A steam generator, in the sense of an auxiliary boiler on a ship, is a form of boiler

Boiler (steam generator)

A boiler or steam generator is a device used to create steam by applying heat energy to water. Although the definitions are somewhat flexible, it can be said that older steam generators were commonly termed boilers and worked at low to medium pressure but, at pressures above this, it is more...

. It uses high-pressure superheated steam

Superheated steam

Superheated steam is steam at a temperature higher than water's boiling point. If saturated steam is heated at constant pressure, its temperature will also remain constant as the steam quality increases towards 100% Dry Saturated Steam. Continued heat input will then generate superheated steam...

 from the main steam supply used for propulsion, and uses this to generate low pressure saturated steam

Saturated steam

In thermodynamics, the state of saturation of a fluid indicates that the fluid is at its boiling point temperature. This term can be applied in several ways:*Saturated liquid: fully in the liquid state but is about to vaporize....

. This is then used to power auxiliary steam engines around the ship such as winches or pumps, or to supply any steam requirement on the ship that does not require high superheating, such as boiler feedwater

Boiler feedwater

Boiler feedwater is water used to supply a boiler to generate steam or hot water. At thermal power stations the feedwater is usually stored, pre-heated and conditioned in a feedwater tank and forwarded into the boiler by a boiler feedwater pump....

 and freshwater evaporator

Evaporator (marine)

An evaporator, distiller or distilling apparatus is a piece of ship's equipment used to produce fresh drinking water from sea water by distillation...

s.

Low-pressure steam generators

A typical generator produces around 10,000 lb/hour at around 120 psi, using inlet steam supplied at 600 psi. They are constructed as a cylindrical steel pressure vessel, either horizontal or vertical, containing a series of heating coils. There is a strong resemblance to the design of evaporators, although steam generators can be rather simpler as they use treated feedwater, rather than scale-producing seawater.