Document VgmJx3NOJ7xepwr9k9K29mZ4

Heating Ventilating Aik Conditioning Guide 1939 switch. All heaters shguld 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. 6. 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. Chapter 40. Electrical Heating 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 Fig. 3. Ceiling Mounted Unit heater Fig. 4. Wall Mounted Unit Heater Thermal / Power supply Fig. 5. Floor Mounted Unit Heater Fig. 6.- Wiring Diagram for Unit Heater 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 served 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 evaporization 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 21). 760 Fig. 7. Wiring Diagram of a Selecting Control for a Fan System constant unless some compensating action is performed by control. Auto matic variation of the electrical heat input synchronized properly with the air flow has been successfully applied to central fan systems. A typical wiring diagram of an automatic modulating system for central fan heating is indicated in Fig. 7. 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 permits the occupant to maintain any desired temperature independent of the rest of the building. ELECTRIC STEAM HEATING Electric steam heating 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 761