VAV FAN COILS AT CIBSE SEMINAR
24/06/2008
VAV Fan Coils were featured in a one day presentation organised by CIBSE at their engineering centre in Balham a few weeks a go.
Barry Trewhitt, Product Manager at TROX UK Ltd, gave a detailed analysis of the potential energy savings that can be achieved with this new system. He commented that up until now fan coils have run at 100%, i.e. constant volume irrespective of the cooling or heating requirements. The analysis showed that by varying the air volume as the loads changed could make significant energy and carbon emission savings. On a specific office project in London TROX analysed the energy consumption and found that with VAV fan coils a staggering 75% reduction in motor power could be achieved. This represented an annual reduction in carbon emissions for each fan coil unit from 166kg CO2 to 42kg CO2.
With VAV fan coil units maintaining good air distribution is of paramount importance because there can be a risk of diffusers “dumping” at the lower air volumes. There are two ways of overcoming this, firstly careful selection of the diffusers and possibly combining the components into a complete fan coil system where a single manufacturer takes responsibility for performance of the whole system. Secondly there are anti dumping devices typical of which is the TROX Varyset plenum. This is a very simple device comprising of a segmented plenum box and counter balanced air operated damper – see picture 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.
