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Old 02-02-13, 08:42 PM   #81
GaryGary
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Hi,
I finished up the first cut at one version of a heat exchanger to recover heat from the dryer exhaust, and did some testing on it.

The heat exchanger is made from some used twinwall polycarbonate glazing I had around. Its a stackup of 14 sheets of the twinwall with spaces between each twinwall sheet. The dryer air travels up through the spaces and the room air travels down through the twin wall -- a counter flow HX.

As it is, it manages to extract energy equal to about 32% of the electricity that goes into the dryer.

I see some opportunity for changes that might make it do a lot better. The biggest one would be to get it to the point where it condenses out a good fraction of the water in the dryer exhaust stream. This would not only recover the heat that went into vaporizing that water in the dryer, but it (I think) would allow you to vent an electric dryer inside without concerns about humidity. If you include all the energy sinks and sources (including your furnace having to heat air the dryer pulls into the house), this appears to me to reduce the dryer energy use by about 80%!

There is a full description of the heat exchanger and results from a test on it here:
Testing a Prototype Dryer Heat Recovery Heat Exchanger

Would appreciate any comments or suggestions or pointing out any errors.

Gary

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Old 02-03-13, 02:46 PM   #82
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You're doing a fantastic job analyzing and documenting with numbers and graphs your results. Kudos!

I think you're right that getting the water to condense is where a lot of heat is released. It's a hard nut to crack without some kind of heat pump arrangement. But I think I have come up with an idea that "might" work. My first thought was that you could just change the room air ducting so that it uses outside cold air to chill the heated dryer air in the exchanger. The dryer would still heat the room air and exhaust the less hot less humid air to the room. But the more I thought about the more it seems like this idea would end up as an energy wash. You definitely would get more condensation this way because you would be using much colder air to chill the dryer air. But most of that heat released from condensation would be used to heat the outside air. Soooo, in theory its pretty much a wash.

But what if you used the outside air in the heat exchanger just to cool the hot air to just above the dew point? Since most of the energy released at condensation comes exactly at the transition temperature you could use a second heat exchanger using room temperature air to do the final cooling to the dew point. There would certainly be a lot of details to work out that you have a lot more expertise about. But theoretically this seems like it could work well. This kind of arrangement wouldn't work in summer. You'd have to dry things outdoors then.

EDIT:
Actually you wouldn't need a second exchanger unless you wanted one just to optimize the different cooling phases. You could get by with just one exchanger if you used some automatic damper valves that controled the input and output of the cooling air. You could use some kind of dew point computer using temperature and humidity to divert the incoming and outgoing outside air to incoming and outgoing room air just above the dew point.

Last edited by Exeric; 02-03-13 at 03:11 PM..
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Old 02-03-13, 03:22 PM   #83
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Scratch the use of one exchanger. You need two. I thought about it and one wouldn't work. You can't treat dryer air as a single block of hot air that suddenly transitions in temp. It's a continuous transition from input to output.

The rest of it you would still need. That is, you would need a dewpoint computer that adapts to changing outside air temperatures and that moves damper valves.
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Old 02-04-13, 10:07 AM   #84
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Quote:
Originally Posted by Exeric View Post
You're doing a fantastic job analyzing and documenting with numbers and graphs your results. Kudos!

I think you're right that getting the water to condense is where a lot of heat is released. It's a hard nut to crack without some kind of heat pump arrangement. But I think I have come up with an idea that "might" work. My first thought was that you could just change the room air ducting so that it uses outside cold air to chill the heated dryer air in the exchanger. The dryer would still heat the room air and exhaust the less hot less humid air to the room. But the more I thought about the more it seems like this idea would end up as an energy wash. You definitely would get more condensation this way because you would be using much colder air to chill the dryer air. But most of that heat released from condensation would be used to heat the outside air. Soooo, in theory its pretty much a wash.

But what if you used the outside air in the heat exchanger just to cool the hot air to just above the dew point? Since most of the energy released at condensation comes exactly at the transition temperature you could use a second heat exchanger using room temperature air to do the final cooling to the dew point. There would certainly be a lot of details to work out that you have a lot more expertise about. But theoretically this seems like it could work well. This kind of arrangement wouldn't work in summer. You'd have to dry things outdoors then.

EDIT:
Actually you wouldn't need a second exchanger unless you wanted one just to optimize the different cooling phases. You could get by with just one exchanger if you used some automatic damper valves that controled the input and output of the cooling air. You could use some kind of dew point computer using temperature and humidity to divert the incoming and outgoing outside air to incoming and outgoing room air just above the dew point.
Thanks,
That's a thought, but I'd like to avoid the 2nd heat exchanger if possible.
Maybe there is some way to make use of the cold outside air temps.

One thing that puzzles me is that the air comes out of the dryer at about 135F and 80% humidity. The dew point for this air is about 125F, so the room air at 70 to 90F is still way below the dew point of the dryer air. Seems like this would result in lots of condensation?
Dew Point Calculator

I've been thinking of reversing the direction of the dryer flow through the HX such that it flow downward. Idea being that for any droplets, gravity and the dryer air flow velocity would be in the same downward direction, and that an droplets would collect at the bottom of the heat exchanger??

Seems like it would be a real win if we could figure out a way to get enough water out of the dryer exhaust to allow it to be vented inside the house without humidity problems.

Gary
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Old 02-04-13, 11:23 AM   #85
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Quote:
Originally Posted by GaryGary View Post
One thing that puzzles me is that the air comes out of the dryer at about 135F and 80% humidity. The dew point for this air is about 125F, so the room air at 70 to 90F is still way below the dew point of the dryer air. Seems like this would result in lots of condensation?
Dew Point Calculator

I've been thinking of reversing the direction of the dryer flow through the HX such that it flow downward. Idea being that for any droplets, gravity and the dryer air flow velocity would be in the same downward direction, and that an droplets would collect at the bottom of the heat exchanger??
Gary
Yeah, I was thinking the same thing about using gravity to aid in water collection. I think that's a good idea. Have you tried just increasing the cfm rate of room air thru the HX just to find out if you have the right proportion of room air to cool the dryer air?

I agree that another exchanger with sensors and flapper valves seems kind of Rube Goldburg. I'm sure that its just a matter of getting everything right in one HX to get the results you want. Another thing to experiment with after all the obvious stuff gets tried, (which you probably have more to try), is to add some turbulence to the air, such as change of direction in air flow at the point where your best estimate is for the dew point to be in the HX. This might help to extract water just like gravity would help. But I think you probably have more general stuff to try before you get to that. Good luck.
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Old 02-04-13, 04:47 PM   #86
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To elaborate on the turbulence idea: I was thinking that any moisture that is starting to precipitate out needs to be coaxed onto the sidewalls of the duct. This could be done by a helical duct with the air flowing in the downward direction. You don't want the water to be in the center of the air stream even if its already condensed because then it will just escape with the air.

The point where you collect the water should be at the bottom of that (for example) helical duct section. You could use a p-trap at the bottom to keep the air seal and have the duct turn back up, or at least sideways. If the water has successfully stuck to the sidewalls of the duct then it will just drop into the p-trap at the bottom and separate from the air that moves in a new direction at that point. I think this might really help the efficiency of the water collection. This section of the dryer air duct wouldn't be part of the HX, or at least wouldn't need to be.

Last edited by Exeric; 02-04-13 at 04:56 PM..
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Old 03-04-13, 10:37 PM   #87
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Daox, Have you considered using pop rivets and some high temp RTV to seal around the rivets? I hate blowing all that heat outside. When my kids were young I vented the electric dryer in the house in the winter and outside in the summer. My wife was not all for it but it worked for me. I have been looking into making a HX for my new home and your Idea has got me thinking. Thanks and good luck, let us know what you come up with.
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Old 03-05-13, 06:39 AM   #88
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These days, we have a lot of ways so that we can dry our clothes. The clothes dryer heat recovery system is very useful. But there is something bothering in my mind, what is the advantage and disadvantage of this system?
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Old 04-02-13, 03:36 PM   #89
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Gary, have you been using your heat exchanger for a while now? Any updates on how it is working out?
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Old 04-03-13, 10:13 PM   #90
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Did not read thru this entire thread, so maybe direct venting into living space already covered.

Driers (have 2 connected) are in basement, used only in winter, solar clothes dryer (aka clotheslines) used in the summer.

Driers are ventied directly into the basement, and have been for the last 35 years. Ducts into a box with a couple of 16" by 20 inch furnace filters. Workshop and lots of power tools inthe basement, never have had a rusting or humidity problem. Never use the driers in the summer, as the solar option is best as seasons permits.

I'm in Seattle area. My parents in central IL have vented their dryer directly into the finished basement during the heating season ever since they had a dryer in the early 1960's, zero problems. Pop only uses a big box with 2-3 layers of window screen to catch the lint.

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