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Old 12-07-12, 09:56 PM   #41
GaryGary
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Hi,
Very interesting stuff on recovering dryer heat -- pumping all that heat outdoors just seems like such a waste.

I just added a set of DIY plans for an HRV made from Coroplast. Maybe there is a way to adapt this for use as a heat exchanger for dryer heat recovery?
DIY Heat Recovery Ventilator (HRV) Plans

I think the Coroplast should hold up to the moisture -- not sure if the dryer exhaust temp is too high, but probably OK. A good lint filter would be needed upstream of the HX.


Another thought would be to supply the dryer air from a free warm source. I've been working on a sort of sunspace the last few days, and the inside air temperature gets as high as 120F when the sunspace is not ventilated -- an attic might be another source in some climates.

For those who have not seen it already, Randy did this for his dryer.
Converting a Clothes Dryer to Use Solar Heated Attic Air For Drying


Gary

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Old 12-08-12, 09:07 PM   #42
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You'd need to be very careful to be sure you aren't adding too much static pressure to the system because the blowers installed in dryers expect reasonably straight ductwork with soft curves and little restriction at the outlet. If you reduce the flow too much you'll end up taking longer to dry the clothes and use more energy to the point where it might not help even if you are extracting some of it.
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Old 12-10-12, 10:02 AM   #43
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Hi,
I took some measurements on our dryer doing a fairly small load:

Maytag Neptune, 2002, electric.

Wet load weight 7.9 lbs, Dry weight 4.2 lbs, Water weight 3.7 lbs

Outlet duct 4 inch dia, 0.0872 sf
Outlet velocity (very constant through load) = 1850 fpm
Flow rate (0.087sf)(1850 ft/min) = 161 cfm

Time to dry 48 minutes including 6 minute "cool down" -- cool down is fan running, but no heat.

Temperature -- I've got a plot of the logged temps, but don't know how to include it here. Anyway, it built up from 70F to about 140F over first 7 minutes (I believe heat element was on all of this time), then temp cylced up and down about 140 F (turning element off and on) until the start of the "cool down".

Also (sort of) measured relative humidity, but did not want to leave the logger in the high temperature stream to long as its not designed for those conditions. I believe the RH was about 42% near start and about 24% about half way through.




Sensible Energy Vented Out of House By Dryer Duct:
--------------------------------------------------------
Flow rate = (0.0872 ft^2)(1850 ft/min) = 161 cfm
Say that average temp was 140F for about 42 minutes (leave out cool down).
Heat Rate = ((161 ft^3/min)(0.062 lb/ft^3)(60 min/hr)(140F 65F)(0.24 BTU/lb-F) = 10,800 BTU/hr
Or, for 42 mins, 7550 BTU (2.2 KWH)

This agrees almost exactly with the Canada study that says 2.23 KWH per load for 416 loads a year and 930 KWH total for year.

Energy to heat air pulled in from outside:
--------------------------------------------
Infiltration Energy = (161 ft^3/min)(48 min)(0.062 lb/ft^3)(70F 30F)(0.24 BTU/lb-F) = 4600 BTU or 1.3 KWH -- for this small load.

Total air pulled into house = (161 cfm)(48 min) = 7700 cf (about 1/3 rd of a house full).


Latent Energy in Water Vapor Vented Out Dryer Duct:
---------------------------------------------------------
Heat that could be recovered if the water vapor in the outlet stream were converted to water inside the house:

Heat = (3.7 lb)(970 BTU/lb) = 3590 BTU or about 1 KWH.



So, for this not full sized load, about 2.2 + 1.3 + 1 = 4.5 KWH of energy per load that could be recovered if you got all of it.

For us, this is our largest single electrical load and is a significant fraction of our monthly bill.


Still thinking about whether something could be done with a heat exchanger made for Coroplast. The Coroplast is polypropylene, which is a good high temp and chemical resistant plastic. I think its about $20 a 4 by 8 sheet. Comes in thicknesses up to 10 mm.

Two sheet of Coroplast would offer 128 sf of heat exchange area -- about the same as 120 ft of 4 inch duct.

The flow area could be quite large, so, I don't think that back pressure would be an issue?

Some sort of condensation drain would be needed.

The dryer already provides a blower for one side of the HX. Other side could be natural convection or a fan of some sort?

Any thoughts on a design?

Gary
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Old 12-10-12, 10:14 AM   #44
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Great info as usual Gary!

Personally, I think I'd be less concerned with the heat exchanger design and more concerned with creating a lint filter/trap so it doesn't plug up the heat exchanger.

I think the coroplast exchanger is a good way to go though. We've talked in some pretty good depth about DIY HRVs here: http://ecorenovator.org/forum/conser...exchanger.html
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Old 12-10-12, 12:54 PM   #45
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Thanks for the thoughts everyone.

The heat exchanger Gary provided is intriguing since the exhaust air side could be cleaned easier since that air wouldn't be running through the individual Coroplast tubes. Great for a HVR, but the lent filter requirement (I agree its a huge issue Daox) for it still concerns me. As a result, I'm still leaning towards a co-axial heat exchanger since it would require only cleaning out a single pipe with a brush (or hopefully water hose) to remove the lent. I know the efficiency would be lower, especially if the exhaust pipe doesn't have some way to add turbulence and break up the laminar airflow but I'm willing to take that hit in exchange for not needing a lint filter and worrying about excessive back-pressure on an already long vent run.

What I had in mind was a co-axial heat exchanger 16 feet long, with a 4" AL inner tube and a 6-8" insulated flex-duct outer tube. That would give a surface area of ~2400 sq. in or 1.55m2. over which to conduct the heat through thin-walled AL. A quick look online shows that a basic overall heat transfer coefficient for AL in an air heat exchanger (Forced Convection (flowing) Gas - Condensing Vapor Water) is 10-50W/m2K. So for a 42K temp differential (thanks for the basic numbers above Gary) I could expect anywhere from ~650W to 3.2kW of energy transfer.
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Old 12-10-12, 02:33 PM   #46
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Quote:
Originally Posted by Daox View Post
Great info as usual Gary!

Personally, I think I'd be less concerned with the heat exchanger design and more concerned with creating a lint filter/trap so it doesn't plug up the heat exchanger.

I think the coroplast exchanger is a good way to go though. We've talked in some pretty good depth about DIY HRVs here: http://ecorenovator.org/forum/conser...exchanger.html
Wow -- 24 pages -- I've got my work cut out for me

Gary
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Old 12-11-12, 09:57 AM   #47
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Hi,

Went through the 24 pages of stuff on the HRV -- interesting.

Couple of thoughts/questions relative to the dryer heat recovery.

- It seems like the Coroplast HX by Besant that I posted earlier would work nicely for the dryer and be relatively easy and cheap to do. But, I wonder about fire safety -- there are lots of dryer fires every year. When the dryer is operating normally, the exhaust temps are well with within the Coroplast OK range, but I guess a partial blockage could raise the temperatures. I would think that dryers have an overtemperature shutdown?
Its not so much that I would worry about a fire on ours as we keep after the lint filter, but I'd like to do a design that does not have any issues.

- The Dave M. HX that uses alum sheets seems like a good design -- perhaps a little more expensive, but efficient and does not have the fire issue?

- Also like the coaxial pipes, but I don't really have the length to do that, and we should get a reading on how well it works when Trekker builds his.

- This is probably my ignorance, but I don't see why the filter is a big obstacle? It seems like a filter in the entry plenum of the HX could do a good job of getting the lint and also hep spread the airflow out to all the passages more evenly?


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Old 12-11-12, 11:08 AM   #48
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If the filter is cheap and easy to replace I don't see it being a big deal. I'd just want to make sure it filters quite well. You really don't want to have to take apart the heat exchanger and clean it out if you don't have to.

But, nobody has ever put a filter on a dryer heat exchanger. So, we don't really know how bad it is for sure.
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Old 12-11-12, 02:43 PM   #49
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I was in Home Depot this morning and checked the alum flashing -- they have 50 ft rolls 14 inches wide for $43 -- 58 cents a sqft. Not so bad.
The Corplast is about the same price per sqft here, but you get twice as much HX area from a sqft.

I think I will give the David M. HX (or a variation on it) a try for the dryer and see how it goes.

For the edge spacers, I'm thinking about taking a plastic lumber deck board and ripping it into half inch (or a bit less) strips to make waterproof edge spacers. Does this seem like a good idea? Any other ideas for the edge spacers? It would be nice if the spacers did not require a table saw to make.

Gary
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Old 12-15-12, 09:50 AM   #50
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Quote:
Originally Posted by GaryGary View Post
I was in Home Depot this morning and checked the alum flashing -- they have 50 ft rolls 14 inches wide for $43 -- 58 cents a sqft. Not so bad.
The Corplast is about the same price per sqft here, but you get twice as much HX area from a sqft.

I think I will give the David M. HX (or a variation on it) a try for the dryer and see how it goes.

For the edge spacers, I'm thinking about taking a plastic lumber deck board and ripping it into half inch (or a bit less) strips to make waterproof edge spacers. Does this seem like a good idea? Any other ideas for the edge spacers? It would be nice if the spacers did not require a table saw to make.

Gary
Decided to try a sort of throwaway prototype to see how well the dryer heat recovery HX works before trying the "production" one.

When I ordered the twinwall glazing for the greenhouse, it came packed between two used 6 by 9 ft sheets of twinwall -- I've cut these up into eighteen 3 by 2 ft panels, and patched the holes with alum tape. Started to assemble them into the HX stack last night, and it seems to be going well. Hope to do some more on it today. Its not so elegant, but its a lot of heat transfer area, so maybe it will work.

The first hookup will be running the dryer exhaust through one side of the HX then outside and with the dryer fan doing the pushing. A small blower will push room air through the other side.

If the first try gets good recovery, I'd like to go to having the 2nd side of the HX air path pull air from outside, through the HX, and then into the laundry room near the dryer. Idea being that the dryer will get the preheated air from the HX. This should allow the dryer to use less heating element time, and also keep the dryer from pushing furnace heated room air outside??

I found this paper posted earlier by Fornax to be quite helpful:
http://www.ewp.rpi.edu/hartford/user...0Exchanger.pdf

They also used a twinwall heat exchanger stack with a lot of heat transfer area and got efficiencies up close to 90%.

edit -- corrected the link above -- I think it works now.


Gary


Last edited by GaryGary; 12-15-12 at 02:04 PM..
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