- A double evaporation boiler uses two independent systems for steam generation and therefore avoids any contamination between the primary and secondary feedwater.
- The primary circuit is in effect a conventional watertube boiler which provides steam to the heating coils of a steam-to-steam generator, which is the secondary system.
- The main reason for the adoption of this design of boilers is to allow use of modern high efficiency watertube boilers witghout fear of damage through contamination by cargo or fuel oils.The basic design consists of a D-Type boiler design upon which is mounted a Steam/Steam generator drum.
- The steam generated by the main boiler heats water in the Steam/Steam generator which produces steam requirements.
- The primary drum is initially filled with high quality feed water and suitably dosed.
- The main reasonfor the adoption of double evaporation boilers in tankers are concerned of damage caused by oil and sulphur which enters the feed systems through leaky steam heating coils used for heating the oil in cargo tanks.
1. High cost: Manganese bronze is a costly alloy, making it less economical for large propellers. 2. Low strength-to-weight ratio: Compared to other propeller materials like nickel-aluminum bronze or stainless steel, manganese bronze has a lower strength-to-weight ratio. 3. Susceptible to corrosion: Manganese bronze can corrode in seawater, especially when exposed to high velocities and turbulence. 4. Poor cavitation resistance: Manganese bronze is more prone to cavitation damage than other materials. 5. Difficult to cast and machine: Manganese bronze is challenging to cast and machine, making it less desirable for complex propeller geometries. 6. Limited weldability: Manganese bronze has poor weldability, making repairs and modifications difficult. Nickel-aluminum bronze or stainless steel are commonly used for propellers due to their: - High strength and durability - Excellent corrosion resistance - Good cavitation resistance - Ease of casting and machining - Weldability

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