Two Types of Energy Efficient Fume Hoods - Low Flow and Low Velocity
by Pete Caudle, Lab Crafters, Inc.

With the current focus on conservation and renewable resources, an increasingly popular offering in the laboratory fume hood market is the energy efficient fume hood. These devices have been designed to offer superior performance to the traditional by-pass fume hood while operating more efficiently, and ultimately saving precious operating dollars for fume hood owners. According to the Scientific Equipment and Furniture Association (SEFA)1, energy efficient hoods can be broken into two distinctly different categories: Low Flow fume hoods, and Low Velocity fume hoods (also known as high performance fume hoods). But are they really different from more traditional fume hoods? According to industry experts, they are, in fact, very different. Although the waters have been severely muddied by fume hood manufacturers, proper testing of the products will ultimately give owners the products they desire.
Let’s begin with a look at Low Flow hoods, which have been on the market for several years. These units do save energy, and provide a safe working environment for the user, by typically limiting the sash height when operating at face velocities less than the traditional benchmark of 100 feet per minute (FPM). The important point to remember about Low Flow hoods is that the working height of the sash is critical to their containment. These hoods are not designed to be operated safely at reduced air volumes unless the sash is stopped down to the operating position, which limits the user's open working area within the hood. Operating the hood with the sash at a higher position can result in spillage, creating a potentially hazardous environment for the user.

The second type of energy efficient hood as defined by SEFA is the Low Velocity fume hood, capable of operating at an average face velocity of 60 fpm or less with the vertical sash fully open to the setup position, no less than 25 inches from the work surface. An important distinction between Low Flow and Low Velocity hoods is further established with the critical testing requirements placed on the Low Velocity hoods. SEFA states that while operating at 60 fpm or less with the sash fully open, the Low Velocity units must maintain a containment level of 0.05 ppm o f SF6 (sulfur hexafluoride) tracer gas at a release rate of 4.0 LPM, when tested in accordance with ASHRAE 110-95. SEFA further notes, in section 4.3 of SEFA 1 – Fume Hood Recommended Practices, Low Flow fume hoods that achieve a reduction in air flow by restricting the sash opening area do not qualify as Low Velocity fume hoods. Essentially, Low Velocity hoods are designed to safely contain at lower airflows with a larger open working sash area. Under this definition, only a few fume hoods on the market are truly Low Velocity.

A common tactic is to offer a Low Flow hood for a Low Velocity fume hood specification. To ensure that a Low Velocity fume hood is provided, stringent testing criteria must be specified. The specification should require onsite ASHRAE testing at 60 FPM or less with the sash fully open, and all hoods should be field tested to verify the containment of the installed product in relationship with the space in which it is installed. This upfront evaluation of the space and fume hoods will show potential flaws in the fume hood and HVAC system performance.

It is important for owners and designers to put the burden of producing safe, quality products, on the fume hood manufacturers. Remember that a fume hood is a safety device! “Prior to purchase investigations” will benefit both the owner and designer and should include the following. First of all, ask the manufacturers for a list of their Low Velocity fume hood installations. This information should include the number of hoods that were installed and a contact reference from each project. Also, determining the sizes, services, and availability that can be offered is important to assure that the manufacturer’s product does not conflict with design parameters. Last but not least, each manufacturer should provide third-party independent ASHRAE 110 testing. This testing should be done out of the manufacturing facility in an “as-installed” or “as-used” application. These three criteria will help narrow down choices between true Low Velocity fume hoods.

Low Velocity/High Performance fume hoods are definitely a growing trend in the laboratory ventilation safety market. By allowing the user to have a larger open working sash height and reduced air flow/velocity, while still providing safe containment, these hoods provide the owner with cost savings and a better working environment. Each traditional fume hood uses the equivalent of three households' worth of energy annually; the Low Velocity fume hood can cut this energy consumption in half. The performance criteria called for in the specifications will separate the various types of hoods, and establishing the correct performance and testing requirements will help ensure that end users get the product and performance they expect.

1 – All Scientific Equipment and Furniture Association references can be found in SEFA 1 Fume Hoods Recommended Practice, section 4.3 Energy Efficient Fume Hood.
Pete Caudle, Midwest Sales Manager for Lab Crafters, Inc., Ronkonkoma, NY, holds a BS in Biology and Chemistry from Missouri Valley College. He has been working with the design of fume hoods and laboratory spaces since 2004. Mr. Caudle is a firm believer in designing facilities for functionality, sustainability, but most importantly, safety. Lab Crafters is locally represented by H&B Specialized Products, Minneapolis, Minnesota.