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Knowledge of Marine diesel engine cooling system.

Date:2018-03-02 Author: Click:

In the diesel engine, the heat released by fuel is about 30% ~ 33%, which is dispersed to the outside by the cylinder, cylinder head and piston.
In order to dissipate the heat, a sufficient number of cooling media are required to force the continuous flow through the heating element, which ensures the normal stable temperature of the heating parts by cooling.
Therefore, the cooling system is set up in most diesel engines to ensure sufficient and continuous cooling medium flow and proper cooling medium temperature.
    

The function and manner of cooling.

From the point of view of energy utilization, the cooling of diesel engine is an energy loss which should be avoided, but it is necessary to ensure the normal operation of diesel engine.
Diesel engine cooling has the following effects: first, cooling can keep the working temperature of heated material does not exceed the allowed limit, which can guarantee enough strength in the condition of high temperature heating components;
Secondly, the cooling can ensure the proper temperature difference between the internal and external walls of the heating parts, and reduce the thermal stress of the heating parts.
In addition, the cooling can also guarantee the normal working status of the moving parts such as the proper clearance between the piston and the cylinder liner and the lubricating film of the working face of the cylinder wall.
The cooling effect is achieved by cooling the system.
In management, the two requirements of diesel engine cooling should be taken into account, which will not cause the diesel engine to be too cold and the diesel engine will not overheat due to lack of cooling.
In modern times, from reducing cooling loss as far as possible in order to make full use of the combustion energy, adiabatic engine research is being carried out in both at home and abroad, the corresponding development of a batch of high temperature resistant material, such as ceramic material, etc.
At present, the cooling mode of diesel engine is divided into forced liquid cooling and air cooling.
Most diesel engines use the former.
 

2. Cooling medium.

In the diesel engine forced liquid cooling system, the coolant usually has three kinds, such as fresh water, sea water and lubricating oil.
The water quality of fresh water is stable, the heat transfer effect is good and the water treatment can be used to solve the defects of corrosion and scaling. Therefore, it is an ideal cooling medium widely used at present.
Diesel engine's requirements for fresh water quality are generally free of impurity fresh water or distilled water.
If the total hardness of fresh water is not more than 10(German degree), pH value of 6.5 ~ 8, and chloride content is not more than 50 x 10-6.
When using distilled water or ion exchanger totally deionized water as cooling water, must pay special attention to water treatment was carried out on the water, and regularly test, to ensure that the concentration of the water treatment agent reach the prescribed scope.
Otherwise, the corrosion caused by insufficient concentration is more serious than the use of ordinary hard water (due to the lack of ordinary hard water).
The water quality of the water are difficult to control and its corrosion and fouling problem more outstanding, to reduce the corrosion and fouling should limit the outlet temperature of the water should not be more than 45 ℃, so now rarely used cooling seawater directly on diesel engine;
Small lubricating oil of specific heat, heat transfer effect is poorer, high temperature condition is easy to produce coking in the cooling chamber, but it does not exist because of leakage and pollution risk of crankcase oil, thus suitable for piston cooling medium.
 

The composition and equipment of the cooling system.

The general diesel engine cooling system USES fresh water to force the cooling of the diesel engine, and then the seawater is forced to cool the fresh water and other hot liquids (such as lubricating oil, pressurized air, etc.).
In the system layout, the former is closed loop, the latter is an open system.
The cooling system of the two is called the closed cooling system.
Due to the different working conditions of the closed fresh water cooling system, the temperature, pressure and basic composition of the cooling liquid are different.
Thus, the cooling system of each heating element is usually composed of several separate systems.
Generally divided into cylinder liner and cylinder head, piston, injector three closed fresh water cooling system.
 

4. Maintenance and management of cooling system.


First, adjust the working parameters of the cooling system and ensure normal:

(1) the outlet pressure of the weak pump should be adjusted to the normal working range.
Generally, fresh water pressure should be higher than seawater pressure to prevent the water from leaking into fresh water when the cold is leaking, causing it to deteriorate.
  

(2) the fresh water temperature shall be adjusted to the normal working range according to the specifications.
Don't make fresh water outlet temperature is too low (made into heat loss was increased, the thermal stress, low temperature corrosion) or high (the cylinder wall lubricating oil film evaporation, cylinder wall wear, cooling cavity vaporize, cylinder liner seal rapidly aging).
For the high speed diesel engine, average exit temperature can be controlled at 70 ℃ ~ 80 ℃ (not burning sulfur crude oil), low speed machine can be controlled in 60 ℃ ~ 70 ℃;
Import and export temperature difference is not more than 12 ℃.
General fresh water outlet temperature is close to allowable limit.
 

(3) the water outlet temperature should not exceed 50 ℃, in order to avoid salt precipitation and sediment into scale, affect the heat transfer.
 

(4) in the operation, adjust the water temperature water bypass valve in the line can be used to regulate the amount of sea water into fresh water cooler, or use fresh water bypass valve in the line to adjust into fresh water cooler fresh water or seawater temperature adjustment.
Modern newly-built ships are equipped with automatic temperature adjusting devices for fresh water and lubricating oil, and their regulating valves are installed in the pipeline of fresh water and lubricating oil, to control the moisture and lubricating oil of the inlet cooler.
 

(5) check the flow of cooling water in each cylinder. If the cooling water flow needs to be adjusted, the outlet valve of the fresh water pump should be adjusted, and the adjustment speed should be as slow as possible.
The inlet valve of the fresh water pump should always be in full open position.
     

(6) when the pressure fluctuation of the cylinder cooling water is found to be ineffective, it is usually caused by the presence of gas in the system, and the cause should be identified and eliminated as soon as possible.
Secondly, the water level of expansion tank and fresh water circulation tank is checked regularly.
If the water level is reduced too fast, it should be quickly identified and eliminated.
When preparing the vehicle, the fresh water pump should be used to drive the cooling system, and the fresh water should be circulated within the system for 15min ~ 30min.
When needed, but also for warming cylinder at the same time, the water temperature of 45 ℃ or so.
The warm cylinder is good for the internal combustion of the cylinder, easy to start, and can make the lubricating oil evenly spread to prevent serious wear of the cylinder liner, and can reduce the thermal stress of the cylinder wall.
In order to control the seawater system to ensure that the temperature of fresh water does not fluctuate excessively, the water pump or controller flow should be turned off in advance.
After parking, the cooling water should be allowed to circulate for 20min ~ 30min in the system, so that the temperature of the cylinder will gradually decrease, so as to prevent the evaporation or carbon deposition of the oil film on the cylinder wall surface, and reduce the thermal stress.
Also pay attention to check whether the seawater filter and seawater valve are blocked by debris.
While sailing in the cold region, we should strengthen the management of the water piping system, prevent the bottom valve was ice card to death, and shall ensure that the water into the cooler temperature (25 ℃).
Should be regularly (preferably once a week) to check quality of cooling water, detecting the concentration of the water treatment additives (such as corrosion inhibitor) should be within the scope prescribed in the prospectus, pH (at 20 ℃ should be 7 ~ 10) and chloride concentration (not more than 50 parts per million (PPM).
Through the changes of these indicators, the working state of the cooling system can be judged.
If the concentration of chloride increases, it indicates that there is water leakage.
A lower pH indicates an exhaust leakage.


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