1906.] on The Steam Turbine on Land and at Sea. 329 



Reviewing more closely the motion of the steam through the 

 blades of a compound turbine, we see that the portion of its course 

 during which it is travelling at relatively high velocity, and in close 

 proximity to the blades, is short in comparision with the total length 

 of its travel within the turbine. The passage-ways between the 

 blades constitute virtually jets of rectangular cross section, but having 

 easy curves, and the frictional losses are consequently small. After 

 leaving the blades, it traverses the intervening space in the form of 

 an annular cylinder with a spiral motion, the angle of pitch being 

 about oO^ to a plane normal to the axis ; and, as the succeeding blades 

 are moving in a similar direction to this flow, we see that the velocity 

 with which the steam is cut by their frontal edges is much less — in 

 fact, less than one half the velocity at which the steam has issued 

 from the previous blades. From this we see how small is the loss 

 due to the cutting of the steam by the frontal edges in the compound 

 turbine, and also how small is the velocity with which drops of water 

 strike the metal of the blades. 



This is an important feature. 



It has been shown by experiment that if drops of pure water, 

 arising from the condensation of expanding steam, impinge on brass 

 at a greater velocity than about 500 feet per second there results a 

 slow wearing away of the metal. It is very slow, and would require 

 about ten years to erode the surface to a depth of ^^ inch. In the 

 compound turbine the striking- velocity is much below this figure, 

 and the preservation of their form and smoothness of surface has 

 been found to be practically indefinite. 



It appears that the erosive power of drops of pure water moving 

 at high velocity increases rapidly with the velocity, it may probably 

 be as the square. Experiment has shown that if saturated steam at 

 100 lb. pressure be allowed to flow through a divergent jet into a 

 good vacuum, attaining a velocity of about 4500 feet per second, and 

 allowed to impinge on a stationary brass blade, the blade will be cut 

 through in a few hours, and the hardest steel will be slowly eroded. 

 The action seems to be the result of the intense local pressure from 

 the bombardment of the drops, which may exceed 100 tons. 



Owing to the receding velocity of the blades from the blast, and 

 consequently reduced striking velocity, the erosion of the blades in 

 impact turbines is much reduced, and in compound turbines there is 

 complete immunity from such erosion. 



It may be asked, how is it that the steam turbine in the larger 

 sizes is niore economical in steam per horse-power developed than the 

 best triple or quadruple expansion reciprocating engine ? The reason 

 is, that all large steam turbines are able to take full advantage of the 

 whole expansive energy of the steam, even when expanding to the 

 very attenuated vapour densities produced by the best condensers. 

 It is indeed easy to construct the low-pressure portion of the turbine 

 to deal effectively with the very attenuated vapour, whereas the re- 



