Cryogenics

The word "Cryogenics" is derived from two Greek words: "kryos", which means cold or freezing, and "genes", which means being born of or generated. Unfortunately, the word now seems to be more concerned with the practice of dipping steel into liquid nitrogen to make it harder; a modern form of snake-oil (though the conversion of austenite to martensite can be assisted with cryogenic annealing at the time of tempering). Cryogenics is the branch of physics concerned with the production and maintenance of extremely low temperatures, and with the effects that occur under such conditions.

By using a succession of liquefied gases, a substance may be cooled to as low as 4.2 degrees Kelvin, the boiling point of liquid helium. Still lower temperatures may be reached by successive magnetization and demagnetization. Temperatures as low as about one millionth of a degree above absolute zero on the Kelvin scale were attained in 1991 by using infrared lasers first to reduce the energy level of cesium atoms in a vacuum and then to induce further energy loss in the atoms. Some unusual conditions, notably superconductivity, superfluidity, and large-scale quantum mechanical effects such as macroscopic bose-einstein condensation, prevail at cryogenic temperatures.

Refrigeration

  • Pressure Change Cooling

    The expansion or contraction of a gas under pressure is adiabatic, meaning that there is a temperature change associated with the volume change. For an ideal gas, this relationship is expressed in the equation PV=nRT. By performing mechanical work on a gas to change its volume, the temperature can be forced to change as well.

    Pressure Change cooling techniques use a compressor to increase the pressure of a gas, raising its temperature. The hot, pressurized gas then circulates through a radiator, giving off its heat to the outside air. The pressurized gas, now at room temperature, is then allowed to expand and lose pressure, thereby cooling. The cold gas then circulates through a radiator that removes heat from the inside of the refrigerator, warming up the low pressure gas. The warmed gas is then fed into the compressor once again to renew the cycle.

  • Phase Change Cooling

    Changes of state, between the three phases of matter, are accompanied by thermal changes for those compounds that undergo 1st order phase transitions. This is what makes the phase stable at a given temperature, the fact that heat must be added without an increase in temperature to overcome the energy barrier that governs the phase change. The heat that must be put into a solid at the melting point to change it into a liquid at the same temperature is called the latent heat of fusion. The heat that must be put into a liquid at the boiling point to change it into a gas at the same temperature is called the latent heat of vaporization. The reverse processes release the same amount of heat.

    Evaporative cooling can be easily accomplished with the loss of material but the phase change cycle is more commonly combined with compressor- driven refrigeration to reduce the extremities of temperature required to pump a given amount of heat.

  • The Vapor-Compression Refrigeration Cycle

    The standard method for achieving refrigeration is through the vapor- compression cycle. The compressor takes low pressure, low temperature refrigerant gas and compresses it to high pressure, high temperature gas. This is accomplished with a piston system or rotary vanes. From the compressor, the hot, high pressure gas travels through the discharge line into the condenser. The condenser is the part of the system where the heat is lost by condensation. Air passing over the coils removes the heat from the refrigerant so that it has all condensed to liquid at the end of the condenser. As the refrigerant leaves the condenser, it has cooled, and condensed to liquid, but is still under high pressure. In order for the liquid to absorb the necessary heat in the evaporator, its pressure must be reduced, which is accomplished within the expansion device. The evaporator is the component of the cycle which actually absorbs the heat from the cold space. It is just a radiator, often with a fan to circulate air. The refrigerant then returns to the compressor as a low pressure, low temperature gas.

    The refrigerant used must have the characteristics that allow condensation at room temperature. Ammonia was used before the discovery of freon, but leaks in such systems were often fatal. Freons, as we are now aware, were not the perfect solution either.

  • Thermoelectric Cooling

    When a temperature gradient is imposed upon a conductor, the resulting redistribution of electrons within the conductor create a potential difference between the hot end and the cold end. If we consider the electrons within the conductor to behave as a gas, then the thermal motion at the hot end will cause the density of electrons to be less than that at the cold end. This sets up a concentration gradient of electrons between the hot and cold ends so that closing the circuit results in a flow of current that will continue as long as the temperature gradient is maintained. This is known as the Seebeck effect. The electrons will create resistive heating as they move through the conductor, thereby bringing the conductor to thermal equilibrium, or limiting the efficiency of the thermal gradient is maintained.

    The electric field that develops across the conductor is proportional to the temperature gradient, so that:

    (4.2.1)

    The coefficient, Q, is called the thermopower; it can be either positive or negative, depending on the temperature and conductor (some metals have both positive and negative thermopowers depending on temperature). The difference in thermopowers is due mainly to the scattering of electrons as then move through the lattice. In semiconductors where the majority carriers are holes (p-type), the thermopower will be negative. Thermocouples use this generated voltage to measure the temperature of a system.

    The reverse procedure, to force a current through a conductor, will generate a thermal gradient within the conductor. This is known as the Peltier effect, and is used to build thermoelectric cooling devices.

    The efficiency of thermoelectric devices is related to the thermal conductivity, the electrical conductivity, and the thermopower. Increases in thermal and electrical conductivities lead to an increased efficiency but they are negatively correlated with the thermopower. Thus, if we consider a spectrum from insulators, through semiconductors, to metallic conductors, the optimal efficiency for thermoelectric devices occurs in the semiconductor range. Generally, a positive thermoelement will be placed with a junction to a negative thermoelement to improve the efficiency.

  • Vortex Cooling

    The Ranque-Hilsch vortex tube is a device that separates a flow of gas into two streams, one hot and the other cold, in the absence of any moving parts. The gas comes into a tube tangentially, orthogonal to the major access of the tube, and moves toward either end in a swirling pattern. On one side of the inlet port there is an orifice that only lets through air that is in the center of the tube. At the other end of the tube, a stopcock is positioned to regulate the relative amounts of air that come out of either end.

    Although the mechanisms of action are still a subject of research, it is thought that the two dominant mechanisms are a kinetic separation due to the vortex and a radial pressure gradient that enables turbulent fluctuations to transport heat outwards. Temperature differentials of about 100C can be generated with pressures of about 600 kPa.


    Fig. 14.1.1 Hilsch Vortex Tube (reproduced from Scientific American).

  • Thermoacoustic Cooling

    Because the expansion and contraction of a gas in sound waves is adiabatic, a temperature gradient exists between the compressions and rarefactions of the gas. By creating a standing wave pattern of acoustic energy within a conical resonator, thermal contacts can be made at the appropriate locations to tap into that temperature difference. This phenomenon has been used to build refrigeration devices and the technology is currently being developed for applications requiring robust architecture.

  • Aside: The Einstein-Szilard Refrigeration Company

    Two of the great thinkers of the twentieth century also made a significant, but rarely appreciated, contribution to refrigeration technology. Sparked by a news story where the refrigerant (ammonia) leaked from a home refrigerator and killed a family, Leo Szilard and Albert Einstein thought that their knowledge of thermodynamics ought to be useful in making these machines more robust. They quickly discovered that the weak link was the mechanical compressor; building robust fittings to moving parts was a difficult engineering problem. The two scientists sat down and decided to look for ways to build a refrigerator with no moving parts.

    They developed three different cooling technologies that were unlike anything currently available. None of the devices had moving parts so that the refrigerant could be hermetically sealed without the danger of bearings or seals becoming leaky. One machine used an absorption design, another used an electromagnetic pump, and the third depended on evaporation and the pressure available from tap water.

    The absorption cooler, shown is figure 14.1.2, uses a heat source to drive butane through a refrigeration cycle. On the far right, liquid butane vaporizes in the presence of ammonia removing heat from the inside of the refrigerator. The gas travels through the apparatus and is driven to the central chamber, where water absorbs the ammonia and butane condenses back into a liquid, giving off heat.


    Fig. 14.1.2 The Einstein-Szilard Absorption Cycle.

    Einstein, with his seven years as a patent clerk, was keenly aware of the virtues of an elegant design. Szilard, trained as an engineer, was not so encumbered. Partly in response to the diabolical complexity of the absorption design, Einstein came up with an ingenious immersion cooler that ran off of the pressure of a water-tap. The design, shown in figure 14.1.3, creates a vacuum from water flow that is used to evaporate methanol and cool chamber 13. The methanol is dissolved in a small amount of water in chamber 1 and used up. Unfortunately, the unreliability of German water pressure made the device untenable.


    Fig. 14.1.3 The Einstein-Szilard Tap-water Immersion Cooler.

    The most successful design used a conventional compression-expansion cooling cycle but substituted the mechanical compressor with an electromagnetic pump that had no working parts. The pump, shown in figure 14.1.4, uses a travelling electromagnetic field to move a liquid metal. The device was actually built by A.E.G. (the German Electric Company) but the discovery of freon, a non-toxic refrigerant, made it uneconomical to procede. The electromagnetic pump did find wide usage several years later when Szilard and others went on to build nuclear reactors.


    Fig. 14.1.4 The Einstein-Szilard Electromagnetic Pump.

    In all, two scientists who are renowned for the impractical nature of their discoveries, filed 45 patents relating to refrigeration technology. In fact, Szilard's early research on a nuclear chain reaction was funded by the revenue generated from this venture.

    Cryogenic Liquifaction

    Although most gases cool down when they are expanded, this is not always the case. There is a temperature dependence that determines whether a particular gas will heat or cool when undergoing expansion. The Joule-Thomson coefficient signifies this temperature dependence. A gas that is above its Joule-Thomson inversion temperature (it has a negative JT coefficient) will heat upon expansion while the opposite is true for gases below their JT inversion temperature.

    For gases that have a positive JT coefficient at room temperature (e.g. air), liquifaction can be achieved by a single Joule-Thomson expansion cycle. In this cycle, the gas is compressed isothermally (heat is given off to a coolant) and then rapidly expanded. Some gas will condense as liquid; this is collected and the remaining gas is put through the cycle once again.

    For gases that have a negative JT coefficient at room temperature (e.g. helium), a multi-step Joule-Thomson expansion cycle is necessary. The first cycle generates a cryogen that is used to pre-cool the gas for the second cycle. For example, liquid nitrogen can be used to cool helium gas below its JT inversion temperature. Expansion of this cold helium can then be used to get liquid helium.


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    Document last updated Mar. 16, 1999.
    Copyright © 1999, Ken Muldrew.