The dimensions of the tubes used in the construction of heat exchangers usually respond to Birmingham Wire Gage (BWG) Standards. The tubes are designated by their outside diameters and a code that is related to the wall thickness. The table included in App. H shows the dimensional characteristics of tubes according to BWG Standards.
Tube diameters ranging from 1/4 in (6.35 mm) to 1.5 in (38 mm) may be used. However, almost all heat exchangers are constructed with 3/4-in (19-mm) or 1-in (25.4-mm) tubes.