148 BUCKINGHAM: REHEAT FACTOR OF TURBINES 



in which e is the stage efficiency at the given point in the turbine, 

 is the absolute temperature of the steam at this pressure, and 

 K 2 is a constant fixed by the relation of the scales of H and <p. If 

 the stage efficiency is constant, as may be the case when the tur- 

 bine has a large number of similar stages, the expansion line is 

 concave upward, because decreases as the pressure and total 

 heat fall. 



Let Ho be the total isentropic heat-drop available between the 

 initial state and the final pressure of the steam. Then if the 

 combined efficiency of a number of stages in series were the same 

 as their separate stage efficiency e, the heat drop of the steam in 

 passing through the turbine would be 



H=eH 



In reality, the combined efficiency is greater than the stage effi- 

 ciency because the reheat in each stage except the last is produced 

 at or above the initial temperature of the next following stage, 

 and is partially available for reconversion into work. This 

 fact is expressed on the Mollier diagram, within the saturation 

 field, by the curvature of the expansion line, which ends at a lower 

 point on the final isopiestic than if it had the uniform slope 

 given by 



— K 2 tan j8 = — — 2 



1 — € 



in which 2 is the temperature of the exhaust. The heat drop in 

 the turbine is therefore expressed by the equation 



H = R e Ho 



in which R is the "reheat factor," a quantity slightly greater than 

 unity. 



On the very approximate assumption that the isopiestics 

 radiate from a single point, a simple equation may be deduced 

 for the expansion line of wet steam; and it follows from this equa- 

 tion that the reheat factor for the expansion of wet steam between 

 the pressures pi and p 2 , at which the absolute saturation tempera- 

 tures are 0i and 2 satisfies the equation 



2 \ 



R = 



<! 





