Rotary expansible chamber devices – Interengaging rotating members – Helical or herringbone
Reexamination Certificate
2001-08-15
2002-12-17
Vrablik, John J. (Department: 3748)
Rotary expansible chamber devices
Interengaging rotating members
Helical or herringbone
C418S001000, C029S888023
Reexamination Certificate
active
06494699
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to screw compressors, and more particularly to axial unloading lift valves for screw compressors.
BACKGROUND OF THE INVENTION
Axial unloading lift valves are commonly used in screw compressors to vary the compression load produced by the screws. One or more valves are arranged axially towards the discharge side of the screws and the load is varied by selectively opening and closing the valves. Opening the valves to “unload” the compressor reduces the effective working length of the screws by opening communication pathways between portions of the screw and the low-pressure suction end of the compressor. The open pathways allow the pressure to equalize so that compression does not occur over the portions of the screw communicating with the suction end of the compressor. When the valves are closed to “load” the compressor, no pressure equalization occurs over the axial length of the screw. Therefore, the full working length of the screws is utilized for compression. The angular location of the valves around the discharge ends of the screws determines how much of the axial working length of the screws is used or eliminated when the valves are closed or opened.
FIGS. 1 and 2
are schematic representations showing a portion of a prior art compressor
10
having an axial unloading lift valve
14
.
FIG. 1
shows the valve
14
in the loaded condition and
FIG. 2
shows the valve
14
in the unloaded condition. The compressor
10
includes a pair of screws
15
,
16
(only one is shown in
FIGS. 1 and 2
) mounted for rotation in a screw housing
22
. The interior of the screw housing
22
defines a compression chamber
24
where the fluid is compressed by the screws
15
,
16
, as is understood by those skilled in the art. A discharge housing
26
supports the discharge end of the screws
15
,
16
and is coupled to one end of the screw housing
22
. A suction housing
30
supports the suction end of the screws
15
,
16
and is coupled to the other end of the screw housing
22
.
The axial unloading valve
14
typically includes a cylindrically-shaped valve member
34
housed in a valve chamber
38
. The valve chamber
38
is formed in the discharge housing
26
so that one end of the valve chamber
38
communicates both with the compression chamber
24
and with a vent passageway
42
. The vent passageway
42
is connected to a suction cavity
46
formed in the suction housing
30
. The other end of the valve chamber
38
communicates with a high-pressure fluid supply that controls the positioning of the valve member
34
. The high-pressure fluid supply is typically either high-pressure lubricating oil or refrigerant that has been discharged from the compressor.
To load the compressor
10
, the valve
14
is closed by flooding the valve chamber
38
with high-pressure fluid. The fluid in the valve chamber
38
forces the valve member
34
toward the screw housing
22
until the valve member
34
abuts the screw housing
22
, as shown in FIG.
1
. When the valve member
34
is in the position shown in
FIG. 1
, there is no communication, and therefore no pressure equalization, between the suction cavity
46
and the compression chamber
24
. Because there is no pressure equalization, the entire working length of the screws
15
,
16
is utilized and maximum compression loading is generated by the compressor
10
.
To unload the compressor
10
, the valve
14
is opened by draining the fluid from the valve chamber
38
. The high-pressure fluid in the compression chamber
24
forces the valve member
34
away from the screw housing
22
, as shown in FIG.
2
. When the valve member
34
is in the position shown in
FIG. 2
, the passageway
42
provides communication, and therefore pressure equalization, between the compression chamber
24
and the suction cavity
46
. This pressure equalization reduces the effective working length of the screws
15
,
16
, thereby reducing the compression load generated by the compressor
10
.
SUMMARY OF THE INVENTION
For the axial unloading valve
14
to function properly, the valve member
34
must be carefully manufactured and installed.
FIG. 3
shows a prior art valve member
34
in greater detail. The valve member
34
is substantially cylindrical and includes opposing first and second axial sealing surfaces
50
and
54
, respectively. A radial sealing and positioning surface
58
extends between the axial sealing surfaces
50
and
54
.
With this symmetrical configuration, the valve member
34
could be installed in the valve chamber
38
in two ways. Therefore, both the first and the second axial sealing surfaces
50
and
54
must be machined to tight axial run-out tolerances to ensure that, regardless of how the valve member
34
is installed, proper axial sealing occurs when the valve
14
is closed. The term “run-out” is well-known to those in manufacturing and in this situation is generally understood to refer to the perpendicularity between a longitudinal axis
62
and each of the axial sealing surfaces
50
and
54
. In addition to sealing concerns, the tight run-out tolerance ensures that no portion of the valve member
34
will interfere with the 5 screws
15
,
16
when the compressor
10
is operating at full load (i.e., when the valve
14
is closed). This is especially important on compressors having small axial screw endplay with respect to the discharge housing
26
. Maintaining the tight axial run-out tolerances requires expensive precision machining and, because both axial sealing surfaces
50
and
54
must be tightly toleranced, two separate machine setups are required for two separate precision machining operations. This significantly increases the manufacturing cost of the valve member
34
.
One way to eliminate the need for two tightly-toleranced axial sealing surfaces
50
,
54
on the valve member
34
is to change the design.
FIG. 4
illustrates an alternative prior art valve member
66
that has only one axial sealing surface
70
. Additionally, the radial sealing surface
74
and the radial positioning surface
78
are separate surfaces. This ensures that the valve member
66
can only be installed in one way, thereby eliminating the need for a second axial sealing surface with a tight run-out tolerance.
While only one precision machining setup is necessary for achieving the desired run-out tolerance on the single axial sealing surface
70
, a separate machining operation is still required to form the radial positioning surface
78
. This second operation need not be precision machining, but nonetheless requires a second machine setup. The two separate machine setups required to manufacture the different radial surfaces
74
and
78
can create tolerance stack-up problems and often mandate the use of a gasket
82
to prevent leakage. The use of the gasket
82
also adds to the cost of the compressor
10
and increases the number of parts that may require periodic replacement.
The present invention provides an improved valve member for an axial unloading lift valve. The improved valve member has only one axial sealing surface requiring a tight run-out tolerance. Therefore, only one machine setup is needed to produce the sealing surfaces of the improved valve member. Additionally, the valve member of the present invention includes features that facilitate proper assembly and ensure that the valve member is properly installed. No gaskets are required to seal the valve member. Thus, the valve member of the present invention provides a less-expensive and more reliable valve member than the prior-art valve members described above.
More specifically, the invention provides a screw compressor having a housing, a drive screw supported by the housing, and an idler screw supported by the housing. The drive screw and idler screw assembly have a low-pressure end and a high-pressure end. The drive screw, driven by an outside force, drives the idler screw, to which the drive screw is operably engaged. Rotation of the screws moves a fluid from the low-pressure end to the high-pressure end.
Anderson Gregory Gordon
Sjoholm Lars Ivan
Michael & Best & Friedrich LLP
Thermo King Corporation
Vrablik John J.
LandOfFree
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