Document 8VykQpkexeXj0n6o58k7YoY7o

American Society of Heating and Ventilating Engineers Guide, 1934 set up by the system rather than to have the heaters discharge at random and in counter directions. DIRECTION OF DISCHARGE Various types and makes of unit heaters are illustrated in the Catalog Section of The Guide. Usually hot blasts of air in working zones are objectionable, so heaters mounted on the floor should have their discharge Fig. 1. Unit Heater Connections Where Condensation Is Returned to Vacuum Pump Fig. 2. Unit Heater Connections Where Condensation Is Returned to Boiler Through Wet Return IFig. 3. Unit Heater Connections Where Condensation s Returned to Condensation Pump or Hot Well outlets above the head line and suspended heaters should be placed in such manner and turned in such direction that the heated air stream will not be objectionable in the working zone. In the interest of economy, however, the elevation of the heater outlet and the direction of discharge should be so arranged that the heated air shall be brought as close to the head line as possible, yet not into the working zone. In general, the higher the elevation of the unit, the greater the volume and-velocity required to bring the warm air down to the working zone, and conse quently, the lower the required temperature of the air leaving the.unit. 176 Chapter 13--Unit Heaters and Ventilators 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 heaters 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 belpw 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 heaters 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 heater coils. QUIETNESS OF OPERATION 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. UNIT HEATER CONNECTIONS Piping connections for unit heaters aje similar to those for other types of fan-blast heaters. Typical connections cure shown in Figs. 1, 2 and 3. One-pipe gravity and vapor systems are not recommended for unit heater work. For two-pipe gravity, or pump and receiver systems the return from each unit should be fitted with a heavy-duty or blast trap and an 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. On vacuum 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 heavy-duty trap 177