What is a blade? The blade being repeatedly mentioned in the articles on steam turbines, we shall define that term below.

The words "blade" and "vane" are known to be synonyms. Blades or vanes are curved metallic parts of the turbine whose function is to deflect or change the direction of a steam jet. There are both moving blades on which the work of the steam is done and guide blades which reverse the direction of the steam jet so that more work might be derived from it.

Types of Turbines. The three fundamental types of steam turbines are: 1) impulse, 2) reaction, 3) impulse-and-reaction.

The terms "impulse" and "reaction" have specific meanings in turbine engineering practice, these specific meanings greatly differing from those of the same words as they are used both in physics and in everyday life.

An impulsive force or "impulse" is the force produced on an object when a fluid jet strikes the object. That is how a turbine specialist would define the above-mentioned term.

A well-known characteristic of an impulsive force is that the fluid jet which strikes an object and thereby produces the force leaves the object at the same or at a less velocity than that with which it strikes the object. When the fluid stream strikes an object which is so shaped that it reverses the direction of the stream, a much greater impulsive force is produced than when the direction of the stream is not reversed. This occurs in spite of the fact that the stream may leave the object with the same velocity as that with which it approached the object.

An impulse turbine (see fig.) is such a turbine whose operation almost entirely depends on the impulsive force of a steam jet or jets striking the buckets of the turbine rotor. Hence, an impulse turbine is so designed that the expansion of the steam that passes through it and makes it work occurs almost entirely in its stationary nozzles. Practically no expansion of steam occurs in its moving blades. The steam jet from the stationary nozzles or blades strikes the rotor vanes and thus, causes the rotor to revolve by virtue of the "push" produced.

A reactive force or "reaction" is the force to be produced on an object when a fluid jet leaves the object at a greater velocity than that with which the object is approached. The above is a specific turbine engineering definition.

A reaction turbine (see fig.) is the one to depend principally on the reaction force of the steam jets as they leave the turbine's moving blades at greater velocities than those at which they approached the blades. Hence, a reaction turbine is so designed that about half of the steam passing through it and causing it to do work occurs in the moving blades and about half of it in the stationary guide vanes. The guide vanes and moving blades of a reaction turbine are designed in such a manner that the steam flows into the blades without striking them. That is possible provided the circumferential speed of the moving blades is the same as the velocity of the steam stream to enter them. The moving blades are so designed that the steam leaves them at a higher velocity than that at which it enters. Thus, the rotation of the rotor is produced by reaction.

The difference between an impulse and a reaction turbine is, therefore, that there is no appreciable expansion of steam in the moving blades of the first and considerable steam expansion in the moving blades of the second. Furthermore, it follows that in impulse turbines there is practically no difference between the pressure of the steam en­tering the moving blades and that of the steam leaving them. In reaction turbines there is a difference between these entering and leaving pressures.

An impulse-and-reасtiоn turbine is the one that has some of its blading designed and arranged as in an impulse turbine and some as in a reaction turbine.

A number of the largest turbines, now in use, are of this type.