HEAT EXCHANGERS

In a ventilation system in a house or building, it is very profitable to recycle the heat contained in the exhaust air and use it to heat the supply air. There are several methods for achieving such heat recovery.

BATTERY HEAT

Water, or water mixed with glycol, circulates between a water battery in the exhaust air duct and a water battery in the supply air duct. In the exhaust air duct, the liquid is heated to then deliver the heat to the air in the supply air duct. The fluid moves in a closed system and there is no risk of transmitting contaminants between exhaust air and supply air. This type is therefore preferable if you have to be absolutely sure that no transmission takes place, for example. in hospitals, laboratories and businesses with very smells. The battery heat exchanger is also suitable if the supply air fan and the extract air fan are not placed in the same unit without the extract air fan eg. sits on the wind and the supply air unit sits in the basement. The heat recovery can be regulated by increasing or decreasing the water flow. The battery heat exchanger has no moving parts. Low efficiency (45-50%).

CHAMBER CHANGER

A chamber is divided by a damper in two parts. The exhaust air first heats up one part of the chamber, then the damper switches the air flow so that the supply air can be heated by the heated part. There is a great risk that contaminants and odors can be transferred between exhaust air and supply air. The only moving part in the chamber changer is the damper. High efficiency (80-90%).

CROSS-FLOW EXCHANGER

The cross-flow heat exchanger, which has a square cross-section, has a temperature efficiency of about 60% for equal air flow on the supply air and exhaust air sides. 60% of the exhaust air heat is thus transferred to the supply air. One way to improve the efficiency of cross flow exchangers is to connect two pieces in series. Then the pressure drop increases slightly and the efficiency increases to about 70%.

 

COUNTER CURRENT HEAT

The counter-current heat exchanger which is becoming the most common alternative is the further development of the cross-flow heat exchanger and a very good compromise with high recycling, usually over 80%, relatively low pressure drop and no moving parts. It has a cross section that allows the air streams to have a longer path to switch the heat. In order to obtain as much heat exchange as possible, the air flows need to be supplied in the opposite direction, hence the name "counter-current heat exchanger".

ROTATING HEAT EXCHANGER

In a rotary heat exchanger, heat is transferred between the exhaust air and the supply air. The system is not complete and there is a risk of contamination and odor being transferred between exhaust air and supply air. There should be a so-called clean-blowing sector built in which makes the air transfer as small as possible. The degree of heat recovery can be controlled by increasing or decreasing the rotational speed. The freezing risk in the heat exchanger is small. The rotary heat exchanger is driven by an electric motor with belt drive to the rotor. High efficiency (75-90%).

PLATE HEAT

This type of changer is usually made of aluminum, but also plastic is present as a material for small heat exchangers. In the heat exchanger, exhaust air and supply air on each side pass through a number of plates or slats. Every other channel is exhaust air and every other is supply air. The hot exhaust air heats the aluminum plates on either side of the supply air duct and the cold supply air is thereby heated up.

 

The longer the channel is, the higher the heat transfer and thus the efficiency, but unfortunately the pressure drop for the air also increases and the fans must work harder to maintain the air flow. In some operating modes it becomes a certain condensation in a plate heat exchanger and therefore it must be equipped with recovery of the condensate or a condensation drain.

 

The drain should have a water trap to prevent drainage of sewage into the unit. In order to avoid "chuck sound" due to underpressure in the unit in relation to the drain, it is advisable to mount a so-called chuck top in the condensation drain. Due to the condensation water, there is also a great risk of frosting and it is therefore necessary with some kind of defrosting system. In case of smaller airflows, one can instantly close the supply air fan to allow the exhaust air heat alone to heat the freezing.

 

When this is done, the supply air fan starts automatically again. With larger airflows, the freezing can be regulated with a bypass damper which controls the intake of outdoor air. Even a preheating battery for electric or water heating can be used to prevent freezing. It is usually a very flexible system. The heat recovery is best regulated by means of a stepless bypass damper that controls the air flow past the exchanger when no recycling is needed.

 

A big difference to the rotating type is that the air flows are completely separated and normally no odor return to the supply air from the extract air. The rotary exchanger, which normally has a slightly higher recovery, has several moving parts and an extra motor, which means that the service cost increases and seen from an LCC (Life Cycle Cost) perspective, the counter-current exchanger in particular will become more attractive in relation to the rotating heat exchanger.

 

The plate heat exchanger contains no moving parts. In essence, there are two different types of plate heat exchangers: cross flow heat exchangers and counter flow heat exchangers.

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