Scotch Fire Tube Boilers
Ships main propulsion engines through about 1900 were reciprocating engines operating at 200 psi or less. The typical boiler that provided steam to the engine were coal fired, fire tube Scotch marine boilers. In this design, hot combustion gasses flows through tubes surrounded by water. The disadvantage to using this type of boiler is that that there are large surface areas exposed to internal steam pressure. To balance this mechanical force, internal supports such as tie rods were used to counteract the force.
The museum ship USS Olympia (Spanish American War) has fire tube boilers and reciprocating steam engines. It may be viewed and toured at Penn Landing in Philadelphia.
Above - Sectional views of a fire tube, Scotch marine boiler.
Left - Steam icebreaker Stettin 1933 that used Scotch marine boilers, now a museum ship in Hamburg Germany.
Water Tube Boilers
Shift to Water Tube Boilers
Steam turbines were the typical steam engines after about 1900. In this design, water is circulated through tubes surrounded by hot combustion gasses. Turbines require dry steam. Water tube boilers are more easily designed to provide dry steam including designed with superheater section and even separately fired superheat sections to ensure the steam provided is dry as well as greater heat content. Moisture carryover in steam to turbines would result in erosion of the blades. Steam nozzles used in turbines accelerate steam to exceed sonic velocity for the steam medium it operates in. High velocity moisture impinging on blades causes the erosion. In water tube boilers, steam pressure is contained in small diameter tubes and steam/water “ drums” that are better able to withstand the pressure.
Superheated steam is steam heated to higher temperatures than the saturated steam it came from.
above - Schematic of a water tube boiler. Babcock and Wilcock boiler.
left - steam turbine