Perhaps the most important design consideration in laboratories with fume cupboards is speed of response and stability.
Speed of Response
As the operator moves the sash, the variable air volume control system should respond by reaching airflow equilibrium within 1 second. Slower speeds of response may create a hazard for personnel. To demonstrate this further the following video tests different speeds. Video courtesy of Phoenix Controls.
Dynamic containment testing similar to what is shown is prescribed in ANSI Z9.5 and ASHRAE Standard 110. Our local standard AS 2243 Part 8 has a requirement for up to 5 seconds. As demonstrated in the video too slow a speed will cause a loss of containment in certain situations.
Manifolding laboratory exhausts is a sustainable design option for mechanical engineers. Rather than enforcing a factory like aesthetic look to the roof with many exhaust stacks and fans, owners can have a single exhaust plenum, fans and stack assembly providing a tranquil setting for the research facility.
When fume cupboard exhausts are manifolded a new variable is introduced to the control system – a pesky instability caused by rapid changes in duct static pressure. While the engineer might consider oversizing ductwork to reduce static pressure, this is rarely possible due to severe spatial and budget restrictions. The best solution is to have pressure independence in the air control valves with a less than 1 second response to static pressure changes.
When both speed of response and stability are under control the manifolded laboratory exhaust system will be safe, efficient and provide years of consistent operation.
Design information and benefits of manifolding laboratory exhaust refer to http://www.i2sl.org/documents/toolkit/bp_manifold_508.pdf
Specific queries for controls design and commissioning contact John Penny. Email: email@example.com