Reducing the Exhaust Velocity for High Volume Laboratory
Exhausts - A Case Study for the U.C. Davis Robert Mondavi Institute
for Wine and Food Science
Brad C. Cochran, Cermak Peterka
Petersen, Inc.
The current exhaust design guidelines in the ANSI/AIHA Z9.5 Standard
for Laboratory Ventilation suggests that the minimum exit velocity
from an exhaust stack should be at least 15 m/s (3000 fpm). This
recommendation does not take into account the total volume flow
from the exhaust system. Since plume rise is a function of the vertical
momentum of the exhaust, volume flow exit velocity, one would expect
that for higher volume flow rates the exit velocity can be reduced.
For most exhaust configurations an increase in the plume rise will
reduce re-entrainment of the exhaust at nearby receptor locations.
However, this plume rise comes at a cost of increased horsepower
requirements for the exhaust fans. This leads to greater equipment
and maintenance costs and higher energy consumption over the entire
lifetime of the system.
This paper will describe a case study that evaluated the impact
of reducing the exit velocity from the ANSI/AIHA Z9.5 suggested
exit velocity of 3000 fpm down to 2000 fpm.
Findings:
The results of a wind tunnel based air quality assessment indicated
that the maximum normalized concentration predicted to occur at
nearby air intake location increased by approximately 15 percent
when the exit velocity was reduced from 3000 fpm to 2500 fpm. When
the exit velocity was further reduced to 2000 fpm the maximum normalized
concentration was only increased by an additional 2 to 3 percent.
(Note, the larger step change that occurred when the exit velocity
was reduced to 2500 fpm may be a result of the discreet wind speeds
included in the analysis.)
Further analysis indicated that a stack height increase of 5 ft
was necessary to reduce intake concentrations of the laboratory
exhaust for the 2000 fpm and 2500 fpm exit velocity configurations
to the level measured with a 3000 fpm.
The higher stack will provide significant energy savings by reducing
air velocity and the associated pressure drops. The small increase
in stack first cost will result in significant reductions in energy
costs over the life of the system. There will also be slight reduction
in fan horsepower, which could reduce the fan motor size. This could
also reduce the demand on the normal and emergency power systems,
possibly allowing smaller generator and fuel storage systems.
Labs21 Connection:
Reducing long-term energy consumption, while maintaining adequate
air quality, is a balancing act that requires a site specific evaluation
of the exhaust system performance. Extending the typical air quality
assessment to include an evaluation of the designed exit velocity
provides the design team will valuable information that allows them
to identify opportunities to optimize equipment costs and energy
consumption while maintaining a stack design that fits within their
design concepts.
The presentation will challenge a typical "rule of thumb"
used widely throughout the industry and present an alternative way
of determining design criteria.
Biographies:
Brad Cochran, Associate at CPP, has over fifteen 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 has 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.
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