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American Society of Heating and Ventilating Engineers Guide, 1934 w D05 CO OO O' N C-J CN 8 >< 0fc5d <M K 3g u a< h Cl, .h f>i< Oz ^ wi 5z --Ccs K* hS i te to o jS & Qh cS &O 2M si| C std sQ <e H n c 02H 4iS <o as 1 -s. QM 2H< H 2CO uO Bjj .3uccqr a* H< 0-3 3g2 3 3-2 > 'O 3 89 NC>l Th* oo oo 3 O' 3 3 OO OO O' O' O' 9 174 ATo/e.-- To determine capacity at any steam pressure and entering temperature, m ultiply constant from table by rated capacity a t 00 F entering and 2 lb pressure. Chapter 13--Unit Heaters and Ventilators A rapid recirculation or turnover of the air in the room will give fuel economy. This requires the selection of heaters having a liberal air capacity for the required heat output, which in turn means a relatively low final temperature. Extremely low final temperatures can be had only at the expense of larger heaters and increased power, so that an eco nomic limit is imposed. Contributing conditions vary too widely to permit of a suggested standard, but in general for heating purposes it is advisable to use a delivery temperature not more than 70 deg above the average room temperature desired. OUTPUT OF UNIT HEATERS It is standard practice to rate unit heaters in Btu per hour at a given temperature of air entering the heater and at a given steam pressure maintained in the coil. Steam at 2 lb pressure and air entering at 60 F are used as the standard basis of rating1. The capacity of a heater increases as the steam pressure increases, and decreases as the entering air temperature increases. The heat capacity for any condition of steam pressure and entering air . temperature may be calculated approximately from any given rating by the use of factors in Tables 1 and 2. Table 1 is for blow-through and Table 2 is for draw-through unit heaters. These tables are accurate within 5 per cent. The ratings customarily published for unit heaters apply only for recirculation and free discharge, unless otherwise noted in the rating tables. If outside air intakes, filters or ducts on the discharge side are used with the heater, proper consideration should be given to the reduc tion in air and heat capacity that will result because of this added resistance. The percentage of this reduction in capacity will depend upon the characteristics of the heater and on the type, design, and speed of the fans employed, so that no specific percentage of reduction can be assigned for all heaters for a given added resistance. In general, however, disc or propeller fan units will have a larger reduction in capacity than housed fan units for a given added resistance, and a given heater will have a larger reduction in capacity as the fan speed is lowered. When confronted with this problem the ratings under the conditions expected should be secured from the manufacturer. When steam supplied to the heaters contains superheat, the capacity of the heater will be but slightly less than with saturated steam at the same pressure. Recent tests indicate that the reduction of capacity from this cause is negligible for superheat up to 50 deg and will not exceed 3J4 per cent for any degree of superheat. Heaters may be distributed through the central portions of a room discharging toward exposed surfaces, or may be spaced around the walls, discharging along the walls and inward as well, when there are con siderable roof losses. In general, it is better to direct the discharge from the unit heaters in such fashion that rotational circulation of the entire room content is `See A.S.H.V.E. Standard Code for Testing and Rating Steam Unit Heaters. (A.S.H.V.E. Trans actions. Vol. 36. 1930). 175