Explanation Properties and use of steam
introduction
In both the open and closed thermodynamic systems the transfer and conversion of energy needs a working substance. In power plant such as internal combustion engines and steam turbines its purpose is to receive heat energy from the fuel and then release it in the form of external work. Steam is an excellent working substance.
It can carry large amounts of heat energy. It is produced from water which is plentiful and it is environmentally friendly. In most of our larger power stations, the electrical generators are driven by steam turbines. Steam is also widely used as a heat source in industrial processes and in hospitals for central heating and the sterilization of equipment.
Phases of a substance
the substance H2O can exist in three different states. It can exist as a solid in the form of ice, as a liquid which is water and as a gas, which is of course steam. These different states are known as phases. When a substance is of the same nature throughout its mass, it is said to be of a single phase. If two or more phases can exist together, the substance is then said to be two-phase mixture. In a single phase the substance is said to be homogenous and in a two phase mixture it is said to be heterogeneous.
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| FIG 1 |
When two systems are at different temperatures, the transfer of energy from one system to the other is called heat transfer. For a block of hot metal cooling in air, heat is transferred from the hot metal to the cool air. The principle of conservation of energy may be stated as
energy cannot be created nor can it be destroyed
and since heat is a form of energy, this law applies to heat transfer problems. A more convenient way of expressing this law when referring to heat transfer problems is in figure 2: |
| FIG 2 |
Fluids consist of a very large number of molecules moving in random directions within the fluid. When the fluid is heated, the speeds of the molecules are increased, increasing the kinetic energy of the molecules. There is also an increase in volume due to an increase in the average distance between
molecules, causing the potential energy of the fluid to increase. The internal energy, U, of a fluid is the sum of the internal kinetic and potential energies of the molecules of a fluid, measured in joules. It is not usual to state the internal energy of a fluid as a particular value in heat transfer problems, since it is normally only the change in internal energy that is required.
The amount of internal energy of a fluid depends on:
(a) the type of fluid; in gases the molecules are well separated and move with high velocities, thus a gaseous fluid has higher internal energy than the same mass of a liquid
(b) the mass of a fluid; the greater the mass, the greater the number of molecules and hence the greater the internal energy
(c) the temperature; the higher the temperature the greater the velocity of the molecules
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| FIG 3 |
Saturation temperature (ts⁰C)
When water receives heat energy, its temperature rises. This, you may recall, is known as sensible heat because its flow can be sensed by a temperature measuring device and that the specific heat capacity of water cw is 4187 J kg⁻¹ K⁻¹. Eventually a condition is reached where the water cannot absorb any more heat energy without undergoing a change of phase. It is then said to be saturated with sensible heat and is known as saturated water.
As heat is added to saturated water, it is turned into saturated steam. The amount of heat required to turn 1 kg of saturated water into saturated steam is called the specific latent heat of vaporisation, and is given the symbol, hfg. The total specific enthalpy of steam at saturation temperature, hg, is given by:
the specific sensible heat + the specific latent heat of vaporization
hg = hf +hfg
hg = specific enthalpy of dry saturated steam, i.e. dry steam at ts⁰C
hfg =specific enthalpy of vaporisation, i.e. the specific latent heat
hf =specific enthalpy of saturated water, i.e. water at ts⁰C
Dry saturated steam
This is another term which sounds rather strange. Dry saturated steam is steam which has just received all of its latent heat, hfg so that it is dry, but still at the saturation temperature, ts⁰C. The enthalpy per kilogram, hg and specific volume, vg of dry saturated steam at any given pressure is given in steam property tables.
The units of specific volume are m³ kg⁻¹, i.e. cubic metres per kilogram.
Enthalpy
If saturated water continues to receive heat energy, a change of phase starts to take place. The water begins to evaporate and the temperature stays constant at the boiling point, or saturation temperature, ts⁰C whilst the change is taking place
The amount of heat energy required to change 1 kg of saturated water completely into steam is of course its specific latent heat of vaporisation. It is also called the specific enthalpy of vaporisation which is given the symbol hfg
The value of the specific enthalpy of vaporisation depends on the pressure at which the steam is being generated. At a normal pressure of 101.325 kPa or 1.01325 bar where the saturation temperature is 100 ⁰C its value is 2256.7 kJ kg⁻¹.
The sum of the internal energy and the pressure energy of a fluid is called the enthalpy of the fluid, denoted by the symbol H and measured in joules. The product of pressure p and volume V gives the pressure energy, or work done pressure energy = pV joules
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| FIG 4 |
H = U + pV
specific enthalpy hfg =enthalpy /mass = H /m
hw = specific enthalpy of water below its saturation temperature
hf = specific enthalpy of saturated water, i.e. water at ts⁰C
hfg = specific enthalpy of vaporisation, i.e. the specific latent heat
hg = specific enthalpy of dry saturated steam, i.e. dry steam at ts⁰C
hsup = specific enthalpy of superheated steam
As the pressure and saturation temperature increase, the specific enthalpy of vaporisation hfg becomes less, falling to zero at the critical pressure and temperature. Its value at any pressure may be obtained from the column headed hfg in steam property tables.
Sensible Heat
The specific enthalpy of water, hf, at temperature 𝛳°C is the quantity of heat needed to raise 1 kg of water from 0°C to𝛳°C, and is called the sensible heat of the water. Its value is given by:
specific heat capacity of water (c) × temperature change(𝛳)
hf = c 𝛳
Wet steam and its dryness fraction (x)
As the steam bubbles rise out of the water in a boiler, they carry with them small droplets of water. This is known as wet steam. Wet steam has not received all of the latent heat required to change it completely to dry steam. The amount of latent heat which it has received is given by its dryness fraction, x.
For example, if wet steam has a dryness fraction of x = 0.9, this means that it has received 90% of its latent heat and that one tenth of its mass will be made up of water droplets.
Superheated steam
If dry saturated steam continues to receive heat energy, its temperature will start to rise again. It is then known as superheated steam, which is of course a vapour until its temperature exceeds 374.15 8C, the critical temperature. Thereafter it becomes supercritical steam, as we have described. The number of degrees by which the temperature
of superheated steam exceeds its saturation temperature, ts⁰C, is known as its degrees of superheat. The enthalpy per kilogram, hsup and specific volume, vsup of superheated steamat given temperatures and pressure are given in steam property tables.
The temperature v. enthalpy diagram, Figure 5, shows how the various values of specific enthalpy values of saturated water, hf , dry saturated steam, hg, and vaporisation hfg, vary with saturation temperature and pressure.
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| FIG 5 |
- A vapour is a substance in its gaseous phase below its critical temperature
- The boiling point of a liquid is also called its saturation temperature.
- The dryness fraction of wet steam is the fraction of the latent heat of vaporisation that it has received.
- Degrees of superheat is the number of degrees above saturation temperature.
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