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

Figure 1 is an example of a chart (not an actual manufacturer’s chart) demonstrating the process of selecting a replacement cap tube with a different inside diameter.

First, determine the ID of the original cap tube. (There are tools available which will allow you to measure the actual ID of the original cap tube.) Then, find the original cap tube ID on the left hand column of the chart, reading across to find the conversion factor under the cap tube size of the replacement unit. Lastly, multiply the given length of the original cap tube by the conversion factor to determine the correct cap tube length to use.

For example: An original cap tube is 9-feet long with an inside diameter of 0.040 inch, but a replacement cap tube with an inside diameter of 0.036 inch is readily available.

Locate 0.040 inch in the left-hand column. Reading across, the conversion factor for the replacement cap tube with an inside diameter of 0.036 inch is 0.62 inch. Multiply 9 feet by 0.62 to yield a new length of 5.58 feet.

Using a replacement cap tube with a different ID is possible as long as you follow the directions offered by the replacement cap tube manufacturer.

Publication date: 3/3/2014

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