Oils and their influence on system design
Due to the high discharge gas temperatures, special requirements apply to the thermal stability of the oil. BITZER uses oils specially optimised for the use with R717.
Danger of chemical decomposition of the oil!
Only use the oils approved by Bitzer!
Operation with comparable mineral oils and poly-alpha-olefin (PAO) oils is only authorised after individual consultation with Bitzer and at the system operator’s own risk!
The polyolester oils developed for synthetic refrigerants are not suitable for the use with R717!
The high solubility of R717 in many oils leads to high refrigerant retention (due to wet operation or insufficient oil temperature), strong outgassing and therefore heavy wear on bearings and sliding ring seals.
Only use the oils approved by BITZER!
Systems with flooded evaporator
In R717 systems with flooded evaporator, mineral oil, poly-alpha-olefin (PAO) and alkylbenzene oil are used:
- Advantages:
- These oils are virtually not soluble in R717.
- Oil dilution due to increased refrigerant solution in the oil is excluded.
- Good conditions for the oil separation and, therefore, low oil migration.
- The low density of R717 has a positive effect on the oil transport in the system. The oil settles below the liquid R717 at the lowest point of the liquid container and can, for example, be extracted at the bottom of the flooded evaporator or the suction accumulator. Depending on the system, a reprocessing and/or automatic return into the compressor is possible. However, there is no return to the compressor with the suction gas flow!
Systems with "dry" and semi-flooded evaporators
Polyalkylene glycol oil, soluble in R717, is increasingly used in R717 systems with "dry" and semi-flooded evaporators:
- Advantages:
- These oils are soluble in R717.
- Favourable viscosity characteristics and a particularly low vapour pressure → low oil carry-over and good oil return.
- Good flowability in the evaporator has a positive effect on heat transfer and automatic oil return.
- Disadvantages:
Polyalkylene glycol oil is strongly hygroscopic.
High water content in R717 systems reduces the protective effect and leads to significant corrosion in the compressor, impaired lubricity and reduced thermal and chemical stability of the oil.
High moisture content in the system and in the refrigerant must be avoided under all circumstances!
Use R717 with a water content of max. 400 ppm!
For "dry" or semi-flooded evaporators, insoluble oils are also suitable. Due to the high discharge gas temperature and the low liquid density of R717, oil transport on the high pressure side is unproblematic. However, special requirements apply to the oil return from the evaporator.
The following design criteria must be taken into account:
- Dry evaporators:
- Use an expansion valve with particularly stable control characteristics!
- Use optimised injection distribution and pipe length!
- Sufficiently high suction gas velocity and turbulence – if necessary, use a multiple-circuit cooler for capacity control.
- Use oil with favourable viscosity characteristics for the respective application range.
- Semi-flooded evaporators:
- Install a high-quality injection system.
- If required, install a downstream suction accumulator.
- Implement a separate oil return from the lower liquid level of the evaporator or suction accumulator combined with an oil rectifier (heat exchanger) → return of the oil into the suction gas flow directly at the screw compressor or into the crankcase of reciprocating compressors!
- Use oil with a pour point below the lowest evaporation temperature!
In the further course of the document only examples and applications with oils that are insoluble in R717 are described.