# Airflow into exhaust opening for dust extract

When air enters a duct through a hood having any shape other than that of a perfect bellmouth, a vena contracta is formed (see Figure 3.41).

This is a point where unwanted air velocity is attained i. e. veloc­ity above that needed in the duct to carry away the dust.

 Coefficient of entry

 Figure 3.45 Hood losses

 Pl =

 Ce2Ps

 Or as a percentage of the velocity pressure in the duct.

 Equ 3.37

Q

But — is the velocity in the duct after the vena contracta and

Equals 1.29^/p^ at normal temperature and barometric pres­sure where pv is the velocity pressure in the duct.

So,

1 x129/[T — — ■&-

X1.29VPv — i_29 /z-

1-29VpT

Or

Equ 3.36

With

Pv = mean velocity pressure in duct after vena

Contracta (Pa)

Ps = static side hole depression taken in position

Specified to be clear of the vena contracta

Many tests have indicated that on a given extract opening, the value of Ce increases with velocity, indicating some Reynolds Number dependence.

Posted in Fans Ventilation A Practical Guide