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Old 10-31-16, 01:03 PM   #21
IdleMind
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A few days ago I charged it and experimented with settings to determine charge amount, fan speed, and txv adjustment. At end of day I had charged with approx 2.70 oz of propane, 3.5v power to fan, and had turned out the txv adjuster screw 2 turns to increase flow to evaporator. The txv adjuster never made much of a change in SH, perhaps the txv is too big for this small compressor. Although it did the job as good or better than the cap tube.

After setting over night to stabilize, I started it up and took this data.
At 13 min the bottom 3 evap tubes were frosted over, and by 19 min they had thawed out. By end of the test the compressor case was up to 197F which I consider on high end of acceptable.


I thought the fan was running too slow and that if I speeded it up it would increase SH and lower SC. So I cooled down the water and changed the fan power supply from 3.5 to 9 volts and took following data.

Only the bottom tube frosted briefly before thawing. It performed better at heating water. The SH responded like I expected, but the SC did not change much. Maybe the condenser is just too big for the compressor.

Not sure about next step, maybe let out some propane. But I have a very old 8000 btu window AC that is not being used, so I could try its compressor just as an experiment.

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Old 11-01-16, 11:33 AM   #22
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Getting closer

By increasing the volume of air thru the evaporator you have introduced more heat into the refrigerant 62 Deg vs 57 Deg. ergo more efficiency. The result is less power 0.479Kwh vs 0.575 Kwh.

IMHO larger compressor Big NO
The existing larger evaporator YES
Moving more air thru the evaporator BIG Yes (fan shroud. bigger fan, 2 fans)
(How much more free heat can you get into the refrigerant.??)
(beware of diminishing returns)
Calculate the BTUs see how your stacking up.

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Old 11-01-16, 11:48 AM   #23
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Taking this to it's logical conclusion, is the most efficient setup going to be a huge evaporator that requires minimal airflow assistance in order to absorb heat from the ambient air?
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Old 11-01-16, 12:49 PM   #24
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Fionn,

What you just described is a deep freezer. No fans at all, big huge evaporator and condenser. Small compressor on a thermostat. For something that doesn't move lots of BTU per hour, it works well. At the other end of the spectrum, this is how ground source heat pumps work when installed in a hydronic heating system. They are some of the most efficient heating systems that can be built using refrigerant phase change.

IdleMind,

I believe you are still overcharged. The TXV seems to be doing its job as best it can, even though you tried eliminating the superheat control by dialing it down. If it doesn't have a MOP charge in the bulb, the valve has some other means of limiting your max evaporator pressure. It probably saved your compressor from flooding or being severely overheated. If you run a larger compressor, you may not be so lucky.

I would attempt to get the superheat and subcooling values fairly equal after the loop stabilizes. If you are using the stock evaporator and compressor together, they were engineered to slowly freeze the evaporator while extracting water from ambient air. Running more airflow and a higher saturation temperature will get you more BTU throughput IF the compressor can move the extra mass of refrigerant. Since you are using a more efficient blower, you are saving energy on that end. The trade-off here is that the compressor isn't ingesting nice, cool vapor. As a result, IT GETS HOT.

On the other (high pressure) end of the loop, you seem to have a nice, large condenser. Being at the bottom of the tank, it is in contact with the coolest water. Even though it is not completely immersed in water, and having to move heat through a steel plate, it still looks to not be a limiting factor. Running a larger compressor would probably work, but that would be using more energy per minute to move the same refrigerant.

The problem you face here is to build up enough high pressure to move heat, but not so much that this heat accumulates in the compressor. With a compressor this small, it is going to have a fairly linear power draw versus head pressure profile. You will have to choose between operating modes. You can run a drag racing profile, where the unit runs at high pressure for less time (maybe saving a few watts) and then rests to save the compressor from overheating, or a long-distance profile, where the unit takes extra time to get going by building up pressure slower. The difference lies in the system charge and how much liquid is backed up behind the expansion valve. The TXV will make sure the evaporator is behaving well by maintaining constant superheat.
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Old 11-01-16, 05:41 PM   #25
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Is P1 the pressure on the suction. I'm tending to agree with Jeff5may overcharged. My suction pressure was around 30 psi. Your current should be a lot lower.

Reduce the charge and let us know

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Old 11-02-16, 10:41 AM   #26
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Randen--I have done some btu calculations, will try to sumarise.
6 gal of H2O weighs 50 lbs, and takes 500 btu's (.147 kwh) to raise 10F. So I look at 10F range of my data and take a ratio to .147 kwh.
I selected ranges in the neighborhood of 96 to 106. I try to read the thermometer in the water tank to 0.5F but there might be +/- 0.5F at each reading so these numbers could have up to 10% error. Note I deleted some data lines from the tables I posted just to keep the size down, the following data points do exist on unedited tables.

In the first table (3.5v to fan) going from 96.0F to 106.0F took .155kwh or 106% of theoretical.
In the second table (9v going to fan) going from 97.0 to 107.0F took 0.147kwh or 100% of theoretical.

Go back to my post on 10-18-16 which still has cap tube
Going from 96.0 to 106.0F took .098 kwh or 67% of theoretical.---This is the best test to date.

The next table posted 10-20-16 where I let out some gas to reduce power and subcooling.
Going from 97.0 to 107.0F took .101 kwh or 69% of theoretical.

I need to repeat this using the resistance heater because I think that would be a better comparison since some energy goes to heating the steel tank.


The reason I experimented with different fan speeds to see how it might affect SC, SH, and compresser temp. The air temp out of evap is measured with a thermometer opposite the fan near the bottom of the evaporator so it gets the coldest air out. Performance was better with 9v Vs 3.5v to fan by 6%. I have run it with 18v to fan and it seems to not be much different than 9v, but intend to stick with 18v because I already have too many variables.

I will respond to the other recent comments soon.
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Old 11-02-16, 11:36 AM   #27
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So I did let out a small amount of gas, since I only started with 2.7 oz it seems unlikely it could be overcharged. And I will be using 18v to the fan which you can see by the T air out of evap is now only about 4 to 6 F lower than air in.



I shut it down at 95F water temp because it didn't seem to make an improvement.
Going from 85.0 to 95.0F took .153 kwh or 104% of theoretical.

In most of the tests i have run there is enough SC & SH to know that there has been a full change of phase, does any excess get any real benefit.

My concern is the gas out of the compressor has been running in the 160 to 170 range around 25F over saturation temp. It makes me think something is wrong with compressor. I am not planning on throwing a bigger compressor on because I want more power, it's just that I have one that I can try.

The P1 is suction pressure, for the winter when my basement air is around 55F I'd like to see it around 60psi which would have a saturation temp of 37F. In the summer when ambient is around 70F and humid I think it should be around 75psi with 48F saturation temp.
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Old 02-13-17, 03:01 PM   #28
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This little HP has been up and running for 2 months, I've just been lazy about posting how I finished it up. It's been a lot of trial and error. When I last posted I was trying to get a TXV to work but I never got COP much over 1.0. So swapped in & out compressors & various cap tubes. Then I got rid of the dehumidifier evaporator and replaced it with a coil of copper tube in a 5 gallon bucket of water. I also replaced the compressor with a small refrigerator compressor. I set it up so I could swap my various cap tubes in & out with flare connections. By chance more than science I eventually got it working to my satisfaction. I kept lots of COP data along the way so I could make good comparisons of various cap tube sizes vs performance. Here's a photo of my test set up.


Then I glued together sections of thin wall 4 inch pvc tube & filled with 6 gallons of water as the working heat source. I also coiled up about 15 feet of 1/4 copper tube to insert into this pvc tube for the evaporator. Here's a photo of the evaporator.

And here's how it looks up & running.


This only feeds the hot water to the bathroom sink. It's located so that I needed to plumb about 8 feet of water line to where it joins the pipe right under the sink. I used 1/4 copper tube to keep the volume of water in the pipe low so it it would deliver hot water quickly. I measured the faucet flow rate before the change (1.6 gpm) then with the restriction for the small tubing it dropped to 1.3 gpm which works really well.

It draws between 250 & 290 watts. When run on the resistance heater it draws 1450 watts. Where it is installed it is on an original circuit with and old screw-in 15 amp fuse. There is enough other stuff on the circuit that I could not use the resistance heater. So besides being more efficient, I don't have the electric capacity for a resistance heater anyway.

Now that I have the kitchen & bathroom sinks both with small heat pumps, I can turn off the main 40 gallon gas water heater except for 15 min before using the shower.

A few more notes on the passive heat source. The last month is about as cold as it gets and the basement temp has been at 55F. The water in heat source runs between 50 & 52F. When the heat pump is running it can drop down to 45F, but only right near the evaporator coil while the temp out of the evaporator gets down to 30/32F. In the summer when the basement air gets up to 65/70, it should work even better.
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Old 02-14-17, 09:13 AM   #29
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Looks like you're steadily cranking away on this!
What do your present configuration and pressures look like?
When you first started i wondered if you might have some blockage (maybe scale) from all the soldering on the condenser but 3/8 is really large for that size compressor and you would likely have discovered it by now anyway.
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Old 02-14-17, 03:43 PM   #30
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Along the way, I did conclude that 1/4 tube would be better than 3/8 for the condenser. With it buried under silicone caulk and great stuff, I do not intend to replace it.

Anyway, here's the last data I took before installing. I started the test in the morning so all the temps are stable. With it on the bench, I can insert a thermometer into the top water outlet & get good water temps which I use for calculating COP. The overall COP is calculated over the whole cycle. The column with incremental COP is from one data read to the next, so not real accurate, but interesting how it improves as the pressures come up and then declines as the hot water temp comes up and the bucket water temp drops.

After installing and filling the tank, I set the various water shut-off valves so there was no flow thru the tank and brought it up to temp using the resistance heater then let it alone for a day to measure watts just to maintain at temperature. Then repeated using the heat pump. It took 389 watt hrs per day with resistance heater and 285 watts per day with HP. So in that comparison COP is 1.36. In those cases it is maintaining water temp between about 98 & 103. I believe the COP will be better for heating cold water which is now coming in at 50F.

When I installed it, I added a liquid filter that you can see just behind the high pressure gauge in the photo in my last post. It took 3 oz of charge more than without the filter and the performance changed, I think for the better. I guess a little more pressure drop, Just thought that was interesting. Here's some data after installing. The water temp is from a thermocouple pasted to the outside of the tank next to the thermostat, it's not accurate enough to use for COP calculation.

After running for 2 months now it seems to have lost a little charge. The low pressure tends to run at 60 and the high is a little lower too and the watts never get over 290. I think it is better at this lower charge.

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