Document ZnwyJDxwV4yo30rYQngQVQnXd

of and 1936American Society Heating Ventilating Engineers Guide, 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. Units 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 convector. AIR TEMPERATURES1 For recirculating heaters with intakes at the floor level, the temperature to be maintained in the room should be considered as the temperature of the air entering the heater. Where outside air is introduced, the tem perature of the mixture must be calculated and used as the entering air temperature to 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 unit heaters taking air at some distance above the floor, the temperature variation from floor to ceiling may reach as much as 1 deg for each foot of elevation during the periods when the maximum capacity of the heaters is required. Thus this allowance should be made in calculating the capacity of suspended heaters. Unit heaters taking in recirculated air at the floor level should maintain temperature differentials of less than 0.5 deg per foot of elevation when the maximum capacity of the heaters is required. This temperature difference per foot of elevation is less than the corresponding variations for spaces heated by direct radiation. Unit heaters save fuel because of their ability to circulate air at a lower average temperature than the air circulated by direct radiators; however, the unit heaters must circulate more air in any given time than is needed with direct radiators. 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 economic limit is imposed. In general, for heating purposes it is advisable to use a delivery temperature not more than 70 F above the average room temperature desired, and considerably less where possible. Since the delivery temperature increases with increase in steam pressure with high pressure steam, units should be selected that have a minimum heating surface in order to provide outlet air not in excess of 70 F above average room temperature. OUTPUT OF HEATERS It is standard practice to rate unit heaters in Btu pet 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 ^Temperature Gradient Observations in a Large Heated Space, by G. L. Larson. D. W. Nelson, and 0. .C. Cromer (A.S.H.V.E. Transactions, Vol. 39, 1933). Tests of Three Heating Systems in an Industrial Type of Building, by G. L. Larson, D. W. Nelson, and John James (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, November. 1934). 222 'Q^~^zfTl2__ Unit Heaters, Ventilators, Coolers, and Air Conditioners used as the standard basis of rating2. The capacity of a heater ?r eases as the steam pressure increases, and decreases as the entering '*> temperature increases. The heat capacity for any condition of steam ^toesnre and entering air temperature may be calculated approximately ? any given rating by the use of factors in Tables 1 and 2. Table 1 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 circulation 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 ^The^ercentage 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 V/2 per cent for any degree of superheat. DIRECTION OF DISCHARGE 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, especially when there are considerable 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 set up by the system rather than to have the heaters discharge at random and in counter directions. ._ Various types and makes of unit heaters are illustrated in the Catalog Section of this edition. Usually hot blasts of air in working zones are objectionable, so heaters mounted on the floor should have their discharge 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. *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). 223