18 PROPERTIES OF STEAM AND AMMONIA 



gation leads to the number 971 .7. which is probably quite close to the 

 truth, though if anything slightly low. 



Entropy. An expression for the entropy of superheated steam is 

 readily obtained from the fundamental equation, 



d q -c.dT-AT()dp. 

 Dividing by T, 



From the characteristic equation 



B . mn 



dv\ B 

 ff)--i 



Introducing this and the expression for c p in the preceding equation, the 

 result i- 



The integration of this exact differential equation gives the following 

 equation for the entropy 



s=a\ag.T+0T- l --AB\og.p-4^P(i + 2ap*) + s . (E) 



The constant s is found by applying the equation at the saturation 

 limit. The value thus determined is s = 0.08108. 



For the range 32-2i2 F., within which Calendar's formula for 

 the heat of the liquid is surely applicable, there are available two inde- 

 pendent methods of calculating the entropy of saturated steam, i. The 

 tut ropy of the liquid s' is determined by the integration of Callendar's 



equation for i' and the entropy of vaporization is added. 2. Corre- 



sponding saturation values of p and T are substituted directly in the 

 preceding formula for s. The two methods give substantially identical 

 result-. 



Above 212 F. the entropy s" of saturated steam is calculated from 

 formula (E) and the entropy of the liquid s' is obtained by the relation 



s' - s" - - 



T 



Integration of Callendar's ^''-equation gives the following formula 

 for 5': 



s' = 2.3623 log T+ 0.0045775 log (/+ 4) - 0.00022609 T 



+ 0.00000013867 r 2 - 6.28787. 



