Comparison of blowing and exhausting

It is important to realise the great difference in effect of distance from the opening when comparing blowing and exhausting. At

31 diameters from the opening, 10% of the initial velocity is still maintained when blowing, but the distance for this 10% velocity contour is only 0.8 diameters when exhausting, see Figure 3.40.

Vena contracta Figure 3.41 Formation of a vena contracta

The acceleration of the air to this excess velocity requires pres­sure and is shown as residual static pressure at the point where the airstream fills the duct at normal velocity. Normal duct ve­locity in average dust extract systems is from 16 to 23 m/s.

A common method of measurement in the USA is to drill one or more holes preferably as small as 1.6 mm diameter, free from burrs on the inside, at one duct diameter from the throat for all tapered entrances. For open ended or flanged ended open­ings, the hole is drilled at these duct diameters from the end. (See Figure 3.42.)

"N

Flanged

Press a rubber tube, connected to a pressure gauge, tightly against the hole and read the static depression.

Then

Q = 1.29^/p^ xAxCe

At normal temperatures and barometric pressures, where:

Q	= extract flowrate (m3/s)
Ps	= static depression (Pa)
A	= cross-sectional area of duct at point of mea­
	Surement (m2)
Ce	= coefficient of entry, which varies from 0.6 to
	0.98 in commercial work

C — Q e 1.29^/i% xA

1 : Q 1.29Jh~s X A

I /«" I —F — tLIe_____ ;;___ II___

LU ~

SrD

On area AiCt

Figure 3.40 Comparison of velocity contour distance when blowing and ex­hausting

Posted in Fans Ventilation A Practical Guide