A technician wants to open a fitting on a chiller's purge unit while the machine is running, on the assumption that the whole machine is under a vacuum. What is wrong with that assumption?
- AThe purge unit holds its own separate charge that is always above atmospheric pressure
The purge does not hold a separate refrigerant charge of its own; what is inside it came from the chiller.
- BThe purge unit's suction line is the one part of the machine that runs at chilled water pressure
The purge is connected to the refrigerant side of the machine and never sees chilled water pressure.
- The purge takes suction from the condenser, which is above atmospheric while the machine runs
- DThe purge unit stores refrigerant under pressure only during the few seconds of each cycle
Whatever is in the purge drum stays there between cycles, and it is at condenser pressure rather than briefly pressurized.
Why C is correct
The pressure profile across a running machine is what decides this. The condenser sits above atmospheric at load, and the purge is connected to it, so purge piping is under positive pressure whenever the machine is running. Assuming the whole machine is under a vacuum is the mistake, and it produces a release directly into an occupied machine room.
What this question is testing
This tests the pressure profile applied to personal safety rather than to leak direction. The exam reuses the same fact in both settings because candidates tend to learn the vacuum story as a blanket statement about the whole machine.
On the job
Technicians who have absorbed the idea that chillers run in a vacuum sometimes treat any fitting on the machine as safe to crack, and the condenser end of a loaded machine will prove them wrong immediately. The habit that avoids it is simple: gauge the section before opening it, whatever you believe about the machine. Purge work is scheduled with the machine down, the purge isolated and its contents recovered.
Memory technique
The purge lives on the warm end of the machine, where the pressure pushes out rather than pulling in.
Exam tip
Vacuum on the cold end, positive pressure on the hot end. Gauge the section before you open it.
Where to look it up
The condenser of a running low-pressure chiller is slightly above atmospheric, and the purge unit is connected to it.