Powered Plenum Bypass - Reduce Laboratory Exhaust Fan Energy and Maintain Safety

Glenn Schuyler, M.A.Sc., P.Eng., RWDI
Joel Good, M.A.Sc., P.Eng., LEED AP, RWDI

A great many authors have discussed the savings to be had in reducing laboratory exhaust fan energy . The lion's share of discussion has been around ways to lower the amount of air exhausted. This discussion is valuable; much of it pointing out that reduction of ventilation air in laboratories must be subordinated to safety. Variable volume systems and implementation of night setback have provided significant savings. One limitation that has been cited is the need to maintain a safe level of dispersion from the exhaust stacks, to avoid negative impacts at intakes on or near the laboratory . One method to overcome this limitation has been proposed and has been implemented in a few cases. The method consists of providing an active control of fan speed based on wind speed and direction information from anemometer instruments on site . This method, while increasing the energy savings, requires a significant investment in equipment and commissioning effort, and itself has practical limits due to the variability of wind currents around buildings, and the need to stabilize fan speed.

Another approach which the authors think may be more practical is to provide bypass air to the main exhaust stack, downstream of the fan, via a secondary fan not connected to the building's exhaust plenum. Many buildings are designed with exhaust plenum pressures in the range of -4 to -6 inches of static pressure (-1000 to -1500 Pa). A typical bypass damper allows outside air into the plenum which then is exhausted through the fan against that pressure. By supplying bypass air directly to the main exhaust stack, the bypass fan is not required to work against the negative pressure of the plenum, reducing the electrical consumption by a percentage only slightly less than the turndown ratio itself. By simply slaving the bypass fan to the primary exhaust fan, the volume flow from the stack can be maintained, and the primary exhaust fan can be turned down to any level, appropriate for the building, without fear of reducing dispersion from the stack. No monitoring of wind conditions with anemometers is required for this method of fan control. At least one fan manufacturer has recognized this saving and provides a specialized fan for the purpose.

The paper will compare the potential savings and energy implications from this system with the active feedback system, using wind tunnel measurements and weather information from a typical campus laboratory.

Biographies:

Glenn has been with RWDI since 1981 and is a Vice President and Principal. He holds a Masters Degree in Aerospace Engineering from the University of Toronto, and is a professional engineer in the Province of Ontario. He is a Fellow of the American Society of Heating Refrigeration and Air-Conditioning Engineers, and a member of the International Society for Pharmaceutical Engineers. Glenn has worked on a wide variety of projects, including exhaust dispersion and stack design, environmental noise, fume hood performance, ventilation, wind energy, and sustainable design. He is a frequent presenter for ASHRAE and Labs 21.

Joel Good has been with RWDI since 2007 as a Building Performance Design Consultant. He has a Masters Degree in Environmental Engineering at Dalhousie University in Halifax, Nova Scotia, and is a Professional Engineer in the Province of British Columbia. He is a member of the American Society of Heating, Refrigeration, and Air-Conditioning Engineers, the Canadian Green Building Council, and the International Building Performance Simulation Association. Joel has worked on numerous studies for building energy usage, natural ventilation, daylighting, and master planning. He is a LEED Accredited Professional.

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