Document OJMqprMDxw52rZqn2eKNJm9xM

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 Heat Emission of Occupants The heat and moisture given off by human beings under different states of activity are shown in various tables and figures of Chapter 2 which covers the physical and physiological principles of air conditioning. It will be observed from these data that the rate of sensible and latent heat emission by human beings varies greatly depending upon state of activity. In many applications this component becomes a large per centage of total load. Metabolic rates are markedly variable for some extreme environmental conditions and this is another important factor which must be considered in cooling load computations. Heat Introduced by Outside Air An allowance must be made for the heat and moisture in the outside air introduced for ventilation purposes or entering the building through cracks, doors, and other places where infiltration might occur. The volume of air entering due to infiltration may be estimated from data given in Chapters 5 and 6. Information on the amount of out side air required for ventilation will be found in Chapter 2. In the event the volume of air entering, an enclosure due to infiltration exceeds that required for ventilation, the former should be used as a basis for determining the portion of the load contributed by outside air. Where volume of air required for ventilation exceeds that due to infiltration it is assumed that a slight positive pressure will exist within the enclosure with a resulting exfiltration instead of infiltration. In this case the air required for ventilation is used in determining outside air load. The heat gain resulting from outside air introduced may be determined by Equation 3: II ~ Qv (ho-hi) (3) where H = heat to.be removed from outside air entering the building, Btu per hour. Q = volume of outside air entering building, cubic feet per hour. v ~ cubic feet of outside air per pound of dry air. hQ = enthalpy of outside air, Btu per pound of dry air. hi *= enthalpy of inside air, Btu per pound of dry air. . The latent heat gain resulting from outside air introduced may be determined by Equation 4: N' Hi = -2- hie (Wo -- WO ' (4) where Hi -- latent heat to be removed, Btu per hour. ' hig = latent heat of evaporation at temperature at which water is condensed, Btu per pound. . Wo -- humidity ratio of outside air, pounds water per pound dry air. Wi =; humidity ratio, of inside air, pounds water per pound dry air.- Heat Emission of Appliances Heat generating appliances which give off either sensible heat or both sensible and latent heat in an air conditioned enclosure may be divided 158 CHAPTER 7. COOLING LOAD into three general classes of equipment or devices: (1) electrical ap pliances, (2) gas appliances, and (3) steam heating appliances. In the first group may be found such devices as lights11, fans, motors, toasters, waffle irons, etc. The wattages are usually marked on the name plates and it is only necessary to multiply the aggregate wattage by 3.413 (Btu per watthour) in order to estimate the heat added to the conditioned space by such devices in Btu per hour. Electric motors are usually rated in units of horsepower output. To determine the corresponding input, which is the rate at which heat is added to the conditioned space by full-load operation of such, motors, some idea of motor efficiency is necessary. The aggregate input in horsepower should then be multiplied by 2546 (Btu per horsepower hour). When motor efficiencies are not known, the data in Table 9 may be.used. Table 9. Heat Generated by Motors Nameplate Rating Horsepower K to H J^to3 3 to 20 Heat Gaik in Btu feb Hour pb& Horsepower Connected Load in Same Room 4250 3700 2950 Connected Load Outside of Room 1700 1150 400 In the second group belong such appliances as coffee urns, gas ranges, steam tables, broilers, hot plates, etc. For heat generating capacities of such appliances refer to Table 10.' Considerable judgment must be followed in the use of data given in Table 10. Consideration must be given to time of day when appliances are used and-the heat they contribute at time of peak load. Only-those appliances in use at the time of the peak load need be considered. Con sideration must also be given to the way appliances are installed, whether products of combustion are vented to a flue, whether they escape into the space to be conditioned, or whether appliances are hooded allowing part of ifhe heat to escape through a stack. There are no generally accepted data available on-the effects of venting and shielding heating appliances but it is believed that, when they are properly, hooded with-a positive fan iiexhaust system through the hood,50 per cent of the heat .will be carried away and' 50 per cent dissipated in the space to be-conditioned. Where latent as well as sensible heat is given off, it is usually safe to assume that all latent heat will be. removed by a properly designed and operated vent or hood. ILLUSTRATION - From the foregoing discussion it is obvious that the determination of the maximum cooiing load is rather complicated by reason of the variable "Cooler Footcandles for Air Conditiomng;- by W.-G.-Darley (A.SiH.V.E. Transactions, Vol. 46. 1940, p. 367). Lighting and Air Conditioning Design Factors. Report of I. B. S.--A.S.H.V.Ei Joint Com mittee on Lighting) in Air Conditioning (A.S.H.V.E.. Journal Section. Heating, Piping and. Atr Con ditioning, September,;1941. p.,605). -Lighting and Air.Conditioning,.by Howard M. Sharp (Heating and Ventilating, November. 1942, p. 35). 159