In this article we will discuss about:- 1. Formation of Steam 2. Conditions of Steam 3. Properties 4. Enthalpy.
Formation of Steam:
Generation of One Kg of Steam at a Given Pressure from Water Initially At 0°C:
Let us decide upon the pressure under which one kg of water is to be heated. After fixing the pressure we can know the saturation temperature for this pressure from steam tables and thus, water should be heated to this temperature before steam will be generated.
Fig. 3-7 illustrates the three stages in the formation of steam, at some constant pressure, namely:
(1) Introducing Stage:
During this stage 1 kg of water at 0°C is pumped into the cylinder against an absolute pressure P, the pressure of steam generation. This pressure is caused by the weight placed on piston and pressure of atmosphere. The energy expended by the pump to deliver 1 kg of water of volume ‘a’ m3 at 0°C equals P × a (joules). This energy appears as pressure energy of the water in the cylinder and could be made to do work by virtue of its pressure.
(2) Warming Stage:
During this stage heat is added to the water so that its temperature from 0°C is raised to ts (the temperature at which steam will begin to form under absolute pressure P) and to increase the volume of 1 kg of water from ‘a’ to vf, where vf is the volume of 1 kg of water at the temperature ts and absolute pressure P. The heat supplied during this warming stage is called Sensible Heat because it can be detected by the sense of touch and produces a rise in temperature (ts – 0)°C to be seen on a thermometer.
ADVERTISEMENTS:
The sensible heat supplied in the warming stage is used for two purposes:
(i) To increase the temperature of 1 kg of water from 0°C to ts. This heat is utilized for increasing the internal energy of water.
(ii) To do external work dw (vf – a) Joules in increasing the volume of water from ‘a’ to vf against the absolute pressure P which acts constantly on the piston.
By First Law of Thermodynamics as applied to non-flow process, heat supplied = change in internal energy + external work done by water.
ADVERTISEMENTS:
∴ Sensible heat = hf — (uf – uo) + P (vf – a).
If the changes in internal energy are reckoned from 0°C, u0 will be zero and we get sensible heat-
hf = uf + P (vf – a).
(3) Evaporating Stage:
ADVERTISEMENTS:
During this stage further heat is supplied to 1 kg of water at tsat, the saturation temperature corresponding to the constant pressure P. The volume changes from vf to vg. During this stage heat is added at constant temperature and this heat is utilized in changing the state of water. The external work done during this stage = P (vg – vf).
Because the heat added during this stage cannot be recorded by a rise in temperature on the thermometer, it is called Latent Heat (hidden heat) or heat of vaporization i.e., hfg.
The latent heat is, therefore, used for two purposes:
(i) To overcome the internal molecular resistance of the water by changing it from water into steam.
ADVERTISEMENTS:
(ii) To force back the piston to increase the volume from that of water to that of steam.
By First Law of Thermodynamics as applied to non-flow process, heat supplied = change in internal energy + external work done.
∴ Heat of vaporization = hfg = (ug – uf) + P (vg – vf).
Thus, we see that energy is supplied in three stages to generate steam at a pressure P from water at 0°C.
Conditions of Steam:
Steam may occur in any one of the following three conditions:
(i) Saturated steam which may be either dry or wet
(ii) Superheated steam
(iii) Supersaturated steam.
Saturated steam is any steam which cannot have heat abstracted from it or be compressed at constant temperature without partially condensing. Saturated steam is a vapour at the temperature corresponding to the boiling point of the liquid at the imposed pressure. We have seen that for each liquid, a certain definite boiling point exists for each pressure.
A substance which is in the vapour state in a confined space and is in contact with some of the same substance in liquid state is always at the same temperature as the liquid and is saturated vapour. When heat is applied to saturated vapour and to the liquid with which it is in contact, more liquid evaporates but the temperature remains constant. Similarly if heat is removed more of the vapour condenses but the temperature will remain constant.
Dry Saturated Steam and Wet Steam:
If saturated steam does not contain any water, it is known as Dry Saturated Steam. It contains just sufficient heat energy to maintain all of the water in a gaseous state. If saturated steam contains liquid particles, it is known as Wet Steam.
It does not contain sufficient heat energy to maintain all water in the gaseous state. If some of the heat energy is absorbed from the dry saturated steam, the steam becomes wet. In practice it is difficult to get an absolutely dry saturated steam if it is produced by boiling, because some of water particles are carried away with the steam.
If the temperature of the steam is greater than that of the boiling point corresponding to the pressure of steam generation, the steam is known as Superheated Steam. The temperature of the steam may be increased above the saturation temperature by adding heat to steam, after all the water has been vaporized or after the steam has been separated from contact with water.
If superheated steam is brought in contact with water it will give up part of its heat to the water. If the superheated steam contains sufficient heat energy, in that case, the water will evaporate. If superheated steam does not contain sufficient heat to evaporate all the water into steam, it will give up all its additional heat known as superheat and some of the water will not be converted into steam.
The amount of superheat in steam is given in terms of the difference between its temperature and that of the saturated steam at the pressure of steam generation. If the temperature of steam is 100°C higher than the saturation temperature corresponding to the pressure of steam generation, we say that the steam has 100°C of superheat.
Supersaturated steam at a particular saturation pressure has temperature less and density greater than the corresponding values given in steam tables. This condition of steam is obtained when it is cooled by its own expansion until it contains less heat energy than the saturated steam under the same conditions. This state of steam is very unstable and the steam soon resumes the saturated condition; such a state of steam occurs in expansion in a nozzle.
Properties of Steam:
The properties of steam are interrelated. If we know certain properties, the other properties may be found out. For example, if the pressure of saturated steam is observed by the pressure gauge, its temperature can be found from steam tables in which the results of various experiments have been tabulated.
All other properties of a given mass of steam can be known when any two properties such as the pressure and dryness fraction of steam for saturated steam and pressure and degree of superheat for superheated steam are known.
If the steam is dry and saturated then only pressure should be known to determine all the properties. Thus, in order to observe the above two properties of steam, pressure gauges and steam calorimeters or thermometers are used at suitable points.
Dryness Fraction of Saturated Steam:
We have seen that steam in contact with water contains liquid particles in suspension. Thus, the steam consists of dry saturated steam and water particles in suspension. The dryness fraction of steam is defined as the ratio of the mass of dry steam in a certain quantity of steam to the mass of total wet steam. It is generally denoted by the letter x.
The dryness fraction of a wet steam may also be defined as the amount of dry steam in unit amount of wet steam.
If 1 kg of wet mercury vapour contains 0.12 kg of droplets of liquid mercury, it has a dryness fraction of (1 – 0.12) = 0.88.
The values tabulated in the steam tables are determined accurately by experiments. These values form the basis for many calculations concerned with steam engineering. These tables are to be used because vapours do not obey general gas law. The values given in the tables are for one kg of dry saturated steam but these values can also be employed for wet steam calculations.
In order to determine the properties of steam at some intermediate pressure between those given in tables, we interpolate assuming the linear relation between these values. The method is very simple and at the same time accurate.
If is denoted by the letter hf in steam tables. It is the quantity of heat in kJ required to raise the temperature of 1 kg of water from 0°C to the saturation temperature at which water begins to boil at the given pressure P. The pressures are given in bar (105 N/m2) absolute.
In changing water to steam under constant pressure, the temperature of water must be brought upto its boiling point at the given pressure before it can evaporate. Sometimes sensible heat is called the liquid heat or liquid enthalpy. Sensible heat of water hf may be found approximately by multiplying its specific heat by its saturation temperature.
Actually the specific heat of water is not constant but it increases with increase in the saturation temperature i.e., with increase in the pressure. The value of hf given in the steam tables accounts for the variation in the specific heat of water.
It is always, preferable to refer to the tables for accurate values. It should be noted that enthalpy values given in tables are not absolute values. These are simple changes in values from the reference state. It values of enthalpy will be positive and below the reference state values of enthalpy if given will be negative. A similar situation arises with temperature measurements. It should be noted that with vapours, other than steam, tables are prepared having their own reference states.
It is denoted by the letter hfg in steam tables. It is the quantity of heat required to convert 1 kg of water at saturation temperature for a given pressure to one kg of dry saturated steam, at that pressure. The value of latent heat of vaporization decreases as the pressure increases and it becomes zero when the critical pressure is reached.
The enthalpy of a vapour depends on how the vapour is heated. The enthalpy given in the tables is for heating at constant pressure.
Hsat = hg = hf + hfg.
Enthalpy of Steam:
If the dryness fraction of steam is known, with the help of steam tables we can get the value of total heat for wet steam by the formula given below:
Hwet = hf + x hfg
where Hwet = enthalpy of 1 kg of wet steam
hf = sensible heat of 1 kg of steam
x = dryness fraction of steam
and hfg = enthalpy of vaporization 1 kg of dry saturated steam.
Enthalpy of Superheated Steam:
From dry saturated condition a vapour receives superheat and its temperature rises above saturation temperature tsat. It has now entered the superheat phase. The enthalpy added during the superheat phase is the superheat enthalpy. The total enthalpy of superheated vapour will be the sum of the enthalpy of dry saturated vapour and the superheat enthalpy.
When steam is superheated, its temperature is known and when its pressure is known the enthalpy of 1 kg of steam can be obtained by the use of the formula given below:
Hsup = hf + hfg + Cp (tsup – tsat)
where Hsup = enthalpy of 1 kg of superheated steam
hf = sensible heat of 1 kg of steam
hfg = enthalpy of vaporization 1 kg of dry saturated steam
Cp = mean specific heat of superheated steam at constant pressure
tsup = temperature of superheated steam
and tsat = saturation temperature corresponding to the pressure of steam generation.
Since we know that-
hg = hf + hfg
where hg is the enthalpy of one kg of dry saturated steam, the above formula can be written as
Hsup = hg + Cp (tsup – tsat).
The difference (tsup – tsat) is called the degree of superheat, e.g., steam at a pressure of 10 bar has a saturation temperature of 179.9°C and if the temperature of steam is 200°C the degree of superheat is 200 – 179.9 = 20.1°C.
The value of mean specific heat of superheated steam Cp depends upon the degree of superheat and the pressure of steam generation. The average value of Cp for superheated steam is 2.0934 kJ/kg-K. The values of h for a given value of pressure P and temperature tsup.