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Post by ChrisJ on Sept 3, 2013 15:06:11 GMT
Here are two much better photos showing the damage to my CA-A machines valve seat. This in no way looks like wear from fluid or anything mechanical to me. It definitely looks like some kind of erosion. I also find it facinating that part of the seat looks totally undamaged. Whats with that!? Please remember this had a guide for the needle which I sawed off for a better view of the seat. I did not touch the seat with anything and yet the damaged area looks like something was keeping it clean somehow. Best I could tell the needle looked perfectly fine. Attachments:
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Post by cablehack on Sept 4, 2013 0:20:58 GMT
I just wonder how well this fridge was operating when it was last in use. That's impressive damage.
The uneven aspect of wear makes me think there is some kind of reaction with the seat material and methyl formate. It's a different pattern of wear to mine, but still only concentrated around part of the seat - not all of it.
Abrasive wear wouldn't give those jagged uneven edges.
It's obvious that it's a waste of time trying to restore a CA without doing the capillary conversion.
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Post by ChrisJ on Sept 4, 2013 1:00:23 GMT
I wonder if its a reaction that takes place when the needle is resting against it. As in, did this happen when the unit was in storage for 20, 30, 40+ years and it worked fine when it was put out of service?
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Post by cablehack on Sept 4, 2013 1:07:57 GMT
Good point, I didn't even think of that...if there was some kind of electrolytic action because the seat and needle are different metals. If they were touching, as they are at rest, and the methyl formate acted as some kind of electrolyte, then it's a plausible theory.
I know my CA-1 has been in use until recently, so that might explain the different wear pattern - as compared to a CA that has been out of use for many years.
You could test the theory by soaking a piece of paper in methyl formate and placing it between two coins of dissimilar metals; eg, nickel and bronze and see if a voltage is developed.
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Post by allan on Sept 4, 2013 13:50:17 GMT
Good point, I didn't even think of that...if there was some kind of electrolytic action because the seat and needle are different metals. If they were touching, as they are at rest, and the methyl formate acted as some kind of electrolyte, then it's a plausible theory. I know my CA-1 has been in use until recently, so that might explain the different wear pattern - as compared to a CA that has been out of use for many years. You could test the theory by soaking a piece of paper in methyl formate and placing it between two coins of dissimilar metals; eg, nickel and bronze and see if a voltage is developed. I wonder if the seat problem was something that developed very early. The reason I say this is because the form B machines that I have had experience with all have the thin wire stuck in the seat body. If you examine one closely you can see that the wire is friction fit in one of 2 cast openings on either side of the seat opening. This passage allows the wire to go in at an angle and the end of the wire is actually in the path of MF flow. Not sure but it seems like the opposite end of that wire would have to be in contact with the side of the float. This arrangement could surely ground the entire assembly, the float, float linkage, needle, seat and the liquid MF,at least in theory. GE had to have some reason to go to all this trouble to change the seat and add the holes for this wire. |
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Post by ChrisJ on Sept 4, 2013 15:38:56 GMT
Here is something I found from Copeland
"Flash gas in the liquid line has a detrimental effect on
system performance in several ways. It increases the
pressure drop due to friction, reduces the capacity of the
expansion device, may erode the expansion valve pin and
seat, can cause excessive noise, and may cause erratic
feeding of the liquid refrigerant to the evaporator."
Bad heater = low charge = flash gas? CK units don't suffer from it, and they don't have a heater. We also know the methyl formate loves to condense in the compressor oil. This of course does not agree with cablehack's CA-1 unit though. Perhaps even with a heater its still an issue with methyl formate in general. It would explain why the damage to my seat looks like the only clean, untarnished area while I would think arcing would cause burn spots? Another thing methyl formate has that SO2 does not, NCG issues. How would holding the float down more than normal effect flash gas?
The wire in the B units does make me wonder, however perhaps they believed it was caused by electrical potential and where wrong? Of course it also seems there are far more B units around than A. Less produced or did many A's die a very early death?
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Post by cablehack on Sept 4, 2013 23:35:22 GMT
What is flash gas?
The way my CA-1B seat was worn was like Allan's form B seat - and these both had the wire.
We need to see some more form A seats to see if it's true the wire affects the wear pattern.
That we missed out on the form A machines in Australia, going straight from DR to CA form B does make me wonder if GE knew something; either that, or there were just a lot less of them and the form B model just happened to be available after the local DR stock had all been sold. For the CA's on ebay, I haven't noticed one form to be less likely to be working than the other....but then "working" can mean many things.
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Post by ChrisJ on Sept 5, 2013 0:55:24 GMT
I only learned about this today so you might want to look it up separately. However my understanding is flash gas is when liquid refrigerant flashes to a gas for different reasons. Flash gas apparently erodes the seat and or needle on TXVs and is somewhat of a common problem. From what I could tell a TXV operates much like a float valve, only by remove temperature instead of liquid level. It also appeared, if you do not give a TXV a clean gas free liquid supply it also causes problems. Here s a wiki page on flash gas although it doesn't give too much info on it. en.wikipedia.org/wiki/Flash-gas |
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Post by coldspaces on Sept 5, 2013 2:50:13 GMT
From a modern repair manual I have.
The term "flash gas" is used to indicate the portion of the refrigerant which instantly (flashes) and turns into a vapor as it passes through the refrigerant control orifice. The instant vaporizing of some of the liquid refrigerant (flash gas) cools the rest of the liquid to the evaporating temperature.
The amount of flash gas depends on temperature of the refrigerant in the liquid line and the pressure in the evaporator. Flash gas reduces the valve capacity.
One method used to reduce "flash gas" is to clamp the liquid line to the suction line. This is often called a heat exchanger.
Since the liquid coming from the condenser often quite warm and the vapor coming from the evaporator is quite cold, clamping the two lines together causes a heat transfer from the liquid line to the suction line.
Cooling the liquid in the liquid line decreases the "flash gas" and increases the heat absorbing capacity of the refrigerant. Raising the temperature of the vapor line
decreases the possibility of any liquid refrigerant entering the compressor.
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