Understanding VAV Control of Cabinet Fume Hoods and Resulting Average Face Velocity Effect

 

This blog (and future blogs from this author) is intended to bring forth discussion regarding field experiences in the design, specification and commissioning of Critical Airflow Control Environments. I have been involved in the Air Movement industry in one capacity or another for over thirty years.  I have plenty of stories to share with you and I hope along the way you may be able to contribute your own experiences so that we can all gain a firmer grasp on items of discussion such as this one I will address today.

 

Being involved in Fume Hood Performance verification can be an enlightening experience – as some of you may have already found.  The anticipated performance versus what actually happens can be somewhat frustrating at times, and other times can be very satisfying. 

 

So what makes the difference between the performance that results in what the design and specifications intend and what we end up with?  To answer that we need to understand the various factors that our route from Alpha to Omega.  This can be said for any evaluation in the field, however, for Fume Hood Cabinetry we need to understand the nature of the beast. 

 

Specifications, Codes and Governing Bodies that guide the design and performance for Fume Hoods in general are wide and varied.  My experience reveals that some are very simple so as to cover a wide variety of Hoods, while others are quite detailed.  These factors in my opinion are some of the prime factors that create a “grey” area when attempting to get fume hood performance criteria to match other less complicated system components tested in a system.  It’s not that the written intent is wrong, but it is in my opinion that we require some sensibility in applying the intent with what we are trying to do in the field for the owner.

 

“Air is a compressible fluid.”  Hmmm, I have had the opportunity of working with a good Engineering Consultant who can at times be very expressive with simple statements – that was one of his.  Those five words yet can shed a light on plenty of “unexplained” field verification when trying to meet specifications. 

 

The owner/consultant have their design constraints, the fume hood cabinet manufacturer has its specifications to meet with regards to construction and “As-Built” verification, the sheet metal contractor has his job to do with the ductwork layout in the field, the mechanical firm has their task in getting the Mechanical Equipment to perform as per specification, and finally the actual physical location of the fume hood cabinet within the space it occupies. 

 

All these components of the field installation will in combination have a corrosive effect in being able to achieve repeated performance for the fume hoods which in all appearances on paper should be the same.  In fact this rarely occurs.When aligned properly the aforementioned criteria will produce more repeatable performance, but there are other factors that need to be considered. 

 

Fume Hood average face velocity performance requires great attention to detail when field testing.  The proper average face velocity is based upon a traverse being conducted with time as a major component.  If we were to view the velocity of one of the point locations over a period of time then you would rarely see a stable, constant velocity. This is the inherent nature of air itself. 

 

Consider what we are trying to do here and the path that “piece of air” is taking – we are measuring a directionally non-linear, compressible fluid moving from a large open space (relative to the sash open area that is) with a fairly abrupt entry, then into a relatively smaller open area with semi-turbulent flow characteristics (hood “roll”), then accelerating through thinner points of the hood chamber (baffles) to collect in a slightly larger area behind the baffle, then meet again with other “pieces of air” at the hood duct connection at a greatly increase velocity again, then through a volume control device which may be moving to maintain the constant volume of air as the remaining components of the exhaust system are changing (be that fan speed or manifold duct system). 

 

Whew!! Now imagine that “piece of air” is your car with you in it driving down the highway during peak traffic periods. You might say “that’s not fair, that’s not what we are really dealing with in our well-designed systems”.  I have to apologize for using that analogy but I am afraid it helps draw a clear picture of what is happening in relatively simple terms – it mimics a traffic jam where we wonder why we keep stopping and starting – can’t we all just drive a one speed???? Flow is flow right??       

  

So Dave, what’s your point.  My point is that we need to have some recognition of field-performance criteria when we use terms such as average face velocity as a measurement in a pass/fail test.  I consult with fume hood manufacturers regularly and their “opinion” is similar – has been so and remains so.  The true test of a hood performance is capture and containment.  In fact, quite often the capture face velocity of a hood should reflect the type of materials being used within the hood. 

 

For instance, there are times when a reduced face velocity is acceptable and other times when it definitely is not.  The ability of the hood interior being able to contain the effluent being exhausted might be volume-related instead.  Other times, the materials inside the hood will require a large capture velocity to ensure the material does not just ‘gather or pool’ at the cabinet base.
 

I have made statements covering a lot of points that do have testing and numbers to back them up.  The purpose of this blog is to promote discussion. So please I do invite you to respond if you wish, positively, negatively, to add more, complain, whatever.

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