Document 4axLMZ4V39R9vV43oMpy9a1qQ

790 CHAPTER 43 1946 Guide this is not a loss and usually can be compensated for by adjustment of air quantities between the various spaces. Heating Load Methods of calculating the heating load are shown in Chapter^ 14. Many of the factors outlined previously under Cooling Load, such as zoning, non-simultaneous peaks, and diversity, apply in the reverse manner due to the heating requirement instead of the cooling requirement. However, these factors enter into the heating load picture from the stand point of control of inside conditions, over-all performance and economy of operation more than from a capacity of equipment standpoint. It is not only necessary to heat a building or space to its design conditions when there is but the merest fraction of normal occupancy, practically no lights, internal heat, or solar radiation, but it is also necessary to provide capacity to heat the building quickly when sudden cold follows relatively warm weather, or after a week-end or holiday shut-down. Central Systems for Comfort Air Conditioning 791 to summer cooling in mind. Thus, if the air conditioning apparatus can be set to take inside air, process it, and return it to the conditioned space completely saturated at the apparatus dew-point, the cooling load require ments can be exactly met both as to removal of energy and simultaneous removal of moisture. In actual practice with commercial apparatus, it is rarely possible to obtain complete saturation and there may be several degrees difference, between- the dry-bulb and wet-bulb temperatures of the,air returned to the conditioned space. This causes no difficulty. In fact, the only special significance of the apparatus dew-point is that, when it exists, it provides Fig. 12. Diagram of Mollier Chart Illustrating Example 1 However, in normal operation during week-ends and holidays, buildings are usually kept at a holding temperature to prevent the freezing of services and conserve fuel. In many cases it requires less fuel to keep a building or space at a temperature of 50 to 65 F for some time than to shut the system down and then bring the temperature up again. Apparatus Dew-Point The method of locating the condition line for a given air conditioning problem has been explained in Chapter 3, Examples 12 and 13. Briefly, the method consists in estimating the net energy gain (or loss) per hour ;and the net moisture gain (or loss) per hour from data on location, ex posure, construction, appliances, occupants, ventilation requirements,, inside and outside design conditions. In computing the quantities of energy and .moisture introduced and displaced by the ventilating air, only that portion of the ventilating air admitted directly to the con ditioned space is considered. With this understanding, the ratio of the net energy gain (or loss) to the net moisture gain (or loss) determines the. slope of (he condition line through the state point of the inside air on the. Mollier Chart. , The condition line may or may not cross the saturation curve. If it does, the intersection is called the apparatus dew-point, with application .AIR TEMP.(SATURATED BELOW ROOM DW-POIN7)-0F Fig. 13. Heat and Moisture Absorbing Power of Air Introduced Into a Room Held at 80 F and 50 Per Cent Relative Humidity (60.1 F Room Dew-point) and Various Sensible Heat Factors, Latent Heat Factors, and Multiplier Factors Corresponding to the Supply Air Conditions a convenient control point at which to regulate the operation of the apparatus, provided complete saturation is attainable. ' In order to illustrate the effect of incomplete saturation in the conditioning apparatus, consider the cooling load problem of Chapter 3, Example 12. In this problem, 114,510 Btu of energy and 16.018 lb of moisture per hour are to be removed simultaneously! * The slope of the condition line is determined by.the ratio q = 114,510 -f- 16.018 = 7149 Btu per pound of water and the apparatus dew-point is 58.08 F, as shown in Fig. 12. (Point B), But suppose that the air delivered, by the apparatus is only 95 per cent saturated. Then the temperature at which the condition line crosses the 95 per cent saturation curve is the proper temperature at which to regulate the apparatus. This temperature is easily found to be 59.65 F dry-biilb temperature (Point A in Fig. 12). Of course, a some what larger, quantity of air will-have'to be recirculated, namely 114^510 -s- (31.514 -- 25.631) = 19,460 lb of dry air per hour, where the enthalpy of the air at 95 per cent saturation on the condition line is 25.631 Btu per pound of dry air and h = 31.514 for the