The Overlapping Sash Bypass System

 The Overlapping Sash Bypass System

Is it a Universal Bypass for all Fume Hood Exhaust Strategies?


Dr. Robert K. Haugen, Director of Product and Technology Development

Flow Sciences

2025 Mercantile Drive

Leland, North Carolina 28451

Over the last year, there has been a lot of discussion regarding fume hood bypasses.  A bypass is an open space usually located above the fume hood sash that allows an alternate path for air to enter the fume hood as the sash is closed.

As the figure 1 shows, the alternate air route that opens moderates the face velocity increase as the sash closes.

As routine as this definition seems, this cavity design has started many debates in the fume hood industry.  It is really the size and placement of this cavity that starts design conversations as fume hood extract technology has focused on saving exhaust volume and therefore hood energy costs.  (Exhaust energy can cost $5 to $17 per CFM per year!)

As HVAC needs have placed downward pressure on hood exhaust volume, a much smaller bypass using a smaller opening has become popular on reduced volume hoods.  This bypass design is usually employed on Variable Air Volume (VAV) and low constant volume (LCV) fumes hoods:

Such a hood uses less total volume as the sash is lowered and therefore does not need to moderate velocity increases, as the damper-modulated air flow does this.  Such a hood design works well with new VAV installations.

However what about EXISTING labs that wish to change from older constant volume systems to newer VAV systems?  In order to get the most out of a VAV system, volume must be reduced as the sash closes and this system therefore requires duct dampers and a hood bypass of reduced size.  This can be accomplished by altering the hood top front interior by adding some sort of chemically resistant panel across the bypass opening to reduce its size.  This procedure can be tedious and may require very involved safety steps if the hood interiors are contaminated.  Some university lab administrators are mindful of this and recommend that such hood bypass retrofits should not be done at all! 1

Because this bypass modification problem will only get worse as more and more existing constant volume systems are upgraded to VAV, Flow Sciences recommends a third type of bypass for all future fume hood installations, the overlapping sash bypass (OSB).

Here’s what an OSB bypass looks like at three different sash settings:

Note as this sash is raised, the air continues to flow through the bypass in a more and more twisted path, reducing the proportional air volume traveling through the bypass.  In a constant volume system, such a bypass steadily consumes a larger proportion of air as the sash is closed.  In a VAV system, the OSB allows reduced bypass throughput at lower sash openings quite successfully!

The OSB is the only bypass design that works well without modification of the hood interior when hoods are transitioned from constant volume to variable air volume exhaust. 

If constant volume OSB hoods are added to a facility to match a constant volume strategy, these hoods will work fine and can later be modified to VAV use without bypass alteration (great cost savings) when facility budgets allow a systemic VAV exhaust upgrade.

Here at Flow Sciences, we have just completed a comparison of full open, OSB, and a 3” restricted bypass design to see how all three systems operate both under Constant Volume and VAV systems.  The graphs reproduced below are for 80 FPM full open constant volume and 80 FPM VAV applications.  We have also run these tests at other face velocities with similar results.

The curious aspect of these three lines is that only small differences exist between them until sash height is about 9” above the airfoil!  (10” represents total sash opening including the 1” below the airfoil.)  In other words, above this 9” opening, face velocity is about the same for all bypass systems.  The OSB bypass is slightly more tempered over this range.

Some newer fume hood full bypass constant volume specifications require the face velocity to stay at or below a certain number.  A good example is the Montana State University specification of 150 FPM written by Dale Hitchings.2 Hitchings advocates in this specification that the sash should not be operated in a constant volume mode below the point where face velocity exceeds 150 FPM due to concerns regarding turbulence and containment.  Both the 3” and full open bypass reach the 150 FPM limit in the above example at a 14” sash opening.  According to Hitchings, it is not safe to use the sash opened below this level.  The OSB Fume hood does not reach 150 FPM under the same conditions until 10”, a lower sash height which provides more of a barrier between the researcher and the hood interior.  For these reasons, the OSB appears to be the best bypass choice under these constant volume conditions.

On a related issue, the vectored downwash of OSB air behind the fume hood sash (see any panel in Figure 3) greatly enhances containment at low and high velocities, which may address concerns voiced by Hitchings about “instability” above 150 FPM.

This graph illustrates the influence of bypasses on total extract volume at a constant face velocity of 80 FPM at decreasing sash positions (VAV).  Obviously, the full bypass design is not intended for VAV and shows more than twice the extract volume at a 2” total face opening at 80 FPM (smaller oval).  This system would therefore cost more energy at lower sash settings.  There is little difference in extract volume at any sash height between OSB and 3” restricted bypasses (larger oval)


Overall Conclusion: The Overlapping Sash Bypass Works Well in all Configurations!

The OSB is the best bypass system available in today’s lab design environment market for either constant volume or variable air volume systems. Only the OSB bypass system works well in both VAV and CV environments without modification: the following chart summarizes these data:


  1. USCSB Chemical Fume Hood Guide, 2007, University of California, Santa Barbara
  2. Montana State University Task Order NIST 2001-06, Aug. 31,2001, sections 2.26-2.34, Dale T Hitchings. Safelab Corporation
  3. Special Containment Testing of 6’ Bench Hood Model FAF723655; Flow Sciences,3/27/2017, available on request

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