Heat exchangers on board ships are mainly coolers where a hot liquid is cooled by seawater. There are some instances where liquid heating is required, such as heavy fuel oil and seawater heaters for tank cleaning.
Although being heat exchangers, the main condenser for a steamship and the evaporator/distiller are dealt with separately The heat exchange process is accomplished by having the two liquids pass on either side of a conducting surface. The heat from the hot liquid passes to the cold liquid and the conducting surface,
i.e. the tube wall, is at a temperature between the two. It is usual for marine heat exchangers to have the two liquids flowing in opposite directions, i.e. counter or contraflow. This arrangement provides a fairly constant temperature difference between the two liquids and therefore the maximum heat transfer for the available surface area.
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Coolers
Coolers at sea fall into two groups, shell and tube and the plate type. Both are considered below.
Shell and tube
In the shell and tube design, a tube bundle or stack is fitted into a shell. The end plates are sealed at either end of the shell andprovision is made at one end for expansion. The tubes are sealed into the tube plate at either end and provide a passageway for the cooling liquid. Headers or water boxes surround the tube plates and enclose the shell. They are arranged for either a single pass or a double pass of cooling liquid.
The tube bundle has baffles fitted which serve to direct the liquid to be cooled up and down over the tubes as it passes along the cooler. The joint arrangements at the tube plate ends are different. At the fixed end, gaskets are fitted between either side of the tube plate and the shell and end cover.
At the other end, the tube plate is free to move with seals fitted on either side of a safety expansion ring. Should either liquid leak past the seal it will pass out of the cooler and be visible. There will be no intermixing or contamination.
Plate type
The plate-type heat exchanger is made up of some pressed plates surrounded by seals and held together in a frame. The inlet and outlet branches for each liquid are attached to one end plate. The seals between the plates provide passageways between adjacent plates for the cooling and hot liquid.
The plates have various designs of corrugations to aid heat transfer and provide support for the large, flat surface. A double seal arrangement is provided at each branch point with a drain hole to detect leakage and prevent intermixing or contamination.
Operation
Temperature control of coolers is usually achieved by adjusting the cooling liquid outlet valve. The inlet valve is left open and this ensures a constant pressure within the cooler. This is particularly important with seawater cooling where reducing pressure could lead to aeration or air collecting within the cooler.
Air remaining in a cooler will considerably reduce the cooling effect. Vents are provided in the highest points of coolers which should be opened on first filling and occasionally afterwards. Vertical mounting of single-pass coolers will ensure automatic venting.
Positioning the inlet cooling water branch facing downwards and the outlet branch upwards will achieve automatic venting with horizontally mounted coolers. Drain plugs are also fitted at the lowest point in coolers.
Maintenance
Clean heat transfer surfaces are the main requirements for satisfactory operation. With seawater cooling the main problem is fouling of the surfaces, i.e. the presence of marine plant and animal growth.
With shell and tube coolers the end covers are removed to give access to the tubes for cleaning. Special tools are usually provided by the cooler manufacturer for cleaning the tubes. The end covers can also be cleaned. Tube leakage can result from corrosion. This can be checked for, or identified, by having the shell side of the cooler circulated while the cooling water is shut off and the end covers removed.
Any seepage into the tubes will indicate the leak. It is also possible to introduce fluorescent dyes into the shell-side liquid: any seepage will show under ultraviolet light as a bright green glow. Leaking tubes can be temporarily plugged at each end or removed and replaced with a new tube.
Heaters
Heaters, such as those used for heavy oil, are shell and tube-type units, similar in construction to coolers. The heating medium in most cases is condensing steam.
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