It is very cost efficient, in a ventilation system, to recover the heat in the exhaust air and use it to heat the supply air.

There are several ways of recovering this heat.

 

PLATE HEAT EXCHANGER:

 

This type of exchanger is usually made of aluminium. Plastic is however used in small heat exchangers. Exhaust air and supply air pass on each side of a number of plates or discs in the heat exchanger. The ducts carry alternately exhaust air and supply air. The warm exhaust air heats the aluminium plates on the exhaust air side and the cooler supply air is therefore heated on its side of the plate. The longer the duct, the higher is the heat transfer and therefore the efficiency achieved. This however also increases the pressure drop and the fans need to work harder to maintain air flow.

Condensation can form under some conditions in a plate heat exchanger.

 

The exchanger must therefore be fitted with condensate recycling or a condensation drain. The drain should have a water lock to avoid drain odours being drawn into the unit. A gurgle stopping device should be fitted to the condensation drain to avoid gurgling due to the lower pressure in the unit than in the drain.

 

Condensation also represents a risk of freezing. Some form of defrosting system is therefore required. At lower air flows, the supply air fan can be shut down for a short space of time to allow the exhaust air heat to warm and melt any frost. The supply air fan then automatically starts again. At higher air flows, freezing can be controlled by a bypass-valve which controls the intake of outdoor air. A preheater coil for elctrical or water heating can be used to prevent freezing. This usually results in a very flexible system.

 

Heat recycling is best controlled using a stepless bypass valve which controls air flow past the exchanger when no recycling is required.

A big difference between this and the rotating type is that air flows are fully separated. There is therefore normally no leakage of smell to the supply air from the exhaust air. The rotating exchanger, which normally achieves higher levels of heat recovery, has a number of moving parts and a motor. This means higher service costs. The counter flow heat exchanger is therefore, from a LCC (Life Cycle Cost) perspective, a more attractive option than a rotating heat exchanger. Plate heat exchangers contain no moving parts.

 

There are two different types of plate heat exchanger: cross flow heat exchanger and counter flow heat exchanger.

 

CROSS FLOW HEAT EXCHANGER:

 

The cross flow heat exchanger has a square cross section and an efficiency of around 60 %, where the air flow on the supply air and the exhaust air side are equal. 60 % of the exhaust air heat is transferred into the supply air. One way of improving the efficiency of cross flow heat exchangers is to connect two in series. The pressure drop increases and efficiency rises to around 70 %.

 

COUNTER FLOW HEAT EXCHANGER:

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Counter flow heat exchangers are beginning to be the most common alternative. The counter flow heat exchanger is a development of the cross flow heat exchanger and is a very good compromise, giving high recovery levels of usually more than 80 %, relatively low pressure drops and has no moving parts. It has a cross section, which means that the area where air flows and heat is exchanged, is bigger.

 

The highest levels of heat exchange are achieved where the air is supplied in opposite directions, thus the name 'counter flow heat exchanger'. 

 

ROTATING HEAT EXCHANGER:

 

In a rotating heat exchanger, heat is transferred directly between the exhaust air and the supply air. The system is not fully sealed and there is a risk that contaminants and smell are transferred between the exhaust air and the supply air. A purge sector should be built-in to the unit, which ensures that air transfer is as little as possible. Heat recovery rates can be regulated by increasing or reducing the rotational speed. The risk of freezing is small in this type of heat exchanger. The rotating heat exchanger is driven by an electric motor and a belt to the rotor.

 

High efficiency (75-90 %).

 

COIL HEAT EXCHANGER:

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Water or water mixed with glycol circulates between a water coil in the exhaust air duct and a water coil in the supply air duct. The fluid is heated in the exhaust air duct, which then releases the heat to the air in the supply air duct. The fluid flows in a closed system and there is no risk of contaminants being transferred between the exhaust air and supply air.

 

This type of heat exchanger is the preferred option when you need to be completely sure that no transfer takes place, for example in a hospital, laboratory and by activities where there is a great deal of smell. The coil heat exchanger is also well suited to installations where the supply air fan and the exhaust air fan are not located in the same unit, for example where the exhaust air fan is outside and the supply air unit is located in the basement.

Heat recovery rates can be regulated by increasing or reducing the water flow rate. Coil heat exchanger contain no moving parts.

Low efficiency (45-50 %).

 

CHAMBER HEAT EXHCANGER:
 

A chamber is subdivided by a damper into two parts.The exhaust air first heats up one part of the chamber, then the damper changes the air flow so that the supply air can be heated by the heated section. There is however a high risk that contaminants and smell can be transferred between the exhaust air and supply air. The only moving part in the chanber heat exchanger is the damper.

 

High efficiency (80-90 %).