Variable volume HVAC systems can offer large energy savings for laboratory applications, but they present special design and operational problems. Although the design considerations are similar to those for a conventional VAV air-conditioning system, more careful analysis of safety issues is required for VAV systems. VAV fume hood control systems have received considerable review and analysis due to their proven energy savings. [Baird, 2001] [Rios, 1999]
VAV control maintains a constant face velocity to ensure safety and save energy by varying exhaust and make-up air volume. SAMA Standard LF-10-1980 (SAMA 1980)suggests that for most commonly used Class B fume hoods; 100 fpm (0.508 m/s) is a reasonable average face velocity for safe operation. Generally, a modulating damper in the fume hood exhaust duct is used to maintain the precise air flow necessary for a given sash position, providing continuous on-line control of the flow through the fume hood. The supply and make-up air are controlled to provide a negative balance of air flow into the room. [Wendes, 1990; Laboratory Control and Safety..., 1994]
According to Wendes (1990), additional desired features and benefits of VAV systems include:
·Remote monitoring of sash position, which allows user compliance and energy waste to be monitored,
·Remote monitoring of face velocity and laboratory pressurization, which provides the facility operator with valuable energy and safety information,
·Integrated control of total exhaust and supply volumes to maintain both laboratory room temperature and negative pressurization,
·Ability to solve chronic negative building pressure problems by reducing average exhaust volume, and
·Emergency exhaust mode actuation with a push button that overrides normal system operation for maximum exhaust in case of a lab spill or fire.