Safe Options for Variable Speed Exhaust Fans
Gregory R. Johnson, PE, Newcomb
Traditionally, constant speed exhaust fans have been used in laboratory
exhaust systems to maintain constant discharge velocity out of exhaust
stacks. Even in large manifolded variable volume exhaust systems,
used in most facilities, constant speed fans with outdoor air make
up dampers are often used to maintain this constant discharge velocity.
Laboratory exhaust fan motors can represent a significant electrical
load, and designing systems that can stage or vary the speed of
the fans represent an opportunity to reduce energy usage and cost.
During the presentation, we will briefly review the basics and
standards guiding laboratory exhaust system discharge design and
how these standards lead towards constant speed fans. We will then
review design strategies and control sequences to stage or provide
variable volume fans while still minimizing potential of re-entrainment
of the exhaust plume. We will compare design drawings, first cost,
operating cost, performance, and other advantages and disadvantages
of five design options.
Lessons learned include ways to reduce fan energy in laboratory
exhaust systems while maintaining a system that safely discharges
exhaust effluent away from building intakes and users. It is possible
to stage fans or use variable speed fans in a variety of options,
to achieve this goal without excessive cost, complexity or significantly
reducing reliability. Each option has advantages and disadvantages
that should carefully be considered for each building application.
Some specific issues that require review include: first cost, energy
cost, maintenance requirements, system complexity stack discharge
performance, acoustics, space requirements, reliability, and aesthetics.
During the presentation, we will show a single 130,000 square foot
laboratory building designed with each of the five-exhaust system
options for comparison. Analysis will include data for each option
with respect to first cost, energy cost, life cycle costs, maintenance
requirements, system complexity, stack discharge performance, acoustics,
space requirements, reliability, and aesthetics. We will review
how system requirements in the test facility influenced the results
and how differences in these requirements in other facilities would
affect the best exhaust system choice for those facilities.
Given the justifiable concerns regarding laboratory safety, the
industry has been rather conservative regarding laboratory exhaust
systems, including the fans and discharge arrangements. With less
expensive and more capable control systems, increased energy costs,
and a growing emphasis on sustainable design, designers should explore
options to reduce energy usage wherever possible, including exhaust
system design. The presentation will provide valuable information
regarding options for incorporating these new design approaches
into laboratory exhaust systems.
Many aspects of the Labs21 approach are reflected in the presentation.
The energy usage of the tradition laboratory exhaust system is significant.
Reducing this energy usage, while maintaining occupant safety, minimizes
the impact of the facility on the environment, which is a fundamental
aspect of the Labs21 approach. The presentation will include life
cycle cost comparisons to show costs and savings as a decision making
tool in selecting the best system for a facility.
Greg Johnson, PE, earned his Bachelor of Mechanical Engineering
degree from the Georgia Institute of Technology in 1988. During
his 15 years with Newcomb & Boyd, he has developed particular
expertise in the design of mechanical systems for laboratory and
research facilities. His experience includes laboratory projects
for CDC, EPA, Emory University, as well as numerous other universities
across the country, totaling more than 800 thousand square feet
and over $108 million in construction value. He is a LEED
accredited professional and has designed four projects registered
in the USGBC LEED program. He has previously presented at
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