Book - School of Science and Technology

438 Ductwork designthe duct with no disturbance to the established velocity profile across the section and thatexcessive turbulance is avoided. The principles to be followed in the design of such ducts are:. Avoidance of sudden restrictions or enlargements, or any arrangement producingabrupt changes of velocity.. Bends to be kept to a minimum, but where required should be radiused, not mitred.Deflectors may be fitted to improve the air flow characteristics.. Where branches occur, they should be taken off at a gradual angle to avoid abruptturning.. Sharp edges should be avoided, as these will be the cause of noise which may travel aconsiderable distance through the duct system.. Rectangular ducts should be as nearly as possible square: the more they depart therefromthe more uneconomic they become.. The ductwork configuration must allow the air flow to be regulated without the needfor excessive throttling, which will lead to noise being generated.These principles apply to all ductwork classifications but are clearly more important thehigher the air velocity.Ductwork classificationThe current classification for ductwork is given in an HVCA specification* according tothe operating pressure, air velocity and leakage rate. These are summarised in Table 16.1.The air leakage rates quoted are satisfactory for all general ventilation and air-conditioningapplications, but where hazardous or obnoxious substances are handled, the ducting mustbe completely air-tight. Specially designed and constructed ductwork is required for pressureshigher than those stated.Galvanised steel ductsThe gauges of metal and form of construction commonly used are set out in the HVCAspecification* for all of the classifications.Ducts of galvanised sheet steel are often priced by weight. Sheet metal is made in metricthickness and the mass of metal will thus merely be thickness times area times 7800 kg/m 3(for steel) or whatever other specific mass is appropriate. One of the aims when designingTable 16.1 Ductwork classificationsDuctpressureclassStatic pressure limitPositive (Pa)Negative (Pa)Mean airvelocity(max.)(m/s)Air leakage limit a(litre/s m 2 ductsurface area)Low 500 500 10 Class A 0:027 p 0:65Medium 1000 750 20 Class B 0:009 p 0:65High 2000 750 40 Class C 0:003 p 0:652500 750 40 Class D 0:001 p 0:65a p, differential pressure (Pa).* Heating and Ventilating Contractors Association, Specification DW/144.