I suppose everyone has a different method of diagnosis. Okay, I’ll play this game. Lets diagnose this system for fun!Quote:
I am confused about what you are suggesting. Of course I would diagnose the unit!! Usually when a customer says this may have happened means it definitely did
First of all, customers can provide useful information. However, they also have a propensity to volunteer misleading and useless information too. I personally don’t place the importance any relevant information from a customer above my own judgment.
One of the standard questions that I usually ask is ‘when they noticed the fault occur’ (in this instance, the temperature becoming warmer). The answer is often relevant later in the diagnosis. Eg. if the fault turns out to be a leak, the size of the leak can be approximated by the duration and intensity of the symptoms.
If a customer said that they thought there were a possibility of mixed/contaminated refrigerant then I would have immediately asked them to elaborate on it. Sometimes answers can be surprising... “Three years ago it had a gas leak and the guy who came out was trying to figure out what gas it took. So he may have put the wrong gas in, but it’s been working good since he fixed it until the other day”
It would be unwise to dismiss the customers claim so we’ll entertain the possibility of there being contamination of the refrigerant. According to the OP the system is performing as follows: (I'm including metric because that's what I am most familiar with)
Evap. Discharge Temp: 10’F (-12.2’C)
Suction pressure: 3psi (21kPa)
Discharge Pressure: 150psi (1034kPa)
SST: -44.6’F (-42.6’C)
SDT: 70.1’F (21.2’C)
Outside Air Temp: 20’F (-6.7’C)
Observations: Frosting/ice between TXV and evap. Refrigerant bubbling through sight glass.
Action taken: Initiated defrost cycle. Added refrigerant R404a to system.
Resultant change to system operation:
Evap. Discharge Temp: 15’F (-26.1’C)
Suction pressure: 5psi (34kPa)
Discharge Pressure: 250psi (1724kPa)
SST: -40.3’F (-40.2C)
SDT: 103.8’F (39.9’C)
Refrigerant still bubbling through sight glass but not as much.
From the above, it should be clear that the evaporator is starving of refrigerant. The bubbles through the sight glass are likely a result of the lack of load on the system and the resulting lower volume of the refrigerant due to the outside ambient conditions. It was most likely a little undercharged as well.
We can rule out the system being low on refrigerant at this point. If it were low on refrigerant then the sight glass would have to be just about empty of liquid for the suction pressure to be at 5psi with the TXV wide open with a discharge pressure of 250psi.
Okay, so let’s entertain the claim of refrigerant contamination...
What would the TXV do if R134a, R22 or even both were mixed with R404a?
At a suction pressure of 5psi the temperature at the TXV sensor would have to be about -40’F for it to be throttling back. The saturated temperature of R134a at 5psi is -3.5’F and R22 is -30’F. It would have to have a mix of R410a (-50.3’F) before the TXV would throttle back and in that case the evap discharge temp would be lower than measured not to mention the discharge pressure.
So if there were significant refrigerant contamination then the symptoms presented would be different. The refrigerant would be flooding back to the compressor and the suction pressure would be much higher. There may still be contamination of the refrigerant but it would not be the cause of the current failure condition.
So we can rule out refrigerant contamination for now.
Clearly there must be an issue with the TXV. At this point I would have measured the superheat (which would no doubt be very high) and adjusted the TXV to confirm that the problem is with it. The power element is probably losing its charge so would require a new TXV.
If the superheat measurement was okay then the problem would most likely be with the CPRV. However, considering the symptoms I would say the fault was with the TXV.
With all that said and the data given i would have to say I would agree with you. But am curious about a few other ideas. What is the placement of the txv sensing bulb. How is it strapped did the guy that installed it zip tie it to the bottom of the suction line? What type of valve is it? Pressure limiting maybe? Does this unit have a CPR? Is the coil clean or filthy? Could something like hp 80 be in the unit? Is there a fan cycle control? Is it working? I live in south Florida so diagnosing units that operate in a cold climate is a foreign idea to me. It was 87 degrees here yesterday. We have 2 seasons hot and hotter and humid.
Also, we should assume that the system was working and then there was a breakdown situation. If it were a chronic fault then the OP surely would have mentioned it…
I live in Sydney, Australia. Today it was about 86'F and in winter 41'F would be considered extremely cold! However, I work on systems that use chilled water in a building for cooling condensers, so am familiar with the effect of over condensation of refrigerant.Quote:
I live in south Florida so diagnosing units that operate in a cold climate is a foreign idea to me. It was 87 degrees here yesterday. We have 2 seasons hot and hotter and humid.
Have to pull charge and replace drier evacuate and charge with correct refrigerant before any diagnostics can be attempted. No data can be trusted at this point and is therefore useless. Right now we don't know what the SST, SCT are.
If you have a proper understanding of the fundamentals of refrigeration and how the various components function then you can diagnose without first changing parts. The diagnosis may require stages where parts are changed but not where parts are changed prior to making a diagnosis.
Eg. Diagnose faulty/locked compressor. Install new compressor. Run system then identify cause of compressor failure - improperly secured TXV sensor.
Could fractionation be a possible problem?
So there may or may not be an issue of refrigerant contamination.
However, there is valid system data that could be measured to substantiate the claim of refrigerant contamination being the cause of the problem.
That data is:
Temperature at the TXV sensor
The expected refrigerant is known, R404a.
Knowing how a TXV works, I could determine if the problem was due to refrigerant contamination, and so should you.
However, fractionation would be highly possible if the refrigerant was contaminated with say R407c, R410a or even any other refrigerant I suppose.
To rule fractionation out as the cause it would be a simple matter of measuring the suction pressure and temperature at the TXV sensor on this particular system. If contamination were the case I would expect the refrigerant to be contaminated with R410a.
The symptoms and measurements provided by the OP don't point to it in my opinion.
The reason why I joined in this discussion was because I was questioning the method of diagnosis. I don't visit this site much and the idea of changing parts without diagnosis was surprising and foreign to me. But I suppose you guys in the USA do things a little different to here. Not that it's a bad thing, just different. Much like the language, we say prawn you say shrimp!
All good and understood. :)