Flooded evaporator with enhanced oil return means

A lubricant composition with decreased solubility in a liquid refrigerant and increased foaming characteristics useful in refrigeration systems to promote oil return to a compressor. A lubricant composition comprises a base lubricant, synthetic paraffinic oil, and a foaming agent. These ingredients are blended in predetermined amounts, so that when the lubricant is used in the refrigeration system, an oil rich layer will form atop the liquid refrigerant in the system's flooded evaporator, and a thick, stable foam will form when vaporized refrigerant boils up through the oil rich layer. The oil foam is then drawn through a connection at the top of the flooded evaporator and returned to the compressor.

BACKGROUND OF THE INVENTION 
This invention pertains to a refrigeration apparatus having a flooded 
evaporator, and more particularly to lubricants with decreased solubility 
and fluorocarbon refrigerants and increased foaming characteristics. 
In refrigeration systems with an oil lubricated compressor and a flooded 
evaporator, oil from the sump of a compressor can migrate to a flooded 
evaporator under certain low load operating conditions. Lubricant return 
to the compressor can be inadequate if the lubricant does not entrain with 
the refrigerant gas passing through the evaporator. Oil return problems 
occur when oil remains dissolved in the refrigerant. 
Lubricant return problems have been experienced particularly in water 
chillers having a screw compressor with a flooded evaporator. Oil injected 
into the working chamber of a screw compressor, is atomized and becomes 
entrained in the refrigerant gas undergoing compression. Oil separators 
can be used in such systems to remove most of the oil that is entrained in 
the refrigerant gas as it leaves the compressor. However, as much as 10% 
of the oil continues through the refrigeration circuit until it becomes 
dissolved in the liquid refrigerant in the flooded evaporator. The 
dissolved oil accumulates in the evaporator which not only starves the 
compressor of needed oil, but also affects the heat exchange function of 
the evaporator. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a lubricant composition with 
decreased solubility in a fluorocarbon refrigerant and increased foaming 
characteristics. 
Another object of the invention is to provide a method for decreasing the 
solubility of a lubricant in a fluorocarbon refrigerant by selecting a 
base lubricant, adding paraffinic oil stocks and promoting the foaming of 
the lubricant by adding a foaming agent. 
It is a further object of the invention to provide a method of using the 
above lubricant in a fluorocarbon refrigeration system having a flooded 
evaporator to facilitate oil return to the compressor. 
Another object of the invention is to provide a refrigeration apparatus 
that offers adequate oil return from a flooded evaporator to a screw 
compressor. 
These and other objects of the invention are provided by a novel 
refrigeration apparatus that includes a lubricant composition for use in a 
refrigeration system to foster oil return from the system's flooded 
evaporator to its compressor. The solubility of the lubricant in 
refrigerant is decreased so that an oil rich layer is formed on top of the 
liquid refrigerant in the evaporator. A foaming addititive in the 
lubricant allows a thick, stable oil foam to form when vaporizing 
refrigerant boils up through the oil rich layer. Carry-over of the foam 
through an outlet port at the top of the evaporator results in lubricant 
return to the compressor. This invention solves the problem of the 
lubricant remaining dissolved in the liquid refrigerant held in flooded 
evaporators by facilitating return of the lubricant to the compressor 
sump, thereby maintaining the proper amount of lubricant in the 
compressor.

DETAILED DESCRIPTION OF THE INVENTION 
A refrigeration system 10 embodying the invention is represented 
schematically in FIG. 1. A refrigeration compressor 12 discharges 
compressed refrigerant 14 through an oil separator 16 which attempts to 
remove as much lubricating oil 18 as possible from the discharged 
refrigerant 14. The separated oil 18 is returned to compressor 12 where 
it's needed, while refrigerant 14 is conveyed to a condenser 20. In 
condenser 20, refrigerant 14 is condensed by heat exchanger tubes 22 
conveying a coolant 24. The condensed liquid refrigerant 26 leaves 
condenser 20 and passes through an expansion device 28 before entering a 
flooded evaporator 30. Expansion device 28 represents any device for 
reducing the refrigerant's pressure. Examples of expansion device 28 
include orifice plates, capillary tubes, and expansion valves. Evaporator 
30 includes heat exchanger tubes 32 conveying a working fluid 34 that is 
cooled by the liquid refrigerant 36. In the process of absorbing heat from 
the working fluid 34, the liquid refrigerant 36 vaporizes. The vaporized 
refrigerant 38 exits evaporator 30 through an outlet port 40 near the top 
of evaporator 30 and returns to a suction port 42 of compressor 12 to 
complete a refrigeration cycle. 
Although compressor 12 is a screw compressor, compressor 12 represents 
other types of compressors as well, such as centrifugal compressors, 
scroll compressors and reciprocating compressors. However, the invention 
is especially useful in a refrigeration system having a screw compressor, 
because the relatively high need for lubrication of a screw compressor's 
intermeshed rotors 44 and its capacity varying slide valve 46. 
Evaporator 30 is referred to as a flooded evaporator because heat exchanger 
tubes 32 are substantially flooded with liquid refrigerant 36. Refrigerant 
36 is a fluorocarbon refrigerant such as "FREON, R-22" which is a 
trademark for chlorodifluoromethane. Refrigerant 36 in evaporator 30 is 
mixed with some oil since separator 16 is unable to completely separate 
all the entrained oil from refrigerant 14. To transfer the oil from 
evaporator 30 back to compressor 12, an oil rich foam 48 is generated 
which floats on top of refrigerant 36 and rises up to the outlet port 40 
which is fluid communication with suction port 42 of compressor 12 by way 
of a suction line 50. 
This oil rich foam 48 is generated by blending a base lubricant with a 
paraffinic oil stocks. The base lubricant provides the necessary 
lubricating properties for compressor 12, while the paraffinic oil stocks 
reduce the base lubricant's solubility in liquid refrigerant. Thus the oil 
composition has a tendency to separate from the liquid refrigerant 36 and 
produce an oil film 52 that floats on top of refrigerant 36 due to the 
oil's lower specific gravity. A foaming agent is added to increase the 
oil's tendency to foam. As the liquid refrigerant 36 is vaporized by heat 
exchanger tubes 32, a boiling action generates vaporous refrigerant 
bubbles 54 that rise upwardly through oil film 52 to produce the oil rich 
foam 48. The boiling action is enhanced by providing heat exchanger tubes 
32 with a nucleate boiling surface 56 which in turn further promotes foam 
generation. The definition of nucleate boiling surface 56 is disclosed in 
U.S. Pat. No. 3,696,861 which is specifically incorporated by reference 
herein. 
A suitable base lubricant is preferably a refined mineral oil, such as 
those sold under the trademark "CALUMET R030". The mineral oil's 
solubility in R-22 is represented by saturation curve 58 of FIG. 2. Below 
curve 58, the refrigerant is saturated, causing some mineral oil to 
separate from the refrigerant. Above curve 58, the mineral oil and 
refrigerant produce a homogeneous mixture. 
Adding paraffinic oil stocks reduces the mineral oil's solubility in the 
refrigerant as indicated by saturation curve 60. Likewise, above curve 60 
the refrigerant mixture is homogeneous and below curve 60 the mineral oil 
and paraffinic oil stocks tend to separate from the refrigerant. The 
paraffinic oil stocks, which can be natural or synthetic, are preferably 
comprised of synthetic polyalphaolefin, but other paraffinic oil species 
may also be used, such as severely or mildly hydro-treated oils. 
The combined percentage of paraffinic oil stocks and base lubricant to the 
refrigerant should be 4% to 8% by weight, and preferably 5% to 6%. And the 
proportions of paraffinic oil stocks to base lubricant should be such that 
the refrigerant is saturated at a temperature of 40.degree. F. to 
45.degree. F. The percentage of paraffinic oil stocks to base lubricant 
should be 5% to 50% by weight. 
Suitable foaming agents are organosiloxanes, such as those disclosed in 
U.S. Pat. No 3,792,755, which is specifically incorporated by reference 
herein. Dimethylpolysiloxane is one preferred organosiloxane due to its 
relatively low viscosity of 20 centistokes which enhances its solubility 
in oil. Other examples of preferred organosiloxanes include 
methylethylpolysiloxane, diethylpolysiloxane, and 
trifluoroproplemethylpolysiloxane. 
The desired quantity of foaming agent is that which produces a thick, 
stable foam 48 as the refrigerant bubbles 54 pass through the oil rich 
layer 52 in evaporator 30. Foam 48 should be thick enough to reach outlet 
port 40 of the evaporator 30, so the oil is readily returned to compressor 
12. A suitable weight concentration of the organosiloxane foaming agent, 
dimethylpolysiloxane, is 10 to 200 parts per million of the combined base 
lubricant and paraffinic oil stocks. 
Although the invention is described with respect to a preferred embodiment, 
modifications thereto will be apparent to those skilled in the art. 
Therefore, the scope of the invention is to be determined by reference to 
the claims which follow.