Perhaps the most important operational requirement in chemistry laboratories is the rapid disturbances from the opening and closing of fume cupboard sashes. Such disturbances require a fast response from the airflow control system to maintain stable and precise conditions in both the laboratory and fume cupboard.
Speed of Response In Less Than 1 sec
As the operator moves the fume cupboard sash, the 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.
Rather than enforcing a factory like aesthetic look to the roof with many dedicated exhaust stacks and fans, owners can now operate a single manifolded exhaust plenum, fans and stack assembly.
With manifolding an exhaust system a new variable is introduced to control – a pesky instability caused by rapid changes in duct static pressure caused by lowering and raising sashes by operators on fume cupboards. While the engineer might consider oversizing ductwork to reduce static pressure, this is rarely possible due to spatial and budget restrictions. It is better to have a mechanical pressure independent assembly built into the air control valve to deliver instant response to pressure changes.
When both speed of response and stability of airflow are under control the manifold laboratory exhaust system can be safe and efficient.