Document zYk7R1reMdz2g66x0xJ6M74n

786 CHAPTER 43 1946 Guide Design Conditions Physiological principles and design conditions are covered in Chapter 12. A detailed study of these'is-beyond the scope of this chapter other than certain recommendations with regard to their application. Where extreme or unusual outdoor conditions prevail for long periods of time, such as in the tropics, at high altitudes, in regions of extremely low humidities, etc., due allowance must be made for the fact that, the people have become acclimatized, to a certain degree at least, to these conditions and the inside conditions should be selected accordingly. A change in the inside conditions from a set standard sometimes is warranted from an economic standpoint/ It is possible at times to make substantial savings in initial and operating costs by maintaining a lower Central Systems Jor Comfort Air Conditioning 787 occur during these times can be used provided the cumulative effect or heat gain lag is taken into consideration in the estimate. Outdoor Air Standards affecting the quantity of outdoor air have been established in Chapter 12. These standards relate the minimum amount of outside air to be introduced into the conditioned space for both the number of occupants and the type of occupancy, i.e., people smoking, etc. This, of course, is the common-sense approach to the problem, but there are some cases, such as those spaces having a very low occupancy with regard to the cubical content where mustiness may develop unless a sufficient air change is provided. Where such a condition exists in a few spaces out of several which are being handled by a central system, the averaging ability of the central system may cope with the situation satisfactorily. This is Fig. 7. Central System with Central Fan and Conditioner and with Individual'Zone Fans N . temperature and- higher humidity in the conditioned space while main taining the same effective temperature. In winter, the matter of condensation oh windows, walls', etc., is of extreme importance. Humidities low enough to avoid this should be maintained, and when it is necessary to carry the higher humidities, double glass should be used or suitable means of handling the condensa tion, should be provided. During intermediate seasons the use of refrigeration as a. source of cooling may be undesirable from an operating cost standpoint depending on the local energy rate structure and demand charges. As a result the use,of outdoor air, either directly or indirectly, as a source of cooling-may be required and. the system may have to operate on-an unusual basis. with.regard to the temperatures and humidities that can be maintained.; This situation should be carefully investigated. As a rule, provision should be made for introducing all outdoor air into the. system during intermediate seasons. .. .. With regard to outside conditions it must.be noted that where systems are to operate at certain hours of the day only, the outside conditions that due to the fact that the return air from the particular space is mixed thoroughly with.the return air from all.the other spaces and all the out side air supplied. ... It should be noted that the minimum quantity of outdoor air is affected by infiltration and leakage. Infiltration will reduce the quantity to be introduced by the system, while leakage may have to be offset by an increase in the quantity of outdoor air. Most summer and year 'round air conditioning systems should be so designed ;and arranged that a quantity of outside air at least equal to the quantity of cooled or dehumidified air can be taken in when desired. In the intermediate seasons and in the cooling season it is more economical to take all outside air into the dehumidifier when the outdoor air wet- bulb temperature is lower than the inside wet-bulb temperature to be maintained.; Also, where using a spray type dehumidifier or air washer, whenever the outdoor wet-bulb temperature is below the apparatus dew point temperature required, the system can be operated on an evaporative cooling basis, dispensing with the need for refrigeration though cooling may be required. Automatic controls are available for accomplishing this and on reasonably large systems are justifiable from an economic stand point. - '