91.  State advantages and disadvantages of the open cooling system?

Advantage

The system is simple and very cheap. The seawater is available.

Disadvantages

- The seawater contains many scale components, so it is easy to precipitate the scale on the cooling surfaces. Because of this, it reduces ability of the heat exchange therefore, the parts of the diesel engine cooled by the seawater are easily cracked.

- Because the seawater contains many scale and salt, it will cause erosion and electrolytic corrosion.

- The temperature of the sea water going out of the engine is very low (It is limited to about 45 ¸ 50o C) this lead to:

+ Thermal stress of the cylinders, cylinder covers and piston crowns are very high.

 + Waste energy for cooling water is high.

+ The operators cannot reuse the thermal energy of the cooling water.

- Effect of the cooling depends on the environment condition that the ship is operating.

95.       State feature of closed cooling system?

In the closed cooling system, both fresh and seawater are used.

The fresh water is pumped to cool the engine (such as cylinders, cylinder covers...) and other auxiliary machinery such as the air compressors... The fresh water going out of the engine at a high temperature returns to the suction pipe of the fresh water pump and then, it is cooled in the fresh water cooler before it is pumped to the engine again and that makes a closed circuit.

The seawater is taken into the ship to cool the fresh water, the lubricating oil, the supercharged air and other machinery and then is discharged overboard

The temperature of the fresh water can be regulated by a thermostatic control valve.

Steam generated in the system will go up to the expansion tank. The fresh water in the system will be replenished from the expansion tank to the suction pipe of the fresh water pump.

96.       State advantages and disadvantages of the closed cooling system?

Advantages

- Reduce waste energy for cooling water.

- Reduce rust, scale on the cooling surfaces of the engine.

- Due to high quality of the fresh water (in comparison with seawater), the engine can be operated with a high temperature of the cooling water (60 ¸ 85oC). This reduces thermal stress of the cylinders and piston crowns. In addition, the thermal energy of the cooling water can be reused so the efficiency of the propulsion plant will be increased.

- The cooling water temperature can be automatically regulated.

- Effect of the cooling mostly does not depend on the environment condition.

Disadvantages

- The system is complex and expensive.

100. State functions of compressed air system?

- To start and reverse the main engine.

- To start the diesel generators.

- To supply compressed air for the remote control system and other automatic control equipment.

- To supply compressed air for air whistle (air horn).

- To clean seachest.

- To clean and dry parts of machinery when repairing, tighten and loosen bolts and nuts...

101. State requirement of main compressor and emergency air compressor?

- There must be at least two air compressors in the system and one of them is driven by an electric motor. Besides these, an emergency air compressor driven by independent energy must be equipped and this air compressor can be started needless compressed air.

- Depending on pressure of air used in the system, the air compressors with one stage or multiple stages are equipped. The purpose of using the multiple-stage air compressors is to reduce compressed air temperature at the end of the compression stroke.

- To decrease the compressed air temperature to a suitable one (about 70 ¸ 80o C), compressed air coolers are used. The compressed air coolers can be installed inside or outside of the air compressors. If without cooling, the temperature of the compressed air can be raised to 200o C.

- Capacity of the air compressor must be designed to fully charge air reservoir from 5 KG/cm2 with charging time of less than one hour.

- The air compressors must be provided with drain valves on discharge side of both stages.

- Safety valves must be installed on both stages of the air compressors.

102. State requirements of safety valve and compressed air pipe?

Safety valve

- Besides the charge and supply valve, the air reservoirs must be provided with safety valve, pressure gauge, drain valve; a manhole for inspecting and repairing must also be installed. On some ships, the air reservoirs can be provided with pressure switch to control the start and stop of the air compressors.

- Hydraulic test must be carried out before using or after working period of the air reservoirs.

   Ptest=1.5Pmax

- Inside inspection      5 years/time

Compressed air pipe

- Unwelding high-pressure pipes and zinc galvanized are used in the system

- Air pressure in the system depends on different duties:

For starting engines: P = 18 ¸ 30 KG/cm2

For the control system: P = 9 - 11 KG/cm2

103. State requirements of air reservoir?

- There must be at least two main air reservoirs for the system.

- Total capacity of the main air reservoir must be enough to start continuously main engine without air charging the reservoirs as following condition:

- For direct reversing the main diesel engine, capacity of air reservoirs must be enough for at least twelve times of starting of the engine

Z=12C

106. State functions and all requirements of ballast system?

Its function is to increase stability for the ship; it ensures the balance for the ship in every condition.

The ballast system is used when the ship is sailing in ballast, loading of cargoes on the ship is not balanced, or in some special cases.