The size of the cap tube is fairly critical.
Unlike orifices, such as expansion valve seats,capillary tubes depend on their length as well as their diameter to determine their total restriction.
The relationship between these two factors are shown in the following charts.
A change in diameter on a percentage basis can change the flow more than an equal change in length.

To illustrate, changing the diameter by .005" as between .026" I.D. and .031" I.D.can double the flow.
Restriction can also be changed by lengthening or shortening the cap tub.
The longer the tube, the slower the flow; the shorter the tube, the faster the flow. The general flow curve graph (right) shows what happens to the flow of refrigerant through a cap tube as the length is changed.
This curve is not meant to give specific flows but to simply illustrate what happens with all cap tubes so that the general flow pattern can be understood.
By following the flow curve from left to right it can be seen that for the very longest length the flow is the smallest.
Then as the cap tube length is decreased, the flow increases slowly until critical point "L" is reached length causes ever increasing flow.

From the study of this typical curve, certain pertinent conclusions can be reached that directly affect the field application of capillary tubes.
On the graph, the section above the critical point "L" is marked as extra long lengths.
Attempting to increase restriction (i.e. reduce flow) by increasing length into this region is not only uneconomical but frequently hopeless.
In addition, tubes in this range may not be responsive enough to changes in head pressures during operation.
All in all, tube lengths in this range should be avoided where possible.
Continuing down the graph, the section below critical point "S" should be avoided like the plague.
In this range, the tube is so short that even small changes in length will cause very
large increases in flow.
This is caused by the fact that the length no longer affects the flow and the tube now beings to act more like an orifice than a capillary tube.
But, without the other components necessary to control an orifice, such as are present in an expansion valve or high side float, a
very short cap tube will give wildly erratic operation under varying ambients and loads.

All of this would be meaningless without some definite way to use this information. Although the critical points will vary depending on the I.D. of the cap tubing being used, a very safe operating rule-of-thumb can be offered.
Keep the cap tube no shorter than 5 ft. and no longer
than 16 ft.

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