Rotary Displacement Compression Heat Transfer Systems Incorporating Highly Fluorinated Refrigerant-Synthetic Oil Lubricant Compositions

FIG. 1 is a schematic block diagram illustrating a typical, oil-flooded, twin screw, compression heat transfer system.

FIG. 2 is a fragmentary schematic view of a typical compressor with a portion of its casing broken away to expose its internal components;

FIG. 3 is a graphical diagram indicating the solubility of the refrigerant and the lubricant oil mixture at various temperatures and concentrations;

FIG. 4 is a similar diagram using an alternative oil; and

FIG. 5 is a similar diagram with still a different oil ingredient.

In FIG. 1 the components of an oil-flooded twin screw, compression system are labeled for purposes of ready identification. In the system illustrated, an oil pump 10 pumps a synthetic oil to be described later, via a line 11, to an oil cooler 12 which supplies it to a compressor 13 via line 12a. The lubricating, cooling, and rotor-sealing oil which is exposed to the refrigerant vapor in the compressor, is returned from the compressor 13 to an oil separator 14 via a line 15. Compressor 13 receives the refrigerant in a gaseous state from the evaporator 16 via a line 7. The vapor is pressurized and "work energy"-heated in the compressor and, after being separated from the oil at separator 14, is moved through a line 18 to a condensor 19 where it condenses or liquifies as a result of the combination of increased pressure and loss of heat. From condensor 19 the liquid proceeds, via a line 20, through an expansion valve 21 to the evaporator 16 where, as a result of its loss of pressure and absorption of heat, it vaporizes. Typically, the refrigerant evaporating temperature is in the temperature range minus 40.degree. C. to plus 20.degree. C., and the refrigerant condensing temperature is in the range plus 30.degree. C. to plus 90.degree. C. The oil separator 14 is capable of separating out a great proportion of the lubricating oil and is aided in this task because, at the temperatures involved, the oil to be more particularly described resists dissolving the refrigerant to be described.

FIG. 2 diagrammatically illustrates a typical rotary screw compressor. Here a female rotor 22, which typically may have six lobes, is mated with a male rotor 23 which typically may have four lobes. A thrust bearing is indicated at 24 and a radial bearing at 25. The sliding valve actuating piston is indicated at 26 and the control cylinder at 27. A shaft seal is indicated at 28. The suction port which connects with line 7 is indicated at 29 and the discharge port, which communicates with line 15, is indicated at 30. Oil supplied by the external pump 10 is injected along with the suctioned gas from line 7 to lubricate and seal compressor 13. In rotary screw compressors the lobes, bearings, and gears must be lubricated and sealed by the oil. In addition, as noted, the volume of oil injected must be sufficient to cool the compressor.

In a closed system the refrigerant gas and the oil will, to some extent, be mutually soluble or completely miscible dependent on the temperature and the concentration of the oil. At compressor temperatures and pressures when oil floods the compressor, there is a maximum surface exposure of the oil to the refrigerant gas. A liquid oil-rich phase which is essentially oil and a refrigerant-rich gas phase which is essentially refrigerant is the result. The gas phase moves to the condenser along with the portion of the liquid phase which the oil separator cannot remove. Normally, at the condenser, the oil will be present in a concentration of less than 5% whereas in the compressor it will be present in far greater concentration.

In FIG. 3 a miscibility graph has been disclosed which demonstrates an unexpected solubility characteristic in the system's operative temperature ranges. The graph was plotted following tests which indicated what is considered to have been demonstrated by the tests. Here the upper graph line x indicates total immiscibility or insolubility above the line at various temperatures and percentages by weight of oil in the oil-refrigerant. The line x indicates inverse solubility in moving from a downward slope with lower concentrations of oil in the 5% range back to an upward slope at higher compressor temperatures with higher concentrations of oil. Line y plots a lower temperature immiscibility or insolubility of the refrigerant in the oil at other weight concentrations and temperatures below the compressor operating temperatures, but not at concentrations below 5%. Both the lines x and y, with a weight concentration of oil up to 5% demonstrate a soluble zone or zone of complete miscibility where a single phase of liquid, i.e. oil in the refrigerant, is encountered from evaporator discharge temperatures of minus 62.2.degree. C. to plus 20.degree. C. at concentrations of oil around 5%. In the compressor, at the oil separator, and at the condenser inlet, the oil-rich phase and the refrigerant-rich phase form essentially two discrete layers which are insoluble in this two phase zone. The oil-rich layer, because of this, is more efficiently separated from the gas at the high temperature and pressure conditions of oil separator 14. Further, the oil will effectively resist dissolution of the refrigerant-rich component in the compressor at temperatures within the range 30.degree.-93.degree. C., and above. It is believed the concentration of the oil in the one phase will be in the range of about 100-60% while the concentration of oil in the other phase will be in the range of about 0-20%.

At the same time, the dissolved oil can have a good return from the evaporator at the operative temperature range involved, minus 40.degree. C. to plus 20.degree. C., since only one phase of liquid is involved when the oil is present in the range of 20 parts per million to 5% oil.

To alleviate any tendency of the oil to accumulate at the evaporator, an oil still of well known character can be employed to tend to separate the oil from the vapor at the evaporator and return it to the compressor inlet. In some systems the oil can be separated at such a still and the need for an oil separator between the compressor and condenser obviated. In other oil injected screw compression systems, a refrigerant pump injects the oil-refrigerant solution directly to the screw compressor suction, making an oil still, as such, unnecessary.

The foregoing miscibility curves were obtained with refrigerant R-134a. The synthetic oil used was a 150 ISO polyoxyalkylene diol glycol having the generalized formula ##STR1## In this formula the integer n has a value of from 5 to 100. R is hydrogen or an alkyl group containing 1-4 carbon atoms. This would also include a copolymer of random distribution and block distribution. Oils of this formulation, having ISO viscosities of 100 and 150, have been tested and found to have the inverse solubility characteristics depicted in FIG. 3. The lubricant was found to remain fluid at low temperatures and the time required for the refrigerant and oil to reach equilibrium was not excessive.

Other oils exhibiting substantially the same inversely soluble characteristic depicted in FIG. 3 are the polyoxyalkylene glycol monols of the structure ##STR2## where R is hydrogen or an aklyl group containing 1-4 carbon atoms, R' is selected from the group consisting of linear or branched chain alkyl groups having 1 to about 24 carbon atoms, preferably about 4 to about 8 carbon atoms, and arylalkyl groups wherein the arylalkyl groups have about 6 to about 30 carbon atoms, preferably about 6 to about 15 carbon atoms, and contain a reactive hydrogen atom. Examples of compounds containing a reactive hydrogen atom are alcohols, phenol triols, primary and secondary amines, and carboxylic and sulfonic acids and their amides, and where n=3-70. FIG. 4 discloses a graph which illustrates inverse solubility where the oil used was a 100 ISO oil of this monol formulation.

FIG. 5 discloses a diagram wherein the miscibility of a glycerol-initiated polyoxypropylene glycol triol with respect to refrigerant R134a is depicted. The triol is the T-1000 product marketed by Olin Corporation of Cheshire, Conn. This product has an average molecular weight of approximately 1000 and is designated 182 ISO. Its viscosity at 40.degree. C. is 180 centistokes.

A general formula for this lubricant and the other lubricants is as follows: ##STR3## Z is the residue of an initiator compound having 1 to 8 active hydrogens,

R.sup.1 is hydrogen or methyl when R.sup.2 is methyl,

R.sup.2 is hydrogen, methyl, or ethyl when R.sup.1 is hydrogen,

n is a number from 1 to 100,

m is an integer having a value from 1 to about 8.

It is expected that additives for various purposes will be added to the base lubricants disclosed which, however, constitute at least a substantial portion of the complete oil sufficient to impart the inverse solubility characteristic depicted. The complete oil will be essentially water free as usual. It may include a compatible commercially available ester (or esters) for purposes of increasing miscibility at the condenser and evaporator. The complete oil injected for most applications will have a viscosity providing at least 4 centistokes as a sealant in the compressor and preferably 6 centistokes, while providing at least 6 centistokes and preferably 10 centistokes at the compressor bearings. For a few applications the viscosity could, it is believed be 3 centistokes. Thus, a base oil of at least about 50 ISO will be selected for most applications. The operative temperature ranges normal for most rotary displacement oil-flooded compressor operations, whether of the screw or vane type, are in the range plus 30.degree. C. to plus 90.degree. C., although operating temperatures above this, for instance, up to 121.degree. C., are considered practical and within the insoluble zone above line x in either FIG. 3 or FIG. 4. The operative range normal for evaporator operation is minus 40.degree. C. to plus 20.degree. C. and both FIGS. 3 and 4 demonstrate that the soluble zone at oil concentrations of under five percent, and, in the case of FIG. 4, under 10 percent, makes possible the desired operation of the system. In FIG. 4 the soluble zone has been cross-hatched between minus 70.degree. F. and plus 110.degree. F., but it is believed the zone extends both above and below the cross-hatched area with a concentration of oil of 10% or less. In FIG. 5 the oil and refrigerant are soluble in the operative range for normal evaporator operation at 15 and 20 percent oil content by weight and above, should the oil content reach such proportions.

While preferred embodiments of the invention have been described in detail, it will be apparent to those skilled in the art that the disclosed embodiments may be modified. Therefore, the foregoing description in all aspects is to be considered exemplary rather than limiting in any way, and the true scope of the invention is that defined in the following claims.