Document VJKrpJvNdYXQvvE5B89VmZx6j

HEATING VENTILATING AIR CONDITIONING CUIDE 1944 Equation 1 may be used for calculating the quantity of air forced through ventilation openings by the wind, or for determining the proper size of such openings to produce given results: Q = EA V (1) where Q = air flow, cubic feet per minute. A = free area of inlet openings, square feet. V -- wind velocity, feet per minute, = miles per hour X 88. E = effectiveness of openings. (E should be taken at 0.50 to 0.60 for perpendicular winds and 0.25 to 0.35 for diagonal winds1.)) The accuracy of the results obtained by the use of Equation 1 depends upon the placing of the openings, as the formula assumes that ventilating openings have a flow coefficielit slightly greater than that of a squareedged orifice. If the openings are not advantageously placed with respect to the wind, the flow per unit area of the opening^will be less and, if unusually well placed, the flow will be slightly more than that given by the formula. Inlets should be placed to face directly into the prevailing wind, while outlets should be placed in one of the five places listed: 1. On the side of the building directly opposite the direction of the prevailing wind. 2. On the roof in the low pressure area caused by the jump of the wind (see Fig. 1). 3. On the sides adjacent to the windward face where low pressure areas occur. 4. In a monitor on the side opposite from the wind. 5. In roof ventilators or stacks. TEMPERATURE DIFFERENCE FORCES' The stack effect produced within a building when the outdoor tem perature is lower is due to the difference in weight of the warm column of air within the building and the cooler air outside. The flow due to stack effect is proportional to the square root of the draft head, or approximately: yQ = 9.4 A -------- H (t - t,,) (2) where Q = air flow, cubic feet per minute. A = free area of inlets or outlets (assumed equal), square feet. H = height from inlets to outlets, feet. t = average temperature of indoor air in height H, degrees Fahrenheit. to = temperature of outdoor air, degrees Fahrenheit. 9.4 = constant of proportionality, including a value of 65 per cent for effectiveness of openings. This should be reduced to 50 per cent (constant = 7.2) if conditions are not favorable. J' HEAT REMOVAL In problems of heat removal, knowing the amount of heat to be re moved and having selected a desirable temperature difference, the amount 'Predetermining Aixation of Industrial Buildings, by W. C, Randall and E. W. Conover (A.S.H.V.E. Transactions. VoL 37, 1931, p. 605). 'Neutral Zone in Ventilation, by J. E. Emswiler (A.S.H.V.E Transactions, VoL 32, 1926, p. 59). 752 CHAPTER 42. NATURAL VENTILATION of air to be passed through the building per minute to maintain this tem perature difference can be determined by means of Equation 3. H = 0.0175 Q (f - to) (3) - where H -- heat removed, Btu per minute. Q = air flow, cubic feet per minute. t--/0= inside-outside temperature difference, degrees Fahrenheit. Fig. 1. The Jump of Wind from Windward Face of Building. (A--Length of Suction Area; B--Point of Maximum Intensity of Suction; C--Point of Maximum Pressure) EFFECT OF UNEQUAL OPENINGS The largest flow per unit area of openings is obtained when inlets and outlets are equal, and the equations given previously are based on this condition. Increasing outlets over inlets, or vice-versa, will increase the air flow, but not in proportion to the added area. When solving problems having an unequal distribution of openings, use the smaller area, either inlet or outlet, in the equations and add the increase as determined from Fig. 2; 753