Refrigerants - History Of Development

1. Introduction:

  The development of refrigeration and air conditioning industry depended to a large extent on the development of refrigerants to suit various applications and the development of various system components. At present the industry is dominated by the vapour compression refrigeration systems, even though the vapour absorption systems have also been developed commercially. The success of vapour compression refrigeration systems owes a lot to the development of suitable refrigerants and compressors. The theoretical thermodynamic efficiency of a vapour compression system depends mainly on the operating temperatures. However, important practical issues such as the system design, size, initial and operating costs, safety, reliability, and serviceability etc. depend very much on the type of refrigerant and compressor selected for a given application. This lesson presents a brief history of refrigerants and compressors. The emphasis here is mainly on vapour compression refrigeration systems, as these are the most commonly used systems, and refrigerants and compressors play a critical role here. The other popular type of refrigeration system, namely the vapour absorption type has seen fewer changes in terms of refrigerant development, and relatively a smaller number of problems exist in these systems as far as the refrigerants are concerned.

2. History of Refrigerant development:

  In general, a refrigerant may be defined as “substance that acts as a cooling medium by extracting heat from another body or substance”. Under this general definition, many bodies or substances may be called as refrigerants, e.g. ice, cold water, cold air etc. In closed cycle vapour compression, absorption systems, air cycle refrigeration systems the refrigerant is a working fluid that undergoes cyclic changes. In a thermoelectric system the current carrying electrons may be treated as a refrigerant. However, normally by refrigerants we mean the working fluids that undergo condensation and evaporation as in compression and absorption systems. The history that we are talking about essentially refers to these substances. Since these substances have to evaporate and condense at required temperatures (which may broadly lie in the range of –100^oC to +100^oC) at reasonable pressures, they have to be essentially volatile. Hence, the development of refrigerants started with the search for suitable, volatile substances. Historically the development of these refrigerants can be divided into three distinct phases, namely:

A.                  Refrigerants prior to the development of CFCs

B.                  The synthetic fluorocarbon (FC) based refrigerants

C.                  Refrigerants in the aftermath of stratospheric ozone layer depletion

3. Refrigerants prior to the development of CFCs

  Punkahs 

           Water is one of the earliest substances to be used as a refrigerant, albeit not in a closed system. Production of cold by evaporation of water dates to 3000 B.C. Archaeological findings show pictures of Egyptian slaves waving fans in front of earthenware jars to accelerate the evaporation of water from the porous surfaces of the pots, thereby producing cold water. Of course, the use of “punkahs” for body cooling in hot summer is very well known in countries like India. Production of ice by nocturnal cooling is also well known. People also had some knowledge of producing sub-zero temperatures using “refrigerant mixtures”. It is believed that as early as 4th Century AD people in India were using mixtures of salts (sodium nitrate, sodium chloride etc.) and water to produce temperatures as low as –20^oC. However, these natural refrigeration systems working with water have many limitations and hence were confined to a small number of applications.

                                                        

 Salt Ice Making Machine

           
Water was the first refrigerant to be used in a continuous refrigeration system by William Cullen (1710-1790) in 1755. William Cullen is also the first man to have scientifically observed the production of low temperatures by evaporation of ethyl ether in 1748. Oliver Evans (1755-1819) proposed the use of a volatile fluid in a closed cycle to produce ice from water. He described a practical system that uses ethyl ether as the refrigerant. As already mentioned, the credit for building the first vapour compression refrigeration system goes to Jakob Perkins (1766-1849). Perkins used sulphuric (ethyl) ether obtained from India rubber as refrigerant. Early commercial refrigerating machines developed by James Harrison (1816-1893) also used ethyl ether as refrigerant. Alexander Twining (1801-1884) also developed refrigerating machines using ethyl ether.  After these developments, ethyl ether was used as refrigerant for several years for ice making, in breweries etc. Ether machines were gradually replaced by ammonia and carbon dioxide-based machines, even though they were used for a longer time in tropical countries such as India.

              Ethyl ether appeared to be a good refrigerant in the beginning, as it was easier to handle it since it exists as a liquid at ordinary temperatures and atmospheric pressure. Ethyl ether has a normal boiling point (NBP) of 34.5^oC, this indicates that in order to obtain low temperatures, the evaporator pressure must be lower than one atmosphere, i.e., operation in vacuum. Operation of a system in vacuum may lead to the danger of outside air leaking into the system resulting in the formation of a potentially explosive mixture. On the other hand a relatively high normal boiling point indicates lower pressures in the condenser, or for a given pressure the condenser can be operated at higher condensing temperatures. This is the reason behind the longer use of ether in tropical countries with high ambient temperatures. Eventually due to the high NBP, toxicity and flammability problems ethyl ether was replaced by other refrigerants. Charles Tellier (1828-1913) introduced dimethyl ether (NBP = 23.6^oC) in 1864. However, this refrigerant did not become popular, as it is also toxic and inflammable. 

             In 1866, the American T.S.C. Lowe (1832-1913) introduced carbon dioxide compressor. However, it enjoyed commercial success only in 1880s due largely to the efforts of German scientists Franz Windhausen (1829-1904) and Carl von Linde (1842-1934). Carbon dioxide has excellent thermodynamic and thermophysical properties, however, it has a low critical temperature (31.7^oC) and very high operating pressures. Since it is non-flammable and non-toxic it found wide applications principally for marine refrigeration. It was also used for refrigeration applications on land. Carbon dioxide was used successfully for about sixty years however, it was completely replaced by CFCs. It is ironic to note that ever since the problem of ozone layer depletion was found, carbon dioxide is steadily making a comeback by replacing the synthetic CFCs/HCFCs/HFCs etc. 

Carbon Dioxide as refrigerant

            One of the landmark events in the history of refrigerants is the introduction of ammonia. The American David Boyle (1837-1891) was granted the first patent for ammonia compressor in 1872. He made the first single acting vertical compressor in 1873. However, the credit for successfully commercializing ammonia systems goes to Carl von Linde (1842-1934)of Germany, who introduced these compressors in Munich in 1876. Linde is credited with perfecting the ammonia refrigeration technology and owing to his pioneering efforts; ammonia has become one of the most important refrigerants to be developed. Ammonia has a NBP of 33.3^oC, hence, the operating pressures are much higher than atmospheric. Ammonia has excellent thermodynamic and thermophysical properties. It is easily available and inexpensive. However, ammonia is toxic and has a strong smell and slight flammability. In addition, it is not compatible with some of the common materials of construction such as copper. Though these are considered to be some of its disadvantages, ammonia has stood the test of time and the onslaught of CFCs due to its excellent properties. At present ammonia is used in large refrigeration systems (both vapour compression and vapour absorption) and also in small absorption refrigerators (triple fluid vapour absorption). 

Ammonia as refrigerant

              In 1874, Raoul Pictet (1846-1929) introduced sulphur dioxide (NBP=10.0^oC). Sulphur dioxide was an important refrigerant and was widely used in small refrigeration systems such as domestic refrigerators due to its small refrigerating effect. Sulphur dioxide has the advantage of being an auto-lubricant. In addition, it is not only non-flammable, but actually acts as a flame extinguisher. However, in the presence of water vapour it produces sulphuric acid, which is highly corrosive. The problem of corrosion was overcome by an airtight sealed compressor (both motor and compressor are mounted in the same outer casing). However, after about sixty years of use in appliances such as domestic refrigerators, sulphur dioxide was replaced by CFCs.

Sulphur Dioxide as refrigerant

             In addition to the above, other fluids such as methyl chloride, ethyl chloride, iso-butane,propane, ethyl alcohol, methyl and ethyl amines, carbon tetra chloride, methylene chloride, gasoline etc. were tried but discarded due to one reason or other.

4. The synthetic CFCs/HCFCs:

           Almost all the refrigerants used in the early stages of refrigeration suffered from one problem or other. Most of these problems were linked to safety issues such as toxicity, flammability, high operating pressures etc. As a result large-scale commercialization of refrigeration systems was hampered. Hence it was felt that “refrigeration industry needs a new refrigerant if they expect to get anywhere”. The task of finding a “safe” refrigerant was taken up by the American Thomas Midgley, Jr., in 1928. Midgley was already famous for the invention of tetra ethyl lead, an important anti-knock agent for petrol engines. Midgley along with his associates Albert L. Henne and Robert R. McNary at the Frigidaire Laboratories (Dayton, Ohio, USA) began a systematic study of the periodic table. From the periodic table they quickly eliminated all those substances yielding insufficient volatility. They then eliminated those elements resulting in unstable and toxic gases as well as the inert gases, based on their very low boiling points. They were finally left with eight elements: carbon, nitrogen, oxygen, sulphur, hydrogen, fluorine, chlorine and bromine. These eight elements clustered at an intersecting row and column of the periodic table, with fluorine at the intersection. Midgley and his colleagues then made three interesting observations:

A.                  Flammability decreases from left to right for the eight elements

B.                  Toxicity generally decreases from the heavy elements at the bottom to the lighter elements at the top

C.                  Every known refrigerant at that time was made from the combination of those eight “Midgley” elements.

          A look at the refrigerants discussed above shows that all of them are made up of seven out of the eight elements identified by Midgley (fluorine was not used till then). Other researchers have repeated Midgley’s search with more modern search methods and databases, but arrived at the same conclusions (almost all the currently used refrigerants are made up of Midgley elements, only exception is Iodine, studies are being carried out on refrigerants containing iodine in addition to some of the Midgley elements). Based on their study, Midgely and his colleagues have developed a whole range of new refrigerants, which are obtained by partial replacement of hydrogen atoms in hydrocarbons by fluorine and chlorine. They have shown how fluorination and chlorination of hydrocarbons can be varied to obtain desired boiling points (volatility) and also how properties such as toxicity, flammability are influenced by the composition. The first commercial refrigerant to come out of Midgley’s study is Freon-12 in 1931. Freon-12 with a chemical formula CCl2F2, is obtained by replacing the four atoms of hydrogen in methane (CH4) by two atoms of chlorine and two atoms of fluorine. Freon-12 has a normal boiling point of 29.8^oC and is one of the most famous and popular synthetic refrigerants. It was exclusively used in small domestic refrigerators, air conditioners, water coolers etc for almost sixty years. Freon-11 (CCl3F) used in large centrifugal air conditioning systems was introduced in 1932. This is followed by Freon-22 (CHClF2) and a whole series of synthetic refrigerants to suit a wide variety of applications.

          Due to the emergence of a large number of refrigerants in addition to the existence of the older refrigerants, it has become essential to work out a numbering system for refrigerants. Thus all refrigerants were indicated with ‘R’ followed by a unique number (thus Freon-12 is changed to R12 etc). The numbering of refrigerants was done based on certain guidelines. For all synthetic refrigerants the number (e.g. 11, 12, 22) denotes the chemical composition. The number of all inorganic refrigerants begins with ‘7’ followed by their molecular weight. Thus R-717 denotes ammonia (ammonia is inorganic and its molecular weight is 17), R-718 denotes water etc.. Refrigerant mixtures begin with the number 4 (zeotropic) or 5 (azeotropic), e.g. R-500, R-502 etc.

          The introduction of CFCs and related compounds has revolutionized the field of refrigeration and air conditioning. Most of the problems associated with early refrigerants such as toxicity, flammability, and material incompatibility were eliminated completely. Also, Freons are highly stable compounds. In addition, by cleverly manipulating the composition a whole range of refrigerants best suited for a particular application could be obtained. In addition to all this, a vigorous promotion of these refrigerants as “wonder gases” and “ideal refrigerants” saw rapid growth of Freons and equally rapid exit of conventional refrigerants such as carbon dioxide, sulphur dioxide etc. Only ammonia among the older refrigerants survived the Freon magic. The Freons enjoyed complete domination for about fifty years, until the Ozone Layer Depletion issue was raised by Rowland and Molina in 1974. Rowland and Molina in their now famous theory argued that the highly stable chlorofluorocarbons cause the depletion of stratospheric ozone layer. Subsequent studies and observations confirmed Rowland and Molina theory on stratospheric ozone depletion by chlorine containing CFCs. In view of the seriousness of the problem on global scale, several countries have agreed to ban the harmful Ozone Depleting Substances, ODS (CFCs and others) in a phase-wise manner under Montreal Protocol. Subsequently almost all countries of the world have agreed to the plan of CFC phase-out. In addition to the ozone layer depletion, the CFCs and related substances were also found to contribute significantly to the problem of “global warming”. This once again brought the scientists back to the search for “safe” refrigerants. The “safety” now refers to not only the immediate personal safety issues such as flammability, toxicity etc., but also the long-term environmental issues such as ozone layer depletion and global warming.

Satellite Image of Ozone Layer Depletion 

5. Refrigerants in the aftermath of Ozone Layer Depletion:

HFC-134a refrigerant

         The most important requirement for refrigerants in the aftermath of ozone layer depletion is that it should be a non-Ozone Depleting Substance (non-ODS). Out of this requirement two alternatives have emerged. The first one is to look for zero ODP synthetic refrigerants and the second one is to look for “natural” substances. Introduction of hydrofluorocarbons (HFCs) and their mixtures belong to the first route, while the re-introduction of carbon dioxide (in a supercritical cycle), water and various hydrocarbons and their mixtures belong to the second route. The increased use of ammonia and use of other refrigeration cycles such as air cycle refrigeration systems and absorption systems also come under the second route. Both these routes have found their proponents and opponents. HFC-134a (synthetic substance) and hydrocarbons (natural substances) have emerged as alternatives to Freon-12. No clear pure fluid alternative has been found as yet for the other popular refrigerant HCFC-22. However, several mixtures consisting of synthetic and natural refrigerants are being used and suggested for future use. Table 2.1 shows the list of refrigerants being replaced and their alternatives. Mention must be made here about the other environmental problem, global warming. In general, the non-ODS synthetic refrigerants such as HFC-134a have high global warming potential (GWP), hence they face an uncertain future. Since the global warming impact of a refrigerant also depends on the energy efficiency of the system using the refrigerant (indirect effect), the efficiency issue has become important in the design of new refrigeration systems. Though the issues of ozone layer depletion and global warming has led to several problems, they have also had beneficial effects of making people realize the importance of environmental friendliness of technologies. It is expected that with the greater awareness more responsible designs will emerge which will ultimately benefit the whole mankind.