Document 1gY4Z9O88KoqGajv9w8jNa27d

American Society of Heating and Ventilating Engineers Guide, 1936 Chapter 22--Fan Systems of Heating . Fig. 3. Heating Unit and Fan Arranged for Outside Air Circulation (Case A) ' Case A, (Fig. 3) All of the air circulated to be drawn from outside the building, in which case In = to. II, ~ 0.24 (/, - t0) M0. _. . .. (10) Hi = 0.24 (tt - h) M0 (ID Example 6. Theheat loss H for a .certain factory , building is 70Q.000 Btu per hour. .The mean inside temperature f to be maintained is 65 F. The assumed outside air. tem perature to is 0 F\tz= 0, - ly = h and is assumed to be 140 K. The temperature living the tempering coil is assumed to be 35 F. Required, Hi and H,. From Equation 4, M` = 'TOO,000 0,24 (140 - 65) = 38,889 lb. per hour. Hi = 0.24 X (35 - 0) X 38,889 = 326,667 Btu per hour. Hi = 0.24 X (140 - 35) X 38,889 = 980,003 Btu per hour. Hi + Hi = 326,667 + 980,003 = 1,306,670 Btu per hour. i- . .^Steam, " --]L^Automatic Valve . Air Leaving Fan at ty . Air Returned from .Heated Space ELEVATION Fig. 4:f . Arrangement for Recirculation (Case B) . ; Case B.:: (Fig.' 4) All of the air is to bejrecirculated, in which case ti = /. MT = 38,8891b . Mi = 0.24 (It - h) Mr. Hi = 0.24 (140 - 65) X 38,889 = 700,000 Btu. per hour. This example illustrates the. saving in fuel consumption by th$ recir culation of the air. The heat to be supplied the apparatus is the same as that required for a direct system of heating and is equal to the heat loss of the building {Hi = IT), in the example 700,000 Btu per hour 'as compared with 1,306,670 for Case A. 396 a f i * \ Fig. 5. Combination of Recirculated Air and Outside Air (CaseC) Qase 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 (ty - t) The temperature of the resulting mixture of outside and recirculated air entering the tempering coil is: M0to -j- Mrt t* = M (12) Example 7. Assuming that a positive supply of outside air (do = 0.0864) is required for ventilation at the rate of 90,000 cu ft per hour in the preceding example, then Ma = 0.0864 X 90,000 = 7776 lb per hour are required, measured at 65 F. Mr = M - Mo = 38,889 - 7776 = 31,113 lb : ' h _f7776 X 0 d- 31,113 --------- 38,889 X 65 = 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. 397