Document wrBoB66eZNEzB1yvJQ77OYzwB

American Society of Heating and Ventilating Engineers Guide, 1936 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 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 HU, - tt) (2) Q = air flow in cubic feet per minute. A = free area of inlets or outlets (assumed equal) in square feet. H = height from inlets to outlets, in feet. 11 = average temperature of indoor air in height H, in degrees Fahrenheit. 1% = temperature of outdoor air, in 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. In some cases the necessary air-flow will be known from the require ments of the building occupancy, and the area necessary for certain assumed temperature differences may be calculated. Or the areas may be fixed by the building construction, and the maximum air flow for various differences between indoor and outdoor temperatures may be calculated. In any case, the conditions which give the minimum air flow are those which control the design, as the system must have ample capacity even under the most unfavorable conditions which are those of mild or warm weather. TYPES OF OPENINGS The engineering problems of a natural ventilation system consist of the design,, location, and control of ventilating openings to best utilize- the *See Airation of Industrial Buildings, by W. C. Randall (A.S.H.V.E. Transactions, Vol. 34, 1928). *See Neutral Zone in Ventilation, by J. E. Emswiler (A.S.H.V.E. Transactions, Vol. 32,1926), and Predetermining Airation of Industrial Buildings, by W. C. Randall and E. W. Conover (A.S.H.V.E. Trans actions, Vol. 37, 1931). Chapter 4--Natural Ventilation in,,agtuorcacluvpeanntciyla.tioTnhefortcyepse,sinofaocpceonrdinagnsce with thp f bui,d,- 1. Windows, doors, monitor openings, and skylights. 2. Roof ventilators. 3. Stacks connecting to registers. 4. Specially designed inlet or outlet openings. Windows, Doors and Skylights * . e _____ , , ,,,, TO,,u ,,,, nmw'Hin 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 or bottom. Whatever the form and type of window used, the amount of clear area that can be made available is the factor of greatest importance in ventilation.. All types of sash (double-hung, top, center or bottom horizontal pivoted, or vertical, pivoted) have about the same air flow capacity for the same clear area. Air leakage through dosed windows is important during high winds (Chapter 6). 91