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MEMPHIS91 06-25-16 08:52 PM

The Ultimate Dehumidifier
Hey guys so in my area of the world we have this terrible things called humidity. It was what keeps us from using swamp coolers and cooling towers and really all the cheap ways of cooling your home. Thus the huge need for AC. WELL if you are brave (or stupid) enough to set your thermostat higher in order to "sweat it out" your in for quite the fight. Why, well because of the moisture in the air. Heat + moisture = death. So we (maybe just I???) run a dehumidifier to keep the moisture lower but as you may know these handy power hogs also produce heat. Even though it is still cheaper to run the dehumidifier more and AC less, it still really sucks to be heating up the air. This is where multistage compressors are REALLY nice but seeing as I don't have one of those and my AC uses a whooping 4,000 watts of power. SO here is why I am asking input on a design of a homemade heat pump dehumidifier.

Main goal: The create a dehumidifier that will use less or equal to the same power the current one uses (4.5A 120V) WHILE cooling the air instead of heating it WHILE still sucking out just as much if not more moisture from the air.

Budget: Low as normal

Bonuses: It would be awesome if it could heat in the summer time.

Design: So basically I'm building a mini split that is designed more around condensing than moving massive BTUs.
Thoughts: Maybe I could have an undersized condenser that freezes up and then a powerful fan kicks on and melt the ice, then turns off again to form more ice? Turning the compressor on and off a lot seems like a bad idea.

Condenser: Jeff said straight fin might be the best for dehumidification. Thoughts?

Refrigerate: I'm thinking R290 or R152A, but open to suggestions.

Evap: I can go either air or do another DX borehole, this time maybe do 2 boreholes?

I have a 6,500 btu rotary compressor, txv, and several air evaps ready to go.

I will get picture up as soon as the build starts.
Thanks guys, I look forward to sharing tons of data and pictures!

mejunkhound 06-25-16 10:46 PM

You do not want to form ice, requires heat of fusion to be withdrawn from evaporator.

For best efficiency, you only want the evap coils to be just below (about 3C) the dew point. Use a RelHumidity sensor from scrap dehumidifier.

re: Evap: I can go either air or do another DX borehole, this time maybe do 2 boreholes?
In dehumidifier usage, any borehole would be the CONDENSOR.
Evaporator is where you boil the refrigerant, which pulls heat from the air to boil the refrigerant and cools the coils to condense air moisture.

jeff5may 06-26-16 02:10 AM

You have your heat exchangers mixed up. The evaporator is the cold one in the suction side, the condenser is the hot one on the discharge side, of the refrigerant loop.

What you have been doing with your DX water heater setup seems ideal for your purpose: big coil indoors with low airflow to grab lots of water vapor, condenser heating not indoor air, compressor located outdoors. With these smaller setups, the compressor is usually the weak link in the heat transfer equation. Due to the limited (nearly constant) displacement, it can only move so much gas.

With your half ton compressor, you can move around 6.3 megajoules of heat per hour. This translates to about 3 pints of condensed water per hour in a perfect process. i would shoot for a liter per hour tops. This is what you can hope for.

MEMPHIS91 06-26-16 05:33 AM

Mejunkhound, gotcha, I can fine tune the charge to set the EVAP to right at the dew point.
Thanks you and Jeff are totally right I did get the evap and condenser mixed up. I knew there was a good reason I was getting advise.

Jeff, OK so large Evap, should I go straight fins? And borehole for the CONDENSER.
So I should look for a 5,000 btu compressor instead.
Also I am thinking that I should be looking for a refrigerant with a dew point at as low a pressure as I can find. Less pressure should equal less power consumption. That is unless I want it the be able to heat as well.

I really wish I used more hot water and the heat pump water heater could do all the moisture removal. Then I would have yet another unit running. Or I guess I could put in more valves and route the borehole into the evap I already have. A dual purpose system.......

Also what if a slowed the fan down on my existing 4 ton central unit air handler? Would that cool slower therefore remove more moisture? The unit comes on like 40 times a day and only runs for 5 minutes or so. I have worked so hard on the house now my unit seems over sized. Lol

jeff5may 06-26-16 12:11 PM

The two ton coil you rigged in to the hpwh is plenty large enough...but for a water grabber. You seem to have indicated it was doing a good job in that regard without overcooling the house. The thing you want to deal with is the high pressure side.

I still like the idea of the desiccant waterfall. A number of members have been interested in them but I haven't seen the results of anything yet from individuals. The basic idea is this: you pump a working fluid (salt water) between a water grabber indoors and a warmed evaporation unit outside. At lower temperature, the salt naturally wants to be a weak solution, at warmer temperature it naturally wants to be a stronger solution. I did a proof of concept experimental rig using calcium chloride ice melting salt, and found it works pretty darned well. I commented about it in another thread here. That method would use a whole lot less energy than a refrigeration cycle would to get the water vapor outdoors.

This liquid desiccant idea could work in tandem with your hpwh / dehumidifying rig you have going now. If the interior RH is too high and your water is hot enough, you could use the waste heat generated to heat your outdoor water shedder. This is known as "double effect" as you are reusing a waste product in the process to further the process. It is a proven way to gain another 30% or more efficiency from your system. Used all the time in industrial processes with waste heat products.

EDIT: Found the thread. Be my guest.
The interesting begins about page 3 or 4. Leads to a discussion on the Coolerado and DEVAP units. Read that one too.

MEMPHIS91 06-26-16 12:46 PM

Jeff, yes the 2 ton coil is a great water grabber, but it doesn't like to let it go. For the 3 hours that that unit runs it does not produce much water out of the drain. It all gets caught in the spine fins and just evaporates back into the air. So that is why I was thinking of using the straight fin coils or make my own like I did with the food dehydrator with not fins only super sized set of copper tubes running vertical so that the water quickly falls off the coils and into the drain.
I also like the idea of it adding COLD air into the house. Yes the salt system has grabbed my attention several times in the past, but it will also add heat to the house and seems to be a lot of work + a lot to maintain. I will visit the idea again but for the time being I would like to finish running this phase changing system idea.

jeff5may 06-26-16 06:32 PM

i hear you. Just throwing around ideas like usual. If you want to build another evap coil, save the bristle coil for further experiments. I would shut off the fan to see what if first though... As oversized as it is, you may not be able to freeze the thing completely before the water heater gets hot... The world may never know...

Otoh, a junk 2 ton a-coil wouldn't be hard to find, and would behave better for your purposes.

DEnd 06-26-16 06:44 PM

How leaky is your home? Just like with heating and air conditioning the cheapest way to reduce humidity levels is to prevent the humidity from entering in the first place. If your home is really leaky then the vapor pressure difference is going to drive moisture and air into your house. This is not really good for your structure, or health.

mejunkhound 06-26-16 10:28 PM

And for the LOW cost option, do what pappy did in the 1940's and 50's.

Take a couple pair of old nylons (old pantyhose would work great now), fill them with calcium chloride, and hang over a bucket or floor drain
- gallon or so per day per 2 foot of sock leg per day in central IL summers in the basement.

MEMPHIS91 06-27-16 11:03 AM

Mejunkhound, did you see this with your own eyes? That's a lot of water! And require no power. Can you give me more info?

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