6. Thermodynamic Cycles

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6. Thermodynamic Cycles

• Objective
• Classification of Thermodynamics Cycles
• Analysis Calculation of Power Cycles
• Carnot Vapor Cycle, Rankie Cycle,
  Regeneration Rankie Cycle,Reheat Rankie Cycle
• Cogeneration
• Gas Refrigeration Cycle
• Vapor-Compression Refrigeration Cycle
• Refrigerant
• Other Refrigeration Cycles

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6.1 Classification of Thermodynamics Cycles
Power Cycle ()
Heat Energy
Mechanical Energy
Heat Pump Cycle (-)
Refrigeration Cycle keep low temperature of heat
source with low temperature Heat Pump Cycle keep
high temperature of heat source with high
temperature
Working Fluid
Gas Cycle no phase-change of working fluid
during cycle Vapor Cycle phase-change of working
fluid during cycle
Combustion form
Inner Combustion Outer Combustion
Combustion occurs in system Combustion occurs out of system
Gas is also the working fluid. The heat is transferred to working fluid through heat exchanger.
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6.2 Carnot Vapor Cycle
Several impracticalities are associated with this
cycle
1. It is impractical to design a compressor that
will handle two phases for isentropic
compression process(4-1). 2. The quality of steam
decrease during isentropic expansion process(2-3)
which do harm to turbine blades.
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6.2 Carnot Vapor Cycle
3. The critical point limits the maximum
temperature used in the cycle which also limits
the thermal efficiency. 4. The specific volume of
steam is much higher than that of water which
needs big equipments and large amount of work
input.
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6.2 Carnot Vapor Cycle
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6.3 Rankine Vapor Cycle
Principle
4-6 Constant pressure heat addition in a
boiler 6-1 to Superheat Vapor 1-2 Isentropic
expansion in a turbine 2-3 Constant
pressure heat rejection in a
condenser 3-4 Isentropic compression in a
pump
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6.3 Rankine Vapor Cycle
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6.3 Rankine Vapor Cycle
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6.3 Rankine Vapor Cycle
Efficiency
4-5-6-1 Constant pressure heat addition in a
boiler
1-2 Isentropic expansion in a turbine
2-3 Constant pressure heat rejection in a
condenser
3-4 Isentropic compression in a pump
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6.3 Rankine Vapor Cycle
Because of uncompressibility of water
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6.3 Rankine Vapor Cycle
Definition
d the steam required to generate work of
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6.3 Rankine Vapor Cycle
Influencing factors
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6.3 Rankine Vapor Cycle
1. - Pressure of Steam, Turbine Inlet
-Unchange
Two Cycles ? 3-4-5-1-2-3 ? 3-4-5-1-2-3
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6.3 Rankine Vapor Cycle
Disadvantages
1.
decrease the turbine efficiency and erodes the
turbine blades.
Increase of requirements on pressure vessels and
equipment investment.
2.
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6.3 Rankine Vapor Cycle
2. - Temperature of Steam, Turbine Inlet
-Unchange
Two Cycles ? 3-4-5-6-1-2-3 ?
3-4-5-6-1-2-3
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6.3 Rankine Vapor Cycle
Advantages
i
ii
it decreases the moisture content of the steam at
the turbine exit.
Disadvantages
Superheating temperature is limited by
metallurgical considerations.
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6.3 Rankine Vapor Cycle
3. - Condenser Pressure, Turbine Exit
-Unchange
Two Cycles ? 1-2-3-4-5-6-1 ?
1-2-3-4-5-6-1
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6.3 Rankine Vapor Cycle
i
Disadvantages
ii
i Condense pressure is limited by the sink
temperature. ii It increases the moisture
content which is highly undesirable.
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6.3 Rankine Vapor Cycle
Example

• Consider a steam power plant operating on the
  ideal Rankine
• cycle. The steam enters the turbine at 2.5MPa and
  350? and
• is condensed in the condenser at pressure of
  70kPa. Determine
• The thermal efficiency of this power plant
• The thermal efficiency if steam is condensed at
  10kPa
• The thermal efficiency if steam is superheated to
  600 ?
• The thermal efficiency if the boiler pressure is
  raised to 15MPa while the turbine inlet
  temperature is maintain at 600 ?

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State 1
State 2
Ideal Rankine Cycle
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State 3
State 4
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6.3 Rankine Vapor Cycle
Actual cycle
Irreversibility

• Flow friction
• Heat transfer under temperature
• difference
• Heat loss to the surroundings

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6.3 Rankine Vapor Cycle
Actual Rankine Vapor Cycle
Turbine Efficiency
Consumed Steam kg/h
Ideal Cycle
Actual Cycle
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6.3 Rankine Vapor Cycle
Mechanical Efficiency
Relative Effective Efficiency
Effective Power
Boiler Efficiency
Equipment Efficiency
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6.4 Improvement to Rankine Cycle
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???????? ???????????????,????? Transfer heat to
the feedwater from the expanding steam in a heat
exchanger built into the turbine ,called
Regeneration.
Disadvantages It is difficult to control the
temperature The dryness is small
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6.4 Improvement to Rankine Cycle
Ideal Regenerative Cycle
Regenerative Cycle 1-7-d-3-4-5-6-1 General
Carnot Cycle3-4-5-7-d-3 Ideal Carnot Cycle
5-7-2-e-5
Same Efficiency
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Ideal Regenerative Cycle
Extracting Regeneration
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Ideal Regenerative Cycle
gt0
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Ideal Regenerative Cycle
1
Turbine
Boiler
Regenerator
7
2
Mixing Chamber
8
Cond- enser
9
4
6
5
3
Pump II
Pump I
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6.3.2 Ideal Reheat Cycle
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?????????????????
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Ideal Reheat Cycle
intermediate pressure
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6.4 Improvement to Rankine Cycle
Extracting Regeneration
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6.4 Improvement to Rankine Cycle
Cogeneration

• Definition
• Cogeneration is the production of more
  than one
• useful form of energy from the same energy
  source.
• electric power
• heat in low quality

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6.5 Gas Refrigeration Cycle
Ideal Reversed Carnot Cycle
T1 Temperature of heat source with high
temperature, surrounding temperature T2
Temperature of heat source with low
temperature, cold source q1 Heat
rejected to the surroundings q2 Heat absorbed
from cold source w0 Work input
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6.5 Gas Refrigeration Cycle
1-2 Isotropic Compress 2-3 Isotonic Heat
Rejection to Surrounding 3-4 Isotropic
Expansion 4-1 Isotonic Heat Absorption
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6.5 Gas Refrigeration Cycle
Cp Constant, Ideal Gas

• Heat Absorbed from Cold Source

• Heat Rejected to the condenser

• Work of Compressor

• Work of Turbine

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6.5 Gas Refrigeration Cycle
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6.5 Gas Refrigeration Cycle
T
3
3
4
2
5
5
1
6
g
m
n
k
s
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Vapor-Compression Refrigeration Cycle

• Shortcomings of Gas-Compression Refrigeration
  Cycle
• 1.small Refrigeration-Coefficient because
  heat absorption
• and rejection are not isothermal
  process
• 2.Lower refrigeration capability of
  refrigerant (gas)

• Sorefrigerant is changed to Vapor
• The highest efficiency is that of Vapor
  Carnot Reverse Cycle

Impracticalities 1.Large moisture content is
highly undesirable for compressor and
turbine. 2.Work output is limited by liquid
expansion in the turbine.
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Vapor-Compression Refrigeration Cycle

• Sopractical vapor-compression refrigeration
  cycle is

2
2
3
4
3
1
4
1
5
6
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Vapor-Compression Refrigeration Cycle
1-2 Isotropic compress to superheated vapor
2-3-4 Isotonic condensed to saturated liquid
4-5 Isentropic expansion in a turbine
4-6 Isotropic expansion through throttle to humidity vapor
5-1 Constant pressure heat absorption in a cool source to dry saturate vapor
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Vapor-Compression Refrigeration Cycle
Throttle
? fluid with low quality is difficult to be
compressed. ? work loss is relatively small ?
easily adjust pressure of fluid and
temperature of cold source
Work difference between Turbine and throttle
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Vapor-Compression Refrigeration Cycle
Regeneration more realistic cycle
Advantages 1. 2. 3.Superheated vapor is
desirable
T
2
Super- cooled Liquid
3
4
4
Superheated Vapor
1
1
5
5
s
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Vapor-Compression Refrigeration Cycle
Conditions
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Vapor-Compression Refrigeration Cycle
Irreversibility 1-2
Isotropic Compress Efficiency
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6.7 Refrigerant
Definition The work fluid cycling flowing in
refrigeration system while transferring energy
with surrounding in order to refrigerate.

• Thermodynamic Request
• Critical temperature should be much higher than
  temperature
• of surroundings.
• ? steam easier be condensed
• ? larger range of latent heat
• ? heat absorption and heat rejection
  closer to
• isothermal process

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6.7 Refrigerant

• Thermodynamic Request
• Solidification temperature should be lower than
  evaporation
• temperature to prevent blocking the pipes.
• Larger latent heat is more desirable.
• appropriate saturate pressure
• small
• being nontoxic ,non-corrosive, nonflammable,
  chemically steady
• low cost

Environment Safety Request
Ammonia ? , Feron ???
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6.8 Absorption Refrigeration System
Definition The form of refrigeration that
inexpensive thermal energy instead of mechanical
energy or electric power is consumed to transfer
heat form low temperature to high temperature
is absorption refrigeration.
Geothermal Energy Solar Energy
Absorption refrigeration system involves
the absorption of a refrigerant by a transport
medium .
Ammonia Water NH3- H2O
Water lithium bromide H2O - LiBr
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6.8 Absorption Refrigeration System
Principle
NH3
Weak
Rich
NH3
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6.8 Absorption Refrigeration System
Thermodynamic Analysis
Thermal Efficiency
Advantage A liquid is compressed instead of a
vapor , and thus the work input for absorption
refrigeration system is very small.
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6.9 Vapor-Jet Refrigeration System
Principle
Diffuser
Nozzle
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6.9 Vapor-Jet Refrigeration System
T
1
5
3
4
1
2
2
5
s
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6.9 Vapor-Jet Refrigeration System
Thermodynamic Analysis
Thermal Efficiency
Disadvantage Irreversibility such as mixture
process and heat transfer with temperature
difference Large exergy loss
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6.10 Liquefaction of Gases

• The liquefaction of gases has always been
  an important area of
• refrigeration since many important scientific and
  engineering process
• at cryogenic temperature depend on liquefied gas.
• Example
• separation of oxygen and nitrogen from air
• preparation of liquid propellants for rockets
• the study of material properties at low
  temperature
• the study of exciting phenomenon such as
  superconductivity

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6.10.1 Min. Work in Liquefaction of Gases
Gas-Liquid Coefficient
Quality at State 4
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6.10.2 Linde Cycle
Principle
P2
P1
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6.10.2 Linde Cycle
Thermodynamic Analysis
Take the Heat Exchanger,
Expansion Valve, Separator as
system. Liquid y kg gas (1-y) kg
Heat of liquefaction y kg
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6.10.2 Linde Cycle
Irreversibility in liquefaction of gas ? heat
loss in heat exchanger q ? non-adiabatic, heat
addition from surrounding q
Thermodynamic Analysis
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6.10.2 Linde Cycle
Irreversibility in compression of gas ?
isothermal compression 1-2 ? isothermal
efficiency (0.59)
Thermodynamic Analysis
Actual work consumption
cannot be treated as Ideal Gas
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6.10.3 Claude Cycle
Thermodynamic Analysis
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Piston expander Turbine
Considering mechanical efficiency