Perhaps the most important health and safety control measure in science laboratories is the fume cupboard. Raising and lowering a sash by an operator will normally cause a major disturbance on a control system. This requires a fast response to maintain stability and precision.
Speed of response
As an operator moves the fume cupboard sash, the control system needs to respond by reaching stable airflow control in less than 1 second. A slower speed of response may create a hazard for personnel.
Dynamic containment testing is prescribed in ANSI Z9.5 and ASHRAE Standard 110. Australia’s standard is AS 2243 Part 8.
Pressure independent control
Rather than enforcing a factory like aesthetic look to the roof with many dedicated exhaust stacks and fans, owners and designers can now operate an energy efficient manifolded laboratory exhaust system. This consists of a roof plenum, redundant fan set and stack assembly.
However, when designing a manifolding exhaust system, a new variable is introduced to control – a pesky instability caused by rapid changes in ductwork static pressure due to lowering and raising fume cupboard sashes by operators.
While the engineer might consider oversizing ductwork to reduce static pressure, this is rarely possible due to spatial and budget restrictions. Instead, we recommend greater control by designing in a venturi air valve at each fume cupboard with an integral mechanical pressure independent assembly. These valves are designed to reliably deliver instant response to pressure changes.
When both speed of response and stability of flow are under control, the system will be safe and resilient.
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