Document 6BZbDOBz6R99ve3v9EwRXMLYE

American Society of Heating and Ventilating Engineers Guide, 1934 Note.-- T o determ ine capacity a t any steam pressure and entering tem perature, m u ltip ly constant fro m table by rated capacity a t 00 F entering and 2 lb pressure. Chapter 13--Unit Heaters and Ventilators capacity with outlet velocities of from 300 to 800 fpm, and these may be placed from 30 to 100 ft apart. HEATINC MEDIA Unit heaters are made to operate with hot water, or with steam at high or low pressure. When high pressure steam is used, the heater must be especially designed for that purpose. The present day tendency is to use steam at low pressure when the boiler supplies steam for heating purposes only, and when the trans mission lines are short. Many industrial plants, however, generate steam at high pressure either for long distance transmission or for process uses. When such high pressure steam is available in sufficient quantity for the peak load, it is economical usually to use high pressure unit heaters. Thus the line pressure may be turned into the units directly without reducing valves and the condensation may be trapped to overhead returns when desired. Smaller coils may be used in high pressure heaters to lessen their cost and to produce a final temperature which will not be too high. ESTIMATING HEAT LOSSES The heat losses of a building to be equipped with unit heaters are determined in the same manner as for any other heating system, excepting so far as the unit heaters may change the air temperature at the ceiling or at the mean height of the walls. (See Chapter 7). Unit heaters may be arranged to recirculate the air or to supply warmed air from the outside for ventilation or to make up air exhausted. If all or a part of the air is to be taken in from out-of-doors, the heat necessary to warm this air from the outside temperature to the inside temperature must be added to the transmission or other losses. Unit heaters of the number and size needed to furnish the total heat required are then selected; from the manufacturers' rating tables, using these ratings at the steam pressure to be used and at the temperature at which the air will enter the heater. ENTERING AND DELIVERY TEMPERATURES For recirculating heaters with intakes at the floor level, the temperature to be maintained in the room should be used as the temperature of the air entering the heater. Where suspended heaters are used without any intake boxes extending down to the floor level, a higher entering air temperature should be used than that at which the room is to be main tained. With suspended heaters taking in air at some distance above the floor, the temperature variation from floor to ceiling may reach as much as 1J4 deg for each foot of elevation during periods when the maximum capacity of the heaters is required. Unit heaters taking in recirculated air at the floor level should maintain temperature differentials of less than 1 deg per foot of elevation when the maximum capacity of the heaters is required. These temperature differences per foot of elevation are less than the corresponding variations per foot of elevation for spaces heated by direct radiation. 173