What are the two types of atomizers used with main propulsion boilers?

The two types of atomizers used with main propulsion boilers are mechanical and steam atomizers. Mechanical atomizers rely on mechanical means to create a fine mist of fuel, thereby enhancing combustion efficiency. They operate using mechanical energy to break up the liquid fuel into fine droplets. This approach helps to ensure that the fuel is well mixed with the air, which is crucial for effective combustion in the boiler.

Steam atomizers, on the other hand, utilize steam pressure to atomize the fuel. The steam creates a fine spray of fuel droplets, which further facilitates the mixing with the air for combustion. This method is often used because it provides better atomization than mechanical atomizers in certain conditions, leading to more efficient fuel consumption and better boiler performance.

The other answer choices do not accurately describe the atomizers used in main propulsion boilers. Electric and pneumatic atomizers are not standard classifications for the types used in this context, while the terms pressure and gravity atomizers or liquid and vapor atomizers do not reflect the specific technologies applied in marine propulsion systems. Hence, mechanical and steam atomizers are the correct and relevant types associated with boiler operation.