Low-Flow Fume Hood Project: A Look at Safety, Containment Requirements, and Test Methods
Low-Flow Fume Hood Project: A Look at Safety, Containment Requirements, and Test Methods
This is a brief discussion of fume hood technology. To learn more, click on the corresponding phrase.
OverviewA Brief Explanation of Fume HoodsWhy Should We Be Concerned About Fume Hood Technology?Fume Hood Standards: A Puzzling FactorA Look at Fume Hood Testing Procedures
Face VelocityIs Face Velocity the Right Measurement?The ASHRAE 110 Protocol
ASHRAE 110 Issues
ConclusionRelated LinksThe Environmental Energy Technologies Division of Lawrence Berkeley National Laboratory has developed a promising new Low-Flow Fume Hood technology that significantly reduces the amount of energy consumed, while still protecting the workers health. Patent pending, the prototype has already passed the ASHRAE 110 test, an accepted industry standard. This undertaking is revolutionary, and I had the opportunity to be a part of it this summer. My research involved three primary areas:
Fume Hood Regulations and Standards
Containment Issues
Testing Protocol
The history of fume hoods can be traced back to the days when Thomas Edison used his fireplace chimney to exhaust fumes from his laboratory. Fume hoods are ventilated, enclosed workspaces designed to capture, contain, and exhaust fumes, vapors and matter generated within. They control chemical fumes and aerosols within pharmaceutical, hospital, educational, and industrial laboratories, protecting those that work with chemical hazards.
To gain a better idea of the fume hood, one might equate it with the hood of a kitchen stove. In a stove hood, the vapors and fumes of the food being prepared are drawn away and exhausted by the fans. Likewise, in the fume hood, harmful chemicals emitted from experiments are drawn away from the worker and exhausted by fans. Their purpose is to improve the level of safety for people conducting experiments with products that could adversely affect the health and lives of laboratory workers. In many cases scientists completely rely upon the fume hood as a protective device.
Fume hoods are designed to capture, contain, and exhaust vapors away from
the laboratory and its workers.
Their primary purpose is to protect workers from harmful chemicals that could
adversely affect their health and lives, thus demonstrating containment.
In terms of energy consumption, fume hoods exhaust 1000 to 1500 cfm and most are run continuously,
which translates into big bucks and extremely high energy usage.Many laboratories rely on face velocity as a measure of performance safety for fume hoods, but
this may not be a good measure of containment.
Hood regulating authorities are hard to define. Quite simply, there are more standards, methods, and recommendations then there are regulations and laws. The majority of these organizations, including ASHRAE have no binding on U.S. businesses. Nevertheless, the following is a condensed version of these organizations and their standards.
ASHRAE, American Society of Heating, Refrigerating, and Air-Conditioning Engineers
ANSI, American National Standards Institute
OSHA, Occupational Safety and Health Administration
Cal-OSHA, California's version of OSHA
Prudent Practices, a report published by the National Science Foundation
Industrial Ventilation, a manual provided by the American Conference of Governmental Industrial Hygienists
NFPA 45, National Fire Protection Agency
| ORGANIZATION | FACE VELOCITY | TEST METHOD |
|---|---|---|
| ASHRAE | ASHRAE 110 | |
| OSHA | 60-100 fpm | |
| Cal-OSHA | 100 fpm | ASHRAE 110 |
| Prudent Practices | 80-100 fpm | ASHRAE 110 |
| NPFA 45 | 80-120 fpm | ASHRAE 110 |
| ANSI | ASHRAE 110 |
As you can see, not one of these organizations seems to be in agreement, at
least as far as face velocity and testing are concerned. Everyone has a different opinion
or no opinion at all.
When we talk about fume hood performance testing, what we're looking for can be summed up in one word: What do we mean by containment? Well, in the simplest of terms, containment means that the hood works. Hazardous materials are kept within the fume and not allowed to leak or spill out into the breathing zone of the user.
Face Velocity is a measure of the average velocity at which air is drawn through the face to the hood exhaust. Recently, the question has come up as to whether people are putting too much value on face velocity. Many laboratories use it as a measure of fume hood performance.
This test involves forming a grid pattern by equally dividing the design hood opening into vertical and horizontal dimensions. Velocity readings are taken with a calibrated anemometer fixed at the center of the grid spaces. An average of the readings is taken. If the hood meets a 100 fpm requirement, it passes. But is this true? Is it ?
In a recent study conducted performed by Dale Hitchings, 59% of the hoods passed face velocity criteria. Only 13% of those same hoods met tracer gas industry standards
Had the face velocity measurements alone been used to determine adequate containment, 46% of the hoods would have passed based on face velocity
According to an article written by Dale Hitchings, 30% - 50% of hoods leaking excessive levels of contaminants pass the traditional face velocity tests
One investigator found that in a properly designed laboratory, fume hoods
with face velocities as low as 50 fpm provided protection factors 2,200 times greater as reported in an article by Coahmin Connell
than hoods with face velocities of 150 fpm.
That same article mentions another study which indicated that with the exception of one particular type of hood operation, there was
no difference in hood containment with face velocites between 59 and 138 fpm.
Obiviously, face velocity is not an effective indication of good containment and fume hood performance.
As these statistics indicate, there have been cases when a hood passed the 100 fpm requirement and still failed
to protect the worker. Then too, there have been cases where hoods operating far below the 100 fpm requirement
have performed better than those meeting the standard. In short, there is one conclusion:
The most widely accepted protocol in industry today and the only nationally recognized quantitative evaluation proposed by a U.S. organization, this three-part evaluation process consists of a face velocity test, flow visualization, and a tracer gas test.
The face velocity test proceeds as described above.
Flow Visualization is a two-fold process. It consists of a local or small-scale smoke test and a large scale smoke test.
The tracer gas test is a more elaborate set-up. A quantitative method, it involves placing a gas detector in a mannequin's mouth in front of the hood and injecting a tracer gas (SF6) into the hood.Contrary to popular belief, the ASHRAE method is not a cure for all fume hood ills. To begin with, although this method is recognized as the standard for fume hoods, none of the standards are binding on U.S. businesses. Nor does the standard specify what is acceptable. In fact the ASHRAE standard states
This standard defines a reproducible method of testing laboratory fume hood. It does not define safe procedures.The standard further states
The procedure is a performance test method and does not constitute a performance specification. It is analogous to a method of chemical analysis, which prescribes how to analyze for a chemical, constituent, not how much of that substance should be present. Another analogy would be a method for measuring airflow; it prescribes how the flow should be measured, not how much it should be.Furthermore, the procedure is both time-consuming and expensive. It requires complicated equipment such as mannequins, purpose built tracer gas ejectors, and electron capture instrumentation. For these reasons, some laboratories fail to check their hoods using the entire ASHRAE test procedure. (Who's going to carry a mannequin around to test 100 fume hoods?) They simply rely upon the face velocity and/or flow visualization as verification that the hood works. The forward of the ASHRAE 110 test does state
The flow visualization and face velocity tests can be conducted without the tracer gas as a combination of qualitative velocity measurement and a qualitative evaluation of hood performance. This portion of the standard could be used in the testing and balancing of new facilities and periodic tests of many hoods at a large facility.However, as the statistics presented above suggest, face velocity is not a viable method of performance testing.
Fume Hood Technology is a continuous study, and more research will have to be performed, specifically in terms of performance testing. However, I believe that the following needs must be met in the fume hood industry.
American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE)
American National Standards Institute (ANSI)
American Industrial Hygiene Association (AIHA)
Scientific Equipment and Furniture Association (SEFA)
Prudent Practices
Phoenix Controls Laboratory Standards and Guidelines
Fume Hood Protocol Comparisons by Caoimhin P. Connell
An Improved Laboratory Fume Hood Performance Test: Beyond Instantaneous Face Velocity by Dale Hitchings