Safely Retrofitting Existing Laboratory Exhaust Systems for VAV Operation

Brad Cochran, CPP, Inc.

There is often great energy savings potential associated with retrofitting existing constant volume laboratory exhaust systems in existing laboratories. However, there is almost always concern that such a retrofit will significantly impact the ongoing science in these existing laboratories. Often these labs are heavily used and any down time comes at an extreme premium. Therefore, there may be reluctance to consider the retrograde even if the energy savings potential is significant. The objective of this presentation is to describe the process involved in implementing a VAV exhaust system into an existing operating laboratory with minimal or no down time within the lab.

The implementation of a VAV exhaust system into an existing laboratory is a five step process: 1) Dispersion modeling; 2) installation of VFD's and/or anemometer (if required); 3) pre-functional testing; 4) sequence of operation development; and 5) implementation and commissioning.

The dispersion modeling is the backbone of the design and all work is done outside of the lab environment with no interruption to the lab. Dispersion modeling provides information on how much a fan can be safely turned down and when. Rules of thumb, such as a minimum exit velocity of 3000 fpm, persistence (we've always done it that way before), or qualitative judgement (it just seems like we can), while sometimes employed, should be avoided when defining safe operation for laboratory exhaust fans.

All pre-functional and implementation activities are carried out when the laboratory ventilation load is at its minimum, often late at night and/or on weekends. This presentation will discuss the specific tests that are included in the analysis, however, it is important to note that the duct static pressure set-point is maintained throughout the testing. Thus, none of the labs should experience any impact during either the pre-functional testing or during the implementation.

If the exhaust fans are already equipped with VFD's then no additional equipment is required for the fan system. An anemometer may be needed if implementing a wind responsive VAV exhaust system, but this is attached to the BMS and does not require any shut down of the HVAC system. If the fans are not currently equipped with VFD's, it may be possible to install the units on one fan at a time during low load conditions and keep the system operating. If not, the laboratory may need to shut down for a short period for installation of the VFD's.

The final step is the commissioning which involves collecting trend data on the laboratory exhaust system performance over a two to three week period. If anomalies are noted, small "tweeks" to the code may be required.

Learning Objectives

  • How VAV exhaust systems can be safely implemented into existing operating laboratories with little or no downtime within the labs.
  • That dispersion modeling should be used to define minimum volume flow rates out of the laboratory exhaust fans.
  • What types of dispersion models should be used to define minimum volume flow rates out of the laboratory exhaust fans and the pro's and con's of each.
  • The five step process involved in SAFELY implementing a VAV exhaust system into an operating laboratory.


Mr. Brad Cochran has over 25 years of experience in dispersion modeling of laboratory exhaust systems. During the past decade, Brad has helped define new design techniques to minimize the energy requirements for laboratory exhaust stacks by utilizing VAV technologies. He has successfully implemented VAV exhaust systems throughout the US, Canada, and the UK. He has authored and presented several papers on laboratory exhaust design for ASHRAE, I2SL, LabWize, R&D Magazine, CMCA, CSHEMA, IFMA, RPIC.


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