Fourth: The piston pressure of each cylinder.
It will be seen that an engine so arranged is able to get substantially the same pressure in each of the second, third and fourth cylinders, as in the first (see [Fig. 16a]), and by condensing the discharge from the fourth cylinder a most economical use of steam is provided for. The Steam Turbine.—We must now consider an entirely new use of steam as a motive power. Heretofore we have been considering steam as a matter of pressure only, in the development of power. It has been observed that when the pressure of steam decreases at the same temperature it is because it has a greater volume, or a greater volume results.
When steam issues from the end of a pipe its velocity depends on its pressure. The higher the pressure the greater its velocity. The elastic character of steam is shown by its action when ejected from the end of a pipe, by the gradually enlarging area of the discharging column.
In a reciprocating engine the power is derived from the pressure of the steam; in a turbine the power results from the impact force of the steam jet. Such being the case velocity in the movement of the steam is of first importance.
Pressure and Velocity.—To show the effectiveness of velocity, as compared with pressure, examine [Fig. 17]. A is a pipe discharging steam at a pressure of 100 pounds. To hold the steam in the pipe would require a pressure of 100 pounds against the disk B, when held at 1, the first position.
Suppose, now, the disk is moved away from the end of the pipe to position 2. The steam, in issuing forth, strikes the disk over a larger area, and in escaping it expands, with the result that its velocity from 1 to 2 is greater than the movement of the steam within the pipe that same distance.