Fume Hood Face Velocity

[esanders77]

I have a client that has asked me to investigate problems with a fume hood in one of their labs. They are only able to reach an average of 46fpm across the face with the sash fully open.
I'm stuck. The airflow into the lab is good, the duct sizes are good, the exhaust fan (multiple hoods go to one fan) is of the proper size.
Any suggestions on what else I can check?

 

[mauricestoker]

Does the fume hood have an exhaust damper where it comes off the gang? Also, is this a VAV hood or bypass? By full open, do you mean 18", or a different opening height? Is the exhaust fan sized for all hoods running simultaneously? Is the face velocity assumed to be 100 fpm? Last, what does the IH sticker have lsited for the prior year? I'm guessing this is an older hood and not compliant with current NFPA 45.

 

[esanders77]

No there is no damper, it's full open, constant volume. The full open sash height is 24". According to the design documents, nameplate, and maintenance staff the fan is designed for all hoods to be run at full open simultaneously.
I didn't check the IH sticker, but I will....what info can I get from this?
The hood is about 10 years old.

 

[imok2]

When was the last time the exhaust fan was cleaned and serviced and is the face velocity ok with the other fume hoods?

 

[cdxx139]

The hoods I typically see are certified at the 100 fpm around 16" sash height. I believe the thinking is that keeping it at that height vs full height better protects the face of the scientist working.

I wouldn't take anybody's word on how it is designed. If you have the height and the fpm, you can get the cfm's of each of the hoods and compare against the fan. Old balancing reports would come in handy.

knowledge is power

 

[daly1]

"...full height better protects the face of the scientist working"

be very careful and read ANSI/NFPA-45 for yourself. OSHA requires you to provide adequate protection from vapors, explosion/blast,and splash. No where in any code or regulation does it state that a sash height of 16" or 18" provides an acceptable explosion/blast or splash control barrier.

The only approved explosion/blast and splash control barriers are horizontal sashes. 45 gives one the inmpression that there are optional barriers but there aren't. Then read what a General Purpose Chemical Fume Hood is and learn that solvents and reactive chemicals are used in every hood. Finally--read how 45 tells you there is a likelihood, not a probability, of an explosion/blast event over the life of the fume hood.

Ignore peple that trot out poorly reasoned thoughts like 16" sash height provides head or neck protection..for people of what height? How does a design that provides some "protection" for a 6' tall person--but provides no protections for someone 5'-1" tall. How does your design accommodate people of varying heights? Some people would call providing varying levels of safety that depend on the workers stature--illegal? Read the ADA.

also FV was abandoned by OSHA in 1991. make sure you do 110 testing while using a test to fail approach.

 

[mauricestoker]

The sash height needs to be set between design and acceptance criteria or you may as well start out by taking a crap in the fume hood. While height is recognized as an issue, so is width. If budget/schedule allows, and potential exposure is a concern (exposure above action level, failure to meet Z9.5 or ACGIH) I'd recommend including the IH/CIH for agreement on an IH accpetance testing protocol. The 110 will address standard ergonomics and position; in situ testing for probable exposure helps address variation in user ergonomics. Facotry cert for 110 should be included if replacing the hoods.

Fortunately, with existing labs, you can obtain the signature of the laboratory director on compatibility of usage, and quantity. The MSDS's would also be available under EPCRA.

Defining the criteria and scope of any repair to an older fume hood is just as important: if from one end of the exhaust to the other, is the discharge 10' above maintenance surface and at proper discharge velocity, does the fan need to be on emergency power, does the control system include purge mode, are there improperly installed fire dampers in the exhaust, are general lab exhaust rates included and acceptable, does the FH have cfm/fpm face indicator, is prior acceptance testing and annual testing indicated as per IH sticker, are relative differential pressure requirements met, would Z9.5 be used for acceptance, blah, blah, blah.