BE1029166B1 - CRANKCASE FOR TURBOMACHINE COMPRESSOR - Google Patents

Abstract

Casing (34) for a compressor for a turbomachine and in particular for an aircraft turbojet, the casing (34) comprising: an outer shroud (35) made of composite material and possibly formed of several angular shroud segments; an annular row of platforms (42) fixed to the shroud (35), each platform (42) supporting one or more stator vanes (32); and a ring (46), possibly formed of several angular ring segments, and having a layer of abradable material (44).

Description

Description

CASING FOR TURBOMACHINE COMPRESSOR The invention relates to the design of a turbomachine, in particular an aircraft turbojet or an aircraft turboprop. The invention relates in particular to the casing of a high-speed compressor. PRIOR ART In a turbomachine, and in particular in its compressor(s), the air flow is compressed by alternating rotor blades and stator blades. The stator vanes extend radially inwards from an outer shroud which delimits the passageway of the air flow. Document EP 1 426 559 A1 describes an example of compressor architecture of such a turbomachine. The shroud which holds the stator vanes has flanges upstream and downstream in order to fix it to the upstream casing and to the downstream inter-compressor casing. Thus, the shroud forms on the one hand a vector transmitting the axial forces and the torque between the various casings to which it is fixed, and on the other hand it supports the forces undergone by the stator vanes inside the compressor. These two sources of forces generate areas of stress concentration in particular at the level of the anchoring of the blades in the shroud. The usual response to ensure the mechanical strength in these stress zones is to provide additional thicknesses or reinforcement parts and therefore additional weight. Summary of the invention Technical problem The aim of the invention is to overcome the drawbacks set out above and in particular to propose a compressor and a lighter/lighter turbomachine without compromising their mechanical strength. Technical Solution The subject of the invention is a compressor casing for an aircraft turbojet equipped with a fan driven by a reduction gear, the casing comprising: an outer shroud made of composite material and optionally formed of several angular shroud segments; and an annular row of platforms fixed to the shroud, each platform supporting one or more stator vane(s); the casing being remarkable in that it comprises a ring, optionally formed of several angular ring segments, and having a layer of abradable material, the ring resting radially on bearing surfaces of the platforms so that no axial force can be transmitted between the ring and the platforms.

The ferrule may be one-piece annular, in particular having a taper compatible with the assembly of the various elements to be attached to it.

Alternatively, the ferrule can be made up of two half-shells.

The composite material can be optimized with denser plies in areas subject to greater mechanical stress.

The composite material can be of the unidirectional thermosetting or thermoplastic prepreg type deposited by placement of automatic fibers (“automated fiber placement”, AFP) and consolidated in an autoclave or in a closed mold or alternatively 2D or 3D interlock fabrics consolidated by compression molding -resin transfer (“Resin transfer molding”, RTM). The platforms and blades can form angular sectors of blades with several blades.

The layer of abradable material is intended to come face to face with the rotor blade tips of a rotor.

The design of the contact interface between the platforms and the ring avoids the transmission of an axial force and thus avoids the concentration of stresses at the periphery of the platforms.

In the usual assemblies, the platforms of the blades can be welded together or welded to the shell and to the rings.

The weld beads generate mechanically heterogeneous zones with a higher stress concentration coefficient (Kt).

To ensure the mechanical strength, in particular in fatigue, additional thicknesses or reinforcements are therefore usually provided.

The welds then lead to the double disadvantage of a surface that is not completely controlled geometrically for guiding the air flow and of excess weight due to the reinforcement of the structure.

With the design of the invention, the drawbacks associated with the presence of weld beads disappear.

Those skilled in the art will recognize that this function can be achieved by different designs, in particular rounded, wavy surfaces, having an “S” profile, or being chamfered, or by the interposition of a flexible joint, for example made of silicone.

According to advantageous embodiments of the invention, the casing can comprise one or more of the following characteristics, taken individually or according to all the technically possible combinations: - the ring is held radially externally by the shroud and internally by the platforms; - each platform comprises a circumferential end butted to a circumferential end of a circumferentially adjacent platform, and the opposite ends of the two circumferentially adjacent platforms each comprise a housing portion, the two housing portions together forming a housing receiving a fastening element ferrule platforms. This design has the advantage of “pooling” the fixing elements of the different platforms and therefore reducing the weight of the assembly; - the fixing element is a screw or a bolt, provided with a head received in the housing and with a rod, possibly threaded, extending radially through and outside the shell. A nut and a washer can come to tighten the screw or the lock outside the ferrule. The hole in the ferrule through which the screw passes can be significantly larger than the head of the screw or bolt so that no axial or circumferential force is transmitted between the platform and the ferrule via the screw; - the ring is made of a metallic material, comprising an internal groove filled with the layer of abradable material and an external groove forming a radial clearance between the ring and the ferrule. This geometry allows the ring to deform in particular radially, under the effect of thermal expansion (its own or that of the bladed disc arranged radially inside the ring), without mechanical stresses appearing on the platforms. The ring can be provided with axial grooves, angularly distributed regularly or not, to provide a treatment therein (like the example disclosed in EP 2 202 385 A1);

- the surfaces of the ring in contact with the platforms and the surfaces of the platforms in contact with the ring are chamfered surfaces or having a rounded profile. This type of arrangement allows the ring to deform or slide axially relative to the platform, the deformations or the sliding making it possible to maintain the contact between the platforms and the ring, and therefore the continuity of the guide surface of the air, without generating mechanical stress at the interface between the platforms and the ring; - each platform comprises an upstream bearing surface and/or a downstream bearing surface which has (have) radial and/or circumferential contact surfaces with the ring; - the ring has an upstream lip and/or a downstream lip, cooperating with the platform; - a flexible seal, in particular made of silicone, is arranged axially between the platforms and the ring so that no axial force can be transmitted between the platforms and the ring; - the ring is mounted floating in the ferrule. It is thus free to deform under the action of thermal expansion without generating mechanical stresses on the platform or on the shell; - the row of platforms is a first row and the casing comprises a second and optionally a third row of platforms, and the ring is a first ring and the casing comprises a second ring and optionally a third ring, each of the rings being arranged in upstream of a row of respective platforms.

- each ring receives a layer of abradable material arranged upstream of one or each row of stator vanes and arranged on the inner surface of the ring. The abradable material is capable of cooperating with the outer radial ends of a moving wheel of a rotor; - the blades are in one piece with their respective platform, the assembly being made of metal or composite material; - the vanes are made of composite material and/or the platforms are made of composite material, optionally in one piece with the shroud;

- the casing includes a row of variable-pitch vanes. The invention also relates to a platform for fixing one or more stator vane(s) to a turbomachine compressor casing, the platform comprising a housing adapted to receive a fixing element 5 intended for its fixing to a composite shroud, and an upstream bearing surface and/or on a downstream bearing surface, provided with a surface capable of coming into contact with a ring so that no axial force can be transmitted between the ring and the platform. This design has the advantage of “pooling” the fastening elements of the different platforms and therefore of reducing the weight of a turbomachine.

According to an advantageous embodiment of the invention, the platform comprises two opposite circumferential ends each having a housing portion, such that when two adjacent platforms provided with such housing portions are abutted, the housing portions together form a housing intended to receive a element for fixing the platforms to the shell.

According to an advantageous embodiment of the invention, the platform comprises a lug arranged on at least the upstream bearing surface and/or the downstream bearing surface, intended to serve as a circumferential stop for a ring.

According to an advantageous embodiment of the invention, the platform comprises several upstream spans and/or several downstream spans, including one or more external spans and one or more internal spans, preferably arranged in staggered rows seen axially.

The invention also relates to a turbomachine comprising a fan driven by a reduction gear, a low-pressure compressor, a high-pressure compressor and an intermediate casing, arranged between the low-pressure compressor and the high-pressure compressor, the turbomachine being remarkable in that the low-pressure compressor has a casing according to one of the embodiments described above.

According to an advantageous embodiment, the shroud comprises an upstream flange and a downstream flange, respectively to fix it - directly or not - to an upstream casing and to the intermediate casing, the casing comprising an upstream flange and a downstream flange, fixed to the flanges respective and ensuring continuity of the airflow guiding surface with an upstream ring and a downstream platform. The invention finally relates to a method for assembling a casing according to one of the embodiments set out above or of a turbomachine according to one of the embodiments set out above, the method comprising a step of placing compressor stages in the shroud, optionally by stacking alternately an annular row of platforms and a ring. Alternatively, the method comprises a step of assembling platforms to a first shell half-shell, a step of positioning bladed discs axially between two neighboring annular rows of platforms, a step of assembling platforms to a second half ferrule shell, and finally a step of assembling the second half-shell to the first half-shell.

In general, the various objects and embodiments of the invention can be combined with each other according to all possible technical combinations, unless the contrary is explicitly mentioned.

Additional advantages The shell made of composite material and the absence of welding and above all of the reinforcements that it requires saves weight.

The rings, preferably metallic, can allow passive control by thermal expansion of the clearance between the blade tips and the abradable. An abradable is more easily deposited (or redeposited during maintenance) on these rings. These rings also give rise to a limitation of the carcass deformations of the booster under the thrust of the engine.

Another advantage of providing different parts (shell, platform, ring) is that it is possible to replace only one of the parts involved in the event of a manufacturing or usage failure rather than scrapping the entire housing.

Also, the fact of providing the abradable layer on a ring facilitates its deposition compared to a deposition of abradable on a ferrule, which is more restrictive and more cumbersome to handle.

The design in two half-shells allows the assembly of a rotor composed of a bladed drum (“blum”) and a one-piece annular shell can receive one-piece mobile wheels (blisks) assembled in a non-permanent manner.

Brief description of the drawings FIG. 1 represents a turbomachine according to the invention;

Figure 2 shows a schematic sectional view of part of a compressor of the state of the art; Figure 3 shows a sectional view of a compressor housing according to the invention;

Figures 4 and 5 illustrate two design examples for a blade attachment platform according to the invention; Figure 6 shows two examples of a fastener used to secure the platform to the shell.

Description of embodiments

In the following description, the terms “internal” (or “inner”) and “external” (or “outer”) refer to a radial positioning with respect to the axis of rotation of a turbomachine.

The axial direction corresponds to the direction along the axis of rotation of the turbomachine.

The radial direction is perpendicular to the axis of rotation.

Upstream and downstream refer to the main flow direction of the stream in the turbomachine.

The term “integral” is understood as integral in rotation, and in particular rigidly linked.

The term "one-piece" is equivalent to "made from material" and designates different elements made from the same block of material, as opposed to "attached" which means that an element is attached, after its manufacture, to another element.

The figures represent the elements schematically, in particular without all of the assembly or sealing elements.

Certain dimensions, and in particular the radial thickness of the various parts, are exaggerated in order to facilitate understanding of the figures.

FIG. 1 schematically represents a turbomachine 2 which comprises a low-pressure compressor 4 and a high-pressure compressor 6, a combustion chamber 8 and two turbines 10. In operation, the mechanical power of the turbines 10 sets the two compressors 4 and 6 by means of respective shafts 12, 13. The compressors 4, 6 comprise several rows of rotor blades (or rotor blades) associated with rows of stator blades (or stator blades). The rotation of the rotor 15 around its axis of rotation 14 thus makes it possible to generate an air flow and to gradually compress the latter until it enters the combustion chamber 8. A fan 16 is coupled to the shaft 12 and generates an air flow which is divided into a primary flow 18 and a secondary flow 20 passing through an annular duct (partially shown) along the machine to then join the primary flow at the turbine outlet.

Reduction means, such as a reducer 17, for example epicyclic, can reduce the speed of rotation of the fan 16 with respect to the associated turbine.

An upstream casing 22 provided with support arms (or “struts”) holds the reducer 17 and supports the upstream part of the turbomachine. A compressor casing 24 supports the compressor 4. An inter-compressor casing 5 comprising support arms is arranged between the low-pressure compressor 4 and the high-pressure compressor 6.

In operation, the various casings are required to support heavy loads. The fan tends to pull the whole plane forward. The casings are mechanical parts which must therefore each support at least part of the thrust generated by the fan. The outer shroud contributes to the rigidity of the structure and is subjected to some of these forces, materialized schematically by the arrow denoted F in FIG. 1. Stress concentration points are also illustrated, in particular in line with the downstream structural arms. By attaching the blades directly to the shroud, these also undergo part of the mechanical stresses of the shroud. The idea of the invention as described below is to dissociate the blades from the efforts of the shroud and thus separate the functions of guiding the flow, attaching the abradable, integrating the casing treatment thereof. .

Figure 2 is a sectional view of the compressor 4 in a known turbomachine. One can observe there a part of the fan 16 as well as the spout 28 separating the primary 18 and secondary 20 flows. The rotor 15 can comprise several rows of rotor blades 30. The low-pressure compressor 4 comprises at least one stator which contains an annular row of stator vanes 32 intended to straighten the flow of air deflected by the rotating vanes. The low-pressure compressor 4 comprises a casing 34. The casing 34 has a generally axisymmetric shape that can incorporate reinforcements, splines or stiffening ribs. The casing 34 comprises a shroud 35 whose axial ends have fixing flanges 36, for example annular fixing flanges 36 for fixing the separation nose 28 and/or for fixing to an intermediate casing 5 of the turbine engine. The stator vanes 32 extend essentially radially from the casing 34 as far as internal shrouds 38. One of the rows of stator vanes 32 can be of variable orientation around its axis 40. The stator vanes 32 are fixed to the casing 34 at the means of platforms 42. A layer of abradable material 44 is generally provided opposite the rotor blades 30. In such a configuration, the anchoring zones of the stator blades — the housings (counterbore, “bathtubs”) accommodating the platforms 42 — form zones of stress concentrations because they concentrate both the forces collected by the stator vanes induced by the straightening of the air flow 18 and the forces transmitted from the fan 16 or the reducer 17. These zones are therefore generally reinforced with excess thicknesses of material. Added to this are the weld beads from the platforms 42 to the shell 35. All this has a significant weight. Figure 3 describes a housing 34 according to the invention. The ferrule 35 is of generally tubular shape and can be made up of several angular sections such as for example two half-shells, joined in a plane comprising the axis

14. The shell comprises two flanges 36 for fixing it to an upstream casing (22 in lafigure 1 and partially shown in figure 3), and to an intermediate casing (5 in figure 2).

The 35 ferrule has a 35.1 outer surface and an inner surface

35.2. The stator vanes 32 extend radially inward from the shroud

35. The vanes 32 are fixed to the shroud 35 by means of platforms 42. The platforms 42 can be integral with the vanes 32. The platforms 42 are fixed to the shroud 35 with means which are described below extending along the axis A. The rotor blades 30 are partially represented. They can come opposite a layer of abradable material 44. The latter is carried by a ring 46.

The inner ends of the blades 32 are supported by an inner shroud 38 under which can be added a layer of abradable material 48 which can cooperate with wipers of a rotor (15 in FIG. 2).

FIG. 3 illustrates three compression stages, each formed of a pair of rows of rotor 30 and stator 32 blades. It should be noted that the invention is not limited to this number of compression stages.

Flanges 50, 52 can ensure the continuity of the air stream upstream and downstream of these three stages. An enlarged portion of Figure 3 shows in detail the contact between a ring 46 and a platform 42.

The platform 42 comprises an outer surface 42.1 which is in contact with the ferrule 35. Sealing can be achieved there by means of a seal such as that illustrated in document WO 2019/105610 A1. The inner surface

42.2 of the platform is an air guiding surface.

The ring 46 for its part comprises an outer surface 46.1 which can be supported on the ferrule 35 and an inner surface 46.2 which defines an inner groove receiving the layer of abradable material 44.

In this example, a surface 42.3, 46.3 having an "S" profile provided on the platform 42 as well as on the ring 46 ensures the line contact between the - platform 42 and the ring 46. The surface 42.3 is provided on one or more spans 42.4 of the platform 42. The surface 46.3 is provided on a lip 46.4 of the ring 46. When the ring 46—preferably metallic—deforms in operation under the action of thermal expansion (its own or that of the rotor), the point of contact between the ring 46 and the platform 42 can move because the lip 46.4 deforms.

Similarly, the point of contact between the outer surface 46.1 of the ring 46 and the inner surface 35.2 of the ferrule 35 can move, the ring 46 being mounted floating in the ferrule 35. The design of the various surfaces (profiles and clearances ) is suitably chosen to allow expansion of the ring even under the maximum temperature conditions generally admissible in operation.

The ring 46 finally has an external groove 46.5 allowing its free deformation without constraints of contact with the shell 35. This groove 46.5 also forms a plenum which can be used to integrate a casing treatment.

Figure 4 shows a first example of a platform 42, with, at the top an isometric view, in the middle a front view (seen circumferentially) and at the bottom a top view (seen radially). The platform 42 comprises an outer surface 42.1 substantially matching the inner surface 35.2 of the shroud 35, and an inner surface 42.2 from which the blade 32 extends. The platform 42 can be made in one piece with the blade 32. In this example, the platform comprises a single bearing surface 42.4 on each axial side, having a contact surface 42.3 with the rings 46. The bearing surface is radially on the inside of the platform 42. The ring is thus radially “stuck” between the ferrule and the platform , which makes optional any means of assembly of the ring to the platforms or to the ferrule.

If the span 42.4 is here presented as extending circumferentially over the entirety of the platform 42, alternatively, the span 42.4 can be only on a part of the platform and can therefore present circumferential stop surfaces with the ring (see figure 5). A lug 42.4' can be arranged on one and/or the other of the surfaces 42.4, so as to cooperate with a notch of a ring and thus limit or even prevent the circumferential movement of the ring. The ring 46 and its notch 46.6 are partially shown in dotted lines. The platform has two faces at its circumferential ends 42.5, 42.6 which can be partially or completely curved. The continuity of the surface guiding the air flow is ensured by the continuity of the surfaces of the platforms arranged end-to-end. A housing 42.10 that can receive a lock (or screw) is arranged on the outer face 42.1. It is in this example made of two parts 42.7, 42.8 each at one of the ends 42.5, 42.6 of the platform. These two parts can be substantially cylindrical. One of the parts, preferably on the convex side

42.5, comprises an outer portion with a flat 42.9 which forms a radial stop for the lock. Alternatively, the radial stop can be a cover, attached to the outer face 42.1. By assembling the platforms against each other, the parts 42.7, 42.8 form a substantially cylindrical housing 42.10.

Additional housings (not shown) can also be provided on the platform to receive additional assembly elements. Some or all surfaces of platform 42 may remain as cast.

Figure 5 shows a variant for the geometry of the platform 42. In this example, several spans 42.4 are arranged on each side of the platform

42. The number of spans 42.4 can be different between one side and the other. Their positions too. In this example, they are arranged in staggered rows, which can facilitate the positioning of the platforms 42 on the ring 46 or vice versa.

Two circumferentially adjacent platforms 42 can be identical or different.

The surface denoted 42.3' can serve as a circumferential stop for the ring 46, having corresponding indentations to fit into the spans 42.4 of the platform 42.

The variant of FIG. 5 is therefore certainly a platform 42 that is more complex to manufacture, but it can have advantages on the indexing in position of the ring 46. FIG. 6 shows two examples of a fastening element 54 whose head 54.1 can penetrate into the housing and the rod 54.2, possibly threaded, can extend through and beyond the ferrule 35. In the example on the left there is a screw 54. 54.1 is inserted into a half-housing of a platform 42 then an adjacent platform encloses the head 54.1 under the flat 42.9. A nut is then screwed onto the threaded rod 54.2 from the outside.

This technique can be repeated for each platform, on two half-shells of ferrule 35 (180°), then the half-shells are assembled to one another.

Alternatively, when the ferrule is annular in one piece, all the platforms of the same annular row are assembled to the ferrule before a ring is stacked on the platforms.

On the example on the right is a lock 54. This comprises a head 54.1 which is a radial protrusion of the rod 54.2. The assembly is identical to that carried out with the screw.

The bolt has a lower weight than the screw although it requires a precise angular positioning around the axis A so that the head 541 remains well in the housing.

The invention has been described by means of exemplary embodiments but is not limited thereto.

Elements of each embodiment are combinable with elements of other embodiments.

The number of compression stages can be adapted to requirements and thus be 1.2, 30u4, or even more.

One of the stages may include a row of variable-pitch vanes.

Alternatively, this row of variable-pitch vanes can come upstream of the casing 34 (Figure 3 of document WO 2019/105610 A1). If the preferred field of application of the casing according to the invention relates to architectures in which the outer shroud takes up significant forces, as in FIG. 2, where the casing is interposed between two structural casings (5 and 22), the invention is not limited to it.

When the ring is formed of several angular segments, these may be in floating contact with each other circumferentially, or be assembled by snap-fastening (tenon/mortise, dovetail, etc.). The assembly of platforms (with lock or screw) can also be applied to ring segments.

This technique can come in addition or as an alternative to the seats and lugs on the platforms maintaining circumferentially and radially the (segments of) rings.

Claims (18)

Claims 1. Compressor casing (34) for an aircraft turbojet engine (2) provided with a fan driven by a reduction gear, the casing (34) comprising: - an outer shroud (35) made of composite material and optionally formed of several segments ferrule angles; and - an annular row of platforms (42) fixed to the shroud (35), each platform (42) supporting one or more stator vane(s) (32); the casing (34) being characterized in that it comprises: - a ring (46), optionally formed of several angular ring segments, and having a layer of abradable material (44), the ring (46) resting radially on bearing surfaces (42.4) of the platforms (42) so that no axial force can be transmitted between the ring (46) and the platforms (42). 2. Carter (34) according to claim 1, characterized in that the ring (46) is held radially externally by the shell (35) and internally by the platforms (42). 3. Carter (34) according to claim 1 or 2, characterized in that each platform (42) comprises a circumferential end (42.5) abutting a circumferential end (426) of a circumferentially adjacent platform (42), and the ends (42.5 , 42.6) facing the two circumferentially adjacent platforms (42) each comprise a housing portion (42.7, 42.8), the two housing portions (42.7, 42.8) together forming a housing (42.10) receiving a fixing element (54) platforms (42) to the shell (35). 4. Carter (34) according to claim 3, characterized in that the fixing element (54) is a screw or a bolt, provided (e) with a head (54.1) received in the housing (42.10) and d a rod (54.2), optionally threaded, extending radially through and outside the ferrule (35). 5. Housing (34) according to one of the preceding claims, characterized in that the ring (46) is made of a metallic material, comprising an internal groove (46.2) filled with the layer of abradable material (44) and an external groove (46.5) forming a radial clearance between the ring (46) and the ferrule (35). 6. Carter (34) according to one of the preceding claims, characterized in that the surfaces (46.3) of the ring (46) in contact with the platforms (42) and the surfaces (42.3) of the platforms (42) in contact of the ring (46) are surfaces (42.3, 46.3) chamfered or having a rounded profile. 7. Carter (34) according to one of the preceding claims, characterized in that each platform (42) comprises an upstream bearing surface and / or a downstream bearing surface (42.4) which has (have) surfaces (42.3) of radial contact and /or circumferential with the ring. 8. Carter (34) according to one of the preceding claims, characterized in that the ring (46) has an upstream lip and / or a downstream lip (46.4), cooperating with the platform (42). 9. Carter (34) according to one of the preceding claims, characterized in that the ring (46) is mounted floating in the shell (35). 10. Housing (34) according to one of the preceding claims, characterized in that the row of platforms (42) is a first row and the housing (34) comprises a second and optionally a third row of platforms (42), and in that the ring (46) is a first ring and the casing (34) comprises a second ring and optionally a third ring, each of the rings (46) being arranged upstream of a row of respective platforms (42). 11. Platform (42) for fixing one or more stator vane(s) to a turbomachine compressor casing (34), the platform (42) comprising a housing (42.7, 42.8, 42.10) capable of to receive a fixing element (54) intended for its fixing to a composite shroud (35), and an upstream bearing surface and/or on a downstream bearing surface (42.4), provided with a surface (42.3, 42.3') suitable to come into contact with a ring (46) so that no axial force can be transmitted between the ring (46) and the platform (42). 12. Platform (42) according to claim 11, characterized in that it comprises two opposite circumferential ends (425, 42.6) each having a housing portion (42.7, 42.8), such as when two adjacent platforms (42) provided with such portions of housing (42.7, 42.8) are butted, the housing portions (42.7, 42.8) together form a housing (42.10) intended to receive the fixing element (54) of such platforms (42) to the ferrule (35). 13. Platform (42) according to one of claims 11 or 12, characterized in that it comprises a lug (42.4') arranged on at least the upstream bearing surface and/or the downstream bearing surface (42.4), intended to serve as circumferential abutment with a ring (46). 14. Platform (42) according to one of claims 11 to 13, characterized in that it comprises several upstream spans and/or several downstream spans (42.4), including one or more external spans and one or more internal spans, preferably arranged in staggered rows seen axially. 15. Turbomachine (2) comprising a fan (16) driven by a reducer (17), a low-pressure compressor (4), a high-pressure compressor (6) and an intermediate casing (5), arranged between the low-pressure compressor -pressure compressor (4) and the high-pressure compressor (6), the turbomachine being characterized in that the low-pressure compressor (4) has a casing (34) according to one of Claims 1 to 10. 16. Turbomachine (2) according to claim 15, characterized in that the shroud (35) comprises an upstream flange (36) and a downstream flange (36), respectively to fix it - directly or not - to an upstream casing (22 ) and to the intermediate casing (5), the casing (34) comprising an upstream flange (50) and a downstream flange (52), fixed to the respective flanges (36) and ensuring continuity of the air flow guide surface with an upstream ring (46) and a downstream platform (42). 17. A method of assembling a casing (34) according to one of claims 1 to 10 or a turbomachine (2) according to claim 15 or 16 comprising a step of placing compressor stages in a ferrule (35), optionally by alternately stacking an annular row of platforms (42) and a ring (46). 18. Compressor casing (34) for an aircraft turbojet engine (2) equipped with a fan driven by a reduction gear, the casing (34) comprising: - an outer shroud (35) made of composite material and possibly formed of several angular shroud segments; and - an annular row of platforms (42) fixed to the shroud (35), each platform (42) supporting one or more stator vane(s) (32); the casing (34) being characterized in that it comprises: - a ring (46), optionally formed of several angular ring segments, and having a layer of abradable material (44), the ring (46) being mounted floating in the shell (35).