Document jeXyopwG4V8edjVOV1LOb2NQ

748. ---------------- :Chapter.44---------------------------------------------------:--1945 Guide resistors is nickel-chromium wire or ribbon which, in order to avoid oxidation, contains practically no iron. Commercial electric heating elements are made in many types. Some have resistors exposed to the air being heated: The resistofs may be coils of wire or metal ribbon, supported by refractory insulation, or they may be non-metallic rods, mounted on insulators. This type of element is used extensively for operation at high temperatures when radiant heat is desired, also at low temperatures for convection and fan circulation heating, especially in large installations. Some elements have metallic resistors embedded in a refractory insu lating material, encased in a protective sheath of metal. Fin's or extended surfaces may be used to add heat-dissipating area. Elements are macle in many forms, such as strips, rings, plates and tubes. Strip elements are used for clamping to surfaces requiring heat by conduction, and in some types of convection air heaters. Ring and plate elements are used in electric ranges, waffle irons, and in many small air heaters. Tubular elements may be immersed in liquids, cast into metal, and, when formed into coils, used in electric ranges and air heaters. Cloth fabrics woven from flexible resistor wires and asbestos thread are used, for many low temperature purposes such as heating pads and aviators' clothing. Special incandescent lamps are used as heating elements in certain applications where radiant heat is desired. These use carbon or tungsten 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, foot warmers, 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 45. 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 draw cold air from the floor line. The flexibility possible with electric heating elements should discourage the use of secondary mediums for heat transfer. Water and steam add nothing to the efficiency of an electric heater and entail expensive construction and maintenance. Electric Heating . 7b 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. Heaters of this type are manufactured in many designs and sizes, and can be located in the same manner as steam unit heaters. Electric unit heaters are used in industrial plants, sub-stations, power houses, pumping stations, etc., where the power rate for electric heating is found to be favorable. In many large plants, such as flour mills, grain elevators, etc., in which there are a number of small offices, locker rooms, etc., scattered over wide areas, electric unit heaters are frequently economical in such locations. In small unattended stations, where freezing temperatures cannot be permitted, thermostatically-controlled electric unit heaters are frequently used to maintain a temperature above freezing. The best location for the heaters depends upon local circum stances as they can be mounted on the ceiling to direct the air downward, Power supply ,, ., Thermostat l ^Control .___ ____-- Panel N * -- \Vs / -- J1" Heater __ i Hand switch Kl ____ ! Fig. 1. Wiring Diagram for Unit Heater on the side wall about 7 ft from the floor, or near the floor line. Variations in design are necessary for different locations, as with steam unit heaters, see Chapter 21. 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. Portable unit heaters are useful for temporary work, such as drying out damp rooms, or for warming rooms during construction. CENTRAL FAN HEATING 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