EP1722104B1 - Rotor pair for a screw compressor - Google Patents

Description

The invention relates to a pair of rotors for a screw compressor, wherein the rotors are in meshing, non-contact engagement, the rotors each having at least one gear and a steady change in pitch, wherein the shape of the rotors is determined by a screw profile.

Screw compressors have been used successfully for some years on the one hand in compressor construction and on the other hand for the production of vacuum. In them rotate synchronously and in opposite directions two rotors, which mesh with each other without contact. These rotors have screw threads with opposite directions of rotation, often referred to as left and right. There is no use of sealants and lubricants such as oil in the working space of the machines, which explains part of the success. A change in the pitch of the threads causes a change in the volume trapped between the rotors and the housing and thus creates a corresponding compression of the gas trapped in the volume. Over the axial length of the rotors, a pressure gradient forms, i. Distributed over the axial length different pressures occur.

The characteristics of the compressors and the effort required to manufacture the rotors are largely determined by the design of the screw rotors. This shape is characterized by the screw profile. This can be described by the profile outline, which results, for example, in the axis or in the face section.

The DE-OS 199 41 787 looks at the profile outline of the tooth, which results in the axial section. It proposes to provide an area along the contour line with a straight section in which a left-handed and a right-handed Facing the rotor. This area is also called profile intervention. The proposed measure does away with undercuts that make the production complex and expensive. However, this must be compromised in the tightness to be accepted, ie the vacuum technical properties are worse than the ideal. The tooth profiles proposed and cited in this document are based on the gearing law.

From the U.S. Patent 6,371,744 It is known to provide the profile contour line in the end section in sections with a so-called Quimby curve. This results in a relatively high tightness with low backflow. However, rotors with such a profile are expensive to produce.

The invention is therefore based on the object to eliminate the disadvantages of the prior art, to develop a relation to the characteristics of the compressor improved screw rotor pair and to create a design flexibility that goes beyond the prior art.

The object is achieved by a pair of rotors for a screw compressor, which is characterized in that the profile type of the screw profile is different at least two along the rotors axially spaced locations and that adapted to the respective pressure range profile types continuously merge into each other. The term "steady" is to be understood in a mathematical sense. This means that the function describing the slope is always derivable.

The characterizing features in the dependent claims show advantageous developments of the invention.

Screw profiles can not only be characterized by the profile outline as in the prior art. In the context of the invention, it proved to be advantageous to classify the screw profiles in profile types. This arrangement takes place via the sealing effect on the profile engagement, which results from the respective profile type. Different profiles produce the sealing effect at radially different locations of the rotor. There are those which achieve the optimum sealing effect in the circumferential direction at an outer radial region (for example so-called Quimby profiles). Other profiles seal almost completely in the radial direction but over a small part of the circumference. The distance between the two rotors in the axial direction at these sealing points, also referred to as gap height, determines the quality of the seal, but not the basic type.

According to the invention different profile types are now used at different axial locations of the rotors. This ensures that the most suitable for a pressure range profile type is used. This makes it possible to use cost-effective profile types where their shortcomings in terms of the characteristics of the compressor are justifiable.

The presented measures make it possible to change the shape of the gap, which exists on the profile engagement between the rotors, along the rotor axis and in each case to optimally adapt to the pressure conditions and tightness requirements. Basis of the design is thus the gap type and not the gearing law as in the prior art.

Due to the continuous transition from one profile type to the next, the rotors can be manufactured in one piece. Above all, there are no steps and edges in the interior of the screw threads, which would adversely affect their gas-promoting and -compacting performance.

In a further embodiment, the change in pitch of the screw threads takes place in such a way that it is zero in certain areas. This allows an adaptation of the compression curve along the rotor axis to the flow or printing area. Since the compaction is controlled, among other things, via the slope and heat is generated during the compression of gases, the change in slope can influence the heat balance of the compressor.

In a further embodiment, the steady change in slope is such that the slope progression is described by a third degree curve. This allows the simplest possible transition from one pitch to the next.

A further advantage results if the change in pitch and the profile change at least in sections along the axis are not simultaneous. This measure results in a partial decoupling of the change in compression from the change in slope.

The profile type can be constant over an axial region, whereby a plurality of such regions can be present over the rotor length. This represents a further simplification of the production without having to forego the advantages that result from the combination of the profile types.

For some pumped fluids, it makes sense to provide coatings of the rotors, so that they are not destroyed by reactive components of the fluid.

The above-mentioned advantages of this rotor pair are particularly pronounced when the rotor pair is used in a dry-compressing screw vacuum pump. In such pumps, the delivery chamber is free of oils and similar substances which are used in other vacuum pumps, among other things for sealing. Thus, the gaps between the rotors among themselves and between the rotor and housing determine the harmful gas flows against the conveying direction. These flows significantly affect the characteristics of the vacuum pump. in addition comes that vacuum pumps work across different pressure ranges, in which different gas flow types are present. The effect of the column on the gas flow depends on the gas flow type. Thus, a given combination of shape and dimensions of a gap in different pressure ranges differently. The profile types determine the shape and dimensions of the gaps that occur in the gas conveying area. Thus, for example, it is possible to use low cost profile types to be produced where their nip characteristic is compatible with the pressure range. At the same time, the costly profile types are only used in the pressure and thus axial areas of the rotors, where low backflow and good vacuum data are required. Especially in vacuum pumps the heat balance plays an important role. Heat is also created here by the compression of the pumped gas. Since the heat conduction through the gas becomes worse with increasing dilution, the formation of the heat of compression must be controlled, for example by a distribution of the compression to a plurality of axial regions of the rotor. It may also be useful to influence the distribution of the compressed and heated gas by means of the profile type.

• Fig. 1: Rotorpaar in der Draufsicht auf die durch die Rotorenachsen aufgespannte Ebene.
• Fig. 2:
  1. a) Partieller Stirnschnitt entlang I-I' mit einem ersten Profiltyp.
  2. b) Partieller Stirnschnitt entlang II-II' mit einem zweiten Profiltyp.
  3. c) Partieller Stirnschnitt entlang III-III' mit einem dritten Profiltyp.
  4. d) Partieller Stirnschnitt entlang IV-IV' mit einem vierten Profiltyp.
• Fig. 3: Partieller Achsschnitt durch die Ebene der Rotorachsen im Bereich zwischen den Wellen.

In the following, the invention will be explained using the example of a rotor pair for a screw compressor, which has four different profile types. This screw compressor can be used in particular as a vacuum pump. With reference to the drawings, this embodiment of the invention will be illustrated.

• Fig. 1 : Rotor pair in plan view on the plane spanned by the rotor axes.
• Fig. 2 :
  1. a) Partial end cut along II 'with a first profile type.
  2. b) Partial end cut along II-II 'with a second profile type.
  3. c) Partial incision along III-III 'with a third profile type.
  4. d) Partial end cut along IV-IV 'with a fourth profile type.
• Fig. 3 Partial axial section through the plane of the rotor axes in the area between the waves.

The FIG. 1 shows the rotor pair in a view of the spanned by the rotor axes plane. The rotor pair is formed by a left rotor 1 and a right rotor 2, these each having a shaft 3, 4. On this wave screw flights are applied as pump-active structures, which are formed in the right-hand 5 and left-hand 6. By reducing the pitch of the screw threads g on g ', a compression of the pumped medium, for example a gas mixture, can be achieved. The rotors are in meshing, non-contact engagement. At this engagement is located between the rotors of the profile engagement gap. The medium is required in the figure from bottom to top and compacted. This is indicated by the block arrow.

FIG. 2a shows the partial frontal section through the rotor pair at height I-I '. The rotors 1 and 2 mesh with each other, ie in this sight, the screw threads overlap 5 and 6 and each push close to the shaft 3 or 4. Dark colored and marked 8 is the area of the highest sealing effect between the rotors. The designated 9 area has only a small sealing effect, which is indicated by the two broad arrows that symbolize gas flow. The shape of the high sealing area is characteristic of the profile type. The shown shape of the region 8 is almost exclusively radially between the two waves. The high sealing effect in this area is paid for by a stronger overflow at the areas 9. Such a type of profile is simple and inexpensive to manufacture. It is used in the embodiment at a point of high pressure, where the overflow 9 leads to a distribution of the gas heated by the compression. This influences the heat balance of the vacuum pump.

The in FIG. 3 The axial section shown illustrates the meaning of profile type and gap height. In this section, the curves 10 and 11 describe the helical profile of the rotors 1 and 2. There is at least one gap S between the rotors. For comparison, the curves 10 'and 11' are shown, which correspond to a different design of the rotors. Due to the higher distance of the curves results in a larger gap width S '. Although this deteriorates the quality of the gasket, in both cases it is the same type of profile.

The FIGS. 2b to 2d , each showing the partial frontal section at three other axial locations of the rotors, illustrate the different profile types. FIG. 2b shows in a section on the rotor height II-II 'an "S" shaped course of the region of highest sealing effect 8 while there are two areas 9, in which gas can flow over between the rotors. This type of profile represents a compromise of manufacturing and vacuum requirements.

Figure 2c as a section through the rotors at height III-III 'shows a transitional shape or mixed shape between the profile types 2b and 2d. The sealing area 8 is formed predominantly along the circumference (as in FIG. 2d), but also has an extent in the radial direction, the latter not lying directly between the shaft axes (therefore, as in FIG. 2b).

Figure 2d as a section through the rotors at height IV-IV 'turn shows a profile type with a divided into two subregions sealing area 8. It is formed along the touching circumferential lines of the rotors. The shown shape of the region 8 is generated by a so-called Quimby curve in the profile of the tooth on the engagement gap. The occurring gaps are small, the sealing effect is altogether high, which is why this profile type is very well suited for high pressures. However, this type of profile is comparatively complicated and expensive to manufacture.

The four in the FIGS. 2a to 2d According to the invention, the profile types shown overlap each other continuously, so that different profile types are present at at least two axial locations of the rotors. In this case, a profile type can be constant over a longer axial section before the transition to another profile type begins. The continuous transition of the profile types results in mixed variants of the profile types.

Claims (7)

1. A rotor pair for a screw compressor, wherein the rotors (1, 2) are in intermeshing contactless arrangement, each with at least one gear, with a continuous variation in pitch, wherein the shape of the rotor is defined by a screw profile, characterised in that the profile type of the screw profile is different at at least two axially-spaced positions. (20, 21, 22) along the rotors and in that the profile types adapted to the respective pressure region merge continuously into one other.
2. A rotor pair for a screw compressor according to claim 1, characterised in that the variation in pitch is zero in some regions.
3. A rotor pair for a screw compressor according to any of the preceding claims, characterised in that the pitch is described by a curve of the third degree.
4. A rotor pair for a screw compressor according to any of the preceding claims, characterised in that pitch variation and profile variation do not occur simultaneously at least in sections along the axis.
5. A rotor pair for a screw compressor according to any of the preceding claims, characterised in that the profile type is constant over at least one axial region.
6. A rotor pair for a screw compressor according to any of the preceding claims, characterised in that the rotors have a coating of their surfaces.
7. A rotor pair for a screw compressor according to any of the preceding claims, characterised in that the rotor pair (1, 2) is used in a dry-sealed screw vacuum pump.

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