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Myth Buster – Cross Contamination in Thermal Wheels

Myth Buster Blog

By Andrew Bott, Commercial Sales Manager, January 2016

Thermal wheels recover energy from a buildings exhaust air by transferring heat (and humidity) collected in the extract air stream, and depositing it in the supply air flow.

This is achieved by placing a rotating wheel made up of corrugated metal fins wrapped around a central axial, in the supply and extract air streams. As the wheel rotates it passes through the extract (exhaust) air collecting heat energy as it goes. When the (now warm) fins or capillaries pass into the supply air stream the recovered heat energy is dissipated into the supply air stream, and is returned to the building.

Many consultants, contractors and end users hold the belief that as the thermal wheels rotate, and the fins (essentially tiny metal tubes) pass from the extract air to the supply air, they carry with them a small amount of the extract air which would contaminate the supply air with nasty smells and pollutants. General consensus states that there is usually around 4% to 6% of cross contamination occurring within a thermal wheel. Whilst this may be true of some products available on the market, I can report that Nuaire solve this problem with the use of a purge sector.

The purge sector works by allowing a small amount of fresh air to escape into the extract duct. This then purges the contaminated air from the wheel structure preventing it from entering the incoming air. The purge sector is sized accordingly to prevent any energy loss. This in turn keeps the high efficiency of the wheel, with the minimum contamination.

This can reduce the cross contamination from 6% down to 0.04% (depending on conditions). The purge sector is situated on the thermal wheel where the extract air is about to rotate into the incoming air. Whilst I would not recommend the reliance on the purge sector for applications such as laboratory or hospital applications where absolute air stream separation may be more suitable, thermal wheels with a purge sector are perfectly suited any application where you might normally select a traditional plate exchanger.

Thermal wheels can provide much higher recovery efficiencies than traditional plates at higher volumes, so if you are considering an application that has a flow rate of over 0.5m3/sthis could be the perfect solution. If your flow rates are below 0.5m3/s you should consider the XBC range of counter flow heat exchangers.