An Energy Success Story – Running VAV Laboratory Exhaust Stacks without By-Pass

Jeff Reifschneider, CPP, Inc.
Sean Convery, Cator, Ruma & Associates

The University of Colorado Jennie Smoly Caruthers Biotechnology Building (CU BIO), which is now fully constructed and occupied, was designed with the intent of running the exhaust system on VAV with 'no by-pass' and 'no fume-entry.' The major challenge to meeting this goal was the desired positioning of the outside air intakes. Due to architectural considerations, the intakes were to be located on several high building elements, affecting the height of the stacks.

To help meet the design objectives, a wind tunnel modeling study was conducted during which a 1:240 scale model of CU BIO and nearby surroundings within a 1360 ft radius was constructed and placed in CPP's boundary-layer wind tunnel. Two configurations were evaluated: CU BIO alone (Configuration 1) and CU BIO with a future Chemistry Building (Configuration 2). A tracer gas was then released from the model laboratory exhaust stacks and concentrations were measured at the intakes and other sensitive locations. The concentrations were then compared to the design criterion which initially was 'no fume entry,' an impossible design goal because some fumes will always reenter the intakes. A realistic design goal was then established based on the intended chemical utilization and one that was consistent with ASHRAE recommendations. Various stack heights, exit velocities and volume flow rates were evaluated so that the optimum combination of stack height and minimum flow/velocity could be specified. Sufficient concentration data versus wind speed and wind direction were collected so that the probability of exceeding the fume reentry design goal could also be assessed. In the end, minimum stack heights and volume flows/exit velocities were specified such that the exhaust system could run VAV with no by-pass while meeting the fume reentry criterion.

This talk will provide detailed information on the exhaust and intake system as-built design and discuss the challenges that were faced in meeting the original goals of VAV with 'no by-pass' dampers along with 'no-fume reentry.' Information will be provided on how wind tunnel modeling was instrumental in ensuring these project design goals were met.

The original intent of designing the building to run VAV with no by-pass was for energy savings, simplistic controls and first cost savings. Not only are there energy savings from reduced conditioning of outside air typical with VAV systems, this design also allowed a true VAV savings of reduced fan energy by not having any by-pass air or pressure drop caused by induction nozzles.

Learning Objectives

  • Show how a laboratory exhaust system can be designed to operate with VAV with no by-pass dampers.
  • Show how air intake placement can be critical to the final exhaust design and VAV implementation.
  • Show how local wind conditions can be critical to air intake placement and final exhaust design.

Biographies:

Jeff Reifschneider is a Senior Engineer, with CPP since 2004, who conducts dispersion modeling studies related to laboratory exhaust design and regulatory applications. At CPP, Jeff has consulted for UC Denver, the Denver VA Medical Center Replacement Project, the Department of Homeland Security (DHS), and the Centers for Disease Control (CDC). Jeff participated in an ASHRAE research study focused on enhancing plume rise by placing exhaust stacks close to each other, known as stack ganging. Jeff is a member of the Air and Waste Management Association (AWMA) and the Program Director for the Colorado I2SL Chapter.

Sean T. Convery, PE is a Mechanical Principal at Cator, Ruma & Associates in Denver, CO. His 19 years of mechanical design experience include energy-efficient mechanical systems for higher education campuses and research labs. Recent projects include the University of Colorado Boulder's Sustainability, Energy and Environment Complex (LEED Gold Pending), Biotechnology Building (LEED Platinum), and Colorado State University's Suzanne and Walter Scott, Jr. Bioengineering Building (LEED Gold).

 

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