Document aJg9JD1j3zX0EJ3nRd4qazozX

American Society of Heating and Ventilating Engineers Guide, 1935 ' By considering the temperatures in Table 5 to be wet-bulb readings, the total heat of any air-vapor mixture may be obtained from the last column in the table. Enthalpy This total heat of. an air-vapor mixture is not exactly equal to the true heat content or enthalpy of the mixture since the heat content of the ; liquid is not included in Equation 10. With the meaning of heat content .in agreement with present practise in other branches of thermodynamics, "the'true-heat content-of-admixture of dry-air and water vapor-(with 0 Fa's the datum for dry air, and the saturated liquid at 32 F as the datum for the water vapor) is where h = !>* (< - 0) + W *3 = 0.24 (< - 0) + W hs h = the heat content of the mixture, Btu per pound of dry air. t = the dry-bulb temperature, degrees Fahrenheit. W = the weight of vapor per pound of dry air, pounds. = the heat content of the vapor in the mixture, Btu. per pound. (12) The heat content of the water vapor in the mixture may be found in steam charts or tables when the dry-bulb temperature and the partial pressure of the vapor are known. Or, since the heat content of steam at low partial pressures, whether super-heated or saturated, depends only upon temperature, the following empiricalr equation:, (derived from Keenan's Steam Tables, may be used: '' ' As = 1059.2 + 0.45 t (13) Substituting this value of K in Equation 12, the heat content of the mixture is h = 0.24 (f -- 0) + W (1059.2 + 0.45 /) . . (14) An energy equation can be written -that applies, in general, to various air-conditioning processes, and this equation can be used to determine the quantity of heat transferred during such processes. In the most general form, this equation may be explained with the aid of Fig. 1 as follows: The rectangle may represent any apparatus, e.g., a drier, humidifier, dehumidifier,' cooling tower, or the like, by proper choice of the direction of the arrows. In general, a mixture of air and water vapor, such as atmospheric air, enters the apparatus at i and leaves at 3. Water is supplied at some temperature,lj. For the flow of 1 lb of dry air (with accompanying vapor) through the apparatus, provided there is no appreciable change in the elevation or velocity of the fluids and no mechanical energy delivered to or by the apparatus, or where A, + h + (W, - Wt) A, = A, + Rc Eh - Rc = hi - h - (Wi - IF,) A, ., (15) Eh. -- the quantity of heat supplied per pound of dry air, Btu. Rc = the quantity of heat .lost externally by heat transfer from the apparatus, Btu per pound of dry air. 18 i t ,i 'i v3 s