is the fact that water vapor containing entrained liquid droplets is generally incompressible at those pressures. In a reciprocating engine or turbine, if steam doing work cools to a temperature at which liquid droplets form, then the water droplets entrained in the fluid flow will strike the mechanical parts with enough force to bend, crack or fracture them. Superheating and pressure reduction through expansion ensures that the steam flow remains as a compressible gas throughout its passage through a turbine or an engine, preventing damage of the internal moving parts. Saturated steam Saturated steam is steam that is in equilibrium with heated water at the same pressure, i.e., it has not been heated above the boiling point for its pressure. This is in contrast to superheated steam, in which the steam (vapor) has been separated from the water droplets then additional heat has been added. These condensation droplets are a cause of damage to steam turbine blades, the reason why such turbines rely on a supply of dry, superheated steam. Dry steam is saturated steam that has been very slightly superheated. This is not sufficient to change its energy appreciably, but is a sufficient rise in temperature to avoid condensation problems, given the average loss in temperature across the steam supply circuit. Towards the end of the 19th century, when superheating was still a less-than-certain technology, such steam-drying gave the condensation-avoiding benefits of superheating without requiring the sophisticated boiler or lubrication techniques of full superheating. By contrast, water vapor that includes water droplets is described as wet steam. If wet steam is heated further, the droplets evaporate, and at a high enough temperature (which depends on the pressure) all of the water evaporates, the system is in vapor–liquid equilibrium, and it becomes saturated steam. Saturated steam is advantageous in heat transfer due to the high latent heat of vaporization. It is a very efficient mode of heat transfer. In layman's terms, saturated steam is at its dew point at the corresponding temperature and pressure. The typical latent heat of vaporization (or condensation) is for saturated steam at atmospheric pressure. Uses Steam engine Superheated steam was widely used in main line steam locomotives. Saturated steam has three main disadvantages in a steam engine: it contains small droplets of water which have to be periodically drained from the cylinders; being precisely at the boiling point of water for the boiler pressure in