Page:The steam turbine, the Rede lecture 1911 (IA steamturbinerede00parsrich).pdf/28

16 EFFICIENCY of the latter rows 75 to 85 per cent., and considering the whole turbine, approximately 75 per cent, of the energy in the steam is delivered on to the shaft. The expansion curve of the steam lies between the adiabatic and isothermal curves, but nearer the former, because 75 per cent, is converted into work on the shaft and only 25 per cent, is lost by friction and eddies in the steam and therefore converted into heat.

In turbine design the expression of the velocity ratio between the steam and blades may be represented by the integral of the square of the velocity of each row through the turbine, and if, for instance, this integral is numerically equal to 150,000, a usual allowance for land turbines, then we know that, with a boiler pressure of 200 lbs.and a good vacuum, the velocity of the blades will be a little over one half that of the steam, and the turbine will be working close up to that speed which gives the maximum efficiency. In large marine turbines where weight and space are of importance the integral may be