Document Z8o1Rk1kKR7Jm517wL4rx19Vd

456 CHAPTER 25 1965 Guide And Data Book faces depend not only on roof shape and height above ground, but also vary with wind angle and location on the roof with'u respect to the leading edge or ridge. At leading edges of Sat or low-pitched roofs (up to 30 deg pitch) negative pressures of -- 0.8 to -- 1.0 p. may occur, while pressures over the remain?' ing areas may have values of -- 0.2 to -- 0.8 p,. Pressures may have values of + 0.2 or + 0.3 p on the windward side of high pitched roofs, while leeward side pressures may be -- 0.5 to - 0.7 jv. In the prediction of rates of infiltration or natural ventila-' tion, pressure differences across the walls or roof of the build ing are required, and these depend on internal as well as ex ternal pressures. Pressures inside buildings due to wind action alone .will depend- on the resistances of cracks and openings and their location with respect to the wind direction, in large buildings the tightness of internal space separations also may be a factor. If the openings are uniformly distributed around the walls of the building, inside pressures will usually be within plus or minus 0.2 p. If openings on the windward side predominate, inside pressures up to + 0.8 p, may occur, approaching the positive values on the outside. Conversely, if openings on the leeward side predominate, inside pressures may have negative values of 0.2 to 0.4 p, or greater. The selection of appropriate .wind velocities for purposes of calculating pressure differences across the building enclosure due to wind action, is not straightforward since the relation ship between recorded wind and wind at the'building' site varies and is difficult to establish. Data on wind is usually based on records obtained at meteorological stations; often airports, where anemometers are usually on top of buildings in relatively open country. Buildings are located within the surface boundary layer of the earth, where there is a'signifi cant increase in velocity with height. Furthermore, the rela tionship between velocity and height depends strongly on the surface roughness as influenced by the type of terrain; whether fiat open country or water, wooded country or suburbs, or large city buildings. In addition to the problem of translating' measured velocities into those at the building site, there is tiie question of the effect of the vertical velocity gradient on the pressure distribution, especially around tall buildings. Some measurements1 suggest that the variation of static pressures with height at the surface of buildings is much less than the variations in the velocity pressures of the oncoming Sow. Measurements on the Empire State Building* show little difference between pressures at the 36th and 75th floor levels. Shielding of one building by another results in signifi cant reductions in effective wind velocities. At street level among large buildings wind directions and velocities may bear little relation to those measured at the nearest weather station.* In general, the velocity of the wind acting on the sur face of a building is entirely too complicated to be repre sented by anything that is rational. TEMPERATURE DIFFERENCE FORCES When the temperature in a building or room is different from that outside, pressure differences between inside and out side occur as a result of differences in air density, that is, chimney or stack effect. When the inside temperature is higher than outside, this chimney effect produces a negative inside pressure and inward flow of air at lower levels, and a positive inside pressure and outward flow at higher levels. The reverse occurs when the inside temperature is lower than outside. At some level, with temperature forces alone acting, there is a neutral cone or plane where there is no pressure difference between inside and out.4 At any other level the pressure difference depends on the distance from-the neutral cone and. the difference between the densities of inside and outside air. Send on indoor dr at 75 F, 50 poreaat *H, standard atmospheric pressure 29.92I in. Hg. Fig. 2 .... Theoretical Draft in Buildings Due to Chimney Effect This can be expressed: p. - 0.S2PA - JL) (2) where Pc = theoretical pressure difference across enclosure due to chimney effect, inches of water. P " absolute pressure, pounds per square inch. h -- distance from neutral rone, or effective chimney height, feet. ' T, -- absolute temperature outside, Fahrenheit. - Ti *= absolute temperature inside, Fahrenheit. This relationship does not account for pressure losses be tween the neutral zone and other levels due to resistance to flow within the structure. Values of Pe'for any effective chim ney height and indoor to outdoor temperature difference can be obtained from Fig. 2. The level of the neutral zone depends on the vertical dis tribution and resistances to air flow of openings through which inflow and outflow can occur. For example, if there is only one opening, or if one opening is extremely .large relative to the others, the neutral zone will be at or near the center of the opening. If the openings are uniformly distributed vertically, the neutral zone-will be at the mid-height of the enclosure. If there are no space separations, or if their resistance to flow is BTWftll (tiie usual case), a building acts as a single chimney' and the pressure at any level in the building is constant; If the spaces are isolated from one another, each has a separate chimney effect and neutral sone independent of the others. In' most cases there is some degree of interconnection of spaces, and the pressures within a space are affected by the action of the building as a whole. Thus, the vertical distribution of pressure differences across the walls of a building depends on' the resistance to flow of horizontal and vertical space separa tions as well as the vertical distribution of openings. Infiltration and Ventilation 457 With only temperature forces acting, the calculation of the mh&ble level of the neutral zone for simple enclosures with oDenings of known air flow characteristics is straightforward, locomplex buildings, however, the effects of internal space separations, stairwells, elevator shafts, utility ducts, chiro- octs, vents, and mechanical air supply and exhaust systems are difficult to evaluate. Chimneys represent openings at or above the roof a"d, therefore, have the effect of increasing the height of the neutral zone. The effect of a chimney bn the level of the neutral sone is especially important in houses or small httjtHmgg where the exhaust from the chimney represents a substantial proportion of the exfiltration. Exhaust systems also increase the height of the neutral zone, while air supply gystems have the opposite effect. Available data on the levels of neutral zones in various Undq of buiMing* is extremely limited. The neutral zone in a 47-story offi*** building, on which measurements were made in 1934,* was at or somewhat below mid-height over a range of outdoor temperatures. Similar measurements on a 40-etory bunding reported in 1937,* indicate that the neutral was generally around the 24th floor, an elevation about 60 percent of the height of the structure. In both these investi gations the pressure differences above and below the level of the neutral sone were somewhat less than would be calcu lated from Equation 2, probably due to the pressure losses required to maintain the upward flow in the building. More recent measurements* of pressure differences across en trances in tall buildings indicate that the apparent level of the neutral zone is 0.7 of the total building height for conventional hnildingR. In- one unusually tight modem building with pneumatically, sealed windows, the apparent level of the neutral sone was 0.3 of the total height. . It would appear from these results and other observations*, that vertical openings, such as stairwells, elevators and other utility shafts in tall buildings, often are not well sealed and that pressure differences due to chimney action result, to a" large degree from action of the building as a whole. It is clear from - Equation 2 that pressure differences due to chimney effect would be greatly reduced if the sealing of vertical open ings were made really effective. Measurements* on houses indicate that'the neutral zone is Hkely to be well above mid-height and that the effect'of the chimney in raising the level of the neutral sone increases with deereasing outdoor temperature. COMBINED FORCES The pattern of pressure differences across a building en closure depends on the magnitude of all acting pressureforces and the distribution of openings into and within the building. In an actual building, since the relative magnifaiHa of the farces is continually changing, the pressure pattern changes accordingly. In design calculations the limiting conditions, either minimum or maximum, are usually required. For these cases one force may dominate so that it may be neces sary to determine only the relative magnitude of air flow induced' by one or more forces acting singly. In other in stances, particularly in' tall buildings, it may be necessary to consider pressure forces in combination in order to arrive at a rational design. Each building presents a unique problem, but the manner in which the pressure forces act can be considered qualita tively with reference to Fig. 3, which represents tiie distri bution of indoor and outdoor pressures with height for a building in which openings of equivalent area are equally dis tributed above and below mid-height and where there is no .significant resistance inside to flow from top to bottom. The slopes of tiie lines are functions of the densities of indoor and outdoor air. In Fig. 3(a), with inside wanner than outside and pressure differences due to stack action alone, the neutral zone is at mid-height with inflow through all openings below and outflow through all openings above. The effect of a chim ney or mechanical exhaust from tiie building would be to shift the inside pressure line to the left, raising the level of the neutral zone. An excess of supply air over exhaust would have tiie opposite effect. Fig. 3(b) shows pressure differences due to wind alone-with the magnitude of wind effect on windward and leeward sides equal but opposite in sign Fig. 3(c) represents the condition where the wind forces illustrated in Fig. 3(b) have just balanced the temperature difference forces with no pressure difference at the top on tiie windward sides or at the bottom on the leeward sides. Thus the total flow is only slightly more or less than with stack ac tion alone. Beyond this point, as the relative magnitude of pressure differences due to wind increases, the lines represent ing windward and leeward pressures move to the right and left, and the total flow becomes increasingly independent on wind, although the differences in pressure with height due to temperature may result in significant variations in flow with Col STACK ACTION ONLY WITH Neutral zone at mid-height Fig. 3 ...Distribution, of Inside and Outside Pressures Over the Height of a Building