Controlling the pressure drop (Pd) across the openings when designing a natural or mechanical ventilation system for a power generation plant is central to ensuring it works effectively.
Control the neutral layer to ensure the system’s efficiency
All buildings with heat sources have a neutral layer. Openings above the neutral layer will exhaust heat and openings below it will provide air inlet. In general, the further an opening is form the neutral layer the more efficient it will be. It is therefore important to control the position of the neutral layer for maximum ventilation efficiency and this is done by control of the Pd across the openings.
Control Pd by restricting velocity across openings
When designing the ventilation system, you will need to assess the effect that the various obstructions to the flow of air through ventilation openings such as louvres, attenuators or ventilators, will have on the overall effectiveness of the system, as losses (expressed as a k factor) can be significant. Accurate information allows you to manage this by controlling the velocity across the openings, taking control of the Pd.
Design sufficient low-level inlet openings to avoid under-pressure
Under-pressure can result from a design providing insufficient low-level inlet openings. Air will take the path of least resistance and may come through other routes than the designed ventilation openings. It may come in from other areas or bring in dust as it comes from openings not designed for inlet. The Pd across inlet openings should be kept low and should never exceed 20Pa.
Avoid over-pressure with mechanical inlet
When designing a ventilation system with mechanical inlet, particular care should be devoted to the exhaust (whether this is natural or mechanical) in order to ensure that the Pd is kept low.
Why it’s important to get it right: safety and efficiency
In addition to the inefficiencies we have seen, getting it wrong can have serious consequences on the building’s safety. Under or over-pressure can result in difficulty opening doors or, worse, doors being “pushed closed”. This can be very dangerous in a fire situation, making it difficult for people to escape and for the fire service to gain access and deal with the fire.
Paul is a Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.
