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best refrigaration oil for industrial compressors

Black sludge by refrigeration compressor: Carbonization of oil

Black sludge is often found in refrigeration compressors. 15 years ago this problem was related to the use of solvent refined paraffinic and naphthenic base oils. At that time most widely used refrigeration oils were Suniso 3GS (paraffinic base oil) and Shell Clavus 68 (naphthenic base oil).
Due to the fact that the naphthenic and paraffinic base oils oil can absorb more refrigerant when compared to other base oils like PAO or 2 stage cracked hydro carbon, we tend to find more carbonization in the fields where these oils are used.

Carbonization:

We talk about carbonization of oil when the oil captures refrigerant and degas due to the friction heat. The temperature at the moment of a short degassing that takes place in nanoseconds can increase over the carbonization point of the used oil.To avoid carbonization used oil should have a low capability to capture refrigerant and should not easily transfer to a vapor.

 

Carbonized oil in a piston compressor - one of the indication places where oil easily carbonizes is the shaft seal area

oil1 pct2
Carbonisation of oil in the crackcase Carbonisation of the shaft seal

 

 Burned oil on the inside of the seal rings of the shaft seal (screw compressor)

sealrings
Damage of the shaftseal in detail

 
Nowadays there are lubricants formulated with two base oils available on the market.
We find even more issues in the field related to the use of these kinds of oils.

A most widely used combination of two base oils is the POA/AB combination. PAO base oil is formulated with a little AB (alkyd benzene).

Solubility:

The diagram below shows the influence of adding the benzene to the PAO base oil. The graph presents solubility with ammonia at 50°C.
The purple PAO was limited capable to capture NH3. By adding small amount AB base oil (red) will the PAO/AB (light blue) be capable to capture more NH3. The PAO was negative effected by the adding of AB base oil.
The 2 stage Hydrocracked base oil will take the lowest capability to collect NH3.

 

NXTREF-SolubilitywithAmmonia

 

In compressor unit there is a big oil separator (the right vessel) that separates the oil and refrigerant after the compression.
Oil separators can only separate if the oil is in liquid phase, not in vapor phase.

separator

 
Vapor will cause oil loss and the vapored oil will be condensing in the refrigeration system. Condensing oil will act as an isolator what will result in lower effectiveness of the refrigeration system.The diagram below presents the amount of weight loss between various base oils. During degassing of oil local temperature in a short moment can be reached of more then 100°C.

In the diagram we find that the naphthenic base oil and Alkyl Benzene formulation go very easy in vapour.
For the total information the solvent paraffinic base oil can be found between the POA/AB and Naphthenic base oil.
 

NXTREF-VolatilityAmmonia

 

 Again the POA ( purple) will find a negative effect by the adding of AB base oil ( red) to formulate the PAO/AB.(light blue)
We find more lubrication issue by this formulation.
The 2 stage hydro cracked base oil shown again as more stable.

In short:

To avoid lubrication problems of damage on bearings, shaft seals, instable oil pressure of oil loss use a oil which is stable. More important is the use by Compressor with variable drive speed, due to the reason that load on the compressor is also variable.
The base oil can be found on the safety date sheet of your oil.

 

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