Today someone said that fixed-tube-sheet heat exchangers require venting and draining, while U-tube heat exchangers do not need it?

I haven’t heard of such a claim. The inner diameter of the large cover of a floating-head heat exchanger must be larger than the diameter of the shell side; vents for venting and draining are usually provided at the top and bottom of the large cover. In practice, the venting and draining of many heat exchangers are carried out on the pipelines at their inlet and outlet. But I have also seen fixed-plate heat exchangers in which the shell side or tube side has flow from top to bottom; in such cases, it is necessary to provide a drain at the bottom of that shell side or tube side. In a U-tube heat exchanger, the tubes are generally arranged vertically; in such cases, it is sufficient to install vent and drain valves on the inlet and outlet pipelines ； Its shell side is also of equal diameter, and the same applies if its inlet and outlet are located at the top and bottom.

There is no such rule; whether non-condensable gas or condensate is generated, it should be added.