ENGINEERING THERMODYNAMICS SHORT QUESTIONS AND ANSWERS - UNIT III FOR ANNA UNIVERSITY SYLLABUS

1. ME 6301
ENGINEERING
THERMODYNAMICS
unit - iii
[FOR THIRD SEMESTER B.E MECHANICAL
ENGINEERING STUDENTS]
COMPILED BY
BIBIN.C
ASSISTANT PROFESSOR
DEPARTMENT OF MECHANICAL ENGINEERING
RMK COLLEGE OF ENGINEERING AND TECHNOLOGY
GUMMIDIPOONDI TALUK
TIRUVALLUR DIST

2. ME 6301 - ENGINEERING THERMODYNAMICS
UNIT III - PROPERTIES OF PURE SUBSTANCE AND STEAM POWER CYCLE
PROPERTIES OF PURE SUBSTANCES
1. Define pure substance
A pure substanee is a homogenous and invariable chemical composition through
out the mass.
2. What is Triple point?
The point at which Solid, liquid and vapour phases are equilibrium is called as
triple point.
OR
The triple point is merely the point of intersection of sublimation and vapourisation
curves.
3. Define enthalpy of steam.
It is the sum of heat added to water from freezing point to saturation temperature
and the heat absorbed during evaporation.
4. Define latent heat of evaporation or Enthalpy of evaporation.
The amount of heat added during heating of water up to dry steam from boiling
point is known as Latent heat of evaporation or enthalpy of evaporation.
5. Explain the term super heated steam and super heating.
The dry steam is further heated its temperature raises, this process is called as
superheating and the steam obtained is known as superheated steam.
6. Explain heat of super heat or super heat enthalpy.
The heat added to dry steam at 1000 C to convert it into super heated steam at the
temperature Tsup is called as heat of superheat or super heat enthalpy.
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3. ME 6301 - ENGINEERING THERMODYNAMICS
7. Define dryness fraction (or) What is the quality of steam?
It is defined as the ratio of mass of the dry steam to the mass of the total steam.
8. Explain the term critical point, critical temperature and critical pressure.
In the T-S diagram the region left of the waterline, the water exists as liquid. In right
of the dry steam line, the water exists as a super heated steam. In between water and dry
steam line the water exists as a wet steam. At a particular point, the water is directly
converted into dry steam without formation of wet steam. The point is called critical point.
The critical temperature is the temperature above which a substance cannot exist as a
liquid; the critical temperature of water is 374.150 C. The corresponding pressure is called
critical pressure.
9. How do you determine the state of steam?
If V>Vg then super heated steam, V= Vg then dry steam and V< Vg then wet steam.
If S>Sg then super heated steam, S= Sg then dry steam and S< Sg then wet steam.
10.Define heat of vapourisation.
The amount of heat required to convert the liquid water completely into vapour
under this condition is called the heat of vapourisation.
11.Explain the terms, Degree of super heat, degree of sub-cooling.
The difference between the temperature of the superheated vapour and the
saturation temperature at the same pressure. The temperature between the saturation
temperature and the temperature in the sub cooled region of liquid.
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4. ME 6301 - ENGINEERING THERMODYNAMICS
STEAM POWER CYCLE
12.Why Rankine cycle is modified?
The work obtained at the end of the expansion is very less. The work is too
inadequate to overcome the friction. Therefore the adiabatic expansion is terminated at
the point before the end of the expansion in the turbine and pressure decreases suddenly,
while the volume remains constant.
13.What are the assumptions made on the analysis of ideal Rankine cycle?
i. Each component of the working fluid is internally reversible.
ii. All processes of the working fluid are internally reversible.
iii. The pump and turbine operate adiabatically.
iv. Potential and kinetic energy affects are neglected.
v. Condensate leaves the condenser as saturated liquid.
14.What are the various methods used to improve the efficiency of Rankine cycle?
i. Increase the boiler pressure (or) Temperature
ii. Decrease the condenser pressure
iii. Increase the temperature of steam at superheated condition
iv. Reheating the steam
v. Adopting regeneration of steam
15.What are the advantages of reheat cycle?
i. It increases turbine work
ii. It increases the efficiency of the plant
iii. It reduces wear of turbine blades by reducing moisture content in steam.
16.Define specific steam consumption of an ideal Rankine cycle.
It is defined as the mass of steam required per unit power output.
Specific steam consumption = (Steam flow kg/h) / (Power)
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5. ME 6301 - ENGINEERING THERMODYNAMICS
17.What is the effect of reheating the steam on the specific output and the cycle
efficiency?
The specific output and the cycle efficiency are increased due to reheating the steam in
vapour power cycles.
18.What is meant by vapour power cycle? Give some examples
Thermodynamic cycles which uses vapour as the working fluid is called vapour
power cycle.
Example: Carnot cycle, Rankine cycle.
19.Define efficiency ratio.
The ratio of actual cycle efficiency to that of the ideal cycle efficiency is termed as
efficiency ratio.
20.Define overall efficiency.
It is the ratio of the mechanical work to the energy supplied in the fuel. It is also
defined as the product of combustion efficiency and the cycle efficiency.
21.Define specific steam consumption of an ideal Rankine cycle.
It is defined as the mass flow of steam required per unit power output.
22.Name the different components in steam power plant working on Rankine cycle.
Boiler, Turbine, Cooling Tower or Condenser and Pump.
23.What is meant by work ratio? What is the importance of work ratio in vapour cycles?
Work ratio is defined as the ration of network transfer to the positive work transfer.
Work ratio affects the actual cycle efficiency. Comparing two cycles with the same
ideal efficiency, the cycle having lower work ratio would have smaller actual efficiency.
Higher work ratio, smaller the plant size.
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6. ME 6301 - ENGINEERING THERMODYNAMICS
24.Compare Carnot and Rankine cycles
Carnot cycle Rankine cycle
1. Lower work ratio 1. Higher work ratio
2. Specific steam consumption is more 2. Specific steam consumption is less
3. Size of the power plant for a given
output is big
3. Size of the power plant for a given
output is small
4. Higher thermal efficiency 4. Lower thermal efficiency
25.What is the function of feed water heater?
The main function of feed water heater is to increase the temperature of feed water
to the saturation temperature corresponding to the boiler pressure before it enters into the
boiler.
26.Why reheat cycle is not used for low boiler pressure?
At low boiler pressure the reheat cycle efficiency may be less than the Rankine cycle
efficiency. This may be due to the lower temperature during heating.
27.What are the advantages of vapour power cycles over gas power cycle?
i. The isothermal heat transfer is possible in condenser and evaporator
ii. The work ratio is high compared to the gas power cycle.
28.Why carnot cycle cannot be realised in practice for vapour power cycles?
The main difficulty to attain the cycle in practice is that isothermal condensation is
stopped before it reaches to saturated liquid condition. Therefore the compressor has to
deal with a non-homogeneous mixture of water and steam. Because of the large specific
volume of liquid vapour mixture before compression, the compressor size and work input
have to be large. The higher power requirement reduces the plant efficiency as well as
work ratio.
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7. ME 6301 - ENGINEERING THERMODYNAMICS
29.What are the effects of condenser pressure in Rankine cycle?
By lowering the condenser pressure, we can increase the cycle efficiency. The main
disadvantage is lowering the backpressure increases the wetness of steam. Isentropic
compression of a very wet vapour is very difficult.
30.What are the disadvantages of bleeding?
Cost of the plant increased and the work done per kg is reduced which results in
higher boiler capacity for given output.
31.What are the advantages of bleeding?
i. It increases the thermodynamic efficiency as the heat of the bled steam is not
lost in the condenser but is utilized in feed water heating
ii. By bleeding, the volume flow at the low-pressure end is considerably reduced,
this reduces the design difficulties of blades, and also condenser size is reduced.
32.Mention the improvements made to increase the ideal efficiency of Rankine cycle.
i. Lowering the condenser pressure.
ii. Superheated steam is supplied to the turbine.
iii. Increasing the boiler pressure to certain limit.
iv. Implementing reheat and regeneration in the cycle.
33.What are the effects of condenser pressure on the Rankine Cycle?
By lowering the condenser pressure, we can increase the cycle efficiency. The main
disadvantage is lowering the back pressure increase the wetness of steam. Isentropic
compression of a very wet vapour is very difficult.
34.Why reheat cycle is not used for low boiler pressure?
At the low reheat pressure the heat cycle efficiency may be less than the Rankine
cycle efficiency. Since the average temperature during heating will then be low.
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8. ME 6301 - ENGINEERING THERMODYNAMICS
35.What are the disadvantages of reheating?
i. The cost of the plant increases
ii. It increases the condenser capacity due to increased dryness fraction.
36.What are the advantages of reheat cycle?
i. It increases the turbine work.
ii. It increases the heat supply.
iii. It increases the efficiency of the plant.
iv. It reduces the wear on the blade because of low moisture content in LP
v. State of the turbine.
37.What is the purpose of reheating?
The purpose of reheating is to increase the dryness fraction of the steam passing out
of the later stages of the turbine.
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