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Thread: Air to air heat exchanger for electric dryer

  1. #1
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    Default Air to air heat exchanger for electric dryer

    I have been trying to get this posted for some time now but have run into difficulties. The main problem was discovering that the file was too large to upload to the Google 3D Warehouse. It ended up at around 14 megs and when I tried to upload it I was informed that it couldn't be over ten megabytes. I completely fail to understand why I can upload videos to You Tube up to two gigabytes but am restricted to only 10 megs for Sketchup files.

    It took a great deal of time to reduce the file size without compromising the model quality in any way. I had to use every trick I know to reduce byte count for everything from textures to the number of polygons in the model. I was able to do it, just barely.

    Anyway, this unit is an air to air heat exchanger. It is for recovering heat from the exhaust air of an electric clothes dryer while not introducing humidity into the home. It is designed to be built for as low cost as possible while still providing a safe, fire resistant device. It is intended to be easy to build with few critical dimensions and readily available materials. Ordinary wood working tools are sufficient even for the metal work that is required.

    It makes use of regular aluminum drink cans for the majority of the heat exchanger area with only a few small pieces of aluminum edging trim required for some parts. The enclosure is constructed from melamine faced particleboard for ease of construction, low cost, good looks and fire resistance. The fire resistance is very important and the melamine is why. Melamine is not flammable and a 3/4" thick panel has a burn through time of one hour when subjected to direct flame. The melamine MDF panel should not be substituted.

    Note that unlike a condensing dryer this unit is designed to recover heat from a dryer that exhausts heated air and humidity outside. Condensing dryers are not designed to recover heat. The main purpose is to allow venting inside the house when an outside vent isn't practical. Some go as far as using house water to cool the condensor unit and flush the heat down the drain. They are also less efficient than an ordinary dryer as they recirculate slightly damp air in the dryer and take longer to dry clothes.

    Because of this this unit it is less likely to condense water in the heat exchanger. Still, full condensation collection is provided in the design. Not shown is the possibility of including a collection funnel in the bottom of the unit and a hole in the bottom of the condensation collection tray that drains into the funnel and then through a hose to wherever is convenient. It could be arranged to drain outside or into a laudry sink. The condensation tray is very easily removed for emptying by opening the right side door and sliding out the tray.

    Most dimensions are calculated in fractions of an inch for familiarity to wood workers. The only critical dimensions are the spacing and pattern of the actual aluminum cans that make up the core of the heat exchanger. They have been calculated to provide specific air flows to maximize efficiency of heat transfer. Once the heat exchanger core is constructed the rest of the device may simply be constructed to clearance dimensions not smaller than indicated.

    Aluminum sheathing of the exchanger core may be standard aluminum flashing attached with flush screws or simple staples. To save expense regular aluminum cans may be cut open and flattened. That is demonstrated on the back side of the exchanger core.

    There are only a few small areas of combustible material (wood) that are directly exposed to the exhaust air from the dryer. The primary parts that are so exposed are the end plates of the exchanger core. They should be treated with fire resistant paint or they could also be covered with aluminum flashing.

    The other small areas are indicated as bright yellow and should also be painted the same. Otherwise, the remaining wood parts are only exposed to the inside room air path. The rest of the parts exposed to the dryer exhaust are either metal or melamine which is non-combustible. The inlet is also provided with a metal ember catch tray in the case of a lint fire in the dryer.

    The unit is designed to use a standard furnace filter and should not be operated without it. It is to prevent lint build up in the heat exchanger which is the lowest air velocity section. The entire heat exchanger core slides out through the right side door for easy cleaning as required. It is light weight with my best guess at less than 15 lbs. The air seals for the inside air vents on front and back are made from strips of Velcro loop material as it is rugged and easily compressible. It acts in the same way as a brush air seal.

    Not all fasteners have been shown as it will be obvious where they are required.

    The fans shown are ordinary 80 mm computer case fans. They should be powered by a certified power brick capable of supplying 1 amp at 12 volts. Included in the design is a Sail Switch on the dryer air inlet to automatically turn on the fans when the dryer is in use. A sail switch is easily made or bought at any HVAC supplier. The sail switch is an important part of the design and should not be omitted.

    If the fans do not operate when the dryer is running that means the filter needs changing. The sail switch is also a safety indicator since if the air flow falls too low the fans will turn off. As an additional indicator one of the fans (or all of them) should include LED illumination.

    The aluminum cans are prepared for construction by removing the top and bottom using an ordinary belt sander. The ends are sanded just enough to cut through the fold of metal that holds the end caps in place. When done carefully the cans will then snap together to form a tube. The cans should be sealed at the junctions with a thin smear of kitchen and bath type silicone sealer. They should also be sealed into the end plates with the same sealer.

    Estimated efficiency is from 50% to 80% when constructed as shown. The exchanger core requires 96 cans. Total core exchanger area is approximately 23 square feet or 2.17 sq metres. Including the exterior area of the exchanger core the total heat exchange area is approximately 28 square feet (2.6 sq meters). Tray condensation capacity is 1.5 litres. Input and output connections are shown with 4 inch elbows but other shapes may be used. The overall design shouldn’t be altered significantly as it is intended to provide specific flow velocities at various places to maximize heat transfer and minimise lint build up.

    I have done a lot of research and there is nothing on the market anything like this. There are about 77 million electric clothes dryers in North America and if only 10% of these were fitted with a unit that works like this it could save from 1/4 to half a billion dollars per year in energy.

    For the average home with two children it could save up to $200 per year in heating cost if you live in a northern tier state or Canada and use an electric dryer. It will save at the least $50 to $100 per year if you use a dryer at all.

    The unit can be mounted on the wall or placed on the floor. My estimate for construction cost is from $150 to much less if you use cans as the source of metal for the flat sheathing of the exchanger core. It's designed to make that easy.

    I have very carefully engineered this design with attention to ease of construction, safety, minimal material cost and waste and good appearance. I have also carefully designed it for efficient airflows at all points to take maximum advantage of heat transfer. The overall design and dimensions aren't critical at all but should be followed reasonably closely to gain the full advantage of the design.

    The file may be downloaded from here:

    http://sketchup.google.com/3dwarehou...4e451065d1c629

    Pictures:







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  2. #2
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    These are the air flow patterns.





    A couple of detail close ups:



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  3. #3
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    Looks pretty slick, Evan. One little question:- would you think that there be sufficient benefit to installing "bowties" in each of the cans? It would mean twisting 48 left hand and 48 right hand units out of more cans. This SHOULD greatly increase air turbulence, with attendant reduction in the boundry layer thickness, without much increase in pressure drop. To my mind, it SEEMS like it would be worth it--but maybe not. Quite a bit*more work too!
    In our remote Northern communities, they, (Gummint,) should GIVE these away. Deisel-generated power is a tad pricy for clothes dryers!
    Duffy, Gatineau, Quebec

  4. #4
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    Excellent. And needed right now by lots of folks I'm sure.

    You should have a couple folks build one from the plans and provide feedback. Based on that, make and sell an e-book for $8.50 and make some R&D money.

    Finest regards,

    troy

  5. #5
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    ACME!? Anything that's got Wile e coyote's seal of approval works for me! Seriously, looks great!
    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

  6. #6
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    I am certain that there won't be laminar airflow through the cans. The air is entering the cans after passing through a random filter and that will insure it is turbulent flow. Entropy isn't known for reversing itself without encouragement in the form of supplied energy.

    It's such a simple experiment to do that anybody can do it if they wish. I don't think it would be worth the trouble.

    BTW, the SketchUp model is divided into many layers. If you open the layer manager you can turn layers on and off instantly to see inside of the model without taking it apart. One of the layers is the dimensions layer and it is off by default.
    Last edited by Evan; 01-25-2012 at 11:23 PM.
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  7. #7
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    how much do you think the efficiency would benefit from having the hot air take a turn in the unit and flowing opposite to the direction of the air to be heated?

  8. #8
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    Why won't it work with gas dryers?

  9. #9
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    It isn't clear and is hard to show that the first transfer takes place before the dryer air enters the core. It is designed to restrict the amount of air that can flow directly up from the hot air inlet and into the filter. There is a lower resistance path along the front and back sides across the entire core and then up over the right side of the core to the filter. Much of the heat transfer will occur there on all sides. If you have a look at the left side parallel view you will see what I mean. The transfer in the core is intended to present the largest heat difference to the up flowing indoor air where it enters at the lower back.

    It is the heat difference that governs the amount of heat transfer per unit time. I studied many configurations and calculated flow velocities to come up with this configuration. This is about the 5th complete model that I designed. As I studied each one I found better ways to do this and better ways to save material. This is the beauty of 3D modelling. One of the major constraints was keeping the size such that it could be comfortably mounted above the dryer on the wall. Many people don't have room for another appliance on the floor. That meant that the depth in particular had to be kept as shallow as possible.

    This unit doesn't have to be used to warm inside air. It would also serve well to preheat outside incoming air and serve just as well to save energy. Connecting it to an outside air intake would be trivial. If that were done it would be necessary to provide a drain hose for the condensation tray. Using it as an outside air preheater would help eliminate cold drafts around the house that will otherwise be caused by the fact that a clothes dryer pumps out about 6000 cubic feet of air per hour.
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  10. #10
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    The beer can heat exchanger is a pretty popular solar heater design - should work great with the dryer, too. A secondary roll can be a simple addition to make it a pre-heater for water entering the water heater - something that tends to happen on a heavy laundry day. Adding plumbing down the center of the beer/pop cans to carry water to the water heater inlet, possibly to a holding tank, would be the thing to explore. You've already done someting along these lines, as I recall. Because there is little turbulence in the can column much of the waste heat carries on out the back end. If there were a copper pipe in the center then something as simple as a washer shaped disk would turbulate the air as it comes in.

    There should probably be some kind of lint separation prior to sending the hot air into the exchanger else it will become a maintenance issue.

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