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First Cut Prototype for Clothes Dryer Heat Recovery Heat Exchanger

The picture below shows the finished heat exchanger in place. 

Red arrows show hot dryer air coming into bottom corners of HX.
Blue arrows show cooled dryer air leaving HX at upper corners.
Green arrows show room air entering HX via the fan, flowing straight
down the HX and exiting the bottom (hopefully warmer).

The prototype is made from some scrap, well used, twinwall polycarbonate.  About 5/16ths inch thick.

The HX is a stack of 14 of these twinwall sheets.  The sheets are spaced about 1/4 inch away from each other to form 13 channels between each pair of twinwall sheets.  The hot exhaust from the dryer will pass through the passages in the twinwall, and the outside air will pass through the spaces between the twinwall sheets. 

The room air flow through the twinwall itself entering the top and leaving
at the bottom.
The dryer air flows in the 1/4 inch spaces between the twinwall sheets
it enters on the lower left and lower right corners, flows upward
through the channels and exits on the upper right and upper left corners.
The spacers that block the ends of the dryer flow channels have not
been added on this picture yet.

I used this twinwall because it was scrap I had on hand -- it is used twinwall that was used as packing material for some sheets of new twinwall that I ordered.  I think that Coroplast is a better material to use for a permanent version of the HX.  Likewise the spacers I used to space the sheets apart are just wood and might not give a very long life if the environment turns out to be wet with condensing water.  Some of the spacers are made from some left over plastic deck board I had on hand, and these might be descent choice for spacers on a permanent version.

Making the stackup of twinwall layers with spaces between each layer. The grey spacer strip with the silicone on it is about to be placed on the far side. It is plastic deck wood.

Half of the twinwall stackup -- waiting for the silicone to set.

With spacers that block the dryer flow siliconed in place and curing.

The 1/4 inch spacers space apart the twinwall sheets to make flow channels between each pair of twinwall sheets.
Silicone caulk was used for sealing and glue -- it worked quite well with the polycarbonate twinwall, not sure how well it would do with Corbond? 

Until I ran out, I used some left over deck board to rip the spacers from thinking it would be more waterproof than wood.

Building the stack went pretty fast.  It would be helpful to have maybe two vertical pegs per side coming straight up from the work surface and spaced to hold the twinwall sheets in exactly the right place to make a square stack. 

Mounting the twinwall stackup to the plywood backing board. In addition to the two hold down 2 by 2's there is some silicone between the bottom twinwall sheet and the plywood.

Adding the inlet and outlet boxes. Silicone caulk was used to seal all the joints between the HX stack and the inlet and outlet boxes.

Most of it done. The room air entry box is at the left -- the fan that drives the room air will be mounted in the top cover. The inlet and outlet boxes at the corners for the dryer air to enter and leave are mostly done.

I built  about half the stack, and then weighted it until the silicone set, then continued with the 2nd half.

The final HX block.   It is pretty strong -- you don't have to worry about damaging it when moving it around.

The airpath for the room air is through the cells IN the twinwall sheets.  The room air comes into the top of the HX, flows straight through the HX, and exits straight out the bottom of the HX.

Another view of the mostly completed heat exchanger.  The fan pulls room air into the larger box at the top, which then flows down the twnwall sheets.

With the fan installed on the room air inlet box.

The heat exchanger installed. The fan is controlled by the thermal switch to the right of the fan. The switch sensor is located where it senses when hot air is coming in from the dryer duct, and turns the room air fan on.

The dryer air flows in the spaces between the twinwall sheets and in the opposite direction (this is a counterflow heat exchanger).   The hot dryer air flows into the two lower corners (see picture above), through the HX, and then out the two upper corners.  The dryer air was channeled through the spaces between the twinwall sheets to lessen the chance of lint clogging the HX.

Size of each twinwall plate is about 36 inches long about 21 inches wide.

Total heat exchanger area (counting top and bottom of each interior twinwall sheet) is (20 *36)(13*2)/144 = 130 sqft.

Flow area each way is about (14 plates)(20 inches wide net)(0.25 inch high) = 70 sq inches  (about 6 times the area of a dryer duct).

It would have been nice to make the plates longer to allow more complete heat exchange, but I did not have the material to do it.  The heat exchanger in the Danish paper is about twice as long, but the graph they have showing the change in efficiency with length indicates that one half as long would still have achieved about 80% efficiency.

The fan that powers the room air side of the heat exchanger is a Grainger/Dayton 10 inch fan producing 600 cfm with no pressure resistance.  In this application with the pressure resistance of the heat exchanger it produces 204 cfm.  It only draws 27 watts, so its energy use is negligible compared to the dryers 5500 watts.

Comments

If you have any thoughts, comments, suggestions, or questions, please use the comments section on this page...  The results of several dryer test runs with this prototype are also on the same page.

Gary February 1, 2013