portion of the steam that has been so condensed will re-evaporate, though the 

 greater portion of such re -evaporation takes place at the point of exhaust. The 

 latent heat of the water given up in re-evaporation is utilized in changing 

 the condition of the working fluid and is not available for work. Furthermore, 

 a portion of the saturated steam condenses during adiabatic expansion, this 

 condensation increasing as expansion progresses. In high speed engines using 

 saturated steam the condensation due to adiabatic expansion is just about offset 

 by the re-evaporation of the initial condensation. Since superheated steam 

 cannot condense until its temperature has been reduced to that of saturated 

 steam at a corresponding pressure, not only is initial condensation prevented by 

 its use but also such condensation as would occur during expansion. When 

 superheated sufficiently, the steam delivered by the exhaust will be dry. The 

 number of heat units lost in overcoming condensation effects will be the same 

 whether superheated or saturated steam is the working fluid, but in the case of 

 saturated steam the water of condensation has no power to do work, while super- 

 heated steam, even after having given up the equivalent number of heat units 'to 

 correspond to the condensation in the case of saturated steam, would still have 

 the power of expansion and the ability to do work. 



With superheated steam, therefore, a larger proportion of the heat is utilized 

 in the developing of power than with saturated steam, where a large amount is 

 lost in changing the condition of the working fluid. This results in a lower heat 

 consumption in an engine using superheated steam, that is, the expenditure of 

 a less number of heat units in the developing of one indicated horse power. 

 The "heat consumption" furnishes the true basis for the comparison of efficiencies 

 of different engines, just as a comparison of boiler results is based on an 

 evaporation from and at 212 degrees. 



The water consumption of an engine in pounds per indicated horse power 

 is in no sense a true indication of its efficiency. The initial pressures and 

 corresponding temperatures in two different cases may vary widely and thus 

 through the resulting difference in the temperature of the exhaust affect the 

 temperature of condensed steam returned to the boiler. 



The lower the heat consumption of an engine per indicated horse power, 

 the higher its economy. Since the use of superheated steam decreases this heat 

 consumption, as has been shown, the number of heat units to be imparted in 

 generating steam is reduced, this in turn leading to the lowering of the amount 

 of fuel which must be burned. 



No accurate statement can be made as to the saving possible through the 

 use of superheated steam with reciprocating engines. In highly economical plants, 

 where the water consumption per indicated horse power is low, the gain would be less 

 than would result from the use of superheated steam in less economical plants 

 where the water consumption is higher. Broadly speaking, it may vary from 

 3 to 5 per cent for 100 degrees of superheat in large and economical plants using 

 engines in which there is a high ratio of expansion, to from i o to 25 per cent 



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