19 February 2013

PAO vs. 2-stage hydrocracked base oil

PAO and mixture thereof with AB base fluids are most known and popular lubricants in the refrigeration industry, even though PAO alone shows some better results in the area of solubility with ammonia and volatility. Modern producing techniques lead to development of the 2-stage hydrocracked base oils that have even better attributes than PAO and therefore became very suitable for the lubrication in refrigeration. 2-stage hydrocracked base oils are highly recommended for ammonia applications.

PAO/AB mixtures, although very popular, demonstrate some problems related to short life span of the shaft seal, instable oil pressure, mechanical damages inside of the compressor and higher oil consumption. 2-stage hydrocracked oils address all the above mentioned issues.

  • Following base oils are used in the  NH3 applications:
  • Naphthenic base fluidsSolvent refined paraffinic base fluids
  • Alkyl benzene (AB) base fluids
  • 2-stage hydrocracked base fluids
  • PAO
  • PAO/AB base fluids
Standard refrigeration compressors are designed for lubrication with an oil of 68 cSt at 40°C. In the case of higher expected solubility, there’s often an oil with a higher viscosity used. The weakness of the oil with higher viscosity is longer time needed to enter the lubrication points.
Because of constant interaction between the refrigerant and the lubrication in the refrigeration systems, it is always important to consider the influence of the refrigerant on the lubrication oil before choosing a type of oil for the machine.
The diagram below presents the solubility of different types of oil with ammonia at 50°C.
figure1Figure 1.
The diagram above shows that standard PAO (purple bar) has a lower solubility when compared to the naphthenic base oil (green bar). To avoid problems with separation of oil and refrigerant, it is preferred to keep solubility at low.

On today’s refrigeration market there are PAO/AB oil mixtures available (presented by the light blue bar on the diagram). PAO/AB is a combination of the PAO (purple bar) and the AB base oil (red bar). The combination of a PAO with the AB base oil results in the product that demonstrates higher solubility rate with ammonia than the PAO alone. The reason for that is AB base oil breaks the high surface tension of PAO oil. This is why it is not recommended to use PAO/AB mixture, nor the AB base oil in industrial applications.
The 2-stage hydrocracked base oil shows the lowest solubility with ammonia. To avoid all issues with PAO oil and mixtures thereof, we would highly recommend to use the 2-stage hydrocracked base oil in industrial applications.

The concept of oil separation in industrial compressor aggregates
The solubility of the oils with ammonia is important, due to the fact that the higher the pressure above the oil level the deeper the refrigerant will penetrate the oil. It is called the interactive phase of the refrigerant and the oil.

Besides the pressure, the temperature and kind of refrigerant are influencing the interactive phase. Furthermore the construction of the oil separator  has also impact on the interactive phase. By horizontal oil separator the oil level is much lower when compared with the vertical oil separators (Figure 2.).  In the case of lower oil level the refrigerant can enter the oil pump or enter the hydraulic system.  The results can be among others: damaged shaft seals,  instable oil pressure, incorrect capacity control and broken oil pumps.
Figure 2. Oil separators in the industrial compressors
The higher the solubility, the deeper the refrigerant will mix with the oil at the same pressure and temperature, the bigger the chance  to develop the above mentioned side effects.
The volatility of the base oils and weight loss by heating.
In oil separators  oil can only be captured from the refrigerant in liquid phase. If oil is in gas phase it will leave the compressor unit and this phenomenon is called oil carry over or oil consumption. Important characteristic for oil in refrigeration compressor system is the volatility that should be as low as possible to reduce the oil consumption.

In the diagram below, Figure 3., one can see that the standard PAO (purple bar) has a lower volatility comparing to the naphthenic base oil (green bar). The mixture of PAO and AB base oil results in higher volatility (Figure 3.) comparing to PAO itself and therefore in higher oil consumption in the system.

figure3Figure 3.
By adding the AB base oil the volatility of the PAO will be increased.  For this reason we do not recommend the PAO/AB mixture and the AB base oil for use in industrial applications.

Pour point
The last important issue is the pour point and flock point of oil. Pour point of oil is the temperature at  which oil becomes semisolid and loses its flow characteristics. The flock point is the temperature  at which oil separates as a wax deposit from the mixture of refrigerant with refrigerant oil. The oil return at NH3 application is based on the fact that the oil is heavier than NH3. Based on the gravity the oil will be forced to flow to the lowest point of the installation. In the flock point the weight of the oil changes and become  more lighter, therefore the oil return problem will occur. We recommend the use of an oil with a pour point of at least 10°C lower than the lowest temperature in the installation.
For applications operating at  -40°C it is advised to use oil which has a pour point of at least -50°C.  Only then the oil will hold the appropriate weight assuring the proper separation. It is recommended to use the oil that meets above mentioned requirements, for instance NXT-717 lubricant based on 2-stage hydrocracked base oil with a pour point of -54°C. For other refrigerants, i.e. freon, R22 and blended freon it would be recommended to choose NXT-4114.

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