Oil recovery system for low capacity operation of refrigeration systems

An oil recovery system for refrigeration apparatus of the type in which an oil lubricated screw compressor is oriented with a suction inlet in communication with an evaporated chamber positioned under the compressor so that under conditions of low capacity compressor operation, lubricating oil may fall from the compressor through the suction inlet thereof and into the evaporator. The recovery system takes advantage of an existing suction distribution tray in the upper region of the evaporator chamber of such systems, normally used to assure distribution of gaseous refrigerant to the suction inlet of the compressor, by removing oil dropping into the tray and returning it back to the compressor. An existing by-pass eductor loop for returning liquid refrigerant from the evaporator to the compressor is extended by a valve controlled branch to a second eductor for withdrawing the oil from the tray and returning it to the compressor. The valve is controlled so that the oil recovery system is disabled at normal compressor capacity levels.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an oil recovery method and system for 
refrigeration apparatus using a screw compressor and, more particularly, 
to such an oil recovery method and system for low capacity operation of 
the compressor. 
2. Discussion of the Related Art 
Oil lubricated screw compressors are commonly used in refrigeration 
apparatus provided with an oil/refrigerant separator from which oil is fed 
back to the compressor whereas compressed refrigerant is passed from the 
separator, through the condenser, through the evaporator units of the 
system, and back to the suction inlet of the compressor. In certain 
applications, such as in refrigeration apparatus used for chilling water 
and other liquids, for example, efficient and compact packaging of the 
compressor, condenser, evaporator and separator components results in the 
suction inlet of the compressor opening downwardly to the top of the 
evaporator chamber. Because the working screws of the compressor are 
lubricated and in some measure sealed by oil, this geometry of 
refrigeration components presents a potential for oil dropping from the 
compressor through the suction opening thereof to the evaporator chamber. 
During normal operation of refrigeration apparatus of the type mentioned, 
the compressor is operated at adequate gas flow through the compressor 
suction chamber to retain droplets of oil which are present. Under such 
conditions, the oil separator and recovery system provides adequate 
management of the oil in the apparatus. At lower compressor capacities, 
however, the velocity of gases entering the suction chamber of the 
compressor is reduced to a point where oil from the compressor may drop 
into the evaporator chamber. If such low capacity operation occurs for any 
substantial period of time, the oil accumulates in the evaporator and 
results in reduced efficiency of the refrigeration cycle performed by the 
apparatus. Also, the supply of oil needed for compressor lubrication may 
become inadequate. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an oil recovery method and 
system for screw compressor refrigeration apparatus in which lubricating 
oil passing from the compressor through the suction inlet thereof to an 
evaporator chamber is collected and returned directly to the compressor 
without mixing with refrigerant liquid in the evaporator chamber. 
Another object of the invention is to provide such an oil recovery system 
which involves a minimum of structural revision to existing refrigeration 
system components. 
Still another object of the invention is to provide such an oil recovery 
method and system which enables a highly efficient refrigeration cycle 
during high and low capacity operation of the refrigeration compressor and 
maintains adequate lubrication of the compressor. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows, and in part will be obvious from 
the description, or may be learned by the practice of the invention. The 
objects and advantages of the invention will be realized and attained by 
means of the elements and combinations particularly pointed out in the 
appended claims. 
To achieve the objects and in accordance with the purpose of the invention, 
as embodied and broadly described herein, the present invention is 
especially applicable to refrigeration apparatus having an oil lubricated 
compressor with a suction inlet opening to the top of an evaporator 
chamber including a suction trough to control distribution of refrigerant 
gas passing from the evaporator to the suction inlet of the compressor and 
comprises a provision for removing oil dropping into the trough during low 
capacity operation of the compressor and for returning the removed oil 
directly to the compressor. 
In a preferred embodiment of the invention, oil collected in the suction 
trough located near the top of the evaporator chamber is drained from the 
trough by a conduit communicating with an eductor through which compressed 
refrigerant is circulated to draw the oil from the trough. The eductor and 
associated piping is in addition to an existing eductor used for removing 
a small flow of liquid refrigerant and oil from the evaporator chamber and 
returning it to the suction inlet of the compressor for oil return 
purposes. During low capacity operation of the compressor, the oil 
recovery system of the invention is enabled so that oil from the trough 
passes back to the compressor through a port located in the lowest 
pressure region of the compressor intake. During normal operation of the 
compressor at higher capacities, the recovery system of the present 
invention is disabled to ensure efficient operation of the overall 
refrigeration apparatus. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory only and are 
not restrictive of the invention, as claimed. 
The accompanying drawings, which are incorporated in and constitute a part 
of this specification, illustrate an embodiment of the invention and 
together with the description serve to explain the principles of the 
invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference will now be made in detail to the present preferred embodiment of 
the invention, an example of which is illustrated in the accompanying 
drawings. Wherever possible, the same reference numbers will be used 
throughout the drawings to refer to the same or like parts. 
In the illustrated embodiment, the invention is incorporated in a 
refrigeration apparatus intended for liquid chilling applications and 
which is designated generally by the reference numeral 10. The major 
components of the apparatus 10, as well as the relative orientation of 
those components, are shown most clearly in FIG. 1 and include a 
compressor 12, an oil separator 14, a condenser 16 and an evaporator 18. 
The condenser 16 and evaporator 18 are similar in exterior configuration 
in that both are defined respectively by elongated cylindrical bodies 20 
and 22 closed at opposite ends by end plates 24 and 26. The evaporator 22 
is further equipped with a manifold 28 on one end plate 26 thereof by 
which water to be chilled in accordance with the illustrated embodiment is 
circulated through inlet and outlet conduits 30 and 32, respectively. 
As shown generally in FIG. 1, the compressor 12 includes a multi-part 
exterior casing 34 to which an electric motor 36 is connected at one end 
for driving the compressor at varying capacities in a manner to be 
described in more detail below. As shown in FIG. 1, the compressor is 
located on top of the cylindrical body 22 of the evaporator 18 and 
includes a suction inlet 38 in communication with a pipe 40 opening 
through the top of the evaporator 18. A compressor outlet or discharge 
opening 42 is in direct communication with the separator 14. The separator 
14 in the illustrated embodiment is conventional and as such includes a 
downwardly directed refrigerant conduit 44 in communication with the 
interior of the condenser body 20 through a conduit 46 opening through the 
top of the cylindrical body 20 of the condenser 16. The condenser 16, in 
turn, is in communication with the evaporator 18 by a conduit 48 which 
opens through and extends from the bottom of both the condenser body 20 
and the evaporator body 22. 
As shown in FIGS. 3 and 4, the interior of the evaporator body 22 is 
provided with longitudinal heat exchange tubes 50 for bringing water to be 
chilled into heat exchange relationship with refrigerant contained the 
body 22. At the top of the evaporator body interior, an elongated trough 
52 is positioned under the conduit 40 in communication with the suction 
intake 38 of the compressor 12. This trough extends for substantially the 
length of the evaporator body 20 as shown in FIG. 4 and is provided with 
spaced window-like openings 54 about the upper marginal edges thereof. The 
suction trough 52 is conventionally provided in refrigerating apparatus of 
the type illustrated to control distribution of gas from within the body 
22 of the evaporator in passing through the pipe 40 to the suction inlet 
38 of the compressor 12. The conventional trough is typically provided 
with an opening through which any liquid refrigerant collecting in the 
trough passes to the bottom of the evaporator 18. 
The conventional water chilling apparatus further includes a by-pass 
eductor loop by which liquid refrigerant and oil at the bottom of the 
evaporator is withdrawn to the suction inlet of the compressor for oil 
return purposes. In FIG. 1, this eductor loop is shown schematically to 
include a conduit for high pressure refrigerant extending from the inlet 
46 of the condenser 16 to an eductor by which the liquid refrigerant is 
withdrawn from the evaporator and fed back to the suction inlet of the 
compressor. Such eductors are well known and operate to aspirate or 
otherwise draw an educted fluid, the liquid refrigerant and oil in this 
instance, into a high velocity stream of a driving fluid, i.e., the 
compressed refrigerant. In FIG. 1, these conduit and eductor components 
are represented schematically. Specifically, a conduit represented by a 
dotted line 56 extends from the condenser inlet pipe 46 to an eductor 
represented by a cylinder 58 and then to the evaporator outlet pipe 40 in 
communication with the suction inlet of the compressor 12. Liquid 
refrigerant and oil, represented by a dashed line 60 in FIG. 1, is 
withdrawn from the evaporator 18 and passed with the high pressure 
refrigerant back to suction inlet of the compressor 12. 
In accordance with the present invention as it is embodied in the 
illustrated apparatus 10, a provision is made for removing oil which may 
drop from the compressor 12 under conditions during which the compressor 
is operated at low capacity. To this end, a drain pipe 64 is fitted to the 
lower end of the trough 52 in the illustrated embodiment and extends 
through the body 22 of the evaporator as shown in FIG. 3 of the drawings. 
In FIG. 1, the drain pipe 64 is represented by a dashed line 64 to 
represent the passage of oil through the pipe shown in FIG. 3. As shown 
further in FIG. 1, the oil passageway extends to a second eductor 66 to 
which compressed refrigerant is fed through a valve 68. The valve 68 is 
preferably an electrically controlled valve, such as a solenoid valve, 
which may be opened or closed by any appropriate control indicated by the 
legend 70 in FIG. 1. 
From the illustration in FIG. 1, it will be appreciated that the 
refrigerant under pressure supplied to the valve 68 has its origin in the 
refrigerant line 56 described above with respect to the first eductor 58 
for withdrawing liquid from the evaporator 18. In this respect, the 
compressed refrigerant passing to the second eductor 66 passes through a 
flow line which may be characterized as a branch or an extension of the 
eductor by-pass loop including the first eductor 58 and is either 
operative or inoperative depending on whether the valve 68 is opened or 
closed. 
The mixture of oil and compressed refrigerant passing through a conduit 
extending from the second eductor 66, represented by dotted and dashed 
lines 72 and 74 respectively in FIG. 1, is returned to the compressor 12 
for recirculation through the apparatus 10. Unlike the return of 
compressed refrigerant and liquid refrigerant from the evaporator to the 
suction inlet 38, however, and with reference to FIG. 2 of the drawings, 
the mixture of compressed refrigerant and oil 72 and 74 is fed directly 
through a port 76 to the intake end 78 of the working screws 80 of the 
compressor 12. In this respect, the suction inlet 38 of the compressor 12 
opens to a chamber 82 which decreases from a relatively large cross 
sectional flow area at the mouth of the suction inlet 38 to a passageway 
of relatively small cross sectional area at the intake end 78 of the 
screws 80. The pressure decreases from the suction inlet 38 to the inlet 
end of the screws 80 and reaches a minimum level in the region of the port 
76. As a result, the refrigerant flow from the eductor 66 to the 
compressor 12 is maximized, ensuring efficient operation of the second 
eductor 66 even under conditions of relatively low capacity operation of 
the compressor. Also, entry through the port at the intake end avoids 
direct encounter with the dropping oil in the suction inlet 38. 
In the practice of the method of the present invention during operation of 
the refrigeration apparatus 10, under normal conditions of operation, the 
compressor 12 is operated above capacities incurring oil dropout. During 
such normal operation, the velocity of refrigerant gas at the suction 
inlet 38 of the compressor is adequate to prevent any oil from dropping 
into the evaporator 18. Also non-working refrigerant bypass for oil return 
is restricted to that needed for withdrawal of liquid refrigerant from the 
evaporator 18 by closing the valve 68. 
When the capacity of the compressor 12 is reduced to a predetermined level, 
the valve 68 is opened to remove oil from the trough 52 and return it to 
the compressor with compressed refrigerant in the manner mentioned above. 
The control 70 for the valve 68 is, in practice, incorporated as part of 
an electronic control system (not shown) for monitoring and controlling 
operation of the refrigeration apparatus 10. Accordingly the valve 68 will 
be opened only at low capacity conditions and closed under all other 
conditions of operation. In this way parasitic power loss caused by 
unneeded high pressure refrigerant by-pass through the second eductor 66 
will be minimized. Closure of the valve 68 at greater capacities is 
important to efficient normal operation of the apparatus 10 where the flow 
of gaseous refrigerant through the suction inlet 38 prevents oil from 
passing back to the evaporator 18. 
It will be apparent to those skilled in the art that the present invention 
and in construction of the apparatus hereof without departing from the 
scope of spirit of the invention. 
Other embodiments of the invention will be apparent to those skilled in the 
art from consideration of the specification and practice of the invention 
disclosed herein. It is intended that the specification and examples be 
considered as exemplary only, with a true scope and spirit of the 
invention being indicated by the following claims.