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Addressing the Acid Problem

Barry Wilson, Managing Director of Advanced Engineering examines the problems caused by the development of acids in AC&R systems.

We all know that acids within AC&R systems can severely shorten the life of both the compressor and the refrigerant. Until recently, though, there was no full-proof way of counteracting this problem.

In this 2-part article we’ll look at how the acid is formed, how to check if acid is in your system, types of acid and methods of removal, including a new development which may overcome some of the most intractable difficulties.

How is acid formed?
Acids are formed in refrigerants through chemical reactions between the refrigerants and components, lubricating oils or impurities.

This instability of the refrigerant and the formation of accompanying acids is accelerated by elevated temperatures which result from improper operation. Typical causes are a failed condensor fan or clogged airflow path.

Checking for Acid
Because acid can be easily treated before the compressor fails, checking systems for acid – as with routine detection for refrigerant leaks – should be part of any planned maintenance programme.

You can check either the refrigerant oil or the refrigerant itself for the presence of acid. However, since a typical hermetic compressor does not have an oil drain, testing the refrigerant vapour is the only practical method on many refrigeration and air conditioning units. The advantage to testing the actual refrigerant itself is that this offers an accurate, fast and inexpensive answer.

If you decide to use an oil acid test kit, you should be aware that using the wrong type of test kit with an ester-based (POE) oil can result in a false acid reading. This is because as far as the test kit is concerned, the oil behaves just like an acid (in that the ester oil displays amphoteric properties). That is why some manufacturers offer separate test kits or scales for mineral oils and for POE oils.

Meet the acids…
A brief look at the different types of acids present in the system is necessary in order to fully understand the acid removal process.

Depending on the refrigerant and lubricant being used, a refrigeration system can contain two types of acids: organic (such as oleic acid) and inorganic (mineral) acids (such as hydrochloric acid).

Organic acids are soluble in the oil, do not vaporise, and therefore stay predominantly in the liquid oil in the compressor oil sump.

Inorganic acids are only slightly soluble in the oil and are volatile. They can therefore exist both in the oil as well as circulating with refrigerant throughout the system. Because they are much stronger and reactive, inorganic acids are much more damaging to the system. For these reasons, this article will focus primarily on inorganic acids.

What happens during a compressor burn-out?
During a compressor motor burnout, inorganic acids are formed as a result of refrigerant decomposition at abnormal temperatures. Consequently, the refrigerant oil becomes extremely acidic. If all this acid is not removed when the compressor is replaced, the elevated levels of acid will attack the new compressor and cause another compressor motor burnout.

Cleanup normally involves changing the compressor, the oil, and the refrigerant to reduce the acid level. At the same time, a suction line filter-drier is added 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. This is because acid is carried throughout the vapour 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 leads 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. Indeed, an acid concentration of just 50ppm has been found to cause compressor motor burnout in a matter of days.

The answer is to check thoroughly for acid and then make sure that every bit is removed from every part of the system.

In the second article in this series, we look at Combating the Acid Problem.