Document OENY3be0BvZjox0n7B3Xn8evj

368 Chapter 20 , 1945 Guide In certain types^of building constructionTthe effect of solar radiation is still apparent hours after the sun has shifted from that - exposure. In other types having a much lighter construction, the heat gain due to solar radiation decreases markedly with the passing of the sun. Some walls, having been warmed by the sun, may radiate Heat long after the passing of the sun, thus requiring lower inside temperatures to offset the radiant energy. Storage effect is usually present in some degree. Often it can be utilized to great advantage and it has more than once provided an unknown'safety factor. If a space is kept below the design inside temperature for a period of time, the interior walls, floors, furniture and fixtures begin to assume the'temperature of the space. Where the period of time is sufficient, the entire mass may reach the room temperature, rather than just the skin or surface of the item. Thus, when a space has been precooled below the Fig. 10. Induction Unit (Low Pressure Type) Fig. 11. Induction Unit (High Pressure Type) design maximum temperature for a period of time prior to the advent of the peak load, and the heat gain begins to increase to peak conditions, some of the increase is used in raising the temperature of the furniture, fixtures, etc., to the design conditions and the cooling load can be reduced accordingly. However, unless very accurate data with regard to the mass, surface, specific heat, etc., of the items within the space are avail able, due caution must be used in discounting the cooling load for this storage effect. In the absence of reliable data it is often a matter of experience rather than calculation. Where air conditioning supply and return ducts pass through uncon ditioned spaces there will be a transfer of heat from these spaces to the air in the ducts, even though these ducts are well insulated. An allowance should be made for this heat gain and included in the heat estimate so that air can be supplied at a temperature low enough to offset the rise caused by this heat gain (see Chapter 31). There will also be some heat gain to the air in ducts passing through conditioned spaces, but since a cooling effect is produced in the space through which the duct passes, Central Systems for Comfort Air Conditioning 369 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 6. 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 a 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 after a shut-down such as when a sudden cold snap follows relatively warm weather, or after a week-end or holiday. 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 1, Examples 19 and 20. 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 the 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 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 a convenient control point at which to regulate the operation of the apparatus, provided complete saturation is attainable. illustrate me enect oi incomplete saturation in me conaitioning apparatus, consider the cooling load problem of Chapter 1, Example 19. In this problem, 114,600 tu of energy and 15.92 lb of moisture per hour are to be removed simultaneously. The JP of the condition line is determined by the ratio q = 114,600 -s- 15.92 = 7205 Btu P" pound of water and the apparatus dew-point is 58.02 F, as shown in Fig. 12. (Point B).