted below the boiler casting is designed for the connection of standard hose fittings.

All controls are prewired and mounted in place by the manufac­turer. The only electrical connections that need to be made during the installation of the boiler are those to the main power supply and room thermostat.

The sequencing relay switch provides for an incremental loading on the electric service line. This reduces line voltage fluctuation and prevents power surge during energizing.

Minimum installation clearances for the boiler just described are shown in Figure 15-14. Two air openings of 108 in2 (6" X 18") each are required for closet installation.

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers

18"

подпись: 18"

18"

подпись: 18"FRONT VIEW

Figure 15-14 Minimum installation clearances.

A typical piping arrangement for a series-loop, hot-water heat­ing system using an electric boiler of this capacity is shown in Figure 15-15. An outline of the procedure used for filling this sys­tem is as follows:

1. Close all zone valves except the one for the zone to be purged.

2. Open the boiler drain valve.

3. Open the fill valve.

4. Close the purge valve.

5. Vent air from the boiler by manually opening the relief valve.

WATER SUPPLY, CHECK VALVE (IF USED) "

FILL VALVE (MANUAL)’

подпись: water supply, check valve (if used) "
fill valve (manual)'

PURGE VALVE (GATE)

подпись: purge valve (gate)

SUPPLY TO SYSTEM U

подпись: supply to system u

FOR MULTIPLE ZONE OR CIRCUIT OPERATION

подпись: for multiple zone or circuit operation Steam and Hot-Water Space Heating Boilers Steam and Hot-Water Space Heating Boilers

R*~—CxJ

подпись: r*~—cxj■ On a single-circuit job balancing valves and 1 ‘ zone valves not needed

6. Open the valve for the second zone to be purged and close the first.

7. After the second zone is purged, open the third zone valve and close the second.

8. Repeat the procedures described in Steps 6 and 7 for as many zones as are in the system.

9. When all zones have been purged, close the boiler drain cock.

10. When water appears at the relief valve, release the lever and allow the valve to close.

11. Continue filling until pressure gauge reads approximately

12 psi. Close fill valve.

12. Open purge valves.

Where multiple circuits are used without zone valves, balancing valves should be installed for balancing. These can be shut off to purge each circuit individually. For a single-loop system, no addi­tional vents, valves, drains, or other accessories are required.

Smaller electric-fired boilers having outputs ranging from approx­imately 6000 to 20,000 Btu/h are available for individual apartment heating or for residential zoned heating through the use of several units. The basic components of one of these smaller boilers is shown in Figure 15-16. The piping arrangement for a series-loop heating

Steam and Hot-Water Space Heating Boilers

AUTOMATIC AIR VENT

ADJUSTABLE LIMIT CONTROL

PRESET

PRESSURE

CONTROL

DRAIN VALVE

WATER PRESSURE RELIEF VALVE

TRANSFORMER

ELECTRIC HEATING ELEMENT

CIRCULATOR

Steam and Hot-Water Space Heating Boilers

-K

подпись: -k

WATER SUPPLY

подпись: water supply

CHECK VALVE (IF USED)

подпись: check valve (if used)

PURGE VALVE (GATE)

подпись: purge valve (gate) Steam and Hot-Water Space Heating Boilers

FILL VALVE (MANUAL)

подпись: fill valve (manual)

AIR VENT

подпись: air vent
 
RETURN

Y

Figure 15-17 Typical piping arrangement for a hot-water heating system.

System using one of these boilers is illustrated in Figure 15-17. The procedure for filling the system is similar to the one just outlined except for the elimination of Steps 6 to 9.

High-Efficiency Boilers

The development of the high-efficiency (condensing) boiler came about as a direct reaction to the oil crisis in the 1970s. Higher heat­ing oil costs and the public’s desire for greater fuel efficiency resulted in the development of boilers with a much higher efficiency than the conventional ones. Using the ASFUE ratings, the conventional boilers had a fuel efficiency in the 60 percent range, whereas the new high­efficiency boilers have a fuel efficiency rating of 85 percent or higher.

A high-efficiency boiler vents its combustion gases in PVC pipe through a sidewall instead of the chimney. The boiler also requires an induced-draft fan (power vent) and an outside source air-intake for combustion air. Typical high-efficiency (condensing) oil-fired and gas-fired hot water (hydronic) boilers are illustrated in Figures

15- 18 and 15-19.

Steam Boiler Valves, Controls, and Accessories

The boilers used in steam heating systems are fitted with a variety of devices designed to ensure the safe and proper operation of the boiler. These boiler valves, controls, and accessories can be divided by function into two basic categories: (1) indicating or measuring devices, and (2) controlling devices. Sometimes both functions are combined in a single unit.

Steam and Hot-Water Space Heating Boilers

(Courtesy Hydrotherm, Inc.)

Steam boilers operate under high pressures and temperatures. To avoid serious injury from scalding steam or water, never begin any service or repairs shutoff before taking the following precautions:

• Wait for the boiler to cool down to 80°F (27°C) or more.

• Wait for the boiler pressure to drop to 0 psi (0 bar).

N m

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers

Figure 15-19 High-efficiency gas-fired hot water

Boiler. (Courtesy Weii-Mciain)

Disconnect the electrical power before making any electrical connections.

Connect a temporary drain pipe to the control opening to pre­vent exposure to steam discharge.

Principal Steam Boiler Valves, Controls, and Accessories

A steam boiler should be equipped with the following valves, controls, and accessories:

• Water level gauge: A gauge that measures the water level inside the boiler.

• Low-water cutoff: A device that automatically switches off the burner if the boiler water level becomes too low for safe operation.

• Pressure gauge: A gauge that measures the operating pres­sure inside the boiler.

• Safety relief valve: A valve that discharges excess steam when boiler pressure exceeds the maximum pressure limit on the valve.

• High-pressure limit switch: A switch that shuts off the burner when the boiler pressure exceeds a preset level.

• Condensate pump: A small pump used to return conden­sate to the boiler.

Indicating devices include water gauges, pressure gauges, and similar types of devices that provide information about the operat­ing conditions in the boiler. They are used to indicate temperatures, pressures, or water levels that fall outside the design limits of the boiler. Controlling devices include boiler equipment designed to cause changes in these conditions. For example, pressure relief valves are used to relieve excess pressure in the boiler.

Steam Boiler Low-Water Cutoffs

Steam boilers must be provided with a water-level control device to shut off the automatic fuel-burning equipment when the water level in the boiler drops to a level too low for safe operation. This water — level control device is referred to as a low-water cutoff. The two types of low-water cutoff used on steam boilers are the float type and the probe type.

Note

According to Section 4 of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, all resi­dential steam boilers, regardless of their size or location, must have a low-water cutoff device installed.

A float low-water cutoff device consists of a cutoff switch operating in conjunction with a float located in the boiler water

Or in a float chamber installed next to the boiler. The float is con­nected through a linkage to a switch that operates a feed water valve. As the water level falls, the float drops with it until it reaches a point at which the feed water switch is actuated. If the water level continues to fall, a second switch connected to the float by a linkage is actuated, and the automatic fuel-burning equipment is shut off.

A probe low-water cutoff device depends on the flow of a low electrical current to control the operation of the automatic fuel — burning equipment. The electrical current flows from the probe through the water to keep the relay energized. When the water level falls, there will be a point at which the probe loses direct contact with the water. As a result, the contact is broken, and the flow of the electrical current is stopped. This, in turn, causes the relay to be energized, which results in shutting off the fuel-burning equipment. A probe low-water device cannot be used in the direct operation of feed water valves.

There are variety of ways to attach a low-water cutoff to a steam boiler (Figure 15-20). Some are designed to be attached to the glass gauge on the outside of the boiler. In this type of instal­lation, the elevation of the low-water cutoff is already determined by the location of the glass gauge tappings on the side of the boiler.

Note

If there is a horizontal cast line on the outside of a gauge — mounted low-water cutoff, make certain it is located above the minimum safe water level specified by the boiler manufacturer.

Another type of low-water cutoff installation is to attach it directly to the side of the boiler. As shown in Figure 15-20, it is not connected to the water gauge. Its elevation is determined by the location of the pipe tappings on the boiler. The body of the control must be located at or above the manufacturer’s minimum safe water level.

On some boilers, the low-water cutoff is combined with a water feeder to add water to the boiler when the water level falls below the safe operating limit. An example of one of these combined units is shown in Figure 15-21. They are available with either single or dual switch assemblies. The single double-throw switch assembly pro­vides a combination feeder and burner cutoff switch, with an extra terminal for line voltage with single-pole, double-throw service

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers

Figure 15-20 Examples of boiler connections for low-water cutoffs.

(Figure 15-22). The dual switch assembly is used for line voltage burner service and for independent low — (or high-) voltage alarm, feed valve, or pump starter service. When an emergency condition occurs, the switch interrupts the current to the burner and shuts it off. When the emergency has passed, the water feeder takes over and feeds makeup water when needed for normal operation. On units equipped with dual switch assemblies, the alarm, feed valve, or pump starter switch actuates just before the burner switch cuts the

Steam and Hot-Water Space Heating Boilers

GAUGE GLASS CONNECTION

DRAIN VALVE (GATE TYPE)

SWITCH

ASSEMBLY

STEAM SEAL BELLOWS

WATER SEAL BELLOWS

FEED VALVE

STRAINER

REMOVABLE SCREEN

Figure 15-21 Basic components of a boiler water feeder and

Cutoff. (Courtesy Watts Regulator Co.)

Firing. A typical installation of a Watts 60 LWD boiler water feeder and cutoff on a steam boiler is illustrated in Figure 15-23.

A low-water cutoff without a boiler water feeder is usually ade­quate for providing automatic low-water safety protection for most small boilers (Figure 15-24). These units can be installed on any boiler having gauge glass connections.

Direct installation (built-in) low-water cutoffs are available for steam boilers with limited space in the boiler jacket (Figure 15-25). Each unit contains a switch assembly, a float and bellows, and a threaded barrel casting that fits into a 2[5]I2-in tapping in the side of the boiler.

Installing a Low-Water Cutoff

The Watts No. 89 low-water cutoff illustrated in Figure 15-24 is suit­able for low-pressure steam heating boilers with a maximum operat­ing steam pressure of 15 psi. Although installation is relatively simple,

Steam and Hot-Water Space Heating Boilers

WHEN USED FOR BURNER AND ALARM SERVICE

подпись: when used for burner and alarm service

TOP

подпись: top

LINE

подпись: line

LOW-VOLTAGE TRANSFORMER, IF LINE VOLTAGE CANNOT BE USED

подпись: low-voltage transformer, if line voltage cannot be used

ALARM

подпись: alarm Steam and Hot-Water Space Heating BoilersWHEN USED FOR BURNER SWITCH ONLY

TOP

подпись: topLINE

N3dO

©-

N0WW03

E-

BURNER

Figure 15-22 A single-switch assembly and wiring diagram.

(Courtesy Watts Regulator Co.)

BURNER

The instructions should be carefully followed. This is a boiler safety device, and it must operate properly or the boiler can be damaged.

Before installing the Watts low-water cutoff, the gauge glass and cocks must be removed from the boiler. When you have done this, proceed as follows:

SUPPLY

Steam and Hot-Water Space Heating Boilers

Figure 15-23 Typical installation of a boiler water feeder and cutoff.

(Courtesy Watts Regulator Co.)

2. Insert the long nipple into the lower glass gauge tapping on the boiler.

3. Swing the entire float chamber until the nipple is made up tight. Line up the cutoff so that the top of the switch box is level (Figure 15-27).

4. Screw the nipple in the tee carrying the tubing connector the upper gauge glass tapping and pull up tight. Install a compres­sion coupling in the top float chamber tapping (Figure 15-28).

5. Hold the tube bend in position and mark the tube on a level with the top of the top of the hex on the compression cou­pling. Cut off the tube at this mark (Figure 15-29).

6. Slide the ring and nut over the end of the connector tube.

7. Slide the end of the tube into the compression coupling in the float chamber, and tighten both couplings.

Steam and Hot-Water Space Heating Boilers

Figure 15-24 A low-water cutoff. (Courtesy Watts Regulator Co.)

8. Install a drain valve in the bottom float chamber tapping (Figure 15-30).

9. Reinstall the gauge cocks in the end of the tees and replace the gauge glass.

Figure 15-25 Direct — installation low-water

Steam and Hot-Water Space Heating BoilersCutoff. (Courtesy Watts Regulator Co.)

Steam and Hot-Water Space Heating Boilers

Figure 15-26 Installing the short nipple in the float chamber

Tapping. (Courtesy Watts Regulator Co.)

Steam and Hot-Water Space Heating Boilers

Figure 15-27 Installing long nipple into lower-gauge glass tapping in boiler.

(Courtesy Watts Regulator Co.)

Note

подпись: note Steam and Hot-Water Space Heating Boilers

FLARED

CONNECTION

подпись: flared
connection

Figure 15-28 Installing compression coupling.

(Courtesy Watts Regulator Co.)

подпись: figure 15-28 installing compression coupling.
(courtesy watts regulator co.)

Am-

подпись: am-The drain valve on the bottom of the low-water cutoff should be opened once a month (or oftener) during boiler operation to flush out sediments from the float chamber.

Steam and Hot-Water Space Heating Boilers

Figure 15-29 Cutting tube on level with top of

Hex. (Courtesy Watts Regulator Co.)

Steam and Hot-Water Space Heating Boilers

Figure 15-30 Low-water cutoff with drain valve. (Courtesy Watts Regulator Co.)

Table 15-1 Troubleshooting a Low-Water Cutoff

Symptom and Possible Cause Possible Remedy

Burner Fails to Shut off When

Boiler Reaches Low-Water Stage

(a) Switch contacts are (a) Remove switch and operate

Fused together. switch manually to determine

Correct switch opoeration.

Replace switch if defective. Check electrical load to low-water cutoff control to make sure it is within ratings of switch. Overload may be burning out switch.

(b)

подпись: (b)

(b) Float chamber is

Filled with mud, scale, or sediments.

Clean float chamber; Check switch terminals 1 and 2 to see if they are open when water level is below the low-water cutoff control. If not, remove the switch and manually operate the terminals to determine if they will open and close freely. If not, replace switch.

Electric Water Feeder in Low-Water Cutoff Will Not Shut Off

(a)

подпись: (a)

(a) Switch contacts are fused together.

(b) Bellows in float chamber (b) covered with mud, scale, or sediment build up.

Remove switch and operate switch manually to determine correct switch operation. Replace switch if defective.

Open float chamber and clean or replace the bellows. Check switch terminals 3 and 4 to see if they are open when water level is above the low-water cutoff control. If they, remove the switch and manually operate the terminals to determine if they will open and close freely. If not, replace switch.

Fusible Plugs

A fusible plug is used on some boilers as a protection against dan­gerous low-water conditions. Examples of fusible plugs are shown in Figure 15-31. They are generally made of bronze and filled with pure tin. When the temperature (and pressure) in the boiler builds up to about 450°F (the approximate melting point of pure tin), the tin core melts and relieves the pressure within the boiler.

Fusible plugs must comply with ASME standards and are avail­able in several sizes for use on boilers having a steam pressure of less than 250 psi.

Pressure Relief Valves

A steam boiler is equipped with a pressure relief valve (or valves), which opens and releases excess steam at or below the maximum allowable working pressure of the boiler (Figures 15-32 and 15-33). These are pop safety-relief valves designed to comply

(A) Inside type

(B) Steam and Hot-Water Space Heating BoilersOutside type

Figure 15-31 Fusible plugs. (Courtesy Lunkenheimer Co.)

With the requirements of the ASME Boiler and Pressure Vessel Code. They are also sometimes called safety valves, safety relief valves, or simply relief valves.

Pop safety valves exhaust the steam through holes drilled around the top of the spring housing. They function by “popping” wide open at the set pressure, remaining in that position until pressure has dropped the predetermined amount (commonly known as blowback or blowdown), and then snapping shut instantly. Lift levers and drain holes in the discharge side of the valve are required by ASME codes.

On the low-pressure boilers used in resi­dential steam heating systems, the pressure relief valve is commonly preset to open and release steam when a maximum pressure of 15 psi is reached in the boiler. The valve closes when the steam pressure once again falls below 15 psi.

Warning

NEVER use a pressure relief valve with a pres­sure relief rating higher than the maximum working pressure of the boiler. If the boiler exceeds its maximum (safe) working pressure, it could rupture and explode at a pressure less than the rating of the pressure relief valve.

Steam and Hot-Water Space Heating Boilers

Figure 15-33 Pressure relief valve.

(Courtesy Cash Acme)

Pressure Controllers

In many steam heating systems, a line-voltage pressure controller is used to provide operating control, automatic limit protection, or a manual reset limit for protection in pressure systems of up to 300 psi. One such pressure controller is the Honeywell L404 Pressuretrol® Controller shown in Figure 15-34. It is available in a variety of different models.

Steam and Hot-Water Space Heating Boilers

MAIN SCALE PRESSURE DIFFERENTIAL

ADJUSTMENT SCREW ADJUSTMENT SCREW CONDUIT KNOCK OUT

подпись: main scale pressure differential
adjustment screw adjustment screw conduit knock out

Figure 15-34 Honeywell L404 Pressuretrol® Controller.

(Courtesy Honeywell, Inc.)

подпись: figure 15-34 honeywell l404 pressuretrol® controller.
(courtesy honeywell, inc.)

POTENTIOMETER COIL SLIDING CONTACT DIFFERENTIAL SCALE DIFFERENTIAL INDICATOR OPERATING LEVER CONDUIT KNOCKOUT BELLOWS HOUSING U-S INDICATOR MARK UNISON-SEQUENCE ADJUSTMENT DIAL

подпись: potentiometer coil sliding contact differential scale differential indicator operating lever conduit knockout bellows housing u-s indicator mark unison-sequence adjustment dialSome models of the L404 are designed to provide on-off and pro­portional control of steam boilers fired by proportional-type burners. These can be field adjusted to operate either in unison (burner starts
at high fire) or in sequence (burner starts at some firing rate other than high fire). A common bellows assembly located in the bellows housing actuates the stop-start switch (on-off operation) and the wiper of the 185-ohm potentiometer (proportional operation).

The L404 operates strictly as a high-limit pressure safety control on steam heating boilers (see Figure 15-35). It breaks the electrical circuit on pressure rise. A variation of this controller is used for suspension-type unit heaters.

Direct control for a proportional motor operating an automatic burner can be obtained by using a pressure controller with two potentiometers operating in unison. This design makes it possible to control two motors simultaneously. It is also provided with an adjustable throttling range.

Pressure controllers are also available for vapor or vacuum sys­tems. A pressure controller with a bellows-operated mercury switch is manufactured by Honeywell for use on vapor heating systems with pressures of up to 4 psi (see Figure 15-36). It can be used as a boiler high-limit control with cut-in and cutout settings in the vac­uum range. In such installations, the heating system must include a vacuum pump and a siphon loop.

Steam and Hot-Water Space Heating Boilers

/1 1/4-IN BLACK IRON PIPE WITH 1/4-18 NPT EXTERNAL TRICADS ON BOTH ENDS. BEND THE STEAM TRAP (SIPHON LOOP) TO LEVEL THE CONTROLLER.

Figure 15-35 Steam boiler mounting of Honeywell L404 Pressuretrol® Controller. (Courtesy Honeywell, Inc.)

Vacuum Relief Valve

A vacuum-relief valve (Figure 15-37) is an effective means of pro­viding protection against the buildup of excessive vacuum condi­tions in steam heating and steam processing systems. It is also used in combination with temperature-pressure relief valves on water heaters.

Vacuum conditions can often occur after the supply line has been shut off. Steam condenses, and a vacuum can be created that not only affects system operation but also can cause damage to equip­ment. The vacuum relief valve automatically admits air to the system in order to break up the vacuum. These valves operate

Steam and Hot-Water Space Heating Boilers

Adjustable differential on the L404A, B,F; L404L with a 5 to 150 psi. 0 34 to 10,3 kg/cm2 [34 to 134 kPa] operating range and L604A models only.

/2 Trip-free manual reset Lever on the L404C, D and L604L models only.

Figure 15-36 Setting a Honeywell Pressuretrol® Controller.

(Courtesy Honeywell, Inc.)

Under conditions up to a maximum temperature of 250°F. The valve disk should be constructed from a material capable of with­standing high temperatures, particularly when the valve is used in a steam processing system.

Steam and Hot-Water Space Heating Boilers

Figure 15-37 Vacuum relief valve used with a unit heater in a steam

Heating system. (Courtesy Watts Regulator Co.)

Steam Boiler Aquastat

Aquastats have been used on some steam boilers to control temper­ature limits. It is similar in function to the pressure control on a steam boiler or the fan and limit control on a forced-warm-air fur­nace. Aquastats are used much more often on hot-water boilers to control temperature limits or to operate the circulator (pump). See Hot-Water Boiler Aquastats in this chapter.

Blowdown Valve

Sediments and contaminants in the water will settle out over a period of time and accumulate at a low point in the bottom of the boiler. These can be removed through a blowoff valve installed in a line connected to the lowest part of the boiler. The blowoff valve is opened periodically, and the accumulated sediments are drained off.

Steam boilers operating at 15 psig or less require a blowdown valve the full size of the boiler connection. Steam boilers operating at pressures greater than 15 psig require at least two such blowdown valves. One or both of the valves must be the slow opening type.

Each water column and float type low-water cutoff must also be equipped with a blowdown valve.

Foam sometimes forms on the surface of the boiler water. This is usually indicated by drops of water appearing with the steam. This condition is caused by the presence of oil, dissolved salts, or similar organic matter in the water. One method of eliminating this prob­lem is by draining off part of the water in the boiler and adding an equal amount of fresh, clean water. Another method involves blow­ing the foam from the water with a specially connected pipe or hose. The boiler should have a blowdown tapping for this purpose.

Try Cocks

Try cocks are small valves installed on steam boilers at the safe high-water level and at the safe low-water level (Figure 15-38). When the boiler water column is inoperable, try cocks function as a backup system to determine the water level. The water level is determined by opening slightly first one try cock and then the other. The water level is indicated by the color of the plume escaping from the try-cock orifice. A steam plume is characteristically colorless and will indicate that the water level is too low (i. e., below the level of the try-cock orifice). Water, on the other hand, is characterized by a white plume. A false water-level reading will be obtained if the try cock is opened too wide when the level of the water is only slightly below the level of the try-cock orifice. The violent agitation of the water caused by a wide-open try cock will result in some of

Steam and Hot-Water Space Heating Boilers

The water escaping, giving the false impression that the level of the water in boiler is at or above the try-cock fitting.

On large steam boilers a third try cock is often installed between the other two. Sometimes a try cock on these larger boilers is at a level too high to reach. When this is the case, chain-operated, lever­type try cocks are installed.

Steam Boiler Injectors

A steam boiler injector is a device used on some boilers to create accelerated steam circulation in a steam heating system. The essen­tial parts of a boiler injector are shown in Figure 15-39.

This device operates on the induction principle. In operation, steam from the boiler, entering the steam nozzle, passes through it, through the space between the steam nozzle and combining tube, and then out through the overflow. This produces a vacuum, which draws in the water through the water inlet. The incoming cold water condenses the steam traversing the combining tube, and the water jet thus formed is at first driven out through the overflow. As the velocity of the water jet increases, sufficient momentum is obtained to overcome the boiler pressure, with the result that the water enters the delivery tube and passes the main check valve into the boiler.

STEAM

Steam and Hot-Water Space Heating Boilers

The induction principle of the steam injector is the same as the operating principle of hydraulic boosters used in forced-hot-water heating systems, with the exception that fast-flowing hot water is used instead of steam to obtain the inductive action to accomplish accelerated circulation.

Water Gauges

A water gauge is used to check the level of water in the boiler visu­ally (Figure 15-40). If the water level in the boiler is high enough, the glass tube will be approximately % to 3/t full. Check the boiler spec­ifications, because the safe operating level will vary among manu­facturers. If no water is showing in the tube, the boiler must be turned off and refilled to the proper level. Do not add the water until the boiler has had time to cool off (Figure 15-41).

Steam gauge siphon Controls

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating BoilersGauge glass minimum safe water level

Figure 15-41 Water gauge levels. (Courtesy ITT McDonnell & Miller)

Steam and Hot-Water Space Heating Boilers

BONNET RING

подпись: bonnet ring

PLUG———————— ■■■■—

UPPER BODY —

подпись: plug ■■■■—
upper body —

ROD PLATE — GLASS

STUFFING BOX

подпись: rod plate — glass
stuffing box

HANDWHEEL

подпись: handwheelGLASS PACKING GLASS STUFFING — BOX WASHER GLASS

Steam and Hot-Water Space Heating Boilers

DRAIN-UNION

подпись: drain-union

LOWER BODY

подпись: lower body

IHIL RING

Figure 15-42 Water gauge construction

Details. (Courtesy Lunkenheimer Co.)

подпись: ihil ring
figure 15-42 water gauge construction
details. (courtesy lunkenheimer co.)

TAIL

подпись: tailROD

The construction details of a typical water gauge are shown in Figure 15-42. The glass tube of the water gauge must be long enough to cover the safe range of water level in the boiler. The ends of the water gauge are connected to the interior of the boiler by fittings located above the safe high-water level and below the safe low-water level.

Water Columns

A water column (Figure 15-43) is a boiler fitting that combines try cocks, a water gauge, and an alarm whistle in a single unit. A float in the column activates an alarm whistle when the water drops below a safe level.

Steam Gauges

The difference between the pressure found on the inside of the boiler and the pressure on the outside of it is indicated by a steam gauge (Figure 15-44). This is a gauge pressure reading and should not be

Steam and Hot-Water Space Heating Boilers

Confused with absolute pressure. A steam gauge is used to measure the steam pressure at the top of the boiler. Generally, a reading of 12 psi (pounds per square-inch) indicates a dangerous buildup of pressure in low-pressure steam heating boilers. The boiler should be shut down before the pressure exceeds this level.

If the steam gauge is operating properly, the needle (or pointer) will move with each change of pressure inside the boiler. Shut off the steam, and the gauge needle should drop to zero; turn on the steam, and the needle should rise to the correct reading. It is very important that the steam gauge be regularly checked to ensure that it is operating properly.

The steam gauges used on boilers operate on the bent-tube princi­ple, that is, the tendency of a bent or curved tube to assume a straight position when pressure is applied. As shown in Figure 15-45, one end of the curved tube is attached in a fixed position to a pigtail (connec­tor tube), which, in turn, is attached to the boiler. The gauge needle is mounted on a rack-and-pinion gear attached to the free end of the curve tube. The pressure in the curved tube causes its free end to move slightly in its effort to assume a straight position. This slight move­ment is multiplied by the rack-and-pinion gear, causing the needle to rotate and indicate the steam pressure.

Steam and Hot-Water Space Heating Boilers

Steam Gauge Pigtails

A steam gauge pigtail (Figure 15-46) is a length of tubing with one or more loops in it used to connect the steam gauge to the boiler. It functions as a protective device by preventing live boiler steam from coming into contact with working parts of the gauge. The steam is denied passage by condensate, which forms in the loop (or loops) of the pigtail. It is recommended that the steam gauge pigtail be filled with water before it is attached to the boiler. It should be attached to the boiler in a position where heat and vibration will be at a minimum.

Steam and Hot-Water Space Heating Boilers

Figure 15-45 Mechanism of a bent-tube steam gauge.

Hartford Return Connection

15-46

Gauge

подпись: 
15-46
gauge
A Hartford return connection or steam loop (Figure 15-47) is a pip­ing arrangement used in a steam heating system to return conden­sation to the boiler. This is done to prevent excessive water loss in the boiler when a leak occurs in the return line. The Hartford return connection is described in Chapter 8, “Steam Heating Systems.”

Hot-Water Boiler Valves, Controls, and Accessories

The boilers used in hot-water heating systems are also fit­ted with a variety of valves, controls, and other devices designed to ensure the safe and proper operation. Some are similar to those used on steam boilers (e. g., pressure relief valves and low-water cutoffs). They also share the same measuring and controlling functions described for steam boiler valves, controls, and accessories. Figures 15-48 and 15-49 illustrate their arrangement on a hot — water space heating boiler. Figure

Hot water (hydronic) boilers operate under high Steam pressures and temperatures. To avoid serious injury, pigtail.

TO SUPPLY MAIN

Steam and Hot-Water Space Heating Boilers

Never begin any service or repairs shutoff before taking the follow­ing precautions:

• Wait for the boiler to cool down to 80°F (27°C) or more.

• Wait for the boiler pressure to drop to 0 psi (0 bar).

• Disconnect the electrical power before making any electrical connections.

Principal Hot-Water Boiler Valves, Controls, and Accessories

A hot-water boiler should be equipped with the following valves, controls, and accessories:

• Pressure relief valve: A safety valve used to relieve boiler pressure when it exceeds the maximum safe level.

• Low-water cutoff: A device that automatically switches off the burner if the boiler water level becomes too low for safe operation.

• High-pressure limit switch: A safety device used to turn off the burner when boiler pressure exceeds its preset maxi­mum safe operating level.

HJ-UMIM

AQUASTAT

/

TEMP-PRESS

INDICATOR

<F=P_

SIDE

TOP

Figure 15-48 Pipings, valves, controls, and fittings for an oil-fired hot — water (hydronic) boiler. (Courtesy Hydrotherm, Inc.)

FRONT

FLOAT TYPE.-»AIR VENT

 

VENT FITTING WITH — PRESS, RELIEF VALVE

 

Steam and Hot-Water Space Heating Boilers

OPTIONAL

—SUPPLY

TAPPING

 

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers Steam and Hot-Water Space Heating Boilers Steam and Hot-Water Space Heating Boilers

Pressure-relief valve

 

Air vent

 

Expansion tank

 

Supply

 

Return Cock"B" Cock "A"

 

Pressure- reducing (fill) valve

 

Cock "C"

 

Circulator

 

Cold-water inlet

 

Cock "D"

 

Figure 15-49 Pipings, valves, controls, ands fittings on a gas-fired hot-water (hydronic) boiler. (Courtesy Hydrotherm, Inc.)

 

Steam and Hot-Water Space Heating Boilers

• Aquastat: An automatic switching device used to control temperature limits or to operate the circulator (pump).

• Water pressure-reducing valve: A valve used to keep the boiler and system automatically filled with water at the desired operating pressure.

• Air vent: A device that releases air from the system to the atmosphere.

• Expansion tank: A tank used to accommodate the expanded volume of heated water in a hot-water (hydronic) heating system.

• Air separator: A device used to separate trapped air bub­bles and pockets from the water before it circulates through the hot-water heating system.

• Circulator: A pump used to move the water through the hot-water heating system.

Hot-Water Boiler Low-Water Cutoffs

A low-water cutoff device, such as the one shown in Figure 15-50, can also be used in a hot-water heating system to provide protec­tion against a low-water-level condition in the boiler resulting from runaway firing caused by malfunctioning controls or a break in the return piping. The low-water cutoff device should be installed in the piping so that the raised line cast on the float chamber body is on a level with the top of the boiler. The drain valve located directly

Steam and Hot-Water Space Heating Boilers

Figure 15-50 Low-water cutoff used in a forced-hot-water heating system.

(Courtesy Watts Regulator Co.)

Below the low-water cutoff should be opened periodically to flush mud and sediment from the float chamber. It is recommended that this be done at least once a month.

Note

The ASME Boiler and Pressure Vessel Code requires that hot — water space heating boilers be equipped with a low-water cut­off device only if the boiler input is greater than 400,000 Btu/h. However, on coil-type boilers, where the flow of water is required to prevent overheating, a flow switch or similar device can be used instead of the low-water cutoff switch to shut off burner operation.

Some hot-water (hydronic) space heating systems have piping for the radiators, convectors, or tankless water heaters installed below the minimum safe water level of the boiler. A low-water cut­off should be installed in these systems for low-water protection.

The probe-type low-water cutoff illustrated in Figure 15-51 has a green “power on” LED and a “low-water condition” red LED for

Steam and Hot-Water Space Heating Boilers

Immediate recognition of the water level status of the boiler. This probe-type low-water cutoff can be installed in either the boiler tapping or a supply riser. Two models are available: one for a 24 volt burner circuit, the other for a 120-volt circuit.

Figure 15-52

Pressure relief

Valve. (Courtesy Watts Regulator Company)

подпись: 
figure 15-52
pressure relief
valve. (courtesy watts regulator company)
Hot-Water Boiler Pressure Relief Valves

ASME rated pressure relief valves are used on hot-water space heating boilers to relieve pres­sure created by two different conditions: (1) water thermal conditions and (2) steam pres­sure conditions (Figures 15-52 and 15-53). These are exclusively safety devices and nor­mally should not be used as regulating valves or other units intended to regulate or control the flow pressure. Relief valves are intended to pre­vent personal injury and damage to property. These valves start to open at the set pressure and require a certain percentage of over­pressure to open fully. As the pressure drops, they start to close and shut at approximately the set pressure.

As thermal conditions develop inside a hot-water space heating boiler, pressures may be built up to the setting of the relief valve. When the pressure reaches the safety limit setting, the valve functions as a water relief valve and discharges the small amount of water that is expanded in the system.

Steam and Hot-Water Space Heating Boilers

If both water and steam are present in a hot-water space heating boiler, the firing controls probably are malfunctioning or have completely broken down. This may result in runaway firing of the boiler, which could cause the boiler water to reach steam-forming temperatures, creating a steam pressure condition. Under these cir­cumstances, the relief valve functions as a steam safety-relief valve. The steam is discharged at a rate or faster than the boiler can gen­erate it, thus restoring system pressure to a safe level. Although these valves are steam rated and have an emergency Btu steam dis­charge capacity if runaway firing conditions occur, they should not be continuous steam service.

In order to be completely effective, the safety relief valves used on hot-water space heating boilers must be designed to discharge the excessive water pressure created by thermal expansion and also the excessive steam pressure resulting from runaway emergency temperature conditions.

High-Pressure Limit Switch

Figure 15-54 Aquastat with immersed heat — sensitive elements.

подпись: 
figure 15-54 aquastat with immersed heat- sensitive elements.
The high-pressure limit switch is a safety device used to shut down the operation of the gas or oil burner when the pressure boiler pressure exceeds a preset level (commonly 5 to 8 psi). The high-pressure switch is connected to the boiler by a pigtail pipe.

Hot-Water Boiler Aquastats

An aquastat is a device commonly used on hot-water space heating boilers and some steam boilers to control tempera­ture limit or to operate the circulator (hot-water heating system pump). It is similar in function to the pressure con­trol on a steam boiler or the fan and limit control on a forced-warm-air furnace.

Basically, an aquastat is an automatic switching device consisting of a metal — or liquid-filled heat-sensitive element designed to detect temperature drop or rise of the boiler water. Aquastats can be strapped to the hot-water supply riser or mounted so that the heat-sensitive ele­ment is immersed in a boiler well (Figure 15-54).

The type of aquastat used in a heating installation generally depends upon whether it is designed to control

Temperature limit or to switch on the circulator. If the former is the case, the aquastat will close on temperature drop and open on tem­perature rise. The aquastat will close on temperature rise if it is used to operate the circulator.

An example of a strap-on type aquastat is the ITT General Controls L-53 Strap-On Hot Water Control illustrated in Figure 15-55. This type of aquastat responds to water temperature changes as conducted through the supply-riser pipe wall to the temperature — responsive base of the device. The enclosed switching is supplied in three basic types, depending upon their operating principles: direct — action (Figure 15-56), reverse-action (Figure 15-57), and double­action (Figure 15-58) models.

A direct-action (normally closed, N. C.) aquastat used as a high­limit control must always be located on the supply riser where it will be subjected to the maximum boiler water temperature. Its location on the riser will therefore have to be as close to the boiler as possible but ahead of any line shutoff valves.

Base of control must fit

Steam and Hot-Water Space Heating Boilers

Cut off excess strap. ^

Figure 15-55 Snap-on hot-water control. (Courtesy ITT General Controls)

‘ N. C. ‘

9

R — y

F COMM. ’

Јk

7

/

Ј

N. O.

9

M*j"

1

T

I

——————- ——————

=±= =Ј

L ■ ■

^@i

N. O. 1 1

Ф

COMM.

6

COMM.

&

N. C.

Figure 15-56

Direct-action switch operation.

(Courtesy ITT General Controls)

Figure 15-57

Reverse-action switch operation.

(Courtesy ITT General Controls)

Figure 15-58

Double-action switch operation.

(Courtesy ITT General Controls)

Steam and Hot-Water Space Heating Boilers

Figure 15-59 Indicator can be set for a temperature higher than 200°F by removing the screws (A) and holding stop bracket to scale plate and removing the bracket (B). (Courtesy ITT General Controls)

подпись: figure 15-59 indicator can be set for a temperature higher than 200°f by removing the screws (a) and holding stop bracket to scale plate and removing the bracket (b). (courtesy itt general controls)

B

подпись: bDirect-action aquastats will open the circuit on temperature rise. Setting the adjustable scale pointer to a posi­tion on the scale will result in breaking the circuit (Figure 15-59). When the aquastat is used as a high-limit control, the cutout setting should be as low as possible and still ensure proper heating in cold weather. An initial cutout set­ting of 170°F is recommended for a gravity hot-water heating system. An initial setting of 200°F is suggested for a forced-hot-water (hydronic) heating system. The mechanical differential of

The control illustrated in Figure 15-59 is nonadjustable and is approximately 15°F.

A reverse-action (normally open, N. O.) aquastat should be mounted ahead of any valves or traps on the return line when it is used on a unit heater installation. It should be mounted on the largest riser from the boiler if it is used to prevent circulator operation when boiler water temperature is low. A reverse-action aquastat closes the circuit on temperature rise.

A double-action (single-pole double-throw, SPDT) aquastat is used to start circulator operation with a single switch actuation and to function as an operating control to maintain boiler water tem­perature. This type of aquastat should be mounted on the largest riser ahead of any valve but at a point where it will be subjected to maximum boiler water temperatures. The double-action aquastat illustrated in Figure 15-58 opens R to B contacts and closes R to W contacts on temperature rise at scale setting.

A strap-on aquastat can be mounted in any position. When mounting these aquastats, make absolutely certain that the pipe sur­face is clean and free of rust and corrosion. All rough and high spots should be filed smooth. Nothing should be allowed to interfere with the operation of the temperature responsive base of the control.

Pressure-Reducing Valves

Most hot-water heating systems are equipped with a water pressure- reducing valve designed to feed water into the boiler automatically when the pressure in the system drops below the valve setting. When the pressure returns to the minimum pressure setting, the valve auto­matically closes. Thus, the function of a water pressure-reducing valve is to keep the system automatically filled with water at the desired operating pressure (Figure 15-60). These valves are also referred to as pressure-reducing boiler feed valves, water pressure — reducing fill valves, or feed water pressure regulators. Regardless of what they are called, their function remains the same: to deliver water to the boiler at the required pressure. To perform this function,

Steam and Hot-Water Space Heating Boilers

They are always installed in the cold-water supply line. If they are part of a combination pressure-reducing and pressure relief valve (dual control), the pressure relief part of the combination valve is always located between the boiler and the pressure-reducing part.

A manually operated feed valve was used on older hot-water heating systems to provide the same function as the automatic water-pressure-reducing valve. On these older systems, the manu­ally operated feed valve should be partly opened and water added to the boiler when the movable (white or red) arrow on the altitude gauge drops below the setting of the stationary (black) arrow (see Altitude Gauges in this chapter).

Some pressure-reducing valves are equipped with a built-in check valve in the supply inlet to prevent a backflow of contami­nated boiler water into the domestic water supply. This backflow drainage of boiler water generally occurs when the supply pressure falls below the feeder valve setting. Not only can this drainage cause possible damage to the boiler, it can also contaminate the domestic water supply. Some pressure-reducing valves are also equipped with integral strainers to trap foreign matter that may clog the valve. The strainer screen should be removed for cleaning at the beginning of each heating season. This can be done by removing the bottom plug from the strainer and withdrawing the screen. Clean and carefully replace screen and plug.

The principal components of a water-pressure-reducing valve are shown in Figure 15-61. This particular valve is constructed with an integral strainer. It is also possible to use separate units in combina­tion, as Figure 15-62 illustrates. The two units are joined by a short threaded connection. A typical installation in which a water- pressure-reducing valve is used is shown in Figure 15-63.

As mentioned previously, a water-pressure-reducing valve should be installed in the water supply line to the boiler. It should also be installed at a level above the boiler and in a horizontal position. Before installing the valve, flush out the supply pipe to clear it of chips, scale, dirt, and other foreign matter that could interfere with its operation. Install a shutoff valve ahead of the regulator, and then connect the supply line to the inlet (usually marked “in” on the valve casting).

To fill the system, open the shutoff valve located ahead of the pressure-reducing valve. Water will flow into the system until it is full and under pressure. The shutoff valve must always be kept open when the system is in operation.

Water-pressure-reducing valves are usually set by the manufac­turer to feed water to the boiler at approximately 15 lb of pressure.

Steam and Hot-Water Space Heating Boilers

Figure 15-61 Basic components of a water pressure-reducing valve with integral stainless steel strainer. (Courtesy Watts Regulator Co.)

This pressure setting is sufficient for residences and houses up to a three-story building in size. For higher buildings in which the pres­sure may not be sufficient to lift the water to the highest radiator, it may be necessary to reset the water-pressure-reducing valve for higher pressure. To do this, calculate the number of feet from the regulator to the top of the highest radiator. Multiply this by 0.43 and add 3 lb. This will give the pressure needed to raise the water to the highest radiator and keep it under pressure. Loosen the locknut on the valve, and turn the adjustment screw clockwise slowly until the gauge indicates the pressure calculated. When the desired pres­sure is obtained, tighten the locknut.

Steam and Hot-Water Space Heating Boilers

BRONZE BODY RENEWABLE NICKEL STAINLESS STEEL

CONSTRUCTION ALLOY SEAT PERFORATED

STRAINER SCREEN

Figure 15-62 Basic components of a water-pressure-reducing valve with separate strainer unit. (Courtesy Watts Regulator Co.)

подпись: r»Combination Valves

Combination valves (also called dual unit valves or dual control valves) are designed for use in hot-water space heating systems. They combine a pressure-reducing/pressure-regulating valve and a positive relief valve in one body (Figures 15-64 and 15-65). Sometimes an integral bypass valve is also included in the same unit.

The purpose of a combination valve is to provide pressure regula­tion and safety control and to reduce boiler pressure and ensure auto­matic filling when conditions warrant. The valve is installed on the supply line of a boiler with the relief-valve section closest to the boiler.

The regulator side of the combination valve is designed to reduce the incoming water supply to the boiler to the required 14-psi oper­ating pressure for a one-, two-, or three-story house. The pressure

Water pressure-

Steam and Hot-Water Space Heating Boilers

Figure 15-63 Typical installation diagram showing the location of a water-pressure-reducing valve. (Courtesy Watts Regulator Co.)

In the system builds up due to thermal expansion when the boiler is fired. Under normal operating conditions, the expansion tank will absorb the additional pressure. However, if the expansion tank is waterlogged or if the system has no expansion tank, the relief side of the valve will open at 23 psi and drop the pressure back to within safe limits. If the pressure should drop below 14 psi, the reg­ulator will open again and automatically refill the system. A built — in check valve prevents the backflow of contaminated boiler water into the potable water supply.

Exploded views of combination valves are shown in Figures 15-66 and 15-67. The Cash-Acme Type CBL Valve (Figure 15-68) differs from the other two by having a built-in bypass valve. In all other respects, it is similar to the others except that it should be installed as close as possible to the top of the boiler (using close nipples). The principal advantages of having the bypass valve included are:

1. It allows rapid filling of the system.

2. It permits an easy high-pressure test for leaks and system purging.

3. It passes first filling dirty water around the valve seat, ensur­ing a clean, good seating surface.

4. It permits the use of a wide opening, small seated regulator that prevents wire drawing and rapid wearing of the seat.

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers

EXPANSION TANK

Steam and Hot-Water Space Heating Boilers

RISER [

RELIEF REGULATOR

TO DRAIN

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers

O

подпись: o Steam and Hot-Water Space Heating Boilers Steam and Hot-Water Space Heating Boilers

Mm (24-

подпись: mm (24- Steam and Hot-Water Space Heating Boilers

RELIEF SECTION

REGULATOR SECTION

1. Lever pin

19. Closing cap

2. Lift lever

20. Pressure screw

3. Pressure screw cap

21. Locknut

4. Pressure screw

22. Spring chamber

5. Locknut

23. Spring button

6. Spring chamber

24. Pressure spring, black

7. Spring button

25. Gasket, brass

8. Pressure spring,

26. Diaphragm assembly

Stainless steel

27. Cylinder

9. Pull rod nut

28. Body

10. Pull rod

29. Piston assembly

11. Lock washer

30. Piston spring

12. Pressure plate

31. Strainer

13. Gasket, brass

32. Gasket, red fiber

14. Diaphragm

33. Bottom plug

15. Composition seat shell

16. Composition seat

17. Composition seat screw

18. Body seat

<25KGra

X’u C^>

SHAPE \* MERGEFORMAT Steam and Hot-Water Space Heating Boilers

Figure 15-66 An exploded view of Type CQ combination valve.

(CourtesyA. W. Cash Valve Mfg. Corp.)

The most common types of problems encountered when using combination valves are the following:

1. Relief valve drips or cannot be shut off tightly.

2. Boiler pressure rises above the reducing valve setting.

12.Diaphragm gasket, brass

14.Diaphragm gasket, black

RELIEF SECTION

1.Lift lever pin

2. Lift lever

3. Pressure screw cap

4. Pressure screw

5.Locknut

6. Spring chamber

7. Spring button

8. Pressure spring

9. Pull rod nut

10.Pull rod

11.Pressure plate

13.Diaphragms, bronze

15.Seat shell gasket, gray

16.Seat shell

17.Composition seat disc

18.Seat disc screw

19.Body seat

20. Body

Steam and Hot-Water Space Heating Boilers

Figure 15-67 An exploded view of Type A-1 combination valve.

(CourtesyA. W. Cash Valve Mfg. Corp.)

 

Steam and Hot-Water Space Heating Boilers

3. Water escapes from a weep hole on top of the regulator.

4. Water escapes from feeder side of the combination valve.

5. Foreign matter collects on the seat of the relief or regulator section of the valve.

Steam and Hot-Water Space Heating Boilers

Some of the problems listed above (particularly the first two) may not be the fault of the valve itself. For example, a dripping relief valve can be caused by an expansion tank being completely filled up with water, an undersized expansion tank, or a leak in the coil of a tankless or indirect water heater installed in the boiler. These possibilities should be checked before attempting to service or repair the valve. The procedures are outlined in this chapter.

If, by process of elimination, the problem can be traced to the valve, try tapping the side of the valve with a wrench. Sometimes a piece of foreign matter becomes lodged and causes the regulator piston to stick. A sharp tap with a wrench may dislodge it and allow the valve to function properly.

Foreign matter such as dirt, pipe scale, or chips often cause a valve to malfunction by lodging on a seating surface, or nicking or chipping the surface. Valve manufacturers often provide replace­ments or instructions for field servicing and repairing. The latter should be attempted only by a skilled and experienced worker with the proper tools and gauges.

Water seeping from the regulator “weep hole” or from the feeder side of a combustion valve is usually an indication that there is a rupture or leak in the diaphragm. Follow the procedures described in the preceding paragraph for repairing the valve. An occasional flushing of the relief side of a combination valve will reduce the possibility of the type of lime or scale buildup that can cause the valve to fuse shut.

Balancing Valve

Balancing valves are used in a hot-water (hydronic) heating system to equalize the pressure drop in multiple piping circuits. The valves are installed on the return side of each circuit. Balancing valves are illustrated and described in Chapter 10, “Steam and Hot-Water Line Controls” in Volume 2.

Backflow Preventer

Sometimes boiler back-siphonage and back pressure will cause the boiler water to mix with and contaminate the domestic water sup­ply. This unwanted mixing of the two water supplies can be pre­vented by installing a backflow preventer. The Bell & Gossett backflow preventer illustrated in Figures 15-69 and 15-70 consists of two independently operated check valves and an intermediate atmospheric air vent contained in the same housing.

When a back-siphonage condition occurs, the atmospheric vent opens to allow air to enter and break the siphon. Leakage is vented

LEFT BODY RIGHT BODY

Steam and Hot-Water Space Heating Boilers

Figure 15-69 Schematic of the Bell & Gossett backflow preventer.

(Courtesy ITT Bell & Gossett)

Through the air vent if back-pressure occurs with a fouled second check valve.

Altitude Gauges

An altitude gauge mounted on the boiler is used to indicate the water level in open hot-water heating systems. The level (altitude) of the water is indicated on the gauge by the relative positions of two

B&G PRESSURE B&G AUTOMATIC REDUCING VALVE AIR VENT B&G IN-LINE

AIR SEPARATOR

B&G BACKFLOW

PREVENTER

SHUTOFF

VALVE

C. W. FILL

STRAINER

AIR GAP

DRAIN

B&G BOOSTER

I B&G TRIPLE DUTY VALVE

“|/ TO

SYSTEM

Steam and Hot-Water Space Heating Boilers
Steam and Hot-Water Space Heating Boilers

B&G MODEL "B"

OR "DV" ASME TANK

 

RETURN

 

Steam and Hot-Water Space Heating Boilers

Hands or arrows. One hand (usually black) is stationary and is per­manently set when the boiler is filled. The movable hand (usually red or white) is initially set in the same position as the stationary hand after the boiler has been filled with water. Its position will change as the water level in the boiler changes. This movable hand indicates the true level of the water. Efficient operation is being pro­vided as long as the movable hand is directly above the stationary one. In closed hot-water heating systems, automatic valves are used to control boiler water level (see Pressure Relief Valves and Pressure-Reducing Valves in this chapter).

Circulator (Pump)

The circulator (circulating pump) in a hot-water space heating sys­tem operates in a sealed piping circuit (loop) and is always filled with water. Recommendations for their service, maintenance, trou­bleshooting, and repair are contained in Chapter 10, “Steam and Hot-Water Line Controls” in Volume 2.

Air Separator

When a hot-water heating system is filled with cold water, the water contains some air dissolved in solution. The air emerges from solution when the water is heated and moves rapidly through the pipes and radiators making noises in the pipes and radiators. In some cases, air pockets become trapped in the farthest radiators, which prevents them from heating. An air separator is used to trap, separate, and remove this trapped air from the water before it enters the system.

Some systems use inline separators installed in the piping close to the boiler. The inline separator shown in Figure 15-71 consists of two chambers slightly wider than the pipe. Water containing trapped air enters the separator and slows down slightly as it expands into the first chamber. The slowing down of the water causes the air trapped in it to separate, form bubbles, and float to the top of the first cham­ber. The water, free of the air, passes into the second chamber and then into the system piping. The air tapped at the top of the first chamber is vented out of the system through an automatic air vent (installed in the tapping on top of the air separator) or passed into an expansion tank.

The air separator shown in Figure 15-72 is screwed into the boiler supply tapping. The separator traps the air in the top of the boiler section, where the water is hottest and where it travels a long horizontal path at low velocity, permitting the air bubbles to separate. The trapped air escapes through a %-in tapping into an

Steam and Hot-Water Space Heating Boilers

Figure 15-71 Inline air separator. (Courtesy ITT Bell & Gossett)

TRAPPED AIR ESCAPES THROUGH THIS 3/4" TAPPING INTO EXPANSION TANK OR FLOAT VENT

 

SUPPLY

 

SCREW AIR SEPARATOR INTO BOILER NOZZLE

 

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Steam and Hot-Water Space Heating Boilers Steam and Hot-Water Space Heating Boilers

RETURN

Air-cushion expansion tank or through an automatic float vent on systems using diaphragm expansion tanks.

An air separator can only expel air that reaches the boiler. However, sometimes air pockets remain trapped in the piping or radiation, thereby impeding the water flow and reducing the heating performance of the boiler. Such air pockets can be eliminated by installing several different purge fittings in the heating system. A manual air vent is installed on the highest point of the system (auto­matic air vents are not generally required); see the following section.

Some boilers are constructed with built-in air separators, which make the addition of an external unit unnecessary. Figure 15-73 illustrates the working principles of one of these integral units. In this design, the air is diverted to an automatic air vent. A variation of this design provides for the diversion of air to an expansion tank. The manufacturer refers to this device as an air eliminator. Its

Steam and Hot-Water Space Heating Boilers

Function is similar to the air eliminator used on pipelines, but it dif­fers in design and construction.

Purging Air from the System

Built-in or externally installed air separators are designed to sepa­rate air from the water in an operating boiler and vent it either into an expansion tank or to the atmosphere through an automatic air vent.

Air can also be removed from a heating system by purging it from the piping connections to the boiler. This type of purging is standard when the system is being filled with water during setup. Purging instructions are usually included in the owner’s manual or the manufacturer’s installation guide.

Expansion Tanks

A hot-water heating system is completely filled with water. When the water is heated, it expands in volume by as much as 5 percent. The function of the expansion tank in a hot-water heating system is to provide a space to accommodate the increased volume of water. There are two types of expansion tanks: steel tanks and diaphragm tanks (Figure 15-74). Both are described in considerable detail in Chapter 10, “Steam and Hot-Water Line Controls” in Volume 2.

Air Supply and Venting

An adequate air supply must be provided for combustion to boilers that are fired by gas, oil, or coal, and the products of combustion must be vented to the outside atmosphere.

Combustion air is normally supplied through venting ducts or openings in the walls, if the appliance relies on natural ventilation for its air. The requirements for the air supply will depend upon the location and enclosure of the boiler.

Boilers installed in unconfined areas usually obtain adequate air for combustion by means of normal infiltration. However, normal air infiltration will be inadequate for this purpose if the construc­tion of the structure is unusually tight. Under these circumstances, provision must be made for the entry of additional air from out­side the building. Unobstructed openings with a total free area of not less than 1 in2 per 5000 Btu/h of the total input of the boiler are necessary to provide an adequate air supply for combustion purposes.

Some boilers are installed in boiler rooms or enclosures supplied with combustion air from inside the structure. The air supply must enter and leave the boiler room through two openings in an interior

554

подпись: 554

Steam and Hot-Water Space Heating Boilers

Figure 15-74 Typical expansion tank locations.

 

Steam and Hot-Water Space Heating Boilers

COLD

Water

FILL

подпись: cold
water
fill
Steam and Hot-Water Space Heating Boilers

Wall or door. One opening is located near the ceiling, the other opening near the floor. Each opening should have a free area of not less than 1 in2 per 1000 Btu/h of the total input of the boiler.

If a boiler is located in a boiler room or similar enclosure that receives its air supply from outside the building, the two air openings are located on an exterior wall. Each opening must have a free area of not less than 1 in2 per 4000 Btu/h of the total input of the boiler.

The products of combustion must be vented to the outside atmo­sphere. In order to accomplish this, the boiler must be connected to a suitable venting system, which should include a flue pipe and a chimney or stack of adequate size and capacity.

The chimney height is usually governed by the height of the structure. As a general rule-of-thumb, the chimney should extend beyond the high-pressure area caused by the passage of 10 ft of the flashing. If the chimney is too short, it will not extend beyond the high-pressure area caused by the passage of winds over the struc­ture. As a result, a downdraft caused by the pressure difference in the high-pressure area will push air down into the chimney, which will block the escape of flue gases and interfere with the combus­tion process. Figures 15-75 and 15-76 illustrate these principles. The correct chimney height is shown by the dotted lines.

Figure 15-77 shows a type of chimney construction referred to as a Type A vent. This chimney can be either masonry or factory-built construction, but it must be designed and constructed in accor­dance with the standards set forth in national codes. Type A vents are suitable for venting of all gas appliances. These are the only

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers

Steam and Hot-Water Space Heating Boilers

Vents suitable for venting oil-burning equipment. Gas-fired boilers that produce temperatures at the draft diverter not in excess of 550°F may also use a Type B vent (Figure 15-78), which employs a double-wall metal vent pipe. A Type C vent (Figure 15-79) is used to vent gas appliances in attic installations.

The boiler should be located so that the length of flue pipe con­necting the boiler (or draft diverter) to the chimney is as short as possible. Additional information about chimneys and flues is con­tained in Chapter 11, “Gas Furnaces.”

2" OPEN AIR SPACE

CLEARANCE — 2" MIN. BUT NOT LESS THAN —

подпись: clearance - 2" min. but not less than -

FLASHING & ROOF CEMENT

подпись: flashing & roof cement

ROOF LINE

подпись: roof line

ROOF BOARDS

подпись: roof boards Steam and Hot-Water Space Heating Boilers

A = DIA OF FLUE B=A + 4 INCHES C = A + 4 INCHES

D = NOT LESS THAN 24" OR NOT LESS THAN 6" ABOVE ROOF COPING

подпись: a = dia of flue b=a + 4 inches c = a + 4 inches
d = not less than 24" or not less than 6" above roof coping
Figure 15-79 Type C vent. (Courtesy Hydrotherm, Inc.)

Induced-Draft Fans

Sometimes a chimney is too small to provide enough updraft to con­vey the products of combustion from the boiler to the outside atmo­sphere. When it is evident that natural venting will be inadequate, a mechanical draft inducer can be used to increase the capacity of the chimney. These devices are used with both gas — and oil-fired boilers.

FAN SEALED

WHEEL BEARINGS

Steam and Hot-Water Space Heating Boilers

COMBUSTION

Steam and Hot-Water Space Heating Boilers

The two principal types of induced-draft fans are (1) direct-fan draft inducer and (2) induced-flow draft inducer. When a direct-fan draft inducer is used, the gases are drawn through an inlet located on the side, top, or bottom of the unit and discharged as shown in Figure 15-80. The moving parts of an indirect fan draft inducer (Figure 15-81) are located outside the path of the hot flue gases. Cool outside air is introduced through the blower section of the unit. This creates suction at the throat of the inducer body, which, in turn, increases the flow rate of the flue gases.

The draft created by the induced-draft fan should closely match the demand. If the size of the draft inducer is correctly estimated, any dilution and excessive cooling of the flue gases (a condition that may result in condensation) will be greatly reduced. Many boiler manufacturers recommend the size draft inducer to be used with specific boiler models and provide data for making the appropriate calculations. Manufacturers of draft inducers are similarly helpful.

Controlling Excessive Draft (Gas-Fired Boilers)

Sometimes a boiler installation will have excessive draft conditions as a result of oversized chimneys or other factors. Draft conditions in excess of the draft design limits of a gas-fired boiler will reduce its combustion efficiency, resulting in higher fuel costs and possible pilot problems.

Sheet-metal baffle Attached with stove bolts

подпись: sheet-metal baffle attached with stove bolts

MG-2

Barometric double acting

подпись: mg-2
barometric double acting

Figure 15-82 Control of excessive draft of gas-fired boilers.

(Courtesy Bryan Steam Corp.)

подпись: figure 15-82 control of excessive draft of gas-fired boilers.
(courtesy bryan steam corp.)
Steam and Hot-Water Space Heating Boilers Steam and Hot-Water Space Heating Boilers

Suggested baffle dimensions

Barometric

Size

Diameter

Cut sheet

A

B

R

10"

10"

14"

5"

12"

12"

163/4"

6"

14"

14"

19V

7"

16"

16"

22V

8"

18"

18"

25"

9"

20"

20"

28"

10"

24"

24"

33V

12"

подпись: suggested baffle dimensions
barometric
size
diameter cut sheet
 a b r
10" 10" 14" 5"
12" 12" 163/4" 6"
14" 14" 19v 7"
16" 16" 22v 8"
18" 18" 25" 9"
20" 20" 28" 10"
24" 24" 33v 12"

The above dimensions for baffle may vary due to different types of flue connections and tee sizes.

подпись: the above dimensions for baffle may vary due to different types of flue connections and tee sizes.A suggested method of controlling the draft on such installations is to install a sheet-metal baffle, or restrictor, in the flue connection at the boiler (Figure 15-82). This baffle may be inserted in a tee, as shown, and secured with stove bolts after the proper draft is obtained as determined by measurements with a draft gauge. If the barometric control is installed in the side of the breeching by means of a draft control collar, the same method of baffling may be used

With the baffle restricting the breeching between the boiler and the barometric control. In this method of baffling, the breeching between the barometric control and breeching should not be restricted.

The baffle installation is correct if the gate of the barometric control is approximately half open while the gas burner is operat­ing and the draft measurement is at or below the operating design of the boiler. The draft limit for a boiler can be obtained from the manufacturer’s specifications.

Always check with the local gas company regarding the use of baffles. Caution should be exercised to ensure against overrestrict­ing the flue. Always be certain that there is a very slight draft after the baffle is secured in position.

Tankless Water Heaters

Some boilers are manufactured with the option of using a tankless water heater. This device consists of an immersion coil inserted in a steam or hot-water space heating boiler to provide domestic and commercial service hot water. It is called a tankless water heater because no storage tank is used to store the heated water during periods of low demand.

The immersion coil is made of small-diameter copper tubes that are either straight with U-bends at the end or formed in the shape of a spiral (see Figure 15-10). The tankless heater is installed in the nipple port on cast-iron hot-water boilers. On steam boilers, it is installed in the left-hand side of a special back section at a point well below the water line. A uniform water temperature is main­tained by a thermostatic three-way mixing valve installed in the supply line leading from the immersion coil.

Because these immersion coils function as a heat exchanger-type device and operate on an indirect heating principle, they are also var­iously referred to as an indirect water heater, indirect heat exchanger, or heat exchanger coil. In other applications, immersion coils provide hot-water radiant heat from a steam boiler, hot water for snow melt­ing, heated water for pools, or industrial process water.

Additional information about water heaters, including sugges­tions for estimating the hot-water allowance, can be found in Chapter 4, “Water Heaters” of Volume 3.

Leaking Coils

On occasion, the coil of a tankless or indirect water heater installed in a boiler will leak, allowing high-pressure water into the boiler water and resulting in a rise in the boiler pressure. This, in turn, can cause the relief valve to open and fail to close again tightly, or it can cause the boiler pressure to rise above the setting of the pressure — reducing valve. These symptoms are often wrongly attributed to a valve malfunction when the actual cause may be a leaking coil.

You can determine whether the problem is a leaking coil by tak­ing the following steps:

1. Shut off the feed valve and/or break the connection to the coil.

2. Check the pressure reading in the boiler.

3. Wait about 8 hours, and check the boiler pressure reading again.

If the pressure reading has remained approximately the same over the 8-hour period, it is a strong indication that the coil is leaking. If the boiler pressure continues to rise, the problem may lie elsewhere in the system.

Blowing Down a Boiler

Foam sometimes forms on the surface of the boiler water and is usually indicated by drops of water appearing with the steam. This condition is caused by the presence of oil, dissolved salts, or simi­lar organic matter in the water. One method of eliminating this problem is by draining off part of the water in the boiler and adding an equal amount of fresh, clean water. Another method consists of blowing the foam from the water with a specially con­nected pipe or hose. The boiler should have a blowdown tapping for this purpose. The first method is the easiest. The second one (i. e., blowing down the boiler) requires considerable experience.

Boiler Operation, Service, and Maintenance

Always follow the manufacturer’s instructions for boiler operation, service, and maintenance. These instructions are commonly con­tained in the owner’s manual left with the boiler by the installer. If you do not have these instructions, you should contact the local dealer for advice or write to the manufacturer.

The sections that follow contain recommendations for operating and maintaining boilers. Because many of these recommendations specifically pertain to either steam or hot-water space heating boilers, they were divided and listed accordingly.

Note

All service and maintenance work must be performed by those trained in the proper application, installation, and maintenance of plumbing, steam, and electrical equipment and/or systems in accordance with all applicable codes and ordinances. If available, the boiler manufacturer’s service and maintenance instructions should be followed

Safe and Reliable Boiler Performance

Poor boiler performance and unsafe operating conditions are caused by failure to follow proper maintenance procedures, the existence of low water conditions, the use of outdated or damaged safety devices, or operator error. The following rec­ommendations will go a long way toward providing trouble­free boiler operation:

• Always follow the ASME code maintenance standards.

• Maintain an inspection and maintenance log for the boiler. The log should include the date and specific form of main­tenance performed.

• Always follow the boiler manufacturer’s operating instruc­tions for initial setup and startup.

• Never use outdated controls. Many will have a date code stamped on them. Replace the control if its age exceeds the stamped date.

• Always operate the boiler with code safety relief valves.

• Always use safety relief valves rated for the boiler.

• Never bypass the existing boiler controls.

Caution

To prevent serious burns, always allow the boiler sufficient time for its temperature to cool down to at least 80°F (27°C) and for its pressure to drop to 0 psi (0 bar) before servicing. Drain the boiler to a level below that of the lower gauge glass tapping on the low-water cutoff control.

Caution

To prevent electrical shock, always shut off the electrical power to the boiler before making any electrical connections.

Steam Boilers

Operating and maintaining a steam boiler differs in certain respects from the procedures followed for hot-water space heating boilers. Many of these differences are evident in the recommendations that follow.

1. Keep a steam boiler filled to the water level recommended by the manufacturer when not in use.

2. Check the water level in the boiler before starting it. The heat­ing surface can be damaged if the water level is too low.

3. Keep the water level at the center of the water gauge glass during operation. If the water level is too low, use the manual feed valves to add more water. These valves are found in most systems.

4. Always add water to the boiler gradually. If at all possible, avoid adding water to a hot boiler. Never add water to an operating cast-iron boiler. Cold water fed rapidly into such boilers may come in contact with the hot surface of the cast-iron heat exchanger, causing it to crack. Shut off the boiler and wait until it has cooled down before adding water. After the boiler has had sufficient time to cool, slowly add water through the cold-water feed line.

5. Check the low-water cutoff at regular intervals. Sediment or rust accumulating under the float of the low-water cutoff can cause it to malfunction. As a result, a drop in the boiler water level would not register properly.

6. Check all boiler accessories to make certain they are functioning properly. Movable parts should be inspected and oiled regularly.

7. Never allow a low-pressure steam boiler to exceed the upper safe pressure limits recommended by the manufacturer (usu­ally 3- to 4-lb of pressure).

Hot-Water (Hydronic) Boilers

Recommendations for operating and maintaining hot-water (hydronic) boilers are as follows:

1. Keep the boiler and the pipes in the heating system filled with water when not in use. Keeping the pipes filled with water reduces the possibility of rust and corrosion.

2. Check the water level in the boiler before starting it. The heat­ing surface can be damaged if the water level is too low.

3. Always add water to a boiler gradually. Never add water to a hot boiler. Shut the boiler down and allow it to cool first.

4. Check all boiler accessories to make certain they are function­ing properly. Movable parts should be inspected and oiled regularly. Such maintenance should also include the pump in a forced-hot-water heating system.

5. When operating a hot-water space heating boiler, make cer­tain all flow valves are open.

6. Never allow a boiler to exceed the upper safe temperature limit recommended by the manufacturer (usually about 200°F).

Boiler Water

Boiler water should be clean and kept clean for efficient operation. This is true for all boilers. Never add dirty or rusty water to a boiler. Even hard water may eventually interfere with the efficient operation of a boiler and should therefore be chemically treated before being added.

The Steel Boiler Institute adopted a chemical conditioning com­pound for treating water used in low-pressure steam and hot-water space heating boilers. Many manufacturers provide this chemical compound with their boilers. Always treat the water immediately after the boiler and the heating system have been cleaned.

Note

Boiler water contains suspended solids that are held in sus­pension during boiler operation by the circulating water and the action of treatment chemicals. Unless care is taken when draining the boiler to remove these suspended solids along with the water, they remain in the boiler, dry and stick to the heating surfaces, and require chemical cleaning to remove.

A system compatible antifreeze, such as propylene or ethylene glycol, can be used in the water of a hot-water (hydronic) boiler, but only when absolutely necessary. Additional information about the use of antifreeze in boilers is contained in The Hydronics Institute’s Technical Topics Number 2A publication.

Some boiler heat exchangers are made of aluminum instead of steel. Only antifreeze solutions certified for use in aluminum boilers should be used.

Warning

Never use an RV-type antifreeze protection solution or an automotive-type antifreeze. Both types can damage the boiler and other system components.

Cleaning Boilers

A boiler should be inspected at least once every year for the accumu­lation of soot and other deposits that could impair its operation. This inspection should take place before the start of the heating season.

The accumulation of soot will result in improper combustion. The soot can be removed with a chemical cleaner or a flue brush. Many boiler manufacturers provide access to the heating surfaces through a removable top jacket panel and cover plates. The flue brush is used to push the soot down between the sections or fins to collect in the combustion chamber below. It can then be removed from the combustion chamber without much difficulty.

Oil burners, gas burners, and coal stokers should also be inspected for dust accumulations and cleaned. Always follow the manufacturer’s recommendations for cleaning the boiler and auto­matic firing equipment. If you do not have an owner’s manual, con­tact a field representative or write to the manufacturer.

Troubleshooting Boilers

Boilers are subject to numerous problems, many of which are due to improper type and design or poor servicing. The following are some of the problems usually encountered:

1. Boiler does not deliver enough heat. This is a very common complaint, but the boiler may not be causing the problem. The problem may be caused by a problem with the burner or the automatic controls. Even a pipeline with improper pitch will

Cause the heat from the boiler to be blocked or trapped. If the boiler is found to be the cause of insufficient heat, the problem can be traced to any one (or more) of the following causes:

A. Boiler too small for the heating system

B. Improper arrangement of boiler sections in cast-iron boilers

C. Poor draft

D. Poor fuel

E. Heating surfaces covered with soot

2. Boiler delivers no heat. The automatic controls should be checked first. Sometimes a low-water cutoff on steam boilers will shut off the burner or stoker before enough steam has formed. Another possible cause is that too much water is being fed into a steam boiler. As a result, not enough space is provided for the steam to form at the top of the boiler. Too lit­tle water in a hot-water space heating boiler can be caused by the limit control moving down to a lower setting.

3. Too much time is required to get up steam in a steam boiler. This common problem can be traced to the following possible causes:

A. Too little or badly arranged heating surface

B. Heating surfaces covered with soot

C. Heating passages too short

D. Poor fuel or fuel firing

E. Poor draft

F. Boiler too small

G. Boiler defective

4. Boiler is slow to respond to the operation of the dampers. Slow response to damper operation can be caused by any of the following:

A. Air leakage into the chimney or stack

B. Poor fuel or fuel firing

C. Boiler too small

D. Clinkers on grate or ashpit full of ashes (coal-fired boilers)

5. Water line is unsteady. The problem of an unsteady water line may simply be due to connecting the water column to an extremely active section of the boiler. Therefore, it is extremely unlikely that the actual water level in the boiler can be read from the water column. Other possible causes of this problem are:

A. Dirt or grease in the water

B. Varying pressure differences on the system

C. Excessive boiler output

6. Water disappears from the gauge glass. The complete loss of water from the gauge glass could be due to priming (i. e., water globules being carried over into the steam). Other causes include:

A. Foaming

B. Pressure drop too great in return line

C. Improper water gauge connection

D. Valve closed in the return line

7. Water is carried over into the steam main. This problem is usually caused by one of the following:

A. Priming or foaming

B. Water line is too high

C. Outlet connections from boiler too small

D. Steam-liberating surface too small

E. Boiler output excessive

8. Flues require cleaning too frequently. A frequent buildup of soot and dirt in flues can be caused by any of the following:

A. Combustion rate too slow

B. Poor draft

C. Smoky combustion

D. Excess air in firebox

9. Low carbon dioxide. This condition can generally be traced to one of the following causes:

A. Air leakage between cast-iron sections

B. Improper conversion job

C. Problem with burners

10. Smoke from boiler fire door. The following conditions may be the cause of this problem:

A. Dirty or clogged flues

B. Incorrect setting of dampers

C. Poor or defective draft in the chimney

D. Incorrect reduction in the breeching size

Boiler Repairs

Repairs to the boiler itself should be done by an experienced and skilled worker. They should never be attempted while the boiler is under pressure. Always shut down the boiler first and allow it time to cool before beginning any repairs.

Installing Boilers

All new boilers are shipped with a complete set of installation instructions. Usually these instructions will also contain an inven­tory list of the contents in the crates and cartons. Always check the contents off against this list before you do anything else. Missing or damaged parts should be immediately reported to the manufacturer or its local representative.

A boiler should always be located so that the connecting flue pipe between the boiler (or draft diverter) and the chimney is as short as possible. Another important consideration is the recom­mended minimum clearances between the boiler and combustible materials. These factors plus the design considerations of the system will dictate where the boiler is located.

The manufacturer’s installation guide will also contain instruc­tions on how to light and operate the boiler. The instructions will differ in accordance with the automatic fuel-burning equipment used (e. g., oil burner and gas burner).

After completing the installation of the boiler, inspect all the con­trols to make certain they are operating properly. Start and stop the burner or stoker several times by moving the room thermostat setting.

All local codes and regulations take precedence over the installa­tion instructions provided by the manufacturer. In the absence of local codes, the installation must conform with the boiler manufac­turer’s installation instructions plus regulations of the National Fire Protection Association, and the provisions in the latest editions of the National Electrical Code (ANSI/NFPA70) and the National Gas Code (ANSI Z223.1).

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers


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