Book - School of Science and Technology

Thermal insulation of ducts 457static pressure which must be developed by the connected fan. A summation of thevarious system losses, those through straight ductwork, through duct fittings and throughplant components, with reductions where available to take account of static pressureregain, will go to produce the static pressure against which a fan must operate.This, of course, is the net or calculated duty and a margin of 10% is usually added tothat pressure to allow for minor changes in the duct configuration during the buildingperiod, due to revised client requirements and architectural second thoughts. Similarly,margins should be added to the calculated volume of say 5% to cater for ductworkleakage, which is a function of ductwork pressure as noted on p. 438, plus another 5%as an allowance for contingencies in commissioning.Thermal insulation of ductsDucts conveying cooled air in an air-conditioning installation are usually insulated on theoutside so as to reduce temperature rise to a minimum, also to prevent surface condensationfrom the surrounding air. Such insulation must be vapour sealed, i.e. sealed againstingress of atmospheric air, otherwise the insulation will quickly become waterlogged anduseless. Ducts conveying warm air equally in most cases require insulation to reduce lossof heat. Where ducts carrying heated air are run in the conditioned space only, insulationmay be unnecessary.Transmission of heat through bare metal duct walls varies according to air velocity, theU value rising from 5:6 W/m 2 K at 2 m/s to 7:2 W/m 2 K at 15 m/s. The effect of insulationis to reduce the loss such that air velocity in the duct affects the U value only to a minorextent, and the coefficients given in Table 16.6 may be adopted as covering the loss at anypractical air speed. The economic thickness may be estimated for any particular case, andreference should be made to BS 5422.* For practical purposes, the minimum thickness ofinsulation on ductwork carrying warmed or cooled air is given in Table 16.7.Materials for duct insulation, internal and external to the duct, must be such as not tosupport fire. When tested to BS 476: Part 6: 1989 and Part 7: 1997, y the insulating materialmust have a fire propagation index of performance not exceeding 12, with not more than6 from the initial period. The surface flame spread, as defined in Part 7, must meet theFire Authorities' requirements. Some suitable materials are glass fibre, mineral wool andpolyurethane.Table 16.6 U values for insulated ductworkThermal conductivityofinsulation(W/m K)U value (W/m 2 K), for giveninsulation thickness (mm)38 50 750.04 0.9 0.7 0.50.055 1.2 1.0 0.70.07 1.5 1.2 0.8* BS 5422: 1990, Specification for the Use of Thermal Insulating Materials.yBS 476, Fire tests on Building Materials and Structures, Parts 6 and 7.