Document wDdLzVrNQo9RbEoY9BnJD3QoQ

American Society of Heating: and Ventilating Engineers Guide, 1936 10. Calculate the total static pressure of the system. 11. Select the fan, motor, and drive. (See Chapters 17 and 42.) 12. Select the pump and motor. 13. Design the control system. (See Chapter 14.) , ZONING - . The above general outline of procedure will prove satisfactory for the smaller and less complex installations. However, when dealing with airr .conditioning systems for large'buildings, after a proper analysis has been made of the conditions;,to be maintained and the heat loads encountered, it;is generally considered best practice to divide the complete job into a number of suitably sized units. In some cases a unit per floor or group of Fig. 3, Central Dehumidifying Plant and Local Recirculating Fans floors may complete the design satisfactorily, whereas in others it may be advantageous to have separate units for each of the various outside exposures of the building. ; Where ^Ke floor area is large in relation to the outside wall exposure, it is obvious that, provision must be made for the variable load to, which the. outside exposures are subjected. The heat loads on inside rooms are apt to be less variable since the fluctuations of the outside weather conditions are riot directly involved. Such conditions often result in the riatural zoning, or.segregation of rooms having similar exposures and internal heat loads. Variation; in the hours of occupancy in different portions of . a building also frequently require careful zoning, for successful operation, ....... . LOCATION OF APPARATUS .... Availability .of space-forcapparatus and duct work is of primary ,im portance when selecting the type of system for a given design. In general7, 172 . 9--Central Air Conditioning Systems :-..-.oi.t{gi.: for large installations,. the refrigeration eqiiiprneht, because of its size, weight, and operating characteristics, is located in the basement along with the boilers, fire pumps, and other equipment. The air conditioning apparatus is generally located where clean outdoor air is readily available, the designer bearing in mind that supply and return air ducts, steam con nections, water and drain connections, and electrical connections must be made to the equipment proper. , TEMPERATURE OF AIR LEAVING INLETS In comfort conditioning applications, air has been distributed, from properly designed inlets without producing drafts at temperatures varying from approximately 5 to 30_ deg below the required room temperature. Factors influencing the design and selection of air inlets are: ceiling height, contour of ceiling, length of blow, and temperature and quantity of-air to be distributed. Most summer conditioning installations are designed to supply the air to the conditioned space at from 8 to 18 deg below room temperature. Recently the use of specially designed nozzles has indicated the possibility of reducing the air quantity necessary to dissipate a given heat load by introducing the air into the room as much as 30 deg below room temperature. Directional flow inlets which spread the air fanwise permit lower inlet temperatures than single direction inlets. Comfort conditioning systems employing differentials greater than 18 deg require special consideration and design experience because high pressure inlets or nozzles are usually used. Further, care must be taken to allow a sufficient air quantity under all load conditions, to insure good distribution. If winter heating, as well as summer conditioning, is to be accomplished by the same distributing system, the design of the inlets will be influenced as discussed in Chapter 22. Industrial system^ in which drafts are not objectionable usually employ a temperature dif ferential equal to the dew-point depression. AIR QUANTITY REQUIRED For calculating the quantity of air required to absorb a given heat gain the following approximate formulae may be used: M= Hs 60 X 0.24 X (/ - ty) =dQ CD or, assuming a constant value of 0.075 lb for d, Hs X 55.2 y . 60 X (< - ty) (2) where Q = volume of air required, cubic feet per minute. H, = total sensible beat gain, Btu per hour. t = room temperature, degrees Fahrenheit. ty = inlet temperature, degrees Fahrenheit. M = weight of airrequired, pounds per minute. d = density of air at the temperature and relative humidity of the room, pounds per cubic foot. - Example 1. The total sensible heat gain in a restaurant when held at 80. F is 199,736 Btu per hour. Assuming a 12 deg F temperature differential between the entering air and 173