SCREW ROTOR MACHINE

Abstract: The present invention relates to a screw rotor machine comprising a pair of intermeshing rotors (3) having helical lands and intervening grooves and a casing with a working space generally composed of two intersecting bores, each enclosing one of the rotors (3). Each rotor (3) has a cylindrical projection (9) passing through a hole (8) in an end wall (5) at the high pressure end of the machine into a bearing chamber. To prevent gas leakage from the working space to the bearing chamber there is provided an annular element (15; 23) surrounding each rotor projection (9) and non-rotatably fixed to the casing. The annular element is provided with one axial (19; 24) and one radial (20; 21) sealing surface, one (19; 21) cooperating with the casing and the other one (20; 24) cooperating with the rotor projection (9) by a slight or zero contact force. The annular element (15; 23) is movable almost without friction in the direction of said contact force. Since the annular element (15; 23) has only one radial sealing surface (20; 21), cooperating with either the rotor projection (9) or the casing, the annular element will seal effectively even if the axis of the hole (8) in the end wall (5) and that of the rotor projection (9) are non-aligned.

Description: SCREW ROTOR MACHINE The present invention relates to a screw rotor machine for a working fluid comprising a pair of intermeshing ro¬ tors having helical lands and intervening grooves, and a casing with a working space generally composed of two in- tersecting bores, each enclosing one of the rotors, said casing being composed of a low pressure end section, of a high pressure end section carrying bearings for the rotors and of an intermediate barrel section, said barrel section being connected to at least one of said end sections by a detachable joint, said high pressure end section being ' pro vided with a bearing chamber separated from the working . space by an end wall provided with holes, each freely sur¬ rounding a cylindrical projection of a rotor. Due to the necessary manufacturing and running clearan- ces in a machine of the actual kind a leakage path exists for the high pressure gas to escape from the working space to the bearing chamber in the high pressure end section. The gas thereby leaks from a working chamber radially through the axial clearance between the end surfaces of th rotors and the inner surface of the high pressure end wall to the cylindrical rotor projections. From there the leak¬ ing gas flows axially through the radial clearance between each cylindrical rotor projection and the related hole in the end wall into the bearing chamber. To avoid that this gas leakage builds up a pressure in the bearing chamber equivalent to the discharge pressure, as such a pressure would exert an additional axial load to the rotors , provi- sions are usually taken for allowing the gas to escape fro the bearing chamber. Thereby it is important to minimize the gas leakage from the working space to the bearing cham ber in order to reduce the decrease of the efficiency of the machine resulting from the losses of high pressure gas. Owing to the deflection of the rotors resulting from radial forces acting thereon there must be a certain clea¬ rance between each rotor projection and the related hole i the end wall to avoid the risk for seizing therebetween. T reduce the leakage by diminishing the clearances to a mini mum corresponding to said necessary running clearances not only requires narrow tolerances for the cylindrical rotor projections and the related holes in the end section but also necessitates a precise positioning of the low and hig pressure end sections relative to each other. When mountin an end section to the barrel section, the end section, due to the play between the fastening bolts and the respective holes in the end section, is movable to a certain degree before the bolts are drawn tight. The end section therefor has to be displaced and angularly adjusted within the limits of said play in order to adjust the position thereo so that the holes in the end section receiving the rotor projections will be aligned with those of the other end section. When the end section is adjusted to the proper position is has to be fixed in this position by means of guiding pins until the bolts are drawn tight. With this extensive adjusting procedure that is necessary for dimi¬ nishing the clearance to a minimum the manufacturing costs will be increased. Different kinds of contact seals therefore have been used to prevent gas leakage from the high pressure end of the working space to the bearing chamber in the high pres¬ sure end section. See for example the article "Gleitrings- dichtungen fiir Schraubenmaschinen - Stand der Technik" by K.H. Victor published in VDI Berichte 521 entitled "Schrau benmaschinen", pages 49-76, VDI-Verlag GmbH, Dusseldorf, 1984. Many of such contact seals are of complicated con¬ struction with substantial axial length and require supply of oil to cool the seal. The distance between the rotor body and the radial bearings is thereby increased to make space for the sealing ' arrangement which results in a greater deflection of the rotors. Furthermore the contact seals give rise to friction losses which negatively affec the efficiency of the machine. 5 Another way of solving this problem is to use a sealing liquid for blocking the gas leakage. An example of this kind of sealing is disclosed in US patent No. 3 462 072 showing a screw compressor in which oil having a pressure exceeding the discharge pressure is supplied from a pres- 10 sure oil source through a channel in the high pressure end section to annular grooves therein, each of those grooves surrounding one of the shafts . This oil blocks the flow of working fluid along the shafts and thus acts as a sealing liquid between the working space and the bearing chambers 15 in the high pressure end section. A fraction of the oil flows along the shafts into the working space, but most of the oil flows along the shafts in the opposite direction to the bearing chambers and lubricates the bearings . The bearing chambers are drained through a channel in the 20 casing to an opening in the barrel wall of the compressor. Also this method of sealing has disadvantages . The oil drained to the compressor will have a considerably higher temperature than the temperature of the working fluid in the working chamber to which the oil is returned. The con- 25 tact between the working fluid and the oil therefore re¬ sults in heating of the working fluid which decreases the volumetric efficiency. Furthermore the oil has to be acce¬ lerated to the tip speed of the rotor lobes which also con sumes power. Another drawback with a sealing arrangement o 30 this kind is that the oil leaking out into the working space exerts an additional axial load to the rptors . f The object of the present invention therefore is to pre vent gas leakage along the rotor projections at the high Q pressure end of a screw rotor machine as defined, in a way 35 not entailing the disadvantages connected to the solutions according to known techniques . This has been achieved in that a screw rotor machine of the kind introductionally specified is provided with one separate annular element, surrounding each one of the rotor projections and non-rotatably fixed to the casing, said element being provided with one axial and one radial seal¬ ing surface, one surface cooperating with the casing and the other one cooperating with the rotor projection by a slight or zero contact force, the annular element being movable almost without friction in the direction of said contact force. Advantageous embodiments of the invention are specified in the depending claims. By the sealing arrangement according to the invention gas leakage from the working space to the bearing chamber is effectively prevented in a simple and reliable way which requires no supply of fluid for blocking or cooling pur¬ pose. The seal can be made with small axial and radial dimensions and works without any friction losses. The invention therefore is particularly useful when applied to small machines. The annular element cooperates by one sealing surface with the casing and by another sealing surface with the ro¬ tor projection. As the annular element is non-rotatably fixed to the casing the sealing between the element and the casing is established simply by direct contact of surfaces that do not rotate relative each other. At the other seal¬ ing surface of the annular element there is relative motion between said surface and the cooperating surface of the ro¬ tor projection. The clearance between these surfaces is very small and due to the freedom of the annular element to move perpendicular to these cooperating surfaces there is practically no contact force between them. Since the sealing of the rotor projection is established in cooperation with the annular element, the size of the clearance between the projection and the related hole in the end wall has no influence upon the gas leakage. The clearance can therefore be made wide enough to allow a certain disalignment between the axis of the projection an that of the hole. This simplifies the assembly of the machine as the tolerances for the surfaces determining the relative position of said axes do not have to be particula narrow. When mounting the high pressure end section to the barrel section it is therefore sufficient with the preci¬ sion attained -with the fastening bolts, and there will thu be no need for a precise adjustment of the position of the end section. By this the manufacturing costs are reduced. The invention will hereinafter be discribed more in de¬ tail with reference to embodiments shown in the accompanyin drawings in which: Figure 1 is a schematic view of a screw compressor according to the invention. Figure 2 is a fragmentary sectional view of a compresso according to one embodiment of the invention showing a par of the high pressure end. Figure 3 is a view similar to figure 2 but showing an- other embodiment of the invention. Figure 4 is an enlarged view of a detail in figure 2. Figure 5 is an enlarged view of a detail in figure 3. In figure 1 a screw compressor is shown. The compressor comprises a casing with a working space generally composed of two intersecting bores, each enclosing a rotor, of whic only one 3 is shown in the figure. The rotors have helical lands and intervening grooves which intermesh to form chevron-shaped working chambers between the rotors and the casing. The casing is composed of a low pressure end sec- tion 4, a high pressure end section 1 and an intermediate barrel section 2. In figure 2 a part of the high pressure end section 1 i shown. It comprises an end wall 5, a bearing chamber casin 6 and a cover plate 7 and is detachably joined to the bar- rel section 2 by bolts not shown. The end wall 5 is provided with holes 8, each of which freely surrounds a rotor projection 9. Each rotor projec¬ tion is journalled in radial rolling bearing 10 and thrust bearings 11, 12. The outer ring 13 of the radial rolling bearing 10 is mounted to abut the end wall 5. Coaxially with each hole 8 in the end wall 5 there is a recess 14 in the same on the side thereof facing the bearing chamber. I each recess 14 an annular element 15 is provided, which surrounds the rotor projection 9 between the bottom of the recess 14 and the radial rolling bearing 10. The annular element 15 is secured against rotation by means of a pin 16 extending through a slot in the annular element 15 and a bore in the end wall 5. By an 0-ring 17 provided in a groove in the annular element 15 the element 15 is biased against the outer ring 13 of the radial rolling bearing 10. The outer diameter of the annular element 15 is smaller than the diameter of the recess 14 so that the annular ele¬ ment 15 is free to move a certain distance in radial direc¬ tion. The radial position of the annular element 15 there- fore is determined solely by the location of the rotor pro¬ jection 9 and makes it possible to leave only a running clearance between the annular element 15 and the rotor projection 9. Due to the fact that this clearance between the inner surface 20 of the annular element 15 and the rotor projection 9 can be made very narrow an effective sealing action is established therebetween, and as a resul of the freedom of the annular element 15 to move radially, practically no contact forces will be developed if the surfaces contact each other. For lubrication of the bearings oil is introduced to the annular space outside th annular element 15. From there the oil is supplied to the bearings 10, 11, 12 through a channel 18 in the annular element 15. When the compressor is in operation high pressure gas reaches the annular element 15 through the radial clearanc between the hole 8 and the rotor projection 9. The gas is prevented from reaching the bearing chamber by the O-ring 17 between the end wall 5 and the bottom 19 of the groove in the annular element 15 and by the cooperating surfaces of the annular element 15 and the rotor projection 9, so that the pressure in the bearing chamber can be kept at a low level . A proper sealing effect is assured irrespectively of th accuracy of the centering of the end wall 5 in relation to the working space, as the annular element 15 due to its freedom to move radially in relation to the end section will be positioned aligned with the axis of the rotor projection 9 even if the latter is offset to the axis of the hole 8 in the end wall 5. Although the radial contact force between the annular element 15 and the rotor projection 9 will be almost neg¬ ligible it is advantageous to produce the element 15 from wear-resisting material having good anti-friction proper¬ ties in relation to the material in the cooperating rotor projection 9. Figure 3 is a section similar to that of figure 2 but showing another embodiment of the invention. Elements com¬ mon to corresponding elements in figure 2 have the same re ference numerals . In this embodiment each rotor projection 9 is provided with an annular groove 22 receiving the annular member which is made as an elastic split-ring 23 with an axial an radial clearance therebetween. The split-ring 23 is by its own spring effect biased outwards against the hole 8 in th end wall 5. A correct dimensioning of the split-ring 23 in order to attain a well-adjusted spring force is thereby of high importance. The spring force must be great enough to assure that the ring 23 circumferentially contacts the wall of the hole 8. It shall, however, not be particularly grea¬ ter than that as it is essential that the friction between the ring 23 and the hole 8 is low. The small friction force between the radially outer surface 21 of the split-ring 23 and the hole 8 is sufficient to prevent the split-ring 23 from rotating together with the rotor projection 9. The split-ring 23 is positioned to abut with one of its end surfaces 24 against one of the axial side walls of the groove 22 in the rotor projection 9 to sealingly cooperate therewith. As axial displacement of the split-ring 23 with¬ in the groove 22 is counter-acted only by the small fric¬ tion force between the split-ring 23 and the hole 8, the contact force between the split-ring 23 and the abutting surface of the side wall in the annular groove 22 will be negligible. High pressure gas reaching the split-ring 23 through the radial clearance between the rotor projection 9 and the hole 8 in the end wall 5 is prevented from leaking to the bearing chamber by the sealing effect established by the outer surface 21 of the split-ring 23 contacting the wall of the hole 8 on one hand and on the other hand by the re¬ latively rotating axial end surface 24 of the split-ring 23 and the side wall surface of the groove 22 abutting there- upon. The clearance between the radially inner surface of the split-ring 23 and the bottom of the annular groove 22 is large enough to take up any disalignment between the axis of the rotor projection 9 and the axis of the hole 8 in the end wall 5.

Claims: CLAIMS 1. Screw rotor machine for a working fluid comprising a pair of intermeshing rotors (3) having helical lands and intervening grooves, and a casing with a working space ge nerally composed of two intersecting bores, each enclosin one of the rotors (3), said casing being composed of a lo pressure end section (4), of a high pressure end section (1) carrying bearings (10, 11, 12) for the rotors (3) and of an intermediate barrel section (2), said barrel section (2) being connected to at least one of said end sections (1, 4) by a detachable joint, said high pressure end sec¬ tion (1) being provided with a bearing chamber separated from the working space by an end wall (5) provided with holes (8), each freely surrounding a cylindrical projectio (9) of a rotor (3), characterized in one separate annular element (15; 23), surrounding each one of the rotor projec tions (9) and non-rotatably fixed to the casing, said ele¬ ment (15; 23) being provided with one axial (19; 24) and one radial (20; 21) sealing surface, one surface (19; 21) cooperating with the casing and the other one (20; 24) co- operating with the rotor projection (9) by a slight or zer contact force, the annular element (15; 23) being movable almost without friction in the direction of said contact force. 2. Machine as defined in claim 1, in which each of said an nular elements (15; 23) is produced from a wear-resisting material having good anti-friction properties in relation to the material in the cooperating rotor projection (9) . 3. Machine as defined in claim 1 or 2 , in which each of said annular elements (15) surrounds the cooperating rotor projection (9) with a' running clearance only therebetween whereas it is radially movable in the casing and in axial sealing contact therewith. 4. Machine as defined in claim 3, in which said annular element (15) in one end face is provided with a groove enclosing an 0-ring (17) biasing the other end face thereo into direct contact with a surface fixed in the casing. 5. Machine as def ned in claim 4, in which said fixed surface is an end surface of the outer ring (13) of a radial bearing (10), 6. Machine as defined in claim 4 or 5, in which said annu¬ lar element (15) is provided with a channel (18) for suppl of lubricating oil to the bearings (10, 11, 12) on the sid thereof remote from the working space. 7. Machine as defined in claim 2, in which each of said annular elements is shaped as an elastic split-ring (23) biased towards the barrel wall of the hole (8) and radiall extending into an annular groove (22) in the rotor projec¬ tion (9) .

Inventors: CARLSSON ANDERS, JOHANSSON LARS, SOEDERLUND FRITS, CARLSSON, ANDERS, JOHANSSON, LARS, SOEDERLUND, FRITS
Applicants: SVENSKA ROTOR MASKINER AB
Application year: 1988
Main classification: F04C18
IPC codes: F01C19, F04C18