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Old 02-21-10, 01:33 PM   #3
Piwoslaw
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Thanks Ryland A small air-air heat pump (like an old A/C unit) could be hooked up after the heat exchanger, increasing the amount of heat transfered, or reducing the temp difference on both sides of the AC, depending on how you look at it. In fact, there will already be a heat pump in the system we are planning, it will transfer heat from waste air to the hot water tank. That will be the first step, the second - a heat exchanger for intake/waste air, and finally a ground source heat exchanger to preheat (precool in the summer) before the recuperator.

I looked around and found a few pics. Here is the inside of a cross-flow recuperator:



And here is how it works:



Here is a homemade plywood construction that uses 2 cross-flow recuperators in a counter-current set-up (increasing efficiency):



Here's the inside of a large area, counter-current heat exchanger:



It is made of scores of thin sheets of copper or aluminum, stacked 4-5mm apart. The space in between sheets is closed off at every other end (let's call them even) at one end and odd ends at the other. That way the two airstreams don't mix, but exchange heat over an area equal to the area of all of the sheets (minus 2). This is very efficient, but large, heavy and expensive. On the other hand, if space isn't a problem, and the cost is not more than a commercial heat exchanger, then the higher efficiency may make this option worth while.

Here is a pictures of someone's finished project:



You can see the smaller diameter bypass for when you do not want to lose/gain heat.

An idea I got while looking through these projects was to use a long length of thin, flexible tubing inside a larger diameter, insulated tube (like Ben's DIY Solar Hot Water Heat Exchanger). This could be a temporary solution, until a better heat exchanger could be made/installed, after which the tubes could be reused in the house's ventilation system or as a ground source heat exchanger.
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