Document RjRvDnqoxL4QBE15yGEmqKX6V

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 other experimental-data which have ;been. presented,the usual endeavor is to secure not less than 90 per cent efficiency for underground piping. Table 19 can be used as a guide in arriving at the minimum thickness-of loose insulation fills to use for laying out conduit systems. Other factors such as the nurriber.of pipes and their combination of sizes, as well as the standard conduit sizes, are primary controlling factors in the amount and thickness of insulation for use. , ..When sectional insulation is applied to lines in tunnels or:.conduits, usual,practice;is to.apply the most efficient materials in. less in thick ness than that determined by the use of Fig; 6. The data in Fig. 6 are based on conditions of insulation exposed to the air,, whereas normal ground temperature is substituted for air temperature in determining the tem perature difference for use with the chart when applying it for under ground pipe line estimates. ... -v::; : ' ~ ' - . r-' V.; .--v - -: "4.^-. J. 704" Chapter 41 ELECTRICAL HEATING Resistors, Heating Elements, Electric Heaters, Unit Heaters, Central Fan Heating, Electric Steam Heating, Electric Hot Water Heating, Heating, Domestic Water Supply, Industrial Heating, Reversed Cycle Refrigeration, Auxiliary Electric Heating Control, Calculating Capacities, Power Problems, Insulation ELECTRIC heating is steadily assuming a more important place in heating, ventilating and air conditioning installations, accelerated in many territories by the load building efforts of the utilities which usually include reduced rates to encourage such installations. Electrical heating has a logical place in the heating industry because of its features of flexibility, cleanliness, safety, convenience and ease of control. Electrical heating practice has many basic principles in common with fuel heating, but there are also important differences. When heat units are delivered' to each room by wire, no combustion process is necessary, either at a central plant or at the individual room units. The maximum output of an electric heater is a fixed constant, unaffected by the temperature of the' surrounding air and it follows that the maximum total load on an electricalheating system is the total wattage of connected electric heaters, regard less of weather conditions. The real obstacle to the more general adoption of electric heating for buildings is the cost of the electricity itself. Because the heat units produced electrically are more costly, their conservation is of more: relative economic importance than with fuel heating, so that sponsors of electric heating give greater attention to temperature-insu-: lated building construction, and to economy by accurate controls. : - 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, because 100 per cent of the energy applied in the resistor is always transformed' into heat. In electric heating practice the engineer need not be concerned; about efficiencies of heat production, but rather about efficiencies-of heatutilization. It is the engineer's problem to distribute the-electrically produced heat units in such manner as to obtain conditions of maximum: comfort with the minimum consumption of electricity. . ; , 705