Intermittent Cap Tube Blockage

Intermittent Cap Tube Blockage
Intermittent Blockage of the Cap Tube – Diagnosis and Repair
If you have been experiencing intermittent problems whereby the compressor runs but no cooling
takes place, there is a possibility that you have a small piece of debris in the system that is
intermittently blocking the entrance to the capillary tube (cap tube). The classic symptom of this
condition is that the temperature in the box starts rising, but the compressor continues to run.
Turning off and restoring power to the system after a short rest may restore the system to normal
running again, but some time in the future the problem will usually return, although this could be
minutes, hours, days, weeks, months or even years later. Quite often this condition appears after a
rough or boisterous passage.
The cap tube looks like a piece of wire, but in fact is a small tube with a micro-bore down the
center, and this separates the high and low pressure sides of the system. Any small piece of debris
(usually introduced during installation) that finds its way to the end of the cap tube can block it, and
so no refrigerant flows even though the compressor is running. Turn the system off, and the
pressures equalize which releases the debris and unblocks the end of the cap tube. It's just a
matter of luck as to where the offending piece of junk sits in the rest period, and whether or not it
causes a blockage on the next cycle. With the boat at rest in a marina, etc., the problem may not
surface again until you encounter some heavy weather, and then the debris is disturbed from its
resting place to cause havoc and mayhem again. Very frustrating!
So what is the remedy? Evacuating the system with a vacuum pump will simply relocate the
debris, and this may prove to be all that is required, but the only true cure is to replace the
evaporator and install a filter-drier in the liquid line. The filter-drier will stop any more debris getting
to the end of the cap tube (the cap tube is integral with the tubing attached to the evaporator). But
there is something else that is worth trying, as it has worked on numerous occasions. You will need
to locate the section of tubing that incorporates the end of the cap tube and orientate that section
vertically in the hope that the debris has enough mass to be trapped somewhere and cannot get to
the end of the cap tube. The cap tube is run inside the suction tube, and if you follow the tubing
back from the evaporator for approx. 9ft you will come across a section where the cap tube exits
from inside the suction line. It then takes a few turns around the suction line before disappearing
inside a slightly larger tube and then on to a coupling, and it is this area we want to focus on. Once
the cap tube enters the slightly larger tube it extends inside it for a few inches, maybe three or four,
(it depends on who fabricated it) and so it is this section that needs to be fashioned so that it is
vertical. Either way up should work, but I suggest you try first with the cap tube entering from the
top, and if that doesn't work try the other way up. A few sharp taps on the tubing with a screwdriver
handle when vertical will help to settle the debris. If neither method is successful there is no
alternative but to replace the evaporator and add a filter/drier.
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