HVAC: EXHAUST
HEAT RECOVERY FOR EXHAUST SYSTEMSHEAT RECOVERY FOR EXHAUST SYSTEMS AT HARVARDBLACKSTONE (2006) Provides up to 5100 CFM of 100% outside air for ventilation, Enthalpy energy recovery system is 80% effective, ASHRAE requires 50% efficiency for AHU over 5000 cfm.
HOW IT WORKSThrough a heat exchanger, the heated indoor exhaust air passes its heat to incoming, outside cool air, reducing the burden on the heating system. There may be some added expense due to increased ducting, but this is a low technology system that allows significant energy savings. Blackstone's 100% outside air AHU has an 80% effective enthalphic heat recovery wheel that recovers latent and sensible exhaust air heat. Tempered ventilation is distributed to various areas of the building.
[Diagram and description courtesy Bruner Cott]
RESOURCESFUME HOODSIn 2002-2003, the HGCI assessed a 68,000 square foot lab building with the purpose of identifying options for technical energy efficiency improvements. Built in 1969, the building had six floors plus a basement, and twenty-six fume hoods. In 2001, this building's utility bill was just under a half million dollars.
Initial explorations revealed that the building needed upgrades to comply with codes and standards. It soon became apparent that energy efficiency improvements could pay for the cost of the building upgrades. Cost effective options are listed below. They range from keeping the current constant volume system and adding heat recovery, which would yield a $20,000 annual energy savings and a 4.8 year payback, to switching to a variable air system with heat recovery, which would yield nearly $50,000 per year in savings and a 4.25 year payback – 10% of the building's annual utility costs.
It is critical to point out that energy savings related to the ventilation system were due specifically to reducing air flow at the fume hood. Evidence from projects like these and from Labs21 retrofits has shown that it is typically possible to secure laboratory energy reductions of 33-60%. DESIGNConsider shared hoods. Face velocity refers to the amount of air coming into the hood – the greater the face velocity, the more energy is used. Generally 100 feet per minute is recommended. TYPESConventional: (constant volume) movable sash with constant speed exhaust fan with varying face velocities. Bypass: allows air to bypass sash, reducing variable face velocities. VAV: energy saving adaption to vary air volume based on sash position to maintain a constant face velocity, usually set to 100 fpm. Lower life cycle costs over constant volume, and lower energy consumption. RESOURCESLongwood Campus Energy Reduction Program: Fume Hoods (case studies for night-time set backs for labs and a renovation at HSPH) Longwood Campus Energy Reduction Program: Green Labs Resources for Green Labs at Harvard, including The Case for Sustainable Laboratories: First Steps at Harvard University, by Jessica Woolliams, Matthew Lloyd, and John Spengler. BIOLOGICAL SAFETY CABINETSBiological safety cabinets provide a safe working environment by using fans to pull clean air over the chemicals in use. Air is also drawn from under the work surface and through filters in the top of the cabinet. This prevents contaminated air from re-entering the lab. |
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Updated: Tuesday, June 10, 2008 10:45 AM
