Document yb9oaoDnJNO5XLVNGe8ojr653

American Society of Heating .and Ventilating Engineers Guide, 1931 6. Increase the capacity of the heating surface by passing the air over it at velocity. 7. Provide a system by which room temperatures are readily controlled manually by thermostats. ' * In addition to their prime function of heating buildings, unit heaters may be adapted to a number of industrial processes, such as dryjn? curing, etc., with which the use of heated air in rapid circulation uniform distribution is of particular advantage. They may be used moisture absorption, such as fog removal in dye-houses or for the pre): vention of condensation on ceilings or other cold surfaces of buildings^' which process moisture is given off. When such conditions are severe, it is necessary that the heaters draw air from outside in enough volume' tol provide a rapid air change and that they operate in conjunction with'? ventilators or fans for exhausting the moisture-laden air. (See discussion? of condensation in Chapter 33.) Types of Unit Heaters There are many types of unit heaters available. Most of them employ* heating coils to be supplied with steam or hot water. Some are designed primarily for mounting on the floor, whereas others are designed for sus-: pension overhead. Heating surfaces in the form of pipe coils, non-ferrous' tubes or shapes with extended surfaces, cast-iron, and pressed and built-up \ sections of the cartridge or automotive type are all used in unit heater [ construction. Among the unit heaters available are types having from one to four outlets per heater which may be arranged to discharge in selected direc tions and which will project their heating effect over distances of from 30 to 200 ft. from the heater, depending upon the capacity of the heater and upon the design of the fans and outlets. These heaters have been successful when placed as far as 400 ft. from each other. This makes it possible to select the heater location bestsuited to the production layout in factories. There are available propeller fan type heaters of smaller capacity with outlet velocities of from 300 to 600 ft. per minute, and these may be placed from 60 to 100 ft. apart. Heating Mediums Unit heaters are made to operate on hot water, or with steam at high or low pressure. When high pressure steam is used, the heater must of course be especially designed (or that purpose. The present day tendency is to use steam at low pressure when the boiler supplies steam for heating purposes only,' and when the trans mission lines are short. Many industrial plants, however, generate steam at high pressure either for long distance transmission or for process.uses. When such high pressure steam is available in sufficient quantity for the peak load, it is economical usually to use high pressure unit'heaters. Thus the line pressure may be turned into the units directly without reducing valves and the condensation may be trapped to overhead returns when desired. Smaller coils may be used in high pressure heaters to lessen their cost and to produce a final temperature which will not be too high. Steam turbines may be used for fan power, each exhausting directly into its own heater. 134 Chapter 9--Unit Heating and Ventilating Machines Determining Heat to Be Supplied u h At losses of a building to be equipped with unit heaters are T"e . , jn t[ie same manner as for any other heating system, excepting determin heaters may change the air temperature at the ceiling 50 mean height of the walls. (See Chapter 5). or t heaters may be arranged to recirculate the air or to supply ^n", ajr jrom the outside for ventilation or to make up air exhausted. If 11 or a part of the air is to be taken in from out-of-doors, the heat ** t0 warm this air from the outside temperature to the inside neCe^erature must be added to the transmission or other losses. Unit haters of the number and size needed to furnish the total heat required hea selected from the manufacturers' rating tables, using these ,, at the steam pressure to be used and at the temperature at which Se air will enter the heater. Temperature of Air Entering Heater For recirculating heaters with intakes at the floor level, use the tem- nerature to be maintained in the room as the temperature of the air entering 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 mainstained. With suspended heaters taking in air at some distance above the floor, the temperature variation from floor to ceiling may reach as much as 1)4 deg. fahr. for each foot of elevation during periods when the maximum capacity of the heaters is required. Unit heaters taking in recirculated air at the floor level should maintain temperature differentials of less than I deg. pier foot of elevation when the maximum capacity of the heaters is required. These temperature differences per foot of eleva tion are less than the corresponding variations per foot of elevation for spaces heated by direct radiation. DeAliverarypiAdirreTceirmcupelartaiotunreor turnover of the air in the room makes for fuel economy. This calls for 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. RaItitnigsssatannddCaradpapcriaticestice to rate unit heaters in B.t.u. per hour at a given temperature of air entering the heater and at a given steam pressure maintained on the coil. Steam at 2 lb. pressure and air entering at 60 deg. fahr. are used as the standard basis of rating. 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