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Fume Hoods

Fume hoods are a significant contributor to energy consumption in laboratory spaces. Hoods use high-powered fans to exhaust conditioned laboratory air, and can consume the energy of over three homes. An open fume hood is essentially an open door with a big fan pushing the air outdoors. Lab spaces are required to have a high air exchange rate, but in general the air pushed through fume hoods far exceeds these minimum requirements.

 

Energy of air exchange

Commercial buildings require high rates of ventilation to maintain the comfort and safety of occupants. This air exchange is measured in cubic feet/minute (CFM) of air. This air must be heated or cooled and, in the summer, dehumidified, and this takes energy. In fact, energy is required no matter what the weather is outside. MIT buildings, like many commercial buildings, cool air to around 55ºF to dehumidify the air. It is then reheated to a setpoint temperature (usually 65ºF) before building electric load heats it up to comfort levels. That means that even when the weather is perfect, the building is using steam and chilled water to condition the air! For Building 18, the minimum energy flow is about 4 mmBTU/hr at 55ºF, corresponding to about 300 metric tons of CO2 emissions per hour. This is comparable to the emissions from 10 Chevy Tahoes traveling 60 MPH. This rate doubles at both 20ºF and 90ºF.

All of this "air conditioning" requires a lot of energy and produces a lot of CO2. The figure below illustrates the cost and CO2emissions per CFM by month [1]. The annual average is $5.14/CFM, and 45 lbs CO2/CFM. Fans required to move the air contribute an additional $1.95 and 15 lbs/CFM. The minimum air flow through building 18 is 128,200 CFM. Fume hood in the building uses 300-850 CFM, accounting for up to $7000/yr in energy and 27 metric tons of CO2.

Types of fume hoods

There are a wide variety of fume hood types, and a number of strategies for minimizing their energy use. Fume hoods can have vertical (up and down), horizontal (side to side) sash movement, or a combination sash with both. Fume hoods can also either move a constant air volume (CAV) in a period of time or a variable air volume (VAV). CAV fume hoods move the same amount of air regardless of the sash position, so lowering the sash in some cases increases the speed of the air going through the opening (the face velocity) but does not decrease the energy use of the hood. Some hoods (called bypass hoods) have vents at the top of the hood to keep the face velocity more constant, but the volume of air flow is not affected by the sash position. A more energy efficient variant is the two-position hood, which moves a high constant air volume when the sash position is above a small critical value, and a much lower constant air volume with the sash is effectively closed. VAV hoods use sensors to calculate the pressure differential necessary to maintain a constant face velocity. Air volume may be adjusted using a closed-loop or open-loop control. Closed-loop control uses a small air speed sensor embedded in the hood to measure the face velocity and adjust dampers to change the air volume accordingly. Open-loop control hoods use sash position sensors to calculate the necessary air volume. Venturi air valves are uses to control the flow of air between the hood and the exhaust network. Either design drops the air flow through the hood substantially. The hood usually operates at minimum air exchange, if workers properly close the hood when it is not in use. The CAV hood operates at maximum air exchange at all times.

MIT has many different kinds of fume hoods. Look for the black energy monitor on the side of your hood... if you have one, you have a VAV hood. Shut the sash and save energy!

Energy savings by shutting the sash

Fume hoods represent a significant opportunity for energy savings. The MIT Chemistry department undertook a campaign to improve use of VAV fume hoods. They used data collected from fume hood sash position sensors to let users know how much their hoods were used. Fume hood sashes were lowered by 26%, from 16.3 ± 0.9% open to 12.1 ± 0.4% open. This resulted in a net savings of about $24,000/yr in energy in building 18, and $41,000/yr throughout the entire department. The program averted approximately 93 tons of CO2 in building 18 and 160 tons of CO2 department-wide. Savings was achieved with approximately $12,000 in initial investment, and minimal continuing cost.

The Chemistry department mostly has relatively efficient combination-sash designs. It is even more important to shut the sash on single-sash hoods. Almost 20% of the total program savings came from one group with vertical sash hoods. They lowered their average sash position 15%, but because of the wider opening in these hoods they saved a tremendous amount of air exchange.

MIT Department of Chemistry Conservation Program

The Chemistry department used monthly use feedback to lab users to encourage conservation. Some examples of use are shown below. Use peaks during the day and is flat at night. Some labs are better than others at closing the hoods at night, but all have room for improvement.

Still, it is clear that all labs are performing better in 2008 after getting feedback than in 2007. Evidence: The average sash position for the department was less than 7.5% for only 5.5 hours (0.3%) out of the two months considered in 2007, as opposed to 432 hours (30%) in the same period in 2008.

The fume hood program had the support of the dean of the School of Science, the department head, and EHS. Feedback was delivered by the department EHS coordinator through the lab PIs. This wide range of support is a big reason for the program's success.

Resources

  1. Air infiltration energy calculator for MIT
  2. Amanti, Steve. "Potential Energy Savings on the MIT Campus." Senior Thesis, MIT Department of Mechanical Engineering, 2006.
  3. Harris, Asa, Bill Brewer, and Wayne Thomann. "Evaluation of the Benefits of Best Management Practices Used to Conserve Energy in Laboratories." Laboratories for the 21st Century
  4. "Shut the Sash" Results of the lab energy competition. Harvard Green Campus Initiative

 

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