Document 5bez4Q0jY4L1rkNVq2Q9o7KL0

American Society of Heating and Ventilating Engineers Guide, 1932 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 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 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 3}/ 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 rotationaLcirculation of the entire room content is set up by the system rather thantolTave-theJieaters discharge in random and in counter-directions. DIRECTION AND LOCATION 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 504 Chapter 35--Industrial Unit Heaters 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. BOILER CAPACITY FOR UNIT HEATERS 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 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 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 design 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 with equal quietness than large fans. UNIT HEATER CONNECTIONS Piping connections for unit heaters are similar to those for other types of fan-blast heaters. Typical connections are shown in Figs. 1, 2 and 3. 505