Document DG4jwB179wB8Qq7d0jKoee0No

HEATING VENTILATING AIR CONDITIONING GUIDE 1941 The following formula 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: Q = EA V (1) where Q = air flow, cubic feet per minute. A =* free area of inlet (or outlet) openings, square feet. V = wind velocity, feet per minute, -- miles per hour X 88. E = effectiveness of openings. ( should be taken at from 50 to 60 per cent if the inlet openings face the wind and from 25 to 35 per cent if the inlet openings receive the wind at an angle.) If outlet openings, where air leaves a building, are smaller than inlet openings, where air enters a building, the air will be less effective than indicated by the constant E. The accuracy of the results obtained by the use of Formula 1 depends upon the placing of the openings, as the formula assumes that ventilating openings have a flow coefficient slightly greater than that of a square-edge orifice. If the openings are not advantageously placed with respect to the wind, the flow per unit area of the openings 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 following four places: 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. In a monitor on the side opposite from the wind. 4. In roof ventilators or stacks exposed to the full force of the wind1. Forces Due to Stack Effect2 The stack effect produced within a building when the outdoor tempera ture 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: where Q = 9.4 A V H (t - ta) (2) 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. The height between inlets and outlets should be the maximum which the building construction will allow. ' 'Airation of Industrial Buildings, by W. C. Randall (A.S.H.V.E. Transactions. Vol.34; 192S. p. 159). Neutral Zone in Ventilation, by J. E. Emswiler (A.S.H.V.E. Transactions, Vol. 32, 1026, p. 59). Predetermining Airation of Industrial Buildings, by W. C. Randall and E. W. Conover (A.S.H.V.E. Transactions, Vol. 37, 1931. p. 605). 718 CHAPTER 41. NATURAL VENTILATION TYPES OF OPENINGS Types of openings may be classified as: (1) windows, doors, monitor openings and skylights, (2) roof ventilators, (3) stacks connecting to registers, and (4) specially designed inlet or outlet openings. Windows, Doors and Skylights Windows have the advantage of transmitting light, as well as providing ventilating area when open. Their movable parts are arranged to open in 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) various ways; they may open by sliding as in the ordinary double-hung windows, by tilting on horizontal pivots at or near the center, or by swinging on pivots at the top, bottom or side. The proper distribution of the air in spaces to be ventilated is as im portant as that of sufficient air quantity. Advantageous pivoting of sash is very useful for securing good air distribution. Deflectors are sometimes used for the same purpose, and these devices should be considered a part' of the ventilation system. Roof Ventilators -- The function of a roof-ventilator is to provide a storm and weather proof air outlet. If it is of a type which is sensitive to wind action addi- 719