Clothes dryer heat recovery system - Page 9

GaryGary · 02-02-13, 08:42 PM

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

Exeric · 02-03-13, 02:46 PM

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.

Exeric · 02-03-13, 03:22 PM

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.

GaryGary · 02-04-13, 10:07 AM

Quote:

Originally Posted by Exeric

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

sten668 · 03-04-13, 10:37 PM

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.

scottharrison · 03-05-13, 06:39 AM

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?

Daox · 04-02-13, 03:36 PM

Gary, have you been using your heat exchanger for a while now? Any updates on how it is working out?

mejunkhound · 04-03-13, 10:13 PM

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.

6Monty6 · 09-15-13, 10:33 PM

Here is the latest of four video I have on youtube...I am pretty happy with the design: Clothes Dryer Heat Exchanger Part 4 - YouTube[/url]

Or search clothers dryer heat exchanger 6monty6

Let me know what you think!

Thanks,

Mark

iamgeo · 09-15-13, 10:37 PM

There are No links!!!!!!!

02-03-13, 02:46 PM	#2
Exeric Apprentice EcoRenovator Join Date: Aug 2012 Location: California Posts: 274 Thanks: 19 Thanked 37 Times in 28 Posts	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..

04-02-13, 03:36 PM	#7
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Gary, have you been using your heat exchanger for a while now? Any updates on how it is working out? __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

09-15-13, 10:33 PM	#9
6Monty6 Lurking Renovator Join Date: Apr 2012 Location: Atlanta Posts: 7 Thanks: 0 Thanked 1 Time in 1 Post	Clothes Dryer Heat Exchanger Here is the latest of four video I have on youtube...I am pretty happy with the design: Clothes Dryer Heat Exchanger Part 4 - YouTube[/url] Or search clothers dryer heat exchanger 6monty6 Let me know what you think! Thanks, Mark

09-15-13, 10:37 PM	#10
iamgeo Apprentice EcoRenovator Join Date: Aug 2010 Location: Lakehills, Tx Posts: 168 Thanks: 4 Thanked 10 Times in 9 Posts	There are No links!!!!!!! Last edited by iamgeo; 09-15-13 at 10:48 PM..

02-02-13, 08:42 PM	#1
GaryGary Apprentice EcoRenovator Join Date: Nov 2008 Location: SW Montana Posts: 139 Thanks: 1 Thanked 21 Times in 15 Posts	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

02-03-13, 03:22 PM	#3
Exeric Apprentice EcoRenovator Join Date: Aug 2012 Location: California Posts: 274 Thanks: 19 Thanked 37 Times in 28 Posts	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.

03-04-13, 10:37 PM	#5
sten668 Lurking Renovator Join Date: Mar 2013 Location: SE Idaho, USA Posts: 2 Thanks: 0 Thanked 0 Times in 0 Posts	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.

03-05-13, 06:39 AM	#6
scottharrison Lurking Renovator Join Date: Feb 2013 Location: Orange County Posts: 8 Thanks: 0 Thanked 0 Times in 0 Posts	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?

04-03-13, 10:13 PM	#8
mejunkhound Apprentice EcoRenovator Join Date: Apr 2011 Location: PNW Posts: 197 Thanks: 0 Thanked 47 Times in 31 Posts	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.