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CFM Control for Fume Hoods (w/o VAV)?

CFM Control for Fume Hoods (w/o VAV)?

CFM Control for Fume Hoods (w/o VAV)?

(OP)
I have several fume hoods connected to constant speed Backward Inclined/ Backward Curved blower fans. The fan motors run at constant speed (no VFD) and there are no VAV dampers or static pressure controls or monitors in the system.

I don't have the ability to switch OFF the fume hoods when they are not in use; So, in order to reduce the CFM of CONDITIONED air exhausted from the rooms, I'd like to just close the fume hood sashes when the hoods are not being used (AND seal over any bypass dampers).

I realize that BI fans are non-overloading, so I predict that as the sashes are closed, the system static pressure will INCREASE and create a new system curve, resulting in REDUCED flow rate, which should also reduce the HP power draw on the fan motor. Right?

However, here's my question: can I expect any long term ill effects from running the fan in this HIGH STATIC, LOW FLOW condition??

I suspect, given the BI Fan curve, at some point when the flow is too low, the fan will "hunt" or "surge" and this will cause an intermittent load/unload on the fan structure (wheel, shaft, etc), plus make a lot of noise and vibration.

Has anyone had any practical/actual experience with doing this??

RE: CFM Control for Fume Hoods (w/o VAV)?

many problems with what you're considering.

Blocking the bypass will negatively effect the performance of the fume hood. Whatever you do ASHRAE-110 test the fume hood and make sure any changes you make don't bring adverse risk to your workers. Follow the EPA's goal of testing the FH with the sash fully open (the abuse condition by ASHRAE-110) so you provide a FH people of any height/stature can use (ergonomic compliance)

make sure you never reduce the exhaust to below the ANSI/NFPA chapter 45 limits or 25 cfm/sq-ft of internal work surface?

Do your people work with solvents or organic chemistry inside the fume hood? If yes then you need horizontal panels (slide left/right) rather than up/down. This is the only OSHA approved barrier for explosion/blast and splash protection.

There is a company that field converts fume hoods to high performance and allows your fume hoods to operate better at lower CFM's 24/7 while providing total compliance with all user codes/standards.

feel free to email me if you want information on these conversion kits.  

RE: CFM Control for Fume Hoods (w/o VAV)?

I would say what you plan looks dangerous. The whole concept of fume hoods is that you maintain a constant face velocity. By blocking the bypasses the result will be changing face velocities in one fume hood when the sash on another is raised or lowered. You really need to stick with what you have, go for a fully engineered variable speed system or introduce fresh air (either untempered or tempered rather than conditioned) adjacent to the hoods. If you use a plastic crossflow heat exchanger (O% leakage) on the exhaust to temper your new local supply you will save on those conditioning bill but you will need to look at the air balance for the rooms as a whole.

Peter

RE: CFM Control for Fume Hoods (w/o VAV)?

^^ be very careful and research whatever information you are given--since you become legally responsible for the health/safety of everyone that works in the FH.

Myths like what are posted above are dangerous but their simple solution is too alluring for many to resist. Fume Hoods have never been about face velocity but containment. FV is a myth and that can be confirmed when ANSI/AIHA-Z9.5-2003 states "face velocity is not a measure of safety or containment". Pretty simple statement that is totally missed by every engineer that supposedly knows labs and offers a VAV solution. The fact that other ANSI standards rely on FV doesn't make it right

Remember, 100 fpm = 0.000623" WC and as such virtually every movement, hot plate, wind direction/intensity, etc, etc affects fume hood containment. FV controls ignore all these dynamics and should be scrutinized

RE: CFM Control for Fume Hoods (w/o VAV)?

Unless you have some way of also controlling the supply, I wouldn't recommend changing the exchuast too quickly.

Unless you want to own rebalance of all the hoods on the plenum, and make sure you maintain the NFPA 45 requirement for maintaining relative negative flow/pressure at the labs (and maybe trigger off other requirements from the NFPA 45), then I would recommend just leaving the sash shut and don't tough the bypass.

If you are going to retire a fume hood, you should physically remove it from hard ducting, notify the IH and clearly label or remove the FH (because people will try to stick things in it). Then you might be expected to rebalance all the exhaust on the same plenum and match supply, then have the IH do the FV certification of each hood as per Z9.5.
 

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