Floating head tubesheet, flow

The tube-side fluid now flows into the floating head, which acts as a return header for the tubes. The tube-side flow makes a 180° turn and flows back through the top hah of the floating-head tubesheet. The floating head is firmiy attached to the floating-head tubesheet. But why is it that one end of the tubes must be left free to float The reason is thermal expansion—or, more precisely, the differential rate of thermal expansion between the tubes and the shell. 

Selecting Tube Passes. The number of tube passes is kept as low as possible in order to get simple head and tubesheet designs. For even numbers of multiple pass designs, no off-center nozzle on the floating head is required. The flow quantity and the desired minimum tubeside velocity determine the number of tubes per pass and the total area and tube length then fix the number of passes for the desired performance. However, the number of tube passes must be an even integer hence, the tube length is variable. 

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