Safe Options for Variable Speed Exhaust Fans

Gregory R. Johnson, PE, Newcomb & Boyd

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.

Findings:

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.

Labs21 Connection:

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.

Biography:

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 Labs21.

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