Shell and tube heat exchangers are complex and more expensive than ordinary pressure vessels. Also the strength calculation is more difficult. Part of the design process includes the determination of the amount of tubes to be used in the heat exchanger. The number and length of the tubes create the area through which the heat is transferred from one process medium to the other. The outside of the shell and tube heat exchanger is mostly a cylindrical form which make the calculation by hand for the amount of tubes difficult. The number of tubes and the dimensions are required to execute the calculation for the tube sheet.
The Tube Sheet Lay-out page facilitates the calculation of the amount of tubes. The calculation can be done for four (4) pitch patterns (how the tubes are laid out) and for ten (10) types of passes (tube compartments).
Tubesheet sketch, with shell inside diameter and tubes (not to scale):