Tubular heat exchangers and their construction variations

Types used on board ship

• Shell and tube heat exchangers (coolers) for sea-water cooling of lubricating oil (lube oil cooler), freshwater (FW cooler), jacket cooling water, fuel oil (fuel heater/cooler), and for steam condensers.
• Double-pipe (hairpin) heat exchangers, which are two concentric pipes.
• U-tube / multipass shell-and-tube exchangers, and floating-head (floating tube sheet) exchangers to allow for thermal expansion.
• Plate heat exchangers are technically not tubular but are used in some duties; the question concerns tubular ones, so the focus is shell-and-tube.

Construction, flow pattern and baffles depending on the medium

Shell-and-tube construction: a cylindrical shell (e.g. steel, zinc-protected or cupro-nickel lined for sea water), with a bundle of tubes fitted between two tube sheets (headers) and secured by tube expansion/glands, the whole enclosed by channel covers. One fluid flows through the tubes (tube side) and the other through the shell in the space around the tubes (shell side), transferring heat through the tube walls.

Flow pattern: for clean fluids (e.g. oil/fresh water) a number of passes is arranged - the tubes are grouped so the fluid passes back and forth to give multipass; the shell fluid is guided across the tube bundle by baffles. Counter-flow is preferred for efficiency (hot and cold enter opposite ends); where a counter-flow cannot conveniently be arranged, a "two-pass" tube-side with shell fluid cross-flow is used. For sea water (dirty, scale-forming) the sea water is normally put on the tube side so it can be cleaned by rodding out/backflushing and so the tube bundle can be withdrawn - and a spacer/no-differential expansion design (floating head) accommodates the large thermal expansion.

Baffles: transverse baffles (segmental baffles) are fitted in the shell to force the shell-side fluid to flow back and forth across the tube bundle, increasing turbulence, mixing and the heat transfer coefficient, and supporting the long tube bundle to prevent sagging/vibration. Baffle spacing and cut shape differ with the medium: for low-viscosity or clean fluids closer baffles and a larger cut promote turbulence; for viscous oils (which have poor heat transfer and high pressure drop) the baffles are spaced wider and have a reduced cut to limit the pressure drop while still sweeping the tubes. For sea water, fewer/wider baffles reduce pressure drop and erosion.

Depending on the medium:

• Oil/fuel (viscous, poor convection): oil on shell side over a large tube area with wide, partly-cut baffles, or oil on tube side with multipass; materials tolerant of heating.
• Fresh water: may be either side; six-pass or four-pass tube arrangement common.
• Sea water (corrosive, scale forming): on the tube side, so tubes cleaned and selected in cupro-nickel; spacious shell, floating (expansion) heads to allow differential expansion; baffles arranged to maintain good cross-flow without excessive pressure drop.
• Steam (steam condenser): steam on the shell side with the cooling water in tubes; the condensate drains; baffles shaped/nozzles arranged to sweep the tubes and direct the steam.

Distinguishing sketch features: shell and flanged cover with tube bundle and tube sheets, removable floating head, the pattern of baffles (segmental plates with holes), the pass partitions, and the inlet/outlet nozzles for tube-side and shell-side.