Document GKymDLdQjwJoBybG0QgLnKLZx

nmmmmmmrn ..>'7; .American Society dJ Heating and Ventilating Engineers G. uide '-'Ifa though the wind velocity be imperceptible, gravity will effect a \ 'lli in the air in a room, if the difference between the temperature ofth indoors and that out-of-doors is great enough that the warmer 1' air indoors rises and escapes and is replaced by cooler, heavier a; i vll outdoors. r ronif In poorly constructed buildings, without means for agitating the ^ warm air accumulates in spaces near the ceiling, from which point heaui ________________ I 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) \ escapes either with the air that filters out through, the walls and ceiling and through cracks around the tops of windows and doors, or by direct transmission through the walls and roof resulting from difference between the indoor and outdoor temperatures. The volume of air actually lost by exfiltration is normally replacedby cooler, heavier air which tends to stratify in the room at its temperature level, usually near the floor. In this way, regardless of wind velocity or direction, air is constantly being changed whenever indoor and outdoor temperatures are different. The rate of change is dependent. upon the tightness of construction and the temperature difference. This manner of air change, however, could hardly be dignified as a system of ventilation, which term system implies something more than a haphazard result., 420 -Chapter 27-Ventilation Systems 'OBgg.v. i1operates to produce regions of pressure or suction about the lE?.e W f building as compared with the conditions that would exist in gjeHor* a , t^e |jr Were still. The principal effects of the wind when l^^ah-eetly against one face of a building are illustrated in Fig.T. The fiS^lmtotenaty pressure at point C is approximately given by " l^ati0n' P,,. - 0.0004814* (1) ^ Ife jg pressure in inches of water, and M is velocity of wind in miles The mean pressure over the windward face is somewhat less, flgli^Whaps, 75 per cent of the maximum. max;muni suction occurs at point B at roof and sides, and in '`tude is about three-quarters of the maximum pressure as given by (i) The distance A, measuring the extent of the suction regions g*9'sidewalls and roof, caused by the leap, or splash, of the wind from the Wflbiward face, seems to be determined by the height and width of the ^^ndward face, and is independent of the wind velocity. The intensity suction diminishes from the maximum at B to zero at the point ilwiiere the stream lines again flow parallel to the surfaces of the building. P?ijSaxitnum effect of the wind in producing ventilation can be obtained providing openings in the windward face for ingress of air and in the fe feward face, or in sidewalls and roof near the leeward face, for egress of in which case the air moves through the building in the same general ^direction as the wind; or, with no openings in the windward face, but with rp&sgnuigs provided in the suction regions in sidewalls and roof near the fe%ifidward face for egress of air, and openings in the leeward end, or in KpCstdewalls near the leeward end for ingress of air, in which case the air .'^movement within the building is in a direction opposite to that of the i'^ wind. A knowledge of these facts and their application will often result 0*$i&better layout of operations and processes, and pven in a more favor^?|ibie orientation of a building with respect to the prevailing direction of die wind, so that heat and objectionable fumes may be removed more ^^advantageously from an industrial building. 0rpg/fhtn! the wind comes at. an angle to the building, the same general Ply;rules apply. The mean, pressure of the wind on the oblique faces may be |-|i;'Cpnsidered as being approximately equivalent to the normal component 'of the wind, except that when the wind's direction is within 15 or 20 deg. .-^parallelism, the splash from the corner begins to appear, and there will ; j.then be suction near the windward comer. Temperature Difference Excess temperature inside a building over that outside tends to produce ; flow as in a chimney. The total head or force arising from this cause is f given approximately by equation (2). ' .V .. pi = O.OMOBSmD (2) where px is the total force or head in inches of water; m is the vertical See Airaiim o/ Industrial Buildinv. by W. C. Randall (A.S.K.V.E. Transactions, VoI. 34, 1928). 421