GOOD INDOOR AIR QUALITY
09/12/2008
Varyset is a TROX anti-dumping device
Good indoor air quality must be considered alongside good air distribution or room air movement. 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 and gets the occupants complaining about comfort levels. How often do people talk about the air conditioning system not working properly when what they really mean is that the grille or diffuser has been poorly selected? Yes, a simple diffuser costing less than £30.00 can condemn the entire A/C 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 the velocities that will exist in a room in specific circumstances and that is why there is no substitute for real live testing. Clearly not every job is big enough to warrant this type of testing but even in relatively standard applications diffuser performance data supported by such test facilities is a vital consideration.
Through investment in research and development many new ideas and concepts can be effectively introduced to the market place. One such concept is VAV Fan coils systems.
One very serious consideration with VAV fan coils 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 FCU 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 Varyset plenum. This is a very simple device comprising of a segmented plenum box and counter balanced air operated damper – see figure 1. Note the damper covers only 2/3 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. But as the air flow reduces so the damper closes, providing constant velocity to part of the diffuser which 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.
