Page 25 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

2.2 Review the properties of pure substances

2.2.1 Explain the importance of steam for heat transfer and power production

Steam is the most common working fluid used in heat engine cycles because

of its many desirable characteristic, such as low cost, availability, and high enthalpy of
vaporization.

Other working fluids used include sodium, potassium, and mercury for high-

temperature applications and some organic fluids such as benzene and the Freon for
low-temperature applications. The majority of this chapter is devoted to the discussion

of steam power plants, which produce most of the electric power in the world today.

2.2.2 Construct phase and phase changes processes of pure substances

The distinction between steam and liquid is usually made (in an elementary manner) by

stating that both will take up the shape of their containers. However liquid will present a
free surface if it does not completely fill its container. Steam on the other hand will always

fill its container.

With these information, let us consider the following system:

A container is filled with water, and a moveable, frictionless piston is placed on the

container at State 1, as shown in Fig. 2.2.1. As heat is added to the system, the

temperature of the system will increase. Note that the pressure on the system is being
kept constant by the weight of the piston. The continued addition of heat will cause the

temperature of the system to increase until the pressure of the steam generated exactly

balances the pressure of the atmosphere plus the pressure due to the weight of the

piston.

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