A clothes drying closet


Scottish "warm rooms" dry clothes. If 1 lb of 70 F air at 50% RH with vapor pressure Pa = 0.5exp(17.863-9621/(460+70)) = 0.374 "Hg (using a Clausius- Clapeyron approximation) and humidity ratio wa = 0.62198/(29.921/Pa-1) = 0.00788 enters a well-insulated 130 F closet with 90% RH, ie Pc = 4.74 "Hg with wc = 0.117, and wc-wa = 0.109 pounds of water per pound of air flows out, we need 10/0.109 = 92 pounds of air to evaporate 10 pounds of water.
Raising the RH of 92 pounds of 70 F air from 50% to 100% as it leaves the heat exchanger requires evaporating 92x0.00788 = 0.72 pounds of water, ie about 720 vs 10K Btu, for a COP of 10K/720 = 14, vs 3 for a heat pump.
If we start and end with 40 F outdoor air at 40%/100% RH, wa = 0.0021, and we only need 10/(0.117-0.0021) = 87 pounds of air. At 100% RH, 40 F air has ws = wa/0.4 = 0.00525, so we only need to evaporate 87(ws-wa) = 0.27 pounds of water with 270 Btu.
Longer drying times and higher closet temps lower the airflow rate and the size of the heat exchanger... 92 pounds of air in 8 hours at 11.5 lb/h makes cfm = 11.5/(60x0.075) = 2.55 = Cmin, approximately, and E = 0.95 = NTU/(NTU+1) makes NTU = 19 = AU/Cmin = 1.5A/2.55 = 0.588A, with heat transfer area A = 32 ft^2, eg 2 4'x8' layers of plastic film, for 95% heat recovery. Vertical films could have thermosyphoning airflow. We might turn on the closet heater (a 30 watt bulb :-) with a humidistat whenever the RH is more than 50% to make the "dryer" operate automatically, with minimal clothes handling.
Some European dryers seem very efficient, with long drying times. I wonder if they work like this.
Nick
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yup, they do Jim
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I've looked and haven't found one. Got a brand name with performance specs?
Nick
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Apparently not...
http://en.wikipedia.org/wiki/Clothes_dryer
Condensation dryers
Just as in a normal dryer, condensation dryers pass heated air through the load. However, instead of exhausting this air, the dryer uses a heat exchanger to cool the air and condense the water vapor into either a drain pipe or a collection tank. Afterwards, this air is run through the loop again. The heat exchanger uses ambient air as its coolant, therefore the heat produced by the dryer will go into the immediate surroundings instead of the outside.
Because the heat exchange process simply cools the internal air using ambient air, it will not dry the air in the internal loop to as low a level of humidity as the fresh, ambient air. As a consequence of the increased humidity of the air used to dry the load as well as the increased complexity of the design, this type of dryer requires more time and energy than a traditional dryer. However, it is a valid option where long, intricate ducting would be required to vent a traditional dryer.
http://www.greenpeace.org/international/campaigns/climate-change/take_action/12_steps
Clothes dryers
Traditional clothes dryers are very energy intensive. So-called 'condensation' models without an exhaust tube use even more energy.
http://www.lib.niu.edu/ipo/1998/ic980618.html
Instead of drawing heated air through the clothes and venting it outdoors, a no-vent dryer uses a simple condensing process to dry the tumbling clothes. Air inside the dryer is heated and circulated through the damp clothes. Since it is warm air, it absorbs moisture from the clothes as it dries them.
This warm damp air now circulates through a small heat exchanger built into the base of the dryer cabinet. Another small quiet fan draws room air in through separate passages in the heat exchanger. The two air flows do not mix and no conditioned indoor air is lost outdoors.
This room air cools the warm damp dryer air causing the moisture to condense into a tray. This dryer air is reheated and circulated through the tumbling clothes again to absorb more moisture. When the clothes are dry, you slide out the water tray and empty it. It is basically distilled water that you can use on your plants. For greater convenience, install a tube kit to run the condensed water to a drain.
Bob Irving writes:

This still requires space house heating energy to evaporate the water, about 1000 Btu/lb, vs 330 for a heat pump and 72 for a closet with an air-air heat exchanger. It looks like typical condensing dryers also use lots of energy...
http://ths.gardenweb.com/faq/lists/laundry/2004120958010854.html says:
In a condenser dryer, there are two separate "loops". The inside "loop" of air is sealed from the outside environment - air from within the drum is heated, then blown through the tumbling clothes, then the moisture-laden air is passed through a heat exchanger, where the water recondenses. The same dry air is then reheated, where it is again blown through the drum and clothes, and the cycle begins again (this is a more-or-less continuous process).
The outside "loop" in a condenser dryer consists of either air or water. Some condenser dryer models are air-cooled, and use the ambient room air as a heat sink, by blowing it across the outside of the heat exchanger. These dryers will tend to heat the indoor air in one's laundry room significantly. Note however that ONLY heat is released - all MOISTURE is contained within the unit. The condensed water can be either pumped away to a drain line (e.g. into a standpipe shared with the clothes washer) or stored in a container within the dryer to be emptied later (not all models offer both options). All standalone Euro condenser dryers are of this type, i.e. units from Miele, AEG, Bosch, Asko, Malber, and Eurotech.
In "combo washer/dryers" (i.e. machines that can BOTH wash and dry the clothes), the ventless condenser system is also widely used, but in these cases the condensers are water-cooled. During a dry cycle, several gallons of cold water are used to condense the moisture evaporated from the clothes, which again is pumped away through the drain line. Most of the "combos" currently available in North America use this method - i.e. units from Equator, Splendide, Malber, Haier, Quietline, Thor, LG, and Eurotech. Note that unlike the air-cooled design, these models do NOT significantly heat the indoor air in one's laundry room - but on the other hand, the fact that they use extra water during the dry cycle must be taken into consideration, especially for anyone on a very limited (or expensive) water supply.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

[snip]
It's a lot easier just to hang them up.
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And waste 14 times more energy? :-)
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

I seriously doubt that hanging clothes up to dry wastes fourteen times more energy than anything.
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Hanging them outside on a rotary line is working well for me. Never heard of a drying closet. Not sure I'd want to devote indoor space to that.
--
~Donna
http://www.frugalsewing.com
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Donna wrote:

Scandanavians hoist them up so the clothes are near the ceiling, presumably out of your way and where the heat has risen to.
Jeff

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Get one of those cheap folding clothes drying things (kmart?) and put it in the room where the hot-water heater and house-heating boiler are. Clothes dry really fast -- especially with a small fan, on low (*very* low current-draw) pointed at the drying thing.
David
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