Pulse Tube Cryocooler

By,

Utsav rao (U10ME112)

B.Tech. IV

Mechanical Engineering department, SVNIT.

Guided by,

Dr. H.B Naik

Professor

Mechanical Engineering department, SVNIT.

Introduction

Cryogenic is a science to achieve low temperature

Temperature below 123 Kelvin are generally consider in cryogenic range

The cryogenic system whose power of cooling is in few watts are consider as cryocooler, where as the system whose cooling power is kilowatts is know as cryorefregerator

Recuperative and Regenerative are the two basic classification of cryocooler

Joules thomson and byrayton cryocooler

are recuperative type where as Stirling

and GM cryocooler are regenerative type

Cryocoolers are use in cooling of infrared

sensor in missile guidance system,

liquefaction of gases, SQUID (Super

Conducting Quantum Interference

Device), Super conductor etc.

Closed Cycle Cryocoolers

Like heat engine cryocooler make use of a

woking substance which undergoes a

thermodynamic cycle of operation.

In close cycle cryocooler, the working

substance absorbs heat Qc from a sample at

a cold temperature at Tc and rejects heat Qw

at a higher temperature Tw. In this process,

the external work done on the system is W.

The ratio W/Qc is a measure of effectiveness

of the refrigerator.

For ideal carnot cycle COP is given as,

Multi stage cryocooler

Pulse Tube Cryocooler

Reliability of small cryocooler was a problem since long time which was under study from many years. One of the approaches to increase reliability is to eliminate mechanical moving component from the cryocooler. The Stirling type cryocooler have two moving component a piston and a gas displacer, where as the GM type cryocooler has one moving component i.e. displacer.

Cooling effect at one end of a hollow tube with a pulsating pressure at the other end was first observed by Gifford and Longsworth, after that in 1963 they discover new type of cryocooler in which moving component was replaced with gas piston. This is known as pulse tube cryocooler.

The working of Pulse tube cryocoolers can be understood with the help of above Figure Consider a thin walled tube closed at one end and other end open. High and low pressure is alternatively applied at the open end. When high pressure is applied the gas boundary moves from left to right end of tube as the pressure inside increases from Pl to Ph. As the work is done on gas due to compression its temperature increases. Since gas in contact with the wall the closed end gets heated.

Similarly when low pressure is applied at closer end the pressure inside tube decreases from Ph to Pl so the gas boundary moves from right to left which leads to expansion of gas, so gas gets cooled so the wall at the open end cools down. Repeated compression and expansion or pressure pulsing cause heating effect on closed end and cooling effect at the open end of tube.

When the heat exchanger are added at both end, the effect is now substantiated, since warm end heat exchanger (WHE) ensure that the gas is maintained at some steady temperature after compression. Hence on expansion, the cold end heat exchanger shows cooling effect.

To convert cooling effect to useful refrigeration, a highly efficient regenerator is introduced. The WHE is now circulated with cooling water. By this, the heat of compression at the end of high pressure Ph, will be removed from WHE, causing the gas to come back to ambient temperature. Now the gas gets cooled on expansion and the gas will gradually cool both CHE and regenerator. The next batch of incoming gas takes in the cold storage in the regenerator and hence the temperature of the gas entering the pulse tube is slightly less than that of the previous cycle. Thus the CHE is gradually cooled to lower and lower temperature.

To attain the lowest possible temperature, one should use helium gas as a working fluid. The pressure variation produced at the inlet of the pulse tube cryocooler varies sinusoidally as

This pressure oscillation can be produced by using a compressor and a rotary valve. The pressure wave produces a sinusoidal oscillation in the velocity v of the gas in the pulse tube. Let the velocity be given as.

The enthalpy flow at any point in the pulse tube can be averaged over one period of the pressure cycle and is given by

The average enthalpy is proportional to , where is the phase difference between pressure and velocity. The average enthalpy flow is a maximum when the pressure and velocity field at any point are in phase. If heat needs to be extracted from the cold end of the pulse tube, then the velocity and pressure oscillations must be in phase at this end.

Types of Pulse tube Cryocooler

Basic pulse tube cryocooler (BPTC).

Pulse tube refrigeration systems can be classified as either a Stirling type or a GM type according to the method of pressurization and expansion as shown in Figure (a) and (b).

Stirling type cryocooler works on frequency of comperessor which is generally 10-120 Hz and produce high cooling power but its performance is limited at low temperature

GM type cryocooler works on low frequency i.e 1-5 Hz.

There is rotarary valve between compressor and Pulse

tube. As due to valve there is friction loss in it so

cooling power achieve in it less then stirling type but

low temperature below 10 kelvin can be achieved

through it.

Orifice Pulse Tube Cryocooler (OPTC). The Orifice Pulse Tube Cryocooler is a modified version of

basic pulse tube cryocooler. This modification is made by

including an orifice valve and a surge volume at the warm

end of the BPTC. Additional components create an advantage

of in-phase relationship between the mass flow and the

pressure within the pulse tube to enhance the heat transport

mechanism. But the mass flow through the regenerator is

increases leading to degradation of regenerator performance.

This drawback is removed by adding a second orifice i.e.

double inlet PTR.

Double Inlet Pulse Tube Cryocooler (DIPTC). In the DIPTR the hot end of the pulse tube is connected with

the entrance (hot end) of the regenerator by an orifice adjusted

to an optimal value . The double inlet is a bypass for the

regenerator and hence reduces the cooling power. In addition,

the valve is a dissipative device, which leads to a deterioration

of the performance. However, both these disadvantages are

overcome by the fact that the double inlet reduces the

dissipation in the regenerator. As a result, the performance of

the overall system is improved significantly.

Multi-stage type PTC.

It is really impossible to

achieve very low temperature

in a single stage of PTC. So

one PTC can be used to pre-

cool the other. For

temperatures below 30K, it

turns out to be advantageous

to split the system in two i.e.

double-stage PTR. In this

arrangement the hot end of

the second tube is connected

to the cold end of the first

stage. Three-stage PTCs have

also been introduced. With a

three-stage PTC, 1.78K has

been reached using He3 as the

working fluid.

Components

Pulse tube cryocooler The Schematic and photograph of the two stage Pulse Tube refrigerator is

shown below. The cold end of the first stage regenerator forms the warm

end of the second stage regenerator. The warm ends of the Pulse Tubes

and that of the first stage regenerator are mounted to the top flange of the

system. The cold and warm end heat exchangers of the first and second

stages are designed such that it leads to the best performance of Pulse

Tube Cryocooler.

Rotary valve

The rotary valve is one of the critical components of

a GM type pulse tube. It is used to switch high and

low pressure from a helium compressor to the pulse

tube system. The high and low pressure of helium

compressor are connected to the rotary valve

The rotary valve has a rulon part which is made

to rotate with the help of a synchronous motor

against an aluminium block with predefined

passages connecting the high and low pressures

from the helium compressor. The rotational

frequency of the synchronous motor is

controlled using an inverter drive.

Helium Compressor The main function of the compressor is to supply gas

pressurization and depressurization in the closed chamber.

Electrical power is applied to the compressor where this electrical

work is converted into the mechanical energy associated with

sinusoidal pressure waves, resulting in gas motion. In an ideal

compressor, the electrical power provided to the compressor must

be equal to ∫PdV, where the integration is performed over an entire

cycle, P is the compressor pressure, and f is the compressor

frequency. In an actual system, however, the above-mentioned

power (the PdV power) is always less than the actual measured

electrical power due to the associated irreversibilities.

Orifice valve

The role of either the inertance tube or the orifice

valve is to appropriately adjust the phase difference

between the mass flow rateand the pressure. By

controlling the orifice diameter or the inertance tube

diameter and length, the desired phase relationship

can be obtained. In general, the orifice valve is a

needle valve.

Temperature measurement

Silicon diode.

Silicon Diodes are the best choice for general-purpose cryogenic use. The cryogenic temperature sensors are interchangeable (they follow a standard curve). Silicon Diodes are easy to instrument, and are used in a wide variety of cryogenic applications, such as cryo-coolers, laboratory cryogenics, cryo-gas production, and space satellites.

Germanium resistor temperature sensor.

Germanium RTDs have the highest accuracy, reproducibility, and sensitivity from 0.05 K to 100 K. They are resistant to ionizing radiation, but are not recommended for use in magnetic fields. Germanium RTDs are used mostly in research settings when the best accuracy and sensitivity are required. Germanium and Ruthenium Oxide are the only two cryogenic temperature sensors that can be used below 100 mK.

Thermocouples.

Thermocouples can be used over an extremely wide range and in harsh environmental conditions, and follow a standard response curve. Less accurate than other cryogenic temperature sensors, special techniques must be employed when using thermocouples to approach temperature accuracies of 1% of temperature. Thermocouples are used for their small size, extremely wide temperature range (exceeding high temperature limits of Platinum RTDs), and simple temperature measurement methodology.

Platinum resistor temperature sensor.

Platinum RTDs are an industry standard. They follow an industry standard curve from 73 K to 873 K with good sensitivity over the whole range. Platinum RTDs can also be used down to 14 K. Because of their high reproducibility, they are used in many precision metrology applications. Platinum RTDs are inexpensive and require simple instrumentation. They are widely used in cryogenic applications at liquid nitrogen temperatures or greater.

Capacitive sensors.

Capacitance sensors are ideally suited for use as temperature control sensors in strong magnetic fields because they exhibit virtually no magnetic field dependence. Small variations in the capacitance/temperature curves occur upon thermal cycling. It is recommended that temperature in zero field be measured with another cryogenic temperature sensor, and that the capacitance sensor be employed as a control element only.

Regenerator Materials The regenerator is the most important component in pulse tube

refrigerator. Its function is to absorb the heat from the incoming

gas during the forward stroke, and deliver that heat back to the gas

during the return stroke. Ideally, PTC regenerators with no

pressure drop and a heat exchanger effectiveness of 100% are

desired, in order to achieve the maximum enthalpy flow in the

pulse tube. The performances of the real regenerators are of

course far from ideal. Stainless steel wire screens, compunds of

rarer earth material such as Er3Ni and HoCu2 are usually selected

as the regenerator packing material, since they offer higher heat

transfer areas, low pressure drop, high heat capacity, and low

thermal conductivity.

Generally in first stage regenerator lead (Pb) and wire mesh is used

where as in second stage regenerator compound of rarer earth

materials are used

Discussion and conclusion

Cooling through Pulse tube cryocooler is one of the best way because of its high reliability compared to G-M cryocooler and sterling cryocooler. It does not require big infrastructure in compared to other system. Rare earth material regenerator such as Er3Ni plays an important role to achieve low temperature with the use of Helium as a working fluid.

Optimization of the pulse tube cryocooler is a big challenge. In optimization mass flow rate of the working fluid is to be set to achieve low temperature work is going on in the field of increasing of its efficiency and optimizing it with the help of CFD analysis.

Future scope is there in the field of thermo acoustically driven pulse tube cryocooler in which the pressure pulse is generated in thermo acoustic prime mover. Solar energy can also be used to dive the thermo acoustic prime mover.

Thank You