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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