A multistage centrifugal compressor with forward swept, back swept or un-swept impeller blades, in combination

Abstract

The centrifugal compressor comprises a plurality of compressor stages, each including an impeller. To reduce the number of stages or to reduce the compressor speed required to achieve a desired pressure ratio, the impellers include at least two of the following: a set consisting of at least one forward swept impeller; a set consisting of at least one back swept impeller; and a set consisting of at least one un-swept impeller.

Description

A MULTISTAGE CENTRIFUGAL COMPRESSOR WITH FORWARD SWEPT, BACK SWEPT OR UN-SWEPT IMPELLER BLADES, IN COMBINATIONDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure concerns improvements to centrifugal compressors.BACKGROUND ART

[0002] Centrifugal compressors are used to compress gaseous fluids in a variety of applications. In general terms, a centrifugal compressor comprises at least one impeller including a plurality of impeller blades extending from a generally axial inlet to a generally tangential outlet of the impeller. For high pressure ratios, multi-stage compressors are used, which include a plurality of sequentially arranged centrifugal impellers. Each impeller is surrounded by a diffuser, where kinetic energy of gas discharged by the impeller is converted into pressure energy. The slowed-down and pressurized gas is guided by a return channel from the diffuser of an upstream impeller toward the inlet of a downstream impeller. The most downstream impeller, along the gas flow path, is fluidly coupled to a discharge volute through the last diffuser.

[0003] Depending on the shape of the impeller blades at the tangential outlet, centrifugal compressor impellers are classified in back swept impellers and forward swept impellers. A back swept impeller (aka backward swept impeller) is characterized by impeller blades the inclination whereof at the impeller outlet is opposite the direction of rotation of the impeller. A forward swept impeller is characterized by impeller blades which are inclined in a direction concordant with the speed of the impeller tip. Un-swept impellers, aka radial impellers, are sometimes also applied, wherein the inclination of the impeller blade at the tangential outlet of the impeller is oriented in a radial direction, i.e. orthogonal to the rotation axis of the impeller.

[0004] While forward swept impellers provide higher pressure ratios, they have a lower flow stability and their use is therefore nowadays limited to few applications, mainly in single-stage compressors.

[0005] Gases having low molecular weight, such as hydrogen, or other gases of higher molecular weight, e.g. equal to or lower than 10 g / mol, pose some challenges in the compressor designs, since a large number of compressor stages or very high rotational speeds at the blade tips are needed to achieve high pressure ratios. In this respect, particularly challenging is the design of hydrogen compressors.

[0006] To provide high compressor stability, back swept compressor impellers are used, which implies the need of a large number of impellers to achieve the required total pressure ratio.SUMMARY

[0007] The present disclosure is aimed at proposing improvements to multi-stage centrifugal compressors aimed at alleviating or overcoming the limitations and drawbacks of centrifugal compressors of the current art.

[0008] A novel centrifugal compressor is disclosed herein, which comprises: a casing; a gas inlet a gas outlet; and a rotor housed for rotation in the casing. The rotor comprises a plurality of impellers arranged in sequence along a flow path extending from the gas inlet to the gas outlet. The plurality of impellers comprises a combination of at least two of the following sets of impellers having different blade inclinations at the tangential outlet of the impeller. In some embodiments, the compressor comprises a set consisting of at least one forward swept impeller and a set consisting of at least one back swept impeller. In other embodiments, the compressor comprises a set of impellers consisting of at least one forward swept impeller and a set consisting of at least one un-swept impeller. In yet another embodiment, the compressor can include a set consisting of at least one back swept impeller and one un-swept impeller.

[0009] In general, the multi-stage compressor can include a first impeller and a second impeller, wherein the first impeller is a forward swept impeller and the second impeller is a back swept impeller, or the first impeller is an un-swept impeller and the second impeller is a forward swept impeller, or the first impeller is an unswept impeller and the second impeller is a back swept impeller. In each combination, the first impeller can be arranged upstream or downstream of the second impeller, with respect to the general direction of flow of the process gas through the compressor.

[0010] In some embodiments the compressor can include at least one impeller of each category, i.e. a back swept impeller, a forward swept impeller and an un-swept (i.e. radial) impeller in combination and in any sequence.

[0011] In some embodiments, the compressor comprises a combination of at least two of the following sets: a set of forward swept impellers, a set of back swept impellers; a set of un-swept (i.e. radial) impellers. At least one of said at least two sets comprises a plurality of impellers. In some embodiments, each set of impellers comprises a plurality of impellers. The total number of impellers may depend on the pressure ration required and / or on the molecular weight of the process gas.

[0012] Specifically, disclosed herein is a multi-stage centrifugal compressor including a set of forward swept impellers, and a set of back swept impellers; wherein the set of back swept impellers is positioned downstream of the set of forward swept impellers in the direction of flow of process gas through the centrifugal compressor.

[0013] The combination of impellers having different orientations of the blades at the outlet results in improved effectiveness, specifically when the process gas has a low molecular weight. A higher pressure ratio can be achieved with less stages, or the same pressure ratio can be reached with the same number of stages, compared with compressors of the current art, where all impellers have the same blade orientations, typically back swept blades. Alternatively, the same pressure ratio can be achieved with the same number of stages but rotating at a lower rotational speed, compared to compressors of the current art.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Reference is now made briefly to the accompanying drawings, in which:Fig. l illustrates a longitudinal sectional view of a multi-stage centrifugal compressor;Fig.2 illustrates a back swept impeller;Fig.3 illustrates a forward swept impeller;Fig.4 illustrates a forward swept impeller with splitter; andFig.5 illustrates a pressure ratio vs. flowrate comparative diagram.DETAILED DESCRIPTION

[0015] Fig. l illustrates a sectional view of a multi-stage centrifugal compressor 1 along a plane containing the rotation axis A-A of the centrifugal compressor 1. The structure of the compressor illustrated in Fig.1 is by way of example only, and different designs are possible. For instance, a different number of stages can be foreseen. In Fig. l an in-line compressor is shown, wherein the process gas flows from an inlet to an outlet in a single direction through all the stages. In other embodiments the compressor can be a back-to-back compressor, for instance.

[0016] In the embodiment of Fig.l the centrifugal compressor 1 comprises a casing 3 having a gas inlet 5 and a gas outlet 7. The gas inlet 5 is fluidly coupled with an inlet volute 5A and the gas outlet is fluidly coupled with a discharge volute 7A.

[0017] The centrifugal compressor 1 comprises a plurality of compressor stages 9. In the non-limiting exemplary embodiment of Fig.l, the compressor 1 comprises eight compressor stages, labeled from 9.1 to 9.8, which are arranged along a gas flow path extending from the gas inlet 5 to the gas outlet 7 in the direction of flow of the gas processed by the compressor 1.

[0018] Each compressor stage includes an impeller 13 and a diffuser 14. Between each pair of sequentially arranged compressor stages from 9.1 to 9.7 a respective return channel 16 is arranged, to return the partially compressed gas from the diffuser of an upstream compressor stage to the inlet of the next impeller. The return channel of the most downstream compressor stage 9.8 is fluidly coupled to the discharge volute 7A.

[0019] The impellers are grouped to form a rotor 15. In the embodiment of Fig.1 the rotor comprises a shaft 15 A, which is supported between bearings 17, 19. Sealing arrangements 21 and 23 are located inboard of the bearings 17, 19.

[0020] In some embodiments, according to the present disclosure, the impellers 9 of the multi-stage centrifugal compressor 1 include at least one forward swept impeller and at least one back swept impeller. In embodiments, the multi-stage compressor 1 includes a set comprising a plurality of forward swept impellers and a set comprising a plurality of back swept impellers.

[0021] An exemplary back swept impeller 9 (or backward swept impeller) is shown in Fig.2 and labeled 9B. Reference number 25 indicates a hub of the impeller 9B and reference number 27 indicates each blade of the impeller 9B. Arrow co indicates the direction of rotation of the impeller 9B. Flow passages between adjacent blades 27 are labeled 29. VR is the relative speed of the gas exiting each flow passage 25 at the tip of each blade 27, VT is the tangential speed of the blade tip and V is the absolute speed of the gas. The impeller 9B is a back swept impeller. The inclination of the blades 27, and therefore the speed vector VR are oriented opposite the rotation speed VT. The blades 27 have a convexity facing forward in the direction of rotation.

[0022] An exemplary forward swept impeller is shown in Fig.3 and is labeled 9F. Similarly to Fig.2, in Fig.3 reference number 25 indicates a hub of the impeller 9F and reference number 27 indicates each blade of the impeller 9F. Arrow co indicates the direction of rotation of the impeller 9F. Flow passages between adjacent blades 27 are labeled 29. VR is the relative speed of the gas exiting each flow passage 25 at the tip of each blade 27, VT is the tangential speed of the blade tip and V is the absolute speed of the gas. The impeller 9F is a forward swept impeller. The inclination of the blades 27, and therefore the speed vector VR are oriented in the same direction of the rotation speed VT. The blades 27 of impeller 9F have a concavity facing forward in the direction of rotation.

[0023] By way of example, Fig.4 illustrates a forward swept impeller with splitter blades. This impeller has a set of shorter blades labeled 27S interspersed between longer blades labeled 27L. In this embodiment, the impeller with splitter blades is a forward swept impeller, but in other embodiments, the same splitter design can be used in a back swept impeller 9B.

[0024] In some embodiments, the forwards swept impellers 9F are positioned in a more upstream position and the back swept impellers 9B are positioned in a more downstream position, i.e., the forward swept impellers 9F are used for the first compressor stages, between the gas inlet volute 5 A and the back swept impeller 9B. These latter are used for the last compressor stages, between the forward swept impellers 9F and the discharge volute 7A. Providing one or more upstream forward swept impeller can be beneficial in terms of flow rate stabilization, combined with a pressurestabilization offered by the one or more downstream back swept impeller. As a matter of fact, a forward swept impeller reacts to a flow rate reduction with a pressure reduction. Thus, in response to a flow rate reduction, the back swept stage arranged downstream of the forward swept stage will receive a less dense gas at a substantially constant volumetric flowrate. Consequently, the point of operation of the back swept impeller will not shift to the left of the flowrate-vs-pressure ratio diagram. This will result in a more stable operation of the compressor.

[0025] In some embodiments, the multi-stage compressor 1 can include from 1 to 5 forward swept impellers, i.e. impellers with forward swept blades, and from 1 to 5 back swept impellers, i.e. impellers with back swept blades. In some embodiments, an increased stability of operation can be achieved if the number of back swept impellers 9B is higher than the number of forward swept impellers 9F. For instance, between one half and two thirds (for instance five or six) of the impellers are back swept impellers 9B and one third (for instance 3) of the impellers are forward swept impellers 9F.

[0026] Advantages of the combined forward swept and back swept multi-stage centrifugal compressor 1 can be best appreciated from the diagram of Fig.5.

[0027] In the diagram of Fig. 5 the pressure ratio vs. the flowrate is plotted for a plurality of configurations of the multi-stage compressor 1. Curve CB shows the pressure ratio vs. flowrate for a 9-stages multi-stage centrifugal compressor including only back swept blades 9B as a function of the flowrate at a given rotational speed. Curve Cl illustrates the pressure ratio for a compressor having eight stages, whereof the first three include each a forward swept impeller and the last five include each a back swept impeller. Similarly, curve C2 illustrates the pressure ratio vs. flowrate in a multi-stage compressor having nine stages, whereof the first three include forward swept impellers and the last six include back swept impellers.

[0028] As can be appreciated from the curves plotted in Fig.5, the combined back swept an forward swept compressors achieve higher pressure ratios compared to the entirely back swept centrifugal compressor (curve CB), still maintaining a stable operating curve, i.e., a monotonically decreasing curve.

[0029] The increased pressure ratio that can be achieved with the combination of forward swept and back swept impellers is particularly beneficial when low molecular weight gases must be compressed such as for instance, but not exclusively, hydrogen, or other gases, e.g. having a molecular weight between 1 and 10 g / mol.

[0030] A higher pressure ratio can be achieved with the same number of compressor stages, compared with a multi-stage of the current art which uses only back swept impellers. Conversely, the same pressure ratio can be achieved with a smaller number of stages and therefore with a compressor having a smaller footprint.

[0031] While in the above description reference is made to a compressor including an upstream set of forward swept impellers and a downstream set of back swept impellers, in other embodiments, different combinations can be foreseen. For instance, a set of one or more upstream back swept impellers can be followed by a set of one or more downstream forward swept impellers.

[0032] In yet further embodiments, the multistage compressor can include a sequence of more than two sets of back swept impeller(s) (shortly BW) and forward swept impeller(s) (shortly FW). Each set can in turn include one or more impellers. For instance, the compressor may include any one of following sequences of impeller sets: BW-FW-BW; FW-BW-FW.

[0033] In some embodiments, the compressor can include one or more sets of impellers, each including one or more radial (i.e. un-swept) impellers, in combination with one or more forward swept impeller sets and / or one or more back swept impeller sets, each including one or more impellers.

[0034] Referring to Figs. 2 and 3, an un-swept impeller is an impeller wherein the direction VR is radial.

[0035] Thus, for example the compressor can include a set of (one or more) back swept impellers followed by a set of (one or more) radial, i.e. un-swept, impellers, or vice-versa. In other embodiments, the compressor can include a combination of a set of (one or more) forward swept impellers, followed by a set of (one or more) un-swept impellers, in turn followed by a set of (one or more) back swept impellers.

[0036] In yet further embodiments, a combination of a set of (on or more) forward swept impellers followed by a set of (one or more) un-swept impellers is not ruled out.

[0037] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. A multi-stage centrifugal compressor comprising: a casing; a gas inlet a gas outlet; and a rotor housed for rotation in the casing; wherein the rotor comprises a plurality of impellers arranged in sequence along a flow path extending from the gas inlet to the gas outlet; wherein the plurality of impellers comprises a combination of at least: a) a set consisting of at least one forward swept impeller; and a set consisting of at least one back swept impeller; b) or a set consisting of at least one forward swept impeller; and a set consisting of at least one un-swept impeller; or c) a set consisting of at least one back swept impeller and a set consisting of at least one un-swept impeller, positioned downstream of the set consisting of said at least one back swept impeller in a direction of flow of process gas through the centrifugal compressor.

2. The multi-stage centrifugal compressor of claim 1, comprising a set consisting of at least one un-swept impeller, in combination with said set consisting of at least one forward swept impeller and said set consisting of at least one back swept impeller.

3. The multi-stage centrifugal compressor of claim 1 or 2, comprising said set consisting of at least one forward swept impeller, and said set consisting of at least one back swept impeller; wherein the set of at least one back swept impeller is positioned downstream of the set consisting of at least one forward swept impeller in the direction of flow of process gas through the centrifugal compressor.

4. The multi-stage centrifugal compressor of claim 2, wherein said set consisting of at least one un-swept impeller is positioned between the set consisting of at least one forward swept impeller and the set consisting of at least one back swept impeller.

5. The multi-stage centrifugal compressor of claim 2, wherein the setconsisting of at least one un-swept impeller is positioned upstream of the set consisting of at least one back swept impeller in the direction of flow of process gas through the centrifugal compressor.

6. The multi-stage centrifugal compressor of claim 1, comprising said set consisting of at least one forward swept impeller, and said set consisting of at least one back swept impeller; wherein the set consisting of at least one back swept impeller is positioned upstream of the set consisting of at least one forward swept impeller in the direction of flow of process gas through the centrifugal compressor.

7. The multi-stage centrifugal compressor of claim 6, further comprising a set consisting of at least one un-swept impeller positioned between the set consisting of at least one forward swept impeller and the set consisting of at least one back swept impeller.

8. The multi-stage centrifugal compressor of any one of the preceding claims, comprising said set consisting of at least one forward swept impeller and said set consisting of at least one back swept impeller; comprising a number of forward swept impeller smaller than a number of back swept impellers.

9. The multi-stage centrifugal compressor of claim 8, wherein the number of forward swept impellers is comprised between 1 and 5, preferably between 2 and 3; and the number of back swept impellers is comprised between 2 and 10, preferably between 3 and 6.

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