The core of heat recovery is the heat exchanger. There are various types of heat exchangers availabe including cross-flow, counter-flow (includes rotary/wheel) and cross-counter-flow.
Through a heat exchanger, fresh-filtered air flows into the house continuously and an equal volume of stale air simultaneously flows out of the home. These airflows are allowed to pass by each other - separated only by a thin membrane. The longer the two streams flow past each other, the higher the efficiency.
How a Cross-flow Heat Exchanger Works
The supply air does not reach quite as high a temperature as with the counter-flow heat exchanger, as at two corners temperatures with high differences between them encounter each other. The efficiency is therefore less, even when a very large exchange surface area is provided. In an optimal scenario it reaches about 70% efficiency.
The large temperature difference at one part of the surface means this form of heat exchange reaches its maximum even with small surface areas. The devices can be constructed to be very compact. This is in contrast to the counter-flow heat exchanger, which is more effective the longer it is.
How a Counter-flow Heat Exchanger Works
These can be distinguished in that the counter-flow zone comprises the largest part of the device. At the beginning and end of the device there are very small zones with crossed air streams, and here too the problem of joining streams must be resolved. The efficiency in the case of very long dimensions is entirely dependent on the available surface area and in practice reaches 95%.
How a Rotary/Wheel Heat Exchanger Works
A rotary/wheel heat exchanger is considered a counter-flow heat exchanger. The advantage of the rotary heat exchanger is that the air distribution is more straight-forward than in some other counter-flow heat exchangers. In the latter device there is the issue of the complex guiding of air from one side into the counter-flow zone, and then on the other leading out from the counter-flow zone. The counter-flow heat exchanger also involves relatively complex membranes and complex problems of maintaining air tightness between the membranes, as the two air flows are not to mix. However it's on exactly this point that adrawback to the rotary/wheel exchanger arises. While the other devices presented above took pains to ensure the air flows do not come in contact, the rotary exchanger design accepts this will happen. The cell through which the return air flows will have outside air flowing through it just a short time later. Therefore the airtightness is not good due to the moving wheel and it's more susceptible to leakage between the fresh air and stale airstream.
How a Cross-Counter-flow Heat Exchanger Works
The thermally wasteful corners are omitted thanks to the counter-flow zone. The remaining cross-flow zones do not play as crucial a role if the counter-flow zone has sufficient surface area. With this geometry too, an efficiency of up to 95% can be achieved.
Figuratively speaking, the cross-flow heat exchanger is divided in the middle and pulled apart.
The principle of airstreams passing each other (counter-flow) is applied to the space created. The result is the cross-counter-flow heat exchanger.
- This design has several benefits :
- a relatively small size can be achieved;
- the crossing of the air streams at the front and behind resolves the problem of joining the multiple streams;
- the counter-flow area in the middle gives high efficiency
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