AIR-HANDLING PROCESSES Air-Heating Equipment

1.3.1.7Introduction

Many different methods of heating the air for industrial ventilation purposes are possible.

In all warm-air design applications, consideration must be given to the ef­fects of stratification in tall buildings. Stratification increases the roof and high-wall fabric losses and the air change rate by the stack effect, and hence the ventilation loss. These effects may increase the heat loss by 25% over that of a radiant heating system.

In many cases, warm-air heating is the cheapest heating system to install from the initial-cost point of view; the running costs, however, will be higher than for a radiant heating system to provide the same conditions.

The ventilation system can be used to full advantage during the summer months with the heater off to provide outside air and assist in removing the heat gains.

1.3.1.8Selection

The air may have to be heated for one or more of the following reasons:

• Fabric heating

• Fleating makeup air to take care of the ventilation loss

• Comfort heating

• Heating to reduce the incidence of condensation

• Heating to protect goods from damage in store

The selection of equipment to meet the above needs depends on many factors; options include these:

• Direct air warming is achieved by means of air-heater batteries.

• Indirect warming of the air is achieved by the use of radiant heaters.

The heating media used can be classified as

A. Low-temperature hot water (up to 100 °C)

B. Medium-temperature hot water (100 to 120 °C)

C. High-temperature hot water (over 120 °C)

D. Steam

E. Electricity

F. Heat recovery from process hot gases

G. Direct gas fired

H. Heat transfer fluids

I. Direct oil fired j. Solid fuel firing

Note: Items a, b, and c above are also classed as low-pressure, medium-pres — sure or high-pressure hot water.

1.3.1.9 Air-Heating Coils

These are used in conjunction with a ventilation or air-conditioning sys­tem and may be included in one of the following methods of ventilation.

• Natural ventilation.

• Mechanical extract-induced input.

• Mechanical input-forced extract. This arrangement is known as a plenum system.

• Mechanical inlet-mechanical extract.

In order for coils included in any of the above systems to operate effi­ciently, they must be designed to have a uniform air velocity across the whole of the heater’s face area. This is of prime importance, and the manu­facturer’s specifications regarding the maximum and minimum air veloci­ties must be met.

From the energy and noise point of view, care must be taken to prevent undue airflow resistance. This is achieved by not normally having more than five tube rows.

Typical face-area air velocity across extended-surface finned coils is nor­mally less than 3.5 m s-1. However, in the case of a plain heater, a velocity of

4 m s“1 can be used. In certain instances it is possible to use higher velocities; in all cases, however, the design requirements of the manufacturer should al­ways be met.

In a dusty industrial environment, finned heaters are more liable than plain tube heaters to become blocked or coated with dust. If this dust is greasy, it will bake on the high-temperature surfaces, reducing the rate of heat transfer and in­creasing rhe pressure drop across the coils. Special attention must be paid to the provision of easy access to areas for cleaning and maintenance of the coils; the cleaning is readily achieved by using compressed air or steam.

Air heaters in industrial environments require corrosion-protective fin­ishes that are capable of protecting the coil and case from damage by conden­sation, acid vapors, or aggressive chemicals, in the air or the primary medium. If air washers are used with coils placed after them, copper or other noncor­roding metal tubes should be used.

Full consideration of the thermal expansion of the tubes is necessary, with adequate provision for expansion and contraction. It is a wise policy to fit thermometer wells in the pipes near the inlets and outlets of all air- heating batteries, as these provide a useful means of checking the coil performance.

1.3.1.10 Heat Requirements

To determine the heat requirements of makeup air, the following equa­tions can be used.

Fabric loss <ty = X(A U)(0l,, — 0ao) (9.49)

Ventilation loss = O.33NV(0ai- 0dO) (9.50)

Equation (9.50) is a simplified equation that can be used at normal condi­tions. For practical purposes,

C (specific heat capacity of air) = 1.01 kj kg_1°C.

P air density = 1.2 kg in 5

O 33 = (1-Q1^-2)

3600

(The number of seconds in one hour is 3600.) where

<ty = fabric loss through all elements (kW)

<!>,,= ventilation loss (kW)

A = surface area of elements (m2)

II = overall thermal transmittance (Wmr2 °C)

0n = environmental temperature (°C)

0ao = outdoor air temperature (°C)

0M = indoor air temperature (°C)

N = number of air changes per hour

V = volume of room (m3)

The sum of the two losses, ^ and Oi;,wilI give This is the plant load. It must be remembered that the plant load is not necessarily the sum of fabric and ventilation loss, as ductwork losses in adjacent areas have to be considered.

For more exact work, the temperature ratios of Fr and F-, related to the mode of heating can be used. These relate to 100% convective, such as forced warm air to a high-temperature radiant system, which gives 90% radiant and 10% convective.

These factors compensate for the relationship between the inside air and mean surface temperature and provide similar comfort conditions at

The center of the space, regardless of the mode of heating employed. These ratios are

AIR-HANDLING PROCESSES Air-Heating Equipment

(9.51)

And

AIR-HANDLING PROCESSES Air-Heating Equipment

(9.52)

The CIBSE Guide Book A. 9 has seven tables which cover these ratios for different heating modes.

A space isolated from external building surfaces will have its heat gains from internal sources, such as process, lighting, occupants, etc. And as such,

These may be considered as being net sensible and latent heat gains throughout

The year requiring cooling.

To form a heat balance for the flows, Net sensible heat flow from the space = Sensible heat absorbed by the ventilating air.

Hence, Sensible heat = Air-mass flow rate (qm, kg s_l) x Specific heat capacity of humid air at constant volume (cj, which is 1.012 kj kg-1 K_1.

It is normal in duct design to use volume flow rate rather than the mass flow rate, giving

(9.53)

подпись: (9.53)Qv{mV1) = ^(kgs-1) x p (density, kg nr3).

The density of air at 20 °C and 101.325 kPa can be taken as p = 1.205 kg nr3. Supply air to a space can be at any temperature 0S.

The general gas laws show that air density is inversely proportional to its absolute temperature, hence,

P V = mRT

Where

P = the absolute pressure of the air (Pa)

V = volume of air (m3) m = mass of air (kg)

R = gas constant (287.1 J kg-1 K-1)

T = absolute temperature of the air (K)

And

= m (kg) = p (Pa)

AIR-HANDLING PROCESSES Air-Heating Equipment

Assuming that the values of the gas constant, the moisture content, and pressure remain constant in the heating or cooling process.

Let standard air density be p1 and supply air density be p2.

AIR-HANDLING PROCESSES Air-Heating Equipment

Ti

Pi = Plf-y ■

In practice, the air density has to be corrected for the specific supply temperature by

P2 = 1.1906 x ‘^2J2> + m_3 ‘

Substituting into the heat balance equation,

Sensible heat C’&sh» kW) = <lv (m3 s“1) x p2(kg m~3) x c(kj kg’1 K 1 x (0r + 9J K

= <7„x1.19O6x^i^(l. OO48)(0r-flJ) • (9.54)

Hence,

Qb, x 351 x 273 + i,9.5.->)

Or, expressed in terms of volume flow,

KW(273 + 0,) q" 351(0r-0s) ‘ ‘

If the space temperature is above its design value, cooling is required. If the space temperature is below the design value, the supply air must be heated. Hence,

L9’571

<J>Sfl(kW) = . (9.58)

The rates of extract involved in industrial ventilation are by nature of a high volume. It is of interest to consider the energy required to heat one cubic merer of air from, say, an outdoor temperature of -5 °C to be discharged into the space at 20 °C.

The basic equation is

<I>SH (kW) = { }?1 )(~ er)

SH^KW^ (273 + es)

= = 29.94 kW, say 30 kW.

(273 + 20) ‘

The above answer gives the heat requirement for the air alone, and normally the fabric losses would be added to this figure.

It will be appreciated that if 30 kW is required for such a small air quantity, it is important to reduce the airflow rate to a value as low as possible in order to save energy.

This reduction in energy use can be achieved without affecting the required pollution levels in the environment by the improved collection methods that are covered in these guides.

1.3.1.11 Low-Temperature Hot-Water Heating Coils

These are used in comfort heating systems and usually have no more than one or two rows of tubes. Various circuit arrangements are possible, depending on the pumping and control methods used.

As well as ensuring that the required design heating capacity is met, the following factors must also be considered.

• The maximum output efficiency must be met with the minimum of air pressure drop.

• The water pressure drop through the coil is as low as economically possible.

The design resistance of hot-water flow through a coil normally never exceeds

4 kPa in accelerated low-pressure hot-water heating installations.

In the case of high-pressure hot-water installations, the resistance to the water flow is determined by other factors, such as the balancing of circuits.

The heaters are fed from hot-water flow and return mains, and to ensure uni­form distribution of the heating medium, adequate connections to each row or bank of tubes or sections are necessary. To reduce air-locking problems, venting of the heater flow connections should be arranged. Parallel and counterflow are common arrangements with water coils. Counterflow is preferred, as this gives the highest possible mean temperature difference.

1.3.1.12 Steam-Heated Coils

For a steam coil to operate efficiently, it must have all the latent heat in the steam. This is achieved by the use of a steam trap. The correct trap type must be selected for the particular application in order to prevent waterlogging. All con­densate, air, or other noncondensable must be removed from the system without delay; otherwise,

• the rated coil output will fall, and

• corrosion will result, causing premature coil fracture.

The best performance is achieved if the steam is uniformly distributed to the individual tubes. Properly designed and selected steam-distribution tube coils distribute the steam throughout the entire length of all primary tubes. Problems may result with air heaters operating under light load, and these may be overcome by greater sectionalization of the controls. When the entering air temperature is below freezing, the steam supply to the coil should not be modulated. Coils are located in series in the airstream, with each coil sized to be on or completely off in a specific sequence; this de­pends on the entering air temperature. Low temperatures produce the risk of a coil freeze-up. The use of bypass dampers could be considered, but care should be taken to ensure that cold airstrearns do not impinge on the coil through the gaps in the partially closed dampers. Full provision is necessary to accommodate expansion and contraction of the coils.

Provision is necessary for adequate venting of the steam space. Heaters should have pressure gauges fitted at the steam inlet. In order to achieve effi — dent heat transfer, dry steam must be supplied to the battery; superheated steam is not suitable due to the time necessary for it to lose its latent heat to produce condensate.

1.3.1.13 Electric Air Heaters

These have the advantage that they are low-cost units to install; the run­ning costs, however, depending on the source of electricity, are generally higher than other energy sources.

The air velocity through a heater battery should be to the manufacturer’s rated output within its range of safe temperatures. In large units the electrical load is balanced across the three phases of the electrical supply.

The heaters are normally divided into a number of sections in order to provide step control of the unit. Each section of heater elements may consist of two or more rows, each having its individual busbar connections and ade­quate provision for it to be withdrawn for repair or cleaning, with the other elements remaining in operation. Care has to be taken with electrical isolation of each section before withdrawing them from the casing. The heaters should be electrically interlocked with the fan motors, allowing the electric heater to be switched off when the fan stops or when the air velocity is reduced to a level below that for which the heater has been designed. The risk of fire under abnormal operating conditions must be counteracted by the use of a suitably positioned, temperature-sensitive cutoff trip of the manual reset type.

1.3.1.14 Direct-Fired Air Heaters

These may be

• Gas fired

• Oil fired

• Solid fuel fired

Regardless of the fuel used, the following are the requirements of the flue.

Flues must be of the correct cross-sectional area in order to remove the products of combustion in a safe and efficient manner. The flue terminal should be fitted with a bird guard.

Care has to be taken to ensure that downdrafts do not cause the combus­tion products to be liberated in the occupied space. To achieve this, it is es­sential that the stack is of such a height that wind deflecting off adjacent structures does not influence the free passage of the products of combustion.

The appliances must be positioned in a space that has adequately sized combustion air inlet louvers that will not clog with debris and reduce the com­bustion air supply.

In small installations flueless appliances may be used; the use of these is to be discouraged due to the problems of toxic gases and condensate from the flue gas building up in the space.

1.3.1.15 Gas-Fired Heaters

Only standard, approved appliances should be used. These may operate on

• an atmospheric burner or

• a forced jet burner.

In large industrial installations, the latter is the most common arrangement. The burner has a profile plate that controls the rate of combustion air. The warm air delivery fan may be either centrifugal or axial.

In Europe, the gas safety controls must meet the requirements of CEN standards, including flame failure devices, solenoid control valve, pilot con­trols, ignition and governor. Overheat-type thermostats and either a pressure switch or an airflow-proving device are fitted to ensure that the burner will cut off in the event of no air flowing through the heater, such as occurs with fan failure.

Thermostatic control of the gas supply to the heater is required so that the air-off temperature can be controlled. This is achieved by a two-stage control that opens a valve partly on low rate and fully on high rate.

The flued appliance is designed to provide maximum heat transfer. The ef­ficiency can be increased by the use of a condensing unit.

The filter banks incorporated require adequate access for cleaning and re­placing.

The heat exchanger is normally of the welded type using aluminized steel, stainless steel, or similar materials. It should create the minimum of resistance to the air movement, which has to be turbulent in order to give an efficient transfer of heat.

Depending on the burner system employed, some form of a flue system is required to remove the products of combustion from the appliance to the at­mosphere.

In the case of atmospheric type burners, a draft diverter is required on the appliance (in addition to the flue outlet).

The positioning of flue terminations must be selected with care and may be subject to statutory or other regulations.

The flueless appliance has gas supplied to the burner head, and air passes outside the baffles at a designed velocity.

The air paths within these baffles are arranged to provide the correct amount of combustion air. Turndown ratios of up to 35 to 1 are obtainable, providing the correct gas flow for air temperature control.

The control system must satisfy a set sequence, as is required by CEN or other standards. An air switch provides airflow to the unit. A 30-second purge of the unit takes place by the fan; after this period, the ignition spark and pilot gas valve are operated.

On proving the pilot ignition, the main gas valve opens and the burner ig­nites. Once alight, the main burner will modulate to the temperature set by the room thermostat.

If, during the ignition sequence or the running sequence, flame failure oc­curs, a lockout will result, which will require manual resetting.

9.4.1,10 Oil-Fired Heaters

Burners of the vaporizing type are used only on low-output equipment. The most common type encountered is the pressure-jet burner.

All burners should meet the requirements of CEN or other national stan­dards. The units must be complete with safety devices for ignition failure and main flame failure.

For safe operation, the heater should have an overheat thermostat and either a pressure switch or airflow-proving device. This control device ensures that the burner will isolate in the event of restricted or no airflow through the heater.

The removal of the combustion products to the external atmosphere takes place through the flue, which may be

• a direct flue connection type,

• a direct flue with a stabilizer (the stabilizer must be compatible with the

Unit), or

• a fan-assisted or balanced flue.

The oil supply to the heater is thermostat controlled by the air-off temperature.

Posted in INDUSTRIAL VENTILATION DESIGN GUIDEBOOK


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