Using the Dryer Exhaust to Heat the Dryer Intake

[wooferhound]

I was thinking outside the dryer today and was wondering . . .

why couldnt you run the output of the cloths dryer through some kinda heat exchanger thing that would preheat the air going into the intake of the dryer. I'm sure there would be some problem with condensation.

[jimovonz]

There was some discussion of this here:

http://www.fieldlines.com/story/2004/11/29/105615/73

[tecker]

  Most of them have a plastic fan

[jacquesm]

the air going out carries away a lot of humidity, if you feed that back in you need to take the water out first, using a condenser (if that's already built in then it will have a better chance of working).

The whole idea of the warm air passing through is that it carries off the water, if you feed it back in without a condenser you'll end up putting the water back in as well...

[nothing to lose]

I don't think a plastic fan would be a problem, how hot is the dryer exhaust and how hot does the fan have to be to melt? I think there should be a large difference in those temps if that's what was ment.

Condensation I would think could be easily handled by just putting a drip/drain hole for it to run out of. Either collect the water or pipe it outside or whatever.

The biggest thing to worry about I think would be the lint. Even though the dryer has a lint filter a great deal of lint gets past that easily. I have a wood fire clothes dryer

Well, sort of. I modified a gas dryer so all the air intake is at a rear duct I made from a coffee can and have made an elbow and connected a foil type vent hose to the intake. The hose and the open end is heated and draws hot air in from the wood burner the dryer sits next too. Working pretty good when I need alot of heat in the house, and not that well when the house is warm enough I don't need much fire/heat. And of course it would figure this has been a mild winter being as I just did this!

For the vent, we need the humidity anyway to lesson static shock so I just vent to the room through a duct pipe straight up and that has a nylon sock over it as a lint filter. The nylon does get lint and need empted or replaced occasionally although not very often for us here. It was mentioned by someone else that it should fill up and block airflow pretty fast normally, they are a service repairman or something as I recall. Anyway I think we figured out that in normal (gas/electric heat) use the air would be much hotter than what I am using. Being the air would be hotter the lint probably dryer and thus able to escape the dryer more easily. Since my dryer is not running as hot the lint most likely has some moisture in it still and that makes it stickier so the dryers lint filter can better catch it and thus less escapes.

 I have noticed also that the dryers lint filter needs cleaned more often than normal and also less build up in the duct than normal from the lint than normally escapes through the filter.

In my case, it takes a little longer to dry the clothes so the motor runs a bit longer, the heat is free, clean the lint filter in the dryer more often but less blowby of lint escaping, humidify the house and prevent static shocks, the exhaust blowing out the vent also acts as a fan would to move the air around the room and I can turn off the fan I would normally use for that when drying clothes, and the heat of course is still in the room so none was lost making it free. The heat would have been in the room anyway since we were using the woodburner for that purpose.

Is it worth it? Not sure yet? How much does it cost to run the motor the extra amount of time compared to how much would propane have cost to heat the air and use the dryer normally? How much electric is not used by turning off one fan durring the drying time. That is the only real factor. For humidifing the house we could just sit a can of water on top of the wood burner, but the dryer does work better at that.

In our case the other factors are the dryer only cost $10 and was like new (propane only in this area and it was nateraul gas dryer). I could probaly changed it over free or cheap easily, but didn't want to

 Only our cookstove uses propane so we only use a 100lb or 20lb tank. We been just using a 20lb recently since I have to haul the 100lb tank in to be filled in an open truck and didn't have one to use for awhile. Much more expensive using smaller tanks than a 250gal tank to be sure but we don't use enough to get a 250gal tank. At our rate it would last the rest of our lives, or we would go hog wild installing things to use it and that would be a waste!

 SO with what I been doing here I would say the biggest and probably only problem would be the lint build up in the pipe/exchanger if you tried to preheat the incomming cold air with the hot exhaust. You would no dought get water forming inside also but that is simple as tip the pipe downwards and let it run out somewhere to cure that.

 If I go back to normal dryer use after winter I will do that myself. Though I plan to still have a heat source all year long so I may not need the normal dryer here.

 My plan is simply a pipe inside a pipe. Maybe a 6" plastic pipe with a 4" metal duct pipe supported inside. Easy to build and can be any length. I would make some type of external lint trap before the exhaust enters the exchanger pipes. Also pull them apart and check them ocasionally for any build ups.

 I don't think the exhaust is gonna be out enough to hurt anything, may want to check it, but after all it blows through dry cloth after the clothes are dried and does not set them on fire

Also normally dryer vents get connected with those cheapy little plastic vinyl slinky type hoses also. If it don't melt or burn that I don't think 6" pvc is a problem

 So personally I myself would have no concerns about fire hazards other than any that could be caused by lint itself. Just don't put anything against the heaters themselfs rather gas or electric.

[Norm]

  Well according to Volvo farmer (#8) at

http://www.fieldlines.com/story/2004/11/29/105615/73

he posted that a load of clothes contains about 10 pounds of water....like I said the other day using an extractor would remove a lot more of that water.

  I contend that the washer on the final spin cycle gives the clothes 4 or 5 injections of water do a little observing and you'll see the water hose/s (?) jump 3 or 4 or 5 times on the spin cycle (unless yours works different than mine)...okay take them out and weigh them ...now put them back in... shut off the hot and cold water to the machine ....run it thru the spin cycle a couple of more times...then weigh them again..I'll bet you'll see a big difference in weight....now dry them....(it'll probably take a lot less time than it usually does) weigh the clothes after they are dry...very scientific?

   Here is my final thought....rather than go thru this all the time we could get an old washer replace the motor with a 12volt,24volt or whatever you use to the dump load and spin the homebrew extractor instead...rig it up to spin the load...maybe just 20 or 30% faster than original put it on permanent spin ...on or off.

    It'll take a lot less energy to spin clothes almost dry than to use 100% hot air...

    I think it would really work sounds logical doesn't it?

                Fun!

                      ( :>) Norm

     

[BrianK]

One thing that i think needs to be looked at is how the thermal overload switch will react. I think the best idea is to get the cloths as dry as possible before the dryer. I don't really understand the reason for the washer to shoot water 3 or more times on the final spin. You would think that the wash and rinse would have been enough water. I have toyed with the idea of the extractor, but only in my brain. I remember as a kid when we used to go to the laundry mat my mom used the extractor that they had.  you think the washer did good until you hear the what sounds like at least a gallon of water spin out. I think my mom could have almost just throwed them in the basket after that, and the drive home would have dried them.

  .

[ghurd]

Why not just run the 4" output through an 8" input for 6 or 8 feet?  Poor mans heat exchanger.

My dryer exhaust runs through about 25' of 4" thin wall PVC. No other way to do it unless someone wants to come over and cut a 4" hole through 24" of sandstone for me. The water that runs out of the uncemented joints is amazing.

A majority of the lint is stuck in the first 5' (straight up), and its wet.

The couple feet of plastic flex hose connecting the dryer and pipe gets half clogged once a year. Its wet too, but I think from running down the 5' pipe.

In the winter the dryer is connected to a 8" wide particial board box with a big furnace filer on each side. You should see that thing steam! Probably too much for in the house.

G-

[wooferhound]

 I did'nt want to feed the Exhaust back into the Intake.

but make a heat exchanger type of device that would recover some of the heat, and then deal with the resulting condensation from the process. Essentially reusing the same heat a lot.

seems like it would save a LOT of money . . .

[Volvo farmer]

About spray rinses... they really do help rinse the clothes. Look at the rinse water next time you do a load. It's not clear, there's a lot of soap residue and leftover dirt in there. The spray rinse puts clean water on the inside of the spinning basket of clothes and the centrifical force pushes it through the clothes to rinse them better. Those short bursts are only at the beginning of the spin cycle. If you watch the thing work, you'll see at least a 5 minute spin out after the final spray rinse.

You want an efficient spin out? Get a front-loader with a 1000 or 1300 rpm spin speed. The clothes come out pretty darn dry out of our 1000rpm Asko. Perhaps a person could modify a top loader to spin faster but the transmissions and clutches might not last long. Vibration could also be a problem.

The guy with the wood heated dryer echoes my concerns. Lint is going to be a problem. You take the heat out, the water condenses, the cold damp lint drops out of the airflow and into/onto whatever you're using to exchange the heat.

[thunderhead]

Although a heat exchanger will recover some heat, it won't recover very much. The process of drying the clothes will cool the air a lot.  So (for example) air at 20C might be heated to 60C before it hits the clothes, which cool it down to 30C and increase its humidity to near 100%

The outgoing air will only heat the incoming air to 30C, not 60C, so it will only save 25% of the heating energy. The heat exchanger will also produce water, which will need to be drained away if it is not to flood the laundry-room floor.  The exhaust air -  still considerably warmer than 20C and still at 100% relative humidity - still needs to be vented outside.

I suspect this is not done in household dryers because the extra complexity and inconvenience is not considered worth the energy saving.  Given that most offline people could use the hot-water system described a few weeks ago to run their dryers from waste heat or low-grade renewable energy (like thermal solar) they might also consider the condenser/heat-exchanger and drain plumbing not to be worthwhile.

[nothing to lose]

Thank you for this sugestion

I was thinking along the lines of removing more water before drying also. I have 3 working washers outside now, not good ones, but they do work. I may try this when the weather warms, I only have to plug em in and set them to spin I geuss.

What I was thinking along is changing the pulley ratio to make the drum spin much faster. Wash normal, when done use the spinner (as I will call that one), then dry.

[jasonphx]

Hello... I was thinking the same thing as the original poster of this thread.  After some of my own curious research, I found there is in fact such a developed and marketed product that "uses the dryer exhaust to heat the dryer intake."  However, to the best of my knowledge it's sold only in europe by Electrolux (largest appliance maker in the world based in Europe).   Here is a link to the brochure on what they call "Heat Recovery Pipes" for their laundry dryers:

http://www.laundrysystems.electrolux.com/Files/pdf/Heat%20recovery%20pipes.pdf

I loved reading all the prior posts.