17-11-2012, 11:47 AM
DESIGN AND FABRICATION OF PULSE TUBE REFRIGERATION SYSTEM
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ABSTRACT
Cooling effect at one end of a hollow tube with a pulsating pressure in the inside gas, was first observed by Gifford and Longsworth in early sixties. This marks the inception of one of the most promising cryogenic refrigerators known as 'pulse tube refrigerator' (PTR). Cryogenics is the science of low temperature. Cryogenics refers to the entire phenomenon occurring below -150°C or 123K. Cryogenic engineering involves the design and development of systems and components which produce maintain, or utilize low temperatures. Cryocoolers are devices which produce the required refrigeration power at low temperature. The pulse tube refrigerator has been investigated for cooling various types of sensitive sensors such as infrared detectors for missiles, military aircrafts, tanks, night vision equipment and SQUIDs (super conducting quantum interference devices).
Introduction
Cryogenics is the science of low temperature. Cryogenics refers to the entire phenomenon occurring below -150°C or 123K. Cryogenic engineering involves the design and development of systems and components which produce maintain, or utilize low temperatures. Cryocoolers are devices which produce the required refrigeration power at low temperature. There is an increasingly strong need for cryocoolers for various applications. For military purpose, almost all infrared detectors, thermal imagers and night vision devices need some form of cryogenic cooling. The space borne infrared sensors require cryocoolers of particularly high reliability and long life time. In the electronics, cryogenic cooling of the chips of semiconductors brings about significant improvement of performance by drastically reducing the thermal noises. Cryocoolers are main component of the ultra clean high vacuum cryopumps used for high reliability in the manufacture of semiconductors and other thin films. In the fields of medicine and cryobiology, cryocoolers are used for cryogenic surgery and conservation. Cryocoolers are also used as small scale helium liquefiers for the applications such as the cooling of super conducting magnets.
Low cost and high reliability are the crucial factor for the successful applications of cryocoolers in these important domains.
Cooling effect at one end of a hollow tube with a pulsating pressure in the inside gas, was first observed by Gifford and Longsworth in early sixties. This marks the inception of one of the most promising cryogenic refrigerators known as 'pulse tube refrigerator' (PTR). Due to the absence of moving parts in the cold temperature region, and the associated advantages of simplicity and enhanced reliability, the pulse tube system has become one of the most researched topics in the area of cryogenic refrigeration. The pulse tube refrigerator has been investigated for cooling various types of sensitive sensors such as infrared detectors for missiles, military aircrafts, tanks, night
CRYOCOOLERS.
Cryogenic temperatures are achieved and maintained by one or more refrigerating units known as 'cryocoolers'. Cryocoolers can be classified as either recuperative or regenerative cryocoolers.
Recuperative Cryocoolers.
The recuperative coolers use only recuperative heat exchangers and operate with a steady flow of refrigerant through the system. Figure 1.1 shows the schematic of three common recuperative cryocooler cycles. The Joule-Thompson (JT) cryocooler shown in figure 1.1(a) is very much like the vapour compression refrigerator, except for the addition of the main heat exchanger to cover the long temperature span. In vapour compression refrigerators the compression takes place below the critical temperature of the refrigerant. As a result liquefaction at room temperature occurs in the condenser. Expansion of the liquid in the JT capillary, orifice or valve is relatively efficient and provides enough temperature drops that little or no heat exchange with the returning cold, expanded gas is required. Nevertheless, the irreversible expansion of the fluid in the JT valve is less efficient than a reversible expansion with an expansion engine or turbine. Thus the Brayton cryocooler shown in figure 1.1(b) offers the potential for higher efficiency with sacrifice in simplicity, cost, and possibly reliability. The Claude system combines the JT and the Brayton cycle shown in figure 1.1©. It is used primarily for gas liquefaction where liquid may damage the expansion engines or turbines. The use of valves in compressors or the high pressure-ratios needed for recuperative cryocoolers limits the efficiency of the compression process to about 50% and significantly limits the overall efficiency of recuperative refrigerators.
Stirling Refrigerator.
The compressor in the Stirling refrigerator is a valve less type. It creates an oscillating pressure in the system where the amplitude of oscillation is typically about 10 to 30% of the average pressure. In order to provide high power densities and keep the system small, the average pressure is typically in the range of 1 to 30 MPa and frequencies are in the range of 20 to 60 Hz. Helium is almost always used as the working fluid in the regenerative cycles because of its ideal gas properties, its high thermal conductivity, and its high ratio of specific heats.
A pressure oscillation by itself in a system would simply cause temperature to oscillate and produce no refrigeration. The second moving component, the displacer, is required to separate the heating and cooling effects by introducing motion of the gas in the proper phase relationship with the pressure oscillation. When the displacer in figure 1.2(a) is moved downward, the helium gas is displaced to the warm end of the system through the regenerator. The piston in the compressor then compresses the gas, and the heat of compression is removed by heat exchange with the ambient. Next the displacer is moved up to displace the gas through the regenerator to the cold end of the system. The piston then expands the gas, mow located at the cold end, and the cooled gas absorbs heat from the system before the displacer forces the gas back to the warm end through the regenerator. There is little pressure difference across the displacer (only enough to overcome the pressure drop in the regenerator) but there is large temperature difference.
Pulse Tube Refrigerator.
The pulse tube refrigerator, first conceived in the mid 1960s, was of academic interest until 1984. Since then, improvement in its efficiency has occurred rapidly. Unlike the Stirling or Gifford-Mc-Mahon refrigerators, it has no moving parts at the cold end region. One variation has also been developed with no moving parts in the entire system. The lack of cold moving parts has allowed it to solve some of the problems associated
CLASSIFICATION OF PULSE TUBE REFRIGERATOR.
The first report of pulse tube refrigeration by W.E. Gilford and R.C. Longsworth inl963 was enough to excite many researchers due to the potential of high reliability inspite of its simplicity. For several decades then, many researchers have concentrated their efforts on improving the performance of pulse tube refrigerators in various ways. As a result, different configurations of pulse tube refrigerators have been introduced. Several representative configurations are detailed below.
Basic Pulse Tube Refrigerator.
Figure 1.3(a) shows a schematic diagram of a basic pulse tube refrigerator. A basic pulse tube refrigerator consists of a compressor, after cooler, regenerator, cold heat exchanger, hot heat exchanger and pulse tube. The periodic pressurization and expansion produced by the compressor causes the gas to flow back and forth through the regenerator and pulse tube. Figure 1.3(b) depicts the cooling mechanism of a basic pulse tube refrigerator. During the compression process, pressurized gas moves towards the hot heat exchanger located at the closed end of the pulse tube. The gas in the pulse tube experiences near adiabatie compression and associated temperature rise. The gas at the boundary layer exchanges heat with the tube wall. Heat transfer, through the hot end heat exchanger wall, cools the gas in the hot heat exchanger. During the subsequent expansion process, the depressurized gas moves towards the cold heat exchanger. The gas element experiences near adiabatie expansion and an associated temperature drop.
Orifice Pulse Tube Refrigerator.
In figure 1.4, an orifice valve and reservoir have been added at the end of the hot heat exchanger. The reservoir is large enough to be maintained at nearly constant intermediate pressure during operation. The valve and the reservoir cause the gas to flow through the orifice valve at the points of maximum and minimum pressures. Therefore the reservoir improves the phase relationship between pressure and gas motion. The orifice pulse tube creates refrigeration through PV-work as well as surface heat pumping. The gas column in the pulse tube acts like the displacer in the Stirling cycle refrigerator. This PV-work is transferred from the compressor to the cold heat exchanger through the regenerator. It is continuously delivered from the cold end to the hot end of the pulse tube with associated pressure changes. Then this PV-work is dissipated in the valve and transferred as heat in the hot heat exchanger. As a result, both the PV-work transferred by the gas column and the surface heat pumping near the pulse tube wall affects the cooling performance.
[attachment=39921]
ABSTRACT
Cooling effect at one end of a hollow tube with a pulsating pressure in the inside gas, was first observed by Gifford and Longsworth in early sixties. This marks the inception of one of the most promising cryogenic refrigerators known as 'pulse tube refrigerator' (PTR). Cryogenics is the science of low temperature. Cryogenics refers to the entire phenomenon occurring below -150°C or 123K. Cryogenic engineering involves the design and development of systems and components which produce maintain, or utilize low temperatures. Cryocoolers are devices which produce the required refrigeration power at low temperature. The pulse tube refrigerator has been investigated for cooling various types of sensitive sensors such as infrared detectors for missiles, military aircrafts, tanks, night vision equipment and SQUIDs (super conducting quantum interference devices).
Introduction
Cryogenics is the science of low temperature. Cryogenics refers to the entire phenomenon occurring below -150°C or 123K. Cryogenic engineering involves the design and development of systems and components which produce maintain, or utilize low temperatures. Cryocoolers are devices which produce the required refrigeration power at low temperature. There is an increasingly strong need for cryocoolers for various applications. For military purpose, almost all infrared detectors, thermal imagers and night vision devices need some form of cryogenic cooling. The space borne infrared sensors require cryocoolers of particularly high reliability and long life time. In the electronics, cryogenic cooling of the chips of semiconductors brings about significant improvement of performance by drastically reducing the thermal noises. Cryocoolers are main component of the ultra clean high vacuum cryopumps used for high reliability in the manufacture of semiconductors and other thin films. In the fields of medicine and cryobiology, cryocoolers are used for cryogenic surgery and conservation. Cryocoolers are also used as small scale helium liquefiers for the applications such as the cooling of super conducting magnets.
Low cost and high reliability are the crucial factor for the successful applications of cryocoolers in these important domains.
Cooling effect at one end of a hollow tube with a pulsating pressure in the inside gas, was first observed by Gifford and Longsworth in early sixties. This marks the inception of one of the most promising cryogenic refrigerators known as 'pulse tube refrigerator' (PTR). Due to the absence of moving parts in the cold temperature region, and the associated advantages of simplicity and enhanced reliability, the pulse tube system has become one of the most researched topics in the area of cryogenic refrigeration. The pulse tube refrigerator has been investigated for cooling various types of sensitive sensors such as infrared detectors for missiles, military aircrafts, tanks, night
CRYOCOOLERS.
Cryogenic temperatures are achieved and maintained by one or more refrigerating units known as 'cryocoolers'. Cryocoolers can be classified as either recuperative or regenerative cryocoolers.
Recuperative Cryocoolers.
The recuperative coolers use only recuperative heat exchangers and operate with a steady flow of refrigerant through the system. Figure 1.1 shows the schematic of three common recuperative cryocooler cycles. The Joule-Thompson (JT) cryocooler shown in figure 1.1(a) is very much like the vapour compression refrigerator, except for the addition of the main heat exchanger to cover the long temperature span. In vapour compression refrigerators the compression takes place below the critical temperature of the refrigerant. As a result liquefaction at room temperature occurs in the condenser. Expansion of the liquid in the JT capillary, orifice or valve is relatively efficient and provides enough temperature drops that little or no heat exchange with the returning cold, expanded gas is required. Nevertheless, the irreversible expansion of the fluid in the JT valve is less efficient than a reversible expansion with an expansion engine or turbine. Thus the Brayton cryocooler shown in figure 1.1(b) offers the potential for higher efficiency with sacrifice in simplicity, cost, and possibly reliability. The Claude system combines the JT and the Brayton cycle shown in figure 1.1©. It is used primarily for gas liquefaction where liquid may damage the expansion engines or turbines. The use of valves in compressors or the high pressure-ratios needed for recuperative cryocoolers limits the efficiency of the compression process to about 50% and significantly limits the overall efficiency of recuperative refrigerators.
Stirling Refrigerator.
The compressor in the Stirling refrigerator is a valve less type. It creates an oscillating pressure in the system where the amplitude of oscillation is typically about 10 to 30% of the average pressure. In order to provide high power densities and keep the system small, the average pressure is typically in the range of 1 to 30 MPa and frequencies are in the range of 20 to 60 Hz. Helium is almost always used as the working fluid in the regenerative cycles because of its ideal gas properties, its high thermal conductivity, and its high ratio of specific heats.
A pressure oscillation by itself in a system would simply cause temperature to oscillate and produce no refrigeration. The second moving component, the displacer, is required to separate the heating and cooling effects by introducing motion of the gas in the proper phase relationship with the pressure oscillation. When the displacer in figure 1.2(a) is moved downward, the helium gas is displaced to the warm end of the system through the regenerator. The piston in the compressor then compresses the gas, and the heat of compression is removed by heat exchange with the ambient. Next the displacer is moved up to displace the gas through the regenerator to the cold end of the system. The piston then expands the gas, mow located at the cold end, and the cooled gas absorbs heat from the system before the displacer forces the gas back to the warm end through the regenerator. There is little pressure difference across the displacer (only enough to overcome the pressure drop in the regenerator) but there is large temperature difference.
Pulse Tube Refrigerator.
The pulse tube refrigerator, first conceived in the mid 1960s, was of academic interest until 1984. Since then, improvement in its efficiency has occurred rapidly. Unlike the Stirling or Gifford-Mc-Mahon refrigerators, it has no moving parts at the cold end region. One variation has also been developed with no moving parts in the entire system. The lack of cold moving parts has allowed it to solve some of the problems associated
CLASSIFICATION OF PULSE TUBE REFRIGERATOR.
The first report of pulse tube refrigeration by W.E. Gilford and R.C. Longsworth inl963 was enough to excite many researchers due to the potential of high reliability inspite of its simplicity. For several decades then, many researchers have concentrated their efforts on improving the performance of pulse tube refrigerators in various ways. As a result, different configurations of pulse tube refrigerators have been introduced. Several representative configurations are detailed below.
Basic Pulse Tube Refrigerator.
Figure 1.3(a) shows a schematic diagram of a basic pulse tube refrigerator. A basic pulse tube refrigerator consists of a compressor, after cooler, regenerator, cold heat exchanger, hot heat exchanger and pulse tube. The periodic pressurization and expansion produced by the compressor causes the gas to flow back and forth through the regenerator and pulse tube. Figure 1.3(b) depicts the cooling mechanism of a basic pulse tube refrigerator. During the compression process, pressurized gas moves towards the hot heat exchanger located at the closed end of the pulse tube. The gas in the pulse tube experiences near adiabatie compression and associated temperature rise. The gas at the boundary layer exchanges heat with the tube wall. Heat transfer, through the hot end heat exchanger wall, cools the gas in the hot heat exchanger. During the subsequent expansion process, the depressurized gas moves towards the cold heat exchanger. The gas element experiences near adiabatie expansion and an associated temperature drop.
Orifice Pulse Tube Refrigerator.
In figure 1.4, an orifice valve and reservoir have been added at the end of the hot heat exchanger. The reservoir is large enough to be maintained at nearly constant intermediate pressure during operation. The valve and the reservoir cause the gas to flow through the orifice valve at the points of maximum and minimum pressures. Therefore the reservoir improves the phase relationship between pressure and gas motion. The orifice pulse tube creates refrigeration through PV-work as well as surface heat pumping. The gas column in the pulse tube acts like the displacer in the Stirling cycle refrigerator. This PV-work is transferred from the compressor to the cold heat exchanger through the regenerator. It is continuously delivered from the cold end to the hot end of the pulse tube with associated pressure changes. Then this PV-work is dissipated in the valve and transferred as heat in the hot heat exchanger. As a result, both the PV-work transferred by the gas column and the surface heat pumping near the pulse tube wall affects the cooling performance.