Pulse Tube Cryocoolers: Working, Classification, and Phasor Analysis, Lecture notes of Mechanical Engineering

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1

Earlier Lecture

• We have seen the schematic and the working of a Gifford – McMahon Cryocooler ( W. E. Gifford and H. O. Mc Mahon , 1950).
• It has a valve mechanism to generate the pressure pulse. The relation between the pressure pulse and the expander – displacer motion is vital.
• The basic components are Compressor, Flex lines, Regenerator(s), Displacer(s), Valve mechanism.
• A GM system can reach much lower temperatures as compared to a Stirling system.

Topic : Cryocoolers

• Pulse Tube (PT) Cryocooler
• Working of a Pulse Tube Cryocooler
• PT Classification
• Stirling, GM and PT – A Comparison and Applications.
• Phasor Analysis

Outline of the Lecture

• In the earlier lectures, we have seen a regenerative type Stirling and GM systems.
• These systems have a mechanical expander – displacer to displace the working gas.
• The displacers are either free moving or driven by an external mechanism.

Introduction

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Stirling

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G – M

Pulse Tube Cryocooler

• Consider the schematic of a Stirling system as shown in the figure.
• In a Pulse Tube cryocooler, the mechanical displacer is removed and an oscillating gas flow in the thin walled tube produces cooling.
• This gas tube is called as Pulse Tube and this phenomenon is called as Pulse Tube action.

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PT

Cryocooler

PSM

Working of PT Cryocooler

• The components of a PT system are Compressor, Heat exchangers, Regenerator, Pulse tube and Phase Shift Mechanism ( PSM ).
• The details and the requirement of the Phase Shift Mechanism ( PSM ) is explained at the later part of the lecture.
• During pressurization, the high pressure gas flows across the regenerator and into the pulse tube.

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PT

Cryocooler

PSM

Working of PT Cryocooler

• The cold gas, during depressurization, transfers cold to the regenerator matrix, which is used to precool the gas.
• The Hot End temperature at the top is maintained at ambient using water.
• The cooling effect produced at the bottom end, also called as Cold End , is lifted by using a heat exchanger.
• Also, the After Cooler temperature at the top is maintained at ambient.

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PT

Cryocooler

PSM

Working of PT Cryocooler

• The gas movement in the PT does not need any mechanical drive.
• Hence, the vibrations in the PT Cryocooler are less as compared to Stirling and GM Cryocoolers.
• The schematic of the PT Cryocooler, with three heat exchangers, namely, After cooler ( AC ), Cold end heat exchanger ( CHX ), Hot end heat exchanger ( HHX ) is as shown in the next slide.

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PT

Cryocooler

PSM

Pulse Tube Cryocooler

Advantages

• No moving parts in the expander, hence, less vibrations.
• No sealing requirement at low temperature.
• High reliability.

Disadvantages

• No reliability data due to less history.
• Orientation effects.

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PT

Cryocooler

PSM

Pulse Tube Cryocooler

Uses

• Cooling of infrared sensors.
• Space applications.
• Re-condensing of LHe and LN 2.
• Gas liquefaction.

Recent Developments

• Reached below 4 K.
• Miniaturization.

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PT

Cryocooler

PSM

PT Classification

Pulse Tube Cryocooler

Stirling Gifford McMahon

Geometry Phase Shift

Inline (^) U – Type Coaxial (^) Annular

Orifice Inertance Tube Double Inlet Valve

Low Frequency

High Frequency

Very High Frequency

Frequency

Basic

PT Classification

• Depending upon the usage of valves, Pulse Tube cryocooler can either be - Stirling type PT Cryocooler - GM type PT Cryocooler
• Stirling systems are high frequency machines where as, GM systems are of low frequency.
• Each of the systems has its own advantages and disadvantages.

Regenerator Pulse Tube

AC CHX HHX

Valve

PT Classification

• The gas does not change the direction of flow. Hence, the pressure losses are minimum.
• This arrangement delivers best performance as compared to others.
• The cold end is at the center of the system.
• The system is less compact since it occupies huge space (length wise). Inline

PT Classification

• The gas flow undergoes a 180 degree change in flow direction. Due to which the system exhibits pressure drop.
• The cold end is exposed and it is easily accessible.
• The system is more compact and it occupies less space.
• The performance is dependent upon the sharpness of the bend. U – Type