Is You Refrigerant System Constipated?

I am hearing more stories about systems having problems with refrigerant restrictions. Many are from black scale created inside the pipes by brazing without a nitrogen purge. In the old days we often got away with this if we did not take a long time brazing the joints and did not overheat the pipe. Today, however, the POE lubricant and HFC refrigerants scrub off the oxides and carry them around in the system. This is especially problematic with metering devices of all sorts. TEV screens get plugged, orifice metering devices get plugged, and even filter driers get stopped up. The black oxides can also conduct electricity, so if they build up inside the compressor shell they can cause it to ground out. Prevention is the best remedy – not forming the oxides in the first place. You can do this by purging the lines with nitrogen while you braze. Of course you need to let the nitrogen out as you put it in so the system does not pressurize. You can’t braze up a hole under pressure. There are new flow meters that let a trickle through – just enough to keep the air out, which is all you need. No air means no oxygen to form oxides. The primary symptoms of refrigerant restriction are a low suction pressure, a high superheat, and a high subcooling. The head pressure may anywhere from slightly low to kind of high. If someone has tried to fix the problem by adding charge, the high side may be high. Normally, it will be a little low after a few minutes of operation unless the system has been overcharged. If the restriction is anywhere in the liquid line, like the filter drier, there may be a temperature drop at that point due to liquid flashing off.

If your system is constipated with black scale, ex-lax won’t help. You will need to recover the system refrigerant, disassemble it and flush it out using a solvent made specifically for refrigeration systems, such as Qwik Flush by Mainstream Engineering. Using carburetor cleaner will just add to your problems. Re-assemble it with a new filter-drier, pull a good vacuum, and weigh in the manufacturer’s specified charge. You will have to adjust the charge slightly for line length and the amount of refrigerant the filter holds. I recommend using flare filters because they are easier to change, and you won’t have to worry about making any more scale. For more details on this procedure, check out unit 91 Refrigeration System Cleanup in the 2^nd edtion of Fundamentals of HVACR.