I recently returned from a trade show where a number of manufacturers showed me their high-performance (low-flow) fume hoods. There were claims of energy savings ranging from 40% to 80%. These savings sound great, but I had to ask myself: Does this really fit with my experience? Can we really get these kinds of savings just from using high-performance hoods?
So I did some digging1. I learned these energy savings were based on a calculation of reduced flow volumes due to a reduction in face velocity. Some manufacturers claim safe containment at face velocities as low as 50 fpm; and one manufacturer even states they could go as low as 40 fpm. Savings were calculated on a comparison of the high-performance hoods to constant volume fume hoods running at 100 fpm face velocity.
I then conducted an informal survey of 11 different institutions who had recently installed high-performance fume hoods. Every institution surveyed had considered using lower face velocities. However, the lowest face velocity that anyone used was 60 fpm, and this was at only one of the 11 institutions. All other facilities were operating at 70 to 80 fpm. So why don't institutions run their hoods at the minimum face velocity? There are a number of reasons for this.
Variations in face velocities
Both ASHRAE 110 and ANSI Z9.5 allow up to 10% variation in average face velocity and up to +/-20% variation across the face of the hood. This means that while you may have designed for a higher face velocity, the actual velocity may be significantly lower. For example, for a hood operating at 60 fpm, you might see an average face velocity as low as 54 fpm and, in some places on the face of the hood, as low as 44 fpm. In addition, an HVAC system's performance will degrade over time. Some estimates say a typical VAV system may degrade by as much as 30% in its first year. Keep in mind ANSI standards don’t require an alarm until average face velocity reaches 80% of set point. This means at some spots on the face of your hood you could get down to a face velocity of 38 fpm before the user gets an indication something is wrong. All of these variations need to be taken into consideration in designing the system.
The lower the face velocity, the more susceptible a fume hood is to room conditions. This means changes in temperature and room air speed will adversely affect the performance of the hood. Ideally, cross-drafts at the face of the hood should be no greater than 30% of face velocity. With a fume hood running at 60 fpm, cross-drafts should be limited to 18 fpm. For comparison, the lower boundary of perceptible air speed for human beings is around 30 fpm. Temperature differences can change airflow patterns; and a temperature difference between the hood interior and the room can generate a reverse airflow. Therefore, a number of institutions will consider reduced velocities only with extensive CFD modeling of the entire lab.
The typical as-manufactured fume hood test is usually conducted in the manufacturer's lab, a lab designed to ensure optimal performance of the fume hood and not necessarily mimic actual field conditions. While there may be a cross-draft generated, everything else is static. No one sticks an arm in the hood, the mannequin is only one size and what about the effects of other hoods in the room? Real life is different, and the lower face velocities only magnify these differences.
Face velocity isn’t necessarily an indicator of user safety
There are many factors that determine user safety when using a fume hood (chemical toxicity, processes, user practices). It's probably naive to set a single face velocity and expect it to cover every conceivable condition, but many institutions do. "An adequate face velocity is necessary, but isn’t the only criterion to achieve acceptable performance and shall not be used as the only performance indicator," according to ANSI/AIHA Z9.5-2003.
Face velocities of 40 to 60 fpm may sound adequate. But let’s consider the impact. 60 fpm is equivalent to only 0.682 mph. That’s the speed of the air passing through the sash opening that protects the user. What happens if a user pulls their hands or a piece of glassware quickly out of the hood? Do they drag toxic fumes with them? What happens if a reaction generates significant amounts of fumes? Does it push toxic fumes into the breathing zone? It’s possible the quantity of toxic fumes that escape will be below allowable thresholds. But without doing an analysis of what goes on inside the hood, you can’t know this.
Institutions should do a thorough investigation of how hoods are intended to be used, and then check regularly to verify their assumptions. It’s critical to investigate anything that's changed. A system of hazard banding should be developed that takes into account chemicals and other hazards, processes, users, training and past history. With this hazard assessment, very often institutions recognize face velocities can be reduced, while sometimes they stay the same or are increased.
So what are the savings you can expect? 1
It's important to note when factors such as minimum room air change rate, pressurization or temperature control exceed the requirements for fume hood exhaust, such as in a large lab with a single hood, there are no savings that come from reducing hood exhaust volume. Maximum energy savings occur when fume hood makeup air determines the room ventilation rate.
Using a mathematical modeling technique that I developed for Wilson Architects, we simulated how changing fume hood face velocity would affect total CFM for a lab with a large quantity of hoods. What we found, not surprisingly, was when we reduced face velocity from 80 to 60 fpm we saw a 25% reduction in CFM. When we examined using VAV hoods in lieu of constant volume hoods, we saw a reduction of 20%. When we reduced the flow to these hoods to 60 fpm, we saw a total reduction of CFM of 38%.
We also looked at other energy-saving strategies. We looked at sash closers, a shut-the-sash program, night-time setbacks, reduced minimum flow and a reduced face opening. The most efficient combination for this application was a VAV hood with a 16-in vertical opening, a 250 ACH minimum flow and a sash closer. Compared to a constant volume hood, this hood provides a 60% savings, without reducing the face velocity. When we reduced the face velocity to 60 fpm, we saw a reduction in total CFM of 63%. The impact of that 20 fpm reduction in face velocity was only 3%.
Conclusion and opinions
After this analysis you may get the impression that we don't like high-performance fume hoods, but that's not the case. We believe these hoods provide greater protection. We also believe they can reduce energy usage inside the lab. But don't expect that you can install these hoods, turn the exhaust down to the manufacturer's minimum and expect to see energy savings of 40% to 80%.
Do high-performance (low-flow) fume hoods save energy? The answer is a resounding maybe. If they are used as part of an overall HVAC strategy with careful attention given to how hoods are being used, then there may be an opportunity to reduce flows and save energy.
We want to stress that reducing face velocity is only one approach to saving fume hood energy. Can you reduce the quantity of air you are exhausting from your lab and save energy? Definitely, if you engage a qualified lab designer who can review all the options with you, including high-performance hoods, and come up with an approach that doesn't compromise safety.
- High-performance fume hoods only save energy if you reduce the face velocity.
- Face velocity isn’t the sole indicator of user safety. Don't count on being able to reduce your CFM just because you have a better hood.
- Understanding what’s going on in each hood is critical for designing a hood that provides the right amount of protection.
- There are other energy-saving techniques for fume hoods. The greater the savings from these techniques, the lesser the savings from reducing face velocity will be.
Greg is a well-respected leader in the design and planning of sustainable lab and research spaces. An enthusiastic and thoughtful problem-solver, he relishes partnerships that engage all participants in the exciting process of discovery.
1: Fume hood research led by Greg while he was employed as a lab planner at Wilson Architects.