A refrigerant cylinder is connected to the high side of a system for liquid charging and the compressor is then started with the cylinder valve still open. What is the danger in that arrangement?
- AThe cylinder will empty into the system faster than the metering device allows
Charging faster than a metering device would pass is exactly why high side liquid charging is done with the compressor off. The reverse flow is the hazard once it runs.
- BThe refrigerant will fractionate as it passes backward through the hose
Fractionation happens when vapor is drawn off a blend. Liquid moving through a hose in either direction does not separate the components.
- Discharge pressure can push back into the cylinder and overfill it
- DThe liquid line will collapse under the vacuum the compressor creates
Refrigerant tubing does not collapse under any pressure a compressor develops. The pressure difference here drives flow, it does not crush the line.
Why C is correct
With the compressor running, discharge pressure exceeds cylinder pressure, so the flow reverses and refrigerant is pushed back into the cylinder. A cylinder taking a return flow can pass its fill limit without the technician noticing, and it is being warmed at the same time. Liquid charging on the high side is therefore done with the system off, and the cylinder valve is closed before startup.
What this question is testing
This tests understanding of which way refrigerant flows when the machine starts, applied to a charging practice technicians use daily. The hazard follows directly from the pressures involved, which is the reasoning the exam is checking.
On the job
This bites on change-outs where one technician is charging and another switches the disconnect back on, and the first warning is a cylinder that has gained weight on the scale. Keeping a hand on the cylinder valve, and treating the scale reading as something to watch rather than glance at, catches it. Overfilled cylinders are dangerous well after the job ends, since they travel in a hot truck and sit in a hot shop.
Memory technique
Start the compressor and the high side becomes uphill. Your cylinder is now at the bottom of the hill.
Exam tip
Ask which side is higher once the compressor runs. Anything connected to the high side becomes a destination, not a source.
Where to look it up
Charge liquid into the high side only with the compressor off, and close the cylinder valve before startup.