Document LpdD0xrN5eZn1mekRdoeXOeD5

HEATING VENTILATINC AIR CONDITIONING GUIDE 1944 1. Controlling the rate of steam flowing into the radiators. This is accomplished by equipping the radiator inlets with orifices and con trolling the flow of steam through them into the radiator by controlling the difference in absolute pressure between the supply and return. 2. Controlling the temperature of steam in the radiators by varying its pressure. This involves the use of high vacuums to obtain low steam temperatures. This must be supplemented by some other type of control; for low heat output. _ 3. Controlling the length of time steam flows into the radiators by admit ting steam to a heating system intermittently and varying the length of the on and off periods. There are two types of controls for doing this. (1) A clock control providing on and off settings of various lengths, which can be changed in accordance with outside temperatures. In most cases these changes are made automatically by means of a thermostatic bulb, placed outdoors. (2) A control having an outdoor bulb, and a bulb at^tached to the-radiator, which varies the length and frequency of the on intervals in such a way that the radiator temperature is varied according to the outside temperature. In some cases heat supply is controlled by combinations of the above methods. Before the installation of any type of modern temperature control equipment, it is necessary to see that the heating system is put in good operating condition. In general, the heating system in a building is not given the attention that other mechanical equipment is given because it will continue to function, after a fashion, even though changes in piping,, location of radiation, settlement of piping, and the normal wear and tear or other changes have taken place. Through all this depreciation of the system,' it becomes more and more costly to operate and parts of the building have to be greatly overheated in order to prevent underheating in a' small section of the building. Vents, traps, vacuum pumps, and valves should be given a careful inspection and replaced or repaired if required. The piping should be of adequate size and graded properly. The return piping, should have a careful inspection, and any pockets or lifts removed and properly vented. These inspections and repairs are not costly and prevent a much greater outlay in future years. In most cities district heating companies will be willing to make a survey of heating systems and offer recommendations as to operation and changes in piping layout. The selection of control equipment depends upon the type and size of building and the degree of saving possible. 774 CHAPTER 44 Electric ^JJ^eu tin f Resistors, Heating Elements, Electric Heaters, Unit Heaters, Central Fan Heating, Electric Boilers, Electric Hot Water Heating, Heating Domestic Water Supply, Radiant Drying, Reversed Cycle Refrigeration, Auxiliary Electric Heating, Control, Calculating Capacities, Power Problems ELECTRIC heating is steadily assuming a more important place in heating, ventilating and air conditioning installations because it is flexible, clean, safe, convenient and easy to control. It has many basic principles in common with fuel heating, but there are also important. differences. When heat is delivered by wire, no combustion process is necessary, either at a central plant or at the individual room units. The output of an electric heater is a fixed constant, unaffected by the tem perature of the surrounding air and it follows that the total load on an electric heating system is the total wattage of connected electric heaters, regardless of weather conditions. The main obstacle to the more general adoption of electric heating for buildings is the cost of the electricity itself. All heat is a form of energy. Fuels hold stored chemical energy which is released into heat by combustion. Electrical, power is a form of energy which can be released into heat by passing it through a resisting material. Both fuel and electric heating have two divisions: first, the conversion of energy into heat; second, the distribution and practical use of the heat after it is produced. In converting the chemical energy of fuels into heat by combustion, there is necessarily a considerable variation in thermal efficiency. This is not true, however, when converting electric power into heat, as 100 per cent of the energy applied to the resistor is always transformed into heat. In electric heating practice no concern need be given to efficiencies of heat production, but rather to efficiencies of heat utilization. The problem is to distribute the electrically produced heat units in such manner as to obtain conditions of maximum comfort with the minimum, consumption of electricity. DEFINITIONS Definitions of general terms used in fuel heating are given in Chapter. 47. Terms which apply particularly to electric heating are shown on the following page. 775