Unit 3: Properties of Pure Substances 3.1 PURE SUBSTANCE A pure substance is a system which is having homogeneous composition throughout its mass. For example a system comprising steam and water, is homogeneous in composition, since chemical analysis would reveal that hydrogen and oxygen atoms are presents in the ratio 2 : 1 whether the sample be taken from the steam or from the water. 3.2PHASE CHANGE OF A PURE SUBSTANCE Let us consider 1 kg of liquid water at a temperature of 20°C in a cylinder fitted with a piston, which exerts on the water a constant pressure of one atmosphere (1.0132 bar) as shown in Fig. 3.2 Fig 3.2 Phase change of water at constant pressure from liquid to vapour phase As the water is heated slowly its temperature rises until the temperature of the liquid water becomes 100°C. During the process of heating, the volume slightly increases as indicated by the line 1-2 on the temperature-specific volume diagram (Fig. 3.3). The piston starts moving upwards. Fig. 3.2.
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Unit 3: Properties of Pure SubstancesUnit 3: Properties of Pure Substances 3.1 PURE SUBSTANCE A pure substance is a system which is having homogeneous composition throughout its mass.
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Unit 3: Properties of Pure Substances
3.1 PURE SUBSTANCE
A pure substance is a system which is having homogeneous composition throughout its mass. For
example a system comprising steam and water, is homogeneous in composition, since chemical analysis
would reveal that hydrogen and oxygen atoms are presents in the ratio 2 : 1 whether the sample be
taken from the steam or from the water.
3.2PHASE CHANGE OF A PURE SUBSTANCE
Let us consider 1 kg of liquid water at a temperature of 20°C in a cylinder fitted with a piston, which
exerts on the water a constant pressure of one atmosphere (1.0132 bar) as shown in Fig. 3.2
Fig 3.2 Phase change of water at constant pressure from liquid to vapour phase
As the water is heated slowly its temperature rises until the temperature of the liquid water becomes
100°C. During the process of heating, the volume slightly increases as indicated by the line 1-2 on the
7. Volume of superheated steam. As superheated steam behaves like a perfect gas its volume can be
found out in the same way as the gases.
If, vg = Specific volume of dry steam at pressure
Ts = Saturation temperature in K,
Tsup = Temperature of superheated steam in K, and
vsup = Volume of 1 kg of superheated steam at pressure p,
----- 3.6
8. ENTROPY OF WET STEAM.
The total entropy of wet steam is the sum of entropy of water (sf ) and entropy of evaporation (sfg).
9. ENTROPY OF SUPERHEATED STEAM
Total entropy of superheated steam above the freezing point of water.
Ssup = Entropy of dry saturated steam + change of entropy during superheating
3.9. THERMODYNAMIC PROPERTIES OF STEAM AND STEAM TABLES
Following are the thermodynamic properties of steam which are tabulated in the form of table:
p = Absolute pressure (bar or kPa) ;
t s = Saturation temperature (°C) ; hf = Enthalpy of saturated liquid (kJ/kg) ;
hfg = Enthalpy or latent heat of vapourisation (kJ/kg) ;
hg = Enthalpy of saturated vapour (steam) (kJ/kg) ;
sf = Entropy of saturated liquid (kJ/kg K) ; sfg = Entropy of vapourisation (kJ/kg K) ;
sg = Entropy of saturated vapour (steam) (kJ/kg K) ;
vf = Specific volume of saturated liquid (m3/kg) ;
vg = Specific volume of saturated vapour (steam) (m3/kg).
Also, hfg = hg – hf ...... Change of enthalpy during evaporation
sfg = sg – sf ...... Change of entropy during evaporation
vfg = vg – vf ...... Change of volume during evaporation.
The above mentioned properties at different pressures are tabulated in the form of tables as under: The
internal energy of steam (u = h – pv) is also tabulated in some steam tables.
3.10. ENTHALPY-ENTROPY (h-s) CHART OR MOLLIER DIAGRAM
Fig 3.11 .ENTHALPY-ENTROPY (h-s) CHART OR MOLLIER DIAGRAM
3.11 RANKINE CYCLE :
Rankine cycle is the theoretical cycle on which the steam turbine (or engine) works.
Fig 3.12 Rankine cycle
The Rankine cycle is shown in Fig. 3.12. It comprises of the following processes :
Process 1-2 : Reversible adiabatic expansion in the turbine (or steam engine).
Process 2-3 : Constant-pressure transfer of heat in the condenser.
Process 3-4 : Reversible adiabatic pumping process in the feed pump.
Process 4-1 : Constant-pressure transfer of heat in the boiler.
Fig. 3.12 shows the Rankine cycle on p-v, T-s and h-s diagrams (when the saturated steam enters the
turbine, the steam can be wet or superheated also).
Considering 1 kg of fluid : Applying steady flow energy equation (S.F.E.E.) to boiler, turbine, condenser
and pump :
(i) For boiler (as control volume), we get
hf4 + Q1 = h1
∴ Q1 = h1 – hf4
(ii) For turbine (as control volume), we get h1 = WT + h2, where WT = turbine work
∴ WT = h1 – h2
(iii) For condenser, We get
h2 = Q2 + hf3
∴ Q2 = h2 – hf3
(iv) For the feed pump, we get
hf3 + WP = hf4 , where, WP = Pump work
∴ WP = hf4 – hf3
Now, efficiency of Rankine cycle is given by
The feed pump handles liquid water which is incompressible which means with the increase in pressure
its density or specific volume undergoes a little change. Using general property relation for reversible
adiabatic compression, we get
Tds = dh – vdp
ds = 0 ∴ dh = vdp or ∆h = v ∆p ...... (since change in specific volume is negligible)
or hf4 – hf3 = v3 (p1 – p2)
When p is in bar and v is in m3/kg, we have
hf4 – hf3 = v3 (p1 – p2) × 105 J/kg
The feed pump term (hf4 – hf3 ) being a small quantity in comparison with turbine work, WT, is usually
neglected, especially when the boiler pressures are low. Then,
3.12 REGENERATIVE CYCLE
In the Rankine cycle it is observed that the condensate which is fairly at low temperature has an
irreversible mixing with hot boiler water and this results in decrease of cycle efficiency. Methods are,
therefore, adopted to heat the feed water from the hot well of condenser irreversibly by interchange of
heat within the system and thus improving the cycle efficiency. This heating method is called
regenerative feed heat and the cycle is called regenerative cycle.
The principle of regeneration can be practically utilised by extracting steam from the turbine at several
locations and supplying it to the regenerative heaters. The resulting cycle is known as regenerative or
bleeding cycle. The heating arrangement comprises of : (i) For medium capacity turbines—not more
than 3 heaters ; (ii) For high pressure high capacity turbines—not more than 5 to 7 heaters ; and (iii) For
turbines of super critical parameters 8 to 9 heaters. The most advantageous condensate heating
temperature is selected depending on the turbine throttle conditions and this determines the number of
heaters to be used. The final condensate heating temperature is kept 50 to 60°C below the boiler
saturated steam temperature so as to prevent evaporation of water in the feed mains following a drop
in the boiler drum pressure. The conditions of steam bled for each heater are so selected that the
temperature of saturated steam will be 4 to 10°C higher than the final condensate temperature.
Fig. 3.13 (a) shows a diagrammatic layout of a condensing steam power plant in which a surface
condenser is used to condense all the steam that is not extracted for feed water heating. The turbine is
double extracting and the boiler is equipped with a superheater. The cycle diagram (T-s) would appear
as shown in Fig. 3.13 (b). This arrangement constitutes a regenerative cycle.
Fig 3.13 REGENERATIVE CYCLE
Advantages of Regenerative cycle over Simple Rankine cycle :
1. The heating process in the boiler tends to become reversible.
2. The thermal stresses set up in the boiler are minimised. This is due to the fact that temperature
ranges in the boiler are reduced.
3. The thermal efficiency is improved because the average temperature of heat addition to the cycle is
increased.
4. Heat rate is reduced.
5. The blade height is less due to the reduced amount of steam passed through the low pressure stages.
6. Due to many extractions there is an improvement in the turbine drainage and it reduces erosion due
to moisture.
7. A small size condenser is required.
Disadvantages:
1. The plant becomes more complicated.
2. Because of addition of heaters greater maintenance is required.
3. For given power a large capacity boiler is required.
4. The heaters are costly and the gain in thermal efficiency is not much in comparison to the heavier
costs.
3.13 REHEAT CYCLE
For attaining greater thermal efficiencies when the initial pressure of steam was raised beyond 42 bar it
was found that resulting condition of steam after, expansion was increasingly wetter and exceeded in
the safe limit of 12 per cent condensation. It, therefore, became necessary to reheat the steam after
part of expansion was over so that the resulting condition after complete expansion fell within the
region of permissible wetness.
The reheating or resuperheating of steam is now universally used when high pressure and temperature
steam conditions such as 100 to 250 bar and 500°C to 600°C are employed for throttle. For plants of still
higher pressures and temperatures, a double reheating may be used.
In actual practice reheat improves the cycle efficiency by about 5% for a 85/15 bar cycle. A second
reheat will give a much less gain while the initial cost involved would be so high as to prohibit use of two
stage reheat except in case of very high initial throttle conditions. The cost of reheat equipment
consisting of boiler, piping and controls may be 5% to 10% more than that of the conventional boilers
and this additional expenditure is justified only if gain in thermal efficiency is sufficient to promise a
return of this investment. Usually a plant with a base load capacity of 50000 kW and initial steam
pressure of 42 bar would economically justify the extra cost of reheating.
The improvement in thermal efficiency due to reheat is greatly dependent upon the reheat pressure
with respect to the original pressure of steam.
Fig. 3.14 shows a schematic diagram of a theoretical single-stage reheat cycle. The corresponding
representation of ideal reheating process on T-s and h-s chart is shown in Figs. 3.14 (a and b).
a b
Figs 3.14 REHEAT CYCLE
Advantages of ‘Reheating’ :
1. There is an increased output of the turbine.
2. Erosion and corrosion problems in the steam turbine are eliminated/avoided.
3. There is an improvement in the thermal efficiency of the turbines.
4. Final dryness fraction of steam is improved.
5. There is an increase in the nozzle and blade efficiencies.
Disadvantages :
1. Reheating requires more maintenance.
2. The increase in thermal efficiency is not appreciable in comparison to the expenditure incurred in
reheating.
3.14 BINARY VAPOUR CYCLE
If we use steam as the working medium the temperature rise is accompanied by rise in pressure and at critical temperature of 374.15°C the pressure is as high as 225 bar which will create many difficulties in design, operation and control. It would be desirable to use some fluid other than steam which has more desirable thermodynamic properties than water. An ideal fluid for this purpose should have a very high critical temperature combined with low pressure. Mercury, diphenyl oxide and similar compounds, aluminium bromide and zinc ammonium chloride are fluids which possess the required properties in varying degrees. Mercury is the only working fluid which has been successfully used in practice. It has high critical temperature (588.4°C) and correspondingly low critical pressure (21 bar abs.). The mercury alone cannot be used as its saturation temperature at atmospheric pressure is high (357°C). Hence binary vapour cycle is generally used to increase the overall efficiency of the plant. Two fluids (mercury and water) are used in cascade in the binary cycle for production of power. Fig. 3.15 shows the schematic line diagram of binary vapour cycle using mercury and water as working fluids. The processes are represented on T-s diagram as shown in 3.16
Fig. 3.15 BINARY VAPOUR CYCLE
Fig. 3.16 BINARY VAPOUR CYCLE ON T-S DIAGRAM
PROBLEMS 1. A vessel having a capacity of 0.05 m3 contains a mixture of saturated water and saturated steam at a temperature of 245°C. The mass of the liquid present is 10 kg. Find the following : (i) The pressure, (ii) The mass, (iii) The specific volume, (iv) The specific enthalpy, (v) The specific entropy, and (vi) The specific internal energy.
2. Determine the amount of heat, which should be supplied to 2 kg of water at 25°C to convert it into steam at 5 bar and 0.9 dry.
3. What amount of heat would be required to produce 4.4 kg of steam at a pressure of 6 bar and temperature of 250°C from water at 30°C ? Take specific heat for superheated steam as 2.2 kJ/kg K.
3.1000 kg of steam at a pressure of 16 bar and 0.9 dry is generated by a boiler per hour. The steam passes through a superheater via boiler stop valve where its temperature israised to 380°C. If the temperature of feed water is 30°C, determine : (i) The total heat supplied to feed water per hour to produce wet steam. (ii) The total heat absorbed per hour in the superheater. Take specific heat for superheated steam as 2.2 kJ/kg K.
4. A pressure cooker contains 1.5 kg of saturated steam at 5 bar. Find the quantity of heat which must be rejected so as to reduce the quality to 60% dry. Determine the pressure and temperature of the steam at the new state.
5.Find the internal energy of 1 kg of steam at 20 bar when (i) it is superheated, its temperature being 400°C ; (ii) it is wet, its dryness being 0.9. Assume superheated steam to behave as a perfect gas from the commencement of superheating and thus obeys Charle’s law. Specific heat for steam = 2.3 kJ/kg K.
Solution. Mass of steam = 1 kg Pressure of steam, p = 20 bar Temperature of superheated steam = 400°C (Tsup = 400 + 273 = 673 K) Dryness fraction, x = 0.9 Specific heat of superheated steam, cps = 2.3 kJ/kg K
6. A throttling calorimeter is used to measure the dryness fraction of the steam in the steam main which has steam flowing at a pressure of 8 bar. The steam after passingthrough the calorimeter is at 1 bar pressure and 115°C. Calculate the dryness fraction of the steam in the main. Take cps = 2.1 kJ/kg K.
6.In a steam power cycle, the steam supply is at 15 bar and dry and saturated. The condenser pressure is 0.4 bar. Calculate the Carnot and Rankine efficiencies of the cycle. Neglect pump work.
7. In a steam turbine steam at 20 bar, 360°C is expanded to 0.08 bar. It then enters a condenser, where it is condensed to saturated liquid water. The pump feeds back the water into the boiler. Assume ideal processes, find per kg of steam the net work and the cycle efficiency.
8. A simple Rankine cycle works between pressures 28 bar and 0.06 bar, the initial condition of steam being dry saturated. Calculate the cycle efficiency, work ratio and specific steam consumption.
9. In a single-heater regenerative cycle the steam enters the turbine at 30 bar, 400°C and the exhaust pressure is 0.10 bar. The feed water heater is a direct contact type which operates at 5 bar. Find : (i) The efficiency and the steam rate of the cycle. (ii) The increase in mean temperature of heat addition, efficiency and steam rate as compared to the Rankine cycle (without regeneration). Pump work may be neglected.
10. Steam at a pressure of 15 bar and 250°C is expanded through a turbine at first to a pressure of 4 bar. It is then reheated at constant pressure to the initial temperature of 250°C and is finally expanded to 0.1 bar. Using Mollier chart, estimate the work done per kg of steam flowing through the turbine and amount of heat supplied during the process of reheat. Compare the work output when the expansion is direct from 15 bar to 0.1 bar without any reheat. Assume all expansion processes to be isentropic.
UNSOLVED EXAMPLES 1. Find the specific volume, enthalpy and internal energy of wet steam at 18 bar, dryness fraction 0.9. [Ans. 0.0994 m3/kg ; 2605.8 kJ/kg ; 2426.5 kJ/kg] 2. Find the dryness fraction, specific volume and internal energy of steam at 7 bar and enthalpy 2600 kJ/kg. [Ans. 0.921 ; 0.2515 m3/kg, 2420 kJ/kg] 3. Steam at 110 bar has a specific volume of 0.0196 m3/kg, find the temperature, the enthalpy and the internal energy. [Ans. 350°C ; 2889 kJ/kg ; 2673.4 kJ/kg]
4. Steam at 150 bar has an enthalpy of 3309 kJ/kg, find the temperature, the specific volume and the internal energy. [Ans. 500°C ; 0.02078 m3/kg ; 2997.3 kJ/kg] 5. Steam at 19 bar is throttled to 1 bar and the temperature after throttling is found to be 150°C. Calculate the initial dryness fraction of the steam. [Ans. 0.989] 6. Find the internal energy of one kg of steam at 14 bar under the following conditions : (i) When the steam is 0.85 dry ; (ii) When steam is dry and saturated ; and (iii) When the temperature of steam is 300°C. Take cps = 2.25 kJ/kg K. [Ans. (i) 2327.5 kJ/kg ; (ii) 2592.5 kJ/kg ; (iii) 2784 kJ/kg] 7. Calculate the internal energy of 0.3 m3 of steam at 4 bar and 0.95 dryness. If this steam is superheated at constant pressure through 30°C, determine the heat added and change in internal energy. [Ans. 2451 kJ/kg ; 119 kJ ; 107.5 kJ/kg] 8. Water is supplied to the boiler at 15 bar and 80°C and steam is generated at the same pressure at 0.9 dryness. Determine the heat supplied to the steam in passing through the boiler and change in entropy. [Ans. 2260.5 kJ/kg ; 4.92 kJ/kg K] 9. A cylindrical vessel of 5 m3 capacity contains wet steam at 1 bar. The volume of vapour and liquid in the vessel are 4.95 m3 and 0.05 m3 respectively. Heat is transferred to the vessel until the vessel is filled with saturated vapour. Determine the heat transfer during the process. [Ans. 104.93 MJ] 10. A pressure cooker contains 1.5 kg of steam at 5 bar and 0.9 dryness when the gas was switched off. Determine the quantity of heat rejected by the pressure cooker when the pressure in the cooker falls to 1bar. [Ans. – 2355 kJ] 11. A simple Rankine cycle works between pressure of 30 bar and 0.04 bar, the initial condition of steam being dry saturated, calculate the cycle efficiency, work ratio and specific steam consumption. [Ans. 35%, 0.997, 3.84 kg/kWh] 12. A steam power plant works between 40 bar and 0.05 bar. If the steam supplied is dry saturated and the cycle of operation is Rankine, find : (i) Cycle efficiency (ii) Specific steam consumption. [Ans. (i) 35.5%, (ii) 3.8 kg/kWh] 13. Compare the Rankine efficiency of a high pressure plant operating from 80 bar and 400°C and a low pressure plant operating from 40 bar 400°C, if the condenser pressure in both cases is 0.07 bar. [Ans. 0.391 and 0.357] 14. A steam power plant working on Rankine cycle has the range of operation from 40 bar dry saturated to 0.05 bar. Determine : (i) The cycle efficiency (ii) Work ratio (iii) Specific fuel consumption. [Ans. (i) 34.64%, (ii) 0.9957, (iii) 3.8 kg/kWh] 15. In a Rankine cycle, the steam at inlet to turbine is saturated at a pressure of 30 bar and the exhaust pressure is 0.25 bar. Determine : (i) The pump work (ii) Turbine work (iii) Rankine efficiency (iv) Condenser heat flow (v) Dryness at the end of expansion. Assume flow rate of 10 kg/s. [Ans. (i) 30 kW, (ii) 7410 kW, (iii) 29.2%, (iv) 17900 kW, (v) 0.763] 16. In a regenerative cycle the inlet conditions are 40 bar and 400°C. Steam is bled at 10 bar in regenerative heating. The exit pressure is 0.8 bar. Neglecting pump work determine the efficiency of the cycle. [Ans. 0.296]