Document vBGa6YdZ7MD8eE8jzQ63aGBb9

HEATING VENTILATING. AIR CONDITIONING: GUIDE 1940 ;In coordinating the input of heat energy and the volume of air circu-r l.ation, a basic difference between electric, heating and' steam - Heating enters into the problem. Steam is approximately a constant-temperature source of heat-for any given pressure and a'change in, air volume flowing over steam coils does not greatly affect the temperatures of the delivered air. The amount of steam condensed (heat input) varies in proportion to the air volume, but the surface temperature of the steam coils remains about the same. Electric heat is quite different,' having, a constant input Fig. 1. ' Ceiling Mounted Unit heater Fig. .2. Wall Mounted Unit Heater^ Thermal / `Power supply Fig. 3. Floor Mounted Unit Heater -. - Fie. 4. Wiring Diagram for Unit Heater of energy. If the volume of. air flow over electric heating elements is changed, and no change is made in the electrical power connections, there will be a corresponding-change in the temperature' of the air delivered. This occurs because the electrical energy input remains constant and the: surface temperature of the heating elements will, vary as is necessary,to force the air to accept all the heat. With, ejectric heat the total' heat is constant unless some compensating action is performed by control. Auto matic variation of the electrical heat input synchronized, properly with the air flow can be successfully accomplished by various special methods of control. Electric heaters are useful in balancing the heat distribution in central 708 CHAPTER 41. ELECTRICAL HEATINC fan heating systems. Even in those instances where steam is the principal heat.source, the temperature of individual rooms pan bp controlled locally by'separate1 electric booster heaters. These heaters can be installed: in branch ducts or behind the air outlet grilles in each room. With this arrangement, the central heating unit distributes air at an average temper-1 ature, controlled from athermostat centrally located, such as in the maiii return duct. The electric booster heaters may be'controlled by thermo stats mounted in each individual room which- permits the occupant to maintain any desired temperature independent of the rest of the building. ELECTRIC STEAM HEATING . Electric steam healing differs from fuel.heating only in the use of electric boilers to generate steam. Electric steam boilers are entirely automatic and are well adapted to intermittent operation. Small electric boilers usually have heating elements of the enclosed metal resistor type im mersed in the water. Boilers of this construction may be used on either direct or alternating current since the heat is delivered to the water by Fig. 5. Resistance Type Boiler for Steam or Hot Water contact with the hot surfaces. -To lessen the likelihood that the heating elements will burn out, they should.beof substantial construction, with a low heat density per unit of surface area. Provision should' be made for cleaning off deposits of scale which restrict the heat flow. A typical resistance type of hot water or steam boiler is shown in Fig. 5. Large electric boilers are usually Of the type employing water as the resistor. Only alternating current can be used, as direct-current would cause electrolytic deterioration. Large boilers of this kind have electrodes immersed in the water where heat is generated-directly. Electric steam boilers are useful in industrial plants which require limited amounts, of steam for local processes, and for sterilizers, jacketed vessels and pressing machines which need a ready supply of steam. It sometimes is economical to shut down the main plant fuel: burning boilers.when the heating season ends, and to supply steam for summer needs with small electric steam boilers located close to the operation. In general, electric steam heating is confined to auxiliary or other limited applications. If the floating1 system is designed to use electricity exclusively, steam generating or distributing equipment is superfluous. A diagrammatic'arrangement of, an electrode boiler is shown in Fig. 6. . 709