The True Definition of a High-performance Fume Hood
Wed, 06/07/2017 - 9:43am Comments by Beth Mankameyer, Sales Engineer, Labconco Corp.
The ASHRAE 110 test comprises a face velocity profile, smoke visualization test, and a tracer gas test using sulfur hexafluoride and a manikin. Photo: Labconco
High-performance fume hoods take on different names throughout the industry, and therefore the meaning of the term “high-performance fume hood” is often misunderstood. With all the various definitions and industry lingo, the terminology can be tricky. This article will help set things straight.
Air’s relationship with fume hoods
Air is pulled through the fume hood via a blower, creating a negatively pressurized environment to protect the user from harmful vapors. The air travels at a certain speed, or velocity. The velocity of a fume hood is measured in the plane of the sash, and referred to as the face velocity, measured in feet per minute (fpm).
The face velocity is related to the amount of air being pulled through the fume hood. The amount of air is called the volumetric flow rate, measured in cubic feet per minute (CFM). The more air that is pulled through the opening, the faster the air will travel.
Using an example of a garden hose to relate, you can place your thumb over the opening of a hose to make the water spray further because blocking that opening speeds up the water’s velocity. The volume of water being pumped through the hose remains the same.
Since water and air are both fluids, the same physics apply. The face velocity (fpm) is dependent upon both the amount of air going through the hood (CFM) and the size of the opening the air passes through, including the bypass area.
Face velocity set point
Different fume hood models will operate at different published face velocities. Since not all laboratories need the most advanced fume hoods, many manufacturers offer a range from which to choose.
Specific face velocities should be chosen based on several variables. The recommended operating face velocity is typically dictated by the safety officer of the laboratory, not the fume hood manufacturer. A safety officer may say it is acceptable to use a fume hood at 60 fpm with an 18” sash working height, or due to drafty lab conditions or hot plate use, the face velocity requirement may rise.
Fume hoods cause an incredible strain on energy in a laboratory. They take expensive, tempered air and throw it outside – much like having a window open year-round and forcefully blowing air out of it. So if a lower face velocity can be used, then it should. A lower face velocity means lower volumetric flow, which means energy conservation and savings to the bottom line.
Velocity does not equal safety
Prescriptions range widely for required face velocities, however, it has been proven that faster is not necessarily safer. In fact, many laboratory standards specifically state that operating at high velocities (above 150 fpm) can actually create a safety risk due to turbulent air.
If a laboratory is balanced and follows general guidelines for operating a constant volume fume hood, a significant amount of energy may be saved if the fume hood operates lower than 100 fpm face velocity.
A lab can be starved of air, and if it is, the fume hood will not exhaust properly. Be sure there is enough supply air provided to the lab before adding fume hoods or choosing face velocity set points. That is not to say all labs can operate their fume hoods at as low as 60 fpm to relieve the strain on their mechanical system. Fume hood face velocity should be analyzed and carefully specified because there are many benefits to operating at a lower face velocity, and higher face velocities cause their share of risks and unnecessary costs.
The airflow of a high performance fume hood. Photo: Labconco
A real high-performance fume hood
High-performance fume hoods are subjected to many marketing spins, and therefore different names are thrown around in an attempt to attract customers. If a fume hood is referred to as a low velocity, high efficiency, energy efficient, or low exhaust volume, it all references the same thing—some form of the moniker “high performance.”
The Scientific Equipment and Furniture Association (SEFA) has set a definition for high efficiency that any fume hood making one of the aforementioned claims should meet or exceed. According to SEFA, a high-efficiency fume hood must pass the testing requirements described below.
The ASHRAE 110 test is a three-part test to check the containment performance of a fume hood. This test is comprised of a face velocity profile, smoke visualization test, and a tracer gas test using sulfur hexafluoride (SF6) and a manikin.
Per SEFA, the definition of an energy efficient fume hood is as follows: “Low Velocity Laboratory Fume Hoods are hood designs that provide a reduction in the required exhaust air volume, when compared to the volume required for the same size fume hood to operate with a face velocity of 100 fpm through a fully opened vertical sash and provides containment levels equivalent or superior to ASHRAE 110 tracer gas test ratings of 4.0 AM 0.05, and 4.0 AI/AU 0.10, with a face velocity of 60 fpm or less through the fully opened vertical sash. Low Velocity Fume Hoods are also referred to as High Performance Fume Hoods and High Efficiency Fume Hoods.”
The rating described above as “4.0 AM 0.05” means that during the ASHRAE 110 test, the SF6 is released at a rate of 4 liters per minute in an “as manufactured state”—hence the AM—and the acceptable amount of SF6 detected by the sensor in the manikin shall be no greater than an average of 0.05 parts per million (ppm). The rating described above as “4.0 AI/AU 0.10” refers to a release rate of 4 liters per minute, in an “as installed” or “as used” state—hence the AI or AU—and the acceptable amount of SF6 detected by the sensor in the manikin shall be no greater than an average of 0.10 ppm. The test allows for a greater amount of SF6 in the AI or AU test because during the AM test, it is conducted in a controlled test lab. The AI and AU tests are performed in the actual lab environments that may contain cross drafts, equipment inside the hood, and suboptimal supply air conditions.
To truly be high performance, the hood must meet the SEFA definition. Therefore. it must be tested at 60 fpm face velocity with the sash at least 25” open. Some hoods are advertised as being operable at 60 fpm, but without listing a sash position. The manufacturer must demonstrate that it can be operated with the sash at least 25 inches open to earn the high performance label.
Historically, most fume hoods were operated at a face velocity of 100 fpm with the sash fully open. That would be full speed for most fume hoods today, and would constitute an unnecessary energy drain in most laboratories. While there is no magic number for fume hood face velocity in every lab, standards do state ranges—typically 60 to 100 fpm. One reason standards are so vague about dictating a specific face velocity is because 100 fpm on an older style basic hood is vastly different from 100 fpm in some newer hoods.
Furthermore, those hoods will differ from lab to lab depending on the air balance scenario and real life conditions. All of these things need to be considered when planning a lab to ensure the best energy savings from the hood, while still maintaining absolute safety.
Choose equipment scientifically
Be informed, inquire about test results and understand what the test results mean.
It is important to ask manufacturers for test reports. Although test reports may state results at 40 or 50 fpm face velocity, the minimum recommended by standards is still 60 fpm. Lower velocity test reports are intended to provide peace of mind. They show that an additional safety factor is built in to the fume hood design, even though no standard recommends operation below 60 fpm.
When operating laboratory fume hoods with lower face velocity set points, energy savings can really add up—some high-performance fume hoods can even pay for themselves in energy savings in a very short time. Ask the manufacturer to help you focus on upfront equipment savings, annual savings and return on investment for your specific installation.
Since many variables can affect fume hood performance, size your fume hood for the face velocity dictated by your safety officer, or applicable local code. Use the manufacturer’s published performance to determine an acceptable operating range for the fume hood, and ask for test reports to back up that performance data.