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Designing VAV Lab Exhaust Systems to Minimize Energy Consumption While Maintaining Acceptable Air Quality

Brad Cochran, CPP, Inc.
Patrick Plue, Agriculture and Agri-Food Canada

In an effort to save energy costs, most modern laboratories are equipped with Variable Air Volume (VAV) HVAC systems. These systems control the level of fresh air that is brought into the laboratory and conditioned. However, on the exhaust side, these VAV systems are often designed with constant volume fans are either off or running at 100 percent flow. The difference between the building interior airflow and the flow out of the exhaust stack is controlled with by-pass dampers that feed additional air into the exhaust fans to allow them to run at 100 percent flow.

For large laboratory exhaust systems, it may be possible to operate the fans at reduced volume rates, i.e., VAV, and still maintain adequate air quality at all nearby receptor locations. With a VAV exhaust system, the volume flow rate out of the exhaust stack matches the airflow into the building, eliminating the need for by-pass air, resulting in the potential for significant energy savings. This presentation will show that by defining the relationship between fume reentry and exhaust volume flow through the use of a wind-tunnel based air quality assessment, exhaust systems can be designed to operated under VAV.

This paper will describe two methods that can be utilized to operate the exhaust stacks under VAV while at the same time minimizing fume reentry. The first involves a fixed minimum volume flow set point for the exhaust fans. This method is generally applicable for large laboratory exhaust systems (>20 to 30,000 cfm volume flow rates). The second involves a variable set point that is defined by the local wind conditions. The more complex variable-set point method may be necessary to achieve energy savings for smaller laboratory exhaust systems (<10,000 to 20,000 cfm). Case studies that apply both methods will be presented.

Labs21 Connection:

Reduced operating costs and improved environmental quality:
A wind tunnel based air quality assessment can be utilized to obtain accurate concentration distributions from laboratory exhaust stacks at nearby receptor locations. This information can be used to define minimum fan operating conditions that may allow the exhaust system to operate without the need for by-pass air. Reducing exhaust fan loads saves energy costs and greenhouse emissions while maintaining adequate air quality at nearby air intake locations.

Increased health, safety, and productivity:
The described approach will help ensure toxic or odorous fumes do reenter the building through air intakes, windows, or doors.

Enhanced community relations:
When neighbors see an exhaust stack, they wonder what is coming out. Am I safe? Knowledge of the air quality impacts can be used to educate the community and tell them they are not at risk (if true).

Superior recruitment and retention of scientists:
Buildings that have health or odor problems due to fume reentry will not promote recruitment and retention. Conversely, facilities that make special efforts to promote good local and global air quality will provide users with a sense that they are positively changing their environment.

Biographies:

Brad Cochran, Associate at CPP, has over 15 years of experience conducting wind tunnel and numerical modeling studies related to laboratory exhaust design for such clients as Northwestern University, UCLA, the National Institutes of Health, University of Texas Medical Center, Loyola University, Bayer, UC Irvine, UC Davis, and UC Berkeley, to name a few. He was instrumental in the development, and EPA's subsequent approval, of the Equivalent Building Dimension concept. This concept provides clients with greater accuracy in estimating concentrations due to building downwash using EPA's ISC model. He has conducted numerous wind tunnel dispersion studies of "Good Engineering Practice" stack height, building ventilation, and site specific evaluations of environmental impact. He has also worked on the development of a new algorithm to describe plume trajectories under Sea Breeze conditions. Prior to arriving at CPP, Mr. Cochran was involved in pollution diffusion studies for the Lawrence Livermore National Laboratories, erosion and threshold velocity studies under reduced pressure conditions at the NASA Ames Research Facility, and was involved in various pedestrian level wind studies for an environmental group in San Francisco, California, while obtaining a Master of Science Degree in Mechanical and Aeronautical Sciences at the University of California at Davis. Professional organizations include ASME, AWEA, and ASHRAE.

Patrick Plue, P.Eng., has an MS.c. and BS.c. in Agricultural Engineering from the University of Guelph. For the past 15 years he has worked for Agriculture and Agri-Food Canada, Public Works Canada, and the Canadian Food Inspection Agency as an agricultural engineering specialist, as well as project leader/manager for a range of laboratory projects. These include conventional general chemistry labs, animal labs, and high containment labs. Patrick is located at the Central Experimental Farm, in Ottawa, Ontario, Canada.

 

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