Fume Hood Fan Sizing

[emalsyd]

I was called by a customer. They had someone hook up their conventional fume hood and it isn't passing certification. The certifier said he needs to get 500 cfm when the sash is open 18" and 350 cfm when the sash is at 12". Exhaust duct is 10" round. Trying to find out how to size a new fan to get these sash values. The existing fan is rated at 500 cfm @ .5 wc. At 18" sash opening they are getting around 240 cfm now.

 

[DRWeig]

It's a squared thing. If you want to double the flow, you need to quadruple the pressure. That doesn't mean you run out and buy a fan that'll do 500 CFM at 2" WC, because you don't know for sure what the differential pressure across the fan is right now. It has ridden its fan curve back.

One of the more experienced MEs may tell me I'm wrong, so wait around.

1) Measure the differential pressure across the fan with your sash open 18". Then verify that you're still getting 240 CFM thru the hood. You'll need a fan that can deliver 500 CFM at 4X the differential pressure you measured.

OR,

2) Do what I'd do. This is a tiny fan. Buy a big honker (in comparison) and a variable speed control. Set the speed that'll give you what the customer needs.

Best to you,

Goober Dave

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[KiwiMace]

The hood vendor should provide the pressure drop/requirements for the hood, and you need to calculate the rest of the system from make-up through to exhaust. Use the 4 x rule as a check - any filters in the system are often allowed at a fixed final (loaded) dp, which does not count into the square calc. Guessing small fans like this can often lead to unstable operation.

 

[chicopee]

The analysis should entail graphing the fan characteristic and the system performance curves. You may be able to increase the fan rpm, or increase the size of the exhaust duct but again you need to do the analysis mentioned in my first sentence.