Question: How do you determine the sizing of a capillary tube for a system, and how do you clear a plug or restricted cap?

-Richard Ali John, West Indies

Answer: According to the ASHRAE Handbook on Fundamentals, there are six tables and numerous formulas that are used to determine the size of a cap tube for a system. You must first ascertain the load, then which refrigerant, pounds-per-hour circulation, estimate length of heat exchange, estimate absolute head pressure, estimate subcooling, desired length of cap tube (to determine the ID), and alternate size for same flow rate, if desired.

After the cap tube has been sized, it must then be “fine tubed” by subjecting the system to high and low ambients, and loads for any correction in length. If this sounds complicated, it is.

There are recommended cap tube sizes for systems of a given HP, refrigerant and temperature that are sold at all wholesalers. These sizes are for average conditions and may have to be “fine tuned” for specific systems and applications.

Attempting to clear a restricted cap tube should be decided by the unit that is involved and who pays for the time, if the results are negative. I believe there are two methods for clearing a cap tube of its restriction. The first method is to pump a solvent under high hydraulic pressure. The second is to install a small flexible object into the tube and force it through the tube by using hydraulic pressure.

This procedure on a domestic refrigerator is very time consuming. There is no way of knowing how successful you were until you reconnect the tube, evacuate and recharge the system. Since replacing the cap tube may take two or three times longer, some technicians may feel it pays to invest the time to clear the tube. According to many service managers, very few, if any, cap tubes in domestic refrigerators are replaced in the field.

By comparison, cap tubes in commercial systems are very accessible but the evacuation, charging and final checking takes much longer. Unless there are extenuating circumstances, it may not be prudent to attempt to clear a cap tube in commercial systems.

-Henry Ehrens, senior engineer, Sealed Units Parts CO. Inc.