Document zdR5JvqMBLLzxyN8moRzk4ROg

220 CHAPTER 10 1951 Guide Infiltration and Air for Combustion Infiltration in buildings normally supplies the air required for combus tion by fuel-burning appliances, but in some cases weatherstripping, sealing and calking may reduce infiltration to the point that special open ings must be provided to supply adequate air to the heating appliances. NATURAL VENTILATION Ventilation by natural forces finds application in industrial plants, public buildings, schools, dwellings, garages, and in farm buildings. The natural forces available for moving air into, through, and out of buildings are: (a) wind forces, and (ft) the difference in temperature between the air inside and outside a building. The air movement may be caused by either of these forces acting alone, or by a combination of the two, depending upon atmospheric conditions, building design, and location. The ventilating results obtained will vary, from time to time, due to varia tion in the velocity and direction of the wind, and the temperaturedifference. The arrangement, location, and control of the ventilating openings should be such that the two forces act cooperatively rather than in opposition. WIND FORCES In considering the use of natural wind forces for producing ventilation, account must be taken of: (1) average wind velocity; (2) prevailing wind direction; (3) seasonal and daily variations in velocity and direction; and (4) local wind interference by nearby buildings, hills or other obstructions of similar nature. Values are given in Table 3, Chapter 12 for the average wind velocities for the months June to September in various localities throughout the United States, while Table 1, Chapter 11, 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 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 inay 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: where Q = EAV (3) 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.*) 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 square- infiltration and Ventilation 221 edged 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 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. 2). 3. On the sides adjacent to the windward face where low pressure areas occur. Fio. 2. The Jump op Wind fbom Windward Face op Building. (A--Length op Suction Area; B--Point op Maximum Intensity of Suction; C--Point op Maximum Pressure) j 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: e = 9.44VA((-g' (4)