HEAT EXCHANGER WITHIN VENTILATION
I en ventilationsanläggning är det mycket lönsamt att återvinna den värme som finns i frånluften och använda den till att värma upp tilluften. Det finns flera metoder eller typer av värmeväxlare för att åstadkomma en energieffektiv ventilation med värmeåtervinning.
BATTERY HEAT EXCHANGER
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 and then emits that heat to the air in the supply air duct. The liquid moves in a closed system and there is no risk of contamination being transferred between exhaust air and supply air.
This type is therefore preferable if you have to be absolutely sure that no transfer takes place, e.g. in hospitals, laboratories and businesses with a lot of smells. The battery heat exchanger also fits well if the supply air fan and exhaust air fan are not placed in the same unit but the exhaust air fan e.g. is in the attic and the supply air unit is 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%).
A chamber is divided by a damper into two parts. The exhaust air first heats 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 impurities and odors can be transferred between exhaust air and supply air.
The only moving part in the chamber exchanger is the damper. High efficiency (80-90%).
CROSS CURRENT EXCHANGER
The cross-flow heat exchanger, which has a square cross-section, has a temperature efficiency of approx. 60% with equal air flow on the supply air and extract air side. 60% of the exhaust air heat is thus transferred to the supply air. One way to improve the efficiency of cross current exchangers is to connect two in series. Then the pressure drop increases slightly and the efficiency increases to approx. 70%.
COUNTERFLOW HEAT EXCHANGER
The counter-flow heat exchanger, which is becoming the most common option, is a further development of the cross-flow heat exchanger
and a very good compromise with high recovery, usually over 80%, relatively low pressure drop and no moving parts.
It has a cross-section which means that the air currents have a longer path to exchange the heat.
In order to obtain as large a heat exchange as possible, the air flows need to be supplied in the opposite direction, hence the name "counter-flow heat exchanger".
ROTARY HEAT EXCHANGER
In a rotating heat exchanger, heat is transferred between the exhaust air and the supply air. The system is not completely sealed and there is a risk that impurities and odors are transferred between exhaust air and supply air. There should be a so-called clean blowing sector built in which means that the air transfer is as small as possible. The degree of heat recovery can be regulated by increasing or decreasing the rotation speed. The risk of freezing in the heat exchanger is small.
The rotating heat exchanger is driven by an electric motor with a belt drive to the rotor. High efficiency (75-90%).
PLATE HEAT EXCHANGER
This type of exchanger is usually made of aluminium, but plastic is also used as a material for small heat exchangers.
In the heat exchanger, exhaust air and supply air pass on each side of a number of plates or slats. Every other channel is extract air and every other is supply air.
The hot exhaust air heats the aluminum plates on each side of the supply air duct and the cold supply air is thereby heated.
The longer the duct, the higher the heat transfer and thus the efficiency, but unfortunately the pressure drop for the air also increases and the fans have to work harder to maintain the air flow.
In certain operating modes, there is a certain amount of condensation in a plate heat exchanger and therefore it must be equipped with recycling of the condensate or a condensation drain. The drain should have a water trap to prevent sewage odors from entering the unit. In order to avoid "clunk noise" due to negative pressure in the unit in relation to the drain, it is advisable to fit a so-called clack stop in the condensation drain.
Because of the condensed water, there is also a high risk of frost build-up and it is therefore necessary to have some kind of defrosting system.
With smaller air flows, you can momentarily close the supply air fan to allow the exhaust air heat alone to warm away the freezing.
When this is done, the supply air fan automatically starts again. In case of larger air flows, the freezing can be regulated with a bypass damper that controls the intake of outside air. A pre-heating battery for electric or water heating can also be used to prevent freezing.
It is usually a very flexible system. The heat recovery is best regulated using a stepless bypass damper that controls the air flow past the exchanger when no recovery is needed.
A big difference to the rotating type is that the air currents are completely separated and normally there is no odor return to the supply air from the exhaust 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 countercurrent exchanger will above all be more attractive in relation to the rotary heat exchanger. The plate heat exchanger contains no moving parts. Basically, there are two different types of plate heat exchangers: cross-flow heat exchangers and counter-flow heat exchangers.
Read more about our FTX units and how we use heat exchangers to save energy.