A chiller has a leaking evaporator tube and the waterboxes have been removed so the tube sheet is exposed. How does the technician identify the individual tube that is leaking?
- Bring the refrigerant side up with nitrogen inside the limit and check each tube end for escaping gas
- BFill each tube with chilled water in turn and watch the shell sight glass for a change in level
Filling tubes one at a time is unworkable on a bundle with hundreds of tubes, and the shell level would not respond usefully to a single tube being wetted.
- CRun a light down the tubes and look for the one whose inside surface shows a bright scale line
Scale and pitting look much the same on a sound tube and a failed one, and a visible defect is not proof of the leak path.
- DMeasure the water flow through each tube and identify the one carrying the lowest flow rate
Flow measurement through individual tubes is not practical, and a leaking tube does not necessarily carry less water than its neighbors.
Why A is correct
The leak path runs from the shell into the tube, so putting pressure on the shell side sends gas out through the failed tube and into the waterbox where it can be detected at the tube sheet. The pressure has to stay under the machine's limit, which is why the nitrogen goes through a regulator with a relief device set below that limit.
What this question is testing
This tests a practical procedure that follows directly from the pressure limit and from the direction of the leak path. The exam ties tube failures to the same low pressure ceiling that governs any other leak test on these machines.
On the job
This is a slow, methodical job: the technician works across the tube sheet with a detector probe, marking candidates, then confirms with soap. On a large bundle it can take most of a day. Once the tube is identified it gets tapered plugs driven into both ends, which is a temporary measure limited by how many tubes the manufacturer allows to be plugged. The machine is then evacuated and dried before the charge goes back in, because water has been inside the shell.
Memory technique
Pressure in the shell, detector at the tube end. The leak tells you which tube it is.
Exam tip
Any tube leak test on a chiller still has to obey the shell pressure ceiling. The method changes; the limit does not.
Where to look it up
A leaking tube is found by pressurizing the refrigerant side with dry nitrogen, within the appliance's pressure limit, and checking the exposed tube ends.