Tube holes cannot be drilled very close together, since this may structually weaken the tube sheet. The shortest distance between two adjacent tube holes is called the “clearance.” Tubes are laid out in either square or triangular patterns as shown in Figure 3-5. The advantage of square pitch is that the tubes are accessible for external cleaning and cause a lower pressure drop when shell-side fluid flows perpendicularly to the tube axis. The tube pitch is the shortest center-to-center distance between adjacent tubes. The common pitches for square patterns are %-in. OD on 1 -in. and 1-in. OD on l/4-in. For triangular patterns these are %-in. OD on %-in., M-in. OD on 1-in., and 1-in. OD on 1/4-in.
In Figure 3-5c the square-pitch layout has been rotated 45°, yel it is essentially the same as Figure 3-5a. In Figure 3-5d a mechanically cleanable modification of triangular pitch is shown. If the tubes are spread wide enough, it is possible to allow the cleaning lanes indicated.