Principle and Operation of Pulse Tube Refrigeration

1. Principle And Operation
of Pulse Tube
Refrigeration
University of the South Pacific
School of Engineering and Physics
MM321- Refrigeration and Air conditioning
°
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2. Objective
• To demonstrate the principle and operation of the pulse tube
refrigeration system.
• To describe the processes involved and the governing
equations in the basic pulse tube refrigerator
• To briefly describe the modifications made to the basic pulse
tube refrigerator.
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3. Introduction
• A pulse tube refrigerator is a Cryocooler capable of
reaching temperatures of a few tens of Kelvin in a single
stage and a few kelvin in two stage.
• Unlike ordinary VCR cycle, a pulse tube refrigeration
system implements the oscillatory compression and
expansion of gas within a closed volume to achieve the
desired refrigeration.
• In other words, generation of low temperature is
achieved due to compression and expansion of a gas.
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4. Brief History
• Pulse tube refrigeration is a recent innovation.
• They were first reported by Prof. W. Gifford and his
graduate student, R Longworth, of Syracuse University at
around 1960 [1].
• They noticed that a closed end of a pipe became very hot
when there was a pressure oscillation inside, whereas
the open end towards the compressor was cool.
• Connecting such a line to a compressor through a
regenerator produced cooling at one end and heating at
the other.
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5. Literature Review
• It is not generally realized that delivery of a constant
temperature gas into a closed volume, thus increasing
the pressure, will result in large temperature gradients.
• By suitable arrangement of a thermal regenerator, heat
exchangers it is possible to preserve this temperature
gradient in an essentially static state even though there is
a rapid flow of gas in and out of the volume, pressure
variation is great, and pressure oscillation from a
maximum to a minimum occurs many times per minute
[2].
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6. Literature Review
• These temperature gradients are maintained by pulsating gas.
• Therefore, if the hot end of the gradient is cooled to room
temperature, the cold end will descend to a lower
temperature.
• Pulse tube refrigeration is a method which uses this principle
to achieve low temperature refrigeration in small compact
tubes [3].
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7. Literature Review
• Pulse tube refrigerator units operate as closed systems where
no mass is exchanged between the Cryocooler and the
environment.
• The only moving component is the piston ( and the rotary
valve) which oscillates back and forth to generate periodic
pressure oscillation of the working fluid.
• Mostly helium is chosen as the working fluid because it offers
the lowest critical temperature compared to other available
gases.
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8. Literature Review
Working Fluid
• The most common working fluid for the pulse tube
refrigerator is Helium.
• Helium non toxic and environment friendly.
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9. Literature Review
Working Fluid
• Helium was a choice of coolant as its properties allow
components to be kept cool over long distances. At
atmospheric pressure gaseous helium becomes liquid at
around 4.2 K (-269.0°C). However, if cooled below 2.17 K (-
271.0°C), it passes from the fluid to the superfluid state.
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10. Literature Review
Basic Pulse Tube Refrigerator:
• Its basic components include a pulse tube, regenerator, a
pressure wave generator and two heat exchangers and an
after cooler as shown in the figure below.
Fig 1
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11. Literature Review
• The piston, compressor or similar pressure wave generator is
attached to the warm end of the regenerator and provides the
pressure oscillations that drives the refrigeration.
• The regenerator is a periodic flow heat exchanger.
• It absorbs heat from the gas pumped into the pulse tube precooling
it, and stores the heat for half a cycle then transfers it back to
outgoing cold gas in the second half of the cycle cooling the
regenerator.
• The interior of the regenerator tube is filled with either stacked fine
mesh screens or packed spheres to increase its heat capacity.
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12. Literature Review
• The pulse tube is a simple tube, with one open end and
closed end.
• The closed end is the hot end and is capped with a heat
exchanger that cools it to the ambient temperature.
Fig 2
Closed end
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13. Literature Review
• The open end is the cold end.
• It is connected to the regenerator and a cold stage by a
second heat exchanger
Fig 3
Open end
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14. Literature Review
Components
Fig 4
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15. Literature Review
• The pulse tube works by transporting heat against a
temperature gradient in a process called surface heat
pumping [4].
• It occurs in many systems subjected to pressure oscillations.
• The piston compresses the working gas and this high pressure
gas in turn compresses the gas already in the tube acting as a
gas piston.
• At the same time, the temperature of the gas rises as they
undergo adiabatic compression.
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16. Literature Review
• All the gas that was initially in the tube will be compressed to
the hot end.
• The extra gas that flows in from the regenerator has a
pressure gradient.
• The pressure is highest closest to the hot end and lowest at
the bottom of the pulse tube.
Fig 5
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17. Literature Review
• The pressure gradient directly results in a temperature
gradient.
• At the hot end of the pulse tube, the gas conducts its heat to
the heat exchanger and the temperature falls.
• The piston then retracts and the gas undergoes adiabatic
expansion cooling it even more.
• As the expanding gases passes from the pulse tube into the
regenerator, it absorbs heat from the regenerator and pulse
tube walls cooling them.
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18. Literature Review
• The next cycle starts by compressing the gas back through the
precooled regenerator.
• The gas begins at a lower temperature and it therefore
reaches an even lower temperature after finishing its
compression and expansion cycle.
• Record temperatures of 74K have been achieved with the
basic pulse tube refrigerator.
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19. Literature Review
Analysis of Pulse Tube Refrigerator
• If a mass of gas 𝑀𝑐 with specific heat 𝐶 𝑝 is compressed to a
temperature 𝑇 𝑚 just before it enters the hot end heat
exchanger at a temperature 𝑇ℎ, then when it enters this heat
exchanger it will give up an amount of heat equal to the
refrigeration effect 𝑄 𝑟 .
• During the exhaust phase of the cycle, gas leaves the hot end
heat exchanger at temperature 𝑇ℎ and expands to an exhaust
temperature 𝑇𝑎 which is less than 𝑇𝑐 creating a refrigeration
effect 𝑄 𝑟.
𝑄 𝑟 = 𝑀𝑐 𝐶 𝑝(𝑇𝑐 − 𝑇𝑎)
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20. Literature Review
• The relation for the temperature at any point x in the
tube, Tx in terms of the volume of the hot end heat
exchanger, 𝑉ℎ, and the total volume from the top of the
tube point to point x , 𝑉𝑥, for a gas whose ratio of specific
heat is 𝛾, is given by:
𝑇ℎ
𝑇𝑥
= [
𝑉 𝑥
𝑉ℎ
+𝛾−1
𝛾
]
𝛾−1
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21. Literature Review
Orifice Pulse Tube Refrigeration
• The basic pulse tube and more generally the surface heat
pumping technique is of limited use when very low
temperatures are required.
• Modifications made to the basic design involved adding an
orifice outside the heat exchanger and a reservoir closing the
orifice [5].
• With the improvements, a low temperature of 60K was
achieved.
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22. Literature Review
Fig 6 (Orifice Pulse Tube Refrigerator)
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23. Literature Review
• The hot end heat exchanger is equivalent to the condenser
and the cold end heat exchanger is equivalent to the
evaporator in the vapor compression cycle.
• During the PTR operation, most of the heat generated due to
the compression is rejected through the after cooler.
• The rest of the energy that is not rejected is carried to the
regenerator by enthalpy flow 𝐻𝑟𝑔.
• The regenerator enthalpy flow and the additional
refrigeration load are absorbed at the cold heat exchanger.
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24. Literature Review
• The enthalpy flow enters the pulse tube, and travels down the
tube, reaches the HHX and part of this enthalpy is rejected to
the environment.
• The portion of the enthalpy that has not been rejected
through the heat exchanger flows to the reservoir through the
orifice.
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25. Literature Review
Applications
• Military
• Environmental
• Transportation
• Energy
Problems
• Reliability
• Efficiency
• Cost
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26. Conclusion
• Pulse tube refrigeration is still a developing technology in the
field of refrigeration
• We have briefly studied the basic pulse tube and the
modifications made to it by the addition of the orifice tube
and the reservoir.
• Although they are very much similar, the method of analysis is
somewhat different.
• Pulse tube refrigeration is very important in cryogenics
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27. Referencing
• 1. Gifford, W.E. and Longsworth, R.C. Pulse tube refrigeration,
Trans ASME B J Eng Industry 86(1964), pp.264-267.
• 2. Gifford, W.E. and Longsworth, R.C. Pulse tube refrigeration
progress, Advances in cryogenic engineering 3B (1964), pp.69-
79.
• 3. Gifford, W.E. and Longs worth, R.C. Surface heat pumping,
Advances in cryogenic engineering 11(1966), pp.171-179.
• 4. Gifford, W.E. and Kyanka, G.H. Reversible pulse tube
refrigerator, Advances in cryogenic engineering 12(1967),
pp.619-630.
• 5. de Boer, P. C. T., Thermodynamic analysis of the basic pulse-
tube refrigerator, Cryogenics34(1994) ,pp. 699-711 .
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