Document V3dK6b9eqy2opGy9kqjKy98nK

304 W. N. WITHERIDGE of required air-flow rates cannot apply. Furthermore, it would not be if to increase the air-flow rate through such a local exhaust hood to brin general ventilation volumes because the suction power requirement"; exorbitant. General ventilation should be provided by low-velocity, ,,l| equipment. The warning, then, is not to overestimate the capability 6$ hoodas a general ventilating device. Special systems of multiple hoOd; simultaneously in small rooms have been observed in some cases to giy; of air change for the space. This condition is not a property of exhaust instead the result of greatly reduced air space per hood. 1. Exhausting vs. Blowing Figure 4 illustrates the fundamental difference between blowing;^] ing devices. Air blown from a small opening retains its directional effe'. siderable distance beyond the plane of the opening. However, if the if4 4000 LFM Approximately 10% of foco velocity ot olr velocity 30 dlI>TMt *oy -from pruturo Jit at foci of opening both openings BLOWING ExhOUSl Uuct^ .. :rrj. 0\ EXHAUSTING L Approximately IOX of foco voloclty at 400 LFM 0n* diom,t#r wy from exhaust opening Air flows from oil direction* toword an exhaust duct that has no flongs Figure 4. Air-flow characteristics of exhausting and blowing devices. Figure 5. Spherics^ flow into an unflangectanc exhaust hood. reversed so that the same opening (or its equivalent in size) now handles volume of air by exhaust, the flow becomes almost completely nondir|||_ ..its-range-of4nfluenee-is--greaWy-i>educedrT-W8^fprd;hiH'e'toif;fhffe|^fe hoods shouldnotybei'contemplated'foi^any procSiii'^^'dSi^Wr^U^^K^ immediate vicinity-oft the'Sood: ...... &-.,JIlallaXkillelsd2.quation.for~Un0bstTucted--Hoods" This equation is used to determine the relation between the quad flowing into an exhaust hood and the resulting air velocity at a distanttSj the hood centerline. The equation applies only when there is no obsti|| tween the hood opening and the control point in question (see Figures^ VENTILATION 305 Sftftffc'/ V' frm f the equation for both round and rectangular hood open- Q -- V (-- + A ) \b J hood opening in square feet, b = a constant that depends upon hood, Q = quantity of air flow in c.f.m., V = centerline air ve- feet from opening, and X -- distance in feet from hood open- b'i.-Sequation is used frequently with a value of 0.1 for constant 6, ^Pffi^r^^dlhapproximate formula suitable for round or essentially square 4i'jjMjpratant the equation is given in the convenient form: Q = :il)L 7 lists the constants and equations for hood openings of vari- kaes and Baffles Increase the Range of Exhaust Hoods liJKrMPWli u' OTates the beneficial effect of restricting the direction of air f_low qtan'Ji1 opening. The diagrams were constructed on the basis of a point IMS%hich. 0I" coursei does not occur in practice. Figure 7 shows the AW Range* 1.4 Hemisphere m -r> ilh , onge*2.0 1^;Quarter Sphere \\ ^^LL'fl;,fl;'l.inge3. and baffles increase the range of exhaust openings. FV streamlines for a rectangular hood and indicates the W'yi.iijip^5ffli':spherical contours at points close to the hood face. Figure Bt'^fl^ng.TdrRllspot exhaust" hood for arc welding. ftltfDail.i'ffiaih,.Exhaust Hoods. Industrial Press, New York, 1944. ,i SfSk