Ducts and Duct Systems

Air distribution systems based on the forced-air principle of delivery utilize a system of ducts to deliver the heated or cooled air to the various rooms and spaces within the structure. These air ducts are generally rectangular or round pipes made from a variety of differ­ent materials. When these ducts are accurately sized and the duct system correctly designed, the air will be delivered to the rooms and spaces with a minimum of resistance, the result being a more efficient operation with reduced operating costs. The purpose of this chapter is to suggest methods for sizing ducts and designing an efficient duct system.

Methods for sizing fans are described in Chapter 7 of Volume 3 (V entilation and Exhaust Fans)’. Chapter 4 of V olume 1 (Sizing Residential Heating and Air-Conditioning Systems)’ and Chapter 8 of Volume 3 (Air — Conditioning)’ provide information and methods for sizing the heating and cooling units.

Codes and Standards

Always consult local codes and standards first before designing and installing a duct system. Any aspect of a duct system that does not comply with these codes and standards will have to be changed, and these changes could be expensive.

Information about duct sizing, installation methods, air distribu­tion, and air duct design methods is contained in the latest edition of the ASHRAE Handbook of Fundamentals and publications of the Air Conditioning Contractors of America (ACCA). Detailed information about ducts and duct fittings is available from the Commodity Standards Division of the U. S. Department of Commerce.

Types of Duct Systems

The two duct systems most commonly used in forced-warm-air heating are (1) the perimeter duct system and (2) the extended plenum duct system. Both are available in several design modifica­tions and are described in the sections that follow.

Details about the piping arrangements used with gravity warm­air furnaces are included in the section describing these furnaces in Chapter 10 of Volume 1 (Furnace Fundamentals)’.

An excellent source of information for designing a warm-air heating system are the publications and software from the ACCA. The recommended publications are Manual D—Residential Duct Systems and Manual T—Air Distribution Basics for Residential and Small Commercial Buildings. The latter manual provides step — by-step procedures for selecting, sizing, and locating the supply air diffusers, grilles, registers, and return grilles. The Ductsize software available from the ACCA describes how to calculate duct sizes for both supply and return duct systems using either the equal-friction or constant-velocity method.

Perimeter Duct Systems

A perimeter duct system is one in which the supply outlets are located around the perimeter (that is, outer edge) of the structure close to the floor of the outside wall, or on the floor itself. The return grilles are generally placed near the ceiling on the inside wall.

The two basic perimeter duct systems used in warm-air heating are (1) the perimeter-loop duct system, and (2) the radial perimeter duct system.

The perimeter-loop duct system (see Figure 7-1) is characterized by feeder supply ducts that extend outward from the furnace plenum to a loop duct running around the perimeter. Warm-air sup­ply outlets are located in the loop duct.

Ducts and Duct Systems

Ducts and Duct Systems

1. CENTER END REGISTER BOOT

2. REGISTER BOOT

3. ADJUSTABLE ROUND-PIPE SIDE TAKEOFF Figure 7-2 Radial perimeter duct system.

There is no loop duct in the radial perimeter duct system (see Figure 7-2). The feeder supply ducts extend from the furnace plenum to the warm-air supply outlets located on the outside walls or on the floor next to the outside walls.

Extended Plenum Systems

In the extended plenum system (see Figure 7-3), a large rectangular duct extends straight out from the furnace plenum (hence the term extended plenum) and generally in a straight line down the center of the basement, attic, or ceiling. Round or rectangular supply ducts extend as branches from the plenum extension to the warm­air supply outlets. The large extension to the plenum permits a bet­ter airflow rate with reduced resistance because of its large duct diameter. The branching ducts are usually located between joists and can be easily covered with a ceiling.

Crawl-Space Plenum Systems

It is possible to incorporate the entire crawl space into a heating system if the crawl-space walls are tight and well insulated. The heated air is forced down into the crawl space and enters the rooms through perimeter outlets, usually located beneath windows.

Ducts and Duct Systems

1. WALL STACK

10. CENTER REVERSE STACK ELBOW

2. STRAIGHT BOOT

11. RIGHT REVERSE STACK ELBOW

3. END BOOT

12. LEFT REVERSE STACK ELBOW

4. END

13. ROUND VOLUME DAMPER

5. ANGLE BOOT

14. STACK DAMPER

6. ANGLE

15. STACK HEAD

7. ELBOW

16. STACK HEAD

8. ELBOW

17. STARTING COLLAR

9. ELBOW

Figure 7-3 Extended plenum duct system.

This type of duct arrangement may be referred to as a crawl­space plenum system and represents a modification of the extended plenum system. Because the entire crawl space is filled with warm air, this system provides relatively uniform temperatures through­out the structure.

Duct Materials

Ducts are manufactured from a variety of different materials. The material selected will depend on the use for which it is intended. It is very important that the duct material be taken into consideration when designing a duct system because not every material is suitable for all conditions in which ducts are used.

It is possible to purchase ducts manufactured from the following materials:

• Steel

• Galvanized sheet steel

• Aluminum

• Copper

• Glass fiber

• Paper fiber Vitrified clay tile

Plain steel and galvanized sheet steel ducts are available in thick­nesses ranging from 0.0163 to 0.1419 inch. Ducts manufactured from this material are preferred for use in warm-air gravity and forced-circulation warm-air heating systems. Table 7-1 indicates the thicknesses, gauges, and weights in which plain steel and galvanized sheet steel ducts are available.

Aluminum ducts are available in thicknesses ranging from 0.012 to 0.064 inch (see Table 7-2) and are used in the same types of heat­ing systems as steel ducts. Although aluminum ducts are lighter than steel ones, they generally cost more. Aluminum ducts are frequently used in duct systems located on the outside of buildings.

Copper ducts are available in sizes and gauges matching the alu­minum ones and are frequently used in outside ductwork.

Round glass-fiber ducts can be purchased in a number of differ­ent sizes ranging up to 14 inches in diameter with duct walls up to 1 inch in thickness. Square or rectangular glass-fiber ducts can also be made from flat glass-fiber board. Because of their composition, glass-fiber ducts dampen sound.

Paper-fiber ducts are laid in concrete and used in warm-air heat­ing systems. They are accordingly not recommended for use in attics, basements, or other exposed areas.

Vitrified clay tile ducts represent another duct material suitable for installation under a concrete slab. These ducts range in outside diameter from 5V8 to 42V4 inches.

Duct System Components

The components of a typical duct system are illustrated in Figure 7-4. In a forced warm-air heating system, the warm air collects in an area at the top of the furnace called the furnace hood or plenum. An extended plenum duct system will have a large rectangular duct con­nected to the plenum by a starting collar and extending out along the ceiling in a straight line. Round (see Figure 7-3) or square supply ducts are connected to the plenum (or plenum extension) usually by adjustable side takeoffs and extend to either a register boot or an elbow. Changes of direction in the round duct are accomplished with flexible angle ducts. A nonflexible elbow provides the same function in rectangular ducts. A vertical duct or warm-air riser is sometimes referred to as a stack. A warm-air duct that carries the warm air horizontally in a straight line from the furnace plenum to

Table 7-1 Thicknesses, Gauges, and Weights of Plain (Black) and Galvanized Sheet Metal

U. S. Std.

Approximate Thickness (in)

Weight per Square Foot

Gauge

Steel

Iron

Ounces

Pounds

30

0.0123

0.0125

8

0.500

28

0.0153

0.0156

10

0.625

26

0.0184

0.0188

12

0.750

24

0.0245

0.0250

16

1.000

22

0.0306

0.0313

20

1.250

§ 20

V

R-i

0.0368

0.0375

24

1.500

-J—i

(A

^ 18

0.0490

0.0500

32

2.000

S 16

0.0613

0.0625

40

2.500

14

0.0766

0.0781

50

3.125

12

0.1072

0.1094

70

4.375

11

0.1225

0.1250

80

5.000

10

0.1379

0.1406

90

5.625

30

0.0163

0.0165

10.5

0.656

28

0.0193

0.0196

12.5

0.781

26

0.0224

0.0228

14.5

0.906

24

0.0285

0.0290

18.5

1.156

* 22

0.0346

0.0353

22.5

1.406

Heets

2

O

0.0408

0.0415

26.5

1.656

To

"S 18

N

0.0530

0.0540

34.5

2.156

•S 16

0.0653

0.0665

42.5

2.656

-3 14

0.0806

0.0821

52.5

3.281

O 12

0.1112

0.1134

72.5

4.531

11

0.1265

0.1290

82.5

5.156

10

0.1419

0.1446

92.5

5.781

* Galvanized sheets are gauged before galvanizing and are therefore approximately 0.004 inch thicker. (CourtesyASHRAE 1960 Guide)

The stack is often referred to as a leader. Dampers are located in the duct so that the quantity of warm air can be regulated manually or automatically by thermostatic control. Ducts that carry the warm air to the rooms are called supply ducts. All ducts that carry the return air back to the furnace are referred to as return ducts.

Table 7-2 Thicknesses, Gauges, and Weights of 2S Aluminum (Density 0.098 lb/in3)

B.& S. Gauge

Thickness (in)

Weight per Square Foot

Decimal

Nearest Fraction

Pounds

Ounces

28

0.012

V64

2.7

0.169

26

0.016

V64

3.6

0.226

24

0.020

V64

4.5

0.282

22

0.025

V32

5.4

0.353

20

0.032

V32

7.2

0.452

18

0.040

%4

9.0

0.563

16

0.051

%4

11.5

0.720

14

0.064

V16

14.4

0.903

(CourtesyASHRAE 1 960 Guide)

Supply Air Registers, Grilles, and Diffusers

The three basic types of supply air outlets used in an air distribution system are (1) grilles, (2) registers, and (3) diffusers.

Grilles (see Figure 7-5) are used not only to admit the airflow but also to deflect it up or down, or to one side or the other, depending on the direction in which the hand-operated bar moves. They are used primarily on high or low wall locations. Floor grilles are used extensively in gravity warm-air heating systems.

A register (see Figure 7-6) is similar in design and function to the grille but with the added feature of being able to regulate the vol­ume of the air with a damper. They may be located on walls (high or low) or floors. Floor registers are often used when installing a new heating system in an old house. The major objection to floor registers is that they tend to collect dust and trash.

A diffuser (see Figure 7-7) is also used to deflect the airflow, but it differs fundamentally in design from the grille. Diffusers manufac­tured in the form of concentric cones or pyramids are usually mounted on ceilings or walls. Baseboard diffusers are used in perimeter forced warm-air heating systems. The major objection to ceiling diffusers is that they cause drafts when the air is discharged downward and dirt smudges when the air is discharged horizontally across the ceiling.

If the duct system is designed primarily for cooling, outlets are sometimes located on the ceiling or high on the wall; however, satis­factory heating and cooling can be achieved with baseboard outlets placed low on walls by increasing the air volume and velocity and by properly directing the airflow.

Ducts and Duct Systems

700-T DUCT SIZES 4 x 8 TO 36 x 8

721 SIDE TAKE OFFS 4 x 8 , 5 x 8, 6 x 8.

710-T STARTING COLLAR, SIZED SAME/

722 SIDE TAKE OFFS 4 x 8, 5 x 8, 6 x 8.

AS/DUCT

224 — 725 — 726 REVERSE STACK ELBOWS

728-T & 729-T INCREASER-REDUCER

10 x 3’/4, 14 x 3’/4

SECTIONS, SIZES SAME AS

712 ANGLE 10 x 31/4, 12 x 31/4, 14 x 31/4

DUCT (MAX. ING. 10′ FOR

713 ANGLE 10 x 31/4, 12 x 31/4, 14 x 31/4

728-T AND 5′ FOR 729-T)

716 ELBOW 10 x 31/4, 12 x 31/4,

712-T MAIN TRUNK ANGLES, SIZES SAME

14 x 31/4

AS DUCT

717 ELBOW 10 x 31/4, 12 x 31/4, 14 x 31/4

713-T MAIN TRUNK ANGLES, SIZES SAME

720 ELBOW 10 x 31/4, 12 x 31/4, 12 x 31/4,

AS DUCT

14 x 31/4

717-T MAIN TRUNK ELLS, SIZES SAME AS

700 WALL STACK 10 x 31/4, 12 x 31/4

DUCT

14 x 31/4

720-T MAIN TRUNK ELLS, SIZES SAME AS

755 STACK HEAD 4, 5, 6, 8 x 10, 4, 5, 6,

DUCT

8 x 12, 4, 5, 6, 8 x 14

756 STACK HEAD 4, 5, 61 8 x 10, 4, 5, 6,

Figure 7-4 Principal components of a warm-air duct system.

{Courte?/ Clayon and Lambert. Mfg. Co.)

Return Air and Exhaust Air Inlets

Grilles and registers are the two principal types of air inlets used to exhaust the air from a space or to return the air to the centrally located heating or cooling unit. The grilles are generally fixed-angle types because there is no need to direct air circulation when return air is involved.

Ducts and Duct Systems

Figure 7-5 Examples of various grilles. (CourtesyA-J Mfg. Co.)

Duct Run Fittings

Round and rectangular duct run fittings are available in a variety of different shapes and sizes, depending on the requirements of the air distribution system. Some duct run fittings are used only for cool­ing systems; others are designed for use in both heating and cooling systems. Duct run fittings can be purchased from manufacturers

Figure 7-6 Examples of various registers.

Ducts and Duct Systems(Courtesy United States Register Co.)

Ducts and Duct Systems

And local supply houses, or they can be made locally. Making your own fittings requires knowledge of sheet-metal work. Components of a typical duct system are shown in Figure 7-4. Based on their function, these duct run fittings can be divided into the following principal categories:

• Supply-air and return-air bonnet or plenum (see Figure 7-8) Plenum and extended plenum takeoffs (see Figure 7-9)

Ducts and Duct Systems

Figure 7-7 Examples of diffusers. (Courtesy United States Register Co.)

• Trunk duct angles and elbows (see Figure 7-10)

• Stack angles and elbows (see Figure 7-11)

• Boot fittings (see Figures 7-12 and 7-13)

• Wall sections (see Figure 7-14)

Air Supply and Venting

Any boiler or furnace fired with a combustible fuel (for example, coal, oil, gas) must be equipped with a piping system to remove smoke and other low-temperature flue gases and to provide suffi­cient air for combustion. These air supply and venting systems are composed of round pipes and fittings made from sheet metal and resemble ducts in design and construction.

The design and installation of air supply and venting systems are described in the several chapters dealing with furnaces and boilers. See, for example, the appropriate sections of Chapter 11 of Volume 1 (Gas-Fired Furnaces)’.

Duct Dampers

A duct damper is a device used for controlling the direction or vol­ume of air flowing through a duct.

The ASHRAE defines a damper as being a device used to vary the volume of air passing through a confined cross-section by varying

Ducts and Duct Systems

Ducts and Duct Systems

Figure 7-8 Warm-air and return-air bonnet or plenum.

(CourtesyASHRAE 1952 Guide)

Ducts and Duct SystemsThe cross-sectional area. ’In other words, a damper functions as an obstruction to airflow through the duct; however, it is a movable obstruction that can be adjusted to give various-size openings for the passage of air.

The air volume in an air distribution system may have to be changed between the heating operation and the cooling operation because it generally requires more air at 50°F to cool a house to 75°F than warm air at 160°170 °F to heat a house to 75°F. Bearing this in mind, the ductwork and fan should be sized for whichever

Ducts and Duct Systems

Ducts and Duct Systems

Ducts and Duct Systems

EQ. FT.

Figure 7-9 Trunk duct takeoff. (CourtesyASHRAE 1952 Guide)

Ducts and Duct SystemsOperation (that is, heating or cooling) requires the greater air volume. Volume control dampers may then be installed in the duct system to reduce the air volume during the season that requires the smaller volume.

A damper is usually made in the form of a round or rectangular blade and can be either manually operated or motorized. The ASHRAE lists the following three basic types of dampers:

• Volume dampers

• Splitter dampers

• Squeeze dampers

A volume damper or volume-control damper (see Figure 7-15) is installed in a duct to either completely cut off or regulate the flow

‘O*’

V_v

Vv-W/

Ft.

4 TO 15 = 15′

16 TO 27 = 10′

28 TO 41 = 15′

42 TO 52 = 20′

53 TO 64 = 25′

 

подпись: v_v

22 TO 27 = 20′ 28 TO 33 = 25′ 34 TO 42 = 30′ 43 TO 51 = 40′ 52 TO 64 = 50′

 

Ducts and Duct Systems Ducts and Duct Systems

Ducts and Duct Systems

28 TO 42 = 40′

Ducts and Duct Systems Ducts and Duct Systems7 TO 11 = 40′ 12 TO 15 = 55′ 16 TO 21 = 75′ 22 TO 27 = 100′ 28 TO 33 = 125′ 34 TO 42 = 150′

Ducts and Duct Systems

Ducts and Duct Systems

Ducts and Duct Systems

10′ EQ. FT

 

Ducts and Duct Systems

10′ EQ. FT.

 

Ducts and Duct Systems

5′ EQ. FT.

 

Ducts and Duct Systems

25′ EQ. FT

 

Ducts and Duct Systems

10′ EQ. FT

15′ EQ. FT

EQ. FT 10" WIDTH = 40′ 12" " = 50′

14" " = 55′

Ducts and Duct Systems

Ducts and Duct Systems

Figure 7-11 Stock angles and elbows. (CourtesyASHRAE 1952 Guide)

EQ. FT. ‘

10‘ WIDTH = 15′ 12" " = 15′

14" " = 20′

подпись: eq. ft. '
10‘ width = 15' 12" " = 15'
14" " = 20'
 
Of air in the duct. Adjustments of the volume of airflow and resis­tance can be made with a squeeze damper. A splitter damper is a directional device consisting of a blade hinged at one end and used at locations where a branch run leaves the main duct.

Three examples of volume dampers are (1) slide dampers, (2) hit-and-miss dampers, and (3) butterfly dampers. The slide damper is operated by sliding or pushing a metal plate across the duct. It operates at either full open or full closed position, there being no

Ducts and Duct Systems

Ducts and Duct Systems

C

E

подпись: c
 
e
Ducts and Duct Systems

D

подпись: d

45′ EQ. FT.

подпись: 45' eq. ft.
 
Ducts and Duct Systems70′ EQ. FT.

Ducts and Duct Systems

SHAPE \* MERGEFORMAT Ducts and Duct Systems

Ducts and Duct Systems

Ducts and Duct Systems

5′ EQ. FT

подпись: 
5' eq. ft

Ducts and Duct Systems

35′ EQ. FT.

Ducts and Duct Systems

AND 45-DEG ELBOW A

AND 90-DEG ELBOW C

подпись: 
and 90-deg elbow c

B

подпись: 
b

Ducts and Duct Systems

Ducts and Duct Systems Ducts and Duct Systems

Ducts and Duct Systems

G H

Figure 7-13 Combination warm-air boots. (CourtesyASHRAE 1952 Guide)

Ducts and Duct Systems

Practical intermediate setting. A hit-and-miss damper provides vol­ume control with two slotted plates or discs placed adjacent to one another. At full open position, 50 percent of the duct cross-section remains blocked. A butterfly damper consists of a single blade hinged in the middle. At full open position, almost the entire cross­section of the duct is free of blockage.

The adjustment lever for a duct damper should be in an accessi­ble location, and it should be labeled to indicate position settings,

Ducts and Duct Systems

DUCT

Ducts and Duct Systems

Function, and area served. The damper should also be equipped with a positive locking device.

Dampers properly placed in the supply and return ducts of an air distribution system will control the supply of air to a room; how­ever, care should be taken in the placement of these dampers. A damper in a supply duct placed too close to the supply air outlet may disturb the airflow. Excess noise is often the result of improperly placed dampers.

Splitters and guide vanes (see Figure 7-16) are nonmovable sheet-metal partitions placed in stack heads or elbows to reduce air turbulence and to guide the airflow. Air directors are similar devices but are used to direct a portion of the airflow to a branch duct.

A fire damper is a damper designed to close off a duct in order to prevent the spread of flames and smoke. Fire dampers should be placed in branch duct connections that pass through fire walls, fire­rated partitions, or floors. Never install a fire damper in main exhaust ducts and risers. Installation of fire dampers should comply with local ordinances and National Fire Protection Association Standards.

Damper Motors and Actuators

Damper motors or actuators are devices used in heating, ventilat­ing, and air-conditioning systems for the following applications:

Diverting airflow Decreasing airflow

• Controlling ventilation

• Zoning

Motorized dampers are available in a number of different designs, but they can be grouped into two general classes: (1) single­blade dampers and (2) opposed-blade dampers. There are two types of single-blade dampers. One type is opened and closed by the damper motor. It remains closed when not engaged by the damper motor. Another type of single-blade damper uses the damper motor to open it and a spring to close it. An opposed-blade damper has narrow panels covering slots. The panels are opened and closed by the damper motor.

A damper motor or actuator can be used to position a diverting damper when a parallel airflow pattern is required. The damper is used to direct airflow through either the heating or cooling unit. The damper motor is connected to the heat-cool switch on a semi­automatic changeover thermostat or in parallel with the cooling contact of an automatic changeover thermostat.

In modern forced-air heating systems, zoning is commonly accomplished by a thermostat for each zone. When there is a call for heat, the thermostat sends a signal to a control panel. The con­trol panel then opens the appropriate zone damper and turns on the furnace or air handler. Some of the older systems do not have a con­trol panel. Instead, the furnace or air handler is turned on by damper drip switches. A control panel is used in modern heating and air-conditioning systems to control the changeover from heating to cooling or from cooling to heating.

A damper motor or actuator can also be used to operate a bypass damper for the following purposes:

To bypass a cooling coil for dehumidification To bypass the heat exchanger on cooling To bypass the heat exchanger on heating

It is sometimes necessary to decrease the airflow during the heat­ing cycle in some systems. This can be accomplished by using a damper motor or actuator to position a resistance damper.

Systems that require the introduction of outdoor air during the cooling season, but not during the heating season, can use damper motors or actuators to control the ventilation.

Damper motors or actuators are also frequently used in zoning a duct system by opening or closing dampers to the various zones.

The small, compact electric motors used to provide proportional control of dampers can also be used to drive valves or step con­trollers in heating, cooling, and ventilating systems. For this reason, additional information about these motors is included in Chapter 9 of Volume 2 (V alves and Valve Installation)’.

An example of a motor used to drive a damper is found in Figure 7-17. This is a cutaway of a Honeywell modutrol motor,

Figure 7-17 Cutaway view of a modutrol motor.

Ducts and Duct Systems

K

подпись: k(Courtesy Honeywell Tradeline Controls)

Which functions as the drive unit in a modulating control circuit. The following are the basic components of this motor:

• Reversible motor

• Balancing relay Feedback potentiometer

• Gear train

The balancing relay controls the motor, which turns the motor drive shaft through the gear train. The motor is equipped with switches that limit its rotation to 90° or 160°. The gear train and other moving parts are immersed in oil to eliminate the need for periodic lubrication.

The motor is started, stopped, and reversed by the single-pole, double-throw contacts of the balancing relay. The balancing relay consists of two solenoid coils with parallel axes, into which are inserted the legs on the U-shaped armature. The armature is piv­oted at the center so that it can be tilted by the changing magnetic flux of the two coils to energize the relay. A contact arm is fastened to the armature so that it may touch either of the two stationary con­tacts as the armature moves back and forth on its pivot. When the relay is balanced, the contact arm floats between the two contacts, touching neither of them.

A feedback potentiometer consisting of a coil of wire and a slid­ing contact is included in the Honeywell modulating motor. The sliding contact is moved by the motor shaft so that it travels along the coil and establishes contact wherever it touches, according to the position of the motor.

Figure 7-18 shows a typical wiring diagram for a Honeywell modutrol motor. Note that there are two separate circuits in the modulating motor powered from T1 and T2. The motor circuit con­sists of the reversible motor, the rotational limit switches, and the con­tacts of the balancing relay. The control circuit includes the feedback potentiometer, the coils of the balancing relay, and the controller potentiometer.

The control circuit offers two paths for current flow—done through each side of the balancing relay. Increasing resistance in the B leg of the motor control circuit by changing the setting of the con­troller potentiometer will run the motor toward the closed position. Adding resistance to the W leg runs the motor open.

As the motor shaft turns, it moves a wiper over the feedback potentiometer. This makes the resistance in each side of the circuit the same. When the resistances are equal, the current flow through both sides of the balancing relay is equal. The balancing relay contacts open, stopping the motor. The circuit is said to be balanced.

The motor of a damper actuator should be completely sealed and immersed in oil. Such a motor can operate without mainte­nance or service for the life of the unit. An example of this type of motor is found on the ITT General Controls DHO series damper actuator shown in Figure 7-19. These DHO series motors are avail­able in two-position type (energized and deenergized) or three — position type (deenergized, first stage open, and second stage open). The first-stage intermediate position can be adjusted.

It is important that the DHO series damper motors work within the manufacturer’s specified load limit during all phases of opera­tion in order to ensure delivery of rated forces under usual voltage variation. Maximum workload can be determined from the data given in Table 7-3. Load is the deadweight pull at the particular stroke position. The imposed load can be measured with a small spring scale.

The Honeywell M833A damper actuator shown in Figure 7-20 is used to regulate duct damper condition according to zone ther­mostat requirements. It attaches directly to a damper shaft V2 inch in diameter or a 3/8-inch shaft with adapter provided. It mounts in any position directly on a duct, or inside a standard wiring junction box where Class 1 wiring is required.

Installing Damper Motors

The location of a damper motor will be determined by the design of the ductwork. If possible, the motor should be easily accessible for

Та

Ducts and Duct Systems

FACTORY CALIBRATION POTENTIOMETER DO NOT ADJUST

Ducts and Duct Systems

C7031 B, C D OR J

THERMISTOR

SENSOR

S963B REMOTE SET POINT POTENTIO­METER

M7044 OR M7045

ELECTRONIC MODUTROL MOTOR

C7031F

A Power supply, provide overload protection and disconnect ‘ means as required.

Ducts and Duct Systems

Figure 7-19 Damper actuator. (Courtesy ITT General Controls)

Servicing and maintenance. The following suggestions should be considered when seeking a location for a duct motor:

Connection

Position

1

2

3

4

5

Length of

Travel (in)

3.25[1]

3.00*

2.75*

2.50*

2.25*

Max

Return

Max

Return

Max

Return

Max

Return

Max

Return

Load

Force

Load

Force

Load

Force

Load

Force

Load

Force

Specifications

(lbs)

(lbs)

(lbs)

(lbs)

(lbs)

(lbs)

(lbs)

(lbs)

(lbs)

(lbs)

25 VA

15

8V2

17

9V2

19

10

20

11

22

13

40 VA

30

8V2

34

91/2

37

10

42

11

45

13

120 VA

30

81/2

34

91/2.

37

10

42

11

45

13

Length of

Travel (in)

2.625f

2.375f

2.l25f

L.875f

L.625f

25 VA

18

10

21

11

24

13

27

15

30

17

40 VA

36

10

42

11

48

13

54

15

60

17

120 VA

36

10

42

11

48

13

54

15

60

17

* When using dual damper arm, combined load must not exceed load maximums shown f Table for reversed damper arm position.

(Courtesy ITT General Controls)

CONNECTION

POSITIONS

1 2 3 4 5

подпись: connection
positions
1 2 3 4 5
Ducts and Duct Systems

O

Ц

1 1

O

1 1

O

ENERGIZED

POSITION

CONNECTION POSITIONS

5 4 3 2 1

I

■5.25-

^ ENERGIZED POSITION

NORMAL DAMPER ARM POSITION

подпись: ^ energized position
normal damper arm position
REVERSED DAMPER ARM POSITION DUAL DAMPER ARM 11.50

А

Ducts and Duct Systems

LOW-VOLTAGE HEATER WIRES TO W737 PANEL OR 24-V POWER SUPPLY

Ducts and Duct Systems

Figure 7-20 Damper actuator. (Courtesy Honeywell Tradeline Controls)

Honeywell modutrol motors must be mounted with the shaft in a horizontal position.

• A remote balancing relay mounted on a firm support must be used if motor location is subject to severe vibration.

Weatherproof motors should be used when the unit is located in a position exposed to the elements.

Troubleshooting Damper Motors

Although damper motors are completely sealed, they do malfunc­tion from time to time. The principal malfunctions, their possible causes, and suggested remedies are listed in Table 7-4.

Some manufacturers produce special compact control units for mounting on damper motors. These control units (for example, the Honeywell W859 Economizer shown in Figure 7-21) give damper motors the capacity to regulate the outside and recirculated air dampers for the following applications:

Possible Cause

Remedy

Motor not operating.

Power off

Check switches and fuse.

Loose wiring

Check connections.

Motor damaged

Replace actuator.

Motor operates but

Linkage binding

Check for free

Output shaft does not move.

Movement.

Actuator damaged

Replace actuator.

Motor stalls at

Limit switch

Inspect and replace

Maximum travel position.

Damage

If necessary.

(Courtesy ITT General Controls)

• To bring in outside air (when it is colder than the inside air) for cooling or ventilating

• To close outdoor air dampers to minimum position during the heating cycle

• To prevent short cycling of the cooling compressor to avoid coil icing

Blowers (or Fans) for Duct Systems

The ductwork and blower (or fan) are sized for either heating or cooling (whichever requires the greater air volume); however, their correct sizing depends on an accurate determination of the follow­ing facts about a structure or space:

The total heat loss

The total cooling load

Air delivery required (CFM)

External static pressure for ductwork

Once these facts are known, it is simply a matter of referring to the performance data provided by the blower or fan manufacturer and selecting the most suitable equipment for the system.

Ducts and Duct Systems

Figure 7-21 Economizer control. (Courtesy HoneywellTradelne Controls)

Designing a Duct System

The purpose of a duct system is to convey air from the blower or fan to the air supply outlets located in the various rooms and spaces of the structure, and then to return it to its point of origin.

The design of a duct system is determined by the cfm output of the furnace blower. The size (Btu/h output) of the furnace and its blower is determined by calculating the heat loss for the house or structure. The furnace must have an output capacity capable of replacing the heat loss. The specification sheet for the furnace will list the blower speeds (cfm) and outputs (Btu/h).

Accuracy in estimating the resistance to the flow of air through the duct system is important in the selection of a suitable blower or fan. Resistance should be kept as low as possible in the interest of economy; however, underestimating the resistance will result in the failure of the blower or fan to deliver the required volume of air.

Every precaution should be taken in the design of a duct system to ensure a smooth and efficient flow of air. Careful study should be made of the building drawings with consideration being given to the construction of duct locations and clearances.

The following recommendations may be of use to you in designing a duct system:

• Keep all duct runs as short as possible, bearing in mind that the airflow should be conducted as directly as possible between its source and delivery points, with the fewest possible changes in direction.

• Select location for duct outlets that will ensure proper air distribution. For example, locate the supply outlets in the floors along or near the exterior walls and the returns along or in the interior walls.

Provide ducts with cross-sectional areas that will permit air to flow at suitable velocities. The furnace blower must have the capacity to overcome the friction between the moving air and the duct surface.

Use moderate velocities in all ventilating work to avoid waste of power and to reduce noise.

Use lower velocities in schools, churches, theaters, and so on, than in factories and other places where noise due to airflow is not objectionable.

Duct System Calculations

An ideal duct system would be one in which total pressure remained constant. In other words, there would be no pressure losses any­where in the system. Under ideal operating conditions, any change in velocity pressure would be compensated for by an equal change in static pressure, thereby maintaining a constant total pressure (that is, the sum of the velocity and static pressures). A drop in velocity pressure would trigger a corresponding rise in static pressure, and vice versa. Unfortunately, this is not the way things work in actual practice because pressure losses very definitely do occur.

In the ductwork, pressure losses result from the resistance of the ducts to the passage of the air. This resistance occurs as a result of two effects: (1) friction loss and (2) dynamic loss. The former is caused primarily by the friction of the moving air against the surface of the duct. Dynamic loss results from sudden changes of direction (for example, in sharp elbows) in the air stream.

An important aspect of duct system calculations is determining the total external static pressure drop (that is, total resistance) of the duct system. In large part, a blower or fan is selected on the basis of its capacity to operate against the total resistance of the ductwork. This resistance of the ductwork to the flow of air is referred to as the external static pressure (or external static pres­sure drop), because it represents the pressure drop occurring out­side the heating or cooling unit. In addition to external static pressure, a blower or fan must also overcome resistance due to internal static pressure drop caused by the passage of air through heaters, coils, filters, and washers. This data can be obtained from rating tables provided by manufacturers. Duct resistance (that is, external static pressure drop) must be calculated for each duct system.

The equal friction method is frequently used to calculate the external static pressure of a duct system (see Equal Friction Method in this chapter). Two other methods used for making these calcula­tions are (1) the velocity-reduction method, and (2) the static-regain method. Detailed descriptions of both of these methods can be found in the ASHRAE Guide.

Duct Heat Loss and Gain

Duct heat loss or gain is another important factor to consider when designing a duct system. This aspect of heat transmission will depend on some or all of the following factors:

Temperature of the air in the duct Ambient temperature Air velocity in the duct Duct insulation

If the temperature of the air inside the duct is different from the temperature surrounding it (that is, the ambient temperature), then either a loss or gain of heat will occur.

If the ducts are used to convey heat, excessive heat loss from the ducts will reduce the efficiency of the heating system. This will result in a total loss of heating effect if the heat loss occurs in duct­work passing through an unheated area, but it can be considerably reduced with proper duct insulation (see Duct Insulation). Such heat loss can also occur in heated spaces, but here it is a problem of poor air distribution.

The same conditions exist when the ducts carry cool air. When air passes through spaces subject to the cooling effect of air-conditioning, insulating the ducts will effectively reduce the amount of heat gain.

Proper air distribution will also minimize heat gain in spaces that are partially cooled.

Air velocity will also influence the amount of heat gain in the ducts. High air velocities are recommended when the ducts are car­rying cool air because their effect is to reduce the amount of heat gain pickup in the ducts; however, they must be maintained consis­tent with the acoustic requirements of the installation.

Air Leakage

Air leakage through duct seams and holes will result in the loss of a portion of the air flowing through the duct and a proportionate reduction in the heating or cooling effectiveness of the system. Depending on the seriousness of the problem, the ductwork can lose up to V3 of the air supply in this manner. It is usually a matter of poor workmanship and can be corrected by sealing the seam cracks and holes by caulking or soldering.

Duct Insulation

Ducts are insulated to prevent excessive heat loss or gain. If the ducts are used to convey heat, excessive heat loss from the ducts will reduce the efficiency of the heating system. The reverse is true of ducts used in cooling systems. If the ducts are not insulated, they will absorb heat from the air around them, and system performance will be impaired.

To maintain the proper level of performance in a heating or cool­ing system, the following ducts should be insulated:

Supply ducts running through spaces that are neither heated nor cooled (for example, attics, basements, garages, crawl spaces).

• Long supply ducts (particularly those over 45 feet in length). All ducts located on the outside of buildings.

• Cool-air return ducts passing through hot areas (for example, furnace and boiler rooms, kitchens).

Round ducts are insulated with flexible fiberglass insulation. Both flexible and slab (board) insulation are used for rectangular ducts. The latter is made of spun fibrous glass wool. These lightweight, semirigid panels are available with a variety of facings (for example, 0.0025 embossed aluminum foil) for appearance and functional use.

Flexible and slab insulation are also produced as duct liners for absorbing duct system noise. One or both sides are coated to reduce air friction loss and bind surface fibers. They also insulate thermally.

Flexible insulation is secured in place with light-gauge wire ties. Slab insulation is secured to the duct surface with adhesive or mechanical clips.

It is recommended that a vapor barrier be placed between the insulating material and the duct surface to prevent the formation of condensation. The vapor barrier should be used when the tempera­ture of the air inside the duct is lower than the dew-point temperature of the air surrounding the duct.

Equal Friction Method

The equal friction method of sizing ducts is recommended because it does not require a great deal of experience in the selection of proper velocities in the various sections of the duct system. It is nec­essary to select the main duct velocity consistent with good practice from a standpoint of noise for a particular building or application. In this duct-sizing method, the duct design is based primarily on a consistent pressure loss for each foot of duct.

Proportioning for equal friction is more advantageous than reducing the velocity in a haphazard manner because the friction calculation is greatly simplified. In calculating the friction, it is necessary to know only the length of the longest run, the number and size of elbows, and the diameter and velocity of the largest duct. The friction loss is exactly the same as though the entire amount of air were carried the whole distance through the largest duct.

The air velocities listed in Table 7-5 have been found to accomplish satisfactory results in engineering practice. Where quiet operation is essential, the blower or fan should be selected on the basis of a low outlet velocity. This will also result in lower operating costs.

The following steps are involved in sizing ducts by the equal friction method:

1. Compute the total volume (in cubic feet) of the structure.

2. Compute the cubic-foot volume of each room in the structure to be supplied with heated or cooled air. The volume of each room should be expressed as a percentage of the total volume of the structure. For example, a room having a volume of 6000 cubic feet would represent 10 percent of a structure hav­ing a total volume of 60,000 cubic feet.

Designation

Recommended Velocities (fpm)

Residences

Schools, Theaters, Public Buildings

Industrial

Buildings

Outdoor air intakes*

500

500

500

Filters*

250

300

350

Heating coils*

450

500

600

Air washers

500

500

500

Fan outlets

1000-1600

1300-2000

1600-2400

Main ducts

700-900

1000-1300

1200-1800

Branch ducts

600

600-900

800-1000

Branch risers

500

600-700

800

Maximum Velocities (fpm)

Outdoor air intakes*

800

900

1200

Filters*

300

350

350

Heating coils*

500

600

700

Air washers

500

500

500

Fan outlets

1700

1500-2200

1700-2800

Main ducts

800-1200

1100-1600

1300-2200

Branch ducts

700-1000

800-1300

1000-1800

Branch risers

650-800

800-1200

1000-1600

*These velocities are for total face area, not the net free area; other velocities in table are for net free area.

(CourtesyASHRAE 1 960 Guide)

3. Compute the total amount of air to be handled by the blower or fan. This will be the total CFM for the entire structure and can be computed by the air change method:

Building Volume in Cubic Feet

CFM =——— — f——— ———————

Minutes Air Change

4. Determine the portion of the total amount of air to be deliv­ered to each room. This is computed by multiplying the total CFM for the structure by the room volume percentage (see step 2). For example, if the blower or fan handles a required

15,0 CFM, then the room described in step 2 would receive 1500 CFM (15,000 CFM X 10% = 1500 CFM).

l /

подпись: l /3000 CFM l/

3000 CFM

15,000 CFM /

FAN

Ducts and Duct Systems

10,500 CFM 6000 CFM / 1

 

4500 CFM

■ 1500 CFM

/ I / I

1500 CFM 3000 CFM

Figure 7-22 Air velocities of a duct system.

5. Determine the design and location of the duct runs (supply, branch, and return runs) (see Figure 7-22) and then locate the supply air outlets and return air openings for each room to give the most uniform distribution of air. Each outlet should be selected for suitable air velocity and throw (the manufac­turer’s catalog will provide the necessary data).

6. Determine the total CFM for the main supply duct before any branch ducts are reached. This will be equal to the total CFM for the entire structure.

7. Determine the allowable air velocity in the main supply duct (see Table 7-5). For a commercial building such as the one represented by this example (that is, 60,000 cubic feet), the air velocity in the main duct will be 1300 fpm.

8. Determine the static pressure drop from Figure 7-23. Since the supply duct must carry 15,000 CFM, locate 15,000 on the ver­tical scale on the left side of the friction chart in Figure 7-23. This will be found a little over halfway between 10,000 and

20,0 on the scale. Draw a line horizontally across the chart to where it intersects at a point halfway between the diagonals representing 1200 and 1400 fpm (that is, 1300 fpm). This gives a static pressure drop for the main duct of approximately 0.04

Inch of water per 100 feet of duct. Use the static pressure drop of 0.04 inch as a constant for the entire duct system.

9. Use Figure 7-23 to compute the round duct diameter for each branch duct. For example, branch No. 10 in Figure 7-22 will have a diameter of approximately 19 inches. This is determined by finding the 1500 CFM on the left-hand column of the chart in Figure 7-23 (halfway between 1000 and 2000 CFM) and
moving horizontally in a straight line to the right until it inter­sects with the vertical line representing a static pressure drop of

0. 04 inch. The two lines intersect at a point representing an approximate round duct diameter of 19 inches.

10. Using the same data in step 9, determine the air velocity for each branch duct. The air velocity of duct branch No. 10, for example, will be approximately 750 fpm.

11. Size the return air duct system by first determining the amount of return air required. In step 4, the amount of supply air for the structure was stated as being 15,000 CFM. The amount of return air (CFM) required can be determined by subtracting the amount of fresh air intake from the supply air (CFM). The amount of fresh air intake (CFM) can be determined from the following formula:

, Volume of Structure Number of Air

CFM —_____________________ X

60 Min./Hr. Changes per Hour

12. After the duct sizes have been determined, it is necessary to compute the external static pressure of the system so that a suitable fan can be selected which will handle the required volume of air (in this case 15,000 CFM) against the total static pressure of the system. In the equal friction method, the total external static pressure drop of the system is obtained by calculating the external static pressure for the duct run having the highest total resistance. For the duct system shown in Figure 7-22, this can be obtained by adding the total length of the main supply duct from the point where the air enters the system (that is, duct sections numbered 1-6) and the equiva­lent straight pipe length of all elbows and transitions. This total length figure is then multiplied by the 0.04 inch static pressure constant (see step 9) to obtain the total resistance (that is, the total external static pressure drop) for the duct system. If, for example, the total length of duct was found to be 406 feet, the total resistance would be 0.1624 inch (406 ft ^ 100 ft — 4.06 ft X 0.04 — 0.1624 in).

13. The total resistance for the return duct run is determined in the same manner as described for the supply air run. Because it represents less air volume than the supply air (return air — supply air — fresh air intake), it will always be a smaller figure than the static pressure determined for the supply duct run.

As a rule-of-thumb, the return-run static pressure will be about 25 percent of the supply-run static pressure.

14. The total static pressure against which the blower or fan must operate includes the following:

A. Total resistance of the supply air duct system (that is, the total external static pressure drop).

B. Total resistance of the return air duct system (that is, the total external static pressure drop).

C. Total internal static pressure losses (that is, resistance through filters, cooling coils, and other forms of equipment).

Balancing an Air Distribution System

After the air distribution system has been installed, it should be tested to determine whether the air delivery and distribution corre­spond with the system design. Engineers and experienced field workers use a number of different instruments to make the various measurements required for balancing the system. Because these measuring devices are not generally available to the average person, the following simplified balancing procedure is suggested:

1. Open all duct and outlet dampers.

2. Check drafts, noise, and temperature differences from room to room while the fan or blower is operating.

3. Adjust the dampers to provide the greatest uniformity in operating characteristics.

This procedure is an abbreviated form of the one recommended for balancing a warm-air heating system. Read the appropriate sec­tions of Chapter 6 of Volume 1 (“Warm-Air Heating Systems”) for more information.

Duct Maintenance

The ducts used in heating, ventilating, and air-conditioning systems require very little maintenance other than periodic cleaning. Evidence that dust and dirt will accumulate in the ducts is often indicated by dirt streaks from ceiling and wall air supply outlets. Accumulations of dust and dirt in the ducts can be dangerous because they represent potential fire hazards. These are combustible materials that can be ignited when conditions are right. Ducts should be cleaned periodi­cally to prevent these accumulations from forming. Doors should be placed in the ducts to provide access for cleaning.

Corrosion is very seldom a problem unless there is an accumula­tion of condensation over a long period of time. This can be pre­vented by keeping the ducts dry. A vapor barrier will accomplish this purpose (see Duct Insulation).

Check the ducts for cracks, holes, or other damage causing leaks. Leaks are usually found in the return ducts located outside the con­ditioned spaces (for example, in basements, crawl space, attic, wall, floor, and ceiling cavities). These cracks or holes in the return ducts draw in outside air, creating excess pressure inside the house and forcing conditioned air back out through the same cracks or holes. This can result in lowered comfort levels and higher energy costs. Most houses, especially older ones, will also have cracks or holes in the return and supply ducts. Making only partial repairs, such as patching the cracks and holes in the return ducts but not in the supply ducts, can create a pressure imbalance.

A properly designed duct system is one with the supply and return ducts in balance. In other words, the amount of air supplied to the ducts should be equal to the amount of air returned to the furnace air handler. If the supply and return ducts are in balance, the pressure inside the house will be neutral. A pressure imbalance can draw in combustion gases from the furnace, allergens from the outdoor air, dust mites, mold, and other contaminants from the basement and attic or attic craw space.

All ducts running through unconditioned spaces should be insu­lated to minimize conductive heat loss.

Roof Plenum Units

Air distribution systems for commercial and industrial buildings sometimes require variable amounts of outside air for ventilation purposes to be mixed with return air from conditioned spaces within the structure. A roof-mounted insulated plenum to which distribution supply and return air ducts may be connected through a roof opening is required for such an installation. Outside air dampers are designed to adjust from closed to full open and may be manually controlled by a multiposition remote potentiometer. When full open, the dampers will permit the intro­duction of a maximum of 80 percent outside air for ventilation purposes.

The Janitrol roof plenum unit shown in Figure 7-24 is located next to the control box to which it is connected by means of a flex­ible conduit. The construction of the roof base and curb is illus­trated in Figure 7-25. Roof plenum units that do not provide for the addition of outside air to the system are also available.

Ducts and Duct Systems

Figure 7-24 Roof plenum unit. (Courtesy Janitroi)

Mobile Home Duct Systems

Duct kits for adding central air-conditioning to forced warm-air heat­ing systems are available from some manufacturers for installation in mobile homes, and particularly for installation in the double-wide combination mobile homes.

A typical air-conditioning duct kit installation is illustrated in Figure 7-26. The air conditioner is located outdoors and the cool air is supplied to the mobile home by a round (12-inch diameter) flexible duct that leads from the air-conditioning unit to the long metal heating ducts located in the center of the structure. A transition

Ducts and Duct Systems

RETURN TRANSITION

1 SUPPLY TRANSITION EXISTING HEATING

Ducts and Duct Systems

PLATES

Figure 7-26 Duct kit for installation in the double-wide combination

Mobile home. (Courtesy Dornback Industries, Inc.)

Tee with a short length of round, nonflexible metal duct connects the flexible duct leading from the air conditioner to the metal heat­ing duct running the length of the mobile home through a supply air opening (not to be confused with the warm-air registers on the top of the heat supply duct and which are a part of the existing heating system).

A return air grille is located in the floor near the outside wall closest to the air conditioner. As shown in Figure 7-27, the return grille is located over an enclosure that contains a filter for cleaning the air returning to the air conditioner.

The flexible cool-air supply duct is connected by cutting a 16-inch by 16-inch square opening in the flooring directly beneath the main heat supply duct at the chosen location. A smaller 12^/2-inch-wide hole is then cut through the bottom of the heating duct. An adapter plate with a round opening is placed over the opening in the heating duct and secured in place with screws. Tape is then run around the outer edge of the adapter plate to seal it against air leaks (see Figure 7-28). Supply air outlets closest to the air condi­tioner use an adapter plate with a 12-inch opening. Those farthest away use an adapter plate with a 10-inch opening.

RETURN AIR 12" x 20" FL00R GRILL

Ducts and Duct Systems

Cut 12V4" x 201/4" opening in both main and lower floors.

Lower return air duct enclosure through opening from inside of coach.

Screw duct to main floor with #8 x wood screws.

Figure 7-27 Construction details of the return air grille and filter.

(Courtesy Dornback Industries, Inc.)

Before locating the best position for the air conditioner and routing the flexible duct, the positions of the supply air outlets and the return air grille should be determined. When doing this, the following recommendations are offered:

SUPPLY AIR (FURTHEST FROM UNIT)

/^7 , — >/^7

/^7

* /.

V,

Ij

H

1 • • [2]‘ • * ‘ • • ; r • — , •

••• — •

JJC

Ducts and Duct Systems

FASTEN FLEX DUCT TO ADAPTER PLATE WITH METAL TABS— SEAL JOINT WITH TAPE

INSTALL BLANK-OFF PLATES AROUND FLEX DUCT AND ATTACH TO LOWER FLOOR

Figure 7-28 Construction details of a supply air grille.

(Courtesy Dornback Industries, Inc.)

Proprietary Air Distribution Systems

Proprietary air distribution systems are available from several man­ufacturers to provide supplementary heating or cooling in existing installations. They are also available as a complete heating and/or cooling system for installation in a structure where none previously existed or in which the existing system is clearly inadequate. They can also be used in new construction.

The Dunham-Bush Space-Pak air distribution system illustrated in Figure 7-29 is an example of a proprietary system that provides total comfort conditioning, including heating, cooling, air cleaning, and humidifying.

The Dunham-Bush air distribution system is generally an attic or overhead installation, but it may also be installed in basements or any other suitable area with no impairment of its operating performance.

Ducts and Duct Systems

Figure 7-29 Space-Pak air distribution system. (Courtesy Dunham-Bush, Inc.)

Ducts and Duct Systems Ducts and Duct SystemsAttic or overhead installation requires the construction of a mounting platform for the blower unit (see Figure 7-30). It is recommended that isolation pads or strips be placed between the blower coil unit and the mounting platform to prevent vibration transmission through walls or floors (see Figure 7-31). If the blower coil unit is

Ducts and Duct Systems

Figure 7-30 Mounting platform for blower coil in attic or overhead

Installation. (Courtesy Dunham-Bush, Inc.)

Suspended from a ceiling, then both the blower coil and the electric heater are mounted on separate platforms.

The supply air is moved through a 7-inch insulated plenum duct with 2-inch insulated flexible tubing runs to air outlets. All compo­nents in the Dunham-Bush air distribution system snap or twist together. As in all duct systems, the number of tees and elbows should be limited to as few as possible in order to keep system pres­sure drop on larger layouts to a minimum.

A blower coil unit and an attached electric heater (see Figure 7-32) provide either heat or cool air depending on the control setting.

A cross-section of a blower coil unit is shown in Figure 7-33. The float switch is used to interrupt compressor operation when the condensation level exceeds a normal operating level. A further safeguard would be to provide a secondary drain pan.

Duct Furnaces

A duct furnace is essentially a unit heater designed for installation in a duct system. It is usually designed to operate on oil, natural gas, or propane gas, although electric duct heaters are also avail­able and growing in popularity.

An example of a gas-fired (natural gas) furnace is the Janitrol 72 Series duct furnace illustrated in Figure 7-34, which is available in

Figure 7-31 Using isolation strips for vibration

Ducts and Duct SystemsControl. (Courtesy Dunham-Bush, Inc.)

Ducts and Duct Systems

Ducts and Duct Systems

BLOWER COIL UNIT

ELECTRIC HEATING UNITS HEATER ELEMENT

Figure 7-32 Blower-coil unit and electric heater. (Courtesy Dunham-Bush, Inc.)

REFRIGERANT LINE CONNECTIONS

Ducts and Duct Systems

Figure 7-33 Cross-section of blower coil unit. (Courtesy Dunham-Bush, Inc.)

13 sizes depending on the requirements of the installation. These are factory-assembled units inspected and tested before they are shipped. Once they reach their destination, they should be unpacked and the contents carefully checked against the packing list. Missing or damaged parts should be reported to the supplier immediately. This is a procedure you should follow habitually whenever you receive a shipment of equipment.

The design of these Janitrol duct furnaces is AGA-certified for use in a duct system with static pressure up to 2 inches of water and with temperature rises as shown in column 4 of Table 7-6.

Duct furnaces used in conjunction with cooling equipment should be installed in parallel with or on the upstream side of the cooling coils. If a parallel flow arrangement is used, the dampers (or other means of controlling the airflow) should be made tight enough to prevent the circulation of cooled air through the unit (see ANSI Z21.30).

When equipped with a suitable condensation pan, a duct furnace may be AGA-certified for installation downstream from cooling coils, air washers, and evaporative coolers when operating as air — cooling systems. In the Janitrol 72 Series duct furnace illustrated in Figure 7-34, the condensation pan is added in the following manner:

1. Remove the bottom panel of the duct furnace.

2. Remove the circular knockout section in the panel.

3. Place the condensation drain connection of the condensation pan in the opening.

Ducts and Duct Systems

Figure 7-34 Series 72 gas-fired duct furnace. (Courtesy Janitroi)

4. Reinstall the bottom panel.

5. Connect the drain line.

6. Provide disconnect adjacent to bottom panel.

*AGA, Rating (Btu/h)

72-XXX-3

Approx.

Net Wt. (lbs) Ship Wt. (lbs)

72-XXX-4

Approx.

Net Wt. (lbs) Ship Wt. (lbs)

Unit Size

Inputt

Output

Temp. Rise

EDR* Steam

72-100

100,000

80,000

25-100

333

148

160

140

152

72-125

125,000

100,000

25-100

418

168

180

154

166

72-150

150,000

120,000

25-100

500

168

180

158

170

72-175

175,000

140,000

25-90

586

156

178

148

170

72-200

200,000

160,000

25-100

667

210

232

188

210

72-225

225,000

180,000

25-100

750

200

222

188

210

72-250

250,000

200,000

25-100

834

212

300

260

282

72-300

300,000

240,000

25-100

1000

270

298

256

284

72-350

350,000

280,000

25-90

1167

262

290

256

284

72-400

400,000

320,000

25-100

1333

399

440

383

424

72-500

500,000

400,000

25-100

1667

469

518

455

504

72-600

600,000

480,000

25-100

2000

565

622

525

582

72-700

700,000

560,000

25-90

2333

567

624

545

602

* Tabled rating for elevations up to 2000 feet above sea ievei. From 2000 to 7000 feet, input must be reduced 4% per 1000 feet above sea ievei by manually adjusting manifold pres­sure. For elevation above 7000 feet, when ordering from factory, order must state elevation at which unit is to operate. tInput of unit (Btu/h) heat value of gas (Btu/ft3) = gas consumption (ft3/h) tOutput ^ 240 Btu (Courtesy Janitroi)

342

подпись: 342

If space limitations prevent bottom-panel removal, access is pos­sible through the front panel and burners (both of which must be removed before inserting the condensation panel).

Duct furnaces may be installed downstream of evaporative cool­ers or air washers considered as air-cooling systems (that is, operat­ing with chilled water, which delivers air below the ambient air temperature at the duct furnace).

The minimum clearances between the duct furnace (and its draft hood) and the nearest adjacent walls, ceilings, and floors of com­bustible construction should be at least 6 inches (see ANSI Z21.30 and NEPA No. 31, Installation of Oil Burning Equipment 1972). Under certain circumstances the minimum clearance between the bottom of the duct furnace and the floor can be as close as 2 inches, provided the other clearances (that is, the 6-inch minimums) are maintained for the rest of the unit.

When planning duct furnace clearances, consideration must be given to accessibility for the following:

• Cleaning the heat exchanger Removal of burners

• Servicing the controls

The minimum clearance should be 18 inches for front removal of the burners and 10 inches for bottom removal.

The inlet and outlet ducts should be attached to the duct connec­tion flange. An access panel (or removable duct section) should be provided at both the inlet and outlet ducts to provide for servicing the limit and fan control elements. Moreover, such access should be so constructed as to provide for visual inspection of the heat exchangers.

In gas-fired duct furnaces, all gas piping must be run in accor­dance with requirements outlined in the American Gas Association’s publication Installation of Gas Appliances and Gas Piping.

It is recommended that a pipe joint compound certified for use with LPG be used on all pipelines. If possible, run a new gas supply line directly to the duct furnace from the meter. Support the gas line with hangers positioned close enough together to prevent strain on the unit. A trap consisting of a tee with a capped nipple should be provided in the gas line when the unit is installed. After installing the gas line, test it for leaks with a soap solution (never with a flame).

Oil- and gas-fired units must be properly vented. The draft head is built into these units, and the flue pipe must be the same size as the outlet of the flue collector on the duct furnace. Never reduce the size of the flue pipe or install a damper in it. Install the flue pipe to provide minimum clearances of 18 inches between it and com­bustible material. Always examine the chimney for proper con­struction and repair before connecting the flue pipe. Other details concerning venting practices are found in ANSI Z21.30 and NEPA No. 31, Installation of Oil Burning Equipment 1972. It is not nec­essary to vent electric-fired duct furnaces.

All electrical wiring for duct furnaces must be done in accor­dance with the National Electrical Code, ANSI CI-1971, and local code requirements. The unit must be grounded in accordance with these codes.

If a pilot flame is used for ignition, the flame should extend 1 inch beyond the pilot burner. Flame adjustment is made either with the pilot flame adjustment device located on the gas valve (on units without a pilot gas valve) or with the built-in adjusting screw (on units supplied with a pilot gas valve). Pilot gas pressure regulators are used on natural gas-fired units in areas where gas pressure vari­ations are great. If a pilot regulator is supplied with the unit, pilot flame length is adjusted by adjusting the gas pressure regulator. These pressure regulators are not supplied with propane gas-fired units. In any event, the length of the pilot flame on propane gas-fired units is not adjustable. A normal flame on a natural gas-fired unit will be blue in color with an inner cone approximately 1 inch high. On a propane gas-fired unit, a normal flame will be green in color with a distinct inner cone approximately V8 to V4 inch high. Figure 7-35 shows a typical pilot assembly for a Janitrol gas-fired duct furnace.

The thermocouple pilot and thermopilot relay on Janitrol gas-fired duct furnaces can be checked for proper functioning as follows:

1. Read the steps outlined on the operating instruction plate.

2. Start the unit and allow the pilot to heat for at least 3 minutes (do not turn on the main burners at this time).

3. Close the pilot valve, and wait for the pilot relay switch to open and cause the electric gas valve to close.

4. Under normal circumstances, the length of time between the closing of the pilot valve and the opening of the pilot relay switch should be less than 2 minutes.

5. If the length of time is greater than 2 minutes, then the pilot relay must be replaced.

The gas input for a gas-fired duct furnace must not be greater than specified on the rating plate of the unit. Duct furnaces are

ELEMENT

Ducts and Duct Systems

Figure 7-35 Location of fan control element. (CourtesyJanitroi)

Shipped with spuds containing orifices sized for the particular gas (natural gas or propane) with which they are fired.

In natural gas-fired duct furnaces, the main burner gas-pressure regulator must be adjusted for the correct gas input. This may be done either by timing the test dial on the meter or by checking the manifold pressure (see Table 7-7). The following steps are recom­mended for adjusting the gas input:

1. Remove the cap on the top of the regulator.

2. Turn the adjusting screw in (or clockwise) to increase the gas input.

3. Back-out the adjusting screw (turn it counterclockwise) to reduce the gas input.

The gas pressure at the inlet to the regulator on natural gas-fired duct furnaces should not be allowed to exceed 12 inches of water.

Propane gas-fired units must maintain a manifold pressure of 11 inches of water for proper operation. These units are not supplied with appliance gas-pressure regulators.

Some duct furnace installations require the use of a fan control. When this is the case, the element of the fan control should be located as shown in Figure 7-35. Adjust the fan control for an off temperature as low as possible without causing the occupants to experience a feeling of cold air.

A proper maintenance schedule for duct furnaces will increase the life of the equipment and result in more efficient and economical

Table 7-7 Natural Gas Manifold Pressures

Btu per Cubic Foot

Sp. Gr.

Man. Press. (Inches of Water)

Btu per Cubic Foot

Sp. Gr.

Man. Press. (Inches of Water)

900

0.50

3.4

1000

0.55

3.0

0.55

3.7

0.60

3.3

0.60

4.1

0.65

3.6

0.65

4.4

0.70

3.9

925

0.50

3.2

1025

0.55

2.9

0.55

3.5

0.60

3.1

0.60

3.9

0.65

3.4

0.65

4.2

0.70

3.7

950

0.50

3.1

1050

0.55

2.7

0.55

3.4

0.60

3.0

0.60

3.7

0.65

3.2

0.65

4.0

0.70

3.5

0.70

4.3

975

0.50

3.2

1075

0.55

2.6

0.60

3.5

0.60

2.9

0.65

3.8

0.65

3.2

0.70

4.1

0.70

3.3

1100

0.55

2.5

0.60

2.7

0.65

2.9

0.70

3.2

Note:Manifoid pressures on this table are based on orifice sizes as shown in orifice table in installation manual. Pressures given in this table apply to sizes of units. This table does not apply to units used in high-aititude areas. See supplement for high-aititude manifold pressure table.

(Courtesy Janitroi)

Operating characteristics. Frequently check burners, pilot, and the interior and exterior of the heat exchanger for a buildup of residue. Clean the exterior of the heat exchanger with a brush, compressed air, or a heavy-duty vacuum. The following steps are recommended for the more complicated procedure of cleaning the interior of the heat exchanger:

1. Disconnect the flue pipe from the unit.

2. Remove the pilot and burners.

3. Remove the flue collector and heat exchanger tube baffles.

4. Brush the interior of the heat exchanger with a flexible 1V2- inch-diameter bristle brush.

5. Remove debris with a vacuum.

6. Replace parts in reverse order to which they were removed.

On a gas-fired duct furnace, leave the pilot on during the sum­mer (except when the unit is part of an air-conditioning system). Inspect the flue pipe for deterioration at the beginning of the heat­ing season. Make a similar inspection of the heat exchanger (and related components) for carbon deposit, rust, or corrosion. Clean or replace when required.

Never light the pilot on gas-fired duct furnaces until you have first read the lighting instructions on the unit plate. Select and maintain a thermostat setting that provides adequate comfort. Do not keep changing the thermostat setting. It will only result in higher heating costs.

Electric Duct Heaters

Electric duct heaters are designed and manufactured to function in the same manner as oil — or gas-fired duct furnaces. These are prewired factory-assembled units available in a wide range of sizes and heating capacities for a variety of different installations.

Typical duct heater construction is illustrated by the Vulcan unit in Figure 7-36. The frame for the electric resistors is designed for inser­tion into the duct as shown in Figures 7-37 through 7-44. These Vulcan duct heaters are available in five standard supply voltage rat­ings (120, 277 single-phase; 208, 240, and 480 single — or three-phase) or a specific voltage depending on the requirements of the installation.

The control housing is located on the outside of the duct and should be located so that the control panel is accessible for inspection and service. Various standard and optional built-in control compo­nents are available, including (1) staging contactors, (2) control

High nickel chrome alloy resistance wire and corrosion — resistant terminals.

 

Bimetallic, disc-type primary automatic reset thermal cutout.

 

Disc — type, secondary manual reset is standard on all heaters up to 50-kw capacity. Above 50 kw, some models employ thermal links for secondary protection.

 

INSULATORS

 

Pressure electric switches to convert pneumatic signals to electrical signal (optional).

Pilot light visually indicates various heater conditions (opional).

REMOVABLE

CONCENTRIC

KNOCK OUTS

 

Built-in fuses for subcircuit protection on all heaters over 48 amperes total current rating.

 

Two types of secondary protection are available, depending on kw rating of the heater-backup contractors and thermal links.

 

SINGLE-SOURCE ENTRY

 

STEPDOWN TRANSFORMER TO SUPPLY CONTROL VOLTAGE (OPTIONAL)

 

Figure 7-36 Construction of a Vulcan electric duct heater.

(Courtesy Vulcan Radiator Co.)

 

Ducts and Duct Systems

Transformers, (3) sequencers, (4) fuses, and (5) primary and sec­ondary protection.

There are three types of UL-listed thermal cutouts used as safety limit controls in Vulcan duct heaters. These are (1) primary automatic reset, (2) secondary manual reset, or (3) secondary

Ducts and Duct Systems

(Courtesy Vulcan Radiator Co.)

Ducts and Duct Systems

Ducts and Duct Systems

INSULATION

Figure 7-39 Slip-in heater with internally insulated duct.

(Courtesy Vulcan Radiator Co.)

Heat limiters (thermal links). The function of each of these limit controls is to shut off the duct heater when there is no air or when the airflow is too low for efficient operation.

The primary automatic reset, a high-limit control, is a snap — action device that is sensitive to both radiant and convected heat. It is designed to deenergize the duct heater at a preselected, higher — than-normal temperature and to automatically reenergize the unit when it cools.

The secondary manual reset control is a warning device set to open at a temperature higher than the primary reset. It cannot be reset until the heater has cooled below the setpoint, and it must be reset manually.

Ducts and Duct Systems

Figure 7-40 When a heater is smaller than the duct area, the opening between the heater frame and duct must be filled with wire mesh or expanded metal. (Courtesy Vulcan Radiator Co.)

Ducts and Duct Systems

Figure 7-41 When a heater is larger than the duct area, the duct cross-section may be increased by a sheet-metal transition as shown in

This drawing. (Courtesy Vulcan Radiator Co.)

The heat limiters (thermal links) belong to the secondary protec­tion system. These are fusible, one-time protective devices that must be replaced in the event that they cut out.

Duct heater circuits may be subdivided into equal or unequal heating increments or stages in order to more closely match heater output with temperature variations.

Ducts and Duct Systems

Figure 7-42 The installation of a slip-in heater.

(Courtesy Vulcan Radiator Co.)

Ducts and Duct Systems

Ducts and Duct Systems

Figure 7-44 The installation of a canvas connector to minimize

Vibration. (Courtesy Vulcan Radiator Co.)

Posted in Audel HVAC Fundamentals Volume 2 Heating System Components, Gas and Oil Burners, and Automatic Controls


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