Low capture velocity @ lab hood

[edmenold]

Lab hood has had a low capture velocity for years.
Client wants to correct situation.

sash ht-12"
hood-42"wide

Motor is 115v/230v
amps 8.8

direct drive utility set dedicated system to this hood.

average measured FPM is 35 FPM
approx. 6'-0" of ductwork

I believe motor/fan set up is wrong, any comments or advise?

 

[trashcanman]

Usual capture velocity is about 100ft/min., but also depends on what type fumes/gas you are exhausting. Industrial Ventilation Handbook is a very good guide. It also is helpful in determining the sp losses of the hood. From your dimensions, nominal cfm needed = 3500. No way to determine actual CFM (@ 35 fpm) unless you have duct size. Motor Hp is about 1.

 

[quark]

You have to tripple (ideally) the velocity across the sash. 6' is too small to create significant pressure drop. However, your power data is not in line with what it has to be. Did you actually check the ampearage or reading from the nameplate?

Check the following things

1. Condition of the impeller
2. Condition of duct work (leakage into the duct).
3. Position of damper (don't go by the outside marking)
4. Take actual power readings and compare with the theoretical formula BHP = cfmxTP/(6356xeff)

 

[daly1]

don't fall for the 100-fpm myth. Look into the OSHA final ruling on Chemical Exposures in labs (1/31/00) to realize there is no OSHA requirement for 100-minute (or any other face velocity). Provide a fume hood that provides the required user protections from explosion (blast), splash, and vapors. Designs using reduced vertical sash heights should be avoided--since they provide various protections depending solely on the height of the user (definition of ergnonomic non-compliance for ADA compliance).

Always test the fume hood using the ASHRAE-110 test and include dynamic challenges, sash movement, and thermal loading. Look into what the EPA is doing and copy their PPT testing (full sash open, rigorous test) protocols.

Avoid risk to the owner and yourself by researching the above issues and don't simply keep on defaulting to failed solutions.

 

[mauricestoker]

I would first guess that you're fan is not working. At an opening of 3.5 SF and 35 fpm, you could get that flow from a freewheeling fan.
If you are considering only six feet of duct, then the next thing I would ask is why a fume hood is needed at all? If there is an industrial hygienist available, which should be easy to tell, as the sticker on the hood should have their name or initials, then contact them and ask why a failed fume hood is in use. The sticker should be updated annually, so the person responsible should be around. Before going further, talk to the IH responsible for annual cert; if no such person exists, then don't mess with it. Being it's an in-line fan on a fume hood, it might be good to talk to the maintenance mechanic (after reading the MSDS flle that is required to be maintained for the hood by law, if requirement for the hood exists).
Sounds like someone got chumped, as I've seen before, with an in-line fan. If whomever was there before and put an in-line fan and could not get 100 fpm on a six foot run, why buy ownership now?

 

[VibeAC]

Any chance the fan is rotating in the wrong direction?

Andrew Gorton, MSc
Noise & Vibration Consultant

http://www.PapadimosGroup.com