Unit 4
Properties of Steam
Content:
When we provide continuous heat to water then at 100 temperatures and 1 atm pressure, it boils and changes its phase from liquid to vapour. This vapour is known as steam.
Steam contains more energy as it has both sensible heat and latent heat of vaporization. Steam has been a popular mode of conveying energy since the industrial revolution. Steam issued for generating power and also used in process industries such as sugar, paper, fertilizer, refineries, petrochemicals, chemical, food, synthetic fibre and textiles the following characteristics of steam make it so popular and useful to the industry:
• Highest specific heat and latent heat
• Highest heat transfer coefficient
• Easy to control and distribute
• Cheap and inert
Types: Wet steam, Dry steam, Superheated steam
Wet Steam: When steam contains water particles then it is known as Wet steam
Dry Steam: When wet steam is further heated then all water particles get converted into vapour and resulted steam is called dry steam.
Superheated Steam: When dry saturated steam is heated to higher temperatures then steam obtained is in superheated state. This steam is mostly used in Power generation.
Use
(1) Power generation
(2) Heat engines (i.e. steam engines train)
Steam is an invisible gas created by adding heat energy to water. It is liquid water changed to its gaseous state.
Saturated steam - steam in immediate contact with the water from which it is being generated. If the pressure remains constant, any loss of heat or BTUs will result in condensation.
Superheated steam – If more heat is added to dry saturated steam at a constant pressure, increasing its temperature and specific volume, super-heated steam is produced. Heat must be lost, and temperature reduced before condensation occurs.
Flash steam - when condensate, at saturation temperature and pressure, is discharged into a region of lower pressure, it automatically adjusts to the saturated conditions at the lower pressure. In effect, some of the condensate is “re-evaporated” into steam.
The total heat content of a substance is called enthalpy. Actually, it has much broad definition in thermodynamics but for 1st year BME students this definition works just fine. So, total heat content by steam is termed as its enthalpy. It is denoted by ‘H’. SI unit is KJ
‘h‘ is generally used term which represents specific enthalpy, unit for which is KJ/Kg. In steam tables you will see enthalpy written as hi, hg, hig
hi is the enthalpy of liquid that is water at boiling point that’s why subscript ‘l’ is used, point ‘D’ in h-T diagram. Similarly, hg is enthalpy of dry saturated steam, point ‘E’ in h-T diagram and hig is the latent heat, Process D to E in h-T diagram.
Gases (steam is a gas) occupy less space under higher pressure than under lower pressure. This means 1 kilogram of steam occupies different volumes, depending upon its pressure. The term specific volume refers to the volume that one kg of steam occupies at a given pressure and temperature.
Unit is m3 / kg denoted by v
Quality of steam (Dryness fraction)
Dryness fraction in simple words denotes the mass of dry steam in given steam. Or how much steam is dry or in other words how much water vapour is present in steam. It is denoted by ‘x’.
X = M / M+m
Where M=mass of the dry steam
m=mass of water vapour
The use of dryness fraction allows us to know both the mass of dry steam and mass of water vapour.
Now, see
If x = 0.9 that means dry steam is 0.9 kg and water vapour is 0.1 kg in 1 kg of given steam.
Obviously for dry steam, x = 1
Quality is represented in percentage but meaning is same as ‘x’.
If quality of steam is 80%, then it has 80% of dry steam and 20 % water vapour by mass.
The thermodynamics steam tables contain the following tables:
Saturated water and steam temperature tables: In these tables for every temperature the absolute pressure, specific volume for saturated water and saturated steam, specific enthalpy for saturated water and saturated steam and specific entropy for saturated and saturated steam are given.
Saturated water and steam pressure tables: In these tables for every pressure the temperature, specific volume for saturated water and saturated steam, specific enthalpy for saturated water and saturated steam and specific entropy for saturated and saturated steam is given.
Specific volume of superheated steam: In this table for every pressure the saturation temperature and the specific volume at various temperatures are given.
Enthalpy of superheated steam: In this table for every pressure the saturation temperature and the enthalpy of superheated steam at various temperatures are given.
Entropy of superheated steam: In this table for every pressure the saturation temperature and the entropy of superheated steam at various temperatures are given.
Specific volume of superheated steam: In this table at every absolute pressure the specific volume of supercritical steam is given.
Enthalpy of supercritical steam: In this table at every absolute pressure the enthalpy of supercritical steam is given.
Entropy of supercritical steam: In this table at every absolute pressure the entropy of supercritical steam is given.
How to Use the Steam Table
In steam tables the properties of the dry steam are listed and for the wet steam the properties may be calculated from the steam tables of the dry and saturated steam.
For values that are not listed exactly in the tables, the value between two figures can be obtained by linear interpolation. Interpolation is a mathematical tool by which, depending on the interval between two variables, a value in between can be calculated.
The steam table shown above is a saturated water and steam table. As all the other tables are used on the same principle we will only discuss this one. For an absolute pressure of 9 bars, the saturation temperature is 175.4 C. It means that at a temperature of 175.4 and above C all of the steam will be saturated. Of course, any temperature above this will be super heating of the steam.
It must be noted however that at 175.4 C, depending on the latent heat supplied for vaporization, the steam can have any dryness faction. Vg is the specific volume of steam, hf is the specific enthalpy of water, hg is the specific enthalpy of steam, sf is the specific entropy of water, and sg is the specific entropy of steam.
We will now become familiar with this formula:
h = hf + xL
Where x is dryness fraction and L = hg – hf
By the above formula, if we know the dryness fraction of steam, we can calculate the enthalpy of wet steam, and its value would lie between that of the saturated water and saturated steam.
For example, if the dryness fraction is 0.8 for steam at 9 bars absolute pressure in bars.
Referring to the steam table above, hf = 743 kJ/Kg, L = 2031 kJ/Kg,
h = 743 + 0.8 x 2031 = 2367.8 kJ/Kg
