Document 06od4B6dd0KZXQEQx6YL5dd3k

American Society of Heating and Ventilating Engineers Guide, 19j^ evaporating pans or spray nozzles. The cooling coils serve no purpoJ during the intermediate or heating seasons, so in this respect the stw> type equipment is often preferred, in that during certain seasons evapo^ tive cooling will be sufficient to produce the cooling desired. EffectiJ cooling and dehumidification accomplished by surface units are depends upon many variable factors. The air velocity through the unit, jjf; ' temperature, moisture content of the air, water or refrigerant tempera:* ture, and velocity of the water or refrigerant through the tubes mustbf considered in selecting the proper unit for a given design load. If anyojl these factors vary without a corresponding variation of the other factors? the effective work of the coil will increase or decrease, as the case may be| DESIGNING THE SYSTEM The general procedure for the design of a central cooling and ds humidifying system is as follows: 1. Calculate the heat gain for each room or space to be conditioned. (See ChaDtc,! 6 and 8.) 2. Establish the temperature of air leaving the supply inlets. 3. Calculate the quantity of air to be circulated. 4. Estimate the temperature rise in the duct system. 5. Determine the volume of outside air to be introduced. (See Chapter 3.) 6. Calculate the heat to be removed by the cooling and dehumidifying apparatus.' 7. Calculate the size of the reheating equipment. 8. Select cooling equipment and heating equipment from manufacturers' data and performance curves. 9. Design the air distribution system and the air outlets and inlets. 19 and 20.) 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. (See Chaptos .' 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 airconditioning 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 ofi 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 the 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 not directly involved. Such conditions often result in the natural 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 ireful zoning for successful operation. 188 LOCATION OF APPARATUS A ilability of space for apparatus and duct work is of primary imVtance when selecting the type of system for a given design. In general, f0 large installations, the refrigeration equipment, because of its size, ` ht and operating characteristics, is located in the basement along W^th the boilers, fire pumps, and other equipment. The air conditioning m aratus is generally located where clean outdoor air is readily available,' aPP .signer bearing in mind that supply and return air ducts, steam con ations water and drain connections, and electrical connections must be macle to'the equipment proper. TEMPERATURE OF AIR LEAVING ROOM INLETS In comfort conditioning applications, air has been distributed from nroperly 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 9. Industrial systems in which drafts are not objectionable usually employ a temperature dif ferential equal to the dew-point depression. 1 AIR QUANTITY REQUIRED For calculating the quantity of air required to absorb a given heat gain, the following approximate formulae may be used: M =dQ 60 X 0.24 X (1 - ly) or, assuming a constant value of 0.075 lb for d, H3 X 55.2, (1) (2) where Q - volume of air required, cubic feet per minute. __ H, = total sensible heat gain, Btu per hour. t = room temperature, degrees Fahrenheit. ., <y = inlet temperature, degrees Fahrenheit. --- M = weight of air required; pounds per minute. d = density of air at the temperature and relative humidity of the room, pounds per cubic foot. 189 i