FUME HOOD WHITE PAPERS

The Feasibility of Fume Hood Containment at 40 FPM

The Feasibility of Fume Hood Containment at 40 FPM

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

Flow Sciences, Inc. 

2025 Mercantile Drive

Leland, North Carolina 28451

V 1.1; 12/28/2017

Background:

Flow Sciences is a company that designs, tests, and manufactures a very diverse line of laboratory containment devices.

Energy conservation appears to be the most significant issue involving containment devices in the 21st Century.  Over the last two decades, techniques to improve containment while reducing exhaust from fume hoods and other exhaust devices have overwhelmed all other aspects of containment exhaust research.  Innovations have included reduced sash openings, self-closing sashes, reduced volume through variable air volume (VAV), and fume hood nighttime volumetric setbacks. 1

When ball-parking lab exhaust cost, one can use $10/CFM/year as a “first guess”.2 A single 5-foot exhaust hood extracting a classical 100 FPM exhausts about 1000 cubic feet per minute (CFM), for an annual energy expense of $10,000! 3 Most lab people, even users of hoods, are astonished when they first hear these numbers. The number is this large because each cubic foot of air brought into the lab must typically be heated or cooled (money), dehumidified (more money), distributed through ductwork (more money), either redistributed and purified (still more money), or exhausted and replaced with new, outside air (still more money).

Reducing the quantity of air passing through such an exhaust cycle can obviously reduce the expense of operating it. This would be good for economy. This would conserve energy.  This would move us toward sustainability. This would be very good indeed!

One way to reduce exhaust volume is to lower fume hood face velocity. Older standards called out an 80 – 100 feet per minute (FPM) fume hood face velocity as being a “safe” number.4   Newer “high efficiency” fume hoods can operate with documented safe containment down to 60 FPM according to their manufacturers. 5, 6, 7

Unfortunately, all good things have their limits.  Several recent job specifications have called out a 40 FPM face velocity and require testing demonstrating containment at this level. Such a low face velocity could save even more energy, but “passing” the ASHRAE 110-2016 As-Manufactured test, the typically-specified containment test in the US, may not be proof the fume hood will be safe when used in operating labs under working conditions.

40 FPM Tests Already Done on the Flow Sciences Fume Hood:

We will now go through the good and bad news of all of this in a more academic fashion and see if there is a clear path forward to save even more energy and money with a 40 FPM face velocity.

Flow Sciences, in response to customers who requested test data on fume hoods tested at 40FPM, have already run these tests on our Saf T Flow high-efficiency fume hood.

All test data at 40 FPM passed the ASHRAE 110 and HAM tests at the AIHA Z 9.5 levels cited with the chart.  In spite of this, FSI feels tests at these low face velocities need extra clarification. 

Qualifications on the 40 FPM Face Velocity Test Results:

Rather than criticizing a particularly low suggested fume hood face velocity, let us consider possible threats to any well-designed low velocity fume hood by making a ZERO assumption. That is, let’s assume our fume hoods are operated at 0 FPM. (Yike!) What could go wrong? We must also place this gedanken hood into an actual building environment.

This is a producto ad absurdum (PAA) approach.  Assume something absurd to highlight resultant difficulties, then see if similar issues will occur under slightly less absurd conditions (i.e.40 FPM).

Here’s some of the PAA things that will go wrong at 0 FPM:

  • Fumes would not stay inside the fume hood, but would move into the room as air vectors and simple molecular kinetic energy would now have supremacy over the zero fume hood face velocity. This scenario is Dire.
  • “Bad” air from contaminated labs with no fume exhaust would now spread everywhere, carrying contagion into every area of the entire building. This scenario is Dire’s first cousin.
  • Room thermostatic control would be unstable since modern buildings circulate conditioned air into a lab area while consistently pulling “old” air out of an area and then exhausting or recirculating it. Fume hoods are frequently the primary or exclusive exit point for “old” lab air. Without this exhaust it may be impossible to control temperature or humidity in labs. “Sensible Heat” in any lab area is much higher than in an office environment because of illumination, ovens, flame emission spectrometers, hotplates, and computer waste heat. This scenario of meager air changes is uncomfortable and may impact experiments requiring controlled temperatures.
  • Hood interiors and their contents would become corroded under a wide variety of applications that release corrosive acid vapors, free radicals, or transition molecules into the containment area. This scenario is expensive.
  • Stuff could blow up inside fume hoods. Many organic chemicals have lower explosive limits (LEL’s) that when reached, will cause eruptive explosions when in the vicinity of sparks or other sources of activation energy. This scenario is literally explosive if solvents spill, boil over, or are released into the hood interior during reflux.

Of these containment and HVAC issues, only numbers one and two are measured by the HAM and ASHRAE 110 containment data cited. The last three scenarios, thermostatic control, interior hood and contents corrosion, and explosions are dependent on building conditions (HVAC system) and hood minimum flow rates.

A variety of scenarios will occur based on the minimum CFM assigned by the building HVAC engineering group to each hood.  This problem is particularly serious with variable air volume (VAV) fume hoods. As one closes a VAV fume hood sash, the exhaust will be reduced to maintain the face velocity specified (40 FPM, for example).  If no other factors are programmed into the VAV system, this would mean that at full sash closure, many hoods with no bypass would exhaust virtually no air at 40 FPM. A 48” FSI hood with no bypass and a 1” airfoil opening would have the following full-closed exhaust:

Cubic Feet per minute = CFM = 40 FPM X (38.5” X 1”)/ (144 sq”/sq ft) = 10.7 CFM

And additionally:

Air Changes per minute = ACM = 10.7 CFM / ((38.5” X 24” X 48”)/(1728 Cu in/Cu ft) = 10.7 / (44,352/ 1728) = 10.7/25.7 = 0.42 ACM= 25 ACH (Air Changes per Hour).

As stated in the footnotes, I have evidence using the LEL (lower explosive limit) level for acetone and observed corrosion effects of HCl(g), that the air change rate inside a fume hood should never drop below 5 ACM (300 ACH). For the record, my 300 ACH minimum number is not unanimously agreed upon in the literature.8   AIHA Z9.5, for example, cites a range of 150-375 ACH. However, the VAV hood cited above at 25 ACH is way lower than even the range cited by AIHA Z 9.5.

These sash-closed corrosion and explosion scenarios at low exhaust levels are very significant with VAV hoods, as little savings are realized with the CFM-hungry sash open.  It is essential that the minimum air change rate with sash closed be agreed to by lab safety and HVAC engineers because it is likely in new labs that the sash-closed position will be encouraged by lab safety and building engineers. This closed sash setting should maintain enough CFM throughput to clear the hood of explosive or corrosive levels of fumes (150-375 ACH). 10

Here’s the good news. Adding a small bypass to the high efficiency hood will increase the minimum air change rate significantly. Such a properly designed bypass can also improve containment.9 If such a bypass system is combined with an algorithm in the VAV controller to guard against sub-standard minimum CFM, two risk charts for hood operation can be set forth as shown below:

The above charts attempt to approximate what would happen at face velocities above zero and reflect my assessment, based on test room observations, of the five danger areas as velocities are increased. Red is bad.  Yellow could be bad in the presence of high fume-generating procedures. Green will probably work okay in most cases.  Because all lab experiments could encounter a variety of other negative scenarios outside of the five I outlined, green conditions are only green if all other design and safety issues are judged favorable.

Other Conditions Affecting High-Efficiency Hood Containment:

  • Aerodynamic overall room layout

Locations of room make up air ceiling gratings, fume hoods out of high foot traffic areas, closed laboratory doors, hood sash discipline, are all within design requirements.

  • Overall building commissioning and periodic re-checks

With VAV systems, but with CV systems as well, room pressurization, hood flow and system response to sash adjustment, duct integrity, and hood & room sensor accuracy should all be periodically verified.

  • Re-entrainment

Since low volume fume hoods will produce exhaust streams with a higher concentration of fumes, the exhaust system should both dilute exhaust streams and insure the exhaust stack velocity allows the fumes to clear the building and remain free from air intake areas.

  • Building HVAC exhaust alterations must be carefully undertaken

Many times the stewardship of a variable air volume system is not consistent.  Obviously the addition or elimination of exhaust hoods will change airflow parameters and require modification of HVAC control parameters. In any facility with HE low velocity hoods, changing the hoods must require an adjustment of control parameters.

  • Power outages or blown circuits

In the US and overseas, many different things can cause power grid loss or building sector outages. During commissioning and re-checks, HVAC and VAV control and tracking operations should be simple and forthright, and not require re-programming or subsystem resets after a power outage. The easy re-establishment of all HVAC functions should be tested and verified before turning the building over after construction.

Conclusions:

1) Traditional containment testing shows 40 FPM is sufficient at 18” to contain fumes for Flow Sciences constant volume HE fume hoods.

2) Under VAV operating conditions, extra care needs to be taken regarding programmed exhaust volume at full sash closure. While exhaust may be contained, either explosive or corrosive issues within the fume hood may occur under these conditions. This issue is of considerable importance since wide disagreement and uncertainty exists in the literature about how many air changes per hour are adequate in a closed VAV fume hood (150-375 ACH).

3) As hood exhaust numbers are lowered, overall building HVAC issues will require frequent and consistent monitoring of building control systems whenever inevitable additions or changes to the fume hood population occur. With energy conserving VAV or CV systems, adding or removing fume hoods from a lab building may require system-wide adjustments.

Footnotes:

1) http://www.flowsciences.com/fume-hood-energy-savings-low-hanging-fruit

2) This approximation falls between a low of $5.00 by Safelab (http://www.safelab.com/FACT_SHEETS/Fact4.pdf) and a high of $15.00 by Cal OSHA (http://docketpublic.energy.ca.gov/PublicDocuments/17-BSTD-01/TN217908_20170607T113738_6617_Laboratory_Fume_Hoods_Presentation.pdf)

3) Constant volume hoods at 100 FPM face velocity 24 hours a day.

4) Industrial Ventilation A Manual of Recommended Practice, 19th Edition,recommends 80 – 100 fpm face velocity with a full open sash depending on quality of supply air distribution and uniformity of face velocity.

5)Flow Sciences Lab Design brochure, 2017, P2.

6)ANSI/ASHRAE 110-2016 Containment Testing of The 4 Foot Safe-T Flow Chemical Fume Hood Including Data for Ultra-Low Face Velocity of 40 FPM at 18” Sash Opening, Flow Sciences, 2017

7)Summary, Containment Testing of Safe-T Flow Chemical Fume Hoods 5/31/17

8) ANSI/AIHA Z9.5-2012 p25 cites a range of 150 to 375 ACHas range of acceptable minimum CFM for VAV fume hoods.

9) http://www.flowsciences.com/overlapping-sash-bypass/

10) Low volume VAV settings are really experiment-sensitive. Very high volume experiments may cause issues not originally considered during system design.

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