I would like to propose a rule for sizing cap tubes (single stage systems). Based on what I have been seeing in the forums, I would propose the following:
For R404A/R507:
Using a center point of 150 watts load I would guestimate 10 feet (3 M) of .028 inch (.7 mm) cap tube.
For each increase of 15 watts, I would shorten the cap tube by 1 foot (30 cm).
For each decrease of 15 watts, I would lengthen the cap tube by 1 foot (30 cm).
Then for R22/R290 (propane) shorten 1 foot (30 cm).
For .026 cap tube, multiply by .7.
For .031 cap tube, multiply by 1.6.
It is best to stay between 5 feet (1.5 M) and 16 feet (5 M), with center range of 10 feet (3 M) being ideal.
Does this sound about right?
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