Why is high miscibility bad?

[Raadster]

I was told that gases such as HC's are highly miscible with oil and thus are great to use in a system or add to boost oil return.Then reading the HC subforum, i see that if gases are being absorbed by the oil that it will ruin the compressor.Why is this? I was also told that if gases are highly miscible with oil that the oil itself starts to aid in the refrigeration system by acting as a heat carrier.Any clarification is appreciated.

[nike123]

I was told that gases such as HC's are highly miscible with oil and thus are great to use in a system or add to boost oil return.Then reading the HC subforum, i see that if gases are being absorbed by the oil that it will ruin the compressor.Why is this? I was also told that if gases are highly miscible with oil that the oil itself starts to aid in the refrigeration system by acting as a heat carrier.Any clarification is appreciated.

1. When gas is mixed with oil, it is easier for that diluted oil to return to compressor then if that oil is not diluted. Diluted liquids flows easier.

2.If oil is diluted by gas then mixture viscosity is not enough good for proper lubrication of rotating parts.

3. "If gases are highly miscible with oil that the oil itself starts to aid in the refrigeration system"
Not quite right. In fact, oil less systems (like Turbocor compressor) have much better heat transfer.

[US Iceman]

The properties which aid in oil return are also the same which reduce the oil viscosity. In order to provide adequate support for bearing loads the oil viscosity should be greater in an undiluted condition. Therefore, when the oil absorbs some refrigerant the oil viscosity is still sufficient to meet the applied bearing loads.

[NoNickName]

Solubility is good and bad at the same time. High solubility helps the oil to return to compressor, as the posters before me wisely noted. For the same reason, high solubility helps the oil LEAVING the compressor.
Basicly if the refrigerant is highly soluble into oil, more transport of oil will result.

On the other hand, oil is a bad heat transfer medium, compared to refrigerant. Oil-covered tube bundles or coils have a lower heat transfer coefficient than dry tubes.

In evaporators, oil tend to increase its viscosity because of reduced temperature. So, a greater oil carry over tends to leave more oil in the evaporator. If this happens, an increase in discharge temperature may be required or an oil separator on the high side.