Editing Steam turbine (section)

===Blade and stage design===
[[File:Turbines impulse v reaction.svg|thumb|upright=1.25|Schematic diagram outlining the difference between an impulse and a 50% reaction turbine]]

Turbine blades are of two basic types, blades and [[nozzles]]. Blades move entirely due to the impact of steam on them and their profiles do not converge. This results in a steam velocity drop and essentially no pressure drop as steam moves through the blades. A turbine composed of blades alternating with fixed nozzles is called an [[Impulse turbines|impulse turbine]], {{visible anchor|Curtis turbine}}, [[Pressure compounding in turbines|Rateau turbine]], or [[#History|Brown-Curtis turbine]]. Nozzles appear similar to blades, but their profiles converge near the exit. This results in a steam pressure drop and velocity increase as steam moves through the nozzles. Nozzles move due to both the impact of steam on them and the reaction due to the high-velocity steam at the exit. A turbine composed of moving nozzles alternating with fixed nozzles is called a reaction turbine or [[Parsons Marine Steam Turbine Company|Parsons turbine]].

Except for low-power applications, turbine blades are arranged in multiple stages in series, called [[Compounding of steam turbines|compounding]], which greatly improves [[#Turbine efficiency|efficiency]] at low speeds.{{sfn|Parsons|1911|pp=7-8}} A reaction stage is a row of fixed nozzles followed by a row of moving nozzles. Multiple reaction stages divide the pressure drop between the steam inlet and exhaust into numerous small drops, resulting in a '''pressure-compounded''' turbine. Impulse stages may be either pressure-compounded, velocity-compounded, or pressure-velocity compounded. A pressure-compounded impulse stage is a row of fixed nozzles followed by a row of moving blades, with multiple stages for compounding. This is also known as a Rateau turbine, after its inventor. A '''velocity-compounded''' impulse stage (invented by Curtis and also called a "Curtis wheel") is a row of fixed nozzles followed by two or more rows of moving blades alternating with rows of fixed blades. This divides the velocity drop across the stage into several smaller drops.{{sfn|Parsons|1911|pp=20–22}} A series of velocity-compounded impulse stages is called a '''pressure-velocity compounded''' turbine.

[[File:AEG marine steam turbine (Rankin Kennedy, Modern Engines, Vol VI).jpg|thumb|left|upright=1.2|Diagram of an AEG marine steam turbine circa 1905]]

By 1905, when steam turbines were coming into use on fast ships (such as {{HMS|Dreadnought|1906|6}}) and in land-based power applications, it had been determined that it was desirable to use one or more Curtis wheels at the beginning of a multi-stage turbine (where the steam pressure is highest), followed by reaction stages. This was more efficient with high-pressure steam due to reduced leakage between the turbine rotor and the casing.{{sfn|Parsons|1911|pp=23–25}} This is illustrated in the drawing of the German 1905 [[AEG (German company)|AEG]] marine steam turbine. The steam from the [[boiler]]s enters from the right at high pressure through a [[throttle]], controlled manually by an operator (in this case a [[sailor]] known as the throttleman). It passes through five Curtis wheels and numerous reaction stages (the small blades at the edges of the two large rotors in the middle) before exiting at low pressure, almost certainly to a [[surface condenser|condenser]]. The condenser provides a vacuum that maximizes the energy extracted from the steam, and condenses the steam into [[boiler feedwater|feedwater]] to be returned to the boilers. On the left are several additional reaction stages (on two large rotors) that rotate the turbine in reverse for astern operation, with steam admitted by a separate throttle. Since ships are rarely operated in reverse, efficiency is not a priority in astern turbines, so only a few stages are used to save cost.