Designing Safe and Energy Efficient Lab Exhaust Stacks in New Construction

Brad Cochran, CPP, Inc.

The traditional laboratory exhaust system consists of multiple constant volume exhaust fans feeding off of a central manifold; often with n+1 redundancy. If nighttime turndown is applied, one fan may stage off in the evening, but the remainder of the time all but one fan are operating at full speed 24 hours a day, 365 days a year. The difference between the supply air requirements and the exhaust volume flow rate is accounted for with bypass dampers that maintain the desired static pressure within the exhaust duct. There are two issues with this design: 1) If you inspect the operation of the bypass dampers after 5 to 10 of operation you often find that they are no longer functioning properly; either stuck open, stuck closed, or excessively slow in responding to pressure changes. This means that the laboratory has either lost, or has very poor duct static control since this design relies on the functionality of the bypass dampers. 2) The constant volume/staged design is the reason that laboratory exhaust systems are often responsible for up to 30% of the energy consumption of the entire laboratory.

A better approach is to tailor the design of the exhaust system to safely operate under a combination of VAV operation and staging. Dispersion modeling can not only be used in its traditional sense to define the optimum design of the stacks (height and diameter) but, in addition, it can also be used to define the appropriate number and size of fans that should be included in the design and the most energy efficient sequence of operation for the exhaust system.

Ultimately, the exhaust system should be designed such that the speed laboratory exhaust fans are used to control the duct static pressure rather than relying on a bypass damper. A variable frequency drive on an exhaust fan is much more responsive and reliable than a bypass damper. Also, maximizing the number of operating fans, including the redundant fan, can save energy, reduce wear and tear on the fans, and reduce noise within and exterior to the laboratory.

This presentation will discuss how dispersion modeling can be used to optimize the whole laboratory exhaust system, not just the stack height and diameter, through the proper placement of exhaust stacks and air intakes, defining the optimum number and size of fans to meet the building needs, and defining the most energy efficient and SAFE sequence of operation. The result is a system that is not only less expensive to operate, but may cost less to construct, and provides greater long term resiliency.

Learning Objectives

  • The traditional constant volume manifolded exhaust system with n+1 redundancy is not cost or energy efficient nor is it necessarily reliable due to its dependence on bypass dampers to maintain duct static pressure.
  • How dispersion modeling can be used to optimize fan selection and the sequence of operation in new laboratory buildings.
  • Utilizing VFD's on laboratory exhaust systems rather than relying on bypass dampers to maintain the desired duct static pressure can result in a more efficient and reliable exhaust system.
  • How through proper selection of the number of fans and the fan size, the need for bypass dampers can be eliminated from a design.

Biography:

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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