Document O3Xn53EO6e0K3Z1EOpL8jvrOM

226 CHAPTER 11 1953 Guide (4) local wind interference by nearby buildings, hills or other obstructions of similar nature. . ' Values are given in Table 3, Chapter 13 for the average wind velocities for the months,June to September in various localities throughout the United'States, while Table 1, Chapter 12, lists similar values for the winter: In-almost all localities, the summer wind velocities are lower than those in the winter, and in about two-thirds of the localities the prevailing direc tion is different during the summer and winter. While the tables give no average velocities below 5 mph, there will be times when the velocity is Fig. 2. 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) lower, even in localities where the seasonal average is considerably above 5 mph. There are relatively few places where the velocity falls below onehalf of the average for many hours per month. Consequently, if the natural ventilating system is designed for wind velocities of one-half of the average seasonal velocity, it should prove satisfactory in almost every case. Equation 3 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: .infiltration and Ventilation 227 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 perpendicu lar winds, and 0.25 to 0.35 for diagonal winds.)5 , The precision of results obtained by the use of Equation 3, depends upon the placing of the openings, as the formula assumes that ventilating openings have a flow coefficient 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 openings will be less and, if un usually weir 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. 2). 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 FORCES7 The stack effect produced within a building, when the outdoor tempera ture is lower than the indoor temperature, is due to the difference in weight of the warm column of air within the building and cooler air outside. The flow due to stack effect is proportional to the square root of the draft head, . or approximately Q = 9.4A Vh(t; - <,,) (4) 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. fi = average temperature of indoor air in height h, Fahrenheit. fi> = temperature of outdoor air, Fahrenheit. 9.4 = constant of proportionality, including a value of 65 percent for effectiveness of openings. This should be reduced to 50 percent (constant = 7.2) if con ditions are not favorable. HEAT REMOVAL In problems of heat removal, knowing the amount of heat to be removed and having selected a desirable temperature difference, the amount of air to be passed through the building per minute, to maintain this tempera ture difference, can be determined by means of Equation 5. where Q =_____ H____ , v 0.0175 (fi - to) Q -- air flow, cubic feet per minute. B = heat removed, Btu per minute, fi -- to = inside-outside temperature difference, Fahrenheit. (5)