A popular metering device used on many fractional horsepower refrigeration systems is the cap tube. But one field problem encountered with this type of metering device is that it can become plugged with system debris, causing a restriction. When this happens, normally the cap tube will need to be replaced.

When replacing a cap tube, the replacement must match the Btu capacity and flow characteristics of the original. It is usually easiest to obtain the replacement cap tube from the OEM to ensure the correct one is used. However, if that is not an option, a replacement cap tube from another source can be used.

The Btu capacity and flow characteristics of a cap tube are based on the length and inside diameter of its tubing. When choosing a replacement cap tube, it is best to choose one with the same inside diameter and length. However, a cap tube with the same inside diameter may not always be readily available. A cap tube with a different inside diameter may be used if it is cut to the appropriate length as directed by the manufacturer. Remember to always follow any applicable notes and directions provided by the manufacturer.

When installing a cap tube metering device, it is common for a technician to braze the tubing into the system. If care is not taken during this process, brazing material and/or other foreign material can enter the end of the cap tube and cause a partial or total restriction.

New cap tubing is normally shipped with plastic caps on the ends. Leave these caps on until it is time to insert the tubing ends into the connecting tubing. This is helpful when it is necessary to feed the tubing through an opening in the cabinet.

When cutting a cap tube to length, do not use an instrument that can distort the end of the tubing. The best way to cut a cap tube is to use a tool designed especially for cutting this tubing, or to score the tubing with a file at the location to be cut and then flex the tubing back and forth until it breaks. This will prevent the ends from being distorted or plugged.

Be careful where the tubing is placed during the preparation process. Do not place the cap tube in a location where foreign material could easily enter the ends, such as on the floor or on the base of the equipment.

Do not sand the very end of the cap tube. Begin sanding two to three inches back from its end. This serves two purposes:

• It will prevent any grit from the sanding process from entering the tubing, and

• During the brazing process, it may help to impede the brazing material from flowing down the cap tube and into the end.

Insert the cap tube at least two to three inches into the connecting tubing.

During the brazing process use only enough filler material to seal the joint. Avoid overfilling the joint - this may lead to filler material entering the end of the tubing.

Replacing cap tubes can be an easy process. However, it does require attention to detail and some patience. Installing the wrong cap tube or mishandling the cap tube during the installation process can cause further system problems or even cause the tube to be replaced again.

Publication date:01/12/2009