for 100 degrees of superheat where less economical steam motors are in 

 service. 



Experience has unquestionably shown that the use of superheated steam with 

 turbines leads to an appreciable gain in economy. This fact is so well established 

 that engineering practice does not countenance the installation of turbines for use 

 with saturated steam. 



Where saturated steam is used with turbines, even when it is dry upon 

 entering the first stage, the work done in expanding the steam through progres- 

 sive stages causes the condensation of a sufficient amount of steam to give trouble 

 through the presence of water in the low pressure stages. When the entering 

 steam is superheated, the amount of water in low pressure stages of the turbine 

 is reduced to a point where no trouble will be caused. 



If the saturated steam entering a turbine contains moisture, the effect of such 

 moisture is an appreciable lowering of the turbine's economy. It is stated on 

 good authority that one per cent of moisture contained in the steam will reduce 

 the economy approximately 2 per cent. 



The water rate of a large economical steam turbine with superheated steam 

 is reduced about one per cent for every 1 2 degrees of superheat up to 200 degrees 

 of superheat. To superheat one pound of steam 12 degrees requires about 

 7 B. t. u. and if 1050 B. t. u. are required at the boiler to evaporate one pound 

 of water into saturated steam from the temperature of the feed water, the heat 

 required for the superheated steam would be 1057 B. t. u. One per cent of 

 saving, therefore, in the water consumption would correspond to a net saving 

 of about one-third of one per cent in the coal consumption. On this basis 100 

 degrees of superheat with an economical steam turbine would result in somewhat 

 over 3 per cent of saving in the coal for equal boiler efficiencies. As a boiler 

 with a properly designed superheater placed within the setting is more economi- 

 cal for a given capacity than a boiler without a superheater, the minimum gain 

 in the coal consumption would be, say, 4 or 5 per cent as compared to a plant 

 with the same boilers without superheaters. 



The above estimates are on the basis of a thoroughly dry saturated steam 

 or steam just at the point of being superheated or containing a few degrees of 

 superheat. If the saturated steam is moist, the saving due to superheat is more, 

 and ordinarily the gain in economy due to superheated steam for equal boiler 

 efficiencies, as compared with commercially dry steam is, say, 5 per cent for each 

 too degrees of superheat. 



Aside from any thermodynamic gain through the use of superheated steam 

 with turbines, there is an objection to the use of saturated steam from the stand- 

 point of turbine construction. If saturated steam is used, the erosion of the 

 turbine buckets by water carried in the steam may become a serious factor, while 

 with superheated steam this is negligible. 



For any type of steam motor it may be broadly stated that the lower its 

 economy, the greater the saving that will be effected through the use of superheat. 



37 



