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322 W. N. WITHERIDGE IV. Aeromotive Methods and Equipment The forces available for ventilation are here considered as either h| mechanical, as in the discussion of general ventilation. A. NATURAL AEROMOTIVE FORCES Natural ventilation may be (I) gravity, motivated by the thermahidiig convection, or (2) anemotive, created by wind pressure. These two natural operate together in most cases, and a study of their effects has led to th^fs^ of the "neutral zone" in thermal ventilation, modified by experimentally wind-pre3sure corrections. Emswiler and Randall16 gave the following analysis of the combing of wind and thermal forces in producing air flow through a building: Even without any wind, a difference of temperature inside and outside v pressure Btate inside to be less than that outside at the ground, and greater near thl'Toqffl inflow will occur at lower windows and outflow at upper windows. If the building ia*jp open throughout from bottom to top, or if arranged in stories in free communication* trcftjlS to another, and if the temperature difference is great, the pressure difference created IjJS near the ground and also at a point near the roof may be considerable.Thus, in 200 ft. high with 70 F. difference in temperature, this force alone causing inflow at tij|L and outflow at the top, may easily exceed 0.20 in. of water, which -is the equivalent ofaB| mile wind. If the building is multiple storied and there is absolutely no communicauuniSS stories, then the force of temperature difference is effectually nullified, or rather reducfe*: amount proportional to the height of a single story. However, there is always cation by means of stairs and otherwise, so that the force of temperature differec$sp operative in some degree. i If the wind blows against a window of a building that is tightly sealed at aU'oflS of possible ingress or egress of air, there will be no continuous infiltration at thefl because the temporary excess of pressure outside will soon be equalized by a momentarww If the building has an outlet of exactly the same area as the presumed infiltratipillftfra window, and this outlet opens into a region of neutral pressure, then the pressure infffhci will be half as much as that produced by the wind outside. Infiltration will take'plftceMiroi the window, but with only half the force of the wind pressure. Something like,, tnis^^ro! occurs in any building. Whatever air gets in at inflow points must escape MweSroySafaf order to force the air out, there must be a greater pressure inside somewhere .than.$\sj& prevails immediately outside. The inside pressure automatically assumes a valuejjjajajKM the necessary head for outflow through openings available, so that the oulflow.quanLitw 1. Wind Direction, Velocity, and Pressure "' 'Vi Few locations in the United States have what is actually a "preyaili direction',"iFthis isTaken to'm'ean a direction from which trie winaR<g^J the time. Not over 5 per cent of the land receives wind from one genfe|aPij] as much as 50 per cent of the time. Wind data collected by the We&ih; r_.T at stations located on the tops of high buildings cannot be taken as represen " J. E. Emswiler and W. C. Randall, Trane. ASHVE, 34, 527 (1928). i K f'A1`v/t' m K VENTILATION 323 era&X'SsSi.g. fiditions throughout the entire area. The observing stations in large llfioated in order to sample a stratum of air that is unaffected by |||n'epiated by surrounding structures. ^Sies of the winds at these elevated locations are higher than ground no obstructions existed. For those design problems where the iXf.; ..$ant' wind velocity determines the quantity of natural ventilation values about one half the indicated seasonal average velocities Average wind velocities throughout the United States range fr seasons of the year.16 llftre an occasional gust of wind, or the pressure of high wind |w hours of a day would create dangerous reduction of ventilation ISing irritating or hazardous.gases, vapor3, or dusts, the ventilator ed against such pressures. It is evident that a substantial wind f, to the side of a building in which a fan is mounted may seriously of air moved and may even reverse the direction of flow when gating a high static pressure. Some type of baffle, "windbreaker," TABLE 14 Wind Velocity Pressure * if* III Velocity pressure, r feet VP* nute--(.inches-of-.watet),, `lfO- " 0.0005 0.0019 0.0044 0.0077 0.012 0.048 0.11 0.19 0.30 0.44 0.59 0.77 ------- j~2--.. 1.7 ' 0.00048 (m.p.h.)* Visible or physical effects Smoke rises vertically Direction of wind shown by smoke drift Moderate walking speed Fast walking speed Wind felt on face; leaves rustle Wind extends light flag Raises dust and loose paper Crested wavelets form on inland water Umbrellas used with difficulty Whole trees in motion Difficulty walking against wind Breaks twigs offerees Slight structurai-daJnage to- Buildings Considerable structural damage ^ ,'^^fjipr'-should be provided, or the fan should be...selec.ted. for a ^Wpjssufc to compensate for reasonable wind pressures above the JlJi di]ffl.l?assem'bles some of the data needed for consideration of the wind f#*lctor. ljk,t.9'AlhJlSht^'Siimmer Weather Data. The Marley Co., Kansas City, Kan., 1939. V if *