Book - School of Science and Technology

458 Ductwork designTable 16.7 Minimum thickness for ductwork insulationThermal conductivityofinsulation(W/m K)Insulation thickness (mm),for air-to-air temperaturedifference (K)10 25 500.04 38 50 630.055 50 50 750.07 50 75 75The gain or loss of temperature in air conveyed in a length of duct may for practicalpurposes be calculated from the following:H ˆ PLUtwhereH ˆ heat loss or gain (W)P ˆ perimeter of duct (m)L ˆ length of duct (m)U ˆ thermal transmittance of bare or insulated duct (W/m 2 K)t ˆ mean temperature differences between inside and outside of duct (K)The temperature rise or drop along the length of the duct, taking the specific heat capacityof air as unity, is approximately PLt/M where M is the mass of air in circulation (kg/s).This expression yields results which are sufficiently true for practical purposes, though notstrictly so when the difference in temperature along the duct is of such a magnitude as tobring the internal temperature closer and closer to that around it.Insulated ducts located outside a building must be provided with a commercialweather-proof finish such as bitumen coated roofing felt, with well sealed joints, securedwith galvanised wire netting, or some better and more permanent protection. Horizontalducts fitted externally should preferably be circular or, where they must be rectangular, beprovided with a sloping cover to prevent water lying on the upper surface.Sound controlThe increasing amount of mechanical plant in buildings brings with it the problem ofnoise. We here confine our attention to equipment such as fans, compressors, pumps andboilers, and the channels by which sound may be conveyed therefrom to other parts of thebuilding.ABFigure 16.19 Transmission of sound