Document 06jB0pmj9vVkvDGBryee3JVyb

American Society of Heating and Ventilating Engineers Guide, 1936 BOILER CAPACITY The capacity of the boiler should be based on the rated capacity of the heaters at the lowest entering air temperature that will occur, plus an allowance for line losses. Ordinarily for recirculating heaters the lowest entering temperature will occur at the beginning of' the heating period and is usually taken as 40 F, while for ventilators taking air from outdoors the lowest entering temperature will be the extreme outdoor temperature expected in the district. No greater allowance in boiler capacity beyond the calculated heat demand need be added in order to supply unit heaters than for any other type of system. It is unwise to install a single unit heater as the sole load on any boiler, particularly if the unit heater motor is started and stopped by thermostatic control. The wide and sudden fluctuations of load that occur under such conditions would require closer attendance to the boiler than is usually possible in a small installation. Where oil or gas is used to fire the boiler, it is possible by means of a pressurestat to control the boiler, in response to this rapid fluctuation. In most cases, however, and particularly where the boiler is coal-fired, it is advisable to use two or more smaller heating units instead of one large unit. Steam pressures below 5 lb can be used with safety for recirculating unit heaters when their coils are designed for the purpose and when proper provision is made for returning the condensate. If ventilators are to take in air that may be at a temperature below freezing, however, a steam pressure of not less than 5 lb should be maintained on the convector or a corresponding differential in pressure between the supply and returnsbe maintained by means of a vacuum. QUIETNESS In selecting unit heaters, attention should be given to the degree of quietness required for the installation. No given fan speed may be applied as a measure of relative quietness to fans of different designs and proportions. Quietness is a function of' type, diameter, blade form and other variables besides speed, and all these must be taken into account. In general small fans may be run at higher motor speeds than large fans with equal quietness; and centrifugal fans are more easily made quiet than disc or propeller fans. \ PIPING CONNECTIONS Piping connections for unit heaters are similar to those for other types of fan-blast heaters.. Typical connections are shown in Figs. 4 and 5. One-pipe gravity and vapor systems are not recommended for unit heater work. For two-pipe closed gravity return systems the return from each unit should be fitted with a heavy-duty or blast trap, and an automatic air valve should be connected into the return header of each unit. Pressuredrop must be compensated for by elevation of the heater above the water line of the boiler or of the receiver. In pump and receiver systems the air may be eliminated by individual 226 Chapter 12- Unit Heaters, Ventilators, Coolers, and Air Conditioners . valves on the heaters, or it may be carried into the returns the same as f *r vacuum systems and the entire return system be free-vented to the * tmosphere, provided all units, drip points, and radiation are properly trapped to prevent steam entering the returns. On vacuum or open vented systems the return from each unit should be fitted with a large capacity trap to discharge the water of condensation and with a thermostatic air valve for eliminating the air, or with a heavyduty trap for handling both the condensation and the air, provided the air finally can be eliminated at some other point in the return system. For high pressure systems the same kind of traps may be used as with vacuum systems, except that they must be constructed for the pressure used. If the air is to be eliminated at the return header of the unit, a Fig. 4. Unit Heater Connections Where Condensation Is Returned to Vacuum Pump or to an Open Vented Receiver Fig. 6. Unit Heater Connections Where Condensation Is Returned to Boiler Through Wet Return high pressure air valve can be used; otherwise-the air may be passed with the condensate through the high-pressure return trap, with some danger of return pipe corrosion and the problem of its elimination at some other point in the system. The connections for steam and return piping to unit heaters must always be calculated on the basis of the high heat emission or condensation rate of such devices. The pipe-size tables given in Chapter 32 may be used for unit heater work by multiplying EDR values by 240 to get Btu values. All Electric OTHER TYPES OF UNITS The foregoing discussion relates generally to units in which steam or hot water is used as the heating medium. On rare occasions electrical resistances are used as the heating element. These are applied only where electric power is abundant and cheap and where other forms of fuel are scarce and expensive. (See Chapter 39.) Direct Fired A recent development in gas burning equipment is the direct-fired industrial unit heater. These heaters are of the warm-air type and are 227