MEC 451

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MEC 451 Thermodynamics
CHAPTER
Lecture Notes MOHD HAFIZ MOHD NOH HAZRAN HUSAIN
MOHD SUHAIRIL Faculty of Mechanical
Engineering Universiti Teknologi MARA, 40450 Shah
Alam, Selangor
Second Law of Thermodynamics
For students EM 220 and EM 221 only
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Introduction

• A process must satisfy the first law in order to
  occur.

• Satisfying the first law alone does not ensure
  that the process will take place.

• Second law is useful

• provide means for predicting the direction of
  processes,
• establishing conditions for equilibrium,
• determining the best theoretical performance of
  cycles, engines and other devices.

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A cup of hot coffee does not get hotter in a
cooler room.
Transferring heat to a paddle wheel will not
cause it to rotate.
These processes cannot occur even though they are
not in violation of the first law.
Transferring heat to a wire will not generate
electricity.
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Second Law of Thermodynamics
Kelvin-Planck statement

• No heat engine can have a thermal efficiency 100
  percent.

• As for a power plant to operate, the working
  fluid must exchange heat with the environment as
  well as the furnace.

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Heat Engines

• Work can easily be converted to other forms of
  energy, but?

• Heat engine differ considerably from one another,
  but all can be characterized

• they receive heat from a high-temperature source

• they convert part of this heat to work

• they reject the remaining waste heat to a
  low-temperature sink atmosphere

• they operate on a cycle

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The work-producing device that best fit into the
definition of a heat engine is the steam power
plant, which is an external combustion engine.
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Thermal Efficiency

• Represent the magnitude of the energy wasted in
  order to complete the cycle.

• A measure of the performance that is called the
  thermal efficiency.

• Can be expressed in terms of the desired output
  and the required input

• For a heat engine the desired result is the net
  work done and the input is the heat supplied to
  make the cycle operate.

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The thermal efficiency is always less than 1 or
less than 100 percent.
where
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• Applying the first law to the cyclic heat engine

• The cycle thermal efficiency may be written as

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• A thermodynamic temperature scale related to the
  heat transfers between a reversible device and
  the high and low-temperature reservoirs by

• The heat engine that operates on the reversible
  Carnot cycle is called the Carnot Heat Engine in
  which its efficiency is

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Heat Pumps and Refrigerators

• A device that transfers heat from a low
  temperature medium to a high temperature one is
  the heat pump.

• Refrigerator operates exactly like heat pump
  except that the desired output is the amount of
  heat removed out of the system

• The index of performance of a heat pumps or
  refrigerators are expressed in terms of the
  coefficient of performance.

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Carnot Cycle
Process Description
1-2 Reversible isothermal heat addition at high temperature
2-3 Reversible adiabatic expansion from high temperature to low temperature
3-4 Reversible isothermal heat rejection at low temperature
4-1 Reversible adiabatic compression from low temperature to high temperature

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Execution of Carnot cycle in a piston cylinder
device
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• The thermal efficiencies of actual and reversible
  heat engines operating between the same
  temperature limits compare as follows

• The coefficients of performance of actual and
  reversible refrigerators operating between the
  same temperature limits compare as follows

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Example 4.1
Solution
A steam power plant produces 50 MW of net work
while burning fuel to produce 150 MW of heat
energy at the high temperature. Determine the
cycle thermal efficiency and the heat rejected by
the cycle to the surroundings.
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Example 4.2
A Carnot heat engine receives 500 kJ of heat per
cycle from a high-temperature heat reservoir at
652ºC and rejects heat to a low-temperature heat
reservoir at 30ºC. Determine (a) The thermal
efficiency of this Carnot engine (b) The amount
of heat rejected to the low-temperature heat
reservoir
Solution
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Example 4.3
An inventor claims to have developed a
refrigerator that maintains the refrigerated
space at 2ºC while operating in a room where the
temperature is 25ºC and has a COP of 13.5. Is
there any truth to his claim?
Solution
- this claim is also false!
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Supplementary Problem 4.1

• A 600 MW steam power plant, which is cooled by a
  river, has a thermal efficiency of 40 percent.
  Determine the rate of heat transfer to the river
  water. Will the actual heat transfer rate be
  higher or lower than this value? Why?
• 
  
  900 MW

• A steam power plant receives heat from a furnace
  at a rate of 280 GJ/h. Heat losses to the
  surrounding air from the steam as it passes
  through the pipes and other components are
  estimated to be about 8 GJ/h. If the waste heat
  is transferred to the cooling water at a rate of
  145 GJ/h, determine (a) net power output and (b)
  the thermal efficiency of this power plant.
• 
  
  35.3 MW, 45.4

• An air conditioner removes heat steadily from a
  house at a rate of 750 kJ/min while drawing
  electric power at a rate of 6 kW. Determine (a)
  the COP of this air conditioner and (b) the rate
  of heat transfer to the outside air.
• 
  
  2.08, 1110 kJ/min

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• Determine the COP of a heat pump that supplies
  energy to a house at a rate of 8000 kJ/h for each
  kW of electric power it draws. Also, determine
  the rate of energy absorption from the outdoor
  air.
• 
  
  2.22, 4400 kJ/h

1. An inventor claims to have developed a heat
   engine that receives 700 kJ of heat from a source
   at 500 K and produces 300 kJ of net work while
   rejecting the waste heat to a sink at 290 K. Is
   this reasonable claim?

• An air-conditioning system operating on the
  reversed Carnot cycle is required to transfer
  heat from a house at a rate of 750 kJ/min to
  maintain its temperature at 24oC. If the outdoor
  air temperature is 35oC, determine the power
  required to operate this air-conditioning system.
• 
  
  0.463
  kW

• A heat pump is used to heat a house and maintain
  it at 24oC. On a winter day when the outdoor air
  temperature is -5oC, the house is estimated to
  lose heat at a rate of 80,000 kJ/h. Determine the
  minimum power required to operate this heat pump.
• 
  
  2.18
  kW

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Entropy

• The 2nd law states that process occur in a
  certain direction, not in any direction.

• It often leads to the definition of a new
  property called entropy, which is a quantitative
  measure of disorder for a system.

• Entropy can also be explained as a measure of the
  unavailability of heat to perform work in a cycle.

• This relates to the 2nd law since the 2nd law
  predicts that not all heat provided to a cycle
  can be transformed into an equal amount of work,
  some heat rejection must take place.

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Entropy Change

• The entropy change during a reversible process is
  defined as

• For a reversible, adiabatic process

• The reversible, adiabatic process is called an
  isentropic process.

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Entropy Change and Isentropic Processes
The entropy-change and isentropic relations for a
process can be summarized as follows
i. Pure substances Any process ?s s2 s1
(kJ/kg?K) Isentropic process s2 s1
ii. Incompressible substances (liquids and
solids) Any process s2 s1 cav T2/T1
(kJ/kg Isentropic process T2 T1
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iii. Ideal gases a) constant specific
heats (approximate treatment)
for all process
for isentropic process
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Example 4.5
Steam at 1 MPa, 600oC, expands in a turbine to
0.01 MPa. If the process is isentropic, find the
final temperature, the final enthalpy of the
steam, and the turbine work.
Solution
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• Since that the process is isentropic, s2s1

• Work of turbine

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Isentropic Efficiency for Turbine
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Isentropic Efficiency for Compressor
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Example 4.6
Steam at 1 MPa, 600C, expands in a turbine to
0.01 MPa. The isentropic work of the turbine is
1152.2 kJ/kg. If the isentropic efficiency of the
turbine is 90 percent, calculate the actual work.
Find the actual turbine exit temperature or
quality of the steam.
Solution

• Theoretically

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• Obtain h2a from Wa

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Example 4.7
Air enters a compressor and is compressed
adiabatically from 0.1 MPa, 27C, to a final
state of 0.5 MPa. Find the work done on the air
for a compressor isentropic efficiency of 80
percent.

• For isentropic process of IGL

• Then

Solution

• From energy balance

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Supplementary Problems 4.2

• The radiator of a steam heating system has a
  volume of 20 L and is filled with the superheated
  water vapor at 200 kPa and 150oC. At this moment
  both inlet and exit valves to the radiator are
  closed. After a while the temperature of the
  steam drops to 40oC as a result of heat transfer
  to the room air. Determine the entropy change of
  the steam during this process.
• 
  
  -0.132 kJ/.K

• A heavily insulated piston-cylinder device
  contains 0.05 m3 of steam at 300 kPa and 150oC.
  Steam is now compressed in a reversible manner to
  a pressure of 1 MPa. Determine the work done on
  the steam during this process.
• 
  
  16 kJ

• A piston cylinder device contains 1.2 kg of
  nitrogen gas at 120 kPa and 27oC. The gas is now
  compressed slowly in a polytropic process during
  which PV1.3constant. The process ends when the
  volume is reduced by one-half. Determine the
  entropy change of nitrogen during this process.
• 
  
  -0.0617 kJ/kg.K

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• Steam enters an adiabatic turbine at 8 MPa and
  500oC with a mass flow rate of 3 kg/s and leaves
  at 30 kPa. The isentropic efficiency of the
  turbine is 0.90. Neglecting the kinetic energy of
  the steam, determine (a) the temperature at the
  turbine exit and (b) the power output of the
  turbine.
• 
  
  69.09oC,3054 kW

• Refrigerant-R134a enters an adiabatic compressor
  as saturated vapor at 120 kPa at a rate of 0.3
  m3/min and exits at 1 MPa pressure. If the
  isentropic efficiency of the compressor is 80
  percent, determine (a) the temperature of the
  refrigerant at the exit of the compressor and (b)
  the power input, in kW. Also, show the process on
  a T-s diagram with respect to the saturation
  lines.
• 
  
  58.9oC,1.70 kW

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