Ljungstrom Compound Reaction Steam Turbine.—With few exceptions, modern turbines are built as axial-flow turbines with horizontal shafts. In the Ljungstrom turbine, a construc tional peculiarity is that the steam is made to flow radially out wards. In addition, instead of having alternate rows of fixed and moving blades, the guide nozzles are themselves allowed to rotate, but in the opposite direction, so that it is the same in effect as if the casing of a Parsons turbine were allowed to revolve in a direc tion opposite to that of the shaft or rotor. The blades are attached in concentric rings to two discs, mounted on shafts with a com mon horizontal axis, revolving in opposite directions, and driving separate dynamos.
Differences Between Impulse and Reaction Turbines.— The names "impulse" and "reaction" turbines mentioned above, are more popular than scientific, and it is important to possess a clear idea of the fundamental constructional difference in design which places a turbine in one or other category.
A comparison of the longitudinal section of a modern axial-flow horizontal impulse turbine (fig. 5) with that of a similar modern reaction turbine (fig. 7) shows that speaking generally the former is characterized by the presence of definite compartments or cells in which each row of blades revolves. while in the latter the com partments are virtually non-existent. This difference, however, is a matter of expediency for the particular type of blading adopted, and is not a matter of fundamental design. There is no theoretical reason why impulse blading should not be arranged on a barrel like the reaction blading in fig. 7, or why reaction blading should not be arranged with diaphragms and wheels like the impulse blading in fig. 5. This is an important point to be borne in mind.
It is not enough merely to say : "this turbine has nozzles in diaphragms followed by a row of buckets, therefore it is an im pulse turbine," and "that turbine has no diaphragms but has nozzles in both fixed and moving elements, therefore it is a re ' action turbine." The further question arises : "What are the essential character istics of nozzles, and in what way do buck ets or vanes or blades differ from them?" The fundamental constructional differ ence lies in the shape of the steam pas sages or paths between the blades or vanes which are used. The nozzles used in steam
turbines to produce the propelling steam jets are always the same in principle, al though they may differ considerably in their proportions. There is, that is to say, always a narrowing or "convergent" passage connecting a region of higher steam pressure with a region of lower steam pressure, the change in cross section of the passage (fig. 'la) being made gradual in order to obtain high efficiency of conversion of pressure energy into kinetic or velocity energy.
The nozzle may have a divergent extension or "mouthpiece" as in fig. lib and in turbine design has to be "skewed" at an angle to the plane of the wheel, as in figs. 12a and i 2b, but in every case the primary intent is to create a drop in pressure over the two sides of the boundary plate containing the nozzles, and to utilize that pressure drop to produce a steam jet. Having obtained the requisite steam jets, however, the buckets or blades on the rotating wheels are made with the fundamental distinction corresponding to "impulse" or "reaction" design respectively. If the blade-passages of the rotating wheels are "parallel" or nearly parallel (i.e., non convergent) as in fig. 13 the blading is of the "impulse" type, i.e., nozzles followed by buckets. If they are passages of diminishing cross-sectional area (i.e., convergent) the blading is of the "reac tion" type, fig. 8.
Velocity Diagrams.—As a further aid to an understanding of these fundamental differences, study may be made of a useful graphic representation of the velocity and trajectory of the steam jets in their pas sage through the nozzles and blades.
Figure 14a is what is known as a "ve locity diagram" for impulse blading, in which "U," the mean circumferential ve locity with which the blades are moving in front of the nozzles, is represented by BA. OA is the velocity "C," of the steam jet issuing from the nozzles. It will be seen that the blades are running away from the steam jets, so that if it were possible for an observer to be stationed at B (i.e., going round with the blades) he would see the steam jets apparently coming in the direction OB. OB is, in fact, the relative velocity of approach with which the steam impinges on the blades, and the blade inlet angle at B would be made suitable to receive the steam approach ing the blades in this direction.