The Hotline: 08/20/2007
August 20, 2007
By Jeffrey M. Muschlitz
I have been converting many CFC-12 systems over to the HCFCs R-401A and R-409A. Both of the near-azeotropic refrigerants have been working well. I have been using 409A as a drop-in replacement with a new filter drier and good evacuation.
The manufacturer identifies that the mineral oil does not have to be changed in most cases. I am completely satisfied with the performance of 409A, so much so that I now have made it company policy to use it as our recommended replacement for R-12 systems for low and medium temperature applications.
But, please clarify a few technical questions. The proper method for oil changes in most cases is what? Charging the refrigerant in liquid only?
When superheat is calculated, do I use saturated liquid pressure or vapor pressure? I found that if I used the vapor pressure chart, the system was under charged and the evaporator coil would ice up. So I use liquid pressure, but I am confused with the temperature glide. I recommend a sight glass to aid in charging the system.
I also have experienced on some older hermetic systems that the compressor on overload would trip erratically. An OEM overload replacement solved this problem. Or maybe it could have been because I did not change the mineral oil to alkabenzine. The amperage was with the manufacturers FLA [full load amps] when fully charged.
What do you think about all the new refrigerants and which R-12 replacement do you recommend?
From Atofina Chemicals
You have raised several good technical questions about the application of R-409A. Let’s try to address them.
Oil change in a system is generally a fairly difficult proposition and something that is avoided as much as possible. In fact, the success of 409A rests mostly on the fact that it can use the existing oil in the system and avoids the need for an oil change. However, changing the oil is still needed in a few cases.
The procedure to follow when changing oil depends usually on the system and on how well the old oil needs to be removed. Typically, compressor and system manufacturers recommend that you do not leave more than 3-5 percent of the old oil in the system.
For larger systems with semi-hermetic compressors, the procedure involves removing as much oil as possible from the crankcase and replacing it with the same amount of the new oil. At this point, recharge the system with refrigerant and run it for about 24 hours. This makes the two oils mix inside the system.
Next, again remove the oil from the crankcase and repeat until you reach the level of no more than 3-5 percent of the old oil. Depending on how large the system is, oil traps, etc., you may need to repeat this process several times. Here you are simply using the new oil to flush the old oil out of the system.
Incidentally, you need to be able to measure the oil composition (old + new) to determine when you reach the right level. Sending the oil to a laboratory for testing does this; or using a single use, disposable chemical oil tester kit (available through your refrigeration supply house); or by means of a device called a refractometer.
If you are dealing with hermetic compressors, the process is similar, but you may need to braze and unbraze lines, remove the compressor, drain it, etc. to be able to follow the procedure described above.
There is yet another way of changing oil that involves using a flushing machine. There are several commercial units that would push a liquid solvent through the system until the oil is removed. You need to ensure that the cleaning fluid used is compatible with the materials in the seal and that it can be removed completely once you are done. The equipment manufacturers can tell you if they have approved any of these solvents for use in the systems they make.
CHARGING IN LIQUID PHASE ONLY
All blended refrigerants must be charged in liquid phase, some more than others. Refrigerants termed azeotropes are a lot more forgiving in this area, but those called zeotropes are not so forgiving. We recommend liquid charging to avoid any possible problems with any of them.
The problems that you may encounter are due to the fact that the different refrigerants making up a blend do not combine chemically, but just coexist together. So, given the right conditions, they could separate. If you charge in vapor phase, this separation is more prevalent and could result in both charging with the wrong composition and altering the composition of the refrigerant left in the cylinder. Neither one is a desirable option. This will not happen if charging is done in the liquid phase.
Please note that we are not suggesting you charge liquid into the suction side of a compressor. If your compressor is running, you still need to remove liquid from your tank, but make sure that the refrigerant is flashed prior to its introduction in the system. You can do this by purchasing a restrictor for your charging lines (available in your refrigerant supply houses) or by just cracking open your valves to allow a very small quantity of refrigerant through.
Finally, remember that most of the new R-12 replacements, such as 409A, require a lower amount of charge than the original R-12.
You always measure superheat on the gas line after your evaporator and sub-cooling on the liquid line after your condenser. For superheat calculations always use the vapor side of the charts and for sub-cooling always use the liquid side of the charts.
Single component refrigerants evaporate (or condense) at a single temperature for a given pressure. In the case of blends, since the individual components maintain some of their independent properties, by evaporating at a different temperature, they will cause a temperature difference between the inlet and outlet of the evaporator. This difference in temperature is the glide of a blend. Please note that glide and superheat are two different things. The glide occurs as the blend is evaporating. Superheat occurs as the blend is fully in vapor form and increases in temperature.
We have found some instances of icing of components when switching to 409A due to not taking into account the glide. In a few occasions we have seen that you may need to change the location of the sensing bulb of the TXV. You may need to set your system to an average temperature that will take into account that some portions of your coil will be cooler and others will be hotter than you need.
If you have a technical question, fax it to 815-654-7278 or submit it online by visiting The NEWS’ Extra Edition page and clicking on The Hotline link in the left-hand column.
Publication date: 08/20/2007