Varying Exhaust Stack Discharge Velocity to Improve Energy Efficiency and Performance—Case Studies

Gregory Johnson, P.E., Newcomb & Boyd Consultants and Engineers

Objectives:

Traditionally, discharge velocity through laboratory exhaust system stacks has been constant, whether from a traditional centrifugal fan and stack arrangement or through a high plume type fan. The objectives for the presentation will be to show, through the use of two case studies, that varying laboratory exhaust stack discharge velocity may be possible and provides many benefits. The presentation will include the benefits of this system control strategy, including energy savings, and improved acoustical performance, reliability, and stability.

DOE II life cycle cost analysis and video of wind tunnel studies for the specific case studies will be presented and will be used to provide evidence of system performance.

Findings:

Traditionally, discharge velocity through laboratory exhaust system stacks has been constant whether from a traditional centrifugal fan and stack arrangement or through a high plume type fan. This was done to minimize the potential of entrainment of the exhaust plume into the stagnation zone around the building created as wind passed over the facility. ASHRAE provides methodology for estimating the size of these stagnation zones and required stack height and velocity to reject the contaminated exhaust into the wind stream that is unaffected by the building.

The growing use of high plume type fans and their resulting high effective stack height has opened the potential for varying the stack velocity. During the design for a new laboratory facility for The Centers for Disease Control and Prevention, we found that many aspects of the design and the owner's criteria made varying the stack discharge velocity attractive. The energy savings are significant. The approach improved acoustical performance. Most importantly though, for this facility, this strategy permits the exhaust equipment to be oversized to provide the required future growth capacity and redundancy without an energy penalty and with greater stability.

The design team performed a DOE II energy study to analyze the life cycle cost advantages of the approach and wind tunnel modeling of the exhaust to verify that the exhaust plume would be safely carried out of the stagnation zone even at minimum flow.

Labs21 Connection:

This presentation reflects many aspects of the Labs21 approach. The energy savings with the system are significant and are calculated with a life cycle cost approach. Reducing energy use minimizes the impact of the facility on the environment, which is a fundamental aspect of Labs21 approach. The system described in the proposed presentation is a break away from traditional exhaust system design and represents a new approach that is sustainable, while remaining safe. Using analytical tools including the DOE II study and wind tunnel testing to confirm and predict economic and environmental performance, while improving energy efficiency and overall system performance is consistent with the Labs21 vision.

Biography:

Gregory Johnson earned his Bachelor of Mechanical Engineering degree from the Georgia Institute of Technology in 1988. He is a licensed Professional Engineer (PE) in Alabama and Georgia, and is a member of the American Society for Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE).

While attending Georgia Tech, Mr. Johnson served in the U.S. Naval Reserve from 1984 to 1988, and the U.S. Navy from 1988 to 1989 at the U.S. Naval Nuclear Power School, Orlando, Florida, where he held a top secret security clearance. Mr. Johnson joined Newcomb & Boyd in 1989 and was named an Associate of the firm in 1999 and an Associate Partner in 2001.

During his thirteen years with Newcomb & Boyd, Mr. Johnson has developed particular expertise in the design of mechanical systems for laboratory and research facilities. His experience has included projects totaling more than 2.6 million square feet and over $450 million construction value.

Mr. Johnson's specific project experience includes successful projects for clients including the Centers for Disease Control and Prevention, Auburn University, Birmingham-Southern College, Tulane University Medical Center, Emory University and Emory University Hospital, New Orleans Children's Hospital, University of Alabama at Birmingham, the United States Environmental Protection Agency, and the United States Army Medical Command.