3.1 Definition of Pure substance Compressibility charts Properties of two phase systems Const. temperature and Const. pressure heating of water

Thermodynamics

Unit 3

Definition

3.1 Definition of Pure substance, Compressibility charts- Properties of two phase systems - Const. temperature and Const. pressure heating of water

Properties of Pure substances

A substance that has a homogeneously fixed chemical composition is called a pure substance.

In thermodynamics, Steam is considered as a pure substance.

So, in this module, we are going to study about the various processes and properties of steam.

Formation of steam

The above diagram shows the phase change occurring when water from and at 0°C is heated till is converted to steam and further to superheat it.

Temperature is plotted as ordinate and Enthalpy as abscissa.

Phase Change

During part A, water at 0°C is heated till it is converted into saturated liquid at 100°C, ie, the boiling point of water. During this process, water does not undergo any phase change. Hence, the heat supplied is used only for sensible heating of water. It is denoted by hf and is called as enthalpy of liquid.

During part B, water at 100°C is being heated to convert it into steam. Complete conversion of water into steam with no water droplets presence takes time. The heat transfer taking place during this process is utilized to change the phase of water from liquid to gaseous state. This is denoted by hfg and is called as latent heat of vaporisation.

During parts A and B, the total heat transferred is denoted by hg and it is given by

hg = hf + hfg

During part C, the steam is already dry saturated. Any further heating results in superheating of steam. The heat supplied during this process is denoted by hsup and it is given by

hsup = hg + Cp (tsup - tsat)

tsup = temperature of superheated steam

tsat = temperature of saturation of steam

Cp = Specific heat of water at constant pressure.

3.2 P-V-T surface

PROPERTY DIAGRAMS FOR PHASE-CHANGE PROCESSES

In order to understand the change in properties of water and steam during the phase change process, we have to study the property diagrams.

T-v diagram for pure substance:

Critical Point

As we go on increasing the pressure of the system, the saturation line continues to become smaller and smaller, and it becomes a point when the pressure is 22.06 MPa (220.6 bar) for water.

This point is called the critical point, and it is defined as the point where the saturated liquid and saturated vapor states are same or latent heat of vaporisation becomes 0.

The temperature, pressure, and specific volume of a substance at the critical point are called, respectively, the critical temperature Tcr, critical pressure Pcr, and critical specific volume vcr.

The properties of water at critical-point are Pcr = 22.06 MPa, Tcr = 373.95°C, and vcr = 0.003106 m3/kg.

The saturated liquid states can be connected by a line called the saturated liquid line, and saturated vapour states in the same figure can be connected by another line, called the saturated vapour line.

These two lines meet at the critical point, forming a dome as shown in Fig.

All the compressed liquid states are located in the region to the left of the saturated liquid line, called the compressed liquid region.

All the superheated vapour states are located to the right of the saturated vapour line, called the superheated vapour region.

In these two regions, the substance exists in a single phase, a liquid or a vapour.

All the states that involve both phases in equilibrium are located under the dome, called the saturated liquid–vapour mixture region, or the wet region.

2. P-v diagram for pure substance:

Triple point

Under some special conditions, all 3 phases of a pure substance can co-exist in equilibrium

On P-v or T-v diagrams, these triple-phase states form a line called the triple line.

The states on the triple line of a substance have the same pressure and temperature but different specific volumes.

The triple line appears as a point on the P-T diagrams and, therefore, is often called the triple point.

For water, the triple-point temperature and pressure are 0.01°C and 0.6117 kPa, respectively.

3. P-T diagram for pure substance:

The P-T diagram of a pure substance is often called the phase diagram since all three phases are separated from each other by three lines.

The sublimation line separates the solid and vapor regions,

The vaporization line separates the liquid and vapor regions, and

The melting (or fusion) line separates the solid and liquid regions.

These three lines meet at the triple point, where all three phase co-exist in equilibrium.

The vaporization line ends at the critical point because no distinction can be made between liquid and vapor phases above the critical point.

Substances that expand and contract on freezing differ only in the melting line on the P-T diagram.

3.3 Use of steam tables

STEAM TABLES & THEIR USES

For a wide range, it is very difficult and complex to keep track of various relationships of thermodynamic properties.

Hence, these are tabulated to make it easier for engineers to find various values.

For water, these are called as STEAM TABLES.

There are 3 notations used.

Subscript ‘f’ means property of liquid.

Subscript ‘g’ means property of gas.

Subscript ‘fg’ means sum of the property for liquid and gas.

3.4 Mollier’s chart.

ENTHALPY-ENTROPY (h-s) CHART OR MOLLIER DIAGRAM

It was prepared by Dr. Mollier. It shows relation between enthalpy and entropy or water at various pressures. It is very useful. The steam tables are actually tabulated version of Mollier diagram.

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