Book - School of Science and Technology

456 Ductwork designBy these means, the entire system will be inherently balanced and the need for dampersin upstream ducts either much reduced or, ideally, avoided altogether. Ductwork for highvelocity systems is almost always constructed with spiral-wound circular or flat-ovalsections except at plant and final connections to terminals.Computer design ofduct systemsIt is now quite common practice for ductwork sizing calculations, other than those forsimple distribution networks where an equal pressure drop approach is quite adequate, tobe made using computer programs. These are particularly useful when the static regainmethod is used since this is very tedious to apply manually. An example output is shownin Figure 16.18.Fan pressure and fan dutyFrom the various descriptions of volume flow and pressure loss above it will be obviousthat the duct sizing operation is but one part of an exercise to determine the total and theNodes SizingNear / Far limits10/ 110110/ 120120/ 121121/ 125125/ 126FX ////MD/300Duct Size(mm)1000/ 800600/ 600400/ 400740/ 250605/ 200ShapecodeRRRFFDuctlength(m)19.204.300.203.002.00Diversity Flowrate(l/s)0.800 10960.1.000 3250.1.000 1050.1.000 1050.1.000 450.Velocity(m/s)13.679.026.566.124.00TotalK-Factor0.531.231.080.031.12Pressuregradient(Pa/m)1.8291.3191.1731.0600.621Pressure loss induct / damper(Pa) (Pa)171./ 0.67./ 0.29./ 7.4./ 0.37./ 5.TIndexpath135/ 137130/ 140210/ 220220/ 221221/ 225FD / 300VL / 3.0//MW/1000955/ 300600/ 600800/ 800600/ 600835/ 500FRRRF2.004.254.300.203.001.0001.0001.0001.0001.000750.1000.4050.1500.1500.2.812.786.324.174.121.384.191.661.390.000.1910.1440.4750.3050.27737./70./42./15./1./0.T0.T12.14.0.335/ 336335/ 337330/ 340110/ 210210/ 310///SM//735/ 400735/ 400600/ 6001000/ 800800/ 800FFRRR2.002.004.258.9011.101.0001.0001.0000.8001.000900.900.1200.8360.6400.3.473.473.3310.449.991.131.132.610.280.100.2520.2520.2011.0871.13649./49./69./28./19./0.0.0.0.0.TTT430/ 440440/ 450450/ 460430/ 435440/ 445450/ 455///VL / 3.0VL / 3.0VL / 3.0400355315355355355IIIIII2.102.103.402.102.102.101.0001.0001.0001.0001.0001.000850.600.350.250.250.250.6.776.064.492.532.532.530.010.061.265.085.084.781.2831.2070.7960.2370.2370.2373./4./18./20./20./19./0.0.1.6.3.0.TTTT$$$$ fan requirements $$ $$Fan total pressureFan static pressureTotal flowrateAir powerOperating conditions319.9 (Pa)205.9 (Pa)10960.0 (l/s)3505.9 (W)Standard conditions314.8 (Pa)202.7 (Pa)10960.0 (l/s)3450.1 (W)The following TM-8 defined margins are added to give design values10.0 percent on flow for leakage and balancing10.0 percent on pressure for flow margin10.0 percent on pressure for calculation uncertaintiesDesign duty at standard conditions –12056. l/s at 378. Pa total pressureFigure 16.18 Sample computer output for duct sizing problem