Document qmRy4jjvdr04rn7JEgvq6YjzM

556 CHAPTER 30 1948 Guide filaments as resistors, and are designed: to-produce maximum energy in. the infra-red portion of'the'spectrum. ELECTRIC HEATERS ' Electric heaters may be divided into four groups ^conduction, radiant, convection, and induction. Conduction electric heaters, which deliver most of their heat by actual contact with the object; to .be heated, are used in such applications .as aviators' clothing, hot pads,' soil heaters, ice melters, and water heaters. Conduction heaters are useful, in conserving and localizing Heat, delivery at definite points. They are not suitable for general air heating.- Radiant electric heaters, which deliver most of their heat by radiation, have high temperature heating elements and reflectors to concentrate the heat rays in the desired directions. The immediate and pleasant sensation. of .warmth which is caused by radiant , heat makes-this type desirable for temporary use where the heat rays can fall directly upon the body. They,are not satisfactory for general air heating, as radiant heat' rays do not warm the air through which they pass. They must first be absorbed by walls, furniture, or other solid objects which then give up the heat to the "air. For a. discussion of electrically heated panels as applied to radiant heating, see Chapter 31. , Gravity convection electric heaters, designed to induce thermal air circu lation, deliver heat largely by convection, and should be located and used in much the same manner,as steam and hot water radiators or convectors. They generally have heating elements of large area, with moderate surface temperature,'enclosed to give proper-stack effect to draw3 Cold air from the floor line;- The "flexibility possible with'electric heating elements should discourage the use of secondary mediums for heat transfer. Waterand' steam-add nothing to the efficiency of an electric heater and"entail expensive construction and maintenance. Induction Heaters are described in the section on Electronic Heating by Induction and Electronic Means. UNIT HEATERS Electric unit heaters include a built-in fan unit which circulates room air over the heating elements. They are adapted to the same uses as other types of unit heaters where' conditions are favorable to electric heating. They are very adaptable for heating "of small offices, locker rooms, etc.,'in otherwise unheated buildings. In small unattended equip ment rooms, thermostatically controlled electric unit heaters are fre quently used to maintain a temperature above freezing. The best location for electric unit heaters depends upon" local conditions. Various designs and arrangements are available, as with steam unit heaters. See Chapter 26. 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. 1. 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. Electric Heating 557 CENTRAL FANHEATING ; 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 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 43.) ; . 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 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 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 matic 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.