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Old 01-20-15, 10:30 PM   #34
RB855
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(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.

Last edited by RB855; 01-20-15 at 11:13 PM.. Reason: Add info/adjust wording/speeling
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