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Dryer Heat Recapture

Don't throw the heat away.
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"Another possible solution is to suck hot air straight from your attic to use in your dryer."

Damn it, that's BRILLIANT! I wish I'd thought of that! Criminy, I could do that right now with stuff on hand, and just run the dryer on "air only."

I hereby submit that [twitch]'s idea is better than my original one.

elhigh, May 25 2007

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       I haven't totally formed a picture of this yet, but one possible issue jumps out at me.   

       As a result of preheating the incoming air, you're cooling the discharge air. This is likely to result in considerable amounts of condensation in your exhaust duct. Certainly something to consider when identifying materials to make the duct from and doing installations.   

       I suppose the heat exchanger could be completely outdoors, making it simpler to deal with the condensation runoff. That might also permit using two smaller holes instead of one big one if that would help in any way.
half, May 25 2007
  

       Have the pipe exit higher than a typical duct, say level with the top of the dryer, and run the exhaust straight down the side of the house a few feet. The coolest part of the exhaust duct would simply point straight down.   

       That would also permit a longer heat exchange path. I recognize that sometimes the exhaust path is very short indeed, where a dryer backs right up to an outside wall. There's nothing saying the outlet has to be at the bottom, or even on the back.
elhigh, May 25 2007
  

       Agreed.   

       Keeping the coaxial heat exchanger duct design, another consideration is the shape of the duct. A circle encloses a given area with the minimum possible circumference. In this case, the minimum circumference is non-optimal because it is this surface through which the heat exchange will occur.   

       I can see the pure circular tubing model possibly not recovering enough energy to justify the cost of manufacture and installation.
half, May 25 2007
  

       You have completely ignored Lint and Lint build up which will in very short order plug any sort of heat exchanger you might install. This will insulate the pipe eliminating any benefit, harbor bacteria due to the moist environment(even before you start cooling the air) and increase back pressure on the system.   

       While the idea has merit it also has some rather large issues that you have glossed over.
jhomrighaus, May 25 2007
  

       It's a heat exchanger in that the hot exhaust will dump some of its heat to the cool intake, but beyond that it isn't your typical convoluted-path heat exchange device. My idea is to recoup some of the energy that might otherwise be thrown away. At no point do I recommend that there be anything other than a round duct for the exhaust to flow through. Lint buildup shouldn't be any more of an issue for this duct than it is for existing designs. For what it's worth, we have several residential model dryers running at work that exhaust throw ducts ranging in length from eight to 23 feet. That last one's a little extreme, but it gets the job done.   

       I'm just trying to make the dryer feel a little cooler, suck a little less energy out of your home, and use less energy itself in the process.
elhigh, May 27 2007
  

       This is akin to the general problem of ventilating a house without wasting energy. Both can be solved to some extent by the use of heat exchangers in the way that's being suggested. There's a compromise involved: gas phase heat exchangers can be reasonably small, use low power fans, or be efficient - but not all three at once.   

       If you want to reduce the wastage of heat to a considerable extent, you need a considerable volume to fold the paths of the incoming and outgoing air together, with a large surface area of each shared with the other. You'll also need a fairly powerful fan to push the air through this labyrinth - unless it's not merely long, but also spacious, which increases the volume still further.   

       Designing to allow condensation to escape is important, but not difficult.   

       Lastly, there is the question of dryness. If volume isn't an issue - and cost of materials constructing the monster - then it's worth cooling the incoming air to condense out moisture, and then reheating it before feeding it into the dryer. Again, you can use heat exchangers to minimize the energy consumed cooling and reheating - at another cost in volume or fan power or both, of course.   

       I wouldn't be surprised if all these things are already done in some industrial laundries.
Cosh i Pi, May 29 2007
  

       I wouldn't doubt that this is being done on some larger-scale, custom job setup. I don't doubt that, provided the condensation problem can be handled, that this will become household.   

       Another possible solution is to suck hot air straight from your attic to use in your dryer. I assume it's hotter there atleast by a couple of degrees, and up to maybe even hot enough that it doesn't need to be heated anymore! I guess there's one reason to have a hot roof instead of a cool one.
twitch, May 29 2007
  

       [twitch] Condensation is quite easy - the difficulty is the bulkiness of gas phase heat exchangers.
Cosh i Pi, May 29 2007
  

       just about every clothes dryer i've seen in japan must work like this.   

       they run on 100 volts, and have no massive exhaust duct or air intake, so it can be used in a small unventilated room if need be.   

       they do, however, have a hose from which a steady drip of water flows.   

       quiet, very efficient, fully computerized - if only the people of n.america had a clue what they were missing out on!
TIB, May 30 2007
  

       [TIB] They don't work like this, no. They have a closed circuit refrigeration unit to dry the air, use quite a lot more electricity than the ventilated type, and generate quite a lot of heat from the refrigeration unit.   

       They're common enough elsewhere too, but they're not energy efficient.
Cosh i Pi, May 30 2007
  
      
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