Recovery is key
Air handling units can be equipped with heat exchangers. The purpose of a heat exchanger is to capture the heat or cold in the air or water streams to prevent it from being lost. A heat exchanger can also be used in order to transfer heat or cold to other systems.
The commonest type of heat exchangers are:
- Rotary heat exchanger
- Plate heat exchangers – counter-flow or cross-flow
- Coil heat exchangers
Rotary for high efficiency
A rotary heat exchanger consists of a rotating wheel with a multitude of small ducts made of aluminium. The warmer extract air heats the ducts and transfers heat to the colder supply air.
The temperature efficiency is potentially as high as 85% if the supply air and extract air flow at the same rate. Normally, frost never forms inside a rotary heat exchanger. This means that the high temperature efficiency is sustained, regardless of the prevailing outdoor temperature.
A rotary heat exchanger recovers cooling energy just as efficiently, and with hygroscopic or sorption treatment it also recovers moisture, which cuts cooling costs.
A rotary heat exchanger should not be used if the same air handling unit serves mixed businesses, for example an office and restaurant, as it is the same surfaces that come into contact with both the extract air and supply air.
A coil heat exchanger is the ideal choice when large-volume air flows need to be kept separate
Coil heat exchangers are generally used when you want separate air passages for high-volume air flows. This exchanger has a liquid coil in the supply air and the extract air. The liquid in the extract air coil is heated by the extract air and is pumped to the supply air coil which heats the supply air. The temperature efficiency is over 68% at equal supply air and exhaust air.