Lubricant still and reservoir for refrigeration system

A lubricant still for use in a compressor for separating oil from refrigerant, includes a vessel having an inlet for incoming oil laden refrigerant, an outlet for gaseous refrigerant, and an outlet for refrigerant laden oil. A separating structure is provided for separating transitioning oil laden refrigerant from the incoming oil-laden refrigerant, wherein the oil laden refrigerant progressively changes to the refrigerant laden oil closer to the outlet for refrigerant laden oil. A heating device is used for heating the incoming oil laden refrigerant and transitioning oil laden refrigerant, for facilitating the formation of the gaseous refrigerant and the refrigerant laden oil.

TECHNICAL FIELD

This invention is directed to separation of lubricant from refrigerant in compressors, and more particularly, the separation of lubricant from refrigerant in a lubricant still.

BACKGROUND OF THE INVENTION

Screw or helical compressors are commonly used in air conditioning applications to compress refrigerant as part of the refrigeration cycle. Screw compressors are composed of meshing screw or helical rotors. While two rotor configurations are the most common design, screw compressors are also known in the art having three, or more, rotors housed in respective overlapping bores so as to co-act in pairs. The rotors of a typical screw compressor are mounted in bearings at each end in housing end plates at the inlet and discharge side. Refrigerant is compressed by the screw rotors toward the discharge side and discharged through ports and into a discharge line.

In normal applications, a solution or mixture of oil and refrigerant is used for lubricating screw compressor bearings and rotors. This lubricant becomes entrained in the refrigerant while the refrigerant passes through and is compressed. If this entrained lubricant is not separated and recovered by some means, it passes through condenser and liquid line and accumulates in the evaporator where it is mixed with liquid refrigerant. As a result, evaporator heat transfer effectiveness is degraded. Oil foam may also be created, which is entrained in suction flow entering the compressor, reducing the refrigerant flow rate of the compressor. Even worse, lubricant supply for bearing and rotor lubrication is eventually depleted.

In the past, oil separators have been utilized immediately downstream of the compressor. While oil separators do separate the lubricant, they have not always provided fully satisfactory results. As an example, the lubricant removed with such a separator will be at a high pressure, and may have an appreciable amount of refrigerant mixed in with the oil. This lowers its viscosity, degrading its usefulness as a bearing lubricant. The use of a separator can also cause a pressure drop in the compressed refrigerant, which is undesirable. A separator may also radiate sound due to internal pressure pulsations acting on its walls. A separator may also add considerable cost to the system since it is a pressure vessel of considerable size.

Another approach to lubricant separation is by use of a concentrator, or still, attached to the evaporator, also sometimes referred to as a generator as, shown for example in U.S. Pat. No. 6,182,467 B1. In such systems, a portion of the oil and refrigerant mix residing in the evaporator is made to flow into the concentrator, where means are provided for heating the mixture to cause some liquid refrigerant to vaporize. The liquid remaining thereby contains a higher fraction of oil. By suitable choice of the amount of refrigerant vaporized, a liquid with sufficient viscosity for use as a bearing lubricant may be created.

Referring toFIG. 3, such a prior art lubricant still is shown in detail, wherein the still28comprises a pressure tight vessel30, which includes an inlet32for oil laden refrigerant7, drawn off the evaporator, below the liquid level line, an outlet34for gaseous refrigerant, an oil outlet36for out flowing concentrated oil that has undergone separation. Still/reservoir28further includes a coil42through which the hot refrigerant flows for transfer of heat to the incoming oil/refrigerant mixture. Coil42has an inlet38for hot refrigerant and an outlet40for cooled refrigerant having gone through a heat transfer process.

The use of such stills for creating a lubricant from the oily refrigerant mix normally residing in an evaporator is a known art, with the viscosity resulting from still action in the range of 3 to 20 centipoise (cP). However, for some screw compressors, particularly those operated at low speeds, much higher lubricant viscosity of at least 50 cP is required. Approaches to achieving higher viscosity lubricants in conventional stills are less than satisfactory. For example, simply increasing the amount of heat provided to vaporize refrigerant may result in somewhat higher lubricant viscosity but may also incur a severe penalty to system efficiency since the extra heat provided must be accounted for when calculating the system efficiency. Moreover, conventional stills are defective in producing adequate lubricant viscosity during operating transients that result in sudden increases in the influx rate of oil laden refrigerant from the evaporator. During such transients the entering liquid tends to flood the still, mixing with liquid that has resided in the still for some time and lowering its viscosity.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved lubricant still for use with a compressor for the separation of lubricant from refrigerant.

It is another object of this invention to provide an improved lubricant still that produces lubricant of high viscosity and maintains high viscosity during operating transients that result in increased flow of oil laden refrigerant from evaporator to still.

These objects, and others as will become apparent hereinafter, are accomplished by the lubricant still of the present invention for use in a compressor for separating lubricant from refrigerant. The still includes a vessel having an inlet for incoming oil laden refrigerant, an outlet for gaseous refrigerant, and an outlet for refrigerant laden oil. A separating structure is provided for separating transitioning oil laden refrigerant from the incoming oil-laden refrigerant, wherein the oil laden refrigerant progressively changes to refrigerant laden oil closer to the outlet for refrigerant laden oil. A heating device is used for heating the incoming oil laden refrigerant and transitioning oil laden refrigerant, for facilitating the formation of the gaseous refrigerant and the refrigerant laden oil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings in detail there is shown inFIG. 1a schematic view of a refrigerant system1, including a compressor2.

As is known, a flooded style evaporator3delivers primarily gaseous refrigerant to the compressor2through a line4. Gaseous refrigerant is compressed by compressor2, entraining lubricant during its passage through compressor2that is used to lubricate the bearings and rotors of compressor2. From the compressor2, refrigerant with entrained oil passes through a line5to a condenser6. Compressed gaseous refrigerant is cooled in the condenser, transferred into a liquid phase, with oil in mixture or solution, as it passes in line11through an expansion valve (not shown) to evaporator3. At the evaporator3, an environment to be cooled is cooled by the refrigerant in the evaporator. As is shown, it is typical that liquid refrigerant7settles from the refrigerant in the evaporator. This refrigerant7is typically lubricant or oil laden as a result of the oil entrained during the compression process and associated lubrication, and a portion of the oil laden refrigerant is transferred via line8to lubricant still128, described further below, according to the principle of the present invention. A portion of this lubricant is drawn off using an oil pump (not shown) and delivered to compressor2through line13for lubrication of bearings and rotors.

Referring toFIG. 2, an example of how oil enters the refrigerant during the compression process will be further described. Shown inFIG. 2is a screw compressor10, that includes a rotor housing12containing intermeshing screw rotors14and15and bearings17on suction-side of screw rotors, refrigerant inlet18and discharge port20, including a discharge bearing housing22containing discharge-side bearings23and discharge housing24that is connected with a discharge line26. In operation, assuming rotor14to be the driving rotor, rotor14rotates engaging the other rotors15, causing their rotation. The co-action of rotating rotors14and15draws refrigerant gas via suction inlet18into the grooves of rotors14and15that engage to trap and compress volumes of gas and deliver hot compressed refrigerant gas to discharge port20. In addition, and simultaneously, lubricant is injected into the screw rotors for effective lubrication of the rotors, and as a result oil becomes mixed with refrigerant. Simultaneously, lubricant is also delivered to suction bearings17and discharge bearings23. Some or all of bearing lubricant may also leak internally and ultimately be entrained in refrigerant passing through. The hot compressed refrigerant with the lubricant therein moves through the system1described above. The present invention is used to separate the lubricant from the refrigerant for use for lubricating the compressor.

While the present invention is shown being used with a screw compressor, it may also be used with other types of compressors as well.

As shown in FIG.4and also referring toFIG. 1, an embodiment of the lubricant still of the present invention, still128, comprises a pressure tight vessel130, which includes an inlet132for oil laden refrigerant7, drawn off the evaporator, below the liquid level line, an outlet134for gaseous refrigerant and a lubricant outlet136for out-flowing lubricant of high viscosity that has undergone separation. Still128includes a series of liquid reservoirs138created by partitions140acting in concert with portions of the inner wall of pressure tight vessel130. In this embodiment, heat for vaporizing some liquid refrigerant in oil laden refrigerant7is provided by electric heater150, which is in close proximity to the lower wall151of pressure tight vessel130. Other arrangements for electric heaters, including locating them within vessel130, and other means for providing heat, such as providing tubes containing hot liquid or gas refrigerant or hot water, are also compatible with this embodiment of the present invention.

Gaseous refrigerant created by heat acting on liquid in any of reservoirs138rises within vessel130and eventually exits through vent134, which is connected (not shown) to either evaporator3or passage4. Flow of liquid through still128is due to the effect of gravity G, wherein vessel130is tilted downwardly from inlet132, as shown. InFIG. 4flow occurs from right to left, proceeding over the top of each partition140and through each reservoir138in sequence, from138ato138e. The most upstream reservoir138ain the sequence is connected to inlet132and typically contains a high fraction of the oil laden refrigerant7. The most downstream reservoir in the sequence138eis connected to lubricant outlet136and acts as a lubricant reservoir. The construction of partitions140, such that flow occurs over their tops T, is an aspect of the present invention. Oil rich liquid or foam, shown typically as152inFIG. 4, tends to rise to the top of reservoirs138due to buoyancy, because the density of the liquid/foam152is lower than the density of other liquid present in reservoirs138. Thus, oil rich liquid and foam flows in reservoirs138over the tops T of partitions140, over the other liquid in the reservoirs. By this means, the oil concentration of the liquid in reservoirs138increases as flow progresses downstream in the sequence of reservoirs138, from138ato138e. Through this means, a lubricant of high viscosity is developed in the most downstream reservoir138e, which acts as a lubricant reservoir. During operating transients when the influx rate of oil laden refrigerant entering the most upstream reservoir138athrough inlet132increases, the liquid flow rate through still128also increases. However, because the liquid is refrigerant rich, its density is higher than oil rich liquids or oil rich foams152, leading to downstream flow over the tops T of partitions by the more oil rich liquids and foams152, as previously described. Thus, even during such transients, the progression of additional refrigerant rich liquid downstream is hindered and the high viscosity of the lubricant in the most downstream reservoir is substantially maintained.

An additional advantage in vaporizing refrigerant to create a lubricant of high viscosity may be realized by designing the reservoirs138such that their free surface area-to-volume ratio is as high as possible as it is known that the migration of vaporizing refrigerant from a mixture or solution of liquid refrigerant and oil is enhanced as free surface area-to-volume ratio increases. Thus, within the bounds of cost-effective construction, the depth (the measure of the reservoirs138into the page) and length of reservoirs138should be maximized relative to their height.

Another preferred embodiment is shown inFIGS. 5 and 6. Referring toFIG. 5, and also referring toFIG. 1, similar to as described above, a still228comprises a pressure tight vessel230, which includes an inlet232for oil laden refrigerant7, drawn off the evaporator, below the liquid level line, an outlet234for gaseous refrigerant and a lubricant outlet236for out flowing lubricant of high viscosity that has undergone separation. Still228further includes a series of liquid reservoirs238ato238gcreated by partitions240. Reservoirs238ato238gand partitions240are preferably made by stamping their forms in sheet metal of relatively high conductivity such as steel, aluminum or copper to form the entire series of reservoirs238and entire series of partitions240in a single pan-shaped piece242of high conductivity material, having an elongated flattened shape, as shown in FIG.6. In this embodiment heat for vaporizing some liquid refrigerant in oil laden refrigerant7is preferably provided by flow of hot refrigerant gas drawn off the condenser or, as shown inFIG. 1, from a tap39off the discharge line5of compressor2, entering through an inlet260in FIG.5and exiting through outlet262as cooled refrigerant having gone through a heat transfer process. The refrigerant flows through an internal passage defined by the single piece242and a matching bottom piece244, described further below with reference to FIG.6. Pan242is fastened within vessel230on an angle as shown, using conventional means such as, for example, brazing, welding, bolting or shimming.

Gaseous refrigerant created by heat acting on liquid in any of reservoirs238rises within vessel230and eventually exits through vent234, which is connected (not shown) to either evaporator3or passage4. Flow of liquid through still228is due to the effect of gravity G and the orientation of pan242. Referring still toFIG. 5, flow occurs from right to left beginning at inlet232through the series of reservoirs238ato238gand over the series of partitions240, ending in the most downstream reservoir238h. The most upstream reservoir in the sequence238a, connected to inlet232, typically contains a high fraction of the oil laden refrigerant7. The most downstream reservoir238hin the sequence is connected to lubricant outlet236and acts as a lubricant reservoir. The construction of partitions240such that flow occurs over their tops T. In other respects, aspects of the embodiment shown inFIG. 5pertinent to creating and maintaining a lubricant of high viscosity are the same as those of the embodiment shown inFIG. 4, and previously described.

With reference toFIG. 6, the pressure-tight passage for flow of the hot refrigerant gas is made, using the single piece of high conductivity material242(described above) as an upper boundary and part of side boundaries for hot refrigerant gas flow and a single lower piece244, preferably stamped from a single sheet of the same high conductivity material as242is formed from, as a lower boundary and forming part of the side boundaries.242and244are suitably joined in a pressure-tight manner, preferably also by brazing. Inlet260and outlet262may suitably be joined in a pressure tight manner to the assembly of242and244, preferably by brazing or could be formed as an integral part of pieces242and244.

In accordance with another embodiment of the present invention, and referring toFIG. 7, a still328includes at least one flat separating pan344positioned in coil342, dividing the cavity346of vessel330into two Zones A and B, and which is angled downwardly such that liquid will flow over its surface. This division by pan344effectively separates the oil-laden refrigerant from the refrigerant-laden oil, by creating the two separate zones A and B. Accordingly no immediate mixing of oil-laden refrigerant with refrigerant-laden oil occurs, thereby avoiding the pitfalls of the prior art that allows such mixture and effectively dilutes the separation process. Pan344is preferably in intimate contact with coil342to facilitate efficient heat transfer with the oily refrigerant and has an elongated, flattened shape. The flattened shape functions to spread the oil-laden refrigerant out in a thin layer which enhances the distillation process and separation of the lubricant from the oil-laden refrigerant. Accordingly, Zone A located in the upper region of cavity346functions as the distilling region, wherein the oily refrigerant at about 90-95% refrigerant enters from the evaporator at saturation temperature and pressure. Heat is transferred from the hot refrigerant in the coil and causes the refrigerant portion of the oily refrigerant to vaporize and separate from the oil-laden refrigerant. The separated refrigerant vapor exits through outlet334.

In Zone B, lubricant, consisting of oil with about 10-40% refrigerant collects, having moved down pan344into the bottom of vessel330.

As an alternative to coil342, an electric heater348shown by dotted lines can be used to supply the necessary heat to the pan.

In operation, oil laden refrigerant which is 90-95% refrigerant enters vessel330from the evaporator through inlet332onto pan344, and hot refrigerant enters inlet338drawn off the condenser or the compressor discharge line, and circulates through coil342. Heat is transferred from the hot refrigerant in the coil causing the oil laden refrigerant to reach saturation temperature and results in vaporization of the majority of the refrigerant, which exits as a gas through outlet334to the slanted orientation of the pan, liquid flows down the pan through Zone A and drips into Zone B at the bottom of vessel330. Heating by the refrigerant in the coil continues in Zone B but is directed to the refrigerant-laden oil, causing additional boiling off of remaining refrigerant which flows as vapor out of the vessel through outlet334while oil flows out of vessel330through outlet336. Cooled refrigerant in the coil exits the vessel through outlet340.

Referring now toFIG. 8, an alternative of the embodiment ofFIG. 7is shown. InFIG. 8, two pans444aand444bare used, along with the same coil142arrangement and outlets and inlets described above. With the embodiment shown inFIG. 8, a third Zone AB is added intermediate to Zones A and B, that acts to further separate refrigerant from the oil laden refrigerant for exiting of vessel430through outlet434. Zone AB functions in a manner similar to that described above for Zone A, acting as a supplementary step to the process described in Zone A. Pans444aand444bare each slanted downward, wherein pan444ahas less of an incline than pan344and leads the liquid to pan444b. Pan444bis slanted in the opposite direction of pan444a, such that the lower point450of pan444ais almost vertically coincident with the higher point452of pan444b, but sufficiently offset to allow liquid flow from one pan to the next. Operation in Zones A and B, and the remaining inlets and outlets, but for their locations due to the differing pan arrangement; are the same as described above for theFIG. 7embodiment.

Although preferred embodiments of the present invention have been illustrated and described, other changes will occur to those skilled in the art. It is therefore intended that the scope of the present invention is to be limited only by the scope of the appended claims.