Document MJxj9K8X9dvqJOZzaqyLwemry

HEATINC VENTILATING AIR CONDITIONING CUIDE. 1944 418 o a i ta p -- pnB BjnssBjd u n ,, Xbb ,, x ,p -- BujuijB^sp 0 1 . : m, -- -- Hdmnn --i w U.0JJ -- PJ>}Bj *q J 09 < H P ^ d ,, ' PUB CHAPTER 22. UNIT HEATERS, UNIT VENTILATORS. UNIT HUMIDIFIERS being located in an elevated position above the floor will withdraw air from this higher level and discharge the heated air down into the working zone. Unit heaters are customarily rated as free delivery type units. . If out side air intakes, air filters, or ducts on the discharge side are used with the unit a reduction in air and heating capacity 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 so that no specific percentage reduction can be assigned for all heaters at a given added resistance. In general, however, disc or propeller fan type units will experience a larger reduction in capacity than housed centrifugal fan units for a given added resistance and a given heater will have a larger reduction in capacity as the fan speed is lowered. The ratings to be expected under such conditions should be secured from the manufacturer. Boiler Capacity The capacity of the boiler should be based on the rated capacity of the. unit heaters at the lowest entering air temperature and highest fan speed that will occur, plus an allowance for pipe line losses. 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 con ditions would require closer attention 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, and particularly where the boiler is coal-fired, it is advisable to use two or more smaller unit, heaters instead of one large unit. Steam pressures below 5 lb can be used with safety for recirculating unit heaters when their heating surfaces are designed for those pressures, and when proper provision is made for returning the condensate. If units admit air that may be at a temperature below freezing, a steam pressure of not less than 5 lb should be maintained on the heating element,.or a corresponding differential .in pressure between the supply and returns should be maintained by means of a vacuum. Piping Connections Piping connections for unit heaters are similar to those for other types of fan blast heaters. The piping of unit heaters must strictly conform to the system requirements while at the same time permitting the heaters themselves to function as intended. The basic piping principles for steam systems are discussed in Chapter 15. Rapid condensation of steam, especially during heating-up periods, is characteristic of this type of equipment. The return piping must be planned to keep the heating surfaces free of rapid condensation, while the steam piping must be ample to carry a full supply of steam to the surfaces to take the place of that condensed. An adequate size of pipe is therefore essential for all heating surfaces over which there is a flow of forced air. . Especially is' this true where the fan is operated under start-and-stop 419