PROPERTY TABLES

For most substances, the thermodynamic properties are related to one another in a complex manner and cannot be expressed by simple equations. Thus, the properties are presented in tabulated form.

Some thermodynamic properties are easy to measure while others aren’t measured directly and need to be calculated using the relations between them and the measurable properties. The results of these measurements and calculations are then presented in tables. Some of the major properties are discussed below:

Enthalpy

In an open system, Enthalpy (H) can be defined as the amount of energy transferred across a system boundary by a moving flow. It is expressed as follows;

H = U + PV

The term PV represents the flow work and has the units of energy. Thus, H also has the units of energy. Enthalpy is a state function since U, P and V are all state functions, any combination of them must also be a state function. Enthalpy is also an extensive property. Other expressions of enthalpy are as follows;

Saturated Liquid and Saturated Vapor

The properties of saturated liquid and saturated vapor are usually presented in tabulated form and presented in appendices of many Thermodynamics books. These tables are usually either listed as either temperature and pressure tables. Thus, it is convenient to use Saturated Water: Temperature Table when temperature is given, and Saturated Water: Pressure Table when pressure is given.

The superscript L is used to denote the properties of a saturated liquid, and the superscript V to denote the properties of saturated vapor. The difference between the saturated vapor and saturated liquid states are designated by Δ, i.e.,

or;

The quality of a saturated liquid is 0, the quality of a saturated vapor is 1.

It is important to note that quality has no meaning in the subcooled (or, compressed) liquid and superheated vapor regions.

In the two-phase region, application of Gibbs phase rule gives, 2 + F = 1 + 2, thus F = 1. Hence, if one of the independent intensive variables of a two-phase mixture is known it is possible to specify the state of the system.

Superheated Vapor

In the region to the right of the saturated vapor line, a pure substance exists as superheated vapor. In this region

P < P^vap at a given temperature

T > T^sat at a given pressure

This is represented graphically as shown below:

The degrees of superheat is defined as the temperature in excess of the saturation temperature at a given pressure, i.e.,

Degrees of Superheat = T - T^sat

For a superheated vapor, application of Gibbs phase rule gives 1 + F = 1+2 ⇒ F = 2

Thus, two independent intensive variables are required to specify the state of a superheated vapor.

Subcooled (Compressed) Liquid

In the region to the left of the saturated liquid line, a pure substance exists as subcooled (or, compressed) liquid. In this region

P > P^vap at a given temperature (compressed liquid)

T < T^sat at a given pressure (subcooled liquid)

This is represented graphically as shown below:

Note: Compressed liquid implies that the pressure is greater than the saturation pressure for a given temperature. Conversely, subcooled liquid implies that the temperature is lower than the saturation temperature for the given pressure.

As an approximation, properties of compressed liquid are equal to that of a saturated liquid at the given temperature. For a compressed liquid, application of Gibbs phase rule gives 1 + F = 1+2 ⇒ F = 2

Thus, two independent intensive variables are required to specify the state of a compressed (or subcooled) liquid.