A purge unit on a low-pressure chiller cycles and discharges through a line to the outdoors. What is in that discharge?
- AOil vapor carried up from the compressor sump by the rising condenser temperature
Oil is not volatile at these temperatures and does not travel to the top of the condenser as a gas.
- BRefrigerant vapor that the machine has no room for at current condenser pressure
A condenser does not run out of room for refrigerant, and a purge is not a pressure relief device.
- CChilled water vapor that has crossed the tube walls into the refrigerant space
Water crossing the tube walls means a tube has failed, which is a fault rather than a description of normal purge discharge.
- Air and other non-condensable gas, with water vapor and a trace of refrigerant
Why D is correct
The purge exhaust is mostly atmospheric air that leaked into the machine, plus the water vapor that came with it. Refrigerant is meant to be separated out and returned, so a small trace is all that should leave. Understanding the composition explains both why the discharge is not treated as venting and why a poor purge costs the owner refrigerant.
What this question is testing
This tests understanding of the purge's contents rather than its function. The exam separates the two because the composition of the discharge is what determines whether it is a permitted release and how much refrigerant the owner is losing.
On the job
Operators watching a purge cycle for the first time almost always assume the machine is blowing off refrigerant, and the conversation that follows is a good chance to explain why purge run time is worth logging. On a tight machine with a modern purge, the annual refrigerant loss through the exhaust is small. On a leaking machine with an old thermal purge, it can be a significant share of the charge, and the operator's log is the evidence that pays for the repair.
Memory technique
Whatever leaked in, leaks back out through the purge, and it should take as little refrigerant with it as possible.
Exam tip
The purge discharge is the air the machine drank, plus its water, plus a little refrigerant. The last part is what a good purge minimizes.
Where to look it up
Purge exhaust consists of non-condensable gas drawn into the machine, with water vapor and a small amount of refrigerant.