Document O1Y1OyYjE4BekJ05OpNqyMLJj

Heating Ventilating Air Conditioning Guide 1939 Unit Heaters. Ventilators. Air Conditioning. Cooling Units N o te .-- 'T o d e te rm in e c a p a c ity a t a n y e te a ro p re ssu re a n d e n te rin g te m p e ra tu re ,' m u lt ip ly c o n s ta n t fro m ta b le b y ra te d c a p a c ity a t 60 F e n te rin g a n d 2 lb preaaure. ses The heat capacity for any condition of steam pressure and *ncre^n,, jir temperature may be calculated approximately from any given en.t.er' tfre use of factors in Table 1. This table is accurate within 5 P^Jrdt heaters are customarily rated as free delivery type units. If - t ide air intakes, filters, or ducts on the discharge side are used with the ?u t proper consideration should be given to the reduction in air and heat capacity that will result because of this added resistance. The percentage of this reduction in capacity will depend upon the . acteristics 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 widi this problem the ratings under the conditions expected should be secured from the manufacturer. 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, f 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. 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 ventilators 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. 436 437