Note: Descriptions are shown in the official language in which they were submitted.
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D E S C R I P T I O N
Title
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FAIL SAFE MECHANICAL OIL SHUTOFF ARRANGEMENT
FOR SCREW COMPRESSOR
Back round of the Invention
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The present invention relates generally to the art
of compressing a gas in an oil-injected rotary screw
compressor. More specifically, the present invention relates
to apparatus for isolating rotor bearing lubricant passages and
the oil in~ection port, which opens into the working chamber of
an oil inJected screw compressor, from their oil supply upon
compressor shut down,
Screw compressors employed in refrigeration systems
, are comprised of complementary male and female sc~ew rotors
i' disposed within a working chamber defined by a rotor housing.
The working chamber can be characterized as a volume generally
shaped as a pair of parallel intersecting cylindrical bores and
is closely toleranced to the outside length and diameter
dimensions of the intermeshed screw rotor set~ The rotor
housing has low and high pressure ends which define unvalved
suction and discharge ports in open-Elow communication with the
working chamber.
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In operatlon, refrigerant gas at suction pressure
enters the working chamber via the suction port and is
enveloped in a chevron shaped pocket i`ormed between che
counter-rotating screw rotors. The pocket closes, its volume :
decreases and it is displaced toward the high pressure end of
the compressor as the rotors meshingly rotate within the
working chamber. The gas within such a pocket is compressed by
virtue of the decreasing volume in which it is contained until
the pocket opens to the discharge port at the high pressure end
of the working chamber where it is expelled through the
discharge port.
Due to the extremely close tolerances between the
rotor set and the walls of the working chamber, the bearing
arrangement in which the rotor set is mounted is critical to
compressor operation and }ife. This is particularly true
because the bearings and rotors in a screw compressor are
~ub;ect to high and variable axial and radial loads.
Protection and lubrication of rotor bearings is therefore o
paramount concern in the design and operation of rotary screw
compressors.
In addition to being delivered to the rotor
bearings, oil is in many instances injected into the working
chamber of a screw compressor through an injection port to
perform several functions. First, the oil in~ec~ed into the
working chamber acts as a sealant between the rotors and the
surfaces of the working chamber in which the rotors are
dlspo~ed,
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The oil also acts as a lubricant becween the
driving and driven screw rotor. In that regard, one of the two
screw rotors is driven by an external source, such as an
electric ~otor, while the other rotor is driven by virtue of
its meshing relationship with the motor-driv~n rotor. Oil -
injected into the working chamber of the compressor therefore
acts to prevent excessive wear between the driving and driven
rotors.
Finally, injected oil is used to cool the
refrigerant undergoing compression within the working chamber
which in turn reduces the thermal expansion of the rotors that :~
would otherwise occur as a result of the heat generated by the
compression process. Such injection cooling therefor permits
tighter rotor to housing clearances from the outset.
At compressor shut down, when the drive motor is
de-energized, the backflow of discharge pressure gas from the
high (downstream) side of the refrigeration system in which a
screw compressor is employed back through the compressor
discharge port, if allowed to occur, causes the high speed
reverse direction rotation of the no longer driven screw rotors
within the working chamber and causes the compressor to act as
an expander with respect to gas downstream of the discharge
port. Such reverse direction freewheeling of che rotors can
occur at speeds greater than the maximum desLgn RPM of the ;`
rotor set for normal operation. `
Additionally, to the extent gas bAckPlow is cutoPf
at shutdown, such as by a chock valve arrangcment, the initial
rush of downstream discharge pressure gas back through the
compressor toward the low pressure side of the refrigeration
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system may still be sufficient to cause the pressure at the
suction end of the compressor to exceed that which exists
immediately downstream of the discharge port. This situation
can occur when the compressor, acting as an expander in its
reverse direction rotation, pumps against the closed discharge
: check valve, and can result in the developmerlt of large axial
forces on the screw rotor set and rotor bearings in a direction
opposite that which is normally enc~untered and compensated for :~
during compressor operation.
; 10 Also, many screw compressor bearing lubrication
i: schemes are predicated on the development and maintenance of
relatively high pressure downstream of the compressor which is
used to drive lubricating oil from a sump or reservoir to the
rotor bearings and/or in;ection port. The high speed reverse
rotation of the rotor set at compressor shutdown and momentary
; development of relatively higher pressure at the upstream or
low side end of the working chamber, i allowed to occur,
~ could, under some circumstances, cause oil to be sucked from
:~ the bearings or not to be delivered to tha be~rings Ln
:~ 20 sufficient quantity with potentially catascrophic results.
Finally, unless the oil injection port opening into
the working chamber of a screw compressor is isolated from its
~ typically pressurized oil supply upon compressor shutdown, oil
will continue to flow through the injection port into the
working chamber after shutdown, until the system pressures
equalize, by virtue of the pressure differential which exists
between the oil supply and the working chamber at compressor
shutdown. Absent means for reliably isolating the oil
in~ection port ~rom Its oil supply under such circumstances,
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the worklng chamber can become flooded with oil. As a result,
the compressor lubrication system can become starved for oil
due to the dislocation of the oil supply from the oil sump to
the working chamber and insufficient oil may be av~ilable for
delivery to the necessary locations within the compressor when
the compressor next starts with potentially catastrophic
results.
; The need, therefore, continues ~o e~ist for a fail
safe arrangement i'or preventing the continued flow of oil to
the bearings and through the injection port into the working `
chamber of a refrigeration screw compressor upon compressor
shut down and for permitting such oil flow at compressor
startup.
Summar~ of the Invention
It is an object of the present invention to isolate
the bearing lubrication passages and the oil injection port
which opens into the working chamber of a screw compressor from
their oil supply upon compressor shutdown in a manner which is : ~
actuated by the existence of discharge pressure gas immedlately :.
~, ~ downstream of the compressor's working chamber when the `
I compressor is in operation.
: A urther ob~ect of the present invention is to
provide an arrangement which, by the act of compressing gas and ;,
discharging it i'rom the compressor's working chamber upon "~'
compressor s~art up, immcdiately and mechanically places ~he 1. `
,I bearing lubricati.on passAges and oil in~ection port into 10w'I communiFation wlth their oil supply. ~
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It i5 also an object of the present invention to
provide mechanical apparatus for closing the bearing
lubrication passages and oil injection port of a screw
compressor immediately upon compressor shutdown and for opening
them i~mediately upon startup in a manner which, by its use of
, ambient conditions which are inherent within the compressor at
; those respective times, is "fail safe" and eliminates the need
for external check valves, solenoid valves or sensors to
: "prove" oil flow within the compressor.
These and other objects of the present invention,
which will become apparent when the Drawing Figures and the
Description of the Preferred Embodiment hereof are considered,
are accomplished by apparatus disposed within a screw
compressor which shuts off the flow of injection and beflring
` 15 lubrication oil in the compressor ac compressor shutdown and
which permits flow to occur at compressor startup by the use of
the internal pressure differentials and gas flow which are
inherent in the compressor and its operation at those
respective times.
Discharge pressure, which exists immediacely
,~ downstream of the compressor's discharge port when the
compressor is in operation, is used to position a spool valve
against internal compressor suction pressure to a position
' which permits the flow of lubricating oil from an oil supply to ~:
bearing locations and to the oil injection port opening into
the compressor's working chamber. At compressor shutdown the
backflow of discharge preasure gas to the compressor's working
chamber closes an internal discharge check valve causing an
immediate pressure differential to develop across the spool .
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valve. The pressure dLfferential operates to posi~ion the
spool valve to isolate the oil supply from the bearings and
in;ection port. Upon compressor startup discharge pressure
develops downstream of the compressor's working chamber and :-
acts on the spool valve causing it to be positioned to permit
oil flow within the compressor so that oil is immediately
directed to the bearings and oil injection port.
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Brief Description of the Drawin~ FiQures ~
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- Figure 1 is a cross sectional view of the
compressor of the present invention and its schematic
disposition in a refrigeration system.
Figure 2 is an enlarged partial view of the oil ~;
; 15 shutoff valve installation in the co~pressor of Flgure 1.
DescriPtion of the Preferred Embodiment
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Referring concurrently to Drawing Figures 1 and 2, : .
refrigeration system 10 is comprised of a compressor housing
assembly 12, condenser 14, expansion valve 16 and evaporator 18
all of which are serially connected to form a hermetic closed
loop refrigeration system. Rotor housing 20 of compressor ;~
assembly 12 houses a pair of screw rotors one of which, rotor
22, is illustrated. The rotor set is disposed ln working
chamber 24 of the rotor housing which further defines a suction
port 26 and discharge port 28 which are, respectively, the
cntry and exit lo~atlons for refrigerant gas passing through
the working chamber during compressor operation.
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Rotor 22, in the embodiment of Figure 1, is the
driven one of ~he pair of screw rotors and is mounted for
rotation within the rotor housing in bearings 30 and 32. Rotor
22 has a shaft 34 extending from one of its ends which is
driven by motor 36. Bearing housing 38 of the compressor
assembly i5 attached to the discharge end of ro~or housing 20
and serves to house bearing 32 and to close the discharge end
of the working chamber.
Bearing housing 38 defines a discharge passage 40
in flow communication with discharge port 28 which channPls
~ dischar~e gas out of the compressor assembly. Discharge
; passage 40 is also in flow communication with oil separator 42
in which lubricant, which has been carried out of compressor
housing assembly 12 in the discharge gas stream, is separated
from the discharge gas prior to the use of that gas in the
refrigeration system.
It is to be noted that a relatively large amount of
oil is typically carried out of the compressor~s working
chamber in the discharge gas stream in an oil-injected screw
compressor and that as much of that entrained oil as possible
must be removed from the refrigerant gas ~o as not to degrade
downstream refrigeration system performance and to ensure that
sufficient lubricant continues to be available ~o the
compressor.
Disposed in discharge passage 40 is a discharge
; check valve member 41. While check valve member 41 in Figure l
is illustrated as being a spherical member trapped in volume 43
against open spider 45, it will be appreciated chat a very
large number and variety of discharge check valve arrangements
are coneemplated within the scope of the present invent_on.
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The discharge check valve assembly may be disposed in the
bearLng housing or in the discharge piping which connects che
compressor assembly to the oil separator. It must, however,
serve to isolate the compressor's working chamber from the oil ~1
sump 44 upon compresscr shutdown. ~ .:
;~ Compressor assembly 10 defines a plurality of oil ;~
passages including lubrication passages 46 and 48 which
communicate with the bearings that support the screw ~otors -;
: within the compressor assembly and with an oil injection
passage 50 which opens into the compressor's working chamber.
In the smbodiment illustrated in Figures 1 and 2, all three
passages flow into common oil supply passage 52.
Oil supply passage 52 is in flow communication with
sump 44 oi oil separator ~2. It is to be noted that oil
separator 42 and swnp 44 may be integral to the campressor :`~
` assembly and that sump 44 might communicate with supply passage
50 via passages which are entirely internal of the compressor
assembly in such instances. Also, oil sump 44 may be
physically removed and in a vessel separate from oil separator
' 20 42. Once again, however, some means for preventing ~as
; backflow to the working chamber at compressor shutdown must bedisposed between the working chamber and oil separator/sump
wherever the separatorjsump may be located.
Interposed in oil supply passa~e 52 in rotor
: 25 housing 20 is a volume 54 in which a valve member 56 is
disposed. Volume 54, ln addition to being in flow
eommunication with oil supply passage 52 and ther~Eore,
internal eompressor oil passages 46, 48 and 50, i9 in tlow
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communlcation with an area in ~he compressor assembly which is
at a pressure less than discharge pressure and an area within
the compressor assembly which, when the compressor is in
operaeion, is at high side or discharge pressure.
S In that regard, volume 54 communicates through a
passage 58 to area 60 which is a volume within rotor housing 20
that is at suction pressure during compressor operation. Area
60 is in flow communication with suction port 26 within the
compressor assembly and is, in effect, upstream thereof within
the refrigeration system.
As was indicated above, area 60, rather than being
an area of the compressor which is at suction pressure, can be
an area within the compressor which is at an intermediate
pressure. Area 60 will, however, always be an area which is at
less than discharge pressure when the compressor is in
operation. Volume 54 also opens into area 62 within which ls
an area immediately downstream of discharge port 28 that is at
dlscharge pressure. Area 62 is therefore on the high side of
the refrigeration system when the compressor is in operation.
It will be appreciated that valve 56 is slideably .
disposed for axial movement within volume 54 between a first
position, illustrated in Figure 1, in which oil is permitted to
flow through passage 52 and chamber 54 to oil passages 46, 48
and 50 around relieved portion 64 of valve member 56 and a
second position, illustrated in Figure 2, in which an
unrelieved portion of valve 56 blocks the flow oi` oil through
chamber 54. Valve 56 is posicioned to the posi~Lon Lllustrated
ln Figure 1 by the exposure of its high side end face 66 to the
discharge pressure which exists in discharge pressure area 62
whenever the compressor is in operation.
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Low side end face 68 of valve 56, on the other
hand, is exposed, as earlier mentioned, to an area of the
compressor which is at low side or suction pressure through
passage 58. The high to low side pressure differentiaI across
valve 56 which exists whenever the compressor is in operation .
ensures that valve 56 is positioned to permit oil flow through
chamber 54, as is illustrated in Figure 1, at all times during
compressor operation. This assures, in a fail safe manner
which relies on an operating condition which is inherent in the ~ `
compressor when it is in operation,- that oil is permitted to ; ;~
flow fro~ sump 44 to the oil injection port and to the
~ compressor bearings whenever the compressor is operating.
; ~pon de-energization of motor 36 the compressor is
shut down and previously compressed discharge pressure gas will
immediately flow back to the working chamber of the de- ~ ;
energlzed compressor from downstream thereof. The immediate
effect of the backflow of the discharge pressure gas is to
carry check valve member 41 to the position in which it is
illustrated in phantom in Figure 1. ~
As soon as check valve member 41 seats in the ;`
phantom position illustrated in Figure l, the backflow of
previously compressed gas from downstream of the compressor to
the working chamber will stop. The immediate initial backflow
of gas to the working chamber prior to the discharge check
valve having seated will, however, have caused the rotors to
begin to rotate in a direction opposite the direction they are
caused to rotate in operation by motor 36.
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This reverse rotation of the ro~ors has the effect
of evacuating 8as from discharge area 62 as soon as valve
member 41 seats and of lowering the pressure in that area to a
pressure ~hich is less than system low side pressure. This is
because the rotors, which function as a gas expander by virtue
of their reverse direction rota~ion, act to pump gas from the
discharge area against the closed discharge check valve 41
under this condition when it is in its backflow preventing
position.
As che pressure in discharge pressure area 62 drops
under these circumstances the pressure on high pressure end 66
of valve 56 quickly drops to a pressure which is less than the
low side pressure in suction area 60. Under tha~ circumstance
the pressur~ on low side end face 68 of the spool valve will b~ ~
greater than the pressure on the high end side face 66 oE the .
valve and the pressure differential across the valve will act
to move the valve into the position illustrated in ~igure 2.
Once again, it will be appreciated that an ambien~ condition
~ inherent i~ the compressor at a particular point in ics .`
;, 20 operation is used to cause oil flow passage 52 to be closed to
`i flow at an appropriate time.
It will be noted fro~ Figure 2 that valve 56 may be
biased by spring 70 toward the Figure 2 position in which an
unrelieved portion of valve member 56 occludes oil supply
passage 52. It will also be noted that a retainer ring 72 is
disposed in volume 54 and protrudes thereinto per~itting valve
56 to travel no Eurther within volume 54 than to the position
illustrat~d in Figure 2. While spring 70 is not ~andatory, it
`i will preierably be used since in addition to assisting the ~:
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movemen~ of valve 56 to the position in which oil ~low is ~
prevented upon compressor shutdown it assists ln maintaining .~ ~;
the valve in that position as conditions in discharge area 62,
which are somewhat transient by nature at compressor shutdown,
assume a steady state condition.
. When the compressor next starts up subsequent to
;~ having been shutdown, the compression of gas between the screw . :~
; rotors will immediately commence and discharge pressure will
quickly build in discharge pressure area 62 causing valve 56 to
be urged in~o the position illustrated in Figure 1 in which oil
; supply passage 52 is open to flow. Pressure will concurrently
build up in oil separator 42 which will cause oil to flow from
sump 44 through oil supply passage 52 to the compressor .
bearings and oil in~ection port.
~ 15 It will be appreciated that since the oil shutofP
:' arrangement of the present invention Ls mechanical and fail
safe, relying on inherent internal compressor operating
conditions for aceuation at appropriate times, the need for
monitoring the position oi' the shutoff valve and/or the need to
l 20 "prove" oil ~low to the compressor bearings and oil injection :~
.' port at compressor startup is avoided. The arrangement of the
present i~vention likewLse eliminates the need for electrical
or electronic sensing and/or monitoring with respect to oil ,~
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flow during compressor operation and, wieh respect to some
systems, the need to employ a relatively expensive solenoid
operated valve, which is sub~ect to electrical failure, in the
;~, compressQr oil supply line,
I What is claimed is:
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