Please dont refer to the old books. There they have shown the diagram of system which was used in 90s . I have sailed on 2013 built ship and all modern ship nowadays uses this type of system which i will show you in my blog with practical explanation.
The main engine during running produces lots of heat,this heat is tranferred to the jacket water . In my ship jacket water was used to cool cylinder liner ,bore cooling for cylinder cover , exhaust valve and in some engine fuel injector and turbocharger. But nowadays the turbocharger and fuel injector are designed in such a way that there is no need of coolant .
The cooling water enters at the lower end of the cylinder jacket at a temperature of around 74 degree celcius,passing up to connection from top of the jacket to the cylinder cover and then to the exhaust valve cages and seats. In some engine restricted amount of water is used for cooling turbocharger cooling spaces.
As you can see in diagram the heated jacket water at a temperature of 80 to 82 degree celcius comes out of the engine, there is an auto vent valve which release the air from the high temperature water. If the main engine is not running due to in port stay or anchor ,the jacket water is preheated . The preheating is done because of 2 reason
1. It elimates the condenstion of water vapour in the products of combustion because at low temperature dew is formed in liner and after it react with sox of combustion it will form acid causing corrosion wear in liner
2. At low cooling water temperature the liner wall will be cold and now as the engine is started the cylinder oil is injected into the liner and As the liner is cold the cylinder oil will not be heated and its viscocity will be increased and thus difficulty is starting the engine .
For preheating we have ME JW preheater pump and JW preheater . Steam is passed in ME JW preheater to heat the jacket water . During at port stay or anchor, preheater pump is running , JC FW pump is stooped , JW cooler and FWG is bypassed .
When the ship is at sea-sailing , the preheater is bypassed. The high jacket water temperature at 80 degree passes the fresh water generator ,where most of its heat is utilized in heating the s.w in heater of FWG.
The temp inlet to FWG is controlled by controller to about 82 degree . After utilizing the heat the outlet temp of FWG is almost 78degree celcius . After that it goes to the jacket water cooler where some more heat is dropped here . The outlet of the cooler is automatically control to about 75degree.
If suppose FWG is not running then all jacket water will go to the cooler i.e no bypass from cooler . If FWG is in use then it is already gave up some heat to FWG so some amout of water is bypassed from cooler because no need of cooling any furthur in cooler as it is already got cooled in FWG.
Now after cooler it goes into deaerator tank where some air +water goes into the HT expansion tank ,which is located at top platform that is in boiler platform. Now after removing air and expanded in expansion tank the jacket cooling water goes to the lower side of deaerator.
From the deaerator the jcw goes to the suction side of pump.
The HT expansion tank has many purpose such as:
1. The temperaure of water is very high and there may be chance of air lock in the system . If the air is not released from the system then the HT pump or JCFW pump will draw air from the system and failure and the main engine will shut down automatically .
2. Provides a low pressure point for the addition of make up cooling water . If suppose before the deaerator the water is blocked or leaked completely ,so the pump will run dry . so it can act as a makeup of cooling water and the supply of water to the pump and the engine doesnt stop.
3. Allow in increase in water volume as the engine warms up and air is released from the system .
4. Is designed to maintain a constant head in the system so that the pump doesn't lose suction.
Now after cooling in FWG and JCW cooler the temperature drops to 74degree . The coolant jcw now enters at the lower end of cylinder jacket and this process continues.
The cooling media in jacket water cooler is from the LT cooling FW system. One line from LT cooling FW system is taken for jcw cooler of ME.
Check my blog for LT COOLING FW system on ship.
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