Cryocoolers – Introduction
- Cryocoolers are small refrigerators that can reach cryogenic temperatures and provide refrigeration in the temperature range 10 K to 120 K.
- A refrigerator designed to reach cryogenic temperatures is often called a cryocooler.
- In most cases cryocoolers use a cryogenic fluid as the working substance and employ moving parts to cycle the fluid around a thermodynamic cycle. The fluid is typically compressed at room temperature, precooled in a heat exchanger, then expanded at some low temperature. The returning low-pressure fluid passes through the heat exchanger to precool the high-pressure fluid before entering the compressor intake. The cycle is then repeated.
- Cryocoolers do not use the Claude/Collins cycles used by large refrigeration plants but use alternative cycles.
Depending on the end-use application, the basic requirements, as given below, have to be satisfied by the Cryocooler.
- Less weight and small volume.
- Fast cool down time and vibrationless operation.
- More Mean time between maintenance (MTBM), Mean time between failure (MTBF) and Mean time to failure (MTTF).
Types of Cryocooler
Cryocooler may be classified as follows :
- Heat exchangers and regenerators
- Stirling refrigerators
- Gifford – McMahon refrigerators
- Pulse Tube refrigerators
- Joule – Thomson refrigerators
Heat exchangers and regenerators
- Heat exchangers are important components of all cryocoolers. A heat exchanger is a device in which the warm fluid gets cooled due to heat exchange with the cold fluid.
- In most of the cases, the process of heat exchange occurs at a constant pressure.
- It can either be a regenerative or recuperative type of heat exchanger depending on the kind of heat exchange between the fluids.
Recuperative Heat exchanger
• In a recuperative heat exchanger, the flow direction of two fluids is constant and is simultaneous.
• The two fluids are separated by a solid boundary across which the warm and cold fluids exchange heat.
Regenerative Heat exchanger
• In a regenerative heat exchanger, a matrix is used as an intermediate heat exchange medium between the warm and cold fluids.
• The flow is periodic in nature alternating between the warm and cold fluids across the matrix.
• It is important to note that, it is an example of indirect heat transfer.
Pulse Tube Refrigerator
The idea of a pulse tube was first proposed by Gifford and Longsworth in 1961. The effects of sudden expansion and release of refrigerant gas are employed to get a refrigeration effect. Refer to the figure for the circuit of pulse-tube refrigeration. K. Pulse tube cooler is one kind of
cryocooler. The use of pulse tube has been propelled by many requirements of modern-day applications such as adequate refrigeration at specified temperature with low power input, long lifetime, reliable and maintenance-free operation with minimum vibration and noise, compactness, and lightweight
Types of Pulse Tube Refrigerator:
A. Basic Pulse Tube Refrigerator
B. Orifice Pulse Tube Refrigerator
C. Double Inlet Pulse Tube Refrigerator
D. Inertance Tube Pulse Tube Refrigerator
Working of Pulse Tube Refrigerator :
The apparatus consists of a high-pressure gas source, which supplies gas at a temperature equal to the ambient temperature to the base of the pulse tube. Now compressed air is supplied from the air compressor. The compressed air enters the vortex tube through a valve. Further, compressed gas is allowed to enter the pulse tube. Now, the compressed gas acts on an imaginary piston i.e. the separating air column. Thus, the gas inside the pulse tube gets compressed resulting in an increase in the temperature in the pulse tube from top to bottom.
The maximum temperature is at the base. A coolant is circulated to remove heat from the gas, which is now compressed at the base. The coolant rejects heat absorbed to a coolant tank and is pumped back to the pulse tube. Due to cooling by the coolant, gas temperature reduces.
Now, the supply of high-pressure gas is stopped and the inlet valve is closed. The exhaust valve now opens and gas is exhausted lowering temperature inside the tube. A detailed sketch of the pulse tube is shown below in figure . and the temperature profile is shown in the figure.
This lowered temperature causes the refrigerating effect. The air leaving the pulse tube also becomes cooler. It is possible to use this cool air to cool down hot compressed air entering the pulse tube with the help of a heat exchanger. This further lowers the temperature of the pulse tube. Temperature as low as 190°K is achieved by the pulse tube. Thus, a pulse tube is a preferred choice if cooling is to be done in areas with low power generation capacities.
Advantages of Pulse Tube
1) Less moving parts, so no maintenance and fewer losses
2) Low power consumption
3) Multi staging is possible.
4) Longer life
5) No lubrication required
6) Can be used instead of cryogenic cycles
Disadvantages of Pulse Tube
1) Poor coefficient of performance (COP)
2) COP depends on the length of the tube
3) Tubes costly to manufacture.
Application Of Pulse Tube Refrigerator ;
- Liquefaction of gases such as nitrogen, oxygen, hydrogen, helium, natural gas.
- SQUID magnetometers.
- Cooling of super-conducting magnets.
- Cooling of infrared sensors for missile guidance.
- Cryo vacuum pumps.
- Cooling of high-temperature superconductors and semiconductors.
- Gamma-ray sensors for monitoring nuclear activity.
- Preservation of biological materials, blood, biological specimens, etc.
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