THE THERMAL PROPERTIES OF STEAM 19 



Intrinsic Energy. From the defining equation 



the energy u in thermal units is readily obtained by subtraction; thus 



f equations (B) and (D) gives therefore the following 

 t \{'!i> it expression 



(F) 



Computation of the Steam Tables. -The tabulated properties of 

 ope steam volume, entropy, and heat content are cal 



latecl (lirntly from formulas (B), (E), and (D), respectively. The same 

 formulas \\ith corresponding saturation values of p and / inserted give, 

 resperti\< !>-. tin- volume, entropy, and heat content of saturated steam. 

 The pressures of saturated steam are calculated from formula (A). Within 

 the range 32-2l2 1. tin- hc.it mntrnt ;' of the liquid is obtained from 

 Calendar's formula, and the latent heat r is then found by subtraction. 

 according to the relation r- i" - r. For temperatures above 212* F. 

 the latent heat is calculated from the Clapeyron relation 



pdT 



in which -/l 



Values of t' are then obtained by subtraction, since i ' - i" - r. The 



internal latent IK .it p is found from the relation 



P= r- 



and the internal energy u" from the relation 



u" - i" - 



The entropy of the saturated steam 5" having been obtained from the 

 ral formula (E), tin- mtropy of the liquid 5' is found by subtracting 



thus 



In the process of computation the formulas were used to give values 

 of the required magnitudes for temi>crature8 (or pressures) so selected 

 as to give a suitable constant interval, and the intervening values were 

 obtained by interpolation. 



Units and Constants. In these tables the mean B.t.u. is taken as 

 the thermal unit This is defined as il$th of the heat required to raise 

 the temperature of a pound of water from 32 to 212 F. The correspond- 

 ing mean calorie is by Griffiths identified with the 17 {-degree calorie and 

 by Barnes with the i6-degree calorie. 



