Book - School of Science and Technology

Pressure distribution in ducts 455whereV ˆ the chosen velocity (m/s)Q ˆ air volume leaving fan (m 3 /s)A ˆ duct area (m 2 )In a complex system, some attempt should be made to balance the pressure loss throughother duct runs with that of the index run but velocity and possible noise generation wouldhave to borne in mind if any variations from those given in Table 16.4 were extreme.Finally, when all circular duct sizes have been chosen, reference to Table 16.3 should bemade and square or rectangular shapes chosen to suit the space available. In carrying outthis last routine, it is as well to check that unnecessary minor reductions in shape or size arenot being made to suit relatively small changes in velocity, since to do so is uneconomic.It is sometimes advantageous, when sizing a long run of ducting with air supply grillesalong its full length, to take what is called static pressure regain into account. This is done byso reducing the velocity in the duct in stages that the regain in static pressure compensates forthe frictional loss in the duct up to the point of reduction. This method provides anapproximately equal pressure at each grille and this assists in balancing one with another.High velocity systemsWith the use of the higher duct velocities inherent in dual duct, induction unit andvariable air volume systems, etc., as in Table 16.5, the use of static pressure regain ismuch more important, the potential availability being higher. All the static pressure gaincalculated, theoretically, at the expanding duct section will not be available since the totalpressure there will be reduced due to turbulence and added wall friction. Thus:Static regain ˆ (p v1 p v0 ) p twherep v1p v0ˆ inlet velocity pressure at expanderˆ outlet velocity pressure at expanderp t ˆ reduction in total pressureThe reduction in total pressure depends upon the design of the expanding duct sectionand another way of stating the expression above is:Static regain ˆ f ( p v1 p v0 )wheref ˆ a factor representing the `efficiency' of the expanding duct section, say, 75%It is normal practice to use the availability of static pressure regain in sizing the mainducts only of high velocity systems since the branch ducts tend to be self-balancing asa result of the high pressure drop through the terminal fittings, be they double-duct,variable volume, or induction units. The routine adopted in sizing a duct system bythis method is first to consider the penultimate section of the main duct and so to selectthe size that the static pressure regain at the expander is equal to the pressure loss in thefinal section, repeating this procedure in regression, section by section, back to the plantoutlet.