Document vyzqDGq6EO75yzLpEGBj68grw

770 CHAPTER 48 Table 2 .... Thermal Stability of Ruorinated Refrigerants*1 Refrig. muul Formula Approx. Max. Temp for Contmuooi fipoiuft* Oecaapoaftao Rato at SOO F to Steel Fervent/Yr Temp for Fint Trace of Decorapotr- Hoa to Quartz. F 113 11 CecCulj?F-CClF, 12 CC1,F, 114 COFrGClF* 22 CHClFt 13 CC1F, 225 225 250 250 275-300'* Over 300 6 2 _ -- 570 840 1000 550 In the presence at oO, steel end copper, bat net ipdudin* motor insoletion. Bmm on eesled-tabe test date ftoo E. I. riuPont deNemoun 4 Co., Trw. * Bee Refeienua S7. chemical formula. In the third column are listed the approximate maximum temperatures recommended for continuous exposure in the presence of oil. steel, and copper. Tins condition would be representative of refrigeration equipment designed to operate for many years. The recommended temperatures.are based on laboratory testing. Field experience has been in substantial agree ment although there is some indication that these limits are auttle conservative. It b possible that hot gas discharge temperatures higher than those shown could be tolerated, especially if the oil concentration is low. Some actual observed decomposition rates at 400 F for the refrigerant alone in steel are shown in the fourth column. They are Eased on exposures of six days and disregard a higher rate indicated on initial exposure. There is considerable difference in the effect of various metals on the decomposition rate. For ex ample, with Refrigerant 11 at 480 F in steel, the initial rate was 120 and the final rate was 36 percent per year. However, in Inconel the initial decomposition rate was 3 percent per year and in platinum it was leas than 1 percent per year. The tendency of metals to promote thermal decomposition of the flnorinwf re frigerants ta in the following general order: Least decomposition: Inconel < 18-8 stainless steel < nickel < copper < 1340 steei < aluminum < bronze < brass < silver: most decomposition. This order b only approximate and exceptions may be found for individual fluorinated refrigerants or for special conditions of use. Most of the commonly used construction metals--such as steel, cast iron, brass, copper, tin, lead. Aluminum--can be used satisfactorily with the fluorinated refrigerants under normal conditions of use. Magnesium alloys and aluminum containing more theji 2 per cent magnesium are not recommended for use m systems contain-' mg fluorinated refrigerants where water may be present. Zinc is not recommended for use with Refrigerant 113. Experi ence with zinc and the other fluorinated refrigerants h** been limited and no unusual reactivity has been observed. However, zinc b somewhat more chemically reactive than other common construction metals and, therefore, it would naem wise to avoid its use with the fluorinated refrigerants unless adequate fawtlng b. carried out. In the last column, decomposition temperatures are illustrated for moderate conditions. No metals were present in these tests and the exposure time was 30 seconds. Refrigerant 13 was not included in the teste but other evidence indicates that it b sub stantially more stable than Refrigerant 12. The presence of hydro gen in the Refrigerant 22 molecule in place of one of the chlorine atoms in Refrigerant 12 decreases its thermal and alkaline sta bility but makes it less reactive with oU. The thermal stability of the fluorinated refrigerants will probr ably be between the extremes illustrated in the table for a par ticular application. , b. Ammonia. Ammonia b a comparatively stable gas at ordi nary temperatures0 and, therefore, thermal stability b not a problem in ordinary refrigeration usage. Under conditions of high discharge temperatures, there' may.be a slow decomposition to form hydrogen and nitrogen.' Glass does not affect the decompo sition rate. Porcelain, pumice and metals such as iron, nickel and zinc increase the decomposition markedly. The effect of pressure b to decrease the dissociation into hydrogen and nitrogen. - e. Methyl Chloride. Under ordinary conditions, dry methyl chloride b stable toward heat. Methyl chloride may be used with iron, steel, copper, brass, etc., but not with aluminum, mag nesium or zinc. The use of aluminum - particularly etwwid be 1965 Guide And Data'Book avoided since dangerous, spontaneously flammable methyl *h*nunum compounds may be formed. _ d. Other Refrigerants. Sulfur dioxide b thermally stable to'Very high temperatures. Hydrocarbons do not present stability prob lems in refrigeration use. Under suitable conditions of temperature and catalysis, they are thermally decomposed to form hydro carbons of lower molecular weignt. However, these conditions are beyond the range usually encountered in refrigeration. 2. Chemical Reactivity. A chemical reaction is one in which the identity of the materials involved is changed. Usually, but not necessarily, more than* one-material enters into the re action. In general, refrigerants do not have much chemical reactivity. Of course, it is possible for a compound to be un reactive in a refrigerant system and yet have considerable re activity under other conditions. Reactions of most interest in refrigeration probably include hydrolysis, oxidation and re duction, thermal decomposition, reaction with oil, and direct reaction with metals. o. Fluorinated Refrigerants. The fluorinated refrigerants are chemically inert under conditions usually found in refrigerant systems. However, some of the less highly fluorinated members of this group will take part in chemical reactions to a certain ex tent especially at higher temperatures. 'The reaction with water may cause hydrolysis and the forma tion of hydrochloric and hydrofluoric acids and carbon dioxide. Obviously, the formation of these products b not desirable in a refrigerant system and in ordinary usage this reaction b negligible. In a very wet system operatingat abnormally high temperatures, some hydrolysb may occur. Typical hydrolysis rates for some halogensted refrigerants are given in Table 3. With water alone at atmospheric pressure, the rate is too low to be determined by the analytical method used. When catalysed by the presence of steel, the hydrolysb rates are detectable but still quite low. At saturation pressures and a higher temperature, the rates are further increased.-The hydrolysis-b directly de pendent on the amount of water present. The haai for compari son in Table 3 b one liter (more than two pounds) of water. It b clear that the amount of refrigerant hydrolysed in a year b very small when based on the amount of water actually present in si refrigerant system. It has been proposed0 that, so long as mois ture does not appear in liquid or solid form in the system, cor rosion and deterioration of the refrigerant through hydrolysb will not proceed enough to be detectable in 10 to 20 years. Whether or not hydrolysb of the refrigerant b a significant factor, experience' has shown without question that refrigerant systems operate longer and better when they are as dry as pos sible and every effort should be made to keep thm that way. The possibility of chemical reaction between the fluorinated refrigerant and lubricating oil b of direct concern in refrigerant systems. Direct reaction does occur'between Refrigerant 12 and oil in the presence of a steel strip to form hydrochloric acid. Refrigerant 22, and other products as shown by Spauschus and Doderer.** Considerable differences are found'with different re frigerants and different oils. Kvalncs and Parmelee* have studied the decomposition of Refrigerants 12 and 22 in a^led glass tubes. The amount of cleavage was measured by the amount of chloride ion found at the aid .of the test period. Temperature, lubricant and refrigerant type appeared to be the important controlling factors in the formation of the chloride kml At tem peratures of 250 F neither refrigerant decomposed significantly (see Fig. 1). At 300 F.the decomposition of Refrigerant 12 rose Table 3 .... Rate of Hydrolysis In Water*1 Gram* per (Liter of Water) (Year) Refrig erant Formula 113 11 -.,12 21 114 22 CCUFCOFi CCUF CCbFi CHCUF CClFf-CClF* CHClFt 1 Aim. Protture 66 F Water Alone With Steel- Pretsure' ` 122 F With Steel <0.005 <0.005 ' <0.005 <0.01 <0.005 <0.01 60 ' 40 10 28 10. 59 13 0.1 Contaminant Control in Refrigerant Systems 771 fcmificantly while Refrigerant 22 decomposed less than 0.1% ^ 400 days (Figs- 1 and 2). Again, the tests showed that the mount of reaction of Refrigerant 12 was dependent upon the type of lubricant. Four times as much reaction occurred with oue lubricant as with another. The chemical reactivity of the various fluorinated refrigerants m a general way follows the same order in different types of rCtlftj*n Thus, a refrigerant with a low hydrolysb rate will robably have a low reaction rate with oil and, also, it b likely to show up well in thermal stability, if devoid of hydrogen, and to exhibit good chemical stability in general. 6. Ammonia. The principal chemical reactivity of- ammonia involves its water solution. Water reacts with ammonia to form jfpmnninm hydroxide. If the hydroxide b formed it can react with some of the components of oQ to give ammonium soaps lead ing to oil deterioration and the formation of sludge. Ammonium hydroxide reacts rapidly with copper and .copper alloys. Thus, mmmnni* rn be used with these metals only in systems from which water b rigidly excluded. There b some evidence that the presence of air or oxygen accelerates the reactivity of ammonia m refrigeration systems, even in the absence of water. c. Methyl Chloride. Methyl chloride was used successfully as refrigerant for a number of years in spite of a rather high degree - of chemical reactivity. It apparently does not react directly with o3 but in contact with metals can be decomposed to form hydro chloric add.* Hydrolysis bee Table 3) gives methyl alcohol and hydrochloric acid. It b flammable, indicating reactivity with oxygen when ignited. Methyl chloride can react directly with certain metals such as magnesium and aluminum. The product of the reaction with magnesium b used in chemical synthesis to introduce the methyl group into organic compounds. The reac tion with aluminum forms spontaneously flammable products. d. Other Refrigerants. Sulfur dioxide ss a liquid b an active agent in the refining of oib and will react with many of the hydro carbons and other of the compounds found therein. Thus highly refined lubricating oib must be used with sulfur dioxide just as b the esse with many of the fluorocarbon refrigerants. Sulfur dioxide b also a good electrolytic solvent and b corrosive to metals when even limited amounts of water are present. In addition, sulfur dioxide b an excellent solvent for, or b reactive with, many of the materials which are presently used as motor insulation. The above problems as well as tho irritating nature of the gas presently limits the usefulness of this thermally stable refrigerant. The hydrocarbon refrigerants are flammable and react rapidly whh oxygen when ignited. Although reactive under special condi tions ana used widely in chemical manufacture,, the chemical reactivity of the hydrocarbons {other than flammability) b usually not a problem in refrigeration service. Lubricants - The physical properties of the lubricant must be carefully considered, and the selection for equipment made on the basis of desired properties. These factors are covered in Chapter 60. In addition, lubricant selection is based on the type of re frigerant to be used, as well as system design, temperature and pressure. Consideration should also be given to its contact with the materials of construction, because its behavior is depend ent to a great extent on other materials in thesystem. Specifi cations adapted from those for motor oils are not a guarantee for stability in operation and resistance to copper plating.1 In this section it will be the purpose to consider the lubricant r9- 1 . Stability of Refrigerant 22 Control' System Rg. 2 .... Stability of Refrigerant 12 Control Systems' at 250 F and 300 F from the standpoint of its stability, or the tendency of its decomposition products to contribute to contamination of a system. The fundamental work of Steinle**' Elsey, Flowers and Kelley* and others has contributed greatly to the under standing of the primary characteristics required of a refrigera tion grade oil. Usually the oil refrigerant breakdown is started by traces of air and moisture in the system as well as the cata lytic action of Steel, copper, and other materials of construc tion. 1. Stability. A wide variety of petroleum based lubricating oib have been used over the years with varying success in refrigerant systems. In general, during recent years, the usage has been pri marily confined to low viscosity index naphthenic Dose oib due to their reduced tendency to separate wax from oil refrigerant mix tures at low operating temperatures. Naphthenic base oib have a low wax content and are usually further dewaxed. These oib are usually refined to a pale yellow color but ordinarily are not refined to the point where they are water-white, because this seems to reduce the general stability of the oil and in some cases reduces the lubricating qualities. Generally, pale oil has less stability to at mospheric oxygen in the system but when oleoresins and sulfur are kept sufficiently low the tendency of the ofl to deposit copper and react chemically with the refrigerant b kept at a minimum. The paraffin base oib have a high viscosity index (see Chapter 50). They have more wax and they are much more stable to nigh temperatures. Because of the wax content, they are not used for low temperature refrigeration applications. Carbonaceous sludges due to the thermal decomposition of.the organic materials of tbe system are frequently found in refriger ating units. Of particular interest, in this regard, are those formed by the interaction of the lubricating oil and the refriger ant. Philipp and Tiffany developed a procedure whereby a solu tion of sulfur dioxide in oil b sealed in an air-free glass tube and stored at 500-F. Tbe rate at which the ofl darkens is taken as an indication of the stability of the oil towards the refrigerant. With alight modification of this procedure Steinle*1 and Elsey, Flowers and Kelley* studied the behavior of refrigerating oib with various refrigerants. Under equivalent conditions it was found that some oib are more stable towards refrigerants than others. On the othohand, it was also found that some refrigerants are more reactive with oib than others. In cases where significant reactivity occurred, acid* were frequently formed in addition to sludges: Again, increased temperature'accelerated the decomposition of the oil. These investigators found excellent correlation between unit performance ana sealed tube tests. Elsey, Flowers, and Kelley found that sludging occurs when the oil loses hydrogen which combines with the (fluorine in the halogenated hydrocarbon to form hydrogen chloride. Those halocarbons having the greatest number of chlorine atoms in the molecule were the most reactive. These oil sludges are related to the waxes which at times precipU tate out of the refrigerant to clog the orifice of the refrigerant metering device. It has been noticed that the darker a used oil has become the higher its fioc point may be. With any refrigerant it is therefore necessary to choose-the proper lubricant and to design for the lowest possible operating