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Old 06-01-16, 05:38 PM   #21
IdleMind
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I had acquired 2 old dehumidifiers to use for this experiment. I took the larger (40 pint) one apart and have been using it's parts up to now. The photo shows where I am currently at. I am now using the smaller 25 pint dehumidifier. I connected the two coils together as was suggested to make a large evaporator.

I have covered the coils on the tank with black silicone caulk to force the heat into the tank. In this test I filled the tank with 2.5 gal of water and charged the system with 3.0 oz of propane.

After 30 min the tank was up from 65F to over 100F. So it seems good to me. Next I will reassemble the water heater with its insulation and outer shell, do some wiring and install it. And I will be adding two flare connectors so I can move it one half at a time.

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Old 06-07-16, 05:45 PM   #22
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It is installed and working, but not efficiently. I few weeks ago when the heater was installed using the original resistance heater I collected whr data for about a week. After one day it appears that it now uses about 90% of that. Too short of time to get precise. It has about 2.5 oz of propane and I think it would be better with a little more charge. I know I need to collect temp and pressure data before I can ask for advice on charge.

I've wired it so I can swap some connectors around and easily change from heat pump to resistance heater so I can establish a test procedure that would compare different operating conditions. It all takes time and I will eventually get to it.

For now I wanted to show where I was at. And repeat again that my objective is not a better water heater, but to learn some new tricks.

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Old 06-07-16, 09:39 PM   #23
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Fun little mini hpwh project. I'm curious, are you charging gas and adjusting the captube with superheat/subcooling in mind, or just kinda winging it to see what works? Given you have a excellent condenser design (though may potentially trap oil if thr velocity is too low) and a far overkill evap, this thing should be very efficient. You have all the equipment needed to get the measurements to solve for sh and sc. Let me know if you would like some help with that part!
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Old 06-08-16, 08:32 AM   #24
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I do intend to use this for experimenting. I am not yet sure that the condenser is very efficient. Near end of a run cycle I had these readings
Temp out of compressor 143
Temp out of condenser 141
P low 74 psi
P high 315 psi corresponds to 146 vapor temp
So 5F of subcool

I need to get another thermocouple to measure temp out of evap but I can tell there is lots of superheat.

Some things I want to try are taking the original condenser out of the dehumidifier, I don't think I need it and I would like better performance as dehumidifier. With the price of natural gas low compared to electricity this has more value to me as a dehumidifier than as a water heater.

I am just going to leave it run for a few days before I mess with it again.
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Old 06-08-16, 09:36 AM   #25
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I'm guessing water temp and Ambient wetbulb, but add some gas till you get closer to 10* sub cool across the condenser. Get me some low side temp readings and a wetbulb and we can judge the captube. Guessing a vapor temp of 70, wetbulb of 70, water of 100 would put the superheat around 22*, while a target of 10-15 would be better. At the moment it just sounds undercharged.
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Old 06-08-16, 12:36 PM   #26
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30+ degF of superheat is not uncommon with these cap tube driven systems. If you feel the need to push your flow rate, a txv will get the low side in the zone. Since you don't have sky-high discharge temperatures, you are not very undercharged. Remember, max efficiency may not be at max heat transfer rate. As you add charge, compressor current draw increases...the optimum for r290 is a little less charge relative to r22. Propane has less mass but more heat capacity. This means high superheat is not necessarily a bad thing. Keep on doing trials. I would rather slow down a fan than chop down a heat exchanger myself.
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Old 06-08-16, 01:02 PM   #27
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You do have a valid point about having the high superheat, but that only applies when it's in a refrigeration situation. Having 30-plus degrees of superheat when your evaporator temperature is 60 can lead to increased compressor temperatures as well. Compressor requires the vapor to cool it off. When your evaporating in 20 or 30 degrees it's not as big of a deal because it's still coming back to the compressor in a relatively cool temperature. A lot of those little reciprocating compressors in mini fridges and dehumidifiers require less than 70 degree Vapor temperature. A suggested addition to your work there would be a dryer bulb in a vertical position right before your captube to act as a receiver drier, that way you can guarantee a positive liquid seal on the front of the tube.
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Old 06-08-16, 05:57 PM   #28
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You guys have a lot of insights and thanks a lot for your help. You need more data and I like taking data so you will get more soon. Today I ordered another of those 2 channel thermocouple reader. I will add one out of evap and paste one to the steel heater tank. I will also set up a controlled test and record data in a spreadsheet to maintain a record.

The compressor is not overheating, I have a cheap IR temp gun which shows compressor around 120F at end of a heat typical 10 min heat cycle and around 140 at end of long 50 min heat cycle.
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Old 06-08-16, 09:12 PM   #29
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Need a temperature and pressure reading before the captube in the liquid line, a temperature and pressure reading between the evap and compressor (closer to compressor better as it gives true superheat at CP). Need a drybulb and wetbulb reading of the ambient air temp, and water temp at time of reading. A snapshot of these readings at start and at nearly operating temperature will give us a pretty good idea of the current state of things. Also, check your amps on that compressor compared to the RLA/FLA rating on the tag when at operating temperature.

If you also get a drybulb reading of the air coming out of the evaporator, I can check how close it comes to target evaporator exit temperature. That tidbit will help you decide if your airflow is too high/low to be an effective dehumidifier. You should adjust fan speed, not evaporator size! It will be a balancing act between evaporator temperature and superheat to get best results. But remember, the higher the ΔT between condensing and evaporating, the lower the output, and the higher the energy usage. If the fan is a pusher, you want to make sure the coldest surface (inlet) is farthest from fan and the suction closest. If its a puller, it would be the other way around.
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Old 06-09-16, 01:15 AM   #30
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If you haven't already, please try to read through Jake's (Memphis91) build. His HPWH turned out really well, especially considering he knew very little about refrigeration circuits when he began. The optimization of the phase change loop begins about here:

http://ecorenovator.org/forum/geothe...html#post45270

Since you are building an air-source unit, your struggle will be easier. Air coils are much more forgiving until ambient temps drop close to freezing. They are very predictable.

There are so many contributions from members and sources of information to ponder concentrated in that thread. Jake also had a way of guiding the conversation, asking the perfect questions, and providing nearly instant feedback and data to just plain get the thing done fast. Thing about it, he is still following up with usage and performance data a year later. I hope yours turns out as well as his did.

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