Why did making the recovery tank cold expedite recovery of refrigerant?

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Why did making the recovery tank cold expedite the recovery of refrigerant?
Many years ago, I was a student in a HVAC program at a technical college. Sometimes when I and the other students were recovering refrigerant out of an air conditioner into a recovery tank, we would get ice from the ice machines and fill up a tub with ice, and then we would put the recovery tank in ice as we recovered the refrigerant. The placement of the recovery tank in a tub of ice would expedite the migration of the refrigerant from the air-conditioner to the recovery tank. Why did the placement of the recovery tank in ice expedite the migration of the refrigerant from the air-conditioner to the recovery tank?
 
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  • #2
When any gas gets expanded, it tends to cool down and take energy from its surroundings.
As temperature of the liquid-gas mix is reduced, so is the pressure inside the tank, because its volume is constant and also because the pressure of evaporation gets reduced at that reduced temperature.

That is why, when charging any refrigerant into an air conditioner from a storage tank, the tank naturally cools down due to the refrigerant expansion.
Placing that tank into a bucket with warm water, expedites the injection of refrigerant into the equipment.

The process that you describe is the opposite to the one described above.
The vacuum pump tends to increase the pressure and temperature of the gas that is transferring from the AC equipment into the tank.

As you encourage the condensation of that gas inside the recovery tank, the pressure of the gas phase gets drastically reduced, making room for more gas to come inside the tank.
Hence, the cooler the faster the recovery process.
 
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