Another DIY HP Waterheater (pics and info)

[RB855]

Not to take bloody forever to finish a project, but things have once again occurred in development. Discovered that the evaporator between a heatpump and straight cool model differed. The heatpump coil, which I was using, had the suction coming off the air downstream of the coil, which was not very good at extracting all the available superheat. The a/c version has the suction on the front of the coil, allowing higher coil capacities as the superheat could be set much lower. So of course, I changed the coil. Fixed my txv hunting issues and increased system capacity. I achieved almost the same btu at 54*F ambient, as I was getting before in the mid 70s! Also got to play with my new gauges to very accurately setup superheat and subcooling. I have also learned the value of a proper vacuum. My previous equipment was old and mal-cared for, and in no way or form could pull an acceptable vac below 4-5000 microns. Fast forward to good cared for equipment, it took forever to get down to 300 microns, as the system had previously uncaptured air and moisture in it. Took a lot of heating of the drier and compressor to get it to stop off gassing moisture. I have also made an attempt at organizing and documenting the experience in a proper photo album, for your viewing pleasure! Still using the same old server that can be a little slow, so give it a moment.

Its been a rough year, so any project development was pretty much halted. I'm taking a more Me Time centric approach to this year as it seemed like everyone else owned more of me than I did, so hopefully things will start progressing again. Losing track of your life, a little hospital time, and a couple major disasters changes your outlook on things, let me tell ya.

[AC_Hacker]

Originally Posted by RB855

...Discovered that the evaporator between a heatpump and straight cool model differed. The heatpump coil, which I was using, had the suction coming off the air downstream of the coil, which was not very good at extracting all the available superheat. The a/c version has the suction on the front of the coil, allowing higher coil capacities as the superheat could be set much lower. So of course, I changed the coil. Fixed my txv hunting issues and increased system capacity. I achieved almost the same btu at 54*F ambient, as I was getting before in the mid 70s!

Thanks for sharing your results.

Is there any way that you could show with photos what you mean regarding, "The heatpump coil, which I was using, had the suction coming off the air downstream of the coil..."

This is really good info that everybody needs too understand.

Great work!

-AC

[jeff5may]

Originally Posted by Stoker

Hi all,
Been lurking here for a while learning lots. Thanks to all of you.

RB, thanks for that explanation. Perhaps you could help me fill in some blanks in my understanding.

"Above the critical temperature, a liquid cannot be formed by an increase in pressure, even though a solid may be formed under sufficient pressure." That said, you dont ever want to go near this region. To name a few, R-12 233*F, R-22 204*F, R-134 213*F, R-290 206*F, R-410 161*F. So as youll notice, 410 is by far the lowest, but 161*F equates to 688psi! Not a good gas for heating beyond say.. 110*F. (and the reason why desuperheater reclaimers dont work as well these days)"

Having your refrigerant becoming supercritical would be a problem if your condenser became overwhelmed and couldn't reject enough heat to condense it before cap tube / txv. Or if it were to exceed the pressure rating of the machine and potentially rupture something. But what else would happen?

If the fluid failed to condense it would whistle through a cap tube as vapour and equalize pressure between the high side and low side (basically turning your evaporator into another condensor). I'm thinking a txv would do the same because the vapour would be superheated entering the evaporator already. So basically pressure in the evaporator would render it incapable of absorbing more heat. No pressure diff = no phase change= no heat pump.

If the pressure of the fluid in the supercritical condition didn't exceed design spec, would it not just sit there blowing heat until the condensor started to condense again? Or would a critical failure result?

I haven't even started trying to think how efficiency would be affected.

I'm a marine engineer not an hvac guy. My experience is mainly with steam, so I could be making some incorrect assumptions or poor analogies.

When the refrigerant will not condense or it is too hot, the gas builds up on the high side until it does condense. With a cap tube metering device, it passes roughly the same velocity of either gas or liquid. The liquid has much more mass, so more heat flow occurs. Equilibrium results. With gas flowing through the cap tube, much less mass is carried to the low side. The compressor will literally pull a vacuum on the evaporator if it can physically pump the cr. more likely, it will trip out on its thermal overload.

With a txv metered system, the situation is much different. Gas in the liquid line will flow much more quickly through the valve. As the evaporator pressure rises, the valve will close quickly, causing the needle to hammer into the seat. If this happens a lot (flash gas in liquid line), the txv will lose its control mechanism and act as a leaky faucet. Eventually, the compressor will be flooded by a burping evaporator and let out magic smoke. The magic smoke will stay in the sealed system and contaminate everything, causing not only the need for a new compressor, but a decontamination of the entire system. Auto techs call it "the black death".

[RB855]

(There are 2 pictures here. If none/one loads, refresh. My servers slow lol)
AC, the design changes I think affected me mostly because of my use of TXV which was sensing a colder vapor line on the HP version vs the AC version. This unit was originally capillary, so superheat was whatever they dialed it to. Now, this design variant probably isnt a universal thing, more of a per model modification. But, to demonstrate the differences found, heres a couple pictures.
This is the A/C version with the vapor line final passes across the front of the coil. This exposes the warmest possible air to the last passes and makes the most use of the evap. This is an example of a counter current exchange.


This is an example of the heat pump version of the evap. I fully suspect this difference was to improve heat performance. Since flow is reversed, this would make the last pass on the front of the coil instead of the rear, again creating a counter current exchange. The negative to this design is its a co-current exchanger in A/C mode.


Both coils used the same number of rows, same fin per inch, but the plumbing was quite different. Both coils entered as 2 channels, split half way through, and exit as 4 channels. The differences are the location of the inlet and outlet ports, as well as the paths of the gas through the coils. Again, I found the HP version coil to not be nearly as efficient as an evaporator, as the AC version was. Im not sure if it was an actual coil performance difference, or the fact that I was unable to dial in my TXV properly with the HP coil.

Im not sure how far along everyone is with understanding superheat and subcooling. If Im talking greek to anyone who fancies building something like this, read it, learn it, understand it, or it will never be perfect! The heatpump coil I found having to set very high (20-25*F!) superheats to prevent hunting. The problem with that is it was severely starving the coil, reducing capacity, and I had no way to see if liquid was actually leaving the coil at low superheat settings causing the hunting as it flashed by the bulb.
The A/C evap, I was getting very good results at 5* superheat (with some hunting around 2-3* which is expected), but bumped it up to 10* superheat as my charge is slightly larger than the accumulator is designed for, the evap is a little smaller than Id prefer, as well as this unit may experience low ambient conditions. More of a safety factor at a slight loss of capacity to prevent compressor slugging. Its worth noting that the ambient to vapor line difference was only 5*F, so even at 57* it was doing a pretty good job, and might even be able to operate down in the mid-high 40's without defrost cycles. Really need to relocate my high side port into the liquid line to get a proper set on the subcooling now.

For the sake of anyone new to the topic or learning, couple key facts defined:
*Counter current - When your working fluids/gases flow in opposite directions. Your warmest air enters the opposite side of your cold gas inlet resulting in the exiting gas being almost the same temperature as your inlet air. Best way to get the most energy out of your system.
*Co-Current - Your warmest air meets the coldest gas. The result is both exit the other side as a average between the two. Your only getting a percentage of what the system is capable of producing.
*Vapor line - This is the gas line coming out of the evaporator after all the liquid refrigerant has boiled off. It is the larger fairly cold line.
*Liquid line - This is the condensed "room" temperature ref. flows through. This is a smaller, usually warm line.
*TXV - This is a temperature controlled metering device that controls the flow of liquid ref into a lower pressure vapor.
*Capillary - A cheaper static means of metering the ref. Diameter and length is carefully chosen to get the right flow.
*Superheat - How many degrees above boiling temp the vapor leaving the evap is. Superheat's purpose is to make sure all the ref has boiled off.
*Subcooling - How many degrees below condensing temp of the liquid ref is. This is to make sure there are no bubbles when reaching the metering device.

[RB855]

(There are 2 pictures here. If none/one loads, refresh. My servers slow lol)
This isnt from this specific model (it had a smaller version of this), but relevant for heatpumps/coil discussion. This is what we know as a Weld. Its a spun filter drier, a check valve, and 2 sections of capillary tube. The bottom of the drier is attached to the condenser, the open end of the captube to the evap.



In a/c mode, it enters the drier, through the check valve, and through the longer length of cap tube. In heat mode, the flow reverses, entering through the long captube. The checkvalve is now closed forcing the gas to also pass through the shorter captube.

This design allows them to run a lower superheat in a/c mode, and a higher superheat in heat mode. This compensates for coil size, design, and temperatures. This may explain why they can get away with their coil designs, as most other brands dont have this neat little feature (split systems for the most part do have double metering, package are typically single fixed metering).