Heat exchangers onboard vessels can be of the shell and tube type, plate type and fin type. Their type and model are adequate for particular systems and operating conditions taking into account all relevant ambient conditions that may affect their performance.

Shell and tube type heat exchangers are generally used onboard as heaters for fuel and oil and as coolers for steam system (steam dump condenser), refrigeration and air conditioning (condensers). The tube nest is located in a jacket tube and consists of inner tubes that are rolled or welded on the tube plate. The tubes are kept in position by spacers and the fluid is guided into the inner tubes via turnaround baffles from the jacket tube both ends. Depending on the baffles there are different type of fluid flows inside of the heat exchanger like: parallel flow, counter flow and cross flow.

Plate type heat exchangers are generally used as coolers for water cooling systems, lube oil coolers and as heaters for lube oil purifiers. They are characterized by a very compact structure and good heat transfer grading. By varying the number of plates, the performance can be adjusted and they have low filling capacity, low flow resistance and low thermal inertia. Fluids flow alternately through adjacent plate interspaces according with the counter flow principle and embossing on the plates provide good fluid turbulence and plate stability.

Fin type heat exchangers are generally used as evaporators for refrigeration and air conditioning and as coolers for engine’s scavenging air (air coolers) or as heaters (steering gear room or bow thruster room heaters). So, they are generally used for cooling or preheating air or gas. Fins are plugged or rolled into heat exchanger tubes and they are intended to increase the heat transfer surface through convection

There are few typical failures and damages of heat exchangers and generally are: leakages, fouling, cooler malfunction, overload etc.

Leakages are mainly common on shell and tubes heat exchangers as tubes tends to fail due corrosion, cavitation or vibrations. Plate type heat exchangers tends to leak due rubber seals failure, plates misalignment due bending or end plates corrosion. Fin type heat exchangers are generally leaking due corrosion. Is advisable that periodically, heat exchangers are to be tested hydraulically to 1.5 times their maximum working pressure in order to asses its condition.

Fouling is the most common and often type of failure and the possible causes can be: sedimentation of solid particles from the fluids, excessive surface high temperature in oil and fuel which lead to cocking especially on the high asphaltene content fuels or deposits due insufficient water treatment. Fouling can lead to increase flow resistance and implicitly to heat transfer reduction. For example, in case of main engine air coolers fouling of the air passages in an air cooler, which is part of the engine turbocharging system is usually due to oil and oily-water films collecting on the sides of the tubes and tube fins. Lint and similar material adheres to these films of oil or emulsion. The presence of oil may be caused by faulty air filters, or by improperly placed filters which allow the air to pass by the side of the filter element. Sometimes the oil is drawn from the bearing at the blower end.
The presence of moisture is usually the result of high humidity, when the engine is operating in warm air temperatures in conjunction with low sea water temperature. When air coolers become fouled, less heat will be transferred from the air to the cooling water. This is shown by changes in the air and cooling water temperatures. In the pressure drop of the air passing through the air cooler changes will also occur. The amount of change will depend on the degree and nature of the fouling.
The symptoms of air side fouling are, for examples: decrease of air temperature difference across cooler, increase of air pressure drop across cooler, rising scavenge temperature, rising exhaust temperature on all cylinders, a smaller increase in cooling water temperature across the cooler.
The symptoms of water side are, for example: rising scavenge temperature, reduction in the difference of the air temperature across the cooler, rising exhaust gas temperature from all cylinders, increase in the temperature rise of the cooling water if fouling or chocking materially reduces the amount of water flow.

When heat exchanger’s performance are degraded under certain circumstances, they can be cleaned onboard (chemically and/or mechanically). One of the most effective methods for descaling of hard deposit is ultrasonic or in case of chemical washing using a non acidic cleaner.

Shell tube heat exchanger can be deepened into chemical or in ultrasonic bath for cleaning (fuel and oil heaters).

Plate type coolers can be mechanically cleaned or through chemical circulation without disassembling it. Fin type coolers can be chemically cleaned through spraying and then rinsed with fresh water.

Cooler malfunction is often caused by seals failure due improper assembly or accidental damage during assembly. Uneven plates tightening and failure of following the instruction manual lead to plates bending and implicitly to seals failure. Fins bending and damage can lead to air flow obstruction. It is quote impossible to repair the plates onboard and usually this type of failure leads to replacement.

Before opening for maintenance the heat exchangers need to be drained in order to ensure that no large quantities of fluids are trapped inside.
When they are returned to service after maintenance and/or repair they need to be vented of entrapped air on each fluid side through the valves attached to them.
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