How do you determine CFM through ventahoods?

[wormy]

I dont' work with vent a hoods much and would like to know more about them.
I know there's not alot to them, but I figure the more I know the better tech I'll be.

DOes anyone know where I can find info about checking the CFM through a vent a hood system?

I'm wanting to know how to check CFM so I dont' have a major positive/negative pressure on building.
Especially if I question wether someone has tinkered with split sheaves on the blower motors pulley.

[gmcd]

My normal rule of thumb is to take the exposed perimeter length and multiply by 100CFM/foot. Example: a cooking line hood with a wall behind it, open on the two ends and front face: 2.5 feet plus 2.5 feet plus 6 feet long = 11 feet x 100CFM = 1100 cfm minimum. Depends on the type of thing being exhausted- some folks will use 125 cfm per foot to be safe. Normally the hood supplier should have minimum CFM ratings on it for compliance to NFPA-96 and UL.

[air1]

If you know the rpm and static pressure you can use the fan curve to estimate cfm.