Evaporator (marine)

One of the Belfast's evaporators. It is wrapped and also entirely hidden by insulating blankets.
This steam evaporator aboard HMS Belfast distilled up to six tons of fresh water per hour for the boiler and for drinking.

An evaporator, distiller or distilling apparatus is a piece of ship's equipment used to produce fresh drinking water from sea water by distillation. As fresh water is bulky, may spoil in storage, and is an essential supply for any long voyage, the ability to produce more fresh water in mid-ocean is important for any ship.