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Combating the Acid Problem


Barry Wilson, Managing Director of Advanced Engineering continues his two part article on acid in AC&R systems by looking at ways to combat the acid once detected in a system.

Removing Acid Residue.
One way to remove the acidic residue throughout the system is by performing several flushes of the system with refrigerant, as refrigerant will dissolve the oil and reduce the oil and acid concentration by dilution.

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 neutralise the acid by treating the system with an acid treatment that contains a base (or a base dissolved in a liquid carrier). This neutralisation process results in the formation of salts and water as by products of the neutralisation reaction. Typical approaches are to neutralise 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 neutralisation approaches use a base that is a liquid, resulting in the formation of liquid neutralisation products that remain as contaminants in the lubricant. The acid neutralisation process also requires the addition of the proper amount of base to fully neutralise 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 the acid that is trapped on 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 200ppm) 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 133ppm (inorganic acid) was vigorously stirred for 32 hours. The acidity dropped 45% to 73ppm. 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 vaporises 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 minutes. The ordinary filter-drier in the vapour-compression system will absorb 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 the acidic surfaces.

The treatment and acid are vaporised (thereby leaving the oil) and travel through the system where they become absorbed 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 vapour of the R-22 system, and it is clear that the treatment is going into the vapour 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 be vaporised any more and the 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 minutes.

Actual Acid Removal.
However, the key issue is acid removal, and these results are shown in Figure 3. Here it can be seen that when a filter-drier is used, about 7% of the acid is removed in less than 6 minutes and about 18% is removed after 11/2 hours.

What is not shown in the figure is that it takes about 12 to 16 hours 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 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 11/2 hours in the R-134a system compared to an R-22 system.

Burnout Cleanup
If a compressor does burn out, the oil 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 potentially cause another compressor motor burnout.

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 burnout).

Unfortunately, removal of the oil contained in the compressor does not remove all the acid in the system, since acid is carried throughout the vapour-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-neutralising acid treatment is an effective way to remove the residual acid from a system without leaving contaminants or residue in the system.

In the first article in this series, we look at Addressing the Acid Problem.