How do you Separate Oil in Chiller Recovery?

Question: How do you separate oil from refrigerant during the recovery process when dealing with heavy oil migration or oil stacking in evaporators?

Sadly, we can't directly separate oil from refrigerant during the recovery phase. When you have a machine with heavy oil migration or oil stacking in the evaporator, the recovered refrigerant becomes oil latent. During the recovery process, all that oil mixes with the refrigerant naturally.

While recovering the liquid refrigerant from a flooded or falling film evaporator, the migrated oil will get carried into the recovery cylinder. If the chiller has been able to settle, then the oil will be sitting on top of the liquid refrigerant. If the chiller was running shortly before recovery, then the top few inches of the liquid refrigerant will be heavily oil latent. 

If your fortunate and know the chiller charge prior to recovery then the last 5-10% of the charge is where most of the oil charge will be. One strategy is to put that final amount in a separate cylinder to prevent the main cylinders from getting contaminated. 

An important note, Shell & Tube DX and Brazed Plate evaporators usually don't have a liquid recovery port. This means the majority of the oil trapped in these will be left behind as the recovery boils the liquid out of the heat exchanger. Thankfully these are much easier to correct oil migration issues by increasing velocity through the evaporator. 

It is still possible to have large oil volumes recovered with any system. It is specifically the flooded and falling film style evaporators that most techs struggle with and require dedicated oil recovery systems to prevent migration/stacking.

How ever your recovery works out, when it is time to charge the refrigerant back into the system, the cylinder that has the oil in it will need to be charged via the vapor port. Boiling the liquid refrigerant under the oil in the cylinder is the primary way to separate these in the field. 

But wait, since my oil is stacked on top of the refrigerant, why not recover through the liquid port until the last little bit of the refrigerant can be boiled off?

This is a circumstantial decision/risk you'll have to make as to your accuracy and needs of the chiller. My personal practice is to not put that recovered oil back into the chiller. I definitely don't want to do startup in a oil latent state. My choice is to play it safe in this situation. 

Flashing the refrigerant out of the oil moves slowly but using a tank heater to speed up will help. Once you've flashed all refrigerant out as vapor, you're left with pure oil in the cylinder. Tip the cylinder over and drain the remaining oil into a waste container for proper disposal.

It is best for the oil not to have a migration issue in the first place. I will walk you through how these oil recovery systems work in this Chiller Training.
If your already enrolled in the training course, review this lesson on Eductor Systems.