Document LKb48kgXGdwK6BO60YOmgXkpw

JSQ ______ _________ Chapter 44 ' 1945 Guide 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 a change in air volume flowing over steam coils does not greatly affect the temperatues 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 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 electric heat the total heat is constant unless some compensating action is performed by control. Auto- Electric Heating 751 by contact with the hot surfaces. To lessen the likelihood of the heatiiig elements burning out, they should be of substantial construction, with a low heat density per unit of surface area and provision should be made for cleaning off deposits of scale which'restrict the heat flow. A typical resistance type of steam or hot water boiler is shown in Fig. 2. 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. 3. Electric steam boilers are useful in industrial plants which require limited amounts of steam for local processes, and also 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 Fig. 2. Resistance Type Boiler for Steam or Hot Water made variation of the electrical heat input synchronized properly with the air flow can be successfully accomplished by various special methods of control. By-pass dampers as used in steam units will not control electric heat. 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 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 either with direct or alternating current since the heat is delivered to the water burning boilers when the heating season ends, and to supply steam for summer needs with small electric steain boilers located dose to the operation. In general, electric steam heating is confined to auxiliary or other limited applications. If the heating system is designed to use electricity exclusively, steam generating or distributing equipment is superfluous. ELECTRIC HOT WATER HEATING Electric water heating, using an electric boiler in place of a fuel burning boiler, like electric steam heating, is generally confined to auxiliary or other limited applications. The use of insulated water storage tanks, in which to store heat generated by electricity during off-peak hours at extremely low rates, is a development which has some spedal applications. In this system of heating, the primary storage tank is simply a large, well-insulated, pressure type steel tank, equipped with electric heating elements and automatic time switches, which also have automatic limit' controls for temperature and pressure. The heating system installed in the building may be of any standard individual radiator or fan-served: indirect type or with provisions for the heating and humidification phases SjfcS&i&i>