All About Engines

i54 All About Engines without any change of velocity, and hence would exert no force upon them. If, however, the wheel moves at such a rate that the blades have half the speed of the jet, then the steam would leave the blades with no forward velocity relative to them. In other words, it would flow outside-ways, at right angles to the wheel. Thus, supposing the steam to have a velocity of 3,000 feet a second, and the blades were to move at 1,500 feet a second, then the force exerted on the wheel would be due to a velocity drop of 3,000 feet a second, for the blades are so shaped that the whole of the forward velocity would be destroyed at this speed. This is the theoretical con- dition of the highest efficiency of an impulse turbine. Now in order for the blades on an impulse tur- bine wheel, i foot in diameter, to have a velocity of 1,500 feet a second, it would need to make more than 475 revolutions a second, or 23,000 revolutions a minute. Such a velocity is not attained, mainly because no known materials will stand the enormous bursting action which would be set up by centri- fugal force in the wheel. Only by employing the very finest steel for the purposes combined with scientific design of the highest order was de Laval able to obtain running velocities of 1,100 to 1,200 feet per second, and this must be regarded as the high-water mark. The smallest turbine makes 30,000 revolutions a minute, but the wheel is only about 6 inches diameter, so that the speed of the rim is only 875 feet a second: the largest make 9,000 in the same time.