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2.1 Thermodynamic and Cryogenic Cycles Review.
2.1.1 Thermo dynamic Cycles
A thermodynamic cycle is a series of thermodynamic processes which returns a system to
its initial state. Properties depend only on the thermodynamic and thus do not change
over a cycle. Variables such as heat and work are not zero over a cycle, but rather are
process dependent. A thermodynamic process may be definedas the energetic evolution
of a thermodynamic system proceeding from an initial state to a final state. Paths through
the space of thermodynamic variables are often specified by holding certain
thermodynamic variables constant.
Some of the thermodynamic processes are given below:
 An isobaric process is a thermodynamic process in which the pressure stays
constant: p = 0.
 An isochoricprocess, also called an isovolumetric process, is a process during
which volume remains constant.
 An isothermal process is a change in which the temperature of
the system stays constant: T = 0.
 A polytropic process is a thermodynamic process that obeys the relation:
PVn = C,
Where P is pressure, V is volume, n is any real number (the polytropic index),
and C is a constant.
Some of thethermodynamics are: Carnot cycle, Ericsson cycle, Stirling cycle, Otto cycle.
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2.1.2 Cryogenic Cycles
The field of cryogenics advanced during World War II when scientists found that metals
frozen to low temperatures showed more resistance to wear. Cryogenic is the branch of
physics and engineering that involves the study of very low temperature, how to produce
them, how material behave tothose temperature. A process is said to be cryogenic if the
operating temperature is below -1500c or 123k. For most of cryogenic work the temp
range lies between 4k -77k. Specifically Linde and Claude cycle is discussed for the
present study.
 LINDE Cycle
Linde cycle is a typical cryogenic cycle used for liquefaction of gases and is the simplest
of all liquefaction cycle. The schematic ofthe cycle is shown below.
Mixer
Compresso
Liquefied Gas.
Patm
Separator
Liquefied Gas
Figure 2.1 Showing line diagram of LINDE cycle.
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Figure 2.2 Showing T~S diagram of LINDE cycle.
Components Used
1. COMPRESSOR: - It is a device used to reduce the volume of gaseous air and
increase the pressure. Generally for cryogenic application compressor with high
compression ratio are used. Toachieve high stage compression ratio a number of
compressor are used in series rather using a single compressor. It also reduces
work consumption. For present work isothermal compression process is used.
2. HEAT EXCHANGER: - Heat exchangers are devices which transfer heat from
hot fluid stream to cold fluid stream. In heat exchanger hot fluid temperature
decreases and there is increase intemperature of cold fluid. By losing heat hot
fluid is prepared for throttling process and similarly by gaining heat cold fluid
heated up for compression process.
3. VALVE: - A throttling valve is used to reduce the pressure of the compressed air
so that liquid air can be produced and stored. The process is assumed to be
isenthalpic expansion.
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4. SEPARATOR/DISTILLATION COLUMN: - In thischamber air is separated
into desired components like liquid N2, liquid O2 etc and the gaseous part is again
recirculated.
5. MIXER: - It is a device helps to maintain a constant flow rate of air into the
compressor. The extra amount of air is added into incoming stream from
separator. The process is assumed to be isobaric.
A basic differentiation between the various refrigeration cycles lies inthe expansion
device. This may be either an expansion engine like expansion turbine or reciprocating
expansion engine or a throttling valve. The expansion engine approaches an isentropic
process and the valve an isenthalpic process. Isentropic expansion implies an adiabatic
reversible process while isenthalpic expansions are irreversible. In the Linde’s system,
the basic principle of...
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