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 typically found in the field.
Dynamic containment testing is prescribed in ANSI Z9.5 and ASHRAE Standard 110. Australia’s standard AS 2243 Part 8 has a requirement for up to 5 seconds.
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.
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 mechanical engineer might consider oversizing ductwork to reduce static pressure, this is rarely possible due to spatial and budget restrictions. The best control solution is to have mechanical pressure independence in the air control valve itself and a performance of < 1 second response to static pressure changes.
When both speeds of response and stability are under control the manifold laboratory exhaust system will be safe, efficient and consistent operation.