Document DDp16B0md7nXBbbZo0mm8ONqO

HEATINC VENTILATING AIR CONDITIONING GUIDE 1943 of limitations, however, the most serious of which is the lack of ready availability of a. practical source of heat. 1. Well water is the most desirable since its temperature is higher than other sources even in the winter, and thus a large amount of heat may be removed in relation to the weight of water handled. 2. Air may be used but its specific heat is low and its temperature uncertain. When the most heat is needed, the temperature of the air is lowest, thus resulting in the least favorable temperature combination. 3. It has been proposed to obtain heat by freezing water but this is still in the experi mental stage. - Some of the other factors which act as limitations are: the large tem perature spread when using air as a source of heat and when attempting to cool' with even moderately low outside temperatures, the frequent disparity between the size of the cooling load and heating load requiring extra equipment for a complete. heating load, and the relatively high initial cost of equipment as compared to that at present available for heating by conventional means. Because of these limitations, the present application of the system is largely limited to temperate climates, such as Florida and Southern California, or to heating only for intermediate seasons, or to other locali ties which have peculiar advantages as, for instance, the ready availability of well water. In these locations it is frequently possible to do all of the heating necessary with the refrigerationrequipment so that tiie extra cost is only that of reversing the functions of the condenser and evaporator. There are a nuiiiber of reversed systems now in operation, particularly among utility companies, using well water as the source of heat. These systems range in size up to 320 hp. In the case of the largest system in operation at present, the cost of the electrical energy would have to be approximately 0.7 cents per kilowatthour in order to compete vyith oil at 6 cents per gallon. A typical arrangement of a reversed cycle conditioning system where air is used as a source of heat is shown in Fig. 11. If the air seldom drops below freezing, heat is often required in the morning and cooling during the afternoon in order to maintain comfortable conditions in such a system. The arrangement as shown lends itself to automatically changing over as-required. \ j 502 Chapter 26 HEAT TRANSFER SURFACE COILS Coil Applications, Construction and Arrangement,. Steam Coils, Water Coils, Direct-Expansion Coils, Flow Arrange ments, Applications, Calculation of Heat Transfer, Air Flow Resistance, Coil Performance, Selection THE coils described in this chapter are used in air conditioning sys tems for heating or cooling an air stream under forced convection. The surface coil equipment may be made up of a number of banks assembled in the field, or the entire assembly may be factory constructed. The applications of each type of coil are limited to the field within which it is rated. Other limitations are imposed by code regulations, by proper choice of materials for the refrigerants used and the condition of the air handled, or by an economic analysis of the possible alternates on each installation. For heating service, these coils are used as preheaters, reheaters or booster heaters, (see Chapters 21 and 22). The function of the coils is air heating only, but the apparatus assembly may include means for humidi fication and air cleaning. Steam or hot'water are the usual heating media, although others are used in special cases, such as reheating by means of discharge gas from a refrigerating system. Coils are used for air cooling with or without accompanying dehumidi fication. Examples of codling applications without.dehumidification are precooling coils using well water or other relatively high temperature water to reduce the load on the refrigerating machinery,, or vvater cooled coils to remove sensible^ heat in connection with chemical moistureabsorption apparatus. By proper coil selection it is possible to handle both sensible cooling and dehumidification together as further explained later. The apparatus assembly usually includes an air cleaning means to protect the coil from accumulation of dirt and to, keep dust and foreign matter out of the conditioned space. Although cooling and dehumidi fication are thie usual functions, there are cases of cooling coils purposely wetted as an aid to air cleaning and Odor absorption. The usual cooling media used in surface coils are cold water and volatile refrigerants such as dichlorodifluoromethane and methyl chloride, but others are used in special cases. Brines are seldom required for the range of applications covered by this chapter, although there are cases where -low, entering air temperatures with large latent heat loads require a refrigerant temperature so low that water becomes impractical. Some- 503