In previous chapters we discussed the basic steam cycle and various types of naval boilers. At this point, we will bring together all you have learned by discussing the components inside the turbine casing.

In the following paragraphs we will discuss turbine theory, types and classifications of turbines, and turbine construction.

Upon completion of this chapter you will understand how stored energy (heat) in steam is transformed to mechanical energy (work).

TURBINE THEORY

The first documented use of steam power is credited to a Greek mathematician, Hero of Alexandria, almost 2000 years ago. Hero built the first steam-powered engine. His turbine design was the forerunner of the jet engine and demonstrated that steam power could be used to operate other machinery. Hero's turbine (aeolipile) (fig 5-1) consists of a hollow sphere and four canted nozzles. The sphere rotates freely on two feed tubes that carry steam from the boiler. Steam generated in the boiler passes through the feed tubes, into the sphere, and out through the nozzles. As the steam leaves the nozzles, the sphere rotates rapidly.

Down through the ages, the application of the turbine principle has been used in many different types of machines. The water wheel that was used to operate the flour mills in colonial times and the common windmill used to pump water are examples of the turbine principle. In these examples, the power comes from the effect of the wind or a stream of water acting on a set of blades. In a steam

Figure 5-1.-Hero's turbine (aeolipile).

turbine, steam serves the same purpose as the wind or the flowing water.

Two methods are used in turbine design and construction to get the desired results from a turbine. These are the impulse principle and the reaction principle. Both methods convert the thermal energy stored in the steam into useful work, but they differ somewhat in the way they do it. In the following paragraphs we will discuss the two basic turbine principles, the impulse and reaction.

Figure 5-2.-Impulse turbine.

Figure 5-3.-Simple impulse turbine principle.