Monday, February 4, 2013

A Prototype Dryer Heat Recovery Heat Exchanger

I did a first cut prototype of a heat exchanger that extracts heat from our clothes dryer exhaust and uses the recovered heat for space heating.  So, this is a brief description of the prototype followed by some questions dealing with how to make it more efficient.

This is just a prototype -- its made out of scrap material I had on hand.  I wanted to investigate what the real heat recovery potential is and to see how bad potential problems such as lint, water, excessive pressure drop might be.

This is a picture of the made from scrap heat exchanger.

It is a stack of 14 twinwall polycarbonate glazing panels that are spaced apart by 1/4 inch.  The dryer air (red and blue arrows above) flows up through the spaces between the twinwall sheets and the room air (green arrows) is drawn in by the fan on top and pushed down through the cells in the twinwall.  This provides about 130 sf of heat exchange area.

The plot above shows a log of heat exchanger inlet and outlet temperatures for the two flows for a good sized load of laundry.

In its current state, it works pretty well.  At the marked location dryer air enters at 136F and exits at 88 F, and the room air enters at 79F and leaves at 95 F.

The recovered heat is equivalent to about 32% of the electricity that the dryer uses to do the load.  There are a few simple changes that might improve this that are discussed at the details link below.  So, not so bad for a first try?

Pressure drop through the exchanger is low and does not affect dryer operation.  Lint is near non-existent so far, and appears to be a very workable problem.

Oddly, very little water or condensation is produced.  The pictures in the details link below show what happens -- basically just a little fog in places and a drop or two.  This is a good news - bad news thing.  Good in that less water will be easier on the exchanger materials, but bad in that the heat used to evaporate the water in the dryer is not being recovered, and this is about half of the heat.

It dawned on me that if the exchanger could be changed so that it condensed out most of the water in the dryer airstream, then 1) you recover the heat that was used to evaporate the water, and 2) you could likely vent electric dryers inside (with a good lint filter).

The number 2 item is a big one in that it prevents the dryer from pulling in outside air that needs to be heated by your furnace.  On cold winter days, this can be about as much energy as the dryer uses in electricity.

By my numbers, a dryer/heat exchanger set up to do the above could save 80% of the total (electric + HVAC) energy used.  This would be a 1000 KWH saving per year for some families.

So, if you have a moment, please look over the details link below and see if you can figure out: 1) why the current version condenses out so little water, and 2) what could be changed to condense out a large fraction of the water in the dryer exhaust.

All the details on the dryer heat recovery heat exchanger...

This is a good paper that covers the incredible amount of energy wasted in clothes drying and gives a good picture of how dryers work and some potential improvements...



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