Page:Model steam turbines; how to design and build them (IA modelsteamturbin00harrrich).pdf/28

24 Assume the velocity is 1000 feet per second, and that one lb. of steam passes per second. The energy in foot pounds

= $Wv²/2g$ = $1×1000×1000/2×32⋅2$ = 15,500 foot pounds.

If the ratio of expansion is twice that in the first instance, the energy developed will be proportionately increased, but the energy required to double the velocity is equal to

$1×2000×2000/2×32⋅2$ = 62,000 foot pounds,

more than twice times the amount actually available, consequently the steam cannot get away, and accumulates until its pressure (and consequently its volume) is such that the available energy suffices to discharge the steam from the nozzle, and this point is reached, as before stated, when the throat pressure is 58 per cent, of the initial. This explains why a boiler does not instantaneously discharge the whole of the steam when the safety-valve lifts. We are now in a position to understand the function of the diverging part of the nozzle. By progressive enlargement of the nozzle area as the expansion of the steam extended, the as in the direction of flow. The increase of area of the jet along the nozzle as the volume of steam grows, makes the velocity required to discharge it at any given point the same as the velocity actually produced by the degree of expansion at that point. If the nozzle were convergent, only, the steam would