ANS:C - should be less than the inside diameter of the shell.

Baffle spacing typically refers to the distance between adjacent baffles inside a shell and tube heat exchanger. It is not the same as baffle pitch, which refers to the distance between the centers of adjacent baffles. The statement "Baffle spacing should be less than one fifth the diameter of the shell" is generally true. Proper baffle spacing is crucial for effective heat transfer within the heat exchanger. If the spacing is too large, it can lead to bypassing of the fluid and reduced heat transfer efficiency. Keeping the baffle spacing less than one-fifth of the shell diameter helps ensure adequate mixing and turbulence of the fluid flow, maximizing heat transfer. Therefore, the correct statement is: Baffle spacing should be less than one fifth the diameter of the shell.