Document M4BB313wGeQGZ79LOgQ3p0Lj

786 CHAPTER 50 1965 Guide And. Data Book definite amount of the substance that can'be present in. a stated amount of solution when the solution is in equilibrium with its environment. The environmental conditions of tem perature and pressure are essential to the definition and are either stated explicitly, or, if no ambiguity is possible, are implied when numerical values are expressed. The effect of the environmental variables of refrigerant pressure and mix ture temperature is generally reported in the form of pressuretemperature-solubility curves. Solubilities are commonly stated in terms of percent by weight but volume percent and mol percent have also been used. When weight percents are used, these are more properly based on the weight of the whole solution and not on the weight of the oil alone. The high degree of solubility illustrated by the foregoing example has .two consequences with respect to a refrigerant system. First, an oil which contains 25 or 26 percent refriger ant will show a much lower viscosity than would be predicted from standard viscosity curves such as Fig. 1. Second, the Refrigerant 12 pressure of 59 psia in the example is sub stantially lower than the saturation. pressure (94.5 psia) which would have been obtained if the system had been charged without oil. These effects are characteristic of most halogenated refrigerant systems. Table 4 provides a convenient index to the pressure-tem perature-solubiiity curves (and the viscosity-temperature charts) included in this chapter. Units for these charts are in psia, degrees F, and weight percent of dissolved refrigerant. Although in some, but not all instances, the pressure, tem perature, and solubility units given by the original authors have been converted in order to minimise inconvenience in applying the data; a cross index to the original work is listed in the last column of the tabulation. Additional information on the solubility of refrigerants in commercial oils may be obtained from the oil suppliers. Effect of Oil Type on Refrigerant Solubility The solubility of a given refrigerant is not greatly affected by tire type of mineral oil in which it dissolves. Some dif ferences exist, however. A low viscosity oil will usually absorb more of a particular refrigerant than a high viscosity oil under equivalent conditions. Naphthene base oils, at least on a weight basis, are usually better solvents than paraffin base oils. As an illustration of the solubility differences, several of the charts contain curves for naphthene base oils and paraffin base oils plotted on the same diagram. See.Fig. 13 (Refriger ant 13 solubility), or Fig. 22 (Refrigerant 22 solubility). In each case the naphthene and paraffin curves plotted at the same temperature are cot far apart. Sometimes it is necessary to know the solubility of a given refrigerant in an unknown oil; an oil-for which actual solu bility data have not been determined. If. the solubility of the given refrigerant has been determined in any .oil, these data will provide a reasonable approximation for the unknown oil. This assumption, however, does not apply to synthetic lubricants and should be limited to mineral oils that are customarily employed in refrigerant systems. Effect of Dissolved Refrigerant on Viscosity Refrigerants differ in their viscosity reducing effects when the solution concentration is measured in weight percent. However, when the solubility is plotted.in terms of mol per cent it has been shown that the reduction in viscosity at a given mol percent is approximately the for the halo genated Refrigerants 13,13B1, 22, and 115.D,u In comparison with paraffin base oils, naphthene base oils show a somewhat greater reduction in viscosity for the same weight percent dilution. (See Figs.'24 and 26.) '* An index to the viscosity-temperature charts reproduced in this chapter is found in the fourth column of Table 4. The weight percent parameter, or in some inntAnre*^ the pressure parameter may be used to correlate the corresponding solu bility curves listed in the third column of the table. Example 9- A certain paraffin base oil of 150 SUV has a specific gravity of 0.8G and a.viscosity of 37 centipoiaes at 90 F in air. Determine viscosity in contact with'Refrigerant 22 at 105 psia and an oil temperature of 90 F. Solution: Find on Fig. 23, weight percent Refrigerant 22 - 14 percent at 90 F and 105 psia. Then locate on Fig. 24, viscosity of oil-refrigerant mixture 12.5. centipoiaes .at 14 percent Re frigerant 22 and 90 F. A general method for estimating the viscosity redaction of any mineral oil for refrigerant dilution up to 25 weight percent has been developed for solutions of oils containing Refriger ants 12, .13, 21, 22,113,114, 115, and 12-22 mixtures.*;0 This method is based on the, use of specialised viscosity plotting paper of German origin and is said to give results which agree within 2 or 3 percent of experimental values. Effect of Viscosity Reduction on Lubricating and Sealing Functions -: A large reduction-in viscosity may affect the lnhrie*ting function, or more likely, the sealing function of the lubri cant, and this possibility must be taken into account when the initial choice of oil viscosity is maria Gortoao eU*, bo** ea doted by fbecuf&or/ riseos'f/ei converted te'St/V cad Interpolated erf 100 f. Hg. 2,.... Variation of Refrigeration Oil Densities. ' With' Temperature 31 Lubricants in Refrigerant Systems 787 Hearing loads in most compressors are not high and the lubrication function is relatively easy to perform. The prob lem of sealing is not always so easy to accomplish. Hie design of some types of hermetically-sealed units, such as the anglevane rotary type, requires the lubricating fluid to act as a highly efficient wealing agent. In reciprocating compressors, the lubricant film is spread over the entire area of contact between the piston and the cylinder wall, and there is a very huge area to resist leakage from the high- to the low-pressure side. In the single-vane rotary type, however, the only sealing. area is a line contact between the vane and a roller. Viscosity reduction may then be a serious matter. Normally, an oil should be chosen with a viscosity which is low enough to give the necessary sealing properties with the refrigerant used for the entire range of temperatures and pressures encountered. A practical method for determining the mtntmum safe viscosity is to determine the total volu metric efficiency of a given compressor system, using several oils of widely varying viscosity. The oil of lowest viscosity that gives satisfactory' volumetric efficiency should be se lected. These tests should be run at a number of ambient temperatures; for example, 70,90 and 110F. Density of Oil-Refrigerant Solutions For converting viscosity units (e^., from centistokes or SUV -seconds to centipoises or vice-versa) and for other purposes, it is necessary to know the density of the lubri cating fluid. For a rough estimate of the density or specific gravity of an oil-refrigerant solution, one may assume that the solution is ideal and hence the specific volumes of the compo- . neats are additive. The formula for calculating the ideal density (Du), in grams per cu cm or pounds per cii ftj is * - - D" " 1 + WW./d, - 1) (1) where ~ - d. -- density of pure oil at the solution temperature. de *= density of refrigerant liquid at the solution temperature. W -- weight fraction of refri^rant in solution. Fig. 2 shows published values for pure.oil densities over a range of temperatures. These density-temperature curves all have approximately the same-slope and appear merely to be fig. 4 .... Density Correction Factors tor Mineral Oil* Refrigerant 22 Solutions' - rijgpVcwH from one another according to base type and vis cosity. Hence, if the density of a particular oil is known at one temperature but not over a range of temperatures, a reasonable estimate at other temperatures may be obtained by constructing a line paralleling those in Fig. 2;' The actual density of an oil-refrigerant solution may deviate from the ideal by as much as 8 percent. The solutions are usually more, but sometimes less, dense than calcu lated.*,1#a7 Density correction factors for Refrigerant 12 and Refrigerant 22 solutions have been published by Ldffier* and are redrawn in Figs. 3 and 4, respectively. The corrected den sities may be obtained from the relation:' Mixture density *= Dm > (2) where A is the density correction factor read from Fig. 3.or 4 at the desired temperature and refrigerant concentration. Thermodynamics of Refrigerant-Oil Solutions The dissolving.of oil in liquid refrigerant will affect the thermodynamic properties of the working fluid. The vapor Table 5.... Mutual Solubility of Refrigerants and Mineral Oil . (Ammonia and carbon dioxide ere precficofy frrnnkcfbte wftA oflj ' H/gh ' AtodMrfy -. Refrigerant'll Refrigerant 12 Refrigerant 13B1 Refrigerant 21 Refrigerant 113 ' Refrigerant 500 ' Methyl Chloride ( "Methylene Chloride Hydrocarbons- . Intermediate. ` - Mitability Refrigerant 22* Refrigerant 114 "low MitabUSfy' Sulfur Dioxide1' Refrigerant-13. Refrigerant 14 Refrigerant 115.- Refrigerant 152a Refrigerant C318, Refrigerant502 _ ' with aQ in the high temperature tide of the turene which emxatonl imn thoe evaporator two uqmo ,f-arme,di, .. Eachijla*ycrir oooteins both oil end rebigerant but the uppenno* 1layer "her to aB end the bottom leywti much tioner to Eefrigertht *3. -k