Superheated steam …..what is this?
The concept of boiling water and harnessing the energy goes back to the 1st Century when Hiro built the first steam turbine engine. The principle is that the volume of water when boiled as it turns from liquid to gas expands at a ratio of 1600 to 1 aprox. dependent on atmospheric pressure. However as the gas or steam rises from the liquid state it takes with it water droplets, that not yet have converted to gas. This is saturated steam. Saturated steam albeit useful in heating and sterilization has problems in both piston and turbine engines. The liquid particles act like bullets breaking turbine vanes. The water droplets are also abrasive. In a steam piston temperature is very critical as when steam looses temperature it returns to a liquid state thus energy is wasted and not available to push the piston.
The process of superheating in the 1364 and other locomotive engines is done by gathering the steam dome and routing it back through the fire tubes to raise temperature (see illustration right) Interesting it take 1 BTU to raise one pound of water one degree. In heating steam it takes ½ BTU to heat the same mass (1 lb.) of steam one degree. Another factoid is that liquid water boils at aprox. 385F at 200 psi. To make it simple the same fuel that boils the water superheats the steam eliminating the water droplets and makes most use of the volume of steam.
Above illustration courtesy of Wikipedia.