How can a technician speed up removal of the last of the vapor from a cold low-pressure machine without risking the tubes?
- AOpen the machine briefly to the room so air pressure pushes the vapor out to the hose
Opening the machine to the room lets air in and lets refrigerant out, which defeats the purpose of recovery and is a prohibited release. Atmospheric pressure is not a recovery tool.
- BRun the chiller's compressor while the recovery machine is connected to the shell
Running the chiller's own compressor during recovery risks damaging it with no oil return and no load, and it does nothing to move the last of the vapor out of the machine.
- Circulate warm water through the tubes to raise the refrigerant's saturation pressure
- DAdd nitrogen to the shell to sweep the remaining vapor toward the recovery hose faster
Adding nitrogen mixes a non-condensable into the charge being recovered, contaminating the cylinder and leaving the machine working against a partial pressure it cannot remove.
Why C is correct
Warm circulating water raises the temperature of the refrigerant remaining in the vessel, which raises its saturation pressure and increases the amount of vapor the recovery machine can move per minute. Because the water is warm and flowing, the freeze hazard disappears at the same time, so the technique solves two problems at once.
What this question is testing
This tests applied understanding of the pressure temperature relationship during recovery. It also checks that you reject the shortcuts that vent refrigerant or contaminate the recovery cylinder. It also measures whether you reject shortcuts that vent refrigerant or introduce non-condensables, both of which look attractive when a job is running long.
On the job
Every plant that does this work regularly has a way to feed warm water or condenser water through the evaporator bundle during recovery. It turns a job that stalls out overnight into one that finishes the same day, and it keeps the freeze protection built in. If no warm water source exists, the fallback is a very long, patient vapor pull with the water pumps left running so nothing freezes while you wait.
Memory technique
Warm water is your friend twice: it makes the vapor move and it keeps the tubes from splitting.
Exam tip
Any answer that involves venting to the room or adding nitrogen into refrigerant you plan to recover is wrong on the exam and wrong in the field.
Where to look it up
Heat raises saturation pressure, so warm circulating water speeds vapor recovery and prevents freezing at the same time.