Another dehumidifier question

[Calvin Sambrook]

A recent post on the subject of dehumidifiers has reminded me of an idea I had about combining one with a tumble drier vent. A normally vented tumble drier heats up room air, gets it moist and then chucks it out of the house. Fresh air must be drawn in somewhere else and, in winter at least, has a cooling effect on the house.

Dehumidifiers are basically both halves of a fridge in a case so that the air flows over the cooling element, depositing water, and then over the heating element to cool it.

I've wondered about getting a small dehumidifier and rearranging it's internals so that the tumble drier draws its air over the heating part of the dehumidifier and its output air goes over the cooling part. That way the air being ejected from the house would be cooled and the tumble drier wouldn't have to work so hard to heat up the air it uses. Alternatively the tumble drier could be left to draw room air in normally and the heater part of the dehumidifier could be used as a room heater.

Any thoughts? (Other than I've got too much time on my hands, I haven't).

[David in Normandy]

We don't have a tumble dryer, but we do use a dehumidifier to provide a certain amount of low level heating. Our house is very old, stone built with nothing the in the way of damp-proofing or insulation and gets quite humid and very very cold. In theory a dehumidifier should be more than 100% efficient in converting electricity to heat. You get the 100% directly from the electricity and a small amount from the water vapour condensing back as water. It would be interesting to know how much this bonus is. It is a good few years since I did my physics degree so it is too much hassle to get my head around the numbers now. We leave our dehumidifier running 24/7 in Winter. It churns out a surprising amount of heat.

[Jason]

Also known as a condensing dryer.

Ours just contains a heat exchanger with room-temperature air blown through it. It does not need to be chilled, so an air conditioner would be overkill. It only needs to cool the moist air enough to condense the vapour. They work really well, and to my mind are more efficient than non-condensing models, since you get to keep the warmth in the house (even though they are labelled as *less* efficient since the air needs to be pumped around and takes a more complicated fan).

-- Jason

[Andrew Gabriel]

This sort of thing already exists commercially. I think they just run the air around in a closed system. It enables tumble drying at lower temperatures and more efficiently.

[js.b1]

Vented dryer produces a large volume of damp air per unit time; dehumidifier dehumidifies a small volume of damp air per unit time. So you could suffer the dehumidifier playing catchup and any door openings putting the dryer's damp air throughout the house.

The solution is perhaps a vented dryer into a cold bucket of water (they sell such kits) and a dehumidifier to mop up the "damp air spillage" of which there is some.

Alternatively a condensing dryer or a heat pump dehumidifying dryer - which exists, quite expensive and rather slow.

Failing that, create a dehumidifier cabinet - slatted trays for clothing, drip tray at the bottom, dehumidifier inside, close door. I believe someone makes one in stainless steel, but it is quite large - basically back to the victorian solutions.

[Harry Bloomfield]

Calvin Sambrook pretended :

I don't think even a large dehumifier would cope with the quantity of air which needed to be dried for it to be effective.

[Calvin Sambrook]

Well the specific heat to vapourisation of water is about 2200 J/kg which I think works out to about 600Wh per litre, so "not much" is the answer.

[Calvin Sambrook]

Not exactly. A condensing drier doesn't output anything to the external world. I'm thinking of a unit which outputs cold(er) air to the outside. Actually thinking about it some more my idea could perhaps be reduced to an air vent which cools the exiting air and warms the incoming. The tumble drier is attractive because it outputs a relatively warm ait stream. A heat exchanger provides a means of recapturing that heat instead of wasting it.

[Calvin Sambrook]

I think I didn't explain what I meant properly. Rather than trying to dry the air completely I'm thinking more of exchanging heat between the exiting air and the inside air, heat recovery if you like. The tumble dryer output is attractive as it is very warm so will try to heat up the cooling part of the heat exchanger a lot, exactly what you want as that gives maximum efficiency for heating the hot part.

[Jason]

What would be the reason for this? Do you want the heat kept inside or do you just want to recycle the heat just in the machine as much as possible?

Using a condenser, we have absolutely no condensation problems, so keeping the heat in the house is simply a case of letting the air from the machine stay in the house.

[dennis]

ITYM 2200kJ or 2.3MJ per kg.

This is about the correct answer.

[Jason]

You want to recover as much heat as you can from the drier, but you absolutely want the air after that to go outside?

[Jason]

I thought it was 4.2kJ just to raise 1 litre of water by 1C. The specific heat of vapourisation would be much more than this.

[NT]

You might as well just use a heat exchanger Doing so will mean the dryer runs hotter though, if its non thermostatic.

NT

[terry]

At one time one could buy a metal box device/fan that inserted in the four inch exhaust duct of the dryer would blow house air past some sort of exchanger ro recover some of the heat going to outside. Never saw one, don't know how well it worked or how efficient it was in recovering heat. Also don't know how it handled any condensate or clogging due to lint? Some 40 years IIRC they were about $20; maybe $80 to $100 today?