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Old 11-26-12, 07:15 PM   #1341
randen
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Default Update for homemade GSHP for the shop

The 3.5 Ton homemade GSHP ran last heating season and provided enough heat to remain comfortable in the shop. A shop temp average of 17-19 Deg C was maintained The beautiful part about it was the heating bill was only about $350.00 as compared to the oil heating $2800.00.

The heat pump had required some additional tweeking. One of the tube and shell heat-exchangers was weeping a little but the whole unit had to be disconnected and disassembled to get at the leak. That annoying little leak had prevented the inclusion of glycol into the system so the HX (ground loop) can operate into the negative loop temps without the HX freezing.

The other tweek was a TXV instead of the capillery tube. This device modulates the amount of refrigerant into the evaporator adjusted by the temp of the refrigerant leaving the evaporator. I had purchased a TXV off ebay for a 3-4 ton air conditioner and I didn't realize not all are adjustable. I thought what the heck I have it now, I will install it maybe I'll get lucky. It's preset during manu., so maybe it will work. NOT.

After installation and purging the air from the loop and air handler the compressor wouldn't give a pressure differential. Damm. The TXV was installed with fittings so it could be removed and replaced with another or hacked. I guess at his point happiness was the pesky little leak was cured!!!

I disassembled the TXV and figured how to make it adjustable by combining a stem from a gate valve and the internal adjustment spool. The new TXV was installed system vacuumed and tested for leakage. All good. Reloaded the refrigerant and turned it on. It is suprising how sensitive the adjustments can be. As of today I'm looking for that sweet spot maximum heat gain with refrigerant charge and TXV adjustment keeping a close eye on temps presures and current draw. I guess the TXV is doing its job the pressure is more steady reguardless of temps.

So far no increase in performance compared to the capillery tube. Here are some of the numbers: Loop temp in 12 deg C return 11.3 Hot water from HX to air handler 33 deg C return 21. Refrigerant suction 70 psi head 275 psi Refrigerant temps Compressor head temp 72 Deg C after TXV 9 deg C suction temp after loop HX 11.3 deg C Compressor Current draw 13A ( this seems a little higher than with operating with the cap tube)

Today with the out-side air temp around -2 Deg C the shop temp made it to 18 Deg C. I would like to see a plus 20 Deg C. with of coarse a good COP. I will continue to adjust the charge and TXV settings to find the sweet spot.

Randen

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Old 11-26-12, 09:31 PM   #1342
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Default

Randen,
That TXV mod is sweet! I never would have thought of doing it. I have a similar TXV in my project. I have had to repair leaks in it a few times, and every time I adjusted the TXV for more superheat. It sure would be nice to be able to adjust that baby at will.

A word on R290 and superheat. If your TXV came set up for R22, the factory superheat setting is WAAAY too low! R22 likes 6-7K superheat in practice, whereas manufacturers are recommending 14-20K or more superheat for R290 systems. It seems that the propane dissolves in oil, making a kind of oil soda in the condensor side. This actually aids in heat transfer, since liquid propane soda has much better thermal conductivity than just oil. The problems happen on the other side of the TXV when this oil soda rapidly expands. Instead of fogging like R22 and oil, the propane soda fizzes and foams like warm beer! If your superheat setting is too low, the stuff spews right through the evap coil, transferring nearly no heat and killing your temperature split. If you listen to the evaporator, you can hear it! Sounds kind of like a car radiator that is about to spew from overheating.

You will see good high side pressure, yet low discharge temps. In my window AC unit, the SST was below freezing, but the bottom third of the evap hx never frosted up. After a few hours of operation, the suction line was heavily frosted all the way from 1/2 way up the hx to the compressor shell. When the unit defrosted, it behaved OK running through stock R22 cap tube circuit. Since then, I believe I have gone two turns up from the factory setting (it was set high anyway to deal with a distributor and its pressure drop). Once I got the superheat up to around 20 degrees F, I started to notice an improvement. I'm running at about 16-18K superheat now, and I just might turn it up again if i have to. Don't tell the HVAC guy for fear of being bludgeoned.

Last edited by jeff5may; 11-26-12 at 09:36 PM..
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Old 11-26-12, 10:31 PM   #1343
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jeff5may
Thanks the mod did work sweet. No leaks. The gate valve was an antique almost threw it out a couple times. I had bought a new gate valve to use and it was constructed so shotty I couldn't use it with any faith.
I want to get a little more heat. Not sure if I can add more propane or if will make a differance. Possible more liquid to evaporate to gather more heat and more gas to compress to liquid releasing more heat.

Randen
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Old 11-27-12, 12:12 AM   #1344
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Randen,
It looks as if you are having the same issue as I was. When I began with the TXV, I was running suction pressures in the 60 to 75 psi range. I just couldn't figure out why my high side wasn't getting hot enough. So I googled the following:

suction superheat r290

The first 2 hits, from bitzer and purdue, are definitely worth reading. That's where I read about the high superheat needs of R290. It has higher heat capacity than R22, so it doesn't superheat much when being compressed like R22 does. There are 2 main ways to add this superheat: lower your suction pressure or add a suction line hx. It goes against what I know about phase change systems: lower suction pressure and you reduce capacity. But guess what? The rule book lied! The scientists tested it out and it works. Who knows why? So I cranked my superheat WAAY up to see what would happen. Lo and behold, the unit poured out the heat immediately.

Now, my ASACHP suction runs in the 40-50 psi range on mild heating days. It has gotten down to 23 degrees f, at which time the suction pressure was closer to 35 psi. It used to peter out in heating mode around freezing temps. Now, I'm waiting for a cold snap to see how low it will go. If it doesn't pop another joint, that is.
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Old 11-27-12, 12:40 AM   #1345
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Quote:
Originally Posted by jeff5may View Post
...So I googled the following:

suction superheat r290

The first 2 hits, from bitzer and purdue, are definitely worth reading. That's where I read about the high superheat needs of R290. It has higher heat capacity than R22, so it doesn't superheat much when being compressed like R22 does...
This is really great information. I hope that Vlad and Brad_C are checking this out!

Best,

-AC
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Old 11-27-12, 12:48 AM   #1346
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Quote:
Originally Posted by randen View Post
...I had purchased a TXV off ebay for a 3-4 ton air conditioner and I didn't realize not all are adjustable. I thought what the heck I have it now, I will install it maybe I'll get lucky. It's preset during manu., so maybe it will work. NOT.

After installation and purging the air from the loop and air handler the compressor wouldn't give a pressure differential. Damm. The TXV was installed with fittings so it could be removed and replaced with another or hacked. I guess at his point happiness was the pesky little leak was cured!!!

I disassembled the TXV and figured how to make it adjustable by combining a stem from a gate valve and the internal adjustment spool. The new TXV was installed system vacuumed and tested for leakage. All good. Reloaded the refrigerant and turned it on. It is suprising how sensitive the adjustments can be...

This is really some great hacking going on here!

I'd say that it is right up there with Brad_C's stepper-motor controlled electronic expansion valve, and the controller and software he cooked up to control it.

Great hacking, absolutely!

Best,

-AC
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Old 11-27-12, 01:24 AM   #1347
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Quote:
Originally Posted by AC_Hacker View Post
This is really great information. I hope that Vlad and Brad_C are checking this out!
I've been keeping a very irregular eye on things, but it's been a manic winter. Have 10 days away coming up to catch up on what you guys have been upto over (my) winter.

I started back on my system a week or so ago. Water loop is finished and running, I'm just chasing my tail with what could be a tiny vac leak on my system, or a leak in my couplers or vac gauge. I'm losing about 400 microns an hour. Once I get that nailed I'll gas 'er up and wait to see what happens.

I need to dig a bigger injection bore. I'm getting 5-10 psi (oddly enough it varies slowly over the day!) back pressure on the water loop, but I should still be flowing ~30 L/min which is plenty for this system (I have a fleabay hall-effect flow meter on the way). I flow the same amount of water, but at 100W less pump power if I can drop that back pressure below 2psi.

It's amazing when you put the tools down for 6 months how much you lose. Took me a couple of goes to get my bends and brazing back up to scratch.

Randen, that valve hack is a work of art. Really beautiful.
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Old 11-27-12, 01:45 AM   #1348
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Quote:
Originally Posted by jeff5may View Post
Now, my ASACHP suction runs in the 40-50 psi range on mild heating days. It has gotten down to 23 degrees f, at which time the suction pressure was closer to 35 psi. It used to peter out in heating mode around freezing temps. Now, I'm waiting for a cold snap to see how low it will go. If it doesn't pop another joint, that is.
Code:
If you are condensing at the same temperature, but you drop your SST then the compressor has to work harder to compress the gas (higher compression ratio), and therefore you increase your power consumption. That extra power gets dumped into the condenser as heat. Check your compressor current as you lower the suction pressure and watch it slowly climb.
<Edit>Scratch that last part, I think I'm wrong. I need to think about it some more. This forum does not seem to have a "strikeout" feature.


Interesting reading on those links you pointed at though. I'll certainly make some changes to my parameters to play with the superheat and see what sort of gains I can make. Last summer I was aiming around 2-3k, which from those docs is _way_ too low for R290.

Last edited by BradC; 11-27-12 at 03:10 AM..
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Old 11-27-12, 04:20 AM   #1349
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Brad C,AC_Hacker & Jeff5may
Thanks for the complements on the TXV hack. I will admit the compressors pressures are alot more consistant as the head side temps change thru the day. What should I try to increase my heating capacity.??
When the cap tube was installed and running 24/7 I had to reduce my charge to keep the loop-side HX from freezing. Of coarse if I had added the propylene glycol this wouldn't happen.
But if I understand what Vlad was speaking of its heating we are after and not the cooling side. We need to optimize the heating capacities of the unit and what occurs on the evaporation side is less of a concern as long as its picking up enough heat.

With the new TXV do I have an overcharge?? Hopefully I haven't maxed out the capacity of my GSHP. By comparison the commercial unit in my home @ 3 tons drawing 12A is heating a larger area. And the hacked compressor was rated at 3.5 ton and drawing more current and less heat.

Randen

Last edited by randen; 11-27-12 at 05:22 AM.. Reason: better explan.
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Old 11-27-12, 04:40 AM   #1350
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randen, how has your DIY unit compared capacity-wise to your commercial unit? Are you seeing similar output temps, amp draw, COP, etc?

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