The importance of air distribution
The Trox Varyset plenum provides a simple approach to avoid the dumping of air when the flow from a VAV fan-coil unit is reduced.
Good indoor air quality must be considered alongside good air distribution or room air movement, says Terry Farthing of Trox UK.Good indoor air quality must be considered alongside good air distribution or room air movement, says Terry Farthing of Trox UK. Air distribution should be draught-free and include good mixing of air so an even temperature is produced throughout the space. It should also include low air velocities so that the entire space is neutral to the occupants. In other words they should not even notice the air-conditioning system is there.
Conversely poor air distribution creates draughts or high temperature gradients, so occupants complain about comfort levels. How often do people talk about the air-conditioning system not working properly when the reality is that the grille or diffuser has been poorly selected? A simple diffuser costing less than £30 can condemn an entire air-conditioning system worth millions of pounds if air-distribution engineering has not been employed.
On large projects the consulting engineer is more liable to produce a detailed schedule providing specific air-distribution performance criteria that must be met. The ability of manufacturers to demonstrate room air conditions via a mock-up is becoming an increasing consideration. Sophisticated computer analysis such as CFD (computational fluid dynamics) can relatively quickly look at a wide range of variants. However, they cannot accurately predict air velocities in specific circumstances, which is why there is no substitute for real live testing. Not every job warrants such testing, but even in relatively standard applications, data about the per formance of diffusers, performance data supported by such test facilities is vital.
One very serious consideration with VAV fan-coil units is that with standard diffusers there is a risk that the air will dump below, say, 60% of maximum. With conventional VAV the risk of dumping was a major concern for many designers. For VAV FCUs to succeed, this risk needs to be eradicated either through a thorough review of each diffuser’s performance or by utilising anti-dumping devices such as the Trox Varyset plenum. This simple device comprises a segmented plenum box and counter-balanced air-operated damper. The damper covers only two-thirds of the spigot area. At 100% air flow the damper is fully open and air can flow into the plenum box as though it were a conventional unit. As the air flow reduces, so the damper closes to provide constant velocity to the part of the diffuser that maintains the coanda effect. As VAV fan coils become more popular. This will drive the market to a more systemised approach and enforce the importance of good air distribution engineering.
So next time you are feeling uncomfortable don’t blame the air conditioning system, get the air distribution checked out.
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