Document YGaEmK37xbg40yv6VmGvo647O

t\ a jr 1 m 792 CHAPTER 50 Table 6 .... Critical Miscibility Values of Refrigerant 22 with 18 Different Oils os Bow Type Approx O? Vucotrfy of 122 F Crffieof So/ofioo Critical Sotob&fy, Grade No. (Converted to SUV) Temp., F Weight % OS Naphthene Naphthene Naphthene Naphthene Naphthene 75 75 150 200 200 2 3 1 8 7 63 -35 60 21 92 27 116 75 125 68 15 25 20 22 22 Naphthene Naphthene Naphthene Naphthene 200+ 250 250 500 5 4 6 13 131 3 16 136 -4 18 140 + 1 22 252 None* None* Naphthene' " 500 Naphthene 700 Naphthene 1000 Naphthene 1200 12 11 10 9 280 None* None* 320 None* None* 430 61 18 500 48 18 Paraffin Paraffin Paraffin Paraffin Paraffin 200+ 250 300 350 400 18 17 16 15 14 137 None* 148 None* 165 None* None* 210 None* 232 111* 18 * Never completely miteihle it eay temperature. A eecood (inverted) mierihility dome wee observed above 138 P; above thie temperature the cil-Relngeraot 22 mixture eyln aepazated mto two usmiadbt* temperature. And it is also true for the temperature plot, as was shown for the vapor pressure plot on graph (b), that, all points outside -the dome represent nurtures which are com1' pletely miscible, and all points inside the dome * represent' mixtures which form two immiscible solutions. ' More'than one curve of this type may be plotted on a miscibility diagram. Each single dome then represents the immiscible ranges for one refrigerant mixed with one oil. Miscibility curves or diagrams have been published for Refrigerants 13,* 13B1,* 22>t7-1,-" 114,* 502,* and mixtures of Refrigerants 12 and 22.u Typical curves selected from thaw* references are shown in figs. 30 to 35. Miscibility of Refrigerant 22 with Mineral Oils The miscibility of Refrigerant 22 with oils, both mineral and synthetic, has been the subject of numerous studies. Among the synthetic oils, a polybutyi silicate oil-has been reported as showing improved miscibility with Refrigerant'-22 (and algo Refrigerant 13) atlow -temperatures.11 -However;. synthetic oils have not yet won general acceptance for refrigerant sys tems and will not be considered further in this chapter. Several reports have dealt with miscibility differences re sulting from the use of different types and viscosity grades of mineral oil with Refrigerant 22. Fig. 32 shows' one .of the miscibility-diagrams reported by Bosworth.17 Oil A is a paraffin base oil; oil B is a mid-continent oil; and oils C and D are naphthene bare oils. All are 300 SUV dewaxed re frigeration grade oils. The diagram indicates that Refrigerant 22 is less miscible with the paraffinbase oil.' '" " Walker and his coworkers have also reported on the misci bility of different oils with Refrigerant 22.1* Twelve different brand-name oils commonly used for'refrigerating machines were tested, and miscibility curves were constructed for each oiL Four of there curves are shown in Fig. 33,.but only in part. The purpose of Fig. 33 is to illustrate the typical conditions required to form two liquid phases in refrigerant-rich mixtures, such as may exist in an evaporator; hence the horizontal has been magnified in the low oil .concentration range, and 1965 Guide And Data'Book the miscibility curves have been cut off above twelve percent ofl. The oils represented are 150 SUV and 300 SUV viscosity grades of naphthene base refrigerator oils. The curves show the following miscibility relations for Refrigerant 22 mixtures: OS Content of Refrig- Teapeietnre* at WUdi Tiro liquid Phrnt 4f+enr eruting Liquid m the Evaporator 150 SUV 300 SUV ! 2 percent 6 percent 10 percent -28 to --37F + 8 to OF +35 to +23F -- 13 to --16F +26 to +22F +54 to +45F - . Thus Walker's data indicate that, if a Refrigerant 22 sys tem, and especially the evaporator, can be designed to limit the oil content of the circulating refrigerant to two percent or less, there will be little danger of oil reparation, except at fairly low temperatures. An extensive study of the miscibility of Refrigerant 22 with 18 different oils has been reported by H. J. Lofiler.10 Most of these oils were experimental products, intended not so much for ultimate practical use as to provide a working range of low, medium and high viscosity gaders in both, naphthene and paraffin base types. Considerable-variation in miscibility was found. Complete miscibility diagrams are shown in Loffler's original paper; however, only the reported critical values'(as converted) are summarized in Table 6. In general, addition of Refrigerant 12 to Refrigerant 22 systems increases oil solubility and lowers the critical solution temperature. This latter effect appears as a drop of aboutv40 Lubricants in Refrigerant Systems 793 or 50 F when the mixed refrigerants contain 85 percent Re frigerant 22 and 15 percent Refrigerant 12.u*1* It has been reported13 that, for a given weight concentration of oil cooled below the critical solution temperature, the minimum tem perature at which the oil-rich phase will still flow is not as low when the refrigerant mixture contains Refrigerant 12. How ever the Refrigerant 12 addition also narrows the immiscible range and.extends the solubility of the refrigerant on the oilrich pde, e.g., from 18 weight percent refrigerant for pure Refrigerant 22 to 24 weight percent refrigerant for the $5/15 Refrigerant 22-12 mixture, both measured at the minimum Sow temperature. Thus the oil-rich phase is automatically further diluted by refrigerant when Refrigerant 12 is added to Refrigerant 22 and hence the minimum flow temperature actually drops instead of rising. The decrease is from -- 89 F to -- 107 F for the 15 percent Refrigerant 12 addition men tioned above.13 The naphthene base oil tested in this particu lar case had experienced a critical solution temperature of -j- 27 F when diluted with pure Refrigerant 22 and about -- 23 F when diluted with the Refrigerant 22-12 mixture. Partial Miscibility in Evaporators Immiscibility of oil-refrigerant solutions in the evaporator may lead to problems. Some oil always finds its way to the evaporator since oil separators, if they are used at all, are never perfectly efficient, and the condensing gas always carries a certain amount of entrained oil. Just as oil within a system always contains dissolved refrigerant, liquefied refrigerant in the system always contains dissolved oil. A problem of major concern maybe adequate return of oil from the evaporator to the crankcase. This is frequently ac complished by providing for entrainment in the suction gas. But if the refrigerant has poor solvent power at the evaporator temperature and an immiscible oil-rich phase separates, this viscous,-nonvolatile liquid may migrate and collect in pockets or blind passages which are not easily reached by.entraiiiment. Thus, it is possible for the rate of oil return to be affected and in some instances, a particularly obnoxious condition known as oil-logging may occur. Evaporator temperatures also may be affected, particularly in the case of flooded systems,, if a Rg. 18 .. .. Viscosity of Mixture "of '80 SUV Naphthene Base OP arid Refrigerant 13B1" separated oil layer floats on the boiling liquid. The'design of thesystem should take into account all these possibilities, and evaporators should be designed to promote good entrainment. Ofl separators are frequently required in the discharge line to minimize nil circulation when refrigerants of poor solvent power are employed. Parmelee* has shown that the viscosity of the oil, or rather!the oil-rich solution, in the low side of a system is an important factor in providing good oil return. -The viscosity of the oilfrich liquid that accompanies the suction gas will change as it meets rising temperatures ou its way back to the compres sor. Two influences seem to be at work. First, since the'tem peratures are increasing; this in itself would be expected to cause a decrease in viscosity. On the other hand, the. increas ing temperature also tends to drive dissolved refrigerant out of solution^ arid if the suction pressure remains unchanged, this second influence would be expected to cause an increase in viscosity. Parmelee studied the variation of viscosity-with temperature and pressure for five oil-refrigerant solutions at temperatures ranging from --100 F to 70 F. These data are shown:in Figs. 38 through 40, inclusive. The oil in each case was a 150 SUV naphthenic oil and the dissolved refrigerants were Refrigerants 12; 13, 13B1, 22, and 502. In all cases, the viscosities of the solutions passed through maxii-mim values as the temperature was changed at constant pressure, a find ing that was also,consistent with previous data obtained by German workers.-*6,1* According to'Parmelee, the existence of a viscosity maximum is significant,'--since at a given suction pressure.the'Ofl-rich solution will.beoome.most viscous, npt in the coldest regions in the evaporator, bpt atsome intermedi- i J