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Old 12-17-12, 05:01 PM   #61
GaryGary
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Quote:
Originally Posted by eNeufenergy View Post
I'm not allowed to post the link since I'm a newbie so check out: FEMA Dot GOV the doc. is "Clothes Dryer Fires in Residential Buildings (2008-2010)"

Some useful stats. to consider:

Findings
■ An estimated 2,900 clothes dryer fires in residential buildings are reported to U.S. fire
departments each year and cause an estimated 5 deaths, 100 injuries, and $35 million in
property loss.
■ Clothes dryer fire incidence in residential buildings was higher in the fall and winter months,
peaking in January at 11 percent.
■ Failure to clean (34 percent) was the leading factor contributing to the ignition of clothes
dryer fires in residential buildings.
■ Dust, fiber, and lint (28 percent) and clothing not on a person (27 percent) were, by far, the
leading items first ignited in clothes dryer fires in residential buildings.
■ Fifty-four percent of clothes dryer fires in residential buildings were confined to the object of
origin.

And it appears the big take-away is that, as a system, a clothes dryer is inherently unsafe (with standard gas or electric elements) as there are no adequate, simple, or passive ways to keep the thing clean. The dryer manufacturers appear to send the problem "downstream" to the ductwork and pass the blame onto something they have no control over.

Bullet point #4 above: At least the saving grace here: you are likely to not start a clothes dryer fire if you are wearing clothes while you are in the dryer.

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Hi,
Those numbers don't seem that bad to me.

There are 115 million households in the US, and if they typically run the dryer once a day, your chances of have a dryer fire on a given run are about one in 15 million -- about once every 41,000 years. Wish I could say the same for my car.

My take away is that if you take care of your lint filter and you have a look at the vent ducting once in a great while the chances of having a dryer fire are essentially zero.

I guess the HX version made from aluminium sheet would have an advantage here in that its smoother and non flammable.

Gary

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Old 12-17-12, 09:07 PM   #62
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An interesting commercial clothes dryer heat exchanger:

ROTOTHERM - Too Good To Be True? Not Always

"Ramada Achieves 50% Fuel Reductions with Laundry Dryer Heat Recovery"

Gary
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Old 12-17-12, 09:19 PM   #63
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How about this one!

Dryer heat exchanger

"performance is quite good. Access holes for a digital thermometer in the top two compartments show air from the dryer between 110 to 130 F degrees on "medium" heat setting while the preheated air back to the air box measures 112 to 122 degrees F. Another test run with a small load of towels on "high" heat measured 113 to 120 Deg. F to the air box and 123 to 148 Deg. F from the dryer."

I like the nice compact arrangement.


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Old 12-17-12, 11:53 PM   #64
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Nice to see so many ideas being floated around on this idea. I apologize for having restarted this thread and not having gotten around to building my proposed prototype yet—the diy cnc router project is almost done...

Gary, your last link is interesting as I've been toying with an easy way to duct the dryer's intake into the preheated air. Building the box with weatherstripping seems potentially easier than removing the back of the dryer and connecting ducting to the intake to the heating element enclosure. Using cold lower moisture air, especially if it can be warmed up with waste heat, seems better than trying to dry with hotter moist air from say near a bathroom where my dryer is—should allow the clothes to be cooler while experiencing the same retaliative humidity change.

I'm impressed with your cross-flow heat exchanger. I like the simplicity of it, especially for a simple HVR. I'm curious to hear how it works in regards to lint buildup/cleaning. Are you putting a filter before it or just planning on cleaning/washing it out often?


The cyclone separator idea that eNeufenergy might help for lint control, although the lint I notice in the vent line is pretty small, and I'm not sure of an easy cyclone separator to build that I can also insulate.

I'm still planning on moving forward with a axial (or lab condenser style) heat exchanger due to a required long run in an open crawl space and its simplicity and hopeful ease of cleaning. Hopefully will report back after the holidays with some preliminary results.
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Old 12-18-12, 11:07 PM   #65
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Gary Gary, I like your build-up and if you can get anywhere near the 80% efficiency, that would be great. Are you planning on having two blowers, perhaps the existing dryer's with the additional one in your mock-up?

I remember reading somewhere that for a lot of clothes, the high temperatures in modern dryers actually degrade and wear out fabric. Another approach would be to lower the temp. and force the air, like hanging your clothes out to dry on a cool windy day instead of a hot muggy day. I don't know what the difference would be on the HX if you were, in effect, decreasing the delta-T between the two air flows while increasing the velocity.
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Old 12-19-12, 11:16 AM   #66
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Quote:
Originally Posted by theworldtrekker View Post
Nice to see so many ideas being floated around on this idea. I apologize for having restarted this thread and not having gotten around to building my proposed prototype yetthe diy cnc router project is almost done...

Gary, your last link is interesting as I've been toying with an easy way to duct the dryer's intake into the preheated air. Building the box with weatherstripping seems potentially easier than removing the back of the dryer and connecting ducting to the intake to the heating element enclosure. Using cold lower moisture air, especially if it can be warmed up with waste heat, seems better than trying to dry with hotter moist air from say near a bathroom where my dryer isshould allow the clothes to be cooler while experiencing the same retaliative humidity change.

I'm impressed with your cross-flow heat exchanger. I like the simplicity of it, especially for a simple HVR. I'm curious to hear how it works in regards to lint buildup/cleaning. Are you putting a filter before it or just planning on cleaning/washing it out often?


The cyclone separator idea that eNeufenergy might help for lint control, although the lint I notice in the vent line is pretty small, and I'm not sure of an easy cyclone separator to build that I can also insulate.

I'm still planning on moving forward with a axial (or lab condenser style) heat exchanger due to a required long run in an open crawl space and its simplicity and hopeful ease of cleaning. Hopefully will report back after the holidays with some preliminary results.
Hi,
The one in the link is not actually mind -- its Al's.

I did send him an email, and he reports its still working well, and that he has not had any condensation problems.

This is part of the email:
Any problems?

The only problem is getting my wife to understand it's value! She over-drys everything not realizing the run time can be less when the dryer's cool-down cycle is actually still retaining heat even though the heater element is off. She won't use the automatic moisture sensor mode!


Does it condense water from the dry exhaust stream as it cools down? And, if so, has this been a problem?

No.. The dryer cools down more slowly as it recycles heat back. The cloths are still warm when the dryer stops. That gives it plenty of time to dry out the inside of the exchanger before the dryer stops.

--------------
I probably won't be able to do much more on mine until after Xmas, but its getting close.

Gary
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Old 12-19-12, 11:23 AM   #67
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Quote:
Originally Posted by eNeufenergy View Post
Gary Gary, I like your build-up and if you can get anywhere near the 80% efficiency, that would be great. Are you planning on having two blowers, perhaps the existing dryer's with the additional one in your mock-up?

I remember reading somewhere that for a lot of clothes, the high temperatures in modern dryers actually degrade and wear out fabric. Another approach would be to lower the temp. and force the air, like hanging your clothes out to dry on a cool windy day instead of a hot muggy day. I don't know what the difference would be on the HX if you were, in effect, decreasing the delta-T between the two air flows while increasing the velocity.
Hi,
I do plan to have a blower in the outside air path and use the dryers blower for the dryer exhaust path. I've got an Elucent blower that appears to be about the right size -- I think it might be in one of the pictures.

Found this really interesting paper on dryers -- great stuff on using dryers more effectively and also some material on heat recovery. Just the bit on energy involved in drying beyond what is required is amazing.

http://www.aceee.org/files/proceedin...apers/2206.pdf

Title is: Are We Missing Energy Savings in Clothes Dryers?
Paul Bendt, Ecos

One of the things they cover int he paper is not heating the dryer air so hot -- its a good saving.

Gary
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Old 12-23-12, 07:46 PM   #68
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Overview: a 12 foot co-axial heat exchanger does not have the necessary surface area with PVC pipe and the spare small blower I had laying around to extract a reasonable amount of energy from the dryer exhaust...~325watts

So today I acquired some 6" insulated duct and put it over the existing 4" thin walled (~1/8" wall thickness, drainage) PVC pipe. I re-routed my dryer vent line so it is now only about 14 feet long, and managed to get the 6" insulated duct over about 12 feet of that. I ran a load of laundry through the dryer, put a blower on the 6" pipe in the crawl space and got the following numbers:

Dryer exhaust air temp 130F (plastic flex surface temp about 2 feet from dryer), at end of 14' of pipe 112F.
No RH measurements
Dryer estimated 81cfm using handheld anemometer (4" pipe, 4.7m/s measured)
Pipe surface temp where duct began 118F, ended 90.5F
Ambient temp in crawlspace today ~60F
Blower exit temp 85F, estimated 50cfm using handheld anemometer (2.5" pipe, 7.4m/s measured)

The heat exchanger looks like it was recovering about 1108BTU/hr (325wh) using Gary's earlier formula (50cfm)(0.062lb/ft3)(60min/hr)(85F-60F)(0.24BTU/lb-F).

Notes: Judging by the 4" pipe external temps, the blower should have been moving more air, which should improve the above performance numbers some.

Changing the heat conducting material (PVC) to something with a higher thermal conduction coefficient (ie AL) should see a small improvement.

Increasing air turbidity might help ensure that the hottest exhaust air is next to the PVC.

Ensuring the 6" duct isn't resting on the 4" pipe would help, increasing heat exchanging surface area, unlike what occurred in the above test where no spacer was used.
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Old 01-04-13, 12:06 PM   #69
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Seems simple enough..

ProFlex Indoor Dryer Vent Kit-TDIDVKZW at The Home Depot
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Old 01-04-13, 12:14 PM   #70
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Good, but only if:

1) You want tons of humidity in the house.

2) You have an electric dryer. If you have a gas dryer you'll be venting exhaust into your house...

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