10-08-2013, 12:47 PM
Study of Stirling Cryocooler
Introduction
The word cryogenics stems from Greek and means "the production of freezing cold";
however, the term is used today as a synonym for the low-temperature state. In physics,
cryogenics is the study of the production of very low temperature and the behavior of materials
at those temperatures. The branches of physics and engineering that involve the study of very
low temperatures, how to produce them, and how materials behave at those temperatures. The
point on the temperature scale at which refrigeration in the ordinary sense of the term ends and
cryogenics begins is not sharply defined. The workers at the National Bureau of Standards at
Boulder, Colorado have chosen to consider the field of cryogenics as that involving the
temperatures below−150 °C (123K). This is a logical dividing line, since the normal boiling
points of the so-called permanent gases (such as helium, hydrogen, neon, nitrogen, oxygen, and
normal air) lie below −150 °C while the Freon refrigerants, hydrogen sulfide, and other common
refrigerants have boiling points above −150 °C. In cryogenics, the temperatures below 123K are
dealt. Cryogenics is a subset of Refrigeration branch.
Magnetic Resonance Imaging
MRI is a method of imaging objects that uses a strong magnetic field to detect the
relaxation of protons that have been perturbed by a radio-frequency pulse. This magnetic field is
generated by electromagnets, and high field strengths can be achieved by using superconducting
magnets. Traditionally, liquid helium is used to cool the coils because it has a boiling point of
around 4 K at ambient pressure, and cheap metallic superconductors can be used for the coil
wiring. So-called high-temperature superconducting compounds can be made to superconduct
with the use of liquid nitrogen which boils at around 77 K.
Electric Power Transmission
It is difficult to transmit power by overhead cables in big cities, so underground cables
are used. But underground cables get heated and the resistance of the wire increases leading to
waste of power. Superconductors could be used to increase power throughput, although they
would require cryogenic liquids such as nitrogen or helium to cool special alloy-containing
cables to increase power transmission.
Stirling Cryocooler
A well-known and a most commonly used cryocooler is the Stirling cycle cryocooler. This cycle
was first conceived by Robert Stirling in the year of 1815. It was an engine cycle and was aimed
to produce work (engine). This cryocooler is based on the reversed Stirling cycle. Due to this the
final effect obtained is cooling instead of work produced. Whenever a gas is compressed in the
piston cylinder arrangement, it gets heated due to rise in pressure and vice versa when it is
expanded its gets cooled due to reduction in pressure. This principle is used in the Stirling
Cryocooler to produce the refrigerating effect.
Reversed Stirling Cycle
The cycle when reversed in the processes gives cooling effect instead of producing work.
Whenever any gas is compressed, its pressure rises. We know that the pressure and temperature
are in directly proportional to each other. Therefore, when pressure of the gas rises its
temperature also rises accordingly. The reverse case happens when the gas is expanded. During
expansion the pressure of the gas falls which results in reduction of its temperature. Absorption
of heat energy into an expanded gas from the environment results in cooling of the system. This
is the basic principle behind working of the Stirling cryocooler. The working of the reversed
cycle is explained with the help of P vs. V diagram.
Conclusion:
Thus, the study of different types of Stirling cryocooler has been done. The ideal and
actual Stirling cryocooler has been discussed along with the impossibilities in actual cryocooler.
The design of Stirling cryocooler based on Schmidt‟s analysis which gives primary guess of the
dimensions of cryocooler has been completed. As there is loss of heat to the surrounding through
Stirling Cryocooler, there is a huge scope in the research and development of the cryocooler to
improve its efficiency.