Forming acids out of refrigerants

[Peter_1]

Does anyone know an article which describes/explains the forming of acids out of refrigerants in a compressor (not the forming acids out of the oil) due to high temperatures inside the compressor?

[NoNickName]

As far as I know, HFCs do not react to form acids, but either form clathrates or decompose.

[HVACGod]

I cannot cite any specific articles on that subject P_1 although I am relatively certain there are documents available addressing this phenomena from DuPont, Genetron probably all of the major chem producers of refrigerant.

Acid is introduced to the system as a result of hydrolysis* of the refrigerant exposed to both excessive temperatures and some degree of moisture. Presumably, without the benefit of engineering documents on the subject the hydrolysis is contingent upon both level of moisture content as well as the intensity of the temperatures.

According to Hudson Technologies, the refrigerants listed below will become fully saturated, or "wet" at the listed corresponding levels:

To illustrate the range of values among refrigerant types, at 40 degrees Fahrenheit, R-11, R-123 and R-134a will become fully saturated at the following levels:

CFC-11 - 44 ppm
HCFC-123 - 470 ppm
HFC 134a - 600 ppm

From The Source:

The acceptable levels of moisture in new or reclaimed refrigerants are given in ARI 700. These levels are generally more demanding than what is typically feasible and acceptable in an operating system. There is no experimental data available on the maximum permissible moisture levels in an operating system since it is a function of several factors, but ASHRAE has some data on typical levels in a normally operating system. The table below gives a comparison of ARI 700 allowable level and the level typically found in normally operating equipment.



*R113, R114, R134a, R500 data are not available in ASHRAE. Above data is based on similarity with the other refrigerants (R500 is an azeotrope of R12 & R152a).

Testing method for moisture is specified in ARI 700. Based on above discussion and operating experience, the acceptance criteria for moisture should be as follows:

A refrigeration system can contain two types of acids, organic and inorganic, depending on the type of refrigerant and oil being used. Organic acids (such as oleic acid) are formed as a result of decomposition of oil at high temperature in the presence of air as the oxidizing agent. These acids are slow to react, soluble in oil, do not vaporize, and typically found in relatively small quantities in the oil sump. Inorganic acids (such as hydrochloric acid and hydrofluoric acid) are formed as a result of decomposition of refrigerants at high temperature in the presence of moisture. These acids are highly reactive, soluble in water, vaporize, and typically found to be the dominant acids that may be present. Therefore, inorganic acids are the real problem in a refrigerant system, which results in degradation of the equipment internals. The major contributors to acid formation in a system are the presence of moisture and abnormally high temperatures around the compressor i.e. bearings, motor windings, terminations, compressor discharge etc. The presence of acids is specially hazardous in case of semi-hermetic and hermetic compressors, since the acid vapor in refrigerant goes over motor windings and can eventually lead to motor burnout. Therefore the amount of acids in a system should be kept to an absolute minimum, and ARI 700 specifications should be followed strictly, i.e., maximum allowable limit for acid in all refrigerants should be 1 ppm by weight.



*Hydrolysis

Hy*drol"y*sis\, n. [Hydro-, 1 + -lysis.] (Chem.) A chemical process involving the addition of the elements of water. Webster's Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc