COMPRESSOR UNLOADING VALVE
BACKGROUND OF THE INVENTION
[0001] The invention relates to compressors. More
particularly, the invention relates to refrigerant
compressors.
[0002] Screw-type compressors are commonly used in air
conditioning and refrigeration applications. In such a
compressor, intermeshed male and female lobed rotors or screws
are rotated about their axes to pump the working fluid
(refrigerant) from a low pressure inlet end to a high pressure
outlet end. During rotation, sequential lobes of the male
rotor serve as pistons driving refrigerant downstream and
compressing it within the space between an adjacent pair of
female rotor lobes and the housing. Likewise sequential lobes
of the female rotor produce compression of refrigerant within
a space between an adjacent pair of male rotor lobes and the
housing. The interlobe spaces of the male and female rotors in
which compression occurs form compression pockets
(alternatively described as male and female portions of a
common compression pocket joined at a mesh zone). In one
implementation, the male rotor is coaxial with an electric
driving motor and is supported by bearings on inlet and outlet
sides of its lobed working portion. There may be multiple
female rotors engaged to a given male rotor or vice versa.
[0003] When one of the interlobe spaces is exposed to an inlet
port, the refrigerant enters the space essentially at suction
pressure. As the rotors continue to rotate, at some point
during the rotation the space is no longer in communication
with the inlet port and the flow of refrigerant to the space
is cut off. After the inlet port is closed, the refrigerant is
compressed as the rotors continue to rotate. At some point
1

during the rotation, each space intersects the associated
outlet port and the closed compression process terminates. The
inlet port and the outlet port may each be radial, axial, or a
hybrid combination of an axial port and a radial port.
[0004] It is often desirable to temporarily reduce the
refrigerant mass flow through the compressor by delaying the
closing off of the inlet port (with or without a reduction in
the compressor volume index) when full capacity operation is •
not required. Such unloading is often provided by a slide
valve having a valve element with one or more portions whose
positions (as the valve is translated) control the respective
suction side closing and discharge side opening of the
compression pockets. The primary effect of an unloading shift
of the slide valve is to reduce the initial trapped suction
volume (and hence compressor capacity); a reduction in volume
index is a typical side effect. Exemplary slide valves are
disclosed in U.S. Patent Application Publication No.
20040109782 Al and U.S. Patent Nos. 4,249,866 and 6,302,668.
The desired degree to which a compressor may be unloaded is
often application-specific. High degrees of unloading (e.g.,
down to an exemplary 15% of full load capacity) may be
preferred for some applications.
SUMMARY OF THE INVENTION
[0005] According to one aspect of the invention, a compressor
has housing having first and second ports along a flow path.
One or more working elements cooperate with the housing to
define a compression path between suction and discharge
locations along the flow path. An unloading valve has a valve
element having a range between a first condition and a second
condition, the second condition being unloaded relative to the
first condition. Means bias the valve element toward a third
condition intermediate the first and second conditions.
2

[0006] In various implementations, the means may comprise a
first and second springs. The springs may be on opposite sides
of a piston engaged to the valve element.
[0007] The means may be introduced in a reengineering of an
existing compressor configuration and/or a remanufacturing of
an existing compressor. The reengineering may be an iterative
process performed on hardware or as a simulation/calculation.
The reengineering or remanufacturing may comprise adding a
second spring to act against an existing first spring of the
baseline compressor.
[0008] The details of one or more embodiments of the invention
are set forth in the accompanying drawings and the description
below. Other features, objects, and advantages of the
invention will be apparent from the description and drawings,
and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a longitudinal sectional view of a
compressor.
[0010] FIG. 2 is a transverse sectional view of a discharge
plenum of the compressor of FIG. 1, taken along line 2-2.
[0011] FIG. 3 is a sectional view of a slide valve assembly of
the discharge plenum of FIG. 2 in a fully loaded condition,
taken along line 3-3.
[0012] FIG. 4 is a view of the slide valve of FIG. 3 in a
relatively unloaded condition.
[0013] FIG. 5 is a view of the slide valve of FIG. 3 in a
neutral condition more loaded than the FIG. 4 condition and
less loaded than the FIG. 3 condition.
[0014] Like reference numbers and designations in the various
drawings indicate like elements.
3

DETAILED DESCRIPTION
[0015] FIG. 1 shows a compressor 20 having a housing assembly
22 containing a motor 24 driving rotors 26 and 28 having
respective central longitudinal axes 500 and 502. In the
exemplary embodiment, the rotor 26 has a male lobed body or
working portion 30 extending between a first end 31 and a
second end 32. The working portion 30 is enmeshed with a
female lobed body or working portion 34 of the female rotor
28. The working portion 34 has a first end 35 and a second end
36. Each rotor includes shaft portions (e.g., stubs 39, 40,
41, and 42 unitarily formed with the associated working
portion) extending from the first and second ends of the
associated working portion. Each of these shaft stubs is
mounted to the housing by one or more bearing assemblies 44
for rotation about the associated rotor axis.
[0016] In the exemplary embodiment, the motor is an electric
motor having a rotor and a stator. One of the shaft stubs of
one of the rotors 26 and 28 may be coupled to the motor's
rotor so as to permit the motor to drive that rotor about its
axis. When so driven in an operative first direction about the
axis, the rotor drives the other rotor in an opposite second
direction. The exemplary housing assembly 22 includes a rotor
housing 48 having an upstream/inlet end face 4 9 approximately
midway along the motor length and a downstream/discharge end
face 50 essentially coplanar with the rotor body ends 32 and
36. Many other configurations are possible.
[0017] The exemplary housing assembly 22 further comprises a
motor/inlet housing 52 having a compressor inlet/suction port
53 at an upstream end and having a downstream face 54 mounted
to the rotor housing downstream face (e.g., by bolts through
both housing pieces). The assembly 22 further includes an
outlet/discharge housing 56 having an upstream face 57 mounted
4

to the rotor housing downstream face and having an
outlet/discharge port 58. The exemplary rotor housing,
motor/inlet housing, and outlet housing 56 may each be formed
as castings subject to further finish machining.
[0018] Surfaces of the housing assembly 22 combine with the
enmeshed rotor bodies 30 and 34 to define inlet and outlet
ports to compression pockets compressing and driving a
refrigerant flow 504 from a suction (inlet) plenum 60 to a
discharge (outlet) plenum 62 (FIG.2). A series of pairs of
male and female compression pockets are formed by the housing
assembly 22, male rotor body 30 and female rotor body 34. Each
compression pocket is bounded by external surfaces of enmeshed
rotors, by portions of cylindrical surfaces of male and female
rotor bore surfaces in the rotor case and continuations
thereof along a slide valve, and portions of face 57.
[0019] FIG. 2 shows further details of the exemplary flowpath
at the outlet/discharge port 58. A check valve 70 is provided
having a valve element 72 mounted within a boss portion 74 of
the outlet housing 56. The exemplary valve element 72 is a
front sealing poppet having a stem/shaft 7 6 unitarily formed
with and extending downstream from a head 78 along a valve
axis 520., The head has a back/underside surface 80 engaging an
upstream lend of a compression bias spring 82 (e.g., a metallic
coil). The downstream end of the spring engages an
upstream-facing shoulder 8 4 of a bushing/guide 86. The
bushing/guide 8 6 may be unitarily formed with or mounted
relative to the housing and has a central bore 88 slidingly
accommodating the stem for reciprocal movement between an open
condition (not shown) and a closed condition of FIG. 2. The
spring 82 biases the element 72 upstream toward the closed
condition. In the closed condition, an annular peripheral
seating portion 90 of the head upstream surface seats against
5

an annular seat 92 at a downstream end of a port 94 from the
discharge plenum.
[0020] For capacity control/unloading, the compressor has a
slide valve 100 having a valve element 102. The valve element
102 has a portion 104 along the mesh zone between the rotors
(i.e., along the high pressure cusp). The exemplary valve
element has a first portion 106 (FIG. 3) at the discharge
plenum and a second portion 108 at the suction plenum. The
valve element is shiftable to control compressor capacity to
provide unloading. The exemplary valve is shifted via linear
translation parallel to the rotor axes.
[0021] FIG. 3 shows the valve element at an upstream-most
position in its range of motion In this position, the
compression pockets close relatively upstream and capacity is
a relative maximum (e.g., at least 90% of a maximum
displacement volume for the rotors, and often about 99%). FIG.
4 shows the valve element shifted to a downstream-most
position. Capacity is reduced in this unloaded condition
(e.g., to a displacement volume typically less than 40% of the
FIG. 3 displacement volume or the maximum displacement volume,
and often less than 30%). In the exemplary slide valve, shifts
between the two positions are driven by a combination of
spring force and fluid pressure. A main spring 120 biases the
valve element from the loaded to the unloaded positions. In
the exemplary valve, the spring 120 is a metal coil spring
surrounding a shaft 122 coupling the valve element to a piston
124. The piston is mounted within a bore (interior) 126 of a
cylinder 128 formed in a slide case element 130 attached to
the outlet case. The shaft passes through an aperture 132 in
the outlet case. The spring is compressed between an underside
134 of the piston and the outlet case. A proximal portion 136
of the cylinder interior is in pressure-balancing fluid
6

communication with the discharge plenum via clearance between
the aperture and shaft. A headspace 138 is coupled via
electronically-controlled solenoid valves 140 and 142 (shown
schematically) to a high pressure fluid source 144 at or near
discharge conditions (e.g., to an oil separator). A port 146
is schematically shown in the cylinder at the headspace at the
end of a conduit network connecting the valves 140 and 142. In
an exemplary implementation, the portions of the conduit
network may be formed within the castings of the housing
components. The exemplary main spring 12 0 acts with a force
that is relatively insignificant in comparison to the net
force which may developed by fluid pressures. During periods
of non-operation, when fluid pressures are balanced, the main
spring 120 acts as is described below.
[0022] The loaded position/condition of FIG. 3 can be achieved
by coupling the headspace 138 to the source 144 and isolating
it from drain/sink 150 by appropriate control of valves 140
and 142. The unloaded position/condition of FIG. 4 can be
achieved by coupling the headspace 138 to the drain/sink 150
and isolating it from source 144 by appropriate control of
valves 140 and 142. Intermediate (partly loaded) positions,
not shown, can be achieved by alternating connection of
headspace 138 to either the source 144 or the drain/sink 150
using appropriately chosen spans of time for connection to
each, possibly in combination with isolating the headspace 138
from both source 144 and drain/sink 150 for an appropriately
chosen span of time (e.g., via appropriate modulation
techniques).
[0023] For some applications it is desirable to have the
unloaded position/condition of FIG. 4 be such that during
operation the refrigerant mass flow through the compressor is
as low as an exemplary 15% of the mass flow achieved when the
7

slide valve is in the loaded position/condition of FIG. 3.
Said another way, the displacement volume of the position of
FIG. 4 would be an exemplary 15-20% of the displacement volume
of the position of FIG. 3. The displacement volume slightly
above 15% would achieve the 15% flow rate due to internal
leakage. At some start-up conditions, low rates of refrigerant
mass flow may result in discharge pressure may not rising in a
relatively short period of time. Many systems depend on
discharge pressure in source 144 to deliver oil for actuating
slide valve 100 as previously described and for lubricating
rotors and bearings. An inability to rapidly develop adequate
discharge pressure to accomplish these roles may be viewed as
having a negative impact on system performance or may be
detrimental to compressor reliability. The problem may be
particularly serious when the system is started after it has
not operated for a long period of time. In such situations,
residual lubrication on rotors and in bearing cavities may be
substantially diluted, owing to the tendency of many
refrigeration oils to absorb refrigerant over time and thereby
become diluted. During operation, this dilution tendency is
countered by elevated temperatures and by high speed motion of
parts, both of which tend to move refrigerant out of solution
with oil. During a start-up after a long shutdown period it is
therefore, desirable to quickly deliver lubricant to the
compressor.
[0024] To provide rapid start-up it is desirable that the
valve position at start-up be more loaded than the unloaded
position of FIG. 4. Preferably, the start-up position would
correspond to a mass flow rate that is in the range of 25-35%
of that of the loaded position of FIG. 3. A displacement
volume might be 25-50% that of FIG. 3.
8

[0025] According to the present invention, means are provided
for biasing the slide valve from the unloaded end of its range
(FIG. 4) at least partially toward the loaded end of its range
(FIG. 3). An exemplary means includes a spring 160. An
exemplary spring 160 is a compression coil spring within the
headspace 138. The exemplary spring 160 extends from a
proximal lend portion 162 to a distal end portion 164. The
proximal end portion 162 is engaged to a boss 166 of the valve
case 130 in the headspace to securely retain the spring 160.
The exemplary spring 160 has dimensions and a spring constant
such that the distal end 164 engages the face 168 of the
piston 124 in the FIG. 4 unloaded condition but disengages at
some point in the range of travel to the FIG. 3 loaded
condition.
[0026] The spring 160 may come into play, for example, during
a shutdown condition. For example, in a shutdown condition,
pressures may equalize in the suction plenum 60, discharge
plenum 62, cylinder interior proximal portion 136, and
headspace 138. In such a condition, the spring 160 will act to
shift the valve element slightly away from the FIG. 4 unloaded
condition (e.g., to an intermediate condition of FIG. 5). At
shutdown when pressures on each side of the piston are equal,
spring 160 acts on piston 124 in opposition to spring 120,
moving piston 124 and attached slide valve 100 to the position
of FIG. 5 which is slightly more loaded than that of FIG. 3.
The length and spring constant of spring 160 are chosen,
possibly in combination with those of spring 120, so that the
resulting position shown in FIG. 5 corresponds to a
displacement volume that results in discharge pressure rising
rapidly enough to ensure quick delivery of lubricant to the
compressor. The displacement volume corresponding to the
position :of FIG. 5 would typically be in the range of 25-35%
of that of the loaded position of FIG. 3. After start-up, once
9

discharge pressure has risen, the unloaded position of FIG. 4
can automatically be achieved because the action of pressures
acting on faces 168 and 134 of piston 124 and on sides 106 and
108 of slide valve 100 generates sufficient force to overcome
the force provided by spring 160. Alternatively, if desired,
the unloaded position of FIG. 4 can be prevented by coupling
headspace 138 to source 144 as previously described as
adequate pressure in source 144 has now been developed to
allow delivery of fluid to headspace 138.
[0027] The spring 160 may be added in a reengineering or
remanufacturing from a baseline compressor or configuration
thereof. In the baseline, the main spring 160 could have
sufficient length so that start-up would be in the fully
unloaded condition. The main spring 160 may be preserved or
modified in the reengineering or remanufacturing. One
modification would be to shorten it.
[0028] Among many alternatives to a headspace compression
spring 160 would be to have the main spring 120 be neutral at
the FIG. .5 valve condition and go into tension between the
FIG. 4 and FIG. 5 valve conditions. Rather than a coil spring,
the spring 160 could be another form of spring (e.g., a
Belleville washer spring). In another embodiment, the spring
160 could be attached to piston 124 rather than to boss 166 of
valve case 130.
[0029] One or more embodiments of the present invention have
been described. Nevertheless, it will be understood that
various modifications may be made without departing from the
spirit and scope of the invention. For example, in a
reengineering or remanufacturing situation, details of the
existing compressor configuration may particularly influence
10

or dictate details of the implementation. Accordingly, other
embodiments are within the scope of the following claims.
11

CLAIMS
What is claimed is:
1. A compressor apparatus (20) comprising:
a housing (22) having first (53) and second (58) ports
along a flow path;
one or more working elements (26; 28) cooperating with
the housing (22) to define a compression path between suction
(60) and discharge (62) locations along the flow path; and
an unloading valve (100) having:
a valve element (102) having a range between a first
condition and a second condition, the second condition
being unloaded relative to the first condition; and,
means (160) biasing the valve element toward a third
condition intermediate the first and second conditions in
displacement volume, the biasing being from both the
first condition and the second condition.
2. The apparatus of claim 1 wherein:
the unloading valve (100) is a slide valve and the range
is a range of linear translation;
the first, second, and third conditions respectively are
associated with first, second, and third valve element
positions, the third valve element position being closer to
the second valve element position than to the first valve
element position.
3. The apparatus of claim 2 wherein:
the first valve element position has a first displacement
volume;
the second valve element position has a second
displacement volume of 15-20% of the first displacement
volume; and
12


the third valve element position has a third displacement
volume of 25-35% of the first displacement volume.
4. The apparatus of claim 2 wherein the third valve element
position is 5-25% of said range from said second valve element
position to said first valve element position.
5. The apparatus of claim 2 wherein the unloading valve
further comprises:
a cylinder (128);
a piston (124) in the cylinder and mechanically coupled
to the valve element (102); and
a control valve (140; 142) coupled to a headspace (138)
of the cylinder to selectively expose the headspace to a fluid
(144)source.
6. The apparatus of claim 5 wherein the means comprises:
a first spring (120) biasing the valve element from the
first condition toward the third condition; and
a second spring (160) biasing the valve element from the
second condition toward the third condition.
7. The apparatus of claim 6 wherein the means comprises:
the first spring (120) is a first coil spring and
surrounds a shaft (122), the shaft coupling the piston (124)
to the valve element (102); and
a second spring (160) is a second coil spring and is in
the headspace (138).
8. The apparatus of claim 1 wherein the means comprises:
a first spring (120) biasing the valve element from the
first condition toward the third condition; and
a second spring (160) biasing the valve element from the
second condition toward the third condition.
13


9. The apparatus of claim 8 wherein:
the first spring (120) has a lower spring constant than
does the second spring (160).
10. The apparatus of claim 8 wherein:
the first spring (120) is under compression when the
valve element is along an entirety of said range; and
the second spring (160) is under compression at least
when said valve element is everywhere between said second and
third conditions.
11. The apparatus of claim 8 wherein:
the first (120) and second (160) springs are metallic
coil springs.
12. The compressor of claim 1 wherein the one or more working
elements include:
a male-lobed rotor (26) having a first rotational axis
(500); and
a female-lobed rotor (28) having a second rotational axis
(502) and, enmeshed with the first rotor.
13. The :compressor of claim 12 wherein:
in the first condition, the compressor is at least at 90%
of a maximum displacement volume;
in the second condition, the compressor is at less than
20% of the first condition displacement volume; and
in the third condition, the compressor is at 25-50% of
the first: condition displacement volume.
14. The compressor of claim 12 wherein:
in the first condition, the compressor is at least at 90%
of a maximum displacement volume;
14


in the second condition, the compressor is at less than
20% of the first condition displacement volume; and
in the third condition, the exceeds the second condition
displacement volume by 10-40% of said first condition
displacement volume.
15. A compressor apparatus (20) comprising:
a housing (22) having first (53) and second (58) ports
along a flow path;
one or more working elements (26; 28) cooperating with
the housing (22) to define a compression path between suction
(60) and discharge (62) locations along the flow path; and
an unloading valve (100) having:
a valve element (102) having a range between a first
condition and a second condition, the second condition
being unloaded relative to the first condition; and
a first spring (120) biasing the valve element from the
first condition toward a third condition intermediate the
first and second conditions in displacement volume; and
a second spring (160) biasing the valve element from the
second condition toward the third condition.
16. The iapparatus of claim 15 wherein:
the first spring (120) has a lower spring constant than
does the second spring (160) .
17. The apparatus of claim 15 wherein:
the first spring (120) is under compression when the
valve element is along an entirety of said range; and
the second spring (160) is under compression at least
when said valve element is everywhere between said second and
third conditions.
18. The apparatus of claim 15 wherein:
15


the first (120) and second (160) springs are metallic
coil springs.
19. A method for remanufacturing a compressor (20) or
reengineering a configuration of the compressor comprising:
providing an initial such compressor or configuration
having:
a housing (22);
one or more working elements (26; 28) cooperating
with the housing to define a compression path between
suction (60) and discharge (62) locations; and
an unloading slide valve (100) having:
a valve element (102) having a range between a
first condition and a second condition, the second
condition being unloaded relative to the first
condition;
a cylinder (128);
a piston (124) in the cylinder and mechanically
coupled to the valve element; and
a fluid in a headspace (138) of the cylinder,
pressure of the fluid in the headspace producing a force
on the piston and valve element in a direction from the
second condition toward the first condition; and
adapting such compressor or configuration to include
means (160) biasing the valve element toward a third condition
from said second condition, the third condition being
intermediate the first and second conditions in displacement
volume.
20. The method of claim 19 wherein:
16


the adapting includes selecting at least one parameter of
the means to provide a desired neutral location of said valve
element.
21. The method of claim 20 wherein the selecting comprises an
iterative:
varying of said at least one parameter; and
directly or indirectly determining a neutral location of
said valve element.
22. The method of claim 21 wherein:
the varying comprises varying a property of a compression
spring (160) in the headspace (138).
17

A compressor apparatus (20) has a housing (22) having first
(53) and second (58) ports along a flow path. One or more
working elements (26; 28) cooperate with the housing (22) to
define a compression path between suction (60) and discharge
(62) locations along the flow path. An unloading valve (100)
has a valve element (102) having a range between a first
condition and a second condition, the second condition being
unloaded relative to the first condition. Means (120, 160)
bias the valve element toward a third condition intermediate
the first and second conditions.