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Clothes dryer heat recovery heat exchanger

Post in 'The Green Room' started by GaryGary, Feb 4, 2013.

  1. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    I did a prototype heat exchanger to extract heat from my dryer exhaust before it is vented outside the house. The idea was to see how well it might work and to investigate some of the issues like lint accumulation etc.

    Well, it works pretty well considering its all made out of scrap materials. In its current state, it extracts energy equal to about 32% of the electrical energy required to run the dryer.

    It dawned on me that if it could be taken a step further and made to condense much of the water out of the dryer airstream, then, with a good lint filter, it would likely be possible (for an electric dryer) to vent the dryer air inside the house. This is a double win in that you get the free heat from both the hot air and heat of vaporization of the water, and you also keep the dryer from pulling cold outside air into the house that needs to be heated by your furnace. You can check my numbers, but I get that a system like this could cut total clothes drying energy (electric and HVAC) by 80%.
    This could be a thousand KWH a year saving for some families.

    The big questions I have are: 1) why doesn't my current prototype condense out more moisture given that I'm lower the the dryer air to way below it dewpoint?, and 2) how can I change it so it does?
    Any ideas or thoughts would be much appreciated.

    Here are the details: http://www.builditsolar.com/Experimental/DryerHX/DryerHXTest1.htm
    Sorry about the length.

    I do realize that there are a whole raft of other ways to reduce clothes drying energy, but I would like to pursue this one and see how good it can be.

    Gary
    Laszlo likes this.

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  2. maple1

    maple1 Minister of Fire

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    Interesting read, as usual.

    Couple of comments/questions from here:

    -Re. 'It seems like the heat taken out of the dryer air stream should equal the heat gained by the room air stream -- they are the flows on the two sides of the same heat exchanger.'. I'm thinking that your exhanger must be radiating most of the 'lost' heat you found in your measuring? Perhaps insulating it (and maybe piping close by) would get the numbers closer?

    -Re. the humidity issue, I'd suspect it is simply being ventilated out. Did you do any air stream humidity measuring on the outlet-to-room side?

    Keep up the great work!
  3. Grisu

    Grisu Minister of Fire

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  4. Ehouse

    Ehouse Minister of Fire

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    Is it possible that the hot dryer air is warming the exchanger too quickly for it to be a good condensing surface?
  5. Circus

    Circus Member

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    This is to recover heat to preheat house ventilation but should work on a dryer. (search HRV)
    You could duct the fresh, arid, pre warmed, winter air to the intake of the dryer improving efficiency. Then the dryer fan would power the integrated system (within dryer).
    [​IMG]
  6. woodgeek

    woodgeek Minister of Fire

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    Gary, the article you linked to at the bottom of your page was quite interesting...and suggested that up to 2/3rds of the energy used goes into latent heat (if the dryer shuts off when the clothes are dry). Seems to suggest that (w/o condensation) only a third of the energy can be recovered as heat....which is less than I would have thought.

    As for your condensate mystery...can you measure wet bulb and dry bulb temps for your dryer exhaust to test your assumed 80% RH?
  7. nate379

    nate379 Guest

    Do electric dryer take longer than gas ones? My gas dryer takes about 30 mins to dry a normal load of clothes.
  8. TradEddie

    TradEddie Minister of Fire

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    Even though you are reducing the exhaust air below the dewpoint, this doesn't condense all the water, only the excess above the capacity of air at that temperature, the exhaust stream will still be at 100% RH. Returning air at 100% RH / 88F might cause localized condensation problems but to recover all of the latent heat, you'd have to cool that air to below 40F (assuming 70F 30% room air).

    Also, the system is not at atmospheric pressure, so those dewpoint numbers will be off slightly. As the pressure decreases passing through the system, the capacity of the air to retain moisture will increase.

    TE
  9. semipro

    semipro Minister of Fire

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    Very cool Gary.
    I've toyed with doing something similar only I had thought of using water from my well tank (~ 50 degrees F) in a liquid/water heat exchanger, using the dryer's exhaust to preheat my water.
    I believe the commercial condensing dryers that someone else mentioned use tap water for cooling, exhausting the heated water into the sewer.
  10. TradEddie

    TradEddie Minister of Fire

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    What would be the point of using water to cool the air and sending that water to the drain? The heat is still lost. Preheating incoming cold water is an interesting idea at a commercial facility where they'd have continuous water usage, but not very practical in a home unless you already had a circulating pre-heating loop.

    TE
  11. semipro

    semipro Minister of Fire

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    The condensing dryers are built for use in situations where venting is problematic.

    Yeah, I envisioned using it with a solar hot water preheat system.
  12. schlot

    schlot Minister of Fire

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    Very interesting info.
  13. Seasoned Oak

    Seasoned Oak Minister of Fire

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    I vent the dryer directly inside in winter. WIth humidity levels in the 20s it barely makes a dent in the dry air and i capture 100% of the heat as well.
  14. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    I did another run this morning and measured the relative humidity on going into and coming out of the heat exchanger on the room air side, and then worked out the absolute water content of the air going in and coming out. If there were no air leaks in the heat exchanger than the absolute water content should be the same. In fact, there was more water in the room air exit stream, and it was enough to account for 0.75 lbs of our lost water over the whole dryer run -- so, about 10% of the water is, as you suspected, being vented out of the slightly leaky heat exchanger into the room air stream.
    I only have one humidity logger, so I have to switch it back and forth to bet both readings, but I think the readings were pretty good.

    I'm going to try measuring the humidity on the dryer side airflow before and after the heat exchanger. This should identify water that just gets vented outside. I was reluctant to do this as I'm not sure the logger will hold up to the higher temps, but the specs say its OK to 158F -- will see :)

    Gary
  15. GaryGary

    GaryGary Feeling the Heat

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    Hi Wood,
    I'm going to have a try at measuring relative humidity in the dryer airstream before and after the heat exchanger -- I can then convert that to absolute lbs of water per cf of airflow and see how much water is just getting vented out the HX exit vs ending up in the HX itself (or just lost to HX air leaks).
    After reading the specs over, I think my logger is just barely up to the task.

    I do think that the 80% is probably too high. The dryer paper shows 70% as an average on the run they did.

    I agree that recovering the latent heat is key. Also nice that if you get enough of the water out of the dryer air you can (hopefully) vent inside without problems.

    Still seeing no sign of lint -- surprising.

    Gary
  16. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    I suppose one might include a new tank in the cold water line going to the hot water heater.
    Then circulate water from this tank to condense water out of the dryer stream and then return the water to the tank.
    That would put the heat you recovered in condensing the dryer water into preheating domestic water??

    That might be worth (7.88 lbs of water)(970 BTU/lb) = 7700 BTU for the test load I did (which was a large load).
    Such a system would be usable winter or summer as opposed to heat recovery which only helps in the winter.
    But, I suppose you woud not get the whole 7.88 lbs.

    There has to be a Pony here somewhere :)
    Gary
  17. GaryGary

    GaryGary Feeling the Heat

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    We were doing this, and in our dry climate and dry house it worked well.

    It just seems like there are so many people who are negative on the idea even in climates where it works that it won't be something that a lot of people would be willing to do.

    What do you use for capture the lint?
    We used pantyhose, which worked pretty well, but it seems like there might be a better filtering system.

    Gary
  18. pen

    pen There are some who call me...mod. Staff Member

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  19. Mr A

    Mr A Minister of Fire

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    My house is all electric. It takes about 40 minutes with my electric dryer to do heavy materials, jeans, towels. Municuiple utility rates are low at 9 cents KWH
  20. Seasoned Oak

    Seasoned Oak Minister of Fire

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    There is no better filtering system. Its very important to hook the outside vent up as soon as temps stay above freezing and humidity rises. in spring and summer you DO NOT WANT that extra water in the air. But in winter its so dry in my house every little bit of moisture helps. WHat sense does it make in winter to vent your dryer OUTSIDE and pump out a gallon of warm water along with gobs of heat and run a humidifier INSIDE and pump 5 gallons of COLD water inside.
  21. semipro

    semipro Minister of Fire

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    I've thought to use a tornadic separator like those used for wood dust collection with a water bath in the bottom.
    This link is to a guy using one called the "dust deputy" for shop dust collection.
  22. 4acrefarm

    4acrefarm Member

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    I have mine piped in to the side of a rectangular trash can. on top of the trash cans I use hvac filters stacked 2 high with weight on top . I have not noticed any problems with humidity or dust yet.
  23. GaryGary

    GaryGary Feeling the Heat

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    I'm waiting for a 2nd humidity logger to arrive before doing anymore testing, but thought this was interesting.

    I did a load that had a wet weight out of the washer of 18.4 lbs (a good sized load).

    Took the wet clothes out of the washer and laid them out on top of the washer and dryer, and left them overnight.

    Next morning the weight was down to 15.2 lbs -- so, overnight 3.2 lbs of water evaporated out.

    The end fully dry weight after going through the dryer was 9.1 lbs, so total water weight was 18.4 - 9.1 = 9.3 lbs

    So, by letting the load sit overnight, the water weight (and prsumably the dryer energy) was reduced by 34%.
    A 34% energy saving for just being lazy about getting the clothes into the dryer!

    The earlier test run showed a drying energy use of 0.54 KWH/lb of water for the dryer electricity plus 0.3 KWH/lb of water for the furnace fuel needed to reheat the air the dryer sucks into the house. For the 3.2 lbs saved, this would give a 1.7 KWH saving in dryer electricity and a 1 KWH saving in propane for our furnace.

    I guess you could say that the saving was a bit less because the furnace had to supply some heat to drive the evaporation of water, but it also added some welcome moisture to the air over an extended period.

    Gary
  24. woodgeek

    woodgeek Minister of Fire

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    a related question....do you have a HE washer with a high speed spin cycle? My dryer runtime is down significantly since I got one, and air drying is a lot easier/faster/dripless.
  25. Seasoned Oak

    Seasoned Oak Minister of Fire

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    Iv got one of those. THat thing spins at 1800 RPM ,the clothes have very little water left in them.

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