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Old 02-12-14, 04:51 PM   #1
nokiasixteth
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Default Heat pump Dryers

University Students Build Heat Pump Clothes Dryer | EarthTechling

My neighbors could really benifit from one of the heat pump dryers.
They wash 2 and three loads of cloths a day and dry them. Extremely high light bill

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Old 02-12-14, 05:57 PM   #2
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I'd simply appreciate the additional cooling and dehumidification by-products available from a system like that. Hanging my clothes to dry on the solar dryer (clothesline) has all sorts of problems in the tropical climate I live in. The electric dryer I use, although relatively new, is no more efficient than my electric water heater.
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Old 02-12-14, 06:04 PM   #3
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There are a number of heat pump dryers out on the market. I have a LG from several years ago (solid state heat pump) that still works great. Some of the newer ones are compressor based and even more efficient. But none beat a clothesline or drying rack.
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Old 02-12-14, 07:06 PM   #4
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Whats the power consumption on it compaired to a regular dryer ?
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Old 02-12-14, 09:29 PM   #5
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About one half to a third the energy use of a regular dryer. One main flaw with the solid state heat pump dryers is that they quickly lose efficiency as the remaining moisture drops, so best way to use one is to use the timed dry setting and hang the clothes (or iron them) to finish drying. The compressor based units do better.
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Old 02-12-14, 09:42 PM   #6
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What sort of refrigerant do these use in order to produce enough heat rise to dry clothes quickly? I keep seeing stuff about it being tough to get temperatures above the 120-140f range efficienctly or even above 140f at all. My electric dryer has a 5kw strip in it so 17000BTU. What are the specs on a heat pump dryer?
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Old 02-12-14, 11:47 PM   #7
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The solid state ones don't use refrigerant, just some semiconductors. Those have no difficulty at all getting to high temperatures. (Solid state heat pumps don't work very effectively at low temperatures but can operate at temperatures much higher than a compressor heat pump.) Some old compressor based dryers ran on R22. I think the newer ones use R134a since R410a would need insane pressures to work at such a high temperature.

The LG I have has a 250W solid state heat pump along with a 1000W auxiliary heater to get it up to temp quickly. I think the compressor based units are 8000BTU/hr but I'm not sure.
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Old 02-13-14, 04:03 AM   #8
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Quote:
Originally Posted by MN Renovator View Post
What sort of refrigerant do these use in order to produce enough heat rise to dry clothes quickly? I keep seeing stuff about it being tough to get temperatures above the 120-140f range efficienctly or even above 140f at all. My electric dryer has a 5kw strip in it so 17000BTU. What are the specs on a heat pump dryer?
The temperature range of a clothes dryer is a good fit for butane or r134a. The high condenser temp is no problem with the right refrigerant. As in automobiles, all you need is heavier piping on the high side.

R410a was specifically designed for the residential HVAC sector. It has a little more heat capacity than R134a, but at its increased operating pressures, it gets above its triple point sooner. Above around 160 degF, it cannot exist as a liquid. More importantly, associated dew pressures above 140 are like 600 psi+.

In comparison, R134a has a critical temp of 214 degF and 590 psi.
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Old 02-13-14, 11:49 AM   #9
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Temperature is only part of the issue.

The other part is humidity. If you have a mechanism to actively extract moisture, you don't really need to raise the temperature at all.

I have been drying some of my stuff by placing it in front of the indoor-unit output of my mini-split. The air is coming out at about 100F and by the time it gets to the wet towels, it has dropped more... and the towels dry quite fast. And the active de-humidifier is in the yard, and not even part of the drying cycle.

So, I think that if you are determined to make a tumble dryer looking kind of a thing, just hacking a working R22 unit to R290, it would utilize both the warmth of the condenser (120F-ish) and also the active de-humidification evaporator. I mean a donor dryer a donor de-humidifier and some duct tape plus a little brazing along the way, and Bob's your Uncle.

Why try to pioneer new refrigerants?

But it would be so much easier to just build a modest size drying cabinet, and put a dehumidifier right inside. Some of them have a humidity sensor that would shut it down when they get dry enough. A small fan for air circulation would complete the mix.

The bonus is that you wouldn't be piping 5kW-hr worth of heat out the back of the house. You'd just need a drain line for the de-humidifier... any heat generated would 'leak' into the rest of the house.

Why not?

Personally I think that we have become slaves to tumbling electric clothes dryers... they beat our clothes into lint particles (thus shortening the life of clothing) and they waste a huge amount of energy. What exactly is there to recommend them?

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Old 02-13-14, 03:55 PM   #10
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Quote:
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Personally I think that we have become slaves to tumbling electric clothes dryers... they beat our clothes into lint particles (thus shortening the life of clothing) and they waste a huge amount of energy. What exactly is there to recommend them?
-AC
Well, for starters, most folks don't like the scratchy stiffness of line dried clothes. That being said, DW and I both prefer line drying as it triples the life of cotton fabric and saves money.

Where2 is correct in that line drying outside is a problem in the south. We tried and gave up due to humidity, bugs, and pollen. The hornet that came in with my pillow cases and nailed me at 2AM was the final blow.

Indoor drying takes up space but we have more than enough. Last year I put a box fan in the area where we hang clothes. It is tipped up at about 45 degrees into the clothes and runs on low (about 60W).

The fan cuts drying time to less than half and completely eliminates any musty smell. The added moisture is a bonus in the winter. The AC gets the moisture in the summer.

I suggest trying the box fan if drying indoors. The difference is amazing.

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