All About Engines

i68 All About Engines fall of pressure in the moving blades, reaction tur- bines are often referred to as pressure turbines. In the Parsons turbine the moving blades are fixed in grooves round a drum, and between each ring of these blades there is a fixed ring of blades attached to the turbine casing. The space between any two consecutive blades forms a nozzle, and the blades are so designed as to give these nozzles the desired shape. As the steam passes from end to end of the turbine the pressure falls to that of the condenser, and there is none left to convert into velocity. It will be almost at rest relatively to the turbine casing ; and as for every pressure there is a corresponding temperature it is clear that the turbine does work by abstracting heat from the steam with which it is supplied. The turbine is a heat engine just as Watt probably understood the ordinary reciprocating engine to be, and in its broad outline it is subject to the same laws. But instead of steam expanding in a cylinder behind a piston having only a straight line motion back- wards and forwards, necessitating cross head, con- necting rod, and crank, we have steam expanding between innumerable thin blades and applying an even, continuous turning motion to the shaft. More- over, there are no eccentrics, no sliding valves, no masses of metal to be alternately heated and cooled, and no narrow passages through which the steam has to escape after expansion. Since in the impulse turbine it is the velocity of the steam which is being used to produce motion,