Exam Question

### (a) Shell and Tube Type Fresh Water Generator (FWG)
#### Principle of Operation
The shell and tube type fresh water generator, often referred to as a submerged tube evaporator, operates on the fundamental principle that the boiling point of a liquid is dependent on the pressure exerted upon it. By creating a high vacuum (approximately 92-95%) inside a sealed chamber (the evaporator shell), the boiling point of seawater is significantly lowered from 100°C at atmospheric pressure to around 40-50°C. This allows a low-grade heat source, typically the main engine's jacket cooling water (circulating at 70-90°C), to be used as the heating medium to evaporate the seawater without causing the formation of hard scale (e.g., calcium sulphate) which precipitates at higher temperatures.
#### Construction