SLIDE VALVE ACTUATION FOR OVERPRESSURE SAFETY
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

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 5,302,668.
[0005] If, for example, there is a restriction downstream of
the discharge plenum, the discharge pressure may be come
excessive, potentially damaging the compressor. Accordingly,
compressors may include a pressure relief valve. An exemplary
relief valve is positioned in a passageway between the
discharge plenum and the suction plenum and vent gas from the
discharge plenum to the suction plenum when the pressure
difference across the valve exceeds a threshold pressure. U.S.
Patent No. 5,807,081 discloses a bidirectional pressure relief
valve. That also addresses flow reversal conditions.
[0006] Additionally, international application PCT/US05/03813,
filed February 7, 2005 discloses a pressure relief valve
protecting the slide valve piston rings.

[0007] According to one aspect of the invention, a compressor
has an unloading slide valve. The 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. A piston is in a cylinder and mechanically
coupled to the valve element. A control valve is coupled to a
headspace of the cylinder to selectively expose the headspace
to a fluid source, pressure of fluid in the headspace
producing a force on the pinton and. valve element in a
direction from the second condition toward the first
condition. A pressure relief valve couples the headspace to
suction conditions to unload the compressor responsive to an
overpressure.
[0008] In various implementations, the pressure relief valve
may be in addition to another pressure relief valve (e.g., an
internal valve) extending between discharge and suction
conditions. The pressure relief valve may be provided in a
remanufacturing of a compressor or the reengineering of a
compressor configuration from an initial baseline
configuration.
[0009] 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
[0010] FIG. 1 is a longitudinal sectional view of a
compressor.
[0011] PIG. 2 is a transverse sectional view of a discharge
plenum of the compressor of FIG. 1, taken along line 2-2.

[0012] FIG. 3 is a sectional view of a slide valve assembly of
the discharge plenum of PIG. 2 in a fully loaded condition,
taken along line 3-3.
[0013] FIG. 4 is a view of the slide valve of FIG. 3 in a
relatively unloaded condition.
[0014] FIG. 5 is a view of the slide valve of FIG. 3 after a
pressure relief unloading,
[0015] Like reference numbers and designations in the various
drawings indicate like elements.

[0016] 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. For purposes
of illustration, the basic structure of the compressor is
taken from PCT/US05/03813, noted above. However, other
existing or yet-developed compressor configurations are
possible.
[0017] 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 ona or more bearing assemblies 44
for rotation about the associated rotor axis.
[0018] 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 49 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.

[0019] The exemplary housing assembly 22 further comprises a
motor/inlet housing 52 hav: ng 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
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.
[0020] Surfaces of the housing assembly 22 combine with the
enmeshed rotor bodies 3 0 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. Bach
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.
[0021] 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 76 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 end of a compression bias spring 82 (e.g., a metallic
coil). The downstream end of the spring engages an
upstream-facing shoulder 84 of a bushing/guide 86. The
bushing/guide 86 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 upstream
position of the closed condition. In the closed condition, an
annular peripheral seating portion 90 of the head upstream
surface seats against an annular seat 92 at a downstream end
of a port 94 from the discharge plenum.
[0022] A pressure relief valve 96 (FIG. 1) is positioned along
a passageway 98 between the suction, plenum and discharge
plenum. Although shown schematically, the actual passageway 98
may be a bore through the rotor case. An exemplary relief
valve opens when the discharge pressure exceeds the suction
pressure by an exemplary threshold of about 400psi (dependent
upon design operational parameters and housing strength).
[0023] 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 contarol compressor capacity to
provide unloading. The exemplary valve is shifted via linear
translation parallel to the rotor sixes.
[0024] 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 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 loadad 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
communication with the discharge plenum via clearance between
the aperture and shaft. A hoadspace 138 is selectively 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., a high pressure oil
separator) and a drain/sink 146 at or near suction conditions..
The drain/sink 146 may be the suction plenum 60 or may
otherwise be substantially at suction conditions. A port 148
is schematically shown in the cylinder at the headspace at the
end of a conduit network connecting the valves 140 and 142,
source 144, and drain/sink, 150. In an exemplary
implementation, the portions of the conduit network may be
formed within the castings of the housing components.
[0025] The loaded position/condition of FIG. 3 can be achieved
by coupling the headspace 138 to the source. 144 and isolating
it from the drain/sink 146 by appropriate control of valves
140 and 142. The unloaded position/condition of FIG. 4 can be
achieved by coupling the headspace 13 8 to the drain/sink 146
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 146
using appropriately chosen spans of time for connection to
each, possibly in combination with isolating the headspace 13 8
from both source 144 and drain/sink 146 for an appropriately
chosen span of time (e.g., via appropriate modulation
techniques).
[0026] As so far described, the compressor may be of a variety
of existing configurations. FIGS. 3-5 show a second pressure
relief valve 180 in a passageway 182 extending between the
headspace 138 and the suction plenum 60.
[0027] The exemplary valve j.80 is a one-way spring-loaded
pressure relief valve having a predetermined threshold/relief
or pop-off pressure. The exemplary valve 180 is normally
closed, when the pressure in the headspace exceeds that in the
suction plenum 60 by the threshold pressure, this pressure
difference will shift the relief valve element against its
spring force to automatically open the valve 180 and permit
the fluid (e.g., the oil in ;he particular example) to pass
from the headspace to the suction plenum 60 to shift the slide
valve element 102 to the unloaded condition.
[0028] When an overpressure condition occurs, the increased
pressure of refrigerant in the discharge plenum 62 and
cylinder bore proximal portion 136 will increase the fluid
pressure against the piston 124 toward the unloaded condition
of FIG. 4. Given the relative incompressibility of the oil in
the headspace 138, the pressure in the headspace 138 may
increase correspondingly even with little piston movement. If
the difference between the headspace pressure and the suction
pressure (e.g., at the sink 146) exceeds the threshold of the

pressure relief value 180, the pressure relief valve 180 will
open, passing fluid from the headspace 138 to the sink 146 and
allowing the slide valve element 102 to shift. The shift may
typically be essentially all the way to the unloaded condition
(FIG. 5). The shift unloads the compressor and reduces the
further contribution of the compressor to the overpressure
condition.
[0029] The pressure relief valve 180 may open in a number of
situations. One example involves a partial blockage of the
refrigerant circuit along which the compressor is located.
This may occur during compressor operation (e.g., even while
operating at a steady-state) or at start-up. The imposition or
development of a restriction during otherwise steady-state
operation will increase discharge pressure. A sufficient
restriction will cause the discharges pressure to increase
above the threshold of the pressure relief valve 180. The
unloading then caused by the opening of the pressure relief
valve 180 may be effective to address a partial restriction in
the absence of opening of the pressure relief valve 96. In
other situations, the unloading may supplement or complement
operation of the pressure relief valve 96. If, despite the
unloading, the discharge pressure rises to exceed the
threshold pressure of the pressure relief valve 96, the
pressure relief valve 96 will then open and divert compressed
refrigerant from the discharge plenum back 62 to the suction
plenum 60. Because of the unloading, however, the pressure
relief valve 96 will not have to pass as great a refrigerant
flow as it would in the absence of the unloading.
[0030] Other examples involve start-up operation. An exemplary
start-up restriction may been the form of an erroneously
closed service valve.

[0031] As is discussed further below, the pressure relief
valve 180 advantageously opens before the pressure relief
valve 96 does. If the impact of the spring 120 (if any) is
negligible, the pressure will be essentially equal across the
piston 124. Thus, the threshold pressure of the pressure
relief valve 180 may, advantageously, be slightly less than
the threshold pressure of the pressure relief valve 96.
[0032] Depending upon the implementation, one possible
advantage of the pressure relief valve 180 is that it
increases flexibility in choice of the pressure relief valve
96. For example, as noted above, the pressure relief valve 96
and its associated passageway 98 would not need to be as large
as they would be in the absence of the pressure relief valve
180. Thus, when engineering a relatively high capacity
compressor, an existing pressure relief valve 96 from a lower
capacity compressor could be used. This represents a cost
savings.
[0033] Because the pressure relief valve 96 typically passes
gas whereas the pressure relief valve 180 typically passes
liquid oil (more dense), the pressure relief valve 180 may be
small relative to the pressure relief valve 96. Additionally,
the cost savings involved in the size decrease of the pressure
relief valve 96 may outweigh the costs of providing the
pressure relief valve 180.
[0034] Other advantages may relate to avoiding deleterious
effects on the pressure relief valve 96. For example, by
unloading at a pressure sligitly below the threshold pressure
of the pressure relief valve 96, wear is saved. This may
reduce the tendency for residual leaks to develop in the
pressure relief valve 96.

[0035] In a re-engineering situation, when one adds the second
pressure relief valve 180, one might one also change other
aspects of the compressor. As noted above, one might reduce
the size (flow capacity) of the pressure relief valve 96. An
engineering or re-engineering may be an iterative process
performed using actual hardware and/or calculation/simulation.
For examples, parameters of the pressure relief valve 180
(e.g., the threshold/relief pressure and/or the capacity) may
be optimized to provide reliable unloading at as high a
pressure as possible without triggering the pressure relief
valve 96.
[0036] 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
or dictate details of the implementation. Additional pressure
relief features such as those disclosed in PCT/US05/03 813, may
also be provided. Accordingly, other embodiments are within
the scope of the following claims.

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;
a pressure relief valve (96) coupling the suction (60)
and discharge (62) locations to relieve an overpressure in the
discharge location (62) by passing working fluid from the
discharge location (60) to the suction location (62); 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;
a control valve (140) coupled to a headspace (138)of
the cylinder to selectively expose the headspace to a
fluid source (144), pressure of 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
means (180, 182) i or relieving excess pressure in
the headspace by passing fluid from the headspace to the
suction location (62).
2. The apparatus of claim 1 wherein:
the range is a range oi linear translation; and
the means comprises a second pressure relief valve (180).

3. The apparatus of claim 2 wherein the second pressure
relief valve (180) is a spring-loaded one-way valve.
4. The apparatus of claim 1 wherein:
the pressure relief valve (96) has a first
threshold/relief pressure; and
the means (180) has a second threshold/relief pressure
less than the first threshold/relief pressure.
5. 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 (18) having a second rotational axis
(502) and enmeshed with the male-lobed rotor.
6. A compressor apparatus comprising:
a housing (22) having first (5:3) 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 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;
a control valve (110) coupled to a headspace (138)
of the cylinder to selectively expose the headspace to a
fluid source (144), pressure of fluid in the headspace
producing a force on the piston and valve element in a

direction from the second condition toward the first
condition;
a passageway (182) coupling the headspace to the
suction location; and
a pressure relief valve (180) along the passageway.
7. The apparatus of clain. 6 wherein:
the pressure relief velve (180) is a spring-loaded
one-way valve.
8. The apparatus of clain 6 wherein:
a second pressure relief valve (96) couples the suction
(60) and discharge (62) locations to relieve an overpressure
in the discharge location (62) by passing working fluid from
the discharge location (62) to the suction location (60).
9. 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 slice 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 (12 4) 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:
a passageway (182) coupling the headspace to
the suction location; and
a pressure relief valve (180) along the
passageway.
10. The method of claim 9 wherein:
the adapting includes selecting at least one parameter of
the pressure relief valve (180).
11. The method of claim 10 wherein the selecting comprises an
iterative:
varying of said at least one parameter; and
determining a relationship between the unloading and the
triggering of a pressure relief valve (96) between suction
(60) and discharge (62) locations.
12. The method of claim 11 therein::
the varying comprises varying a threshold/relief
pressure.

A compressor (20) has an unloading slide valve (100). The valve has a valve element (102) having a range between a fir condition and a second condition, the second condition being unleaded relative to the first condition. A piston (124) is in a cylinder (128) and mechanically coupled to the valve element. A control valve (140) is coupled to a headspace (138) of the cylinder to selectively expose the headspace to a flu source (144), pressure of fluic in the headspace producing a force on the piston and valve element in a direction from the second condition toward the first condition. The compressor includes a pressure relief valve coupling (180) in the headspace to suction conditions to unload the compressor responsive to an overpressure.