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Specific Operations 10-67 <br /> <br />in push-pull ventilation, a nozzle pushes a jet of air across the vessel surface into an exhaust hood. Effectiveness <br />of a push jet is a function of its momentum which can be related to thc product of the nozzle supply air flow (Qj) <br />and the nozzle exit velocity (VI). For a jet used for plating tanks or other open surface vessels, a push supply flow <br />can be determined from: <br /> <br />Qj ~ 243 ~ <br /> <br />Where: Q.! = push nozzle supply, cfm per foot of push nozzle plenum length <br /> Aj push nozzle exit area, ft2 per foot of push nozzle plenum length <br /> <br />Using this approach, a push nozzle design is first selected and the nozzle exit area (Aj) determk~ed. <br /> <br />Thc push nozzle plenum may be round, rectangular, or square in cross-section. The push nozzle may be a 1/8-inch <br />to 1/4-inch horizontal slot or 1/4-inch diameter drilled holes on 3 and 8 diameter spacing. <br /> <br />It is important that thc air flow from the nozzle be evenly distributed along the length of the supply plenum. To <br />achieve this, the total nozzle exit area should not exceed 33% of the plenum cross-sectional area. Multiple supply <br />plenum inlets should be used where practical. <br /> <br />Tile push nozzle should be located as near the vessel edge as possible to minimize the height above the liquid <br />surface. The nozzle axis can be angled down a maximum of 20° to permit thc jet to cigar obstructions and to <br />maintain the jet at the vessel surface. It is essential any opening between the nozzle and tank be sealed. <br /> <br />An exhaust flow of 75 cfnrqft:~ of vessel area will adequately captare and remove the push jet.(141'142) The exhaust <br />hood opening (ix) should be 0.14 times the distance from the push nozzle to the hood (0.14 W). If multiple slots <br />are used, they should be located within the 0.14 W height. A flanged hood design is to be used where ever practical. <br />The exhaust hood should be located at the vessel edge so as not to leave a gap between the hood and vessel. <br /> <br />D~slgn and location of an open surface vessel encompasses a number of variables. In some cases, vessel shape, <br />room location, cross drafts, etc., may create conditions requiring adjustment of the push and/or pull flow rates in <br />order to achieve effective control. Cross-draft velocities over 75 fi/rain or very wide vessels (8 feet or more) may <br />require increased push and/or pull flows. To account for the effects of these variables, a + 20% flow adjustment <br />should bc designed iuto both the push and the pull flow systems where ever practical. Once designed and installed, <br />push-pull systems can be initially evaluated by use of a visual tracer technique. <br /> <br />AMERCAN CONFERENCE <br />OF GOVERNMENTAL <br />iN'I)[ S IRIAL t-tYGIENISTS <br /> <br />DATE <br /> <br /> PUSH-PULL HOOD <br /> DESIGN DA TA <br /> <br />1-88 [mu g VS,-504.1 <br /> <br /> <br />