[Sketch notes: a U-tube (hairpin) heat exchanger has the tube plates rigidly located; a straight-tube exchanger with fixed tube sheets must accommodate the different axial thermal expansion between the tubes and the shell.] In a heat exchanger with the tube plates rigidly bolted to the shell (fixed tube-sheet type), the tubes and shell expand differently because they are at different temperatures (shell could run hotter or colder). The differential movement is accommodated by:

1. A floating/expansion head design where one tube sheet is not fixed but slides (a "floating head"), which is not "rigidly located".
2. For the fixed-tube-plate (rigid) design: a U-bend (U-tube) so the tubes individually expand freely in the shell, the two tube sheets being at the same end.
3. An expansion loop / bellows in the shell (a "shell expansion joint") to allow the shell to expand axially without overstressing the tube sheets.
4. On a straight-tube unit with rigid plates, a stainless-steel bellows (expansion joint) in the shell to absorb the differential expansion; or the tubes being designed with a slight bow/allowance.

• In practice the common rigidly-located-tube-plate marine heat exchanger is the U-tube type, in which each tube is free to expand because it is bent round at one end, so the differential expansion between the tubes and the shell body is absorbed by the deflection of the tubes & by the shell expanding; if the shell is fixed, an expansion bellows or the free sliding of one tube sheet is used.

Turbulence is induced by fitting "turbulators" or by using a "double/split flow", or by the surface roughness/small bore of the tubes; on the shell side, by baffles. The reason is that a turbulent flow gives much higher convective heat-transfer coefficient than the laminar (streamline) flow that would otherwise develop inside tubes. Laminar flow has a stagnant boundary layer that insulates, so heat transfer is poor; turbulence breaks the boundary layer and brings fresh fluid to the tube wall, greatly increasing heat transfer. Turbulizers (twisted metal inserts) or spiral ribbing are put inside the tubes/maintaining flow area while creating radial mixing and increasing the heat-transfer coefficient with a modest increase in pressure drop, allowing the exchanger to be smaller and more efficient.

The sacrificial (zinc/iron) anodes in sea-water coolers have been made unnecessary (in modern installations) because:

1. The non-corrodible materials: heat exchanger tube plates, tube sheets, water boxes and tubes are now made of materials that do not couple galvanically (e.g. all-titanium, or 90-10 cupro-nickel, or aluminium/brass tubes with compatible plates and a non-corrosive coating), or the water boxes are protected with a cathodic-protection-compatible coating and modern materials do not suffer galvanic corrosion.
2. Cupro-nickel and titanium tube material is highly corrosion resistant to sea water, and the galvanic couple to the steel water box is eliminated by design/coating/lining, so a zinc anode is not needed.
3. The anodes themselves would deplete and require regular renewal and could also cause problems (e.g. coating of tubes with zinc deposits and increased baffle welding); modern design opts out of the anode.

However, in many still-installed iron/steel water-box coolers sacrificial anodes are still used; where they are "discarded" it is due to all-non-corroding construction/coating of the sea-water side.

"Guided flow" in an oil heater (shell-and-tube heat exchanger for heating fuel/lubricating oil) refers to the arrangement of baffles in the shell that forces the oil to flow in a definite, guided (often cross-flow or helical) path across the tubes, rather than allowing it to take an uncontrolled straight path. The baffles direct the oil so it passes over ("sweeping") all the tubes in sequence, ensuring all the heat-transfer surface is used and achieving a high heat-transfer rate with a controlled pressure drop. Guided flow prevents stagnant/dead zones (where oil could coke or overheated local hot spots) and improves the effective mean temperature difference. In oil heaters it is important to ensure even, guided flow so the oil is heated uniformly without coking, and the heater circulates the oil continuously to maintain the correct temperature.