Going the Extra Mile to Reduce Laboratory Exhaust Energy Requirements at the Cal Tech Broad Center

Brad Cochran, CPP, Inc.
Glenn Friedman, Taylor Engineering, LLC

VAV laboratory exhaust systems have been around for about a decade and are now becoming the basis of design for most new laboratories. While these systems are responsible for significantly reducing the total laboratory energy consumption (typically up to 15%) of the laboratory, they have left some energy savings potential on the table.

A typical laboratory exhaust system has a duct static pressure set-point that was set during testing, adjusting and balancing to provide sufficient pressure to drive the system under full load conditions; and often with an often random and significant safety factor applied. However, with a VAV exhaust system, the duct static pressure needed to drive the system reduces as the total volume flow rate reduces. Traditionally, the duct static pressure remains constant throughout the operation, regardless of the current building load. Maintaining a constant duct static, independent of volume flow rate has two negative consequences; first it limits the minimum volume flow rate at which the fan can operate, and second, because the exhaust terminal VAV units damper down to absorb the excess pressure, higher than necessary exhaust fan energy is consumed.

The inefficiencies of artificially elevating duct static pressure during low demand have been solved on the supply side for quite some time. By using valve position feedback from the VAV valves a duct static pressure reset protocol sets the supply duct static pressure just high enough to meet the VAV demand. Now this same technique is being applied on the exhaust side.

This presentation will describe a case study at the Caltech University Broad Centre where VAV exhaust strategies are combined with duct static pressure reduction techniques to drive down energy consumption on their laboratory exhaust system, without impacting air quality at nearby pedestrian locations or building intakes. As an added benefit, through the use of both VAV control and the staging of fans, the exhaust system is being designed without the need for by-pass air. As such, this eliminates the need for by-pass dampers, making the system more efficient, less complicated, and requiring less maintenance.

Prior to the VAV implementation the laboratory exhaust system was operating using the traditional n+1 redundancy configuration, with two of the three exhaust fans operating at approximately 87 BHp. By employing a VAV exhaust system with fan staging while operating at constant duct static pressure the fan energy consumption would have ranged from 22 BHp to 75 BHp. But, by combining the VAV exhaust system, fan staging, and static pressure reset, the fan energy consumption during minimum building demand can be reduced down to as low as ~5 BHp.

Learning Objectives

  • To show how even additional energy savings can be derived from laboratory exhaust systems by reducing duct static pressure in addition to VAV exhaust strategies.
  • To demonstrate how the much additional savings can be achieved by using duct static pressure reset on the exhaust side of a variable air volume vivarium exhaust.
  • Present a case study of the implementation of a VAV exhaust system with static pressure reset at the Caltech Broad Centre; demonstrating the design matrix used to develop the sequence of operation

Biographies:

Brad Cochran is a registered Professional Engineer in the State of Colorado and has over 20 years of experience conducting wind-tunnel and mathematical dispersion modeling assessments related to laboratory exhaust design.

Brad is active in ASHRAE and is currently serving as the lead author of Chapter 9, Exhaust Stack Design, in the upcoming latest edition of the ASHRAE Laboratory Design Guide. He is a voting member of TC9.10 Laboratory Systems.

Glenn Friedman has a Bachelor's of Science in chemical engineering from Berkeley and is a Professional Engineer in the State of California. He ran the family design/build contracting business for 20-years before joining Taylor Engineering as a Principal in 1999. Glenn's industry experience includes being a National Past Chair of ACCA. Glenn is involved ASHRAE and is Fellow and and is Fellow and is the Program and Standards Chair for TC4.1 Load Calculations.

 

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