Document 2jwE5Xyd6aLZb4oe04VnJp1g7

--A---m---e--r--i-c--a--n---S--o--c--ie--t--y---o--f--H---e--a--t--in---g---a--n--d---V---e--n---t-i_l--a--t-i-n--g---E---n--g--i-n--e-e--r--s-- Gim,. . 'A /Steam r u N>r=s^< t Control Valve I Control Valve ^ Recirculated Air j Rolling Shutter- ^ p~' /y/ Chapter 22--Fan Systems of Heating Outside WaU By-pass Damper Fig. 5. Combination of Recirculated Air and Outside Air (Case C) 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 Equations 4 and 5, ngio M Mo + Mr ~H 0.24 (ty - t) The temperature of the resulting mixture of outside and recirculated air entering the tempering coil is: s , _ Motir + Mrl --~ir-- - Example s. Assuming that a positive supplyof outside air (dr, '= 0.0864) is required for ventilation at the rate of 90,000 cu ft per hour in the preceding example, then It. = 0.0864 X 90,000 = 7776 lb per hour are required, measured at 65 F. Mr = Af - M,, = 38,889-^.7776 = 31,113 lb 7776 X 0 + 31,113 X 65 38,889 52 F H, = 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 vehtilating.requirements include the provision for washing and humidifying, the air delivered to the.various rooms of the structure. - .! .' ' .V ` \ . -. - .. In the following cases it is assumed that in addition to; maintaining a mean room, temperature : /,, the heating. and ventilating' apparatus is required to maintain a constant relative humidity in the rooms. ~ 368 Fig 6 Outside Air Circulated; Constant Relative Humidity in Room (Case D) r D (Fig 6) The maximum relative humidity that may be maintained within the without the precipitation of moisture on single glazed sash when the outside Seiature is 30 F is approximately 35 per cent. If the inside temperature / is 70 F, 35 off cent relative humidity corresponds to a dew-pomt temperature of 41 F. (bee JJsychrometric chart.) The installation shown in Fig. 6 contemplates the use of a tempering coil, an air .rasher provided with a water heater, and a reheater. The tempering coil, one section in derth warms the incoming air to approximately 35 F to prevent freezing any of the spray water! The air passing through the spray chamber is saturated and leaves at a tempera ture of,<i = 41 F. The heat to be supplied the reheater is: Hi = 0.24 (tt -- 41)M Btu per hour. The beat to be supplied the tempering coil is: Hi = 0.24 (35 -- to) 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 h, = 35 F and the leaving saturated air at <i. = 41 F (Chapter 1), or: 15.7 -- 8.4 = 7.3 Btu per pound of dry air. The amount of heat required for the washer is: H% = 7.3 M Btu per hour. The total amount of heat required by the apparatus is, therefore: Hi + Hi + Ht Btu per hour. If a washer having a humidifying efficiency.oi 67 per cent without water heater is employed it will be necessary to best 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 369