A running low-pressure chiller has a leak at a joint on its condenser. What is happening at that joint while the machine is loaded?
- ANothing moves either way, because the condenser sits exactly at atmospheric pressure at load
The condenser is above atmospheric at load rather than at it, which is exactly why a leak there behaves differently from one on the evaporator.
- BAir is drawn in, because every part of a low-pressure machine is under a vacuum at all times
The evaporator is under a vacuum; the condenser is not. Treating the whole machine as a vacuum vessel is the most common error on these questions.
- Refrigerant is escaping, because the condenser runs slightly above atmospheric pressure at load
- DWater vapor is drawn in, because the condenser is the coldest surface on the machine at load
The condenser is the hottest part of the machine at load, not the coldest, and nothing is drawing water vapor into it.
Why C is correct
The pressure profile across a running machine decides the direction of every leak on it. The condenser is above atmospheric at load, so a leak there pushes refrigerant out and can be found with an ordinary electronic detector. The same joint reverses direction on shutdown, which is why these machines both lose charge and accumulate air over a season.
What this question is testing
This tests the pressure profile of a running machine applied to leak direction. The exam keeps returning to the vacuum story, and this question checks that the candidate learned it with the exception attached rather than as a blanket statement.
On the job
This explains a result that confuses technicians on their first chiller: the sniffer alarms at one end of the machine and finds nothing at the other, and both readings are correct. It also explains why a chiller can lose charge steadily while its purge log climbs at the same time. Finding the condenser leaks with the machine running, and the evaporator leaks with the shell pressurized during a shutdown, is how experienced crews split the work.
Memory technique
Cold end sucks, hot end blows. The machine is a vacuum machine only at one end.
Exam tip
Evaporator under a vacuum, condenser slightly above. Leak direction follows the pressure at that spot, not the machine's reputation.
Where to look it up
On a running low-pressure chiller the evaporator is below atmospheric pressure and the condenser is slightly above it.