When discussing water cooled chillers, we are dealing with a large volume of refrigerant. We need a fast and efficient way to recover that charge. With basic methods of recovery like direct recovery, it can take too long to move these volumes. In addition, when recovering smaller systems, we struggle with maintaining tank pressure/temperature. Now imagine that at a larger scale. Wrong or inefficient practices will lead to recovery taking days longer than actually needed.
First and foremost, the only time you will move vapor only is after all the liquid has been removed as a final step. In direct recovery this means pumping liquid refrigerant, but an advanced practice that is far more efficient is known as push-pull.
Push-pull has been around for a long time but has very specific applications. Water cooled chillers with flooded evaporators are perfect applications. This is because at shutdown the liquid refrigerant will be pushed to stack in the evaporator. Add a recovery/charging port to the bottom of the shell and tube evaporator and you have amazing access to the liquid refrigerant.
In push-pull the recovery machine pulls vapor from the recovery cylinders vapor port. The pump discharges that vapor to a service port at the top of the evaporator. The vapor pushed into the evaporator pushes the liquid refrigerant out into the recovery cylinder. There will be a hose connected at the bottom of the evaporator to the liquid port of the recovery cylinder.
Using a scale to monitor tank weight, ensure to follow safe recovery practices. A sightglass in the liquid line between the chiller and cylinder will allow you to see when liquid stops flowing. The liquid flowing through the refrigerant hose will cause the hose to bounce, so when the recovery hose goes still then you have likely stopped liquid flow. Keep in mind using large recovery hoses will increase time efficiency.
Once all liquid has been removed from the chiller then a standard direct recovery procedure will get the remaining vapor out. I recommend a subcooler to prevent tanks overheating or building pressure. Always flow water as a safety precaution on your heat exchangers, we can not risk freezing tubes.
Using push-pull recovery in my experience moved refrigerant at twice the speed. I have done side by side tests on equipment to prove the effectiveness of each practice, push-pull vs direct recovery with subcooler pumping liquid, and push-pull always dominated a direct recovery. You do not need a subcooler during push-pull because the recovery machine will pull vapor pressure off the top of the cylinder and keep the chiller pressure up by pumping that vapor back into it.
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