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Heating Ventilating Air Conditioning Guide 1938 Chapter 21. Central Systems for Heating and Humidifying 1 Case C. (Fig. 5) A portion of the air circulated is recirculated air and the remainder, as may be required for ventilating purposes, is drawn in from the outside. According to Equations 4 and 5, M = Mo + Mr H 0.24 (fy -- f) The temperature of the resulting mixture of outside and recirculated air entering the tempering coil is: Mato -f- Mit <x = M (9) Example 7. Assuming that a positive supply of outside air (do = 0.08633) is required for ventilation at the rate of 90,000 cu ft per hour in the preceding example, then Af0 0.08633 X 90,000 = 7776 lb per hour are required, measured at 65 F. MT = M -- M0 = 38,889 - 7776 = 31,113 lb 7776 X 0 + 31,113 X 65 38.889 52 F Hi = 38,889 X 0.24 (140 - 52) = 821,336 Btu. This amount of work may be accomplished with one or more banks of heating units, that is, either a single reheater or a tempering coil and reheater. The three preceding cases refer to installations in which conditioning the air to maintain certain relative humidity requirements does not enter into the problem, as for example, certain types of industrial installations. In practically all modern public buildings, theaters, schools, and in many industrial installations, the ventilating requirements include the provision. for washing and humidifying the air delivered to the various rooms of the structure. In'the following cases it is assumed that in addition to maintaining a mean room temperature t, the heating and ventilating apparatus is required to maintain a constant relative humidity in the rooms. 418 Cose D. (Fig. 6) The maximum relative humidity that may be maintained within the building without the precipitation of moisture on single glazed sash when the outside temperature is 30 F is approximately 35 per cent. If the inside temperature t is 70 F, 35 . per cent relative humidity corresponds to a dew-point temperature of 41 F. (See psychrometric chart.) The installation shown in Fig. 6 contemplates the use of a tempering coil, an air washer provided with a water heater, and a reheater. The tempering coil, one section in depth, warms the incoming air to approximately 35 F to prevent freezing any of the spray twuareteor.f hTh=e4a1irFp.assing through the spray chamber is saturated and leaves at a tempera The heat to be supplied the reheater is: ~ Hi -- 0.24 (It -- 41)M Btu per hour. The heat to be supplied the tempering coil is: Hi = 0.24 (35 -- t0)M Btu per hour. The amount of heat, per pound of air circulated, to be supplied the humidifying washer or humidifier is the difference between the heat content of the assumed dry air entering the washer at a temperature of U, = 35 F and that of the leaving saturated air at 1, = 41 F (Table 6, Chapter 1), or: 15.657 -- 8.397 = 7.26 Btu per pound of dry air. The amount of heat required for the washer is: Ht = 7.26 M Btu per hour. The total amount of heat required by the apparatus is, therefore: Hi + Hi + Hi Btu per hour. If a washer having a humidifying efficiency of 67 per cent without water heater is em ployed it will be necessary to heat the outside air drawn into the apparatus by means of the tempering and preheater coils to such a temperature that the air in passing through 419 S'