vacuum pump

Abstract

A dry vacuum pump (1) comprises an outlet duct (14) that fluidly connects a lubricant housing (2) and a seal-to-seal volume (11), and a pressure gradient control device (15), wherein the pressure gradient control device (15) includes a second check valve (18) located on the outlet duct (13), and when the pressure difference between the upstream and downstream sides of the second check valve (18) exceeds a predetermined opening threshold, and the pressure inside the lubricant housing (2) becomes higher than the pressure inside the seal-to-seal volume (11), purge gas is allowed to pass from the lubricant housing (2) towards the seal-to-seal volume (11).

Description

Technical Field

[0001] The present invention relates to a vacuum pump. More specifically, the present invention relates to a seal between a pump chamber of a vacuum pump and a lubricant housing.

Background Art

[0002] Positive displacement vacuum pumps have one or more stages of pump stages in series, and the gas to be pumped circulates between the suction port and the discharge port. Also called a "roots" pump, a roughing vacuum pump with rotary lobes having two or more lobes is distinguished as a "claw" pump and a screw pump. A "roots blower" type vacuum pump is also known, and this is used upstream of the roughing vacuum pump to enhance the pump capacity under high flow conditions. These vacuum pumps are called "dry" because during operation, a pair of rotors rotate within the stator without mechanically contacting each other or the stator. This makes it possible not to use oil in the pump stage.

[0003] The rotation of a pair of rotors is synchronized by gears, and the pair of rotors are usually guided in rotation by rolling bearings arranged at both ends of the pump chamber. These gears and bearings are lubricated with oil or grease within a lubricant housing. This lubricant housing is isolated from the pump chamber by sealing means, but the rotating shaft can rotate through this sealing means. The sealing device is mainly composed of a rubbing lip seal, an ejector disk, a gas purge, and / or a physical barrier against the lubricant such as a labyrinth seal or a baffle.

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, during the operation of a vacuum pump, especially in applications where a constant amount of gas is periodically brought into a vacuum, the pressure used fluctuates significantly. Although the atmosphere inside the lubricant housing is maintained at atmospheric pressure and not a vacuum, the pressure fluctuations generated by the vacuum pump do have some degree of effect on the atmosphere inside the lubricant housing, albeit to a lesser extent. This is because the sealing means must allow the rotation of the rotating shaft and does not provide complete airtightness in terms of isolation. Furthermore, the contact between the friction lip seal and the rotating parts can deteriorate or even be lost over time due to heat generated by the lip itself and wear caused by the movement of the lip. These movements themselves are caused by changes in the pressure difference, which reduces the effectiveness of the seal. As a result, the transport gas sent into the pump chamber may enter the lubricant housing.

[0005] In the semiconductor, flat panel display, solar power, and thin-film deposition industries, the gases transported by these pumps may contain corrosive substances, precursors, or resin vapors. Certain applications, such as HARP (High Aspect Ratio Process), generate large amounts of solid by-products (powder, paste, fragments). These by-products can enter and accumulate or react in large quantities in the lubricant housing, particularly in the dead volume of sealing devices within baffles and labyrinth seals. This contamination can lead to premature degradation of the lubricant's properties and insufficient lubrication of rolling bearings. Over time, this can result in premature bearing wear and failure, shortening the lifespan of the vacuum pump. This phenomenon is accelerated in periodic pump applications where continuous emptying and refilling of the lubricant housing promotes gas movement and lubricant contamination.

[0006] Therefore, one of the objectives of the present invention is to propose a dry vacuum pump that overcomes, at least partially, the shortcomings of the prior art. [Means for solving the problem]

[0007] For this purpose, the subject matter of the present invention is A dry vacuum pump comprising a lubricant housing, a pump chamber including at least one pump stage, two rotating shafts configured to drive the rotation of a pair of rotors in the pump chamber, at least one first annular lip seal and a second annular lip seal, a gas injection duct fluidly connected to the lubricant housing, and an overpressurizer located in the gas injection duct, The rotating shaft is guided to rotate within a bearing that is lubricated by the lubricant contained in the lubricant housing. The first annular lip seal and the second annular lip seal are arranged in series along each of the rotating shafts and interposed between the lubricant housing and the pump chamber. The lips of the first and second annular lip seals face the same direction, and the pressure of the pump stage in the pump chamber adjacent to the second annular lip seal is higher than the pressure in the inter-seal volume located between the first and second annular lip seals, pressing each lip against the rotating shaft. The aforementioned dry vacuum pump further, An outlet duct that fluidly connects the lubricant housing to the seal-to-seal volume, and The system includes a pressure gradient control device, which includes a second check valve located on the aforementioned outlet duct. The pressure gradient control device is configured to control the flow of purge gas from the lubricant housing towards the seal-interval when the pressure fluctuation between the upstream and downstream sides of the second check valve exceeds a predetermined threshold and the pressure inside the lubricant housing becomes higher than the pressure inside the seal-interval.

[0008] According to the present invention, it is possible to ensure that the lubricant housing is continuously overpressurized relative to the inter-seal volume by, on the one hand, the purge gas supply flow rate and the lubricant housing overpressure device, and on the other hand, the outflow rate of purge gas as a result of leakage through the outflow duct via the first annular lip seal and pressure gradient control device on the lubricant housing side. In other words, the pressure inside the lubricant housing becomes higher than the pressure in the inter-seal volume, and the purge gas continuously leaks from the inter-seal volume below the lip of the second annular lip seal toward the pump chamber.

[0009] Thus, the pump chamber allows the pressure within the interseal volume to be reduced by suction through the leak of the second annular lip seal. One advantage of this configuration is that the pumping of the interseal volume is performed directly by the pump stage of the pump chamber, i.e., the vacuum pump itself.

[0010] When the first annular lip seal wears out (first from the lubricant housing side), the purge gas not only continues to leak through the second check valve but also begins to pass under the lip of the first annular lip seal and reach the interseal volume. Although the rate of purge gas flow leaking from the second check valve decreases, the pressurization maintained within the lubricant housing maintains the direction of the purge gas flow from the lubricant housing towards the pump chamber, thereby ensuring that no particles, powders, or other contaminants of the pumped gas enter the lubricant housing.

[0011] As the lip of the first annular lip seal wears down, the flow rate of purge gas leaking from beneath the lip increases. As the flow rate of purge gas passing beneath the lip increases, the flow rate of purge gas leaking through the second check valve decreases. Therefore, the flow rate discharged from the interseal volume via the outlet duct automatically adapts to the degree of wear of the first annular lip seal.

[0012] Another effect of increasing the flow of purge gas under the lip of the first annular lip seal is that it can also limit the damage to the first annular lip seal on the housing side.

[0013] Furthermore, a vacuum pump may have one or more of the following features, either individually or in combination. The first and second annular lip seals are, for example, single-lip types. The opening threshold for the second check valve is greater than, for example, 40 mbar (4000 Pa), and is in the range of, for example, 40 mbar (4000 Pa) to 100 mbar (10000 Pa), for example, 75 mbar (or 7500 Pa). This threshold allows the second check valve to open before the purge gas begins to leak from the first annular lip seal when the first annular lip seal is new. Therefore, when the first annular lip seal on the housing side is new, most of the purge gas flow will exit through the second check valve.

[0014] The second check valve is equipped with a spring that biases the shutter of the second check valve to a closed seated position. The outlet duct can be fluidly connected to the gas injection duct between the overpressurizer and the lubricant housing in the inter-seal volume. The pressure gradient control device may include a lubricant filter positioned between the second check valve and the lubricant housing.

[0015] The overpressurization device may include a limiting unit and a pressurizing check valve installed in series within the gas injection duct. The opening threshold for a pressurized check valve is, for example, less than 20 millibars (or 2000 Pa), for example, 10 millibars (or 1000 Pa) or less. A pressurized check valve is, for example, a gravity check valve. The vacuum pump may include a casing that at least partially houses the gas injection duct and the gas outlet duct. This casing houses an overpressurizer, a check valve, and a lubricant filter. Other advantages and features of the present invention will become apparent from reading the description of the invention and the accompanying drawings. [Brief explanation of the drawing]

[0016] [Figure 1] It is a schematic diagram of a vacuum pump according to an embodiment of the present invention. [Figure 2] It is a cross-sectional view of the element of the seal clearance volume portion of the vacuum pump of FIG. 1 as viewed from the side. [Figure 3] It is a cross-sectional view of the element of FIG. 2 as viewed from above. [Figure 4A] It is a partial cross-sectional view of the casing region of the vacuum pump of FIG. 1. [Figure 4B] It is a view of the casing of FIG. 4A and the elements contained therein. [Figure 5] It is a graph showing the change in pressure (mbar) as a function of time (seconds) for the pressure (broken line curve P1) in the lubricant housing, the pressure (thin solid line curve P2) in the seal clearance volume, and the pressure (thick solid line curve P3) in the pump chamber of the vacuum pump of FIG. 1.

Mode for Carrying Out the Invention

[0017] Hereinafter, embodiments of the present invention will be described. The following embodiments are illustrative. Although one or more embodiments are referred to in this specification, this does not necessarily mean that each reference relates to the same embodiment, or that the features apply only to one embodiment. Also, without departing from the scope of the present invention defined in the claims, other embodiments can be provided by combining or exchanging the individual features of various embodiments. The "upstream" element means an element located ahead of other elements with respect to the circulation direction of the gas conveyed by the pump. On the other hand, the "downstream" element means an element located behind other elements with respect to the circulation direction of the gas conveyed by the pump.

[0018] The present invention applies to any type of dry vacuum pump having one or at least two pump stages (e.g., one to ten pump stages). The vacuum pump may be a roughing vacuum pump having multiple pump stages and configured to deliver the pumped gas to atmospheric pressure, or it may be a Roots pump or Roots blower type dry vacuum pump having one to three pump stages that are fluidically connected in series upstream of the roughing vacuum pump during use, and whose discharge pressure is the pressure obtained by the roughing vacuum pump.

[0019] Figure 1 shows a vacuum pump according to one embodiment of the present invention. This vacuum pump 1 comprises at least one lubricant housing 2, one pump chamber 3 including at least one pump stage T1 to T6, two rotating shafts 4 (see Figure 3) configured to rotate a pair of rotors 5 within the pump chamber 3, and at least one sealing device 6 disposed between the lubricant housing 2 and the pump chamber 3.

[0020] The vacuum pump 1 may have only one pump stage, or it may have multiple pump stages, such as six pump stages T1 to T6, installed in series between the inlet and outlet of the vacuum pump 1. Each pump stage T1 to T6 has an inlet and an outlet. If the vacuum pump 1 has multiple pump stages, each pump stage is fluidly connected in series successively by interstage channels that fluidly connect the outlet of the preceding pump stage to the inlet of the subsequent pump stage. The pump flow rates of each pump stage T1 to T6 decrease or remain constant depending on their position between the inlet and outlet of the vacuum pump. The flow rate generated at the lowest pressure of the first pump stage T1 corresponds to the maximum flow rate of the pump.

[0021] The two rotating shafts 4 drive the rotation of a pair of rotors 5 within the pump stages T1-T6 of the pump chamber 3, driving the gas to be pumped from the inlet to the outlet of the vacuum pump 1. These rotating shafts 4 are driven by at least one motor M of the vacuum pump 1.

[0022] While the pump is rotating, the transport gas drawn in from the inlet is confined in the space formed by the pair of rotors 5 and the stator, and is driven downstream by the pair of rotors 5. The pair of rotors 5 of the vacuum pump 1 have, for example, lobes of the same shape and employ, for example, a "roots" pump type (with an eight-shaped or bean-shaped cross-section), a three-lobed type, a "claw" type, a screw type, or other similar positive displacement vacuum pump principles. The rotating shaft 4 is guided to rotate within a bearing that is lubricated by a lubricant housed in the lubricant housing 2. The lubricant, such as oil or grease, can in particular lubricate the rolling bearing 7 and / or the synchronous gear 8 that synchronizes the rotating shaft 4.

[0023] The vacuum pump 1 is called "dry" because, during operation, a pair of rotors 5 rotate within the stator without mechanical contact with each other or with the stator, thus allowing the use of oil in the pump chamber 3. The vacuum pump 1 includes, for example, two lubricant housings 2, one at each end of the pump chamber 3, and a sealing device 6. This sealing device 6 is interposed between the lubricant housings 2 at both ends of the pump stages T1 and T6 and the pump stages T1 and T6 in each shaft passage to seal the lubricant.

[0024] The sealing device 6 comprises at least a first annular lip seal 9 and a second annular lip seal 10, which are arranged in series along each rotating shaft 4. Therefore, there are at least two first annular lip seals 9 and two second annular lip seals 10 (see Figure 3). The first annular lip seal 9 is interposed between the lubricant housing 2 and the second annular lip seal 10. The second annular lip seal 10 is interposed between the first annular lip seal 9 and the pump chamber 3.

[0025] The first annular lip seal 9 and the second annular lip seal 10 are, for example, single-lip type (only one lip). The lips of the first and second annular lip seals 9 and 10 rub against a friction ring attached to the rotating shaft 4. As best illustrated in Figure 2, the lips of the first annular lip seal 9 and the second annular lip seal 10 face the same direction, and when the pressures of the pump stages T1 and T6 of the pump chamber 3 adjacent to the second annular lip seal 10 are higher than the pressure in the interseal volume 11 located between the first annular lip seal 9 and the second annular lip seal 10, the lips are pressed against the rotating shaft 4. Conversely, if the pressure in the seal-to-seal volume 11 is higher than the pressure in the pump chamber 3, the lips of the annular lip seals 9 and 10 tend to lift, causing leakage.

[0026] The pump stages T1 and T6 adjacent to the first annular lip seal 9 and the second annular lip seal 10 are, in this example, the first pump stage and the final pump stage. The sealing device 6 may also include a “dynamic” seal, i.e., a non-friction seal such as a seal ring, a labyrinth seal, or a seal including a baffle or a “wall” of gas. Furthermore, the sealing device 6 may also include a deflector disc that is integrally mounted to the rotating shaft 4 and is disc-shaped overall. The sealing device 6 generates an extremely small conductance around the rotating shaft 4, and while allowing the rotation of the rotating shaft 4, it can significantly restrict the passage of lubricant from the lubricant housing 2 to the dry pump stage, and vice versa.

[0027] The vacuum pump 1 further includes a gas injection duct 12 fluidly connected to the lubricant housing 2, an overpressurizer 13 located within the gas injection duct 12, an outlet duct 14 fluidly connecting the lubricant housing 2 to the inter-seal volume 11, and a pressure gradient control device 15. The purge gas injected is, for example, nitrogen.

[0028] The gas injection duct 12 opens into the internal space of the lubricant housing 2, and its injection area is arranged in a phase configuration that minimizes the mixing of oil mist and purge gas. The inlet of the gas injection duct 12 is configured to be fluidly connected to the purge gas supply source. The overpressurizer 13 ensures that a small amount of purge gas flows into the lubricant housing 2 quickly and easily. The maximum injection flow rate of the purge gas is, for example, 3.2 slm (or 5.4 Pa·m) when the lubricant housing 2 is located on the final pump stage T6 side of the multistage vacuum pump. 3 / s), or if located on the lubricant housing 2 side on the first pump stage side of a multistage vacuum pump, or in the case of a Roots blower type vacuum pump, 0.5 slm (or 0.8 Pa·m 3 The flow rate of the purge gas is low, so the lubricant is not agitated by the inflow of the purge gas into the lubricant housing 2, nor is the lubricant drawn towards the pump chamber 3.

[0029] According to one embodiment of the present invention, the overpressurizer 13 comprises a limiting section 16, i.e., a low-conductance orifice (also called a calibration orifice or nozzle), attached in series to the gas injection duct 12, and a pressurizing check valve 17. The pressurizing check valve 17 is interposed between the limiting section 16 and the lubricant housing 2. This limiting unit 16 can limit the maximum value of the purge gas injection flow rate. The pressurized check valve 17 prevents gas from the lubricant housing 2 from entering the gas injection duct 12. Conversely, if the pressure difference between the gas injection duct 12 and the lubricant housing 2 is greater than the threshold to which the pressurized check valve 17 is calibrated, i.e., if the pressure inside the lubricant housing 2 is too low, the pressurized check valve 17 can be opened to supply purge gas from the gas injection duct 12 to the lubricant housing 2.

[0030] The pressurized check valve 17 is adjusted to allow the passage of purge gas toward the lubricant housing 2 when the pressure difference between the upstream and downstream sides of the pressurized check valve 17 exceeds a predetermined opening threshold. This opening threshold is low, for example, around a few mbar, for example, less than 20 mbar (or 2000 Pa), for example, 10 mbar (or 1000 Pa) or less. The pressurized check valve 17 is, for example, a gravity check valve, and its shutter is, for example, a ball, bullet-shaped, or other shaped object, which is held on the sheet in a closed position by the influence of gravity. In this way, the limiting section 16 and the pressurized check valve 17 mechanically control the flow of the injected gas, making implementation simple and inexpensive. Alternatively, the overpressure device 13 may be equipped with a flow controller or a flow meter (mass flow controller).

[0031] On the one hand, the flow rate of the purge gas supply and the flow rate of the purge gas supplied by the overpressure device 13 of the lubricant housing 2, and on the other hand, the flow rate of the purge gas discharged as a result of leakage through the first annular lip seal 9 on the lubricant housing 2 side and the pressure gradient control device 15 and the outlet duct 14, ensure that the lubricant housing 2 is always in an overpressurized state relative to the inter-seal volume 11. In other words, it can be ensured that the pressure inside the lubricant housing 2 is always higher than the pressure in the inter-seal volume 11. Therefore, the lubricant housing 2 can be pressurized with clean gas. The outlet duct 14, for example, fluidly connects the gas injection duct 12 between the overpressurizer 13 and the lubricant housing 2 to the seal-to-seal volume 11.

[0032] The pressure gradient control device 15 includes a second check valve 18 located in the outlet duct 14. The second check valve 18 is calibrated to allow purge gas to pass from the lubricant housing 2 to the seal-to-seal volume 11 when the pressure change between the upstream and downstream sides of the second check valve 18 exceeds a predetermined opening threshold and the pressure inside the lubricant housing 2 is higher than the pressure inside the seal-to-seal volume 11. The second check valve 18 also prevents leaked gas from the pump chamber 3 from entering the lubricant housing 2.

[0033] The opening threshold for the second check valve 18 is greater than, for example, 40 mbar (4000 Pa), and is in the range of, for example, 40 mbar (4000 Pa) to 100 mbar (10000 Pa), for example, 75 mbar (or 7500 Pa). This threshold allows the second check valve 18 to open before the purge gas begins to leak from the first annular lip seal 9 when the first annular lip seal 9 is new. Therefore, when the first annular lip seal 9 on the housing side is new, most of the purge gas flow flows out through the second check valve 18. The second check valve 18 is equipped with a spring 19, such as a compression spring, which biases the shutter of the second check valve 18, which is, for example, a ball, bullet-shaped, or other shaped object, to a closed seated position, in order to more easily achieve a predetermined threshold.

[0034] The pressure gradient control device 15 includes a lubricant filter 20 positioned between the second check valve 18 and the lubricant housing 2, which can capture minute droplets of oil mist that may enter the interseal volume 11 from the lubricant housing 2 via the second check valve 18. The lubricant filter 20, for example, a sintered stainless steel filter, can separate the oil or other lubricant from the gas so that the lubricant does not run out of the lubricant housing 2. If the outlet duct 14 is fluidly connected to the gas injection duct 12 between the overpressurizer 13 and the lubricant housing 2, the lubricant filter 20 can be placed on the gas injection duct 12 between the intersection of the outlet duct 14 and the lubricant housing 2.

[0035] According to one embodiment shown in Figures 4A and 4B, the vacuum pump 1 comprises a casing 21 fixed to the stator of the vacuum pump 1, which at least partially houses a gas injection duct 12 and an outlet duct 14. The casing 21 houses an overpressurization device 13 (particularly a pressurizing check valve 17 and a limiting section 16), a second check valve 18, and optionally a spring 19 that biases the second check valve 18 to the closed position, and / or a lubricant filter 20. Outside the casing 21, a coupling 22 may be provided at the inlet of the gas injection duct 12 for fluid connection of the duct 12 to a purge gas supply source. In another embodiment, the elements of the casing 21 may be located inside the body of the vacuum pump 1.

[0036] During operation, when the first annular lip seal 9 and the second annular lip seal 10 are new, their lips are pressed against the rotating shaft 4. In equilibrium, the pressure inside the lubricant housing 2 is high, causing gas to leak continuously from the lubricant housing 2 through the second check valve 18 into the interseal volume 11, and then leak from the interseal volume 11 below the lip of the second annular lip seal 10 towards the pump chamber 3.

[0037] Therefore, the pump chamber 3 allows the pressure in the interseal volume 11 to be reduced by suction through the leakage of the second annular lip seal 10. One advantage is that the pumping from the interseal volume 11 is performed directly by the pump stages T1-T6 of the pump chamber 3, i.e., by the vacuum pump 1 itself. When the first annular lip seal 9 wears down (first on the lubricant housing 2 side), the purge gas not only continues to leak through the second check valve 18, but also begins to pass under the lip of the first annular lip seal 9 and directly reach the interseal volume 11. Although the rate of purge gas flow leaking through the second check valve 18 decreases, the pressurization maintained within the lubricant housing 2 maintains the direction of flow from the lubricant housing 2 towards the pump chamber 3, thereby ensuring that no particles, powders, or other contaminants of the pumped gas enter the lubricant housing 2.

[0038] As the lip of the first annular lip seal 9 wears down, the flow rate of purge gas leaking from below the lip increases. As the flow rate of purge gas passing below the lip increases, the flow rate of purge gas leaking through the second check valve 18 decreases. Therefore, the presence of the pressure gradient control device 15 and the fact that the pressure inside the lubricant housing 2 is higher than the pressure inside the seal-to-seal volume 11 make it possible to adjust the flow rate of purge gas flowing out of the seal-to-seal volume 11 via the outlet duct 14 according to the degree of wear of the first annular lip seal 9. Another effect obtained as a result of the flow rate of purge gas leaking through the second check valve 18 decreasing in accordance with the increase in the flow rate of purge gas under the lip of the first annular lip seal 9 is that damage to these first annular lip seals 9 is limited.

[0039] These advantages of the present invention can be better understood by referring to the graph in Figure 5. This graph shows the pressure curve in the lubricant housing 2 on the final pump stage T6 side of the multistage vacuum pump 1 (curve P1), the pressure curve in the inter-seal volume 11 (curve P2), and the pressure curve in the second annular lip seal 10 in the final pump stage T6 of the pump chamber (curve P3). In equilibrium, i.e., when the first annular lip seal 9 in this example is new (very tightly sealed), the pressure P1 inside the lubricant housing 2 is approximately 1180 mbar (118,000 Pa), and the pressure P2 inside the seal-to-seal volume 11 is approximately 1080 mbar (108,000 Pa). The difference between these two pressures, 100 mbar (10,000 Pa), is due to the calibration value of the pressurized check valve 17 (approximately 75 mbar (7,500 Pa)) and various conductances.

[0040] The pressure P3 inside the pump chamber is the average of the suction and discharge pressures at the final pump stage T6. In this example, this average is approximately 760 mbar (or 76,000 Pa) in equilibrium. Therefore, the pressure P1 in the lubricant housing 2 is higher than the pressure P2 in the inter-seal volume 11, and the pressure P2 in the inter-seal volume 11 is higher than the pressure P3 in the final pump stage T6. The purge gas flows continuously from the lubricant housing 2 to the inter-seal volume 11 via the second check valve 18, and from the inter-seal volume 11 through the lip of the second annular lip seal 10 to the pump chamber 3.

[0041] When a shut-off valve located upstream of the vacuum pump 1 in the direction of the gas flow to be exhausted is opened at time t1 to evacuate a chamber (or equivalent) fluidly connected to the inlet of the vacuum pump 1, the gas pumped from the chamber rapidly flows into the vacuum pump 1, causing a sharp increase in the pressure at the final pump stage T6 (curve P3). That is, the pressure P3 at the final pump stage T6 becomes almost instantaneously higher than the pressure P2 in the inter-seal volume 11, and consequently, the second annular lip seal 10 tends to close. As a result of the shut-off valve, the overpressure device 13 continues to inject purge gas into the lubricant housing 2 and, consequently, into the seal cavity 11, causing the pressure P1 in the lubricant housing 2 and the pressure P2 in the seal cavity 11 to rise.

[0042] Subsequently, as the chamber volume is evacuated, the pressure P3 at the final pump stage T6 decreases again, falling below the pressure P2 in the inter-seal volume 11 (time t2). This means that the pressure P1 inside the lubricant housing 2 returns to a state where it is higher than the pressure P2 in the seal-to-seal volume 11, and this pressure P2 itself is higher than the pressure P3 in the final pump stage T6. Therefore, the purge gas leaks from the lubricant housing 2 into the seal-to-seal volume 11 via the second check valve 18, and then into the pump chamber 3 from below the lip of the second annular lip seal 10 in the seal-to-seal volume 11. As a result, the pressure P1 inside the lubricant housing 2 and the pressure P2 inside the seal-to-seal volume begin to decrease again.

[0043] The closure of the second annular lip seal 10 and the continuous injection of purge gas by the overpressure device 13 increase the pressure P1 in the lubricant housing 2 and the pressure P2 in the inter-seal volume 11, thereby reducing the time (1.5 seconds in this example) during which the pressure P3 in the pump chamber 3 is higher than the pressure P2 in the inter-seal volume 11. During this time, particles or contaminants may be drawn towards the lubricant housing 2, and the second annular lip seal 10 on the pump side may wear down. Although Figure 1 shows an overpressurizer 13 and a pressure gradient control device 15 for only one of the two lubricant housings 2, the present invention can be applied to one or the other, or both, of the two lubricant housings 2 of the vacuum pump 1. [Explanation of Symbols]

[0044] 1. Dry vacuum pump 2 Lubricant Housing 3 Pump Room 4 rotation axes 5 rotors 6. Sealing device 9. First annular lip seal 10. Second annular lip seal 11 Seal-to-seal volume 12 Gas injection duct 13. Overpressurization device 14 Outlet duct 15 Pressure gradient control device 16 Restriction section 17. Pressurized check valve 18. Second check valve 19 Spring 20 Lubricant filter 21 Casing 22 Coupling M Motor T1-T6 Pump Stages

Claims

1. A dry vacuum pump (1) comprising a lubricant housing (2), a pump chamber (3) including at least one pump stage (T1 to T6), two rotating shafts (4) configured to drive the rotation of a pair of rotors (5) in the pump chamber (3), at least one first annular lip seal (9) and a second annular lip seal (10), a gas injection duct (12) fluidly connected to the lubricant housing (2), and an overpressurizer (13) disposed within the gas injection duct (12), The rotating shaft (4) is guided to rotate within a bearing that is lubricated by the lubricant contained in the lubricant housing (2), The first annular lip seal (9) and the second annular lip seal (10) are interposed between the lubricant housing (2) and the pump chamber (3), and are arranged in series along each of the rotating shafts (4). The lips of the first annular lip seal (9) and the second annular lip seal (10) are facing the same direction, and the pressure of the pump stage (T1, T6) in the pump chamber (3) adjacent to the second annular lip seal (10) is higher than the pressure of the inter-seal volume (11) located between the first annular lip seal (9) and the second annular lip seal (10), pressing the lips of the first annular lip seal (9) and the second annular lip seal (10) against the rotating shaft (4). The aforementioned dry vacuum pump further, The system includes an outlet duct (14) that fluidly connects the lubricant housing (2) to the seal-to-seal volume (11), and a pressure gradient control device (15) that includes a second check valve (18) positioned on the outlet duct (14). A dry vacuum pump characterized in that the pressure gradient control device (15) is configured to control the flow of purge gas from the lubricant housing (2) towards the seal volume (11) when the pressure fluctuation between the upstream and downstream sides of the second check valve (18) exceeds a predetermined opening threshold and the pressure inside the lubricant housing (2) becomes higher than the pressure inside the seal volume (11).

2. A dry vacuum pump (1) according to claim 1, characterized in that the first annular lip seal (9) and the second annular lip seal (10) are single-lip type seals.

3. A dry vacuum pump (1) according to claim 1, characterized in that the second check valve (18) is provided with a spring (19) that biases the shutter of the second check valve (18) to the closed position.

4. A dry vacuum pump (1) according to claim 1, characterized in that the opening threshold of the second check valve (18) is greater than 4000 Pa.

5. A dry vacuum pump (1) according to claim 1, characterized in that the outflow duct (14) fluidly connects the gas injection duct (12) between the overpressure device (13) and the lubricant housing (2) to the seal-to-seal volume (11).

6. A dry vacuum pump according to claim 1, wherein the pressure gradient control device (15) comprises a lubricant filter (20) disposed between the second check valve (18) and the lubricant housing (2).

7. A dry vacuum pump (1) according to claim 1, wherein the overpressure device (13) comprises a limiting unit (16) and a pressure check valve (17) installed in series within the gas injection duct (12).

8. A dry vacuum pump (1) according to claim 7, characterized in that the pressurized check valve (17) is a gravity check valve.

9. A dry vacuum pump (1) according to claim 7, characterized in that the opening threshold of the pressurized check valve (17) is less than 2000 Pa.

10. A vacuum pump (1) according to claim 6, comprising a casing (21) in which the gas injection duct (12) and the outlet duct (14) are at least partially housed, A dry vacuum pump characterized in that the casing (21) houses the overpressure device (13), the second check valve (18), and the lubricant filter (20).

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