Chapter 8-INTRODUCTION TO THERMODYNAMICS 



WORKING SUBSTANCES 



As previously noted, a thermodynamic sys- 

 tem requires a working substance to receive, 

 store, transport, and deliver energy. The work- 

 ing substance is almost always a fluid and is 

 therefore frequently referred to as the working 

 fluid . Water (together with its vapor, steam) is 

 one of the most commonly used working fluids, 

 although air, ammonia, carbon dioxide, and a 

 wide variety of other fluids are used in certain 

 kinds of systems. A working substance may 

 change its physical state during the course of a 

 thermodynamic cycle or it may remain in one 

 state, depending upon the nature of the cycle 

 and the processes involved. 



To understand the behavior of working fluids, 

 we should have some understanding of the laws 

 of perfect gases, of the relationships between 

 liquids and their vapors, and of the ways in 

 which the properties of working fluids may be 

 represented and tabulated. These topics are 

 discussed in the following sections. 



Laws of Perfect Gases 



The relationships of the volume, the absolute 

 pressure, and the absolute temperature in the 

 hypothetical substances known as "perfect 

 gases" were stated by the physicists Boyle and 

 Charles in the form of various gas laws. The 

 laws thus established may be combined and 

 summarized in the general statement: For a 

 given weight of any gas, the product of the 

 absolute pressure and the volume, divided by 

 the absolute temperature, is a constant. Or, in 

 equation form. 



pv 

 "t 



p V p V 

 11_I = _2_2. 



Ti T2 



R 



where 



p = absolute pressure 

 V = total volume 

 T = absolute temperature 

 R = the gas constant 



Although the laws of perfect gases were 

 developed on the basis of experiments made 

 with air and other real gases, later experiments 

 showed that these relationships do not hold pre- 

 cisely for real gases over the entire range of 

 pressures and temperatures. However, air and 

 other gases used as working fluids may be 



treated as perfect gases over quite a wide range 

 of pressures and temperatures without any ap- 

 preciable error being introduced. Values of the 

 gas constant for some common gases are: 



Air 53,3 



Oxygen 48.3 



Nitrogen 55.0 



Hydrogen 766.0 



Helium 386.0 



Liquids and Their Vapors 



When heat is transferred to a liquid, the 

 average velocity of the molecules is increased 

 and the amount of internal kinetic energy stored 

 in the liquid is increased. As the average veloc- 

 ity of the molecules increases, some molecules 

 which are at or near the surface of the liquid 

 momentarily achieve unusually high velocities; 

 and some of these escape from the liquid and 

 enter the space above, where they exist in the 

 vapor state. As more and more of the molecules 

 escape and come into the vapor state, the prob- 

 ability increases that some of the vapor mole- 

 cules will momentarily have unusually low 

 velocities; these molecules will be captured by 

 the liquid. As a result of this exchange of 

 molecules between the liquid and the vapor, a 

 condition of equilibrium is reached and an 

 equilibrium pressure is established. The equi- 

 librium pressure depends upon the molecular 

 structure of the fluid and upon its temperature. 

 For any given fluid, therefore, there is a definite 

 relationship between the temperature and the 

 pressure at which a liquid and its vapor may 

 exist in equilibrium contact with each other. 



As long as the vapor is in contact with the 

 liquid from which it is being generated, the 

 liquid and the vapor will remain at the same 

 temperature. If the liquid and the vapor are in 

 a closed container (such as a boiler with all 

 steam stop valves closed) both the temperature 

 and the pressure of the liquid and its vapor will 

 increase as heat is added. If the vapor is per- 

 mitted to leave the steam space at a rate equal 

 to the evaporation rate, an equilibrium will be 

 established at the equilibrium pressure for the 

 particular temperature. 



The pressure and the temperature which are 

 related in the manner just described are known 

 as the saturation pressure and the saturation 

 temperature. Thus, for any specified pressure 

 there is a corresponding temperature of vapo- 

 rization known as the saturation temperature; 



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