Document DDaNKz9q708d1qwNNJr8dgmX4

HEATING VENTILATING AIR CONDITIONING GUIDE 1941 downward, on the side wall about 7 ft from the floor, or near the floor line. Variations in design are necessary for different locations, but typical arrangements are indicated in Figs. 1 and 2. The arrangement of the wiring circuits is very important for electric unit heaters. In principle they are all the same and include as essential elements an automatic control panel, a thermostat, and a master hand switch. All heaters should be designed with a safety thermal trip wired in series with the magnet coil of the control panel and with the hand switch and thermostat. A typical wiring diagram is shown in Fig. 3. This applies to a single phase power supply, but for 3 phase the only difference is to have a 3-pole panel and a heater arrangement for 3-phase connection. Portable unit heaters are useful for temporary work, such as drying out damp rooms, or for warming rooms during construction. CENTRAL FAN HEATINC Electric heating elements can be used for the prime source of heat in a central fan electric heating system or in the heating phase of a complete Power supply CHAPTER 43. ELECTRIC HEATING 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 electric 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 fan heating systems: Even in those instances where steam is the principal heat source, the temperature of individual rooms can be controlled locally by separate 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 ature, controlled from a thermostat centrally located, such as in the main; return duct. The electric booster heaters may be controlled by thermo- Fig. 3.. Wiring.Diagram for Unit Heater air conditioning system. They can be used in the same manner as steam heating units for tempering, preheating or., reheating ^the air at the main supply fan location and as booster heaters at the delivery terminals of the duct system. In the humidification; phase, of air conditioning electric heating elements can be used to provide moisture by the evaporation of water, or for controlling air. washer- dew-point temperatures when mounted as preheating units on'the intake'side of the air washer. (See Chapter 20.) ' In coordinating the input of heat energy and the volume of air circu lation, 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 achange 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-surfaee temperature of the steam'coils remains about the same. Electric heat is quite different,/having a constant input 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'. 752 Fig. 4. Resistance Type Boiler for Steam or Hot Water stats mounted in each individual room which permit the occupant to maintain any desired temperature independent of the rest of the building. ELECTRIC BOILERS Steam or hot water generating boilers using electric energy are entirely automatic and are well adapted to intermittent operation. Small electric boilers usually have heating elements of the enclosed metal resistor type immersed in the water. Boilers of this construction may be used eitherwith direct or alternating current since the heat is delivered to the water by contact with the hot surfaces. To lessen the likelihood of the heating elements burning out, they should be of substantial construction, with a low heat density pier unit of surface area and provision should be made for cleaning off desposits of scale which restrict the heat' flow. A typical resistance typie of steam or hot water boiler is shown in Fig. 4. ; Large electric boilers are usually of the type employing water as the resistor, using immersed electrodes. With this -type only alternating current can be used, as direct current would cause electrolytic deteriora tion. Such a type of electrode boiler is shown in Fig. 5. 753