Heat Exchanging

On-board ships there are various systems where a requirement for the removal, or addition, of heat to the working fluid exists. The exchange of heat, or heat transfer, is achieved by making use of Coolers/Heat Exchangers where the working fluid or liquid is cooled by seawater.

During heat exchanging, heat is transferred, or exchanged, by means of conduction when two liquids pass on either side of a heat conducting surface.  Heat can also be transferred by means of convection or radiation. Heat transfer by means of conduction will be the focus within this Learner Guide. It must however be noted that Convection and Radiation does play a considerable part, because a heat source near a heat exchanger or cooler might affect its heat transfer abilities negatively.

Fluid flow in shipboard heat exchangers are usually contra-, or countercurrent, flow. This means that the liquid that needs heat added, or removed from it, flows against the liquid providing the source or heat sink.

Heat Exchanger Types

Generally, two types of heat exchangers are used on-board ships:

1. Shell and Tube Heat Exchanger (Countercurrent)

• These heat exchangers, as shown below, are generally used for engine cooling water and lubricating oil. The liquid the heat is exchanged with is traditionally seawater.

• The cold sea water is in contact with the inside of the tubes, the tube plates keeping all the tubes in position, and the water boxes (Plenums). The sea water flows through the tubes, while the liquid requiring heat removal or addition flows through the shell. A straight flow is shown in the diagram but two-pass flow is common in larger heat exchangers. The oil or water being cooled is in contact with the outside of the tubes.

2. Plate Heat Exchanger

• A diagram depicting the flow principle through a plate heat exchanger is shown below:

• Plate heat exchangers are made up from an assembly of identical metal pressings with horizontal or chevron pattern corrugations, each with a nitrile rubber joint. The plates, which are supported beneath and located at the top by parallel metal bars, are held together against an end plate by clamping bolts. Four branch pipes on the end plates, align with ports in the plates through which two fluids pass. Seals around the ports are so arranged that one fluid flows in alternate passages between plates and the second fluid in the intervening passages, usually in opposite directions (countercurrent). The plate corrugations promote turbulence in the flow of both fluids and so encourages efficient heat transfer. Turbulence, as opposed to smooth flow, causes more of the liquid passing between the plates to come into contact with them. It also breaks up the boundary layer of liquid which tends to adhere to the metal and act as a heat barrier when flow is slow.

Cocurrent vs. Countercurrent Flow

The diagrams below depict graphically the difference in input and output temperatures for both cocurrent (parallel) and countercurrent (contra) flows within heat exchangers.

The first diagram depicts cocurrent flow, where the temperature difference between the temperature of the inlets of the hot and cold fluids is initially large, decreases rapidly over a distance X (A to B). In this case, the outlet temperature of the cold fluid will never exceed that of the hot fluid.

The next diagram depicts countercurrent flow. In contrast to cocurrent flow this configuration provides for heat transfer between the hotter portions of the two fluids at one end, as well as between the colder portions at the other. Therefore, the change in temperature difference with respect to X (A to B) is nowhere as large as for cocurrent flow. It can be seen now that for countercurrernt flow the outlet temperature of the cold fluid may now actually exceed the outlet temperature of the hot fluid.

Thus, due to the log mean temperature difference, which is the average temperature difference, for countercurrent flow exceeding that of cocurrent flow, the surface area required to affect a prescribed heat transfer rate is smaller for countercurrent flow heat exchangers than for cocurrent heat exchangers

Systems Using Heat Exchangers

Heat exchangers can be found as part of the following systems on-board a ship:

• Main propulsion units make use of heat exchangers for the purposes of engine jacket water cooling and lube oil cooling.
• Electrical Generation uses heat exchangers for the purposes of diesel generator engine jacket water cooling and lube oil cooling.
• The aftercooler in the starting air compressed air system is a heat exchanger.
• The fuel injection of the main engines requires the pre-heating of fuel i.e. HFO. This heat is made possible by a heat exchanger.
• The evaporator as part of the refrigeration system (phase change), is a heat exchanger
• The evaporator as part of the Air Conditioning system is a heat exchanger (phase change, condenser)

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