Book - School of Science and Technology

454 Ductwork design. Any allowances included for commissioning, etc.. Headings as listed below.LocationsectionVolume EquivalentdiameterRectangularAirvelocityPressuredrop/mDuctlengthFitting VelocitypressurePressuredropDesign Revised sizeType (a) (b) (c) (d) (e) (f) (g) (h) (j) (k) (l) (m)Low velocity systemsThe most convenient method of sizing ducts in a low velocity system is by using the equalpressure loss basis. For this purpose, it is necessary to establish the maximum velocity inthe main duct leaving the fan from Table 16.4 and, knowing the air volume to be carried,locate the intersection point of the volume and velocity lines on the chart, Figure 16.16.From this point, a line followed vertically downwards will indicate the unit pressure loss,Pa/m, on the base scale. The main duct diameter may be read, at the same time, from thechart.If the unit pressure loss so found were to be considered acceptable, the remainder of theduct sections following on (or those in the most disadvantaged duct run, the index run, ina system having many such) would be sized using the same unit rate. However, if that ratewere thought to be too high or too low then another velocity might be chosen as thedatum using the expression:V ˆ Q/ATable 16.4 Maximum velocities for low pressure ducting systemsVelocity (m/s)ApplicationMain ductBranch ductDomestic 3 2Theatres, auditoria, studios 4 3Hotel bedrooms, conference halls, operating theatres 5 3Private offices, libraries, cinemas, hospital wards 6 4General offices, restaurants, department stores 7.5 5Cafeteria, supermarkets, machine rooms 9 6Factories, workshops 10±12 7.5Table 16.5 Maximum air velocities for mediumand high ducting pressure systemsAir quantity(litre/s)MediumVelocity (m/s)HighLess than 100 8 9100±500 9 11500±1500 11 15Over 1500 15 20