Key facts on heat exchangers

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.

Plate heat exchanger channel separation prevents contamination

A cross-flow plate heat exchanger consists of thin aluminium panels – plates – arrayed to form crossed air ducts. The warmer extract air heats the plates and in so doing transfers heat to the colder supply air.

A counter-flow plate heat exchanger is based on the opposite principle of the cross-flow heat exchanger. However, its construction features a parallel section, giving it a larger contact surface than the cross-flow plate heat exchanger.

The temperature efficiency where the supply and extract air flow at identical rates is approx. 65% for a cross-flow plate heat exchanger and up to approx. 80% for a counter-flow plate heat exchanger. The supply air and extract air flow in completely separate ducts, meaning that any odours or particles in the extract air cannot return to the 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 as high as 60% if the supply air and extract air flow at the same rate.