Centrifugal compressor

Details Centrifugal compressor Centrifugal compressor

1999-08-17

2001-05-22

Verdier, Christopher (Department: 3745)

Rotary kinetic fluid motors or pumps

Bearing, seal, or liner between runner portion and static part

Between blade edge and static part

C415S122100, C417S244000, C417S423500

Reexamination Certificate

active

06234749

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a centrifugal compressor, and more particularly to a centrifugal compressor having an abradable layer embedded in a compressor casing inner wall and cut by a rotating impeller.
2. Description of the Related Art
Various centrifugal compressors are known in the art. One type of centrifugal compressor includes a casing, an impeller housed in the casing, and an abradable layer provided on an inner surface of the casing such that it is cut by the impeller rotating in the casing. As the compressor is activated and the impeller rotates, the clearance between the impeller and the abradable layer is eventually adjusted to an optimum value. This type of centrifugal compressor improves an operation efficiency. Such centrifugal compressor is disclosed in, for example, Japanese Patent Application, Laid-Open Publication No. 6-257454 published on Sep. 13, 1994.
Referring to
FIG. 6
of the accompanying drawings, illustrated is another conventional centrifugal compressor. This is a multi-stage centrifugal compressor
61
including a casing
64
and two impellers
62
and
63
mounted on ends of a common rotating shaft
65
. If the teaching of Japanese Patent Application, Laid-Open Publication No. 6-257454 is applied to the illustrated centrifugal compressor
61
, two abradable layers (not shown) will be embedded in the casing inner walls
66
and
67
in the vicinity of both the impellers
62
and
63
respectively.
However, the abradable layer is expensive so that providing the abradable layers for the two impellers
62
and
63
will raise a manufacturing cost of the compressor
61
.
Incidentally, the abradable layer taught in Japanese Patent Application, Laid-Open Publication No. 6-257454 also extends along the impeller
62
,
63
from its front edge
72
to rear edge
79
.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a multi-stage centrifugal compressor which can realize both cost reduction and efficiency improvement.
It is another object of the present invention to provide a centrifugal compressor which can realize both surge limit extension toward the lower flow rate range and compression efficiency improvement.
According to one aspect of the present invention, there is provided a centrifugal compressor including a single rotating shaft, a plurality of impellers mounted on the rotating shaft, an air path for introducing an air accelerated by a first (or upstream) impeller to subsequent (or downstream) impellers, a casing for accommodating the plurality of impellers, and an abradable layer provided in the casing such that it faces the subsequent impellers and is cut by these impellers. This compressor is a single-shaft multi-stage centrifugal compressor. The abradable layers are only provided for the downstream impellers since the effect of the abradable layer is significant when provided for the downstream impellers but not significant when provided for the upstream impeller. When compared with a centrifugal compressor having abradable layers for all the impellers, the compressor according to the invention demonstrates substantially the same efficiency while reducing the manufacturing cost. The abradable layer is expensive so that eliminating the abradable layer for the first upstream impeller contributes to cost reduction.
The inventors made experiments on a multi-stage centrifugal compressor equipped with abradable layers and learned by these experiments that providing the abradable layer only for the downstream impeller will be sufficient. In other words, it is unnecessary to provide an abradable layer for the upstream impeller.
In the arrangement shown in
FIG. 6
, the rotational speed of the upstream impeller
62
is equal to that of the downstream impeller
63
since these impellers
62
and
63
are mounted on the mutual shaft
65
. Therefore, the volumetric flow rate of the impeller
63
is smaller than that of the impeller
62
. As a result, as illustrated in
FIG. 8
of the accompanying drawings, the exit width W2 of the downstream impeller
63
becomes smaller than that W1 of the upstream impeller
62
. When the impeller outlet width W becomes smaller, the impeller-casing clearance &dgr; becomes larger relatively. Accordingly, the ratio &dgr;/W indicative of influence of leakage by the clearance &dgr; at the impeller outlet width W is greater for the downstream impeller
63
than the upstream impeller
62
when &dgr;1=&dgr;2.
Thus, the inventors concluded that providing the abradable layer only for the downstream impeller
63
is enough in view of efficiency improvement since the influence of leakage by the clearance &dgr; is relatively great for the downstream impeller
63
and relatively small for the upstream impeller
62
. Of course, dispensing with one of the two expensive abradable layers will also result in manufacturing cost reduction.
Referring back to
FIG. 6
, the rotating shaft
65
is supported by bearings
69
such that it is allowed to slide in its axial direction to a certain extent in order to suppress vibrations and/or for other reasons. Since the impellers
62
and
63
are mounted on the opposite ends of the rotating shaft
65
with the backs of these impellers facing each other, a high speed flow of air passing the downstream impeller
63
causes the impeller
63
to be attracted toward the casing inner wall
67
. Therefore, the shaft
65
moves to the right in the illustration within the tolerated range.
As a result, even if an abradable layer was provided on an inner wall
66
of the casing
64
near the upstream impeller
62
, the impeller
62
would rotate without contacting the abradable layer since the rotating shaft
65
would be caused to move to the right during operation and the impeller
62
would leave the abradable layer. On the contrary, the downstream impeller
63
is forced against the casing inner wall
67
during operation so that this abradable layer demonstrates its effect in a significant manner.
From this fact also, it can be said that providing the abradable layer only for the downstream impeller
63
suffices in terms of efficiency improvement.
The compressor may only have two impellers, these impellers may be mounted on the mutual shaft such that their backs face each other, and the abradable layer may be provided for the single downstream impeller only. The rotating shaft may be supported such that it is slidable in an axial direction of the shaft within a certain range (e.g., 0.2 mm) relative to the casing.
A pinion may be mounted on the rotating shaft, a large gear may be provided to engage the pinion, and a drive motor may be provided to activate the large gear.
The compressor casing may include an inducer block which defines an intake air path for the downstream impeller, and the abradable layer may be provided at a front end of the inducer block. The abradable layer may be made from Teflon™ mixed with silica (quartz) or mica.
According to the second aspect of the present invention, there is provided a centrifugal compressor including a casing, an impeller housed in the casing, and an abradable layer embedded in the casing inner wall and subjected to impeller blades such that it extends in the range of M-m, which M and m satisfy the equation of 0.2≦m/M≦0.4 where m represents that length on the casing inner wall which corresponds to length of the impeller blade from its front edge to an arbitrary position, and M represents that length on the casing inner wall which corresponds to length of the impeller blade from the front edge to the rear edge.
With this design, there is no abradable layer near the front edge of the impeller blade. Specifically, if the impeller blade length is expressed 100%, then the abradable layer does not extend in the 20-40% area in the vicinity of the front edge of the impeller blade. Consequently, a certain clearance (e.g., 0.2 to 0.4 mm) is formed between the casing inner wall and the impeller blade. On the other hand, there is substantially no clearance between the casi

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