LỊCH SỮ HÌNH THÀNH ĐIỀU HÒA KHÔNG KHÍ

Refrigeration Fundamentals Throughout History: Methods Used to Obtain Colder Temperatures, and Principles Governing Them A literature Seminar Presented by Jesse N Lawrence 1:00 pm, Tuesday, February 25, 2003 Department of Chemistry The University of Alabama There has always been a need or at least a desire to cool some environments below ambient temperatures This paper will explore some of the methods used to produce refrigeration, and to give some history on the subject The properties of many of the refrigerants used in evaporative systems will be discussed, such as what characteristics qualify a liquid as a good candidate for evaporative refrigeration, why the CFCs were successful as refrigerants, and what new compounds are in place to phase them out There will also be some discussion on the cryogenic field, the history of scientist’s attempts to liquefy the components of air and other gases, as well as some methods used to approach near zero absolute temperature Records dating back to around 2000 B.C indicate that people knew of the preserving effects colder temperatures had on food Alexander the Great served his soldiers snow-cooled drinks around 300 BC, and as far back as 755 AD Khalif Madhi provided refrigerated transport across the desert to Mecca using snow as refrigerant.1 Some of the earliest methods of producing cold made use of naturally occurring ice and freezing mixtures such as salt and snow.2 The fact that sodium nitrate lowers the temperature of water upon dissolving was known in the 14th century A.D Naturally occurring ice was either shipped from colder climates or collected in the winter and stored in cold houses, the earliest mention of them dating to about 1000 B.C in some ancient Chinese poems called the Shi Ching.3 These houses were made of various insulating materials, such as straw dirt, and even manure In the 18th century, this ice was typically only available to the rich and powerful In 1806 a man by the name of Frederick Tudor began an ice business by cutting it from the Hudson River and other nearby bodies of water, and selling it at a price that made it available to a greater number of people.4 He eventually shipped it to various localities throughout the world, his first venture being a shipment of 130 tons of ice to the port of St Pierre, Martinique Ice was unknown there, and there were no facilities to store it The attempt would have been disastrous, but Tudor worked with the proprietor of a local eating house, and they concocted and sold ice creams, which had previously been unknown in the West Indies As it was he lost $3500, but construction of an icehouse at St Pierre and the use of sawdust while shipping made the business profitable.3 He also had active shipments to the southern states until they were halted by the civil war Others joined the ice trade, and many entrepreneurs began to ship ice from other places In 1854 it was reported that 156,000 tons of ice were shipped from Boston Ice houses in the U.S typically made use of sawdust as insulation, having walls meter thick This ice trade continued even after the development of artificial ice, generally fueled by claims of natural ice’s superior qualities to a man-made product The trade finally gave way in 1930 The main method used to provide refrigeration relies on the evaporation of some liquid or other, and the bulk of this paper will be devoted to its principles As of 1755, the fact that ether chilled the skin was already known At that time, a Professor of Chemistry, William Cullen, demonstrated the formation of ice in water that was in thermal contact with a body of ether By reducing the pressure above the ether, he caused it to boil, and lower in temperature to the point that it formed ice This was the first half of the refrigeration cycle, and it still remained to find a way to recirculate the ether, for to simply refill the liquid ether vessel when the previous had boiled away would not be cost effective Information on methods to liquefy vapors through compression was gathered in the last half of the 18th century In 1780, two men by the names of J.F.Clouet and G.Monge liquefied sulfur dioxide Ammonia was liquefied in 1787 by van Marum and van Troostwijk The idea of putting condensation and evaporation techniques together to create a cyclic system seems to have been first suggested by Oliver Evans of Philadelphia, but the first cyclic refrigeration machine was made by Jacob Perkins Its description can be found in a patent specification of 18344 There were earlier patents given for refrigeration machines, the first appearing in 1790, but Perkins’ seems to be the first to have been built and put to any use.3 It was intended to be used with any volatile fluid as refrigerant, especially ether It consisted of four main components: an evaporator, compressor, condenser, and expansion valve The compressor pumps vapor into the condenser, which is essentially a heat exchanger The vapor heats upon compression This heated, compressed vapor is cooled by outside air or water, causing it to condense The liquid then flows through the expansion valve, which is essentially a portion of the piping where flow is restricted by some means or other to create a pressure differential across it At the lower pressure side of the valve, the liquid is then in a superheated state, and some of it boils to cool the rest to the boiling point of the lower pressure, which then flows into the evaporator The evaporator is also a heat exchanger, and heat from the substance meant to be cooled causes the refrigerant to evaporate This evaporated vapor is then pulled into the compressor, and the cycle begins again Obviously if the ambient temperature at the condenser is greater than the temperature of the compressed vapor, or if the desired temperature of the substance which needs to be cooled is lower than the boiling temperature of the liquid in the evaporator, the system will be useless Perkins’ invention generated as much excitement as Crystal Pepsi It made no appearance in the literature of the time, and was only casually referred to by Bramwell 50 years later The person most responsible for putting the refrigeration machine into use was James Harrison of Scotland He received little technical training save for some chemistry classes he took while he studied to be a printer at college He noticed the chilling effect of ether when using it to wash type, and around 1850 he invented a hand operated machine that would produce ice In 1856 and 57 he applied for British patents, and built better machines in England These machines were shipped to various places for applications such as the making of ice and the crystallization of paraffin wax from shale oil They were regularly manufactured until the advent of ammonia and carbon dioxide systems, but remained popular in places such as India, where the water used to cool the condenser was generally at a higher temperature than other localities Ether’s normal boiling point is 34.5°C, thus when used to make ice, there must be a pressure less than atmospheric on the low pressure side of the system This can be dangerous because any leaks will bring in oxygen, thus creating a potentially explosive environment Dimethyl ether, with a normal boiling point of –23.6°C was introduced by Carles Tellier in 1864 Sulfur dioxide, with a boiling point of -10°C, was introduced in 1874 Although dimethyl ether was never in general use, sulfur dioxide was used extensively for about 60 years Carl Von Linde was the first to introduce ammonia as a refrigerant in the 1870’s It was advantageous in that having an atmospheric bp of -33.3°C, it could provide much colder temperatures than previously available, but it also exhibited pressures of ten atmospheres or more in the condenser, which required sturdier construction Carbon dioxide systems were developed for use in 1886, and because of its nontoxic nature, it was used extensively on ships until 1955, when it was then replaced by the CFCs.4 Another system which relies upon the evaporation of a liquid is the absorption system It essentially operates in the same manner as a compression system, but instead of going through a compressor the refrigerant is absorbed in some substance or other, and then released to a condenser by heating Thus the absorption chamber, also called the generator, takes the place of the compressor The principles of such a system were demonstrated by Sir John Leslie in 1810 He placed two vessels in a bell jar, one of water and one of sulfuric acid, then evacuated the vessel somewhat with a pump Over a period of time, ice was seen to form on the top of the water vessel The pump was not absolutely necessary in this case; it simply sped along the process Water vapor is readily absorbed by sulfuric acid, and by so doing, the sulfuric vessel continued to ‘pump’ vapor from the water vessel, lowering its temperature until it froze Early systems developed around 1878 using this principle weren’t cyclic; they were designed to have the sulfuric acid removed from the system in order to re-concentrate it by boiling off the absorbed water In this system water acted as the refrigerant A better absorption system was developed in 1859, by Ferdinand Carré employing ammonia as the refrigerant and water as the absorbent In order to make such a system cyclic, a system of valves and containers are built wherein a vessel containing a strong solution of the refrigerant, in this case ammonia, is heated to produce pressurized ammonia vapor which goes to a condenser to become liquid The rest of the system is the same as the compression system, the vapor being re-absorbed in a weak ammonia solution The system of valves and vessels insure that the vapor formed on heating can only go in a ‘forward’ direction Other systems were developed using an aqueous solution of lithium Bromide as absorbent and water as refrigerant.4 So what characteristics make a ‘good’ evaporative refrigerant? Since the system operates on the energy required to evaporate the liquid, it is good to have a high enthalpy of evaporation The refrigerant must also exhibit boiling points at pressures that are appropriate to the materials and seals used to construct the system It is also more economic if the operating pressure in the condenser be well below the critical pressure of the gas, for the closer to the critical pressure one approaches, the more power that is required to compress it Viscosity can also be a factor Typically a lower viscosity will allow smaller pipes, valves, and compressor passages.5 It is also desirable that it be non-toxic, or at least not terribly so Of the refrigerants mentioned, sulfur dioxide is the most toxic An exposure of 0.5-1% of air for five minutes can be fatal Ammonia is next, requiring 30 minutes to produce death at the same concentration Carbon dioxide, methane, ethane and propane are all of very low toxicity, as are the CFC’s It is also best to use a refrigerant that is not flammable Any hydrocarbon such as methane suffers from this requirement, but they otherwise exhibit all of the properties necessary to perform well as a refrigerant These find use in the petroleum industry as they are readily available, and there are plenty of other things to set on fire anyway Ammonia is somewhat flammable, and explosions have occurred If enough hydrogen atoms in an alkane are replaced with halogens, the refrigerant will be non-flammable Methyl chloride, for instance, is difficult to ignite, but it has also caused explosions The detection of leaks can be aided by a strong smell Despite this fact, ammonia is sometimes not used for this very reason, the predominating belief being that a strong smell from a relatively small and harmless leak may induce panic Sulfur dioxide is the only other refrigerant that has a strong odor Carbon dioxide can be tasted at certain concentrations, and some people claim they can smell the CFCs, but the number is few If desired, chemicals such as acrolein have been added to indicate a leak by smell It is also necessary that the refrigerant not react with the materials used in constructing the refrigerating system When using ammonia, copper must be avoided unless water is strictly kept from the system Methyl chloride is reactive with aluminum The CFC’s are generally inert, but the presence of water in the system can form hydrochloric acid when either methyl chloride or R-12 is used Also, the refrigerant should be stable at the system’s operating conditions All the refrigerants mentioned are rather stable, especially carbon dioxide, sulfur dioxide, and the alkanes Ammonia decomposes slightly under certain circumstances It is also necessary that the refrigerant behave appropriately with the lubricating oil in the system The behavior of the lubricating oil with the refrigerant must be considered when designing the system For instance, one must be certain the crankcase oil does not lose too much viscosity if dissolved in the refrigerant, or carried away from the crankcase entirely The basic nature of ammonia tends to saponify some lubricating oils used in refrigerating systems Cost is also a factor Despite its toxic nature and tendency to form sulfuric acid in the presence of water, sulfur dioxide was used extensively because it was inexpensive.4 Obviously, it must also never be solid at any of the conditions of operation, or it will not cycle, and must phase change after leaving the expansion valve The freezing point of ammonia is -78° C, far enough away from its normal bp of –33.3°C Carbon dioxide can be troublesome in that if too much pressure is lost through leakage, it will form a solid that can block the expansion valve.3 All of the earlier evaporative refrigerants had one or more problems associated with them Despite its status as the first refrigerant fluid to be used commercially, ether was not popular for long due to its flammability Some early refrigerants that stayed in use for a longer time, some of which are still in use today, were methyl chloride, sulfur dioxide, carbon dioxide, ammonia, and some hydrocarbons Other early refrigerants that never gained much use were acetone and alcohol, also highly flammable.6 Ammonia has many characteristics that make it useful as a refrigerant Despite its drawbacks mentioned previously, with a boiling point of –33.3° C it exhibits good working pressure, typically not exceeding 200 psi, and has a large latent heat of evaporation It is still widely used Carbon dioxide is a good refrigerant in that it is not toxic or reactive to metals However, the pressures necessary to use it are quite high; generally in the area of 1200 psi or more, and this calls for stronger equipment and better seals Its latent heat of vaporization is not as good as that of ammonia Because of the higher pressures it requires, a smaller volume of vapor is formed, and thus a smaller capacity compressor can be used with it Sulfur dioxide operates at pressures much lower than ammonia, typically about 60 psi, and thus a very large volume of vapor is formed, requiring larger compressors Its latent heat of vaporization is a bit greater than that of CO2 Its lower operating pressure is also disadvantageous because the low pressure side may be below atmospheric pressure, which will allow in air and water vapor at any leak points Water can then freeze and plug up the line, or combine with any SO3 present to form sulfuric acid, which will then corrode the pipes.2,3 Methyl Chloride, CH3Cl, was introduced in the U.S in the 1920’s, and was popular for a time as a replacement for the CFCs during WW II, during which they were not as available It had good operating qualities, but was somewhat flammable and toxic Nevertheless, it had a place even in residential applications at least until the late 1950’s Water has been used at times as a refrigerant, but is limited in its cooling ability by its freezing point of 0° C.3 In light of some of the difficulties exhibited by the earlier refrigerant fluids, it was necessary to find fluids that had better qualities This was done by Midgely, Henne, and McNary of the General Motors Corporation.6 In 1928 Thomas Midgeley was given an assignment to find a refrigerant that had a low toxicity, low flammability, good stability, and an atmospheric boiling point between –40 and 32° F It took him and his associates three days They synthesized the first fluorocarbon refrigerant, using guinea pigs to demonstrate its low toxicity They synthesized all 15 combinations of one carbon with various combinations of chlorine, fluorine, and hydrogen They tested their properties, and finally chose dichlorodifluoromethane as having the most desirable characteristics The new refrigerant was announced at the American Chemical Society’s meeting of April, 1930, at which all other sections of the society adjourned their meetings to attend Midgely’s presentation At the end of his talk, Midgley demonstrated some of the good properties of R-12 by inhaling a lungful of the gas, then using it to extinguish a candle.6 In general the properties of the HCFCs will tend toward increased flammability when the number of hydrogens are increased, and increased toxicicity when the number of chlorine atoms are increased.7 There is a numerical nomenclature system that names all evaporative refrigerants as follows: For the chlorofluorocarbons, the last digit on the right is the number of fluorine atoms contained in the molecule The next to last digit is the number of hydrogen atoms in the molecule plus one The first digit, if the molecule contains more than carbon atom, is the number of carbon atoms minus If the molecule only has one carbon atom, the digit is generally omitted, though some texts may include a zero Thus, R-12 has fluorine atoms, hydrogen atoms, and carbon atom Any open valence spots are filled by chlorine atoms, giving R-12 a molecular formula of CF2Cl2 If some or all of the possible chlorine atoms are replaced with bromine atoms, an extra digit preceded by a B is added to the end, representing the number of bromine atoms, thus R-13 is CClF3, and R-13B1 is CBrF3.6 Isomers are designated by a lower case letter following the digits The most symmetric isomer is named as above, while the next most symmetric isomer will have an ‘a’ following the digits If a still less symmetric isomer is possible, the digits will be followed by a ‘b’, and so on Thus R-134 is CHF2CHF2, whereas R-134a is CH2FCF3 Cyclic compounds are represented by an upper case ‘C’ placed before the digits.4 Azeotropic mixtures of refrigerants are assigned to the 500 series according to the order in which they became commercially available (R-500, R-501, etc.) Zeotropic mixtures are assigned to the 400 series Hydrocarbons are assigned according to rules 1-4, except for butane and isobutane, which are in the 600 series, along with other organic refrigerants 10 Inorganic refrigerants are assigned to the 700 series, using the molecular weight in the numbering Thus ammonia is R-717.6 11 Unsaturated compounds are named according to rules 1-6, but the resulting number has an extra placed in front of it Thus R-1113 is FClC=CF2.8 The initial R- (for refrigerant) that comes before the number portion of the name may be replaced by Freon, CFC, HCFC, and some others Refrigerant mixtures may be used for different reasons For instance, a mixture of R-12 and R-114 is used in systems where the ambient temperature may be excessively high, such as refineries This calls for a stable refrigerant that does not exhibit very high pressure at ambient temperatures Both of these refrigerants are quite stable, and the presence of R-114 (bp 3.77°C) will lower the higher pressures associated with R-12(bp -29.79°C) Also, the R-12 creates a greater cooling capacity than R-114 exhibits alone R-502 was developed to take the place of R22 in systems that had extraordinarily long return lines to the compressor Under these conditions, the vapor can gain more heat than normal on the return, and will then cause unacceptably high temperatures upon compression, resulting in system failure The switch to R-502 reduced the temperature at the compressor by as much as 100°F It had three characteristics that enabled this First, its vapor had a larger heat capacity than that of R-22’s Its heat of vaporization is also less than that of R-22, causing a need to use a higher vapor flow rate to provide the same amount of cooling, which reduces the time the vapor has to pick up additional heat on its return, as well as the amount of heat given off upon condensation Lastly, its vapor also exhibits a lower rise in temperature due to compression than R-22 That is to say, its Joule-Thompson coefficient is not as large R-502 is simply R-22 with R-115 added to it, and one could use R-115 by itself, but this would result in a significant loss in cooling capacity.6 Non-azeotropic blends will exhibit a change in boiling temperature as the composition of the liquid changes This phenomon is known as glide in the refrigeration industry It can enhance a system’s performance If a countercurrent heat exchanger is used in the evaporator, the first refrigerant the medium to be cooled will contact will be the warmer than the last bit of refrigerant it contacts As the medium to be cooled travels along the exchanger, it will be colder, and will experience even colder refrigerant, as the refrigerant entering the heat exchanger will have more of its colder boiling substituent present In this manner the medium receives maximum refrigeration from the refrigerant Counter flow heat exchangers are difficult to build, however, and demand a higher cost.7 The CFCs, of course, have also exhibited some undesirable properties in their interactions when in the upper atmosphere Any molecules having chlorine or bromine undergo chemistry that depletes the ozone present, allowing greater amounts of harmful UV radiation to reach the surface of the planet The process begins by having a chlorine or bromine radical separated from its parent molecule under the influence of UV radiation This radical can then combine with ozone, yielding oxygen and XO, X=Br or Cl Any free oxygen atoms that would have created ozone may instead come upon the XO molecule, forming oxygen and the starting radical all over again The most practical solution to remedy this has been the proposals to lessen the use of CFC’s and eventually halt it altogether Some other ideas have been proposed, one being by a professor at UCLA He suggested providing extra electrons for the radicals by floating a large metal sheet in the stratosphere with balloons The photoelectric effect would provide electrons for the radicals, rendering them inert His suggestions have not been tried The most popular replacements so far for the CFCs have been hydrofluorocarbons Temporary replacements have been molecules with a lower number of chlorine and bromine atoms Thus, R-134a can be a permanent replacement for R-12, as it has no bromine or chlorine Other distant alternatives may lie in the family of fluoroiodocarbons.7 In 1834 Jean Peltier noticed that if current was passed through a junction of dissimilar metals, the junction would either become cooler or hotter,depending on the direction of the current across the junction The thermoelectric effect is best between alloys, most pure substances providing weak results.4 Another novel method to provide colder temperatures appeared in France in1931 George Ranque observed the separation of a stream of compressed air into two streams, one hot and the other cold The device he used has no moving parts, and has come to be known as the Ranque tube, vortex tube, or werbelrhor It works by introducing a stream of compressed air perpendicularly and tangentially into a pipe, where it then forms a vortex One end of the pipe has a plug in it with a hole drilled just in the center of the plug so that only air in the center of the pipe can escape A cone is inserted point first into the other end of the pipe so only air flowing on the outer portion of the pipe can escape at this end Depending on the amount in which the cone is inserted, a stream of air significantly warmer than that introduced by the compressed pipe will issue from this end and a colder stream will issue from the other end If the cone is completely inserted to stop any flow at that end, the temperature at the other will be that of the compressed air As the cone is removed, the flow rate reduces, as does the temperature.4 While no quantitative theory yet exists to explain this effect, the dimensions have been optimized to provide a cold air stream 100° F colder than the compressed air stream, and they are still in use today.3 They are convenient in places such as machine shops where compressed air is readily available Still another method which can be used to provide cooling is the expansion of a gas It was known as early as the 18th century that a reduction in pressure would provide a drop in the temperature of air Richard Trevithik was the first to propose using this phenomenon in refrigeration applications in 1828 The first system to use this principle was made by John Gorrie in Florida in 1844 Many improvements were made upon the design, the process being rather simple First, air is drawn from a room or vessel that one desires to cool It enters a compressor, and increases in pressure and temperature This hot compressed air is then cooled by outside air or water, as the case may be, after which it is allowed to expand again, preferably against a piston or turbine which provides work to the compressor, upon which it cools further, then re-enters the vessel to be cooled A technique similar to this is used to cool jet aircraft For the physicist, cryogenic may mean temperatures close to zero K For our purposes, it will refer to temperatures below about 120 K, which is a bit above the boiling point of natural gas Much of the earliest work at cryogenic temperatures was simply an attempt to liquefy and/or separate various gases The knowledge that had been developed to reach colder temperatures with vapor compression systems enabled scientists to cool compressed gases that had formerly been uncondensable at ambient temperatures As of 1854 Michael Faraday had liquefied many of the gases known at that time He did so by compressing them and cooling to 163 K with solid carbon dioxide and ether All attempts to liquefy gases such as hydrogen, nitrogen, oxygen, and some others had failed In Vienna, Johannes Natterer showed that oxygen, nitrogen, and hydrogen were still gaseous at 3600 atmospheres and 195 K It was considered for a time that these gases were uncondensable until Thomas Andrews discovered the significance of the critical temperature of carbon dioxide during his work from 1861-1869 Efforts were begun again to liquefy these gases, and in December 1877 Louis Cailletet in France and Raoul Pictet in Geneva formed liquid oxygen at almost the same time Cailletet compressed oxygen to 300 atmospheres and cooled it to 244 K with sulfur dioxide Upon expansion, a mist of liquid appeared Neither man made enough to see in a container This was done in 1883 by Sigmund von Wroblewski and Karek Olszewski in Poland They compressed the oxygen and cooled it to 137 K with liquid ethylene boiling under vacuum In 1885 Wroblewski liquified air, finding that liqiud nitrogen and oxygen are completely miscible By the early 1890’s liquid air, oxygen and nitrogen were produced in sufficient quantities to experiment with, and there began to be a commercial demand for these liquids, especially oxygen The methods used to liquefy these gases employed cascade refrigeration systems where one refrigerant was used to cool the condenser of a lower boiling refrigerant, and so on, to reach the temperatures necessary to condense these gases At most these methods produced about 14 liters per hour Various scientists worked independently to produce greater amounts of these liquids, among them being Carl Linde in Germany, and William Hampson in England They all developed similar methods in 1895 All were based on the reduction of temperature experienced by a gas when throttled from a high pressure to a lower one This was known as the Joule-Thompson effect The reduction seen in air is small, only about _ K per atmosphere Linde and Hampson applied for patents within a few days of each other, but Linde was the first to produce enough liquid air to sell His system essentially worked by first compressing the air and cooling it by use of a low temperature refrigerant The gas traveled through an expansion valve, causing it to lower more in temperature, and entered a holding vessel for any liquid that formed on expansion The expanded gas then ran back along the pressurized gas just before the expansion valve, providing additional cooling, and was then sent back to the pump to be re-pressurized, and the cycle would begin again By 1898 all of the known gases had been successfully liquefied with the exception of hydrogen Pictet had claimed to have produced a mist of hydrogen, but this was not generally believed The struggle to reach lower temperatures continued, and help in this regard was made available in 1892 when James Dewar invented the vacuum flask Calculations of the critical state of hydrogen estimated its critical temperature at 30K The coldest temperature then possible was achieved by boiling liquid oxygen at reduced temperatures It was also known that Hydrogen could not be liquefied by the Linde-Hampson process, for its behavior was the opposite of other gases, heating upon expansion instead of cooling Olszewski found that when cooled to a temperature below 190 K, compressed hydrogen would then reverse its behavior, and would cool upon expansion This is known as the inversion temperature for hydrogen Dewar succeeded in liquefying hydrogen in 1898 by cooling the compressed gas to below 190 K with liquefied air, then condensing it with the Linde-Hampson process New gases were being found in the atmosphere, some being easier to liquefy than others Argon was discovered in 1894, and had a boiling point between that of oxygen and nitrogen Helium was extracted from the mineral cleveite in 1895, and the rest of the noble gases were found by separation from liquid air Krypton and xenon could be condensed by liquid air, and neon by liquid hydrogen, but all attempts to liquefy helium failed It has a critical temperature of 5.2 K The freezing point of hydrogen is 13.8 K, and it could thus not be used to liquefy helium by boiling it under reduced pressure The inversion temperature of Helium is 40 K, however, and it was eventually liquefied through the Linde-Hampson process by Kamerlingh Onnes in 1908 Evaporation of helium under reduced pressure produced a temperature of 0.83 K in 1922, and this seemed to be the limit unless a new method for reducing temperature was found Other methods were employed to reach still colder temperatures, such as isothermal magnetisation of a material, gadolinium sulphate for instance, followed by demagnetization This resulted in a temperature of 0.25 K in 1933 and 0.01 K later Other demagnetisation methods resulted in temperatures on the order 0f 10-3 K Even colder temperatures have been reached by the use of lasers on gas molecules, using the momentum of the photons to slow the molecule’s translational speed Zero K is unnattainable as a result of the third law of thermodynamics.4 What causes most gases to cool upon expansion, and why the others, namely hydrogen and helium, heat upon expansion, until they are cooled to their inversion temperature? What is the significance of the inversion temperature? In 1843 James Joule investigated the internal energy change associated with the expansion of a gas by immersing two chambers attached by a valve in a water bath with adiabatic walls One chamber contained a pressurized gas, and the other was evacuated A thermometer was inserted in the water to note a temperature change Since the gas expands into a vacuum, no work, w, is done by the gas as it expands The walls are adiabatic, so the heat, q, is also zero Hence the change in internal energy _U = q + w = This experiment would determine the change in temperature with respect to volume at constant U, _T/_V Typically, the change in temperature with change in volume is very small That coupled with the very small heat capacity of the gas compared to the large heat capacity of the water gave Joule a _T of zero for every reading He concluded that _T/_V was zero regardless of the change in volume, or more specifically, that (∂T/∂V)U was zero The value (∂T/∂V)U is known as the Joule coefficient, commonly given the symbol µJ In 1924 Keyes and Sears used an improved setup and repeated the experiment, this time noting a small but noticeable temperature change In 1853 Joule and William Thompson performed the Joule-Thompson experiment, which was similar to his previous experiment In this experiment, a gas was pushed from a higher pressure cylinder to a lower pressure cylinder through a rigid porous plug At the end of each cylinder was a piston providing the different pressures A thermometer was inserted in each cylinder, and the system had adiabatic walls The gas would be brought to some initial Pi and Ti, and would then be allowed to flow through the plug to the lower pressure Pf, and the final temperature Tf would be read As the walls are adiabatic, the heat exchanged, q, is zero There are two points where work is performed on the system At the high pressure piston the work is performed on the system, and if all of the gas is throttled through, this amount can be calculated as PhVh Where Vh is the volume of the gas when all of it is on the high pressure side The system itself does work on the low pressure piston, which can be calculated as PlVl Thus the total work w on the system can be calculated as PhVh - PlVl For this experiment _U is equal to w instead of zero From the identity U + PV = H, it can be derived that Hh = Hl This is a constant enthalpy process Taking various measurements of _T/_P by starting with the same initial temperature and pressure and varying the final pressure will yield an isenthalpic curve on a pressure-temperature diagram, the slope of a tangential line to which will yield (∂T/∂P)H, the Joule-Thompson coefficient, known as µJ-T Values for gases can range from to –0.1 K/atm, depending on the gas and its initial temperature and pressure This value is critical to the liquefaction of gases in the Linde-Hampson process If the value is negative, the gas heats upon expansion, and liquefaction is not possible The reason why some gases cool upon expansion, while others will heat depends on the intermolecular forces present In any gas, there are attractive and repulsive forces present between the molecules or atoms Keep in mind that when a system goes to a more stable state, it gives off energy; the process is exothermic, whereas if a system goes to a less stable state, energy must come from somewhere for the process to occur In the process of gas expansion, this energy is in the form of heat When cooling is observed upon expansion, the attractive forces predominate A larger average separation of molecules leads to a less stable state, and energy in the form of heat must be taken from the surroundings When heating is observed upon expansion, the repulsive forces predominate, and a greater separation of molecules results in a more stable state, releasing energy in the form of heat What about the fact that a gas that heats on expansion will then cool on expansion if its temperature is brought low enough? When the temperature is lowered, the average kinetic energy of the gas molecules is also lowered Collisions between molecules are not as forceful, and where the collisions at a higher temperature were high energy enough to make repulsion the predominant force, at lower temperature the attractive forces become more significant, and expansion then yields a cooling effect Not only the initial temperature, but the initial pressure chosen will have a bearing on whether or not the Joule or Joule-Thompson coefficient will be positive or negative If you start at a pressure on the isenthalpic curve where heating is observed instead of cooling, you will first observe heating until you reach the inversion point, Where (∂T/∂P)H = After this point on the isenthalpic expansion will yield a cooling effect instead of heating.9 There are many inversion pressures and temperatures possible, depending on the initial conditions of the gas, and a plot of a family of isenthalpic curves will yield a locus of inversion points that will have a maximum inversion temperature at P = Above this temperature it is not possible to observe cooling upon expansion regardless of what pressure is used The following table lists some maximum inversion temperatures: Helium 40 Hydrogen 195 Oxygen 49.7 Nitrogen 621 Argon 723 Carbon Dioxide 15034 So what is the significance of the inversion point? It can be derived from the ideal gas law that ∂U/∂V = ∂H/∂P = for an ideal gas It can also be shown that ∂U/∂V = -CvµJ and ∂H/∂P = -CPµJ-T Cv and CP are the heat capacities at constant volume and pressure, which will always be positive Thus when ∂U/∂V = ∂H/∂P = 0, µJ = µJ-T = 0, which is the inversion point At this point the attractive and repulsive forces are balanced, in a word, and the gas behaves very much like an ideal gas, even though P is not close to zero How does higher pressure affect the temperature change upon expansion? In the same way that higher temperatures correspond to higher energy collisions, making the repulsive forces more significant and thus increasing U of the system, higher pressures can also squeeze the molecules together to a point where the repulsive forces become significant, also increasing U of the system Thus upon expansion this higher U goes into heating the system Of course these same attractive and repulsive forces that change the temperature as the molecules in a gas separate upon expansion also determine the cooling effect observed when a liquid boils, The only difference being the fact that since the molecules have strong enough attractive forces to form a liquid, it is guaranteed that they are the predominate force, and a cooling effect will always be observed upon boiling of a liquid, that is, when the liquid molecules undergo an expansion.9 References Koelet, P C Industrial Refrigeration Principles, Design and Application Marcell dekker, Inc., New York, 1992 Daniels, G W Refrigeration in the Chemical Industry D Van Nostrand Company, New York, 1926 Jordan, R C and Priester, G B Refrigeration and Air Conditioning Prentice-Hall Inc., Englewood Cliffs, NJ 1956 Gosney, W B Principles of Refrigeration Cambridge University Press, New York, 1982 King, G R Modern Refrigeration Practice McGraw-Hill Book Company, New York, 1971 Downing, R C Fluorocarbon Refrigerants Handbook Prentice-Hall Inc., Englewood Cliffs, NJ 1988 Wylie, D and Davenport, J W New Refrigerants for Air conditioning and Refrigeration Systems The Fairmont Press, Inc., Lilburn, GA 1996 Meacock, H M Refrigeration Processes Pergamon Press, New York, 1979 Levine, I N Physical Chemistry, 4th Ed Mcgraw-Hill Inc., New York, 1995