Document 2jLKdv5j0gdwL47wLxjM853Yp

330 W. N. WITHERIDGE air flow. This requires ingenuity in designing enclosures that will not interfere with'ipr1 or hoods with movable sections that can and will be operated as planned. There, is?* creating an "ideal" hood that cannot be used without seriously disrupting tliVjpr schedule, inasmuch as neither worker nor employer will tolerate its use. " Determine Air-Flow Rates tor Each Exhaust Hood 3. Estimate the control or "capture" air velocity required for each operationtiififli hood or enclosure. Tables 8, 9, and 10 suggest control velocities. This part of the. des'ipi?'involves a great deal of judgment. 4. Compute the boundary area of the plane, cylinder, or sphere of influemjewhich the selected "capture" velocity must operate. The ability to conceive hood,;arra in three dimensions is an advantage. 5. Compute the total air flow in c.f.m. for each hood (capture velocity; tifrieT influence) and put this rate on the sketch or diagram of the system for reference .ii(i, 6. Summarize the air flow for each section of the branch and main dudjworlJ form or preferred calculation sheet, or enter the air flows directly upon the diagram-- Establish Air Velocities tor Each Duct Section 7. Select the minimum effective transport velocity for particulate matter (j For transporting of gases and vapors, duct velocities can be as low as is consistentwr and weight limitations. Velocities in the range of 1000 to 2000 li.m. for gases andSi) the usual compromise between excessive duct size and unnecessarily high Jair-lrOr r equirements. ;< ? 'jdf 8. Figure the nearest practical duct diameter to carry the required ci.m.._atHtk velocity. Consideration should be given to the sheet metal fabricator's standard/" ments to avoid unnecessary additional expense. If the available duct sizes are'mii from the theoretically required diameters, compute the actual transport velocity,1, subsequent design steps. _.... ..... ... .... ... _........................... ................................ . Determine the Entrance or Orifice Loss jor Each Hood . 9. Compute the velocity pressures corresponding to the air velocities in eachjjr connected to an exhaust opening (see page 337). 10. Estimate the orifice loss for each hood in percentage of velocity preasure^m pipe (see Figure 19). 11. Compute from items 9 and 10 the entrance losses in terms of inches 65 wate Determine Duct Friction and Transition Losses 12. Compute the losses in elbows, branch to main connections, or other "transBipnl (a) in terms of equivalent straight-pipe diameters (Figures 20 and 21) or'>(^jli*m.. velocity pressures, whichever form corresponds with the data at hand. The equivalent pipe method has the advantage that such transition losses can be addedjdiiieij&l liTM!gM:EiEMSliaBS-JarauaJS'ith-riation_char,ts,,T.he-velocity-head-mt|th,d^^^tbi by those who are .especially familiar with the fundamentals of fluid 13. Obtain the duct friction losses from a chart, table, or formula/^iucneyer^ycju, to use. Figure 22 is used at this.point. A*||:' Balance the Pressure Drop by Adjusting Duct Sizes 14. "Summarize" the losses for each branch to determine where to increase duct sizes to obtain better balance of pressure drop. (This assumes that balanrinffwf] left exclusively to manually operated dampers.) The total pressure drop from :nnytiljm, system to each upstream opening or exhaust hood should be the same) `irBalaiftejS. aohieved. This is so because in actual operation, the air-flow quantities 'thro'ilgHj branches automatically adjust themselves to fulfill this requirement of balancVJ''Ti VENTILATION 331 TABLE 15 Air Velocities for Transporting Various Dusty Materials (Linear feet per minute) il ig powder l' l jjf. i)f i'" jgfl' j vi - ''/.-in, screen) v Dust collecting 4500 30_00 2500 ' 6000 3500 -- 3500 3000 4000 3500 3500 3500 3000 2500 2500 2500 3500 2500 4000 3500 5000 5000 4000 4000 4000 3_500 30_00 30_00 3500 5000 3500 5000 5000 3500 3000 2500 4000 3000 3500 4500 4000 4500 3500 3000 5000 4000 4000 6000 6000 Pneumatic conveying -- -- 6000 -- -- 8500 6000 -- -- 6000 -- 4000 -- 3500 3500 5000 4000 4000 4000 -- -- 6000 '-- 4500 4500 4500 3500 6500 -- -- 9000 -- 5500 -- 5000 5500 7000 6000 6000 6000 -- -- 8000 -- 6000 '6000 6500 5000 5000 5000 4500 4500 7000 8000 6000 7000 2500 3500 2500 3000 3000 3000 4000 6000 5500 6000 4000 7000 7500 9000 6000