Air con recharge and oil

[CliveM]

If an aircon system looses gas (over a period of years rather than a sudden leak), and is tested to make sure there are not any leaks, can the gas be refilled without removing/replacing the oil?

i.e will the oil still be in the system, once the a/c has been recharged or should a recharge always need oil?

 

[Mr Fridge]

A little general information on oil & gas.

When the system is new the compressor is filled with oil equal to the total system quantity, as the system operates the oil mixes with the gas and is distributed around the system as a mist, some of this mist will stick to pipe walls, condenser coils and evaporator coils, liquid refrigerant will always migrate to the coldest place in the system so will some of the oil and that is why it is important to operate your system once a month to prevent oil "logging". When replacing a component the quantity of oil from that component should be replaced (oil travels around the system as a mist, remember) and there are some rule of thumb measurements to provide "topping up" with oil.

If you replace the :
condenser add 30 ml
Evaporator add 30 ml
Receiver / drier add 12 ml
Suction accumulator add 12 ml
Refrigerant lines add 30 ml

And remeber to use the correct oil for your system, there are quite a few on the market, the most popular being PAG oil.

If you disconnect a component while the system is still charged or partly charged (highly illegal, £20,000 fine) a quantity of oil will leave the system under pressure, that oil will have to be replaced but you do not know how much you lost so off with the compressor, drain the oil and replenish it, blow all the lines with oxygen free nitrogen to clear any pockets of oil ( too much oil is just as bad as not enough, one will seize and smash, the other will hydraulic and smash) and then refit the compressor, dehydrate the system down to at least 4 mbar then weigh the correct amount of gas in, the gas must be added by weight and not just guesswork by an engineer who says he knows what hes doing, he can destroy your system.( or blow the fusible link on the super heat switch if you drive a Rolls or Bentley)

Regarding your original question of does it need oil, if it is just a general loss over a period of time, no oil is required, vac the system out, confirm if it has leaks, ( it probably leaked on the shaft seal on the compressor if you did not run the system monthly) if the car has had a front end smash and burst under pressure, oil is required.

You would be amazed how many compressors are changed due to some enginer or mechanics incompetence or in experience.

I hope some one may find the above useful or even interesting, sorry about going off topic a bit.