The turbocharger uses the exhaust gas from the engine to compress new air and charge the engine with a positive pressure higher than ambient conditions.
Exhaust gas temperature convection and compression raises the temperature of the air and this cannot be delivered directly into the engine due to operational limitations exceeding.
As a result, the engine is equipped with a cooler that returns the air temperature to ambient relatively close levels.
Because hot air has a lower density, the mass of air charged into the engine is smaller than when the air is cold.
Thus, the charge air cooler increases the density and lowers the temperature of the charge air.
The compressed charged air exiting the charge air cooler will have a temperature of around 40 to 50 degrees Celsius, down from approximately 150 degrees Celsius.
At cold temperatures, the lower temperature of the air increases the density of the charge air.
Increased charge air density increases scavenging efficiency and allows for more air to be compressed inside the engine cylinder, allowing for more fuel to be burnt inside the combustion chamber, resulting in increased power.
Additionally, the engine is kept at a safe operating temperature. Compression temperature reduction alleviates stress on the piston, piston rings, cylinder liner, and cylinder head.
Additionally, the charge air cooler lowers the exhaust gas temperature.
It has been demonstrated that a one-degree Celsius decrease in scavenging air temperature results in a five- to ten-degree Celsius decrease in exhaust temperature.
This does not mean that cryogenic temperatures may be used to charge the air.
If very cold air reaches the cylinder liner, a severe thermal stress occurs, resulting in cylinder liner failure.
Charge air coolers are installed between the turbocharger compressor outlet and the engine intake or scavenging manifold.
The figure below illustrates the precise position of a charge air cooler.
When the air cooler become fouled, the heat transfer between air and cooling agent, which is normally fresh water, is decreasing and this is indicated by the raise in air temperature after cooler and a raise in the pressure drop across the cooler (U-tube manometer).
Fouling of the air passages in air cooler, which is part of the engine turbocharging system, is usually due to oil film and oily-water films collected 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 which allow the air to pass by the side of the filter element.
Sometimes the oil is drawn from 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.
So, the signs of air coolers on air side are: higher pressure drop across the cooler, higher air temperature after the cooler, higher cooling water outlet temperature, higher underpiston scavenging air temperature, higher exhaust temperature on all cylinders and in worst case scenario turbocharger surging.
If the fouling is on the water side of the cooler the signs are: higher air temperature after the cooler, higher cooling water outlet temperature, higher underpiston scavenging air temperature and higher exhaust temperature on all cylinders.
As a remedy, fouling can be prevented through proper plan maintenance by cleaning the coolers at regular intervals.
The air side of the cooler can be water washed and/or chemically cleaned. On most of the 2 stroke main engines there is a possibility of periodically cleaning the air coolers even during engine operation at low load. On some of the vessel there is designated tank available where a mixture of water and Air Cooler Cleaner chemical are mixed and recirculated through the air cooler for a certain period of time. It is very important to rinse with fresh water the air cooler very well after cleaning and blow with air if possible in order to prevent chemical corrosion of the cooler parts.
For the water side a soft brush can be used to clean inside tubes and in case of hard deposits a long drill bit can be used, but with caution in order not to damage the tubes.
However, if there is no improvement after cleaning then the cooler needs to be removed from the engine and dipped into a chemical bath for several hours.
Usually this procedure, especially for main engines are performed during every vessel special survey (dry docking), when the coolers are dismantled and taken into yard facilities for proper cleaning into chemical ultrasonic bath.
In conclusion, keeping the air coolers clean is very important and must be part of the vessel plan maintenance. The engineers must be familiar with monitoring of coolers performance, manufacturer’s maintenance manual, cleaning procedure and assembling/disassembling procedures. Failure to do so can lead to engine breakdowns, expensive repairs and vessel stoppage which incurs high operational costs for the ship owner.
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