Use and PrevalenceHigh amounts of airflow tend to "drive" sizing (first cost) and energy use of central hvac systems in buildings where hoods are located. As a result, fume hoods are a major factor in making a typical laboratory four to five times more energy intensive than a commercial building. The average hood, at 6 ft wide, exhausting 1,200 cfm 24 hrs/day, consumes 3.5 times more energy than the average house.
Between 500,000 and 1 million hoods are in use in the United States, so aggregate energy use and savings potential is significant. The annual operating cost of U.S. fume hoods ranges from $1 billion to $2 billion, with a corresponding peak electrical demand of 2,300 to 4,600 MW.
Past research showed that increasing the amount and rate of airflow (and, consequently, energy use) does not necessarily improve containment. Instead, errant eddy currents and vortexes can be induced around hood users as air flows around workers and into the hood, reducing its containment effectiveness and potentially compromising safety.
Conventional fume hoods rely solely on pulling air through the hood's open sash from the laboratory, around the worker, and through the hood workspace. The generally accepted "face velocity" is about 100 fpm, depending on hazard level. Computer modeling shows that eddy currents and vortices form inside conventional hoods, reducing the hood's effectiveness.
Hoods exhaust large volumes of air at great expense. The energy to filter, move, cool, or heat, and sometimes clean the air is one of the largest loads in most facilities and tends to drive the sizing (a first cost) and energy use of central hvac systems within the building.
The most common energy-efficient modifications to traditional fume hoods are based on the use of outside (auxiliary) air or vav control techniques. Auxiliary air hoods decrease safety and are virtually no longer sold. Vav increases safety by controlling air flowing through the hood to 100 fpm.
Existing approaches for saving energy in hoods may be complicated and costly to implement. Traditional fume hood designs also may not always address the full range of inherent worker safety issues. Innovation in hood design is hampered by various barriers stemming from existing fume hood testing/rating procedures, entrenched rules of thumb, and ambiguous and often contradictory guidance on safe levels of airflow.
Containment InnovationA new, patented LBNL technology addresses the shortcomings of existing approaches. It was developed by a team of researchers at LBNL's Environmental Energy Technologies Division (EETD). The "Berkeley Hood" uses a "push/pull" approach to contain fumes and move air. Small supply fans located at the top and bottom of the hood's face push air into the hood and into the user's breathing zone, setting up an "air divider" at the hood opening (Figure 1).
Consequently, the exhaust fan can be operated at a much lower flow rate. Because less air is flowing through the hood, the building's environmental conditioning system can be downsized, saving both energy and initial construction costs and thus offsetting the potential added cost of the Berkeley Hood.
The LBNL technology reduces the hood's airflow requirements by up to 70% while enhancing worker safety by supplying most of the exhaust air in front of the hood's operator.
The containment technology's push/pull displacement airflow approach contains fumes and moves airflow through the hood. Displacement air "push" is introduced with supply vents near the top and bottom of a hood's sash opening.
Displacement air "pull" is provided by simultaneously exhausting air from the back and top of the hood. These low-velocity airflows create an air divider between an operator and a hood's contents that separates and distributes airflow at the sash opening. This is unlike the air curtain approach that uses high-velocity airflow.
When the face of a hood is protected by an airflow with low turbulence intensity, the need to exhaust large amounts of air from the hood is largely reduced. The air divider technology is simple, protects the operator, and delivers dramatic cost reductions in a facility's construction and operation. The new hood achieves greater containment and exhaust efficiency, creating a more effective, energy-efficient solution. The team that developed the hood also designed new components for its lighting system that cut lighting energy nearly in half while improving the quality of the light.
The research team has developed several prototypes of the hood. Much of the work of developing the hood's alpha prototypes was funded by the Department of Energy, the California Energy Commission, and the California Institute for Energy Efficiency. A large number of private sector partners have also provided in-kind support, including Labconco, ATMI, Fisher-Nickel/PG&E Food Service Technology, Phoenix Controls/Newmatic Engineering, Siemens Building Technologies, US Filter/Johnson Screens, Jamestown Metal Products, and Tek-Air Systems.
Field TrialsField trials in progress at three sites have provided useful data that increase understanding of how the Berkeley Hood performs under working conditions in functioning labs. The University of California, San Francisco (UCSF); Montana State University (MSU); and San Diego State University (SDSU) are currently hosting tests of the Berkeley Hood. Pacific Gas & Electric (PG&E), MSU, and San Diego Gas & Electric (SDG&E), through SDSU, are funding the field trials.
The first prototype hood was installed at MSU. The four-foot unit at MSU began operating in July 2000; MSU's testing of the hood was funded by the National Institute of Standards and Technology. The MSU effort was part of a larger program at this university to build an environmentally friendly "green" laboratory facility. The building is planned to incorporate state-of-the-art mechanical and electrical systems in order to provide occupants with a high-quality environment and low energy use (Table 1).
Tests conducted according to ASHRAE Standard 110-1995 protocol found that the prototype hood contained smoke and operated at less than 0.10 ppm tracer gas leakage, the maximum level recommended by the American National Standards Institute ANSI/AIHA Z9.5-1992.
The second prototype was installed at UCSF's Department of Pathology, Medical Radiology Center. Labconco, another private-sector partner, provided the fume hood superstructure for this installation. UCSF's contractor, Marina Mechanical and Siemens Controls were also partners in this effort. PG&E funded this field test. Testing began in November 2000. The hood has performed well at UCSF, in some cases exceeding expectations. Table 2 shows the results of several performance tests. The Berkeley Hood was shown to contain test smoke and tracer gas under all conditions down to 33% of full flow. The hood is being operated at 50% of normal flow to provide the operator with a margin of safety.
A third test at SDSU, funded by SDG&E, will begin in the next couple of months.
Safety, Energy, Design, and Maintenance BenefitsThe new hood will address a number of areas of concern in laboratory design and operation. Energy-efficient operation will cut energy costs in half, improve user safety, and reduce unnecessary ventilation. In particular, the last improvement allows building designers to "right-size" the facility's hvac system, thus reducing initial capital cost.
Calculating the hood's potential energy savings begins with an estimate of the laboratory market in the United States. Interviews with industry experts suggest that there are 150,000 laboratories in the nation, with between 500,000 and 1 million fume hoods installed.
LBNL's national savings calculation conservatively assumes a cut in airflow of 50% compared to standard laboratory fume hood installations. Each new hood is calculated to reduce peak electrical peak load by 3.4 kW, and to reduce electricity use by 14.2 MWh/yr. This translates into a per-hood energy bill reduction of over $2,000 per year.
For labs throughout the United States - assuming that three out of conventional four hoods are ultimately replaced with the Berkeley Hood - total annual savings reach $1.2 billion, corresponding to 8,000 gigawatt-hours (GWh), 2 gigawatts of peak electrical capacity (the output of four large power plants), and 73 trillion Btu of heating fuel. These calculations assume 750,000 hoods and electricity prices of $0.065/kWh for electricity, demand charges of $120/kW, and heating fuel costs of $6.29/MBtu.
Other benefits to facilities include:
- Simpler design than state-of-the-art vav fume hood systems offers more certain energy savings and easier, less expensive installations and maintenance.
- Constant-volume operation ensures that energy savings are independent of operator interface.
- Improved containment reduces dangerous airflow patterns.
- Cleanroom air flowing into the operator's breathing zone reduces potential hazard from fumes.
In new construction, designers specifying the new hood could achieve savings in energy, construction, and maintenance costs. The expected higher first cost of the Berkeley Hood (compared to a conventional hood) would be offset by smaller ducts, fans, and central plants, as well as simpler control systems.
In retrofit projects, the new hood provides important hvac system benefits. Many laboratories are "starved for air" (supply, exhaust, or both) as their use of hoods has expanded over the years. Such labs could cause contaminant spills from hoods. Increasing supply and exhaust airflow is costly, which prevents many labs from adding new hoods. Replacing existing hoods with this technology can increase the number of hoods, improve the exhaust performance of existing hoods, or both.
Institutional Barriers And Future WorkWhile developing the hood's design, the project team identified barriers that need to be overcome in order for this new technology to enter the marketplace. The major obstacle arises from the procedures used to test fume hood containment performance. The Berkeley Hood decouples the hood's containment performance from the velocity measured at the face.
However, because of the complexity and expense of more robust testing, many labs use only the face velocity test as a measure of a hood's containment ability. This places the Berkeley Hood at a disadvantage, since it has a lower face velocity, even though its containment has been evaluated as equivalent, or superior, according to other types of tests.
The ASHRAE Standard 110-1995 is the most widely used test method for evaluating a hood's containment performance. It recommends three types of tests, but the ASHRAE standard itself does not stipulate what is "safe." The three tests are: face velocity, flow visualization, and tracer gas testing. Face velocity measures the average velocity at which air is drawn through the face to the hood exhaust. Various regulating bodies do not agree on a specific number for face velocity, although most specify a minimum of between 80 and 100 fpm.
Flow visualization tests, performed with smoke-generating substances, provide a qualitative picture of containment, and rate patterns of smoke from "poor" to "good." Tracer gas testing, the most reliable method for determining a hood's containment, is most often performed using sulfur hexafluoride (SF6).
Other than the full ASHRAE standard, the most commonly used indicator of hood capture and containment is hood face velocity by itself. The LBNL push-pull technique provides excellent containment of tracer gas and smoke, as measured in ASHRAE 110 tests, but hoods using this technology have an "equivalent" face velocity of approximately 30 to 50 fpm.
California's Cal/OSHA requires 100 fpm face velocity for a laboratory-type fume hood (noncarcinogen) to be in compliance, limiting the use of the Berkeley Hood.
The project team has demonstrated that the Berkeley Hood achieved containment levels equivalent to the majority of fume hoods, at exhaust flow reductions of 50% to 70%.
The Berkeley Hood meets the ASHRAE Standard 110 tracer gas test with a containment rating of no greater than 4-AI-0.1 (4 liters/minute of SF6, as-installed, 0.1 ppm). The hood's leakage rate, 0.01 to 0.02 ppm, is far below the 0.1 ppm recommended maximum level of the American Council of Governmental Industrial Hygienists.
To address the issue of restrictive performance standards for laboratory fume hoods, LBNL researchers are participating in a Cal/OSHA advisory subcommittee. The subcommittee is helping Cal/OSHA develop a test compliance standard that provides greater flexibility for alternative designs. The LBNL team is highlighting studies that show that fume hoods, which pass the face velocity test, can nonetheless leak SF6 during tracer gas tests, per the ASHRAE 110-1995 protocol.
With improved performance standards in place, the innovative approach of the Berkeley Hood will gain market acceptance as a safer and more energy-efficient alternative to existing fume hood technology. The plan is to continue development and licensing of the hood, as well as market transformation activities, until the hood becomes a well-established alternative in the marketplace. ES