Mechanical Design for Good Heat Transfer

From a theoretical heat-exchanger perspective, the triangular tube pitch shown in Fig. 26.7 is best. The term pitch refers to the geometry and distance between the holes drilled into the tube-support baffles. 

A triangular pitch means the holes are drilled in the pattern of a 60° equilateral triangle. For example, we might have a 0.75-inch tube (OD) on a 1-inch triangular pattern. The advantages of this tube pitch are 

This all results in reduced capital cost. The disadvantage of this tube pitch is that, except for the outer few rows of tubes, the shell side cannot be cleaned by hydroblasting. Obviously, this sort of arrangement should never be used in a fouling service, unless the deposits can be chemically removed. 

The second example shown in Fig. 26.7 is 0.75-inch tubes, also set on a 1-inch spacing. But this time, the arrangement is a square pattern. This pattern reduces the number of tubes in a shell of a fixed ID. Also, less turbulence is created by the tubes. Some fluid may flow between the tubes without encountering the tubes at all. All this reduces heat-transfer efficiency. But hydroblasting between the tubes is usually possible—depending on the extent of tube bending and twisting. 

The third example shown in Fig. 26.7 is my favorite for sensible-heat transfer in fouling service. This consists of 1-inch tubes set on a 1.5-in rotated square pitch. The pitch layout is the same as that of 

The third example shown in Fig. 22.7 is my favorite for sensible-heat transfer in fouling service. This consists of 1-in tubes set on 1.5-in rotated square pitch. The pitch layout is the same as that of the square pitch. It is just the tube bundle that is rotated by 45° relative to the shell-side flow. The shell-side fluid cannot flow without interference between the tubes. Hence, the tubes promote turbulence, and improve heat transfer. The large tubes, and especially the doubling of the space between the tubes, reduce the heat-transfer surface area that can fit into a shell of a given ID. But the primary objective of the 1-in tubes on a 1.5-in rotated square pitch is that it is relatively easy to hydroblast. 

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