By looking at features, options, and alternatives, fume hood energy impact can be reduced.
The fume hood is arguably the single most important safety feature in laboratories. They are also one of the major energy users and can use three and a half times more energy annually than the average American house.(1) For safety reasons these devices cannot be eliminated from the lab, however, there are energy-optimization strategies that can reduce their environmental impact and enhance their performance.
To better understand the energy-reduction options available, at Payette we started by analyzing a number of recent projects to catalog the range of strategies used in past projects to minimize the energy usage from fume hoods. We also sat down with fume hood manufacturers to discuss features currently available in hood design to reduce energy consumption, and reviewed published research on hood energy reduction to leverage industry knowledge. Pulling all this data together, we generated a fume hood selection flow chart to assist project teams with the various choices and options available. This chart walks through the decision points to consider when selecting fume hoods for a project and covers the range of strategies currently available.
In order to select the best options for a fume hood, project teams need to start by gathering information about the intended activity to be performed in the hood. Understanding their use can help determine the number needed, because the most energy-efficient option is to reduce the number of hoods where possible without reducing safety. One way to eliminate a hood is by using alternate devices that can safely support the intended activities. For example, if storage is the primary use of the fume hood, a ventilated cabinet may be more appropriate and will exhaust less air—therefore having a smaller impact on building’s energy usage. Ventilated storage cabinets require only 60 to 120 ACH,(2) compared with 150 to 375 for hoods.(3) Custom enclosures can often be less expensive and more efficient to house equipment for operational device such as rotary evaporators. Other alternatives to the traditional hood include capture hoods, ventilated benches, snorkels, laminar flow hoods, glove boxes, biosafety cabinets and ductless hoods. All these alternatives should be reviewed with an industrial hygienist to confirm their appropriateness.
When placing a fume hood in a lab, it is also important to understand if the amount of air exhausted by the fume hood will exceed the lab supply air requirement (Figure 1). If the exhaust air of the fume hood is more than the supply air required for the space, then the hood will drive the air requirement, leaving users to pay an energy premium for the additional energy needed to treat and move the additional air volume. Fume hood manufacturers provide airflow data in cubic feet per minute (cfm) based on the size of the hood, the sash opening and the face velocity. If the total volume of air exhausted through the hood exceeds the volume of supply air recommended by the HVAC engineer, the project should investigate options to reduce the energy associated with the hoods. For spaces where lab exhaust does not exceed the supply air need, an understanding of the HVAC system is needed to understand the benefits of energy-saving fume hood features.
Another critical factor to consider in fume hood selection is the integration with the HVAC system. Because lab HVAC systems typically require large volumes of 100% outside air for ventilation, they have much higher air change rates than typical commercial buildings. The heating, cooling and movement of this air consumes significant energy. Systems with energy-recovery devices can greatly reduce the energy required to condition this air. They also have the potential to downsize heating and cooling systems that not only have first-cost implications, but benefit the long-term energy use of the building. While enthalpy wheels are the most efficient energy-recovery devices and are frequently used on general exhaust—because of concerns of cross-contamination with the incoming air stream—lab fume hood exhaust may not have energy recovery or use a less efficient system. Therefore, even when the fume hood is not driving the ventilation rate, reducing the amount of fume hood exhaust may still increase energy savings. When efficient energy recovery is provided on both the general lab and fume hood exhaust, reducing the hood exhaust volume may not yield a great return on investment for energy-reduction strategies. Because of this, the fume hood should be considered as just one member in the overall HVAC system whose performance can contribute to the energy goals of the project or hinder them.
The number of fume hoods is ultimately based on the specific procedures required in the lab. Their placement within the lab should be done with safety and performance of the hood and airflow in mind. There are ways to keep hoods in secluded areas to enable them to perform safely while keeping them as part of larger open spaces to take advantage of the larger air volume, such as the use of alcoves and support spaces without doors. These types of ancillary spaces can contribute to the hoods containment ability while not driving up the air volumes of the building (Figure 2).
Ways to reduce fume hood exhaust volume
There are a number of options for reducing the exhaust volume of fume hoods. Operating alternative devices that are appropriate and use less energy (as mentioned above) is one tool. Another option involves reducing the overall width of the hood by right-sizing it according to use. Reducing a 100 feet per minute (fpm) fume hood, with an 18-in sash opening by 2-ft, users reduce the air volume by 300 cfm and save $1,178 per year.4 Sharing hood use, especially for teaching, is one method to eliminate a hood; (2.5-ft per person or 5-ft wide minimum with 6-ft recommended) and staggering use times is another strategy. Teaching labs also have the opportunity to shut off constant volume hoods when contents have been removed and placed back into the dispensing hood.
Ductless fume hoods that recirculate lab air and use filters to capture fumes, vapors and particulates are one alternative to reduce air volume. Many of these are used for chemical usage that is known to be captured by the filter system. There is also new ductless filter technology available that allows the simultaneous handling of solvents, acids and bases with the same filter and is not as limited by predictable processes.
High-performance fume hoods are designed to operate at lower face velocities of 60 fpm or less, (compared to 100 fpm for standard hoods). Manufacturers employ a variety of techniques including motorized baffles, special airfoils, induced airflows and currents to improve containment at lower face velocities. These are usually deeper and more expensive than standard hoods due to the additional containment techniques. But lower face velocity equals lower cfm and energy savings. A 6-ft fume hood with a face velocity of 100 fpm and an 18-in sash height can cost $3,043 per year to operate, while a face velocity of 60 fpm would have an operational cost of $1,826, saving $1,217 per hood annually.(4)
Airflow reductions can be achieved with variable air volume (VAV) and two-position mechanical systems. They reduce airflow to the hood based on the position of the sash, while constant volume systems draw the same amount of air whether the sash is open or closed. VAV systems have finer modulations that can save the most energy, but have high initial costs. The more hoods there are, the more this system will pay back. VAV systems on fume hoods can reduce cfm by 63% per year, compared to a constant volume system according to Victor Neuman in a 2007 Labs21 presentation(5) If the total number of hoods does not financially substantiate a VAV system, two-position systems should be considered. Two-position systems are less expensive to install and still provide an air volume reduction when the sash is closed. The first position is set for the designed airflow when the sash is open, and the second position is set to exhaust at a minimum flow rate when the sash is closed. To control vapor concentrations inside fume hoods ANSI/AIHA Z9.5 notes 150 to 375 hood ACH have been used when attempting to save energy.(6)
Auto sash operators close the fume hood sash with occupancy sensors that detect when the user is not at the hood after a set period of time. Combined with a VAV or a two-position mechanical system that reduces airflow, significant energy savings can be had. Mott Manufacturing states a 6-ft hood with a face velocity of 100 fpm could save $1,294 annually with a closer and VAV system. Sash closers are not inexpensive and add to the overall cost of the hood. Some facilities will establish fume hood sash management plans with signage and staff training that instruct on proper sash management in lieu of closers or in facilities with existing hoods that do not have closers.
Maintaining a holistic view of fume hoods and the part they play in the air management system of a building is the key to reducing their energy impact. The fume hood selection, its options and arrangement within the lab can be optimized; and when working together will result in significant energy savings for the building.
1. Energy Use and Savings Potential for Laboratory Fume Hoods, by Evan Mills and Dale Sartor, Lawrence Berkeley National Laboratory, 2006
2. Prudent Practices in the Laboratory: Handling and Management of Chemical Hazards, ©2011 National Academy of Sciences
3. ANSI/AIHA Z9.5-2012 Laboratory Ventilation
4. Calculations made using Laurence Berkeley National Lab Fume Hood Energy Model Calculator with Boston as the location: http://fumehoodcalculator.lbl.gov/
5. “VAV and Low Flow: Which Strategies Save More?” 2007 Labs21 presentation by Victor Neuman
6. ANSI/AIHA Z9.5-2012 Laboratory Ventilation
Ronald Blanchard, AIA, LEED, AP BD+C, is an associate and Andrea Love, AIA, LEED, AP BD+C, is a building scientist, both with Payette, Boston, Mass.