DIEMER : THERMAL DIAGRAMS. 



231 



sented by the supplying of heat from the source of heat between tem- 

 peratures Ti and 71, or 212" F., and the temperature corresponding to 

 the initial pressure. The cycle of operations just described is what is 

 known as the Rankine ideal steam-engine cycle, and has been adopted 

 by the British Institute of Civil Engineers as the standard for com- 

 parison of thermodynamic efficiencies of steam-engines. 

 Let us now trace out the corresponding entropy diagram : 

 At state a we have one pound of water whose temperature is known. 

 The entropy of the liquid, c^loQeYZ.^ calculated by the increment 

 method already referred to, may be found in tables prepared by Pea- 

 body and others. 



i> 



Plotting a suitable distance on the Y axis to represent the tem- 

 perature, we plot a suitable distance to scale, horizontally to the right 

 to represent the entropy of the liquid, found as above indicated, thus 

 locating the point a (fig. 2) ; h' is located by adding the entropy of 

 vaporization, that is, ^, or, for complete evaporation, ^. If the X 

 axis has been taken as representing 32^ F., the rectangle underneath 

 a'h' will represent to scale the latent heat of vaporization. This gives 

 us the means for determining the area corresponding to one heat unit. 



Points on the curve h'a are found by calculating the entropy values 

 belonging to the respective temperatures corresponding to selected 

 pressures ; c'cT represents abstraction of heat at constant temperature, 

 and d'a represents supply of heat to the water in the boiler. The area 

 md'rn represents the heat required to heat the feed water from 32" F. 



