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Old 06-15-15, 11:12 PM   #1
MEMPHIS91
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Default Heat Pump Food Dryer

So far this project is in the research phase.
Goals are to create a food dehydrator that uses less than 450 watts of power with out heating up the air as much in my house. My current one pumps out 140F air for 10+ hours a day.
First thoughts were to simply hack a window unit making a box on the condenser coils to push the hot air through the box therefore drying what ever was in it. After doing some digging I found I might be able to make one even more efficient that circulates its own air. Plus a window unit compressor is much to big. I'm on the lookout for an old fridge compressor now.
This article on page 8 has a great picture for what I'm thinking though this one uses a clothes dryer as an example (I am thinking I will need to build a clothes dryer next).
http://www.wasteheat.eu/HP4Drying/Pr...rying_info.pdf

This article I also found very interesting.
Heat pump dehumidifier drying of food - ResearchGate

So far that is all I have. Has anyone seen one of these commercial dryers?
Knowing that this is possible is very encouraging.
I plan to use my current dehumidifier with some ducted taped hoses and drying chamber to see what kind of results I get. Its 76F in the house and my dehumidifier is push 95F air out.


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Old 06-16-15, 01:25 AM   #2
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You'll have better luck with Peltiers. It's much easier to make those work at high temperatures.
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Old 06-16-15, 08:48 PM   #3
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NiHaoMike, yes I can see that it would be easier but the Peltiers are not very efficient. The articles mention that the extreme temperatures are not needed using the "dehumidifier" action of the heat pump. I'm guessing 100-110 would be more than enough, maybe lower.
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Old 06-17-15, 08:35 AM   #4
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That is a very interesting idea. What about using a A/C setup out of a water cooler? Those are quite small.
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Old 06-17-15, 10:38 AM   #5
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Yes a small cooler would be about perfect I think. Those are just a little harder to find than window units. But I live in a college town so I should be able to find one.
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Old 06-17-15, 11:57 AM   #6
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Collegetown equals mini fridges. Portable dehumidification units also. Most draw near 200 watts when running, larger units use more power. Depends on how fast you want to dry your food.
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Old 06-17-15, 01:32 PM   #7
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Oooh yeah, mini-fridge...
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Old 06-19-15, 11:55 AM   #8
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When I was traveling through Turkey, I was struck by the fact that in the countryside, nearly all houses had flat roofs, and that the locals took advantage of the flat roofs to dry fruit.

They didn't build a solar food dryer, they put fruit on the roof to dry. Worked great, the practice is VERY wide spread there.

[EDIT: I just remembered that you are in Mississippi... what is your typical RH?]

I can understand why you wouldn't want your food dryer to heat up the house too much. Why have the food dryer inside at all? I can see it in the cooler season, you could use the heat to help heat your house.

Also, why are you so interested in finding the smallest compressor? That part is not clear to me.

Do you know how many watts of resistant heating is required to dry your food?

If you knew that, you could divide that number by 3 and you would have a good approximation for the compressor that you would need.

As to tiny refrigerator compressors, I'm not convinced that their efficiency is very high. I was interested in building a tiny compressor for experimental use, just a water-in-water-out kind of thing. Seemed like it could find uses around the house. I looked into tiny compressors from those free-standing water coolers. They must he horrible, I see a constant flood them turning up in the thrift stores, all the time. I got one, and was in the process of disemboweling it, so as to re-arrange the parts for my project. and I looked up the energy efficiency rating of the compressor, and as I recall, it was barely over COP = 1. Which is far better than Peltier, but not up to the standard that I would need, to make the project worthwhile. So eventually, the parts made it to the scrap metal recyclers.

I would look for the smallest dehumidifier you can find. I know that 20 pint per day units exist, and that is what I would seek. I think they are about 350 Watts.

If you can't locate a compressor that can give you COP = 2.0 minimum, you might be better with incandescent using light bulbs.

Maybe paint your food dryer black, leave it out in the sun... use a cheap thermostat so that the sun's heat plus your bulb's heat would give you the temp you want. When the sun was not shining, the bulbs would take over the heating job. Fan running constantly, of course.

Are incandescent bulbs still being made?

-AC
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Old 06-19-15, 10:14 PM   #9
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AC, Yes solar is an option and one that I have used from time to time. But with as much as we have to dry we can not afford to miss a day due to clouds/rain, also we dry a lot at night while we sleep. Solar is unreliable here anyway, you CAN dry but if the sun goes behind a cloud the food soaks the moisture right back up if you aren't watching it carefully.



Colder season if fine for the extra heat but at 450watts I can use that power else where and make way more heat.

Dehydrating slower is easier in the food therefore a small, slow compressor is all that is needed. But not exactly tiny either. COP is measured by the machine it is in and the job it is doing, I do understand that a tiny compressor can in fact be too small to really handle the job it is trying to do, but I'm only trying to dry a 18"x 18" x 10" tightly sealed box.

My grandmother past away Wednesday and I needed to get my mind off things so I went to work. I got lucky this week and found a handful of free compressors, the smallest being 6.5 LRA.
I stole a evap coil out of a window unit, a condenser out of a water cooler, and a fan from a refrigerator.


I laid everything out to see how it would fit. Air enters from the food box above the unit through the evap coil and is blown over the compressor and through the condenser coils back up into the bottom of the food box. Thus continuing the loop until the food is dry.



Made me a custom drip pan for the evap. With a piece of plastic and a torch.



And here she is, at least the guts of her.



25 psi suction. 180 psi discharge. 1.5 amps.
It started poring water out with in 5 minutes of running. But I think my cap tube is to long so I'm going to cut some off and see what happens.
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Old 06-19-15, 10:46 PM   #10
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That was quick!

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