Abstract:
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.

Description:
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 to  FIG. 3 , such a prior art lubricant still is shown in detail, wherein the still  28  comprises a pressure tight vessel  30 , which includes an inlet  32  for oil laden refrigerant  7 , drawn off the evaporator, below the liquid level line, an outlet  34  for gaseous refrigerant, an oil outlet  36  for out flowing concentrated oil that has undergone separation. Still/reservoir  28  further includes a coil  42  through which the hot refrigerant flows for transfer of heat to the incoming oil/refrigerant mixture. Coil  42  has an inlet  38  for hot refrigerant and an outlet  40  for 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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the present invention, reference should now be made to the following detailed description thereof taken in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a schematic diagram of a refrigerant system; 
       FIG. 2  is a simplified schematic view of a screw compressor showing the discharge end and connections to the discharge line; 
       FIG. 3  is a simplified schematic view of a prior art still; 
       FIG. 4  is a simplified schematic view of an embodiment of a lubricant still of the present invention; 
       FIG. 5  is a view of the preferred embodiment of a lubricant still of the present invention; 
       FIG. 6  is a perspective view of a component of the lubricant still shown in  FIG. 5 ; 
       FIG. 7  is a view of the an alternative embodiment of the oil still of the present invention; and 
       FIG. 8  is an alternative embodiment of the oil still shown in FIG.  7 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings in detail there is shown in  FIG. 1  a schematic view of a refrigerant system  1 , including a compressor  2 . 
   As is known, a flooded style evaporator  3  delivers primarily gaseous refrigerant to the compressor  2  through a line  4 . Gaseous refrigerant is compressed by compressor  2 , entraining lubricant during its passage through compressor  2  that is used to lubricate the bearings and rotors of compressor  2 . From the compressor  2 , refrigerant with entrained oil passes through a line  5  to a condenser  6 . Compressed gaseous refrigerant is cooled in the condenser, transferred into a liquid phase, with oil in mixture or solution, as it passes in line  11  through an expansion valve (not shown) to evaporator  3 . At the evaporator  3 , an environment to be cooled is cooled by the refrigerant in the evaporator. As is shown, it is typical that liquid refrigerant  7  settles from the refrigerant in the evaporator. This refrigerant  7  is 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 line  8  to lubricant still  128 , 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 compressor  2  through line  13  for lubrication of bearings and rotors. 
   Referring to  FIG. 2 , an example of how oil enters the refrigerant during the compression process will be further described. Shown in  FIG. 2  is a screw compressor  10 , that includes a rotor housing  12  containing intermeshing screw rotors  14  and  15  and bearings  17  on suction-side of screw rotors, refrigerant inlet  18  and discharge port  20 , including a discharge bearing housing  22  containing discharge-side bearings  23  and discharge housing  24  that is connected with a discharge line  26 . In operation, assuming rotor  14  to be the driving rotor, rotor  14  rotates engaging the other rotors  15 , causing their rotation. The co-action of rotating rotors  14  and  15  draws refrigerant gas via suction inlet  18  into the grooves of rotors  14  and  15  that engage to trap and compress volumes of gas and deliver hot compressed refrigerant gas to discharge port  20 . 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 bearings  17  and discharge bearings  23 . 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 system  1  described 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.  4  and also referring to  FIG. 1 , an embodiment of the lubricant still of the present invention, still  128 , comprises a pressure tight vessel  130 , which includes an inlet  132  for oil laden refrigerant  7 , drawn off the evaporator, below the liquid level line, an outlet  134  for gaseous refrigerant and a lubricant outlet  136  for out-flowing lubricant of high viscosity that has undergone separation. Still  128  includes a series of liquid reservoirs  138  created by partitions  140  acting in concert with portions of the inner wall of pressure tight vessel  130 . In this embodiment, heat for vaporizing some liquid refrigerant in oil laden refrigerant  7  is provided by electric heater  150 , which is in close proximity to the lower wall  151  of pressure tight vessel  130 . Other arrangements for electric heaters, including locating them within vessel  130 , 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 reservoirs  138  rises within vessel  130  and eventually exits through vent  134 , which is connected (not shown) to either evaporator  3  or passage  4 . Flow of liquid through still  128  is due to the effect of gravity G, wherein vessel  130  is tilted downwardly from inlet  132 , as shown. In  FIG. 4  flow occurs from right to left, proceeding over the top of each partition  140  and through each reservoir  138  in sequence, from  138   a  to  138   e . The most upstream reservoir  138   a  in the sequence is connected to inlet  132  and typically contains a high fraction of the oil laden refrigerant  7 . The most downstream reservoir in the sequence  138   e  is connected to lubricant outlet  136  and acts as a lubricant reservoir. The construction of partitions  140 , such that flow occurs over their tops T, is an aspect of the present invention. Oil rich liquid or foam, shown typically as  152  in  FIG. 4 , tends to rise to the top of reservoirs  138  due to buoyancy, because the density of the liquid/foam  152  is lower than the density of other liquid present in reservoirs  138 . Thus, oil rich liquid and foam flows in reservoirs  138  over the tops T of partitions  140 , over the other liquid in the reservoirs. By this means, the oil concentration of the liquid in reservoirs  138  increases as flow progresses downstream in the sequence of reservoirs  138 , from  138   a  to  138   e . Through this means, a lubricant of high viscosity is developed in the most downstream reservoir  138   e , which acts as a lubricant reservoir. During operating transients when the influx rate of oil laden refrigerant entering the most upstream reservoir  138   a  through inlet  132  increases, the liquid flow rate through still  128  also increases. However, because the liquid is refrigerant rich, its density is higher than oil rich liquids or oil rich foams  152 , leading to downstream flow over the tops T of partitions by the more oil rich liquids and foams  152 , 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 reservoirs  138  such 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 reservoirs  138  into the page) and length of reservoirs  138  should be maximized relative to their height. 
   Another preferred embodiment is shown in  FIGS. 5 and 6 . Referring to  FIG. 5 , and also referring to  FIG. 1 , similar to as described above, a still  228  comprises a pressure tight vessel  230 , which includes an inlet  232  for oil laden refrigerant  7 , drawn off the evaporator, below the liquid level line, an outlet  234  for gaseous refrigerant and a lubricant outlet  236  for out flowing lubricant of high viscosity that has undergone separation. Still  228  further includes a series of liquid reservoirs  238   a  to  238   g  created by partitions  240 . Reservoirs  238   a  to  238   g  and partitions  240  are 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 reservoirs  238  and entire series of partitions  240  in a single pan-shaped piece  242  of 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 refrigerant  7  is preferably provided by flow of hot refrigerant gas drawn off the condenser or, as shown in  FIG. 1 , from a tap  39  off the discharge line  5  of compressor  2 , entering through an inlet  260  in FIG.  5  and exiting through outlet  262  as cooled refrigerant having gone through a heat transfer process. The refrigerant flows through an internal passage defined by the single piece  242  and a matching bottom piece  244 , described further below with reference to FIG.  6 . Pan  242  is fastened within vessel  230  on 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 reservoirs  238  rises within vessel  230  and eventually exits through vent  234 , which is connected (not shown) to either evaporator  3  or passage  4 . Flow of liquid through still  228  is due to the effect of gravity G and the orientation of pan  242 . Referring still to  FIG. 5 , flow occurs from right to left beginning at inlet  232  through the series of reservoirs  238   a  to  238   g  and over the series of partitions  240 , ending in the most downstream reservoir  238   h . The most upstream reservoir in the sequence  238   a , connected to inlet  232 , typically contains a high fraction of the oil laden refrigerant  7 . The most downstream reservoir  238   h  in the sequence is connected to lubricant outlet  236  and acts as a lubricant reservoir. The construction of partitions  240  such that flow occurs over their tops T. In other respects, aspects of the embodiment shown in  FIG. 5  pertinent to creating and maintaining a lubricant of high viscosity are the same as those of the embodiment shown in  FIG. 4 , and previously described. 
   With reference to  FIG. 6 , the pressure-tight passage for flow of the hot refrigerant gas is made, using the single piece of high conductivity material  242  (described above) as an upper boundary and part of side boundaries for hot refrigerant gas flow and a single lower piece  244 , preferably stamped from a single sheet of the same high conductivity material as  242  is formed from, as a lower boundary and forming part of the side boundaries.  242  and  244  are suitably joined in a pressure-tight manner, preferably also by brazing. Inlet  260  and outlet  262  may suitably be joined in a pressure tight manner to the assembly of  242  and  244 , preferably by brazing or could be formed as an integral part of pieces  242  and  244 . 
   In accordance with another embodiment of the present invention, and referring to  FIG. 7 , a still  328  includes at least one flat separating pan  344  positioned in coil  342 , dividing the cavity  346  of vessel  330  into two Zones A and B, and which is angled downwardly such that liquid will flow over its surface. This division by pan  344  effectively 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. Pan  344  is preferably in intimate contact with coil  342  to 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 cavity  346  functions 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 outlet  334 . 
   In Zone B, lubricant, consisting of oil with about 10-40% refrigerant collects, having moved down pan  344  into the bottom of vessel  330 . 
   As an alternative to coil  342 , an electric heater  348  shown 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 vessel  330  from the evaporator through inlet  332  onto pan  344 , and hot refrigerant enters inlet  338  drawn off the condenser or the compressor discharge line, and circulates through coil  342 . 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 outlet  334  to the slanted orientation of the pan, liquid flows down the pan through Zone A and drips into Zone B at the bottom of vessel  330 . 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 outlet  334  while oil flows out of vessel  330  through outlet  336 . Cooled refrigerant in the coil exits the vessel through outlet  340 . 
   Referring now to  FIG. 8 , an alternative of the embodiment of  FIG. 7  is shown. In  FIG. 8 , two pans  444   a  and  444   b  are used, along with the same coil  142  arrangement and outlets and inlets described above. With the embodiment shown in  FIG. 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 vessel  430  through outlet  434 . 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. Pans  444   a  and  444   b  are each slanted downward, wherein pan  444   a  has less of an incline than pan  344  and leads the liquid to pan  444   b . Pan  444   b  is slanted in the opposite direction of pan  444   a , such that the lower point  450  of pan  444   a  is almost vertically coincident with the higher point  452  of pan  444   b , 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 the  FIG. 7  embodiment. 
   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.