Sometimes a wax buildup or dirt obstructs the passage of refrigerant through a capillary tube. Due to its length, the capillary tube may run through places difficult to reach. In such cases, it is easier to unclog it rather than replacing it.
A capillary tube cleaner can be purchased from most major refrigeration supply houses.
The way the device works is that it forces wax and dirt out of the capillary tube under high pressure. Some of these devices are capable of producing pressures as high as 3,000 psi.
An obvious sign of a clogged capillary tube is that the back pressure reads lower than normal (or even vacuum), the head pressure reads higher than normal, the unit no longer cools while running constantly, and the condenser feels cooler than normal.
To use a capillary tube cleaner, disconnect the capillary tube at both ends. (Flux and apply heat to the brazed joint to remove it.) Connect the tube cleaner to one open end of the capillary tube by using an adapter fitting; then turn the handle to create the pressure necessary to clear the tube. In these devices, either oil or R-11 is used as a pressure fluid. (See fig. 45b.) After removing the obstruction from the tube, install a new filter-drier and silver-braze the tube back into the system before evacuating and charging the unit.
There are some capillary tube replacements on the market called patented tubes. Some of them are available with different sized strainers while some are fitted with a calibrated wire inside to control the flow of refrigerant (see fig. 45c).
If a new capillary tube is needed, it must always be replaced with one having the same inside diameter and the same length; otherwise, evaporator temperature will be affected.
For better understanding, you should know that there are three types of compressors made today: high-temperature types, which produce temperatures down to about 0°F; medium-temperature types, which produce temperatures of about 0°F to -10°F; and low-temperature models which produce temperatures below -10°F.
A capillary tube sizing gauge can be purchased from a major local refrigeration supply house. This is a tool similar in appearance to a spark plug gapping tool used by auto mechanics. It consists of a number of different sized wires to measure the inside diameter of capillary tubes (see fig. 45d).
See the chart on the next page for the required length of tubing based on its inside diameter, the horsepower, and temperature rating of the compressor.
When handling capillary tubes, it is important to remember that
1. capillary tubes are connected to the sealed system mostly by silver brazing (a flared connection is seldom used) and,
2. since capillary tubes are too small in diameter to be cut with a tubing cutter, the usual practice is to score them with the edge of a file. It is then bent carefully until it breaks.
To silver-braze a small capillary tube to a large tubing, place the small tube at least two inches inside and against one wall of the larger tubing. Using a pair of pliers, crimp the opposite wall of the larger tubing until it fits snugly around the capillary tube. Then clean and silver-braze the joint as instructed earlier. (Because small tubing absorbs heat very rapidly, be careful no solder gets far enough inside to block the opening of the small tube and cause a restriction.)