Document jgw5pd29Vw9mOje62BvLBmBnk

American Society of Heating and Ventilating Engineers Guide, 1937 Chapter 9--Central Systems for Heating and Humidifying By-pass Damper Fig. 3. Heating Unit and Fan Arranged for Outside Air Circulation (Case A) Case A. which case (Fig. 3) All of the air circulated to be drawn from outside the building |,, = k,. H, = 0.24 i - to) Mt (10)' Hi = 0.24 (/, - fa) M0 (111 Example 6. The heat loss H for a certain factory building is 700,000 Btu per hour. The mean inside temperature t to be maintained is 65 F. The assumed outside air tem perature to is 0 F; = 0, ty = ti and is assumed to be 140 F. The temperature leaving the tempering coil is assumed to be 35 F. Required, Hi and Ht. From Equation 4, M = 700,000 0.24 (140 -- 65) = 38,889 lb pet hour. H, = 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. Air Returned from Heated Space ,Steam ^Automatic Valve . Air Leaving Fan at ty 'ulley ELEVATION Fig. 4. Arrangement for Recirculation (Case B) `'"''Case B\ (Fig. 4) All ot the air is to be recirculated, in which case h = t. Mr = 38,889 lb Hi -- 0.24 (Ij --.0) Mr Hi = 0.24 (i40 - 65) X 3S.S89 = 700,000 Btu per hour. This example illustrates the saving in fuel consumption by. the recir culation of the air. The heat to be supplied the apparatus is the same as fj 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. 178 Fig. 5. Combination of Recirculated Air and Outside Air (Case C) Case C. (Fig. 5) A portion of the air circulated is recirculated airland the remainder, as may be required for ventilating purposes, is drawn in from the outside. According to Equations 4 and 5, H M = Mo + Mr 0.24 (ty - t) The temperature of the resulting mixture of outside and recirculated air entering the tempering coil is: Moto T' Mrt h=' M (12) 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' M0 = 0.08633 X 90,000 = 7776 lb per hour are required, measured at 65 F. Mr = M - Mo = 38,889 - 7776 = 31,113 lb ( 7776 X 0 + 31,113 X 65 ,, = --------------38^89"----------- = 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 provisionfor 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. .. 179 /