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# Avg.hood face velocity from duct velocity

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## Avg.hood face velocity from duct velocity

(OP)
I was asked to check the average face velocity of a canopy hood. It's located in a very user unfriendly location, but I can measure the duct velocity down stream 5' from the hood with easy. Duct dia is 4", average vel. is 921 ft/min and the hood measures 48" x 54". I beleive thats 80 cubic ft/min. Can I get a fair assessment of the average face velocity from these measurements or will I have to learn to rappel. Thank you.

### RE: Avg.hood face velocity from duct velocity

Yes, all you need to calculate the average face velocity is the flow rate Q, and the face area of the hood. 921 fpm in a 4" diameter duct ~ 80 cfm.

Hood area = 18 ft^2

Hood face velocity = Q / A
= 80 / 18
= 4.5 fpm

This is VERY VERY low.... are you sure you are accurately measuring the velocity in the duct? Are your units correct?

### RE: Avg.hood face velocity from duct velocity

(OP)
Engineer6512, Thanks for the reply and the readings are correct (unfortunately). The system is an old packout bagfilter used to take the dust off of product during the bulk loading process and to keep the dust from excaping the opening between the hood and product container. As you can guess it isn't working. Our company standard is quit a bit higher than 4.5 fpm for box filling. I believe I have the data I need now. Thanks

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