Cover for a compressor plate

Abstract

A cover 109 for a compressor plate 108 of a compressor, the compressor plate at least partially defining a flow passage of the compressor to receive fluid from a first compressor stage of the compress

Description

The present disclosure relates to a cover for a compressor plate, for example a cover for a compressor plate that forms at least part of an interstage passage of a compressor. BACKGROUND In general, for machinery such as compressors, achieving a high performance ratio is important. SUMMARY OF THE INVENTION Examples of this disclosure focus on reducing a drop in the performance of a compressor. In some compressors, fluid is compressed in a first compressor stage and then caused to flow through an interstage passage toward a volute or diffusion chamber. Alternatively or additionally in some compressors, fluid is compressed in a first compressor stage and then caused to flow through an interstage passage (in which it undergo diffusion at a first diffusion stage) to a second compressor stage where the fluid is compressed again (and then possibly undergoing diffusion again at a second diffusion stage) depending on the type of compressor. Some compressors are “single-stage” compressors comprising only one compressor stage whereas some compressors are “two-stage” compressors comprising only a first compressor stage and a second compressor stage, however other compressors are “multi-stage” compressors comprising at least two compressor stages. In this disclosure, by a “first compressor stage” it is meant any stage of a compressor, the label “first” being nomenclature only. Hence any compressor stage may be “first” compressor stage even though having regard to the use of the compressor it may not be first stage at which the fluid is compressed (e.g. it may be a compressor stage subsequent to the first). Similarly, in this disclosure, by “first” and “second” compressor stages it is meant any stages of a compressor, the labels “first” and “second” being nomenclature only. Hence any two compressor stages may be “first” and “second” compressor stages even though having regard to the use of the compressor they may not be first and second stages at which the fluid is compressed having regard to the flow of fluid through the compressor. According to this disclosure, fluid to be compressed enters a compressor and passes through a first compressor stage where it is compressed and then enters an “interstage” flow passage. The interstage flow passage may comprise a diffuser passage comprising one or more diffuser walls. Compressed fluid passing through such a diffuser passage may be diffused (the diffusion process increasing the fluid’s pressure by causing the velocity of the fluid to be reduced). Depending on the type of compressor, the diffused fluid enters a volute (e.g. for a single-stage compressor) or a further compressor stage (e.g. a second compressor stage for a multi-stage compressor) in which the fluid is compressed and diffused again. The compressed fluid exits the second compressor stage and either exits the compressor or passes to a further compressor stage. To assemble a compressor, a compressor plate such as a diffuser plate forming part of the interstage passage (e.g. a first stage diffuser) may be assembled to form part of a compressor unit (e.g. part of the compressor, part of a component of the compressor or part of a compressor housing). Additionally, the compressor plate may function as a seal plate to reduce bleed, or leakage, flow between a first compressor stage and a volute, or between first and second compressor stages, the seal plate being to seal parts of the compressor (e.g. to seal the first stage and a volute, or to seal the first and second stages). Such a seal plate may be a part, or component, of the compressor. The diffuser plate and the seal plate may be the same plate depending on the design of the compressor and in this example one side of the seal plate may be part of the interstage passage, receiving fluid from the first compressor stage to diffuse it. However, the way any compressor plate such as a seal plate and / or a diffuser plate and / or or any other type of compressor plate are assembled as part of the compressor may introduce the potential for a loss of compressor performance depending on how they are assembled as part of the compressor. The present disclosure aims to reduce such potential performance losses. According to this disclosure there is provided a cover for a compressor plate of a compressor. The compressor plate at least partially defines a flow passage of the compressor to receive fluid from a first compressor stage of the compressor, the compressor plate being attachable to a portion of the compressor via a fastener extending through the compressor plate from a first side to a second side. The cover has a first surface and a second surface and the cover is attachable to the compressor plate such that the second surface of the cover is configured to cover the first side of the compressor plate such that the portion of the compressor plate through which the fastener is configured to extend is covered by the cover, and such that the first surface of the cover forms a surface to receive compressed fluid from the first stage of the compressor and forms part of a diffuser for the compressor. The compressor may be multi-stage compressor and the compressor plate may be a seal plate, the seal plate configured to prevent fluid from leaking from the first compressor stage to a second compressor stage of the multi-stage compressor. In this example the cover is therefore a cover for a seal plate of a multi-stage compressor (e.g. a two-stage compressor). The seal plate may be configured to prevent fluid from leaking from a second compressor stage of the compressor to a first compressor stage of the compressor, the seal plate having a first surface configured to receive fluid from the first compressor stage, the seal plate being attachable to a portion of the compressor via a fastener extending through the seal plate from a first side to a second side. The cover has a first surface and a second surface and is attachable to the seal plate such that the second surface of the cover is configured to cover the first side of the seal plate such that the portion of the seal plate through which the fastener is configured to extend is covered by the cover, and such that the first surface of the cover forms a surface to receive compressed fluid from the first stage of the compressor and forms part of a first stage diffuser for the compressor. The cover may be substantially ring-shaped and may comprise a central bore configured to receive a portion of the compressor. The cover may comprise at least one flange protruding away from an edge of the cover in an axial direction of the cover. At least one flange may extend the entire circumference of the cover. At least one flange may be configured to enable the cover to be attached to the compressor plate via a snap or interference fit. The first surface of the cover may comprise a smooth surface. The first surface of the cover may comprise a continuous surface. The first surface of the cover may comprise a low coefficient of friction. According to an example of this disclosure there is provided a compressor unit comprising a compressor plate and the cover as claimed in any preceding claim, wherein the compressor plate is attached to a portion of the compressor unit via the fastener extending through the compressor plate from the first side to the second side, the cover being attached to the compressor plate such that the second surface of the cover covers the first side of the compressor plate such that the portion of the compressor plate through which the fastener extends is covered by the cover and the first surface of the cover forms a surface to receive compressed fluid from the first stage of the compressor and forms part of a diffuser for the compressor. In examples where the compressor plate is a seal plate, the compressor unit therefore comprises a seal plate and the cover described above. The seal plate may be attached to a portion of the compressor unit via the fastener extending through the seal plate from the first side to the second side, and the cover may be attached to the seal plate such that the second surface of the cover covers the first side of the seal plate such that the portion of the seal plate through which the fastener extends is covered by the cover and the first surface of the cover forms a surface to receive compressed fluid from the first stage of the compressor and forms part of a first stage diffuser for the compressor. The fastener may comprise a bolt, and the bolt may be inserted through the compressor plate from the first side to the second side such that the top surface of the head of the bolt is flush with the surface of the first side of the compressor plate. The cover may completely cover the first side of the compressor plate. The compressor unit may comprise a first stage comprising a rotatable compressor blade configured to compress fluid, wherein the compressor plate is mounted to the compressor unit to form part of a flow passage to receive fluid from the first compressor stage and to form a diffuser for the first compressor stage, wherein compressed fluid discharged from the first compressor stage is directed to the diffuser, the diffuser having a channel comprising the first surface of the cover, the channel being configured to diffuse fluid discharged from the first compressor stage. The compressor unit may be a multi-stage compressor and may comprise a first stage and a second stage and the compressor plate may be a seal plate. Each of the first and second stages may comprise a rotatable compressor blade configured to compress fluid. The seal plate may be mounted to the compressor unit in between the first and second compressor stages such that fluid leakage from the second compressor stage to the first compressor stage is prevented and to form a diffuser for the first compressor stage. Compressed fluid discharged from the first compressor stage may be directed to the diffuser, the diffuser having a channel comprising the first surface of the cover, the channel being configured to diffuse fluid discharged from the first compressor stage. The compressor unit may have a return channel comprising a second surface of the seal plate that is opposite the first surface of the seal plate, the return channel being configured to receive diffused fluid from the diffuser and to direct diffused fluid to the second stage of the compressor. According to an example of this disclosure there is provided a process for forming a portion of a compressor. The process comprises providing a compressor plate configured to at least partially define a flow passage for the compressor receive fluid from a first compressor stage, the compressor plate being attachable to a portion of the compressor via a fastener extending through the compressor plate from a first side to a second side. The process further comprises providing a cover having a first surface and a second surface, and attaching the cover to the compressor plate such that the second surface of the cover covers the first side of the compressor plate such that the portion of the compressor plate through which the fastener is configured to extend is covered by the cover and the first surface of the cover forms a surface to receive compressed fluid from the first stage of the compressor and forms part of a diffuser for the compressor. The compressor plate may be a seal plate and the compressor may be a multi-stage compressor in which case the process comprises providing a seal plate configured to prevent fluid from leaking from a second compressor stage of the compressor to a first compressor stage of the compressor. The seal plate has a first surface configured to receive fluid from the first compressor stage. The seal plate is attachable to a portion of the compressor via a fastener extending through the seal plate from a first side to a second side. The process comprises providing a cover having a first surface and a second surface, and attaching the cover to the seal plate such that the second surface of the cover covers the first side of the seal plate such that the portion of the seal plate through which the fastener is configured to extend is covered by the cover and the first surface of the cover forms a surface to receive compressed fluid from the first stage of the compressor and forms part of a first stage diffuser for the compressor. The cover may be attached to the compressor plate via an interference fit. The cover may be attached to the compressor plate via a friction fit. In this way, the presence of fasteners to secure the compressor plate does not lead to a performance drop, even if the compressor was assembled such that the fasteners (e.g. their heads) protruded into the diffuser area, as the cover covers the fasteners that would otherwise be in the diffuser area, and provides a smooth surface to form part of the diffuser flow passage to diffuse the fluid. Use of the cover according to the present disclosure therefore reduces performance losses in a compressor. BRIEF DESCRIPTION OF THE DRAWINGS Examples of the present disclosure will be described in detail with reference to the accompanying drawings, which should not be considered limiting, in which: Figure 1 shows a schematic cutaway of a compressor unit; Figures 2-4 show enlarged schematic cutaways of a portion of a compressor unit; Figure 5 shows a perspective view of an example cover; Figure 6 shows part of a perspective view of an example seal plate with a cover attached; Figure 7 shows an enlarged schematic cutaway of a seal plate with a cover attached; Figure 8 shows a perspective view of a seal plate attached to a portion of a compressor unit housing; and Figure 9 shows a flow chart of an example process. DETAILED DESCRIPTION These drawings should not be considered limiting, rather they are used for explaining and understanding the present disclosure. Figure 1 shows a compressor unit (or “compressor”) 100 illustrating, by means of the arrows, the flow of a fluid through the compressor. The compressor unit 100 in this example is a two-stage centrifugal, or radial, compressor and in this example is a two-stage compressor, meaning that the compressor comprises two compressor “stages,” a “first stage” and a “second stage” to be described later. This is for exemplary and illustrative purposes only; it should be appreciated that the compressor may be a multi-stage compressor comprising any number of stages greater than two. It should further be appreciated that the compressor may be a single-stage compressor having only one compressor stage. The cover described herein is suitable for all such compressors as will be described below. The compressor unit 100 comprises an inlet 101 via which fluid enters the compressor. The compressor comprises a housing 102 for the first stage of the compressor (housing 102 thereby being a first stage compressor housing) and a first stage compressor wheel, or impeller, 103. The first stage impeller 103 is mounted to a shaft 104 of the compressor such that rotation of the shaft 104 causes the first stage impeller 103 to rotate. The first stage of the compressor unit 100 therefore comprises at least the first stage impeller 103 and it will be appreciated that movement of the fluid through the first compressor stage compresses the fluid by virtue of the influence of the rotating first stage impeller 103. Specifically, rotation of the blades of the first stage impeller 103 increases the pressure and the flow rate of the fluid moving through the first compressor stage. The fluid then passes through an interstage passage 105, which is substantially U-shaped in this example and which extends away from the first compressor stage in a first direction, turns approximately 180 degrees, and extends in a second direction which in this example is opposite to the first direction. The interstage passage 105 is configured to receive fluid from the first compressor stage and direct fluid to the second compressor stage (although in examples where the compressor is not a multistage compressor the interstage passage 105 may be configured to direct fluid to a volute or other compressor component). The interstage passage 105 comprises a diffuser to be explained more later, the diffuser being configured to increase the fluid’s pressure by causing the velocity of the fluid to be reduced. This may be caused, at least in part, by the geometry of the interstage passage (e.g. its geometrical design). The compressor comprises a housing 106 for the second stage of the compressor (housing 106 thereby being a second stage compressor housing) and a second stage compressor wheel, or impeller, 107. The second stage impeller 107 is mounted to the shaft 104 of the compressor such that rotation of the shaft 104 causes the second stage impeller 106 to rotate. The second stage of the compressor unit 100 therefore comprises at least the second stage impeller 106 and it will be appreciated that movement of the fluid through the second compressor stage compresses the fluid by virtue of the influence of the rotating second stage impeller 106. Specifically, rotation of the blades of the second stage impeller 106 increases the pressure and the flow rate of the fluid moving through the second compressor stage. It will be further appreciated that the terminology of first and second compressor stages is referring to the fluid through the compressor being compressed first and second instances. In operation, the rotating blades of the compressor stages act to “pull” or drive the flow of fluid through the compressor from an inlet to an outlet thereof. The compressor unit 100 of Figure 1 may be termed a “stacked” compressor as the compressor stages are arranged (e.g. stacked) on top of each other. The interstage passage 105 is configured to direct fluid to the second compressor stage (e.g. to the second stage impeller 107). Put another way, the second stage impeller 107 is configured to receive fluid from the interstage passage 107. However in examples where the compressor is not a multi-stage compressor the interstage passage 105 may be configured to direct fluid to a different part of the compressor (e.g. toward an outlet and / or to a volute). It should be appreciated that the interstage passage 107, which may define a diffuser passage of the compressor, is defined at least in part by the compressor plate 108 which is assembled as part of the compressor unit 100 to define such a passage, the compressor plate (e.g. a side or surface thereof) may also form a wall of the diffuser passage of the interstage passage 107. If fluid were to leak, or bleed, from one part of the compressor unit 100 to another, such as from the second stage (e.g. from the chamber of the compressor unit 100 containing the second stage impeller 106) back to the first stage (e.g. to the chamber of the compressor unit 100 containing the first stage impeller 103) or from an outlet or volute to the first stage, then this could lead to loss of performance of the compressor. To mitigate such losses, the compressor unit 100 comprises a seal plate whose purpose is to seal the first and second compressor stages. In other words, the seal plate may be configured to prevent fluid from leaking from the second compressor stage to the first compressor stage. The compressor plate 108 forming part of the interstage flow passage 107 may be the seal plate (as in the Figure 1 example) and in this disclosure the two terms will be referred to interchangeably, although it should be appreciated that the compressor plate 108 defining the flow passage 107 (e.g. the diffuser flow passage) may or may not have a sealing function depending on the example. The seal plate 108 is to create a seal between the first and second compressor stages such that any instances of fluid bleeding or leaking from the second stage to the first stage is reduced. Such bleeding or leaking fluid may result from the pressure difference or imbalance between the end of the first compressor stage and the beginning of the second compressor stage, when fluid arrives at the inlet of the second compressor stage. Such bleeding or leaking may be termed parasitic loss. Hence, the seal plate 108 may be configured to limit, or reduce, that loss. For single-stage compressors the seal plate 108 may still perform a sealing function, this may be to seal between the first compressor stage and a downstream portion of the compressor (e.g. a volute thereof). The compressor plate 108 is assembled as part of (e.g. fixed, fastened, attached, mounted, and / or secured to) the compressor unit 100. The compressor plate 108 may be attached to part of a housing of the compressor unit 100 (e.g. part of a “compressor housing”). In one example the compressor plate 108 may be attached to the second stage compressor housing 106 (or a downstream part of the compressor housing for a single-stage compressor). In one example the compressor plate 108 may be attached to part of the compressor unit 100 via a fastener (not shown in Figure 1). In such examples, if the fastener protruded into the interstage passage this may present an impediment that would affect the flow of fluid therethrough. As part of the interstage passage forms a diffuser for the first stage compressed fluid, such a protruding element may affect the ability of the compressor to diffuse the first-stage compressed fluid, and may therefore affect the performance of the compressor. Even if the fasteners were recessed into the compressor plate 108 this may still create discontinuities in the surface of the seal plate and impede the seal plate’s ability to diffuse the first stage compressor fluid. According to the disclosure there is provided a cover 109 for the compressor plate 108 which will be described more fully below, which aims to reduce losses and / or inefficiencies and / or drops in performance of the compressor. The cover 109 not only covers the compressor plate 108 but forms a surface that becomes part of the diffuser (e.g. part of the interstage passage for diffusing the first-stage fluid) of the compressor. Figure 2 shows part of the compressor unit 100 of Figure 1 in more detail. Figure 2 shows that fluid discharged from the first stage of the compressor, e.g. fluid that has been compressed by the first stage impeller 103 passes to the interstage passage 105 which comprises a first stage diffuser 110. The geometry of the fist stage diffuser 110 of the interstage passage 105 causes the compressed fluid from the first stage to experience a reduction in velocity and therefore an increase in pressure. The diffuser 110 comprises a flow passage, and the interstage passage 105 comprises the diffuser flow passage. The first stage diffuser flow passage comprises a first surface 109a of the cover 109, as shown in Figure 2. A second surface 109b of the cover 109 faces the compressor plate 108. The compressor plate 108 has two sides and it will be appreciated that the cover 109 covers one side of the compressor plate such that one side of the cover 109 abuts one side of the compressor plate 108. The compressor plate is configured to engage with the first impeller 130 via a tooth-liked engagement although any other engagement may be used. After the first stage diffuser flow passage 110 the interstage flow passage 105 comprises a return channel 111. It will be appreciated from Figure 2 that the return channel 111 is a flow passage that comprises a second surface 108b of the compressor plate 108. The compressor plate 108 may comprise a number of vanes (one of which is shown in Figure 2, denoted 108-1) whose purpose is to direct the fluid flow through the return channel and toward the second compressor stage (although in examples where the compressor is a single-stage compressor the vanes may direct the fluid flow through the return channel and to a different downstream compressor component, such as a volute). The vanes 108-1 may therefore be referred to as return channel vanes. The compressor plate 108 may not comprise vanes however, whether it does depends on the implementation. The cover 109 comprises a flange 109c and it will be appreciated that the return channel 111 comprises the flange 109c in this example. Put another way, a side wall or wall of the return channel 111 comprises an outer surface of the flange 109c of the cover 109. The flange 109c extends in a direction that is orthogonal, or perpendicular, to the main body of the cover 109 and the first and second sides and surfaces thereof. The flange 109c extends in the axial direction whereas the main body of the cover extends in the circumferential and radial directions. It should however be appreciated that in some examples the cover 109 may not comprise a flange. Herein, when the figures show a space between the cover 109 and the compressor plate 108 this should be understood as purely exemplary to illustrate the disclosure. When the disclosure is implemented, it will be appreciated that these surfaces may abut one another and be in direct contact. Figure 3 shows a further enlarged portion of Figure 1, and in a different cross-section showing the vane 108-1 of the compressor plate 108, the compressor plate having first and second sides, 108a and 108b respectively, the first and second sides 108a, 108b of the compressor plate 108 being synonymous with first and second surfaces 108a, 108b of the compressor plate 108. Figure 3 shows how the cover 109 covers the first side 108a of the compressor plate 108. Figure 3 also shows how the flange 109c of the cover 109 engages a side portion 108c, or side surface, of the compressor plate 108, the side surface 108c being orthogonal, or perpendicular, to the first and second surfaces 108a, 108b. Engagement between the flange 109c and the compressor plate 108 may facilitate installation of the cover plate 109 into or onto the compressor plate 108. For example, the flange 109c may enable the cover plate 109 to be attached to the compressor plate 108 via a snap fit or via an interference fit. Accordingly, the dimensions of the cover 109 may be such that the diameter of the flange (e.g. between two diametrically opposed points of the inside surfaces thereof) is approximately equal or equal to the diameter of the compressor plate (e.g. between two diametrically opposed points of the side 108c thereof). Figure 4 shows another enlarged portion of Figure 1, and in a different cross-section. Figure 4 shows that, in one example, the compressor plate 108 is secured to the compressor housing 106 (the second stage compressor housing in the example, although the compressor plate 108 may be secured to any part of the compressor housing) via means of a fastener 115. Figure 4 shows another vane 108-2 of the compressor plate 108. It will be appreciated from these figures that the compressor plate 108 comprises a first surface 108a that is configured to receive fluid from the first compressor stage and that the compressor plate 108 is attachable to a portion of the compressor (e.g. the compressor housing, e.g. the second stage compressor housing) via means of at least one fastener 115 that extends through the compressor plate from a first side 108a of the compressor plate to a second side 108b of the compressor plate. It will be further be appreciated that the cover 109 has a first surface 109 and a second surface 109b and that the cover 109 is attachable to the compressor plate 108 such that the second surface 109b of the cover is configured to cover the first side 108a of the compressor plate 108 such that the portion of the compressor plate 108 through which the fastener 115 is configured to extend, denoted in Figure 4 by 108d, is covered by the cover 109 (e.g. completely covered by the cover 109). It will further be appreciated that, according to this arrangement, the first surface 109a of the cover 109 forms a surface to receive compressed fluid from the first stage of the compressor and therefore that the first surface 109a of the cover 109 forms part of a first stage diffuser for the compressor. As stated above, the first stage diffuser comprises a flow passage, and the first surface 109a of the cover 109 forms at least one side wall, or side, of the diffuser flow passage. It will therefore be appreciated that the cover 109 disclosed herein may be referred to as a “diffuser” or as a “diffuser plate.” Figure 5 shows the cover 109 in isolation. Figure 5 shows the first surface 109a of the cover. The cover 109 defines an axial direction Z and a radial direction R. As shown in Figure 5, the flange 109c in this example protrudes away from an edge of the cover in an axial direction of the cover. The flange 109c in this example is a circumferential flange and extends the entire circumference of the cover 109 (e.g. the flange is continuous around the circumference of the cover). This may be referred to as a single, discontinuous, flange. However this may not necessarily be the case, for example the cover 109 may comprise a plurality of flanges (e.g. all extending in the axial direction) like clips to enable the cover 109 to be secured to the compressor plate 108. As stated above, the or each flange 109c may be configured to enable the cover to be attached to the seal plate via a snap or interference fit. It should also be appreciated that in some examples the cover 109 may not comprise the flange and may be attached to the compressor plate via welding or any form of suitable adhesive. The first surface 109a may comprise a smooth surface. The first surface 109a may comprise a continuous surface. Hence, in one example, the first surface 109a of the cover 109 may comprise a continuous, smooth, surface. The first surface 109a of the cover 109 may comprise a low coefficient of friction. Hence, in one example, the first surface 109a of the cover 109 may comprise a continuous, smooth, surface having a low coefficient of friction. These features enable the first surface 109a of the cover 109 to act as an efficient diffuser (or diffuser plate) which leads to a reduction in compressor losses and an improvement in performance. The smooth surface also means that there will be no obstructions in the diffuser flow path to impede the flow of fluid through the interstage passage. The first surface 109a forms part of the diffuser flow passage and hence may be also be considered or termed a diffuser wall. Figure 6 shows a view of the underside of the cover 109 attached to the compressor plate 108. Figure 6 shows the second side 108b of the compressor plate 108 have a plurality of vanes 108-1 to 108-3 being configured to direct the flow of fluid through the return passage of the interstage passage to the second compressor stage in the Fig. 1 example (or to another part of the compressor in examples where the compressor is a single-stage compressor). Figure 7 shows a side cutaway view of the cover 109 being assembled to the compressor plate 108 and being attached to a portion of the compressor housing 120 via the fastener 115. Figure 8 shows a perspective view of the compressor plate 108 being attached to a portion of the compressor housing 120 via the fastener 115, without the cover 109. Figure 8 shows that the compressor plate in one example may comprise three holes, one of which is receiving the fastener 115 in this example, the other two being denoted by 116 and 117. The skilled person will appreciate that the outer housing 120 and / or the compressor plate 108 are configured to (e.g. their geometry is designed such that) when the compressor plate 108 is assembled to the compressor housing 120 a recess 119 is formed to receive the flange 109c of the cover 109. The recess 119 is a circumferential recess in this example. It will be appreciated from the figures that when the compressor plate 108 is attached to the compressor unit 100 via means of a fastener 115, the fastener 115 is inserted through the compressor plate from the first side 108a of the compressor plate 108 to the second side 108b of the compressor plate 108 such that the top surface of the head of the bolt is flush with the surface of the first side 108a of the compressor plate 108, as shown in Figures 4 and 7. It will also be appreciated from the figures that in some examples the cover completely covers the first side of the compressor plate. Figure 9 shows a flow chart of a process 900. Process 900 is a process for forming a portion of a compressor, such as the compressor 100 described above. At block 901 the process 900 comprises providing a compressor plate configured to at least partially define a flow passage for the compressor to receive fluid from a first compressor stage (although in some examples the compressor plate is a seal plate in which case the seal plate is configured to prevent fluid from leaking from a second compressor stage of the compressor to a first compressor stage of the compressor), the compressor plate having a first surface configured to receive fluid from the first compressor stage, the compressor plate being attachable to a portion of the compressor via a fastener extending through the compressor plate from a first side to a second side. The compressor plate provided as part of block 901 may be the compressor plate 108 as described above. At block 902 the process 900 comprises providing a cover having a first surface and a second surface. The cover provided as part of block 902 may be the cover 109 as described above. At block 903 the process 900 comprises attaching the cover to the compressor plate such that the second surface of the cover covers the first side of the compressor plate such that the portion of the compressor plate through which the fastener is configured to extend is covered by the cover and the first surface of the cover forms a surface to receive compressed fluid from the first stage of the compressor and forms part of a diffuser for the compressor. Block 903 may comprise assembling the cover and compressor plate to form this portion of the compressor as described above in relation to Figures 1-8. The process 900 may comprise attaching the cover to the compressor plate via an interference fit. The process 900 may comprise attaching the cover to the compressor plate via a friction fit (or snap fit). Alternatively, or additionally, the cover may be mounted to, or on, the compressor plate. Alternatively or additionally, the process may comprise heating the cover and then placing the cover onto the compressor plate such that the cover is attached to the compressor plate as it cools. Accordingly, the cover may comprise a metal and / or a metal alloy. For example, the cover may comprise Aluminium and / or stainless steel. The cover may comprise any suitable material. However in other examples the cover may comprise a plastics material. It will therefore be appreciated that the present disclosure provides a cover for a compressor plate which forms part of the diffuser for a compressor unit. More specifically, the cover is to cover the compressor plate such that a surface of the cover forms a diffuser surface. A surface of the cover thereby forms a part or portion of (e.g. a wall of) a diffuser passage, or channel, of the compressor (which is part of an interstage passage of the compressor unit, the interstage passage connecting the first and second compressor stages). The opposing or opposite side of the compressor plate (the side that is not covered by the cover) is part or a portion of (e.g. a wall of) a return passage, or channel, of the compressor (which is part of the interstage passage) and comprises a number of vanes. The compressor plate may be nonrotating and hence these may be considered stator vanes. The vanes are configured to guide the flow of fluid to the second stage of the compressor. The compressor plate may be a seal plate to seal the first stage of the compressor from another portion of the compressor (e.g. another compressor stage or an outlet or a volute etc.). The present disclosure provides for a first stage diffuser passage where no part of a fastener (securing the seal plate to the compressor housing) protrudes into the diffuser passage, thereby improving the performance of the compressor. It has been found that use of the cover according to this disclosure leads to an improvement in the compressor ratio of 6% and an overall improvement in the compressor performance by 3%. The design can be used for various applications utilising compressors, such as HVAC and air compressor systems. The person skilled in the art realizes that the present disclosure by no means is limited to what is explicitly described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. Additionally, variations can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Claims

1. A cover for a compressor plate of a compressor,5 the compressor plate at least partially defining a flow passage of the compressor toreceive fluid from a first compressor stage of the compressor, the compressor plate being attachable to a portion of the compressor via a fastener extending through the compressor plate from a first side to a second side, wherein the cover has a first surface and a second surface and the cover is attachable to the10 compressor plate such that the second surface of the cover is configured to cover the first side of the compressor plate such that the portion of the compressor plate through which the fastener is configured to extend is covered by the cover, and such that the first surface of the cover forms a surface to receive compressed fluid from the first stage of the compressor and forms part of a diffuser for the compressor,15 wherein the cover comprises at least one flange configured to enable the cover to beattached to the compressor plate.

2. The cover of claim 1 wherein the compressor is a multi-stage compressor, and wherein the compressor plate is a seal plate, the seal plate configured to prevent fluid from20 leaking from the first compressor stage to a second compressor stage of the multi-stage compressor.

3. The cover of claim 1 or 2, wherein the cover is substantially ring-shaped, comprising a central bore configured to receive a portion of the compressor.

254. The cover of any preceding claim wherein at least one flange protrudes away from an edge of the cover in an axial direction of the cover.

5. The cover of claim 4 wherein at least one flange extends the entire circumference of 30 the cover.

6. The cover of any preceding claim, wherein at least one flange is configured to enable the cover to be attached to the compressor plate via a snap or interference fit.35 7. The cover of any preceding claim wherein the first surface of the cover comprises asmooth surface having a low coefficient of friction.27 08 258. The cover of claim 7 wherein the first surface of the cover comprises a continuous surface.

9. A compressor unit comprising a compressor plate and the cover as claimed in any preceding claim, wherein the compressor plate is attached to a portion of the compressor unit via the fastener extending through the compressor plate from the first side to the second side, the cover being attached to the compressor plate such that the second surface of the cover covers the first side of the compressor plate such that the portion of the compressor plate through which the fastener extends is covered by the cover and the first surface of the cover forms a surface to receive compressed fluid from the first stage of the compressor and forms part of a diffuser for the compressor.

10. The compressor unit of claim 9, wherein the fastener comprises a bolt, and wherein the bolt is inserted through the compressor plate from the first side to the second side such that the top surface of the head of the bolt is flush with the surface of the first side of the compressor plate.

11. The compressor unit of claim 9 or 10 wherein the cover completely covers the first side of the compressor plate.

12. The compressor unit of any of claims 9-11, wherein the compressor unit comprises a first stage comprising a rotatable compressor blade configured to compress fluid, wherein the compressor plate is mounted to the compressor unit to form part of a flow passage to receive fluid from the first compressor stage and to form a diffuser for the first compressor stage, wherein compressed fluid discharged from the first compressor stage is directed to the diffuser, the diffuser having a channel comprising the first surface of the cover, the channel being configured to diffuse fluid discharged from the first compressor stage.

13. A process for forming a portion of a compressor, the process comprising: providing a compressor plate configured to at least partially define a flow passage for the compressor receive fluid from a first compressor stage, the compressor plate being attachable to a portion of the compressor via a fastener extending through the compressor plate from a first side to a second side;providing a cover having a first surface and a second surface; andattaching the cover to the compressor plate such that the second surface of the cover covers the first side of the compressor plate such that the portion of the compressor plate through which the fastener is configured to extend is covered by the cover and the firstsurface of the cover forms a surface to receive compressed fluid from the first stage of the compressor and forms part of a diffuser for the compressor.

14. The process of claim 13 wherein the cover is attached to the compressor plate via an 5 interference fit.

15. The process of claim 13 wherein the cover is attached to the compressor plate via a friction fit.1027 08 25

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