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Overcoming Barriers to and Taking Advantage of Manifold Exhaust Systems

Lee Chapman, Industrial Design and Construction (IDC)

Objectives:

In my experience designing high performance laboratories, I have encountered various obstacles, yet have recognized many opportunities in the design of manifold laboratory exhaust systems. My objective is to promote the design of manifold exhaust systems by highlighting advantages and discrediting drawbacks associated with such systems.

I will identify apparent barriers to the design of manifold laboratory exhaust systems such as code restrictions, cost implications, and functional limitations and suggest means of overcoming these barriers. I will also identify advantages of manifold systems including opportunity for energy recovery, reduced maintenance, inherent redundancy, and energy efficient control of exhaust stack discharge velocity.

I will give the engineer confidence in designing manifold exhaust systems insofar as code compliance, functional reliability, and innovative design strategies.

Findings:

1. While the International Mechanical Code classifies laboratory fume hood exhaust as a hazardous exhaust system, the exhaust fumes are not necessarily classified as flammable as defined by the Code. Exhaust fumes are considered flammable when they exceed 25% of their Lower Flammability Limit (LFL). It can be proven that the most critical chemical fumes exiting a fume hood, even in a minimum flow condition, are diluted to a concentration well below that which qualifies them as flammable.

2. By employing a manifold exhaust discharge arrangement with on/off control dampers installed in multiple stacks, a minimum exhaust stack discharge velocity can be maintained without imposing a false load on the system, characteristic of Strobic fans, thus saving energy.

3. A manifold exhaust system allows the convenient use of an energy recovery system. A research laboratory uses a large amount of outside air and therefore must exhaust air at nearly the same rate in order to maintain desirable pressures within the building. By using a manifold exhaust arrangement, the energy recovery system including coils, piping, etc. can be simplified.

Labs21 Connection:

The design strategies addressed in this presentation offer credit opportunities in the "Energy and Atmosphere" and "Innovation in Design" categories of the LEEDTM certification process and reflect many aspects of the Lab21 approach.

Adopt Energy and Environmental Performance Goals
A manifold exhaust system typically employs two, three or four central exhaust fans with N+1 redundancy. These fans would be part of a variable air volume lab exhaust/supply system and would be controlled with variable frequency drives (VFDs) offering energy savings at low utilization. Since outside air conditioning and conveying represent a large percentage of a laboratory's total energy consumption, this strategy can have a significant impact on optimizing the energy performance of a lab.

Assess Opportunities From a "Whole Buildings" Approach
Integrating a manifold exhaust system into a high performance lab design offers the opportunity for other energy saving strategies such as energy recovery and a discharge damper control system as described above.

Use Lifecycle Cost Decision-Making
A manifold exhaust system in itself may not provide an attractive payback; however, by "bundling" upgrades such as described above, the overall payback period can be significantly reduced. This aspect of the Labs21 Approach exhibits synergy with the "Whole Buildings" approach.

Employ a Broad Range of Sustainable Energy and Water Efficiency Strategies
A manifold exhaust system designed with a variable air volume lab exhaust/supply system can optimize energy efficiency and provide initial cost savings with the reduced number of exhaust fans and VFDs. Also, as described above, a manifold exhaust system offers a design opportunity for various methods of energy recovery.

Biography:

Lee Chapman graduated from Clemson University with a BS Degree in Mechanical Engineering. I am a registered professional mechanical engineer employed by Industrial Design and Construction (IDC) in Greenville, SC. I have 10 years experience in mechanical design for commercial and industrial buildings including churches, hospitals, libraries, universities, pharmaceutical labs and manufacturing plants. The last three years of my career have been primarily involved in cleanroom design for the microelectronics industry. Most recently I performed lead design responsibilities for the Clemson University Advanced Materials Research Laboratory, a 110,000 square foot facility consisting of physical wet labs, laser labs, instrument labs, electron microscope labs, and lab and administrative offices. This building was designed in pursuit of LEEDTM silver certification and is currently registered with the USGBC. I am currently studying to take the exam to become a LEEDTM accredited professional.

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