The instability of the refrigerant, and thus the formation of acids, is accelerated by elevated temperatures which could be the result of improper operation, such as a failed condenser fan or clogged airflow path.
Checking the system for acid should be a routine maintenance practice, because acid can be easily treated before the compressor fails.
You can check the refrigerant oil for acid, or you can check the refrigerant for the acid. Since a typical hermetic compressor does not have an oil drain, testing the refrigerant vapor is the only practical method on many residential or light commercial a/c units. The advantage to testing the refrigerant for acid is that it is accurate, fast, and inexpensive.
If you decide to use an oil acid test kit, be aware that using the wrong type of test kit with an ester-based (POE) oil can result in a false acid reading, because the oil behaves like an acid to the test kit (that is, the ester oil displays amphoteric properties). That’s why many oil acid test kits have one kit (or one scale) for mineral oils and a different test kit (or scale) for POE oils.
A discussion of the different types of acids present in the system is necessary to fully understand the acid-removal process.
Organic acids are soluble in the oil, do not vaporize, and therefore stay predominately in the liquid oil in the compressor oil sump.
Inorganic acids are only slightly soluble in the oil and are volatile; thus, they can exist both in the oil and can circulate with refrigerant throughout the system. Inorganic acids are much more damaging to the system (they are much stronger and reactive acids) and are the subject of this discussion.
During a compressor motor burnout, inorganic acids are formed as a result of refrigerant decomposition at elevated temperatures. The refrigerant oil thus becomes extremely acidic. If all this acid is not removed when the compressor is replaced, the elevated acid levels will attack the new compressor and cause another compressor motor burnout.
Cleanup normally involves changing the compressor, oil, and the refrigerant to reduce the acid level and adding a suction line filter-drier to catch any acid returning to the compressor from the system. Unfortunately, removal of the oil contained in the compressor does not remove all the acid in the system, because acid is carried throughout the vapor-compression loop by the flowing refrigerant, leaving acidic oil or its residue throughout the system.
This residual acid has been shown to shorten the life of the system since it will lead to accelerated acid formation in the system. This has been supported by evidence that after a burnout the frequency of subsequent burnouts increases.
The inorganic acid in the oil will etch the lacquer insulation from the wire, causing the motor winding to short-out electrically and resulting in a motor burnout. An acid concentration of 50 ppm has been found to cause compressor motor burnout in a matter of days.
However, because of EPA-mandated refrigerant recovery requirements, this is an impractical, costly, and time-consuming task. Furthermore, the refrigerant used in the flushing operation could not be reused without reclamation.
An alternative approach is to neutralize the acid by treating the system with an acid treatment that contains a base (or a base dissolved in a liquid carrier). This neutralization process results in the formation of salts and water as byproducts of the neutralization reaction. Typical approaches are to neutralize the acid with a base, such as potassium hydroxide (KOH). These bases are solid and are dissolved in a non-water solvent.
In such a reaction, the acid and base combine to form a potassium-salt and water. While the water can be removed by the filter-drier in the system, the salt remains trapped in the system and could cause problems.
Other neutralization approaches use a base that is a liquid, resulting in the formation of liquid neutralization products that remain as contaminants in the lubricant. The acid neutralization process also requires addition of the proper amount of base to fully neutralize the acid. Too little base and the refrigerant is still acidic; too much base and the refrigerant is basic.
The ideal method of removing acid is to liberate (or free) the acid from the refrigerant oil and hard surfaces, allowing the filter-drier in the system to remove it.
A filter-drier does an excellent job of removing acid. The problem with relying on the filter-drier to remove the acid is that a significant portion of acid that is trapped on the hard surfaces and in the oil may never get to the filter-drier in a reasonable time, and may contribute to a subsequent burnout.
After changing out a compressor burnout, very high concentrations of inorganic acids (significantly greater than 200 ppm) have been measured in a new compressor’s oil. Agitation of the oil has not been found to release this trapped acid.
In order to demonstrate this, an oil sample with an initial acidity value of 133 ppm (inorganic acid) was vigorously stirred for 32 hrs. The acidity dropped 45% to 73 ppm. While this may seem like a significant drop, it should be pointed out that this level of acid would ultimately lead to a compressor burnout.
However, if the trapped inorganic acid could be liberated in a reasonable time, the filter-drier in the system would remove this acid.
One acid treatment, called “QwikShot,” works by liberating the trapped acid from the oil and acid-contaminated surfaces. The treatment also vaporizes so that it travels throughout the system. When the agitation of the acidic-oil experiment was repeated with this product added to the oil prior to stirring, the acid was completely stripped from the oil after 20 min. The ordinary filter-drier in the vapor-compression system will adsorb the liberated acid and the acid treatment.
Ideally, an acid treatment of this type should be introduced into the compressor’s oil sump so that it can thoroughly mix with the oil during compressor lubrication (the QwikShot oil concentrations are less than 1% of the oil). As the treatment mixes with the oil, it serves to flush the acid from the oil and acidic surfaces.
The treatment and acid are vaporized (thereby leaving the oil) and travel through the system where they become adsorbed on the filter-drier (molecular sieve, carbon, or activated alumina filter-driers all work).
The net result is that the acid is removed and no residue is left in the system. After use, about half of the filter-drier’s capacity will be available for future clean-up of water or acid.
In Figure 1, the treatment is in the vapor of the R-22 system, and it is clear that the treatment is going into the vapor phase as it releases the acid from the compressor oil. However, if the filter-drier is not present, the treatment will remain in the system and eventually reach equilibrium, which means it can’t vaporize any more and acid liberation will stop.
This is also supported by Figure 2, which shows the drop of treatment in the oil. Note that by using the filter-drier, more than 60% of the QwikShot is removed from the oil in less than 6 min.
What is not shown in the figure is that it takes about 12 to 16 hrs to remove all the acid from the system. Also note that without the filter-drier, the acid removal is much slower.
The filter-drier should always be changed when an acid treatment is added to the system. Failure to change the filter-drier could result in a slower reduction in acid removal and complete acid removal may not be achieved.
The experiment described here was repeated for an R-134a system with similar results; in fact, the treatment removed even more acid in 1 1/2 hrs in the R-134a system compared to an R-22 system.
Acid cleanup normally involves changing the compressor oil and the refrigerant to reduce the acid level (and changing the hermetic or semi-hermetic compressor if it did burn out).
Unfortunately, removal of the oil contained in the compressor does not remove all the acid in the system, since acid is carried throughout the vapor-compression loop by the flowing refrigerant. Therefore, acidic oil or its residue remains throughout the system.
This residual acid has been shown to shorten the life of the system, since it will lead to accelerated acid formation in the system.
The use of a non-neutralizing acid treatment is an effective way to remove the residual acid from a system without leaving contaminants or residue in the system.