Blowing systems for H & V

Design schemes Round piping

1. Make a line diagram, or isometric of proposed run of ducts with all branches and outlets shown. On this diagram mark the volumes of air to be delivered by each outlet and the totals to all branches from main duct.

2. For general industrial schemes the piping is sized on the basis of 1.6 Pa/m. In very extensive layouts i. e. with distri­bution ducts up to 120 m -150 m long, it may be increased to around 3.3 Pa/m.

Initial velocity in the duct system will vary from 10 to 11.5 m/s in relatively small layouts, up to 18.5 to 21.5 m/s in extensive in­dustrial systems.

It is important to note that when the system is for a public build­ing, such velocities cannot be used because of air noise in ducts and in the noise generated by the fan. When quietness is essential the maximum air speed in ducts should be kept be­tween 6 to 8.5 m/s, and for less important cases it may be 8.5 to

11.5 m/s.

General: Subdivide the main duct and branches by tapering down after air outlets with reasonable compromise. Included angle of tapers between 2.5° and 10°.

Too many tapers should be avoided, and with small “pops” (say 150 mm dia.) 3 may be taken on each section without reduction. With larger “pops” either a single outlet, or a pair, is usual prac­tice.

Note that the sizing of the ducts on the basis of construction friction per metre does not in itself ensure the flow of the calcu­lated volumes in the various branches. Balancing of resistance is necessary as described later. Size the duct to the nearest

3 Mm in smaller sizes to nearest 6 mm in larger sizes.

Rectangular piping

1. Make a line diagram of system with volumes indicated ex­actly as in scheme A.

2. Assess the sizes of ducts as round piping

3. Convert these round duct diameters into equivalent rect­angular by the Equivalent chart, in Table 3.6, which shows sizes for equal friction at equal volume. One side of rect­angular duct, such as the depth, is kept constant in many cases, or at least so far as is reasonable.

General: The size of the fan, and hence its discharge dimen­sions, are not known at this stage. The initial area of main duct is not necessarily equal to the fan discharge, but of course should never be less in area.

If a fan supplies a main duct which immediately branches into two directions, it is usual to come from the discharge in a rect­angular duct of same area. Then divide into two with each area proportional to the respective air volumes. Finally, transform from these on each side to the area decided in the duct layout assessment.

An adjustable splitter damper is desirable at the junction as flow from the fan discharge is generally uneven.

Posted in Fans Ventilation A Practical Guide


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