Chemical and Physical Properties of Refrigeration Oils.

 

Kinematic Viscosity : Refrigeration Lubricants (as well as oils) are classified by their kinematic viscosity. It is determined through the ASTM D 445 method and according to ISO Kinematic Viscosity.

The viscosity is a physical feature which depends on the temperature. The reference temperature is 40°C and the measurement unit is mm2/s, but also the cSt are in use. Each ISO viscosity is described by its kinematic viscosity at 40°C, with a +/- 10% deviation from the ISO value.

 

DIN 51519 VISCOSITY TABLE

ISO Viscosity

Viscosity at 40°C [mm2/s] Viscosity Limits

Nominal

Mid range Minimum Maximum

ISO VG 2

2.2 1.98 2.42

ISO VG 3

3.2 2.88 3.52

ISO VG 5

4.6 4.14 5.06

ISO VG 7

6.8 6.12 7.48

ISO VG 10

10 9.00

11

ISO VG 15 15 13.50

16.5

ISO VG 22 22 19.80

24.2

ISO VG 32 32 28.80

35.2

ISO VG 46 46 41.40

50.6

ISO VG 68 68 61.20

74.8

ISO VG 100 100 90

110

ISO VG 150

150 135 165

ISO VG 220

220 198 242

ISO VG 320

320 288 352

ISO VG 460

460 414 506
ISO VG 680 680 612

748

ISO VG 1,000 1,000 900

1,100

ISO VG 2,200 2,200 1,980

2,420

ISO VG 3,200 3,200 2,880

3,520

 

The viscosity provides an index of the thickness formed by the lubricant and, consequentially, of the resulting lubricant strength. The viscosity tends to collapse with the increase of temperature and show the opposite result when the temperature decrease.

The Viscosity Index describes perfectly this temperature dependence and it is calculated according to the ratio of the kinematic viscosity at 40°C and 100°C. A sufficiently high viscosity is required to form a lubricating film in the bearing and in the compressor cylinder

However, in the same refrigerant circuit, the oil must have the lowest possible viscosity. Refrigeration Oil which varies viscosity grade are used depending on the type of Compressor and application. The viscosity to be applied is normally specified by the Compressor manufacturer.

 

Flash Point : is the minimum temperature at which a flame generated from the lubricant vapors. It is determined through the ASTM D 92 method and is an indirect parameter, useful to know the oil vapor pressure.

 

Density : defines the mass of the fluid in relation to its volume. A refrigeration lubricant is normally characterized at 15°C. The density is expressed in grams on cubic centimeter (g/cm3) and the reference method is ASTM D 4052. The lubricant density depends strongly on the temperature of the lubricant itself: the volume actually grows with the increase of temperature.

 

Water content : is determined with the Karl Fischer method (ppm = parts per million) through a ASTM E 1064 Coulometer. The water level in refrigeration lubricants must be extremely low in comparison to other oils. In HVAC & R sector lubricants are treated as very anhydrous (with the addition of drying agents).

 

Total Acidity : determines the amount of acid in a lubricant, since the acid can corrode the materials it comes in contact with High acidity levels created by oxidation, hydrolysis or aging are highly undesirable.

The total acidity is expressed in mg KOH/g, it is identified as Total Acid Number (TAN Total Acid Number) and determined by the ASTM D 974 method. A refrigeration lubricant must have a very low acid level if compared to other oils: this value must be around <0.1 mg KOH/g.

 

Pour point : expresses the lowest temperature at which the lubricant continues to be fluid and therefore it defines the lower temperature at which this lubricant can be used. This factor is crucial in the choice of the lubricant.

The pour point is determined according to ASTM D 97. The percentage of refrigerant dissolved in the lubricant has a perceptible impact on this feature, as it reduces the pour point. An estimate of the amount of refrigerant that dissolves with the lubricant is made through the pressure-viscosity-temperature (PVT) graphic art of refrigeration oils, known as Daniel Plots.

 

 

The color : is a specific data and can vary between crystal clear APHA 0 and dark brown APHA 500. It is determined in accordance with ASTM D 1209. These changes can be related to the type of additive applied to the lubricant base (in the case of new oils) or to the presence of contaminants (in the case of waste lubricates).

 

Miscibility with Refrigerants : the miscibility of the lubricants to the different refrigerants is analyzed at different temperatures, for different percentages of lubricant in the refrigerant itself.

It is a fundamental parameter for predicting the transport of oil by the refrigerant gas into the system and the efficiency of the system itself. The separation phase can lead to malfunctions and loss of system performance, with reduced heat exchange, insufficient return of lubricant to the Compressor (due to inadequate lubrication) up to the breakage of the Compressor.

 

 

 

Refrigerant-Lubricant Compatibility : a determining factor for a lubricant is its compatibility with the refrigerants on the market and the compatibility of the refrigerant gas lubricant mixture with the materials that compose the system (such as copper, aluminum and steel). These compatibility must be constant even at high temperatures and pressures (these are conditions that emulate the “stress” situations of a plant).

 

Chemical Stability : a lubricant must be stable in many respects in order to be suitable for refrigerationapplications. In particular, its stability must be excellent to moisture, pH, oxidation, and have a very low residual catalyst. The presence of residual catalyst (even in very low quantity) may decrease critically the stability of a lubricant.

 

Moisture : is a particularly damaging factor in synthetic POE and PAG lubricants, that are hygroscopic and must be well controlled. By forming hydrohalogenic and carbonic acids, humidity can promote the refrigerants and gases decomposition. This can consequently cause the corrosion of the system, by freeing metallic oxides which amplify the decay of the refrigerant and oil. A correctly synthesized lubricant and additive must have a great resistance to these factors.

 

Thermal stability : high temperatures accelerate the decay reactions of gas and lubricant. An excellent thermal aging resistance is therefore one of the key characteristics for a lubricant that has to keep its peculiarities. It also accelerates the action of chemical reactions, favoring the decays due to moisture, acidity, to the presence of residues of reaction and metal of the circuit.

 

Stability andCompatibility Test.

ORAFON lubricants are strictly and constantly monitored to ensure the highest quality standards. Each lubricant is tested according to the ANSI / ASHRAE 97-2007 standard method.

The lubricant decomposition is simulated at high working temperature for long periods and accelerated through the control of temperature, humidity, acidity and the presence of reaction catalysts (residues from the synthesis process or lost by the main system materials such as copper, iron or aluminum).

Resistance to thermal oxidation gives an excellent indication of a lubricant life and of its future behavior in application.

It is prepared by a lubricant-refrigerant mixture, weight 1/1. This is placed in a test tube together with a small metal sheet: an iron tube is made, as well as a copper and an aluminum one.

Once everything has been welded, this is brought to 175°C for 14 days.

 

Examples of tests performed on POE :

 

POE

Refrigerant

R-134a R-410a R-404a

Aspect

Clear Clear Clear

Sludge formation

no no no

Total Acidity (mg KOH/g)

Initial

0.01 0.01

0.01

Final <0.1 <0.1

<0.1

System materials

Copper NC NC

NC

Iron NC NC

NC

Aluminium NC NC

NC

 

Examples of tests performed on different PAG :

 

PAG

Refrigerant

R-134a R-1234yf CO2

Aspect

Clear Clear Clear

Sludge formation

no no no

Total Acidity (mg KOH/g)

Initial

0.02 0.02

0.02

Final <0.1 <0.1

<0.1

System materials Copper NC NC

NC

Iron NC NC

NC

Aluminium NC NC

NC

 

We also test the compatibility of the materials with the lubricants and their additives through a method which is similar to the previous one. It provides a lower operating temperature (130°C) but for a longer time (500 hours). This process is performed for testing other components, also plastic ones.

Each lubricant is tested in pilot system, with different versions of components and refrigerants. In this process, we observe the normal plant operation, its performances and noise.