Fluid injected screw compressor element

Fluid-injected screw-type compressor element comprising two co-operating rotors (2 and 3) which are radially and axially bearing-mounted in a housing (1), whereby this housing confines a rotor chamber (4) in which the rotors (2 and 3) are situated and in which a fluid circuit (11) for the injection of a fluid discharges, characterized in that the radial bearing of at least one rotor is formed by the contact of the rotor concerned with the part of the wall of the housing opposite the radial perimeter of the rotor concerned and/or the co-operation with the other rotor and by maximally one additional radial bearing.

BACKGROUND OF THE INVENTION

The present invention concerns a fluid-injected screw-type compressor element comprising two co-operating rotors which are bearing-mounted in a housing, whereby this housing confines a rotor chamber in which the rotors are situated and in which a fluid circuit for the injection of a fluid discharges.

B. Related Art

Fluid-injected screw-type compressor elements are generally known and work according to the known principle whereby the volume of the compression chambers which are formed between the teeth of the respective rotors meshing into one another gradually decreases while the rotors are turning.

After a certain rotation of the rotors, one of the compression chambers which is formed between the rotors is connected to the outlet of the screw-type compressor element.

Oil-injected screw-type compressor elements are already known at present whereby, for the lubrication, cooling and sealing of the rotors, oil with a relatively high kinematic viscosity of 30 to 70 cSt (=3·10−6to 7·10−6m2/s) at 40° C. according to ISO 46 is injected in the compression chamber.

The good working of such oil-injected screw-type compressor elements requires the use of radial and/or radial/axial roller bearings on either side of both rotors, so that said rotors can be mounted in a rotating manner in such a way that the friction between the rotors and the housing is restricted.

By roller bearings, or what are called anti-friction bearings, are understood bearings which are provided with roller elements which can be formed in various ways, such as in the shape of balls, tapered rollers, cones, cylinders, needles or the like.

The presence of such radial or radial/axial roller bearings is required among others to absorb radial forces exerted on the rotors during the compression, as well as the forces which are exerted on said rotors by the drive, for example in the form of gear wheels, belts or chains. Said roller bearings also make sure that the transmission of vibrations from the rotors to the housing is limited, which is good for the life span of the compressor and also to restrict any noise nuisance.

A disadvantage of such known screw-type compressor elements is that such roller bearings are usually expensive and relatively difficult to mount during the assembly of the screw-type compressor element.

Water-injected compressor elements are already known as well whereby, instead of oil, water is injected in the compression chamber with the same purpose, i.e. to provide for a cooling, lubrication and sealing.

With this type of water-injected screw-type compressor elements, the rotors are radially bearing-mounted with axle journals on either side in the housing by means of water-lubricated slide bearings comprising a corrosion-free bush, for example made of graphite, which bush extends between the respective axle journals of the rotors on the one hand, and the housing of the compressor element on the other hand.

A disadvantage of such water-injected compressor elements is that such water-lubricated slide bearings are usually expensive as the aforesaid bush is always made of a relatively expensive material and as said bearings require a high manufacturing tolerance of the bush.

Another disadvantage of such water-injected screw-type compressor elements is that they often have a relatively complex housing due to the presence of several injection channels for injecting water in the compression chamber and in the slide bearings.

An additional disadvantage of the known water-injected screw-type compressor elements is that the play of the rotors in the housing is relatively large, as a result of which leakage losses occur in the compression chamber between the mutual rotors and between the rotors and the wall of the rotor chamber, which leakage losses are partly absorbed by the injected fluid.

The present invention aims to remedy one or several of the above-mentioned and other disadvantages.

SUMMARY OF THE INVENTION

To this end, the invention concerns a fluid-injected screw-type compressor element comprising two co-operating rotors which are radially and axially bearing-mounted in a housing, whereby this housing confines a rotor chamber in which the rotors are situated and in which a fluid circuit for the injection of a fluid discharges, whereby this screw-type compressor element has the specific characteristic that the radial bearing of at least one rotor is realised by the direct contact of the rotor concerned with the part of the wall of the housing opposite the radial perimeter of the rotor concerned and/or by the co-operation with the other rotor and by maximally one additional radial bearing.

The aforesaid maximally one additional bearing may consist of a roller bearing or a slide bearing.

An advantage of such fluid-injected screw-type compressor element according to the invention is that, as the radial bearing of at least one rotor is realised by the housing and/or the co-operation with the other rotor, said at least one rotor is free of radial bearings on at least one axial rotor end, such that such fluid-injected screw-type compressor element can be made cheaper, simpler and more compact than conventional oil-injected screw-type compressor elements whereby both rotors are provided with radial or radial/axial roller bearings on either side.

Such a fluid-injected screw-type compressor element according to the invention can also be realised in a simpler and cheaper way than the known water-injected screw-type compressor elements whose rotors are provided with radial slide bearings on either side, since considerably less lubrication channels must be provided in the housing than in such water-injected screw-type compressor elements.

Another advantage of a fluid-injected screw-type compressor element according to the invention is that a bad alignment of the rotors is avoided during the mounting, since at least one rotor is radially bearing-mounted by the housing and/or by the co-operation with the other rotor.

Preferably, at least one of the aforesaid rotors is entirely free from any additional radial bearings, and the radial bearing of this at least one rotor is exclusively, realised by the housing and/or the co-operation with the other rotor.

This is advantageous in that the screw-type compressor element is further simplified, whereas the production and material costs are further lowered.

According to a preferred characteristic of the invention, the fluid-injected screw-type compressor element also comprises provisions to restrict any friction between the mutual rotors and/or between the rotors and the housing.

An advantage thereof is that the screw-type compressor element has a better efficiency than the conventional screw-type compressor elements and that the transmission of vibrations from the rotors to the housing is moreover restricted, which reduces the vibro-acoustic emission and promotes the life span of the screw-type compressor element.

The fluid-injected screw-type compressor element which is represented in the FIGURE mainly consists of a housing1and two co-operating rotors, namely a female rotor2and a male rotor3provided in said housing1.

The housing1encloses a rotor chamber4which is provided on one side, called the inlet side, with an inlet5with an inlet opening for the gas to be compressed and on the other side, called the outlet side, with an outlet6for the compressed gas and the injected fluid.

Onto this outlet6is connected an outlet line7which ends up in a fluid separator8in which opens a discharge line9for compressed gas at the top and onto which is connected a fluid line10at the bottom to carry back the separated fluid to the rotor chamber4in which said fluid line10opens via openings10A and10B.

The fluid separator8and the fluid line10are part of a fluid circuit11. As a relatively high pressure, namely the outlet pressure, prevails in the outlet line7during normal operation of the screw-type compressor element, this outlet pressure will also prevail more or less in the fluid separator8, and the fluid line10will form a part of the fluid circuit11in which the prevailing pressure is practically the same as the outlet pressure of the screw-type compressor element.

The female rotor2comprises a helical body12which, in the given example, is provided in the rotor chamber4without any axle journals and additional bearings in such a way that it can be directly rotated, whereas the male rotor3has a helical body13and, in this case, two axle journals14and15with which said male rotor is bearing-mounted in the housing1by means of two bearings16and17which each extend around an axle journal14,15respectively.

According to the invention, the radial bearing of at least one rotor is realised by the contact, in particular more specifically the direct contact of the rotor concerned with the part23or24of the wall of the housing1opposite the radial perimeter25or26of the rotor concerned and/or the co-operation with the other rotor and by maximally one additional radial bearing, whereby this radial bearing may consist of a roller bearing or a slide bearing.

In the example of the FIGURE, the bearing of the female rotor2is exclusively realised by the housing1and the co-operation with the male rotor3, without this female rotor being provided with additional radial bearings.

In particular, said female rotor2is free of bearing-mounted axle journals, and the radial bearing is exclusively guaranteed by the direct contact of the rotor2concerned with the part of the wall21of the housing1opposite the radial perimeter25of the rotor2concerned and the co-operation with the male rotor3, and thus without any additional radial bearings being required, whereas the axial bearing of the female rotor2is exclusively guaranteed by the contact between an end face27or28of the female rotor2concerned and the opposite part29or30of the wall of the housing2, without any additional axial bearing being required.

The fluid-injected screw-type compressor element according to the invention is preferably provided with means to restrict the axial play on the outlet side between the rotors2and3and the housing1.

To that end, opposite the crosscut far end of the axle journal14on the inlet side of the compressor element is formed a closed chamber19in an end part18of the housing1in this case, which is connected to the fluid line10via a branch20, and thus to the part of the fluid circuit11in which a lower pressure than the outlet pressure prevails, such that, while the compressor element is operational, a pressure will be exerted on the crosscut far end of said axle journal14.

In an analogous way, a closed chamber21is formed in the end part18of the housing1, opposite the crosscut end of the body12of the female rotor2on the inlet side of the compressor element, which chamber21is connected to the fluid line10via a branch22, and thus to the part of the fluid circuit11in which a pressure prevails which is practically equal to the outlet pressure, such that, while the compressor element is operational, a pressure will be exerted on this crosscut end of the rotor2as well.

In the given example, the axle journal15of the male rotor3is extended to outside the housing1where it can be coupled to a drive such as a motor or the like, not represented inFIG. 1.

The female rotor2is in this cage not connected to said drive, but it is directly driven by the male rotor3.

According to the invention, however, it is also possible to provide only the female rotor2with an axle journal which extends to outside the housing1and which is coupled to a drive, such that the male rotor3is driven by the female rotor2.

According to a preferred characteristic of the invention, the fluid-injected screw-type compressor element comprises provisions to restrict any friction between the mutual rotors2and3and between the rotors2and3and the housing1, which provisions, according to a preferred aspect of the invention, contain a hard, practically frictionless coating provided over at least a part of the surface of one or both rotors2and3and/or at least a part of the wall of the housing1.

In particular, the aforesaid coating can be provided over at least a part of one or several of the following surfaces:an end face of at least one of the rotors2and3;a contact surface of at least one of the rotors2and3, whereby by a “contact surface” is meant, a collection of points on the surface of a rotor, which points, while the screw-type compressor element is operational, make contact with the rotor co-operating with the latter;a part of the wall of the housing1opposite an aforesaid end face of a rotor2or3;a part of the wall of the housing1opposite the radial perimeter of an aforesaid rotor2or3.

The aforesaid coating may for example consist of what is called a “diamond like carbon” coating (DLC coating), but it can also be made in the form of another coating such as a “Near Frictionless Carbon” coating (NFC coating), a ceramic coating, a metallic coating, a polymer coating or the like.

The aforesaid coatings may be either or not doped with micro particles and/or nano particles of hard and/or lubricating types of fillers.

According to a special aspect of the invention, the fluid which is injected in the rotor chamber4via the openings10A and10B consists of a cooling liquid with a very low viscosity (e.g. a mineral oil), or in other words a cooling liquid whose kinematic viscosity is considerably lower at 40° C. than that of the present oil-injected compressors (6·10−6m2/s), which liquid, in co-operation with the aforesaid coating, causes a tribological effect between the mutual rotors2and3and/or between the rotors2and3and the housing1, so that, despite the very low viscosity, a lubricating and sealing effect is nevertheless obtained during every operational condition of the screw-type compressor element.

Consequently, the injected fluid must only consist of a cooling liquid and it must not have any sealing or lubricating qualities as such.

Thanks to an appropriate application of the provisions for restricting any friction between the rotors2and3, and between the rotors2and3and the housing1, which in this case means an appropriate dimensioning of the coated parts and applying the appropriate combination of coating and cooling liquid, at least one of the rotors2and/or3on at least one axial rotor end will be free of radial bearings, and in particular on at least one axial rotor end it will be free of an axle journal14-15.

Another preferred additional measure to reduce any friction between the mutual rotors2and3and between the rotors2and3and the housing1consists in the characteristic that one or both rotors and/or the wall of the housing1is provided with a relief over at least a part of its surface so as to provide for a morphology with practically frictionless behaviour.

To this end, a texture can be burnt in the surface concerned by means of a laser; the surface concerned can be processed by means of sand-blasting, honing, lapping or grinding or any other surface treatment technique whatsoever.

Such a relief is preferably provided at least on the surfaces that are free of any coating, but this is not strictly necessary according to the invention and, consequently, such a coating may also be provided on a surface which is coated afterwards with an aforesaid practically frictionless coating.

In particular, the aforesaid relief can be provided on at least a part of one or several of the following surfaces:an end face of at least one of the rotors2and3;a contact surface of at least one of the rotors2and3;a part of the wall of the housing1opposite an aforesaid end face of a rotor2or3;a part of the wall of the housing1opposite the radial perimeter of an aforesaid rotor2or3.

The practically frictionless contact between the aforesaid tribological surfaces and the distribution of the bearing-load over a considerably larger area than in the case of for example roller bearings will lead to a considerable reduction of the transfer of peak loads on the housing1. A considerable reduction of the vibro-acoustic noise emission of the screw-type compressor element is thereby moreover obtained as well.

Thanks to the presence of the chambers19and21, an axial pressure is created on the inlet side on the crosscut far end of the axle journal14and the end face of the female rotor2respectively, which pressure counteracts the axial forces which are exerted on the rotors2and3by the compressed gas.

The invention is by no means restricted to the embodiments described by way of example and represented in the accompanying drawings; on the contrary, such a fluid-injected screw-type compressor element can be made in all sorts of shapes and dimensions while still remaining within the scope of the invention.