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How does one size a line which is connected to a laboratory fume hood at one end and a fan/blower at the other. The fan/blower is discharging to atmosphere.

Flow is 250-300 cfm.
Duct length is around 30 m.
Is there a velocity/pressure drop criteria ?
I have come up with a line size of 6" considering a velocity of 15-20 m/s and

This particular fan/blower can develop a head of 50 mm WC. If it is supposed to discharge to atmosphere, then pressure drop in inlet line should be <= 50 mm WC.

What could cause it to deliver lower than desired capacity ? Is shape of the mouth of the hood important ?

[akslzf]

Among other reasons, lower than 'desired' capacity can be because of
1) higher dP - A discharge velocity of 5m/s will be more appropriate
2) positioning of suction hood - is the fume hood much above where the fume is generated?
3) wet bulb temp - What's the actual density of gas and what's the assumed density?