WATER WHEELS AND TURBINES — 498 
A loss which is inevitable is the kinetic energy in the water as it 
leaves the wheel, and this amounts to es per second. 
428. Classification of Turbines.—A turbine consists of two main 
parts, the wheel or runner, and the stationary guides. The wheel consists 
of two plates or rings called crowns, between which lie numerous vanes. 
The guides direct the water on to the vanes of the wheel. 
Turbines may be divided, according to the manner in which the 
water acts on the moving vanes, into two classes, namely, impulse 
turbines and reaction turbines. In impulse turbines the water does not 
fill the passages between the wheel vanes, and there being free access of 
air to these passages, the velocity of the water as it enters them is that 
due to the head. Also the energy of the water as it enters the wheel is 
entirely kinetic. In reaction turbines the water completely fills the 
passages between the guides and between the wheel vanes, and the 
velocity of the water at the entrance to the wheel may be greater or less 
than that due to the head there. Also the energy of the water as it 
enters the wheel is partly kinetic and partly pressure energy. 
An impulse turbine must discharge into the atmosphere, and 
must therefore be clear of the tail race, but a reaction turbine may be 
completely immersed or drowned. 
Another classification of turbines is according to the direction in 
which the water flows through the wheel. This leads to four classes. 
(1) Outward flow turbines, in which the direction of flow is radial and 
outwards. (2) Inward flow turbines, in which the direction of flow is 
radial and inwards. (3) Parallel flow or axial flow turbines, in which 
the direction of flow is parallel to the axis of the wheel. (4) Mixed jlow 
turbines, in which the direction of flow is 
partly radial and partly axial, changing 
from one to the other on the vanes inside 
the wheel. 
The various types are also frequently 
referred to by the names of the engineers 
who were identified with their introduction 
or improvement, as the Fourneyron turbine 
(radial outward flow), the Francis turbine 
(radial inward flow), and the Jonval turbine 
(parallel flow). Mixed flow turbines,. in 
which the water enters in a radial inward 
direction and leaves in an axial direction, Fig. 792. 
are largely used in America, and this type 
is often called the American turbine. Fig. 792 shows the wheel or 
runner of the Victor (American) turbine. 
The Girard turbines are impulse turbines, and they may have either 
radial or axial flow. 
429. Formule for Reaction Turbines.—The notation to be used in 
this Article is partly shown on Figs. 793, 794, and 795, which represent 
outward flow, inward flow, and parallel flow turbines respectively, and 
is the same as was used for the impulse wheel, Art. 426, p. 490. 
Values of the Angles.—The angles 0, and ¢, are assumed in designing 
a turbine. 6, varies from 10° to 25° in inward flow turbines, and from 
