Document MJLM9bOXBQdD5BBdG1ra29L3M

CHAPTER 42 Ifjaturai %Jentiiation Wind Forces, Stack Effect, Openings, Windows, Doors, tSkylights, Roof Yentilatora, Stacks, Principles of Control/ General Rules,' 'Measurements, Dairy Barn Ventilation, Garage 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 (6) 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 loca tion. The ventilating results obtained will vary, from time to-time, due to variation in the velocity and direction of the wind and the heat gener ated in the building. 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 1, Chapter 7 for the average summer wind velocities and the prevailing wind directions in various localities through out the United States, while Table 1, Chapter 6, 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 direction 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 con siderably above 5 mph. There are relatively few places where the velocity falls below one half of the average for many hours per month. Con sequently,'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 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 = EAV (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.)] `Predetermining Airation of Industrial Buildings, by W. C. Randall and E. W. Conover (A.S.H.V.E. Transactions, VoL 37, 1931, p. 605). , ' . Natural Ventilation 727 * 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 coefficient slightly greater than that of a square, 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 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. Fig. 1. The Jump of Wind from Windward Face of Building. (4--Length of Suction Area; B--Point of Maximum Intensity of Suction; C--Point of Maximum Pressure) ' 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 FORCES2 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: ' _______ Q = 9.4 4 y if (i - io) (2) Neutral Zone in Ventilation, by J. E. Emswiler (A.S.H.V.E Transactions, Vol. 32, 1926, p. 59).