Document QnbM28d02VL2DmqD24pOJ9eR

788 CHAPTER 50 1965 Guide And-Data Book can be considered as dissolving the other and ret^irmcbip is frequently, termed the mutual solubility. ' The mutual solubility relationships are commonly ex pressed according to the predominant component, i.e., oil- rich and refrigerantrrich solutions..If the solution tW. ^ formed on muring the refrigerant and oil consists of a wingU liquid phase, the two components are said to be miscible. But if two separate liquid phases appear, it does not neces sarily mean that the oil and refrigerant are insoluble in each other. The two liquid phases are, themselves, solutions and although they,differ in composition, both will be found to contain substantial amounts of oil and refrigerant in mutual solution. Normally, one phase will'be oil-rich, the other, re frigerant-rich. The two solutions which are thus formed are, themselves, immiscible. Refrigerants are designated as being of high, intermediate, or low miscibility according to-thelr mutual solubilities with oils as shown in Table 5. . Completely miscible refrigerants and oils are mutually solu : <a> ble in all proportions at any temperature under consideration. Hence, a mixture of this type always forms a ring!* liquid phase under equilibrium conditions/ no matter how much refrigerant or oil may be present. Examples are Refrigerant 12 and mineral oil, and methyl chloride and mineral oil. In completely miscible mixtures, the oil-rich compositions merge smoothly and continuously into refrigerant-rich com positions as the refrigerant content is increased froin'iero to 100 jjerccot. This type of behavior is illustrated in graph (a) of Fig. 5. On this' graph Pi and P,4 represent the saturation pressures of pure refrigerant at temperatures h arid V respec tively. Point Ei represents one of the infinite number of solu tions which are possible at temperature I,. The coordinates of Ei ire Pi and Fi; these coordinates determine, respectively, the equilibrium' refrigerant pressure'' and .composition of the solution. The equilibrium refrigerantpressurePi is always lower than the corresponding saturation pressure':(Pi) for pressures, for example,.of refrigerantioil solutions at a given -temperature arealways less than,the vapor pressure of pure refrigerant at that" temperature. Thus, in an evaporator the presence of diraolved oil would lead to lower suction pressures and higher'evaporator temperatures than expected from pure refrigerant tables. An enthalpy diagram for R 12-oil solutions over the whole range.of compositions from rsero to 100 percent oil and temperatures from - 40 F to 240 F has been given by Bambach." Spauschus14 hasdeveloped general equations for calculating;thermodynamic' functions of refrigerarit-oil solu tions and' has applied these equations to thie special of R12^petroleuin oil solutions: Mutual,Sojubility of Oils and Refrigerants In dealing with the lubricating and* sealing problems in a compressor^ the lubricating fluid is considered as a solution of X refrigerant dissolved in oil. But in other,parts of the refriger ant system the problem may involve a solution of oil in liquid refrigerant. In both'instances, either the oil or the refrigerant could exist alone as a liquid if the other were not present, hence any. distinction between the dissolving and`dissolved rootpoherit merely reflects" a,point ohview.-. Either of the.liquids tubricants in Refrigerant Systems pure refrigerant at the given temperature, Ci. By increasing the mixture temperature to U one can shift the equilibrium to a new point, Ep Assuming no change in the total mass or volume of the system, the rise in temperature would be expected to vaporise a portion of the dissolved refrigerant. This transfer of dissolved refrigerant to-the-vapor phase should decrease the amount of refrigerant in solution and increase the refrigerant pressure. Hence point Et lies slightly to the left and above Ei, but it should be noted, as .before, that the pressure attained at the new temperature h is always less the saturation pressure of pure refrigerant, P** at at that temperature. Four general rules may be stated for refrigerant-oil mix tures of the completely miscible type: 1. For every solution composition, there is a corresponding vapor pressure which depends only on the temperature of the and increases as the temperature increases.; 2. At given temperature, the solution vapor pressure de pends only on the refrigerant concentration and increases as the concentration increases, but it is always less than the saturated vapor pressure of pure refrigerant at the given temperature. 3. For every refrigerant gas pressure, there is a corresponding refrigerant solubility which depends only on the solution tem perature and decreases as the temperature increases. i r f - 4.' When both the pressure of the refrigerant gas and the tem perature of the solution are fixed, there is only one possible solu tion composition under equilibrium conditions. Partially miscible refrigerants and oils are mutually soluble only to a limit**! extent. Above a certain temperature known as the critical solution temperature (or consolute temperature) oil-refrigerant mixtures in this *l*ss are usually completely mWiViip But below .that temperature there is a maximum value for the solubility of the refrigerant in the oil, andalso a minimum value for the solubilityof the oil in the refrigerant. Between these.two values an immiscible range of composition exists. The existence and breadth of the immiscible range varies with the nature of the two dissolving substances, and may be strongly influenced by. temperature. Examples;.of partially miscible mixtures are sulfur dioxide and oil, Re^ frigerant 22 and oil below its critical solution temperature, and Refrigerant 114 and oil below its critical solution tem perature. The behavior of partially miscible mixtures is illustrated in graph (b) of fig. 5. Point C on this graph represents the critical solution temperature (b). Below this temperature three separate regions may be seen in the diagram. Reading from left to right, a family of three smooth solid line curves r9- 7 -..Viscosity-Temperature Curves for Solutions: of 'Refrigerant VI in 150 SUV Naphthene Base OH* TEMPERATURE . OEGREES FAHftEWtttT fig. 8 .... Viscosity-Temperature Curves for Solutions of Refrigerant 11 in 300 SUV Naphthene Base Oil* represents a region of completely miscible oil-rich solutions; these are followed by a wide break representing a region of partial miscibility in which two immiscible liquid phases exist; and, at the right hand side, the partially miscible region disappears into a. second completely miscible region of re frigerant-rich solutions. A dome-shaped envelope (broken line curve OCR) encloses the partially miscible region; every where outside this dome the refrigerant and oil are completely miscible. Thus in a sense, graph (b) may be regarded as a variant of graph (a) in which the partial miscibility dome (OCR) blots out a substantial portion of the continuous solu bility curves. At all points outside the dome, and especially on the left hand tide, the pressure-temperature-solubility relations are treated as completely miscible mixtures. A concrete wranplw is shown by the solubility curves for Re frigerant 22 in Fig. 23, in which a small portion of the partial miscibility region lies to the right of the broken line in the lower right harid comer. In the partially miw.ihlft range, two important facts should be noted.'One is that at a given temperature, e.g., h, the two irarntM-ihlft solutions which are formed are represented by two equilibrium points, e.g., Ei and Ep These two solutions differ considerably in composition (Fi and F*) but have the same refrigerant pressure, P|. The solution pressure P\ lies not far below the saturation pressure of pure refrigerant, Pi*. Hence it is not uncommon for refrigerant-oil solutions near the par tial miscibility limit to show I*rr reduction in refrigerant pressure than is observed at the same oil concentration with refrigerants of the completely miscible type- A second fact is that the two immiscible solutions which are formed at a given temperature within the partially miscible range retain their identity regardless of the starting compo sition. Thus, a mixture of composition Fi would separate into two solutions, having compositions Fi and Ft at temperature h, always into these same two solutions,' no matter where Ft was taken between Fi and Wp The relative weights of the two solutions, i.e., the ratio of one solution layer to the other, may be computed from the lever rule: weight of oil-rich phase of'composition !Fi/weight of refrigerant-rich phase of com position F,(F,-F,)/(F.-F.).