6 Wilson and Noble, Entropy Diagrams. 



of volume and pressure respectively. K is the point of 

 compression, and C that of release. R is the point where 

 ^K, which is drawn parallel to OX, meets ST. The 

 dotted curve represents a saturation curve through K. It 

 is then necessary to assume the quality of the steam at K, 

 the customary procedure being to assume it dry at this 

 point, i.e., at the beginning of compression.* 



Then the weight of mixture shut in at compression is 



„ pounds, and the total weight in the cylinder will be 



( I + ^) pounds. 



The volume occupied by the clearance steam at any 

 pressure PN during expansion will be 



r^ 

 rR 



rR 



X Pw cubic feet. 



Yn being the total volume occupied by the mixture at 

 the pressure PN. 



Hence, in order to obtain the clearance steam expan- 

 sion curve GHJ, it is only necessary to determine this 



* The error in assuming the steam dry at compression may be seen as 

 follows : — 



Let the fraction of the clearance steam which is water at K be a. 

 Then the percentage correction to be applied to the dryness fraction as 

 formed above will be very approximately 



- looa— % of the dryness fraction. 



rR + rK 



On the figure, if the error in assuming the dryness at K was io% , this 



would be 



- loox — X -15= - 1-5% 

 10 ^ ^'° 



shewing how the error is reduced. Thus in Fig. 3, on this assumption, the 



dryness would be 57'5%, instead of s8"4%, which still leaves a difference of 



5% between the two methods. 



