Oil circuit, oil-free compressor provided with such oil circuit and a method to control lubrication and/or cooling of such oil-free compressor via such oil circuit

Abstract

An oil circuit for lubrication and cooling of an oil-free compressor with an oil reservoir and a rotary oil pump to drive oil to the compressor element and/or the motor via an oil pipe. The rotary oil pump has a rotor mounted on a rotation shaft, and is driven by the motor of the compressor. The oil circuit is provided with a bypass pipe and a pressure-actuated bypass valve which guide a portion of the oil back to the oil reservoir without this portion of the oil passing through the compressor element and/or the motor during its way back to the oil reservoir. The oil circuit is further provided with an oil cooler in the bypass pipe. The bypass valve is in the oil pipe.

Claims

1. An oil circuit for lubrication and cooling of an oil-free compressor comprising a motor with a variable speed and a compressor element driven by said motor, whereby this oil circuit is provided with an oil reservoir with oil and a rotary oil pump configured to drive oil from the oil reservoir through an inlet channel upstream the rotary oil pump to the compressor element and/or the motor via an oil pipe; whereby this rotary oil pump is provided with a rotor mounted on a rotation shaft, whereby this rotary oil pump has a swept volume, and whereby this rotary oil pump is driven by the motor of the compressor element; whereby the oil circuit is further provided with a return pipe configured to guide oil from the compressor element and/or the motor back to the oil reservoir; whereby the oil circuit is further provided with a bypass pipe and a pressure-actuated bypass valve which are configured to directly guide a portion of the oil between the rotary oil pump and the compressor element and/or the motor back to the oil reservoir without this portion of the oil passing through the compressor element and/or the motor during its way back to the oil reservoir; and whereby the oil circuit is further provided with an oil cooler, wherein the oil cooler is placed in the bypass pipe and the bypass valve is placed in the oil pipe.

2. The oil circuit according to claim 1, wherein the oil circuit is provided with only one rotary oil pump.

3. The oil circuit according to claim 1, wherein the oil cooler has a fixed or constant cooling capacity.

4. The oil circuit according to claim 1, wherein the bypass valve is a mechanical valve, preferably a spring-loaded valve.

5. The oil circuit according to claim 1, wherein the inlet channel is provided with a dam with a height that is higher than a height of a centerline of the rotation shaft of the rotary oil pump reduced with a smallest diameter of the rotor of the rotary oil pump divided by two.

6. The oil circuit according to claim 5, wherein the height of the dam is smaller than the height of the centreline of the rotation shaft of the rotary oil pump reduced with a smallest diameter of the rotation shaft of the rotary oil pump divided by two.

7. The oil circuit according to claim 5, wherein the dam is configured such that the rotary oil pump and the inlet channel are able to contain a volume of the oil between the rotary oil pump and the dam which is at least twice the swept volume of the rotary oil pump.

8. The oil circuit according to claim 5, wherein the oil circuit is provided with a sensor configured to register whether oil is present between the rotary oil pump and the dam.

9. The oil circuit according to claim 5, wherein the oil circuit is provided with a fluid connection between the oil reservoir and a space in the inlet channel between the rotary oil pump and the dam, whereby the fluid connection is configured to transfer oil from the oil reservoir to the space between the rotary oil pump and the dam.

10. An oil-free compressor comprising an oil circuit for its lubrication and cooling, whereby this oil-free compressor comprises a motor with a variable speed and a compressor element driven by said motor; whereby this oil circuit is provided with an oil reservoir with oil and a rotary oil pump configured to drive oil from the oil reservoir through an inlet channel upstream the rotary oil pump to the compressor element and/or the motor via an oil pipe; whereby this rotary oil pump is provided with a rotor mounted on a rotation shaft, whereby this rotary oil pump has a swept volume, and whereby this rotary oil pump is driven by the motor of the compressor element; whereby the oil circuit is further provided with a return pipe configured to guide oil from the compressor element and/or the motor back to the oil reservoir; whereby the oil circuit is further provided with a bypass pipe and a pressure-actuated bypass valve which are configured to directly guide a portion of the oil between the rotary oil pump and the compressor element and/or the motor back to the oil reservoir without this portion of the oil passing through the compressor element and/or the motor during its way back to the oil reservoir; and whereby the oil circuit is further provided with an oil cooler, wherein the oil-free compressor is configured such that the oil cooler is placed in the bypass pipe and the bypass valve is placed in the oil pipe.

11. The oil-free compressor according to claim 10, wherein the oil circuit is provided with only one rotary oil pump.

12. The oil-free compressor according to claim 10, wherein the oil cooler has a fixed or constant cooling capacity.

13. The oil-free compressor according to claim 10, wherein the bypass valve is a mechanical valve, preferably a spring-loaded valve.

14. The oil-free compressor according to claim 10, wherein the inlet channel is provided with a dam with a height that is higher than a height of a centreline of the rotation shaft of the rotary oil pump reduced with a smallest diameter of the rotor of the rotary oil pump divided by two.

15. The oil-free compressor according to claim 14, wherein the height of the dam is smaller than the height of the centreline of the rotation shaft of the rotary oil pump reduced with a smallest diameter of the rotation shaft of the rotary oil pump divided by two.

16. The oil-free compressor according to claim 14, wherein the dam is configured such that the rotary oil pump and the inlet channel are able to contain a volume of the oil between the rotary oil pump and the dam which is at least twice the swept volume of the rotary oil pump.

17. The oil-free compressor according to claim 14, wherein the oil circuit is provided with a sensor configured to register whether oil is present between the rotary oil pump and the dam.

18. The oil-free compressor according to claim 14, wherein the oil circuit is provided with a fluid connection between the oil reservoir and a space in the inlet channel between the rotary oil pump and the dam, whereby the fluid connection is configured to transfer oil from the oil reservoir to the space between the rotary oil pump and the dam.

19. The oil-free compressor according to claim 10, wherein the oil-free compressor is an oil-free screw compressor.

20. A method to control lubrication and/or cooling of an oil-free compressor via an oil circuit, whereby this oil-free compressor comprises a motor with a variable speed and a compressor element driven by said motor; whereby this oil circuit is provided with an oil reservoir with oil and a rotary oil pump configured to drive oil from the oil reservoir through an inlet channel upstream the rotary oil pump to the compressor element and/or the motor via an oil pipe; whereby this rotary oil pump is provided with a rotor mounted on a rotation shaft, and whereby this rotary oil pump is driven by the motor of the compressor element; whereby the oil circuit is further provided with a bypass pipe and a pressure-actuated bypass valve through which a portion of the oil between the rotary oil pump and the compressor element and/or the motor is directly guided back to the oil reservoir without this portion of the oil passing through the compressor element and/or motor during its way back to the oil reservoir; and whereby the oil circuit is further provided with an oil cooler, wherein the portion of the pumped oil which is guided back to the oil reservoir through the bypass pipe and the bypass valve, passes through the oil cooler which is placed in the bypass pipe, comprising controlling the bypass valve such that a preset pressure is reached in the oil pipe between the bypass valve and the compressor element and/or motor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) With the intention of better showing the characteristics of the invention, a few preferred embodiments of an oil circuit according to the invention and an oil-free compressor provided with such an oil circuit are described hereinafter, by way of an example without limiting nature, with reference to the accompanying drawings, wherein:

(2) FIG. 1 schematically shows an oil-free compressor provided with an oil circuit according to the invention;

(3) FIG. 2 schematically shows the change of the flow rate of the rotary oil pump as a function of the motor speed;

(4) FIG. 3 shows the change of the pressure in the oil pipe downstream from the bypass valve as a function of the motor speed;

(5) FIG. 4 schematically shows the motor and the rotary oil pump of FIG. 1 in more detail;

(6) FIG. 5 shows a view according to arrow F3 in FIG. 4, whereby a housing of the rotary oil pump is partly cut away;

(7) FIG. 6 shows in more detail the part that is indicated by F4 in FIG. 5;

(8) FIG. 7 shows an alternative embodiment to the part in FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

(9) In this case the oil-free compressor 1 shown in FIG. 1 is a screw compressor device with a screw compressor element 2, a transmission 3 (or ‘gearbox’) and a motor 4 with variable speed, whereby the oil-free compressor 1 is provided with an oil circuit 5 according to the invention.

(10) According to the invention, it is not necessary for the oil-free compressor 1 to be a screw compressor 1, as the compressor element 2 could also be of a different type, e.g. a tooth compressor element, scroll compressor element, vane compressor element, etc.

(11) The compressor element 2 is provided with a housing 6 with an inlet 7 to draw in a gas and an outlet 8 for compressed gas. Two mating helical rotors 9 are mounted on bearings in the housing 6.

(12) The oil circuit 5 will supply the oil-free compressor 1 with oil 11 to lubricate and if need be cool the components of the oil-free compressor 1.

(13) These components are for example the gears in the transmission 3, the bearings on which the helical rotors 9 are mounted in the compressor element 2, etc.

(14) The oil circuit 5 comprises an oil reservoir 10 with oil 11 and an oil pipe 12 to bring the oil 11 to the components of the oil-free compressor 1 to be lubricated and/or cooled.

(15) A rotary oil pump 13 is provided in the oil pipe 12 to be able to pump oil 11 from the oil reservoir 10.

(16) The rotary oil pump 13 is driven by the motor 4 of the compressor element 2.

(17) The rotary oil pump 13 can be connected directly to the shaft of the motor 4 or to a drive shaft. This drive shaft is then connected to the motor 4 via a coupling. Then the gear is mounted on the driveshaft that is driven by the gearbox. One or more compressor elements 2 can be driven via the gearbox.

(18) A bypass valve 14 and a bypass pipe 15, that leads from the oil pipe 12 back to the oil reservoir 10, are provided in the oil pipe 12 downstream from the rotary oil pump 13.

(19) Although in the example shown the bypass valve 14 is affixed in the oil pipe 12, it is not excluded that the bypass valve is affixed in the bypass pipe 15. It is not excluded either that a three-way valve is used that is affixed at the location of the connection of the oil pipe 12 to the bypass pipe 15.

(20) The bypass valve 14 will distribute the oil 11 that is pumped by the rotary oil pump 13: a part will be driven to the components of the oil-free compressor 1 to be lubricated and/or cooled via the oil pipe 12, the other part will be driven back to the oil reservoir 10 via the bypass pipe 15.

(21) In this case, but not necessarily, the bypass valve 14 is a mechanical valve 14.

(22) In a preferred embodiment, the valve 14 is a spring-loaded valve, i.e. the valve 14 comprises a spring or spring element, whereby the spring will open the valve 14 more or less depending on a pressure p upstream or downstream the valve 14.

(23) In this case the valve will be a spring-loaded valve 14 that will close and open the bypass pipe 15 depending on the pressure p downstream of the valve 14. When a certain threshold value of the pressure p is exceeded, the valve 14 will open the bypass pipe 14 so that a portion of the pumped oil 11 will flow via the bypass pipe 15 to the oil reservoir 10.

(24) According to the invention an oil cooler 16 is placed in the bypass pipe 15. This means that the oil 11 that flows via the bypass pipe 15 can be cooled, but that the oil 11 that flows via the oil pipe 12 to the components to be lubricated and/or cooled will not be cooled.

(25) In other words: cooled cold oil 11 will be guided to the oil reservoir 10 via the bypass pipe 15.

(26) In this case the aforementioned oil cooler 16 forms part of a heat exchanger 17. The oil cooler 16 could be a plate cooler for example, but any type of cooler that is suitable for cooling the oil 11 can be used in this invention.

(27) In this case the oil cooler 16 has a fixed or constant cooling capacity for a given oil flow and flow of a coolant. This means that the cooling capacity cannot be adjusted. By adjusting the flow of the coolant, it would indeed be possible to adjust the cooling capacity. However, this is not necessary.

(28) From the bypass valve 14, the oil pipe 12 runs to the components of the oil-free compressor 1 to be lubricated and cooled if need be. Here the oil pipe 12 will be divided into subpipes 18 that may be partly integrated in the compressor element 2.

(29) Furthermore, the oil circuit 5 is provided with a return pipe 19 to carry the oil 11 from the compressor element 2 back to the oil reservoir 10, after it has lubricated and if need be cooled the components.

(30) This oil 11 will have a higher temperature.

(31) In the oil reservoir 10 this hot oil 11 will be mixed with the cooled cold oil 11 that is guided to the oil reservoir 10 via the bypass pipe 15.

(32) The operation of the oil-free compressor 1 with the oil circuit 5 is very simple and as follows.

(33) When the compressor element 2 is driven by the motor 4, the mating rotating helical rotors 9 will draw in and compress air.

(34) During the operation, the different components of the compressor element 2, the transmission 3 and the motor 4 will be lubricated and cooled.

(35) As the rotary oil pump 13 is driven by the motor 4 of the compressor element 2, as of the start-up of the oil-free compressor 1 it will pump oil 11 and drive it to the components of the oil-free compressor 1 to be lubricated and cooled via the oil pipe 12 and subpipes 18.

(36) The change of the flow rate Q of the rotary oil pump 13 as a function of the speed n of the motor 4 is shown in FIG. 2.

(37) As can be seen from this drawing, at low speeds n the rotary oil pump 13 will pump less oil 11 compared to at high speeds n. This is advantageous, as at low speeds n less lubrication and cooling will be required and more at high speeds n.

(38) At low speeds n, all oil 11 that is pumped will be driven to the compressor element 2 and the motor 4, i.e. the bypass valve 14 will close the bypass pipe 15 so that no oil 11 can flow back to the oil reservoir 10 along the bypass pipe 15 and the oil cooler 16. As at low speeds n no cooling is required as the oil 11 will barely warm up, this is not a problem and this will ensure that the oil 11 does not get too cold.

(39) The change of the pressure p in the oil pipe 12 downstream from the bypass valve 14 is shown in FIG. 3.

(40) The pressure will systematically rise in proportion to the speed n, until a specific pressure p′ is reached corresponding to the speed n′.

(41) As of this speed n′ a pressure p′ is reached such that the bypass valve 14 will partially be opened to the bypass pipe 15.

(42) As a result, at higher speeds than n′, a portion of the pumped oil 11 will be driven through the bypass valve 14 via the bypass pipe 15.

(43) This is schematically shown in FIG. 2 whereby the curve is divided into two branches: a portion of the oil flow Q corresponding to zone I will be driven via the oil pipe 12 to the components of the oil-free compressor 1 to be lubricated and cooled, while the other portion of the oil flow Q corresponding to zone II will be driven back to the oil reservoir 10 via the bypass pipe 15.

(44) Because the bypass valve 14 will open, as of the speed n′ the pressure p will no longer rise in proportion to the speed n of the motor 4, but the curve flattens out, as shown in FIG. 3.

(45) The higher the speed n, the more the bypass valve 15 will be pushed open by the higher pressure p downstream from the bypass valve 15 in the oil pipe 12. Indeed, at a higher speed n, the flow rate Q of the rotary oil pump 13 will be greater, so that this pressure p will also rise such that the bypass valve 14 will open more.

(46) The spring characteristics of the spring-loaded bypass valve 14 are chosen such that the bypass valve 14 is controlled by the spring such that a preset pressure p is reached in the oil pipe 12 between the bypass valve 14 and the compressor element 2 and/or the motor 4 according to the curve of FIG. 3.

(47) The oil 11 that is guided via the bypass pipe 15 will pass through and be cooled by the oil cooler 16.

(48) Because the cooled oil 11 that is guided via the bypass pipe 15 comes to the oil reservoir 10, the temperature of the oil 11 in the oil reservoir 10 will fall. This cold(er) oil 11 is then pumped by the rotary oil pump 13 and brought to the compressor element 2 and/or motor 4.

(49) As at high speeds n more heat is generated in the oil-free compressor 1, more cooling will be required which is taken care of precisely by the above method.

(50) At increasing speeds n, the rotary oil pump 13 will always pump more oil 11 from the oil reservoir 10. As the pressure p downstream of the bypass valve 14 will always be higher as a result, this bypass valve 14 will respond to this by always guiding more oil 11 via the bypass pipe 15, so that the pressure p does not rise too high and continues to follow the curve of FIG. 3.

(51) As a result, with increasing speeds n, ever more oil 11 will be cooled, so that the rising temperature of the oil-free compressor 1 can be accommodated at these increasing speeds n.

(52) This is shown in FIG. 2, whereby the zone II always becomes greater at higher speeds n.

(53) The above clearly shows that at low speeds n little or no oil 11 is cooled, while at increasing speeds n ever more oil 11 is cooled.

(54) As a result of this, the oil temperature will be more constant and higher on average, which ensures that the viscosity of the oil 11 will be lower on average so that there are fewer oil losses in the rotary oil pump 13 and at the lubrication locations.

(55) As can be further seen from FIG. 2, at all speeds n the oil flow Q that goes via the bypass pipe 15 and the oil cooler 16 (zone II) will be smaller than the oil flow Q that is driven to the compressor element 2 and/or the motor 4 (zone I).

(56) This means that the oil cooler 16 can have smaller dimensions compared to the known cooling circuits.

(57) The oil 11 of the compressor element 2 and/or the motor 4 will be driven back to the oil reservoir 10 via the return pipe 19.

(58) This oil 11 will have a higher temperature than the oil 11 in the oil reservoir 10.

(59) In addition to this hot oil 11, the cooled oil 11 will also come to the oil reservoir 10 via the bypass pipe 15.

(60) The two will be mixed together in the oil reservoir 10, which will result in an oil 11 at a certain temperature between the temperature of the cooled oil 11 and the hot oil 11.

(61) As of the oil reservoir 10, the rotary oil pump 13 will again pump the oil 11 and the method and control set out above will be followed.

(62) Although in the example shown, a spring-loaded mechanical valve is used as a bypass valve 14, it is possible to use an electronic bypass valve 14 that is controlled by a controller 20.

(63) In FIG. 1, this controller 20 is shown by a dotted line by way of an example. This controller 20 will control the bypass valve 14, for example on the basis of a signal from a pressure sensor 21 that is placed downstream from the bypass valve 14 in the oil pipe 12. The controller 20 will control the bypass valve 14 so that the pressure p, as registered by the pressure sensor 21, will follow the path of the curve of FIG. 3. In other words: the bypass valve 14 is controlled such that a preset pressure p is reached in the oil pipe 12 between the bypass valve 14 and the compressor element 2 and/or the motor 4.

(64) Although in the examples shown and described, the oil circuit 5 is shown separate from the compressor element 2 and the motor 4, it is of course not excluded that the oil circuit 5 is integrated in or physically forms part of the compressor element 2 and/or the motor 4.

(65) In all embodiments shown and described above it is possible that the oil circuit 5 also comprises an oil filter. This oil filter can for example, but not necessarily, be affixed in the oil pipe 12 downstream from the bypass valve 14. The oil filter will collect any contaminants from the oil 11 before sending it to the compressor element 2 and/or the motor 4.

(66) The motor 4 will directly drive the compressor element 2 as well as the rotary oil pump 13. In FIG. 4, it is shown that a rotation shaft 22 of the motor 4 is directly driving the rotary oil pump 13.

(67) The oil circuit 5 will allow that the rotary oil pump 13 pumps up oil 11 from the oil reservoir 10 through an inlet channel 23 before the rotary oil pump 13, after which the oil 11 may be guided through the pipe 12 and the subpipes 18 to the nozzles that are positioned on specific locations in the motor 4 and/or the compressor element 2 for the lubrication and/or cooling of one or more bearings and other components of the oil-free compressor 1.

(68) As the rotary oil pump 13 is driven by the motor 4 of the compressor element 2, it will be at a considerably higher position level than the oil reservoir 10. This means that the inlet channel 23, which is running from the oil reservoir 10 to the rotary oil pump 13, is relatively long.

(69) The rotary oil pump 13 comprises a housing 24 wherein a stator 25 and a rotor 26 are mounted. The rotor 26 is mounted on a rotation shaft 27, which is driven by the rotation shaft 22 of the motor 4.

(70) The rotary oil pump 13 is a gerotor pump, however this is not a prerequisite of the invention.

(71) The housing 24 is provided with an inlet port 28 for oil 11, to which the inlet channel 23 is connected, and with an outlet port 29 for the pumped oil 11.

(72) In FIG. 5, the inlet port 28 and the outlet port 29 are clearly visible.

(73) As shown in FIG. 6, the inlet channel 23 is provided with a dam 30 near the rotary oil pump 13.

(74) By ‘dam 30’ is meant a structure which ensures that, when the motor 4 has stopped, a certain volume of oil 11 will remain in a space 31 in the inlet channel 23 which is dammed by the dam 30.

(75) By ‘near the rotary oil pump 13’ is meant that the aforementioned remaining volume of oil 11 will remain at a location such that the rotary oil pump 13 is able to pump up the oil 11 immediately at the start-up of the rotary oil pump 13.

(76) This means that the aforementioned remaining volume of oil 11 will for example at least partly be present in the rotary oil pump 13 or that the remaining volume of oil 11 will be located in the inlet channel 23 right next to inlet port 28 of the rotary oil pump 13.

(77) In FIG. 6, it is clearly visible that the dam 30 has a minimal height equal to the height A of the centreline 32 of the rotation shaft 27 of the rotary oil pump 13 reduced with half a smallest diameter B of the rotor 26 of the rotary oil pump 13.

(78) By making the dam 30 at least as high as this minimal height, indicated by the line C, enough oil 11 will remain in the by the dam 30 dammed space 31 in the inlet channel 23 between the dam 30 and the rotary oil pump 13, whereby the rotary oil pump 13 is completely wetted internally at start-up of the oil-free compressor 1. Due to this immediate internal wetting of the rotary oil pump 13 with oil 11, the rotor 26 and the stator 25 will be immediately sealed by this oil 11 such that the suction power of the rotary oil pump 13 is immediately maximal.

(79) In this case, and preferably, a height D of the dam 30 is smaller than a maximal height equal to the height A of the centreline 32 of the rotation shaft 27 of the rotary oil pump 13 reduced with half a diameter E of the rotation shaft 27 of the rotary oil pump 13.

(80) If the dam 30 would be higher than this maximal height, indicated by the line F, the level of the remaining oil 11 would be higher than a lowest point of the rotation shaft 27 of the rotary oil pump 13. Because of this, oil 11 would possibly leak via the rotation shaft 27 of the rotary oil pump 13 and/or sealings would need to be provided on the rotation shaft 27 of the rotary oil pump 13 to avoid this.

(81) Next to the minimum height C and maximum height F of the dam 30, the configuration of the dam 30 is such that in this case, and preferably, the volume of the oil 11 which might be present between the rotary oil pump 13 and the dam 30 in the rotary oil pump 13 and the inlet channel 23, is at least twice a swept volume of the rotary oil pump 13.

(82) This has the advantage that immediately enough oil 11 is available in the rotary oil pump 13 and the inlet channel 23 at start-up of the rotary oil pump 13, such that it is not only possible to immediately wet the rotary pump 13 internally, but also to immediately pump up or pump through a volume of oil 11 via the outlet port 29 to the oil circuit 5 and so further to the components of the oil-free compressor 1 to be lubricated and/or cooled.

(83) Despite the dam 30 in FIGS. 5 and 6 being designed as a slanting slope towards the rotor 26 and the stator 25 of the rotary oil pump 13, it is not excluded that the dam 30 has another configuration.

(84) In FIG. 7 an alternative configuration is shown, whereby the dam 30 has a stepped form, whereby the inlet channel 23 is as it were provided with a step 33.

(85) Although this embodiment has the advantage that more oil 11 will remain in the space 31 between the dam 30 and the rotary oil pump 13, it does have the disadvantage that at the suction of the oil 11, the oil 11 so to speak flows down via the step 33, which may result in undesired turbulences. In the embodiments of FIGS. 5 and 6, the oil 11 will so to speak flow down from the dam 30.

(86) The operation of the oil-free compressor 1 is very straightforward and as follows.

(87) For the start-up of the oil-free compressor 1, preferably the following steps are taken: bringing oil 11 into the oil circuit 5 at a position downstream and higher than the rotary oil pump 13 until the space 31 is completely filled with oil 11; and then starting the motor 4.

(88) The oil 11 that is brought into the oil circuit 5 may flow down to the rotary oil pump 13 and fill both the rotary oil pump 13 and the inlet channel 23 in the space 31 between the dam 30 and the rotary oil pump 13 to a level equal to the height D of the dam 30.

(89) When the motor 4 is then started, the compressor element 2 and the rotary oil pump 13 will be driven and the oil 11 that is brought into the oil circuit 5 and is now located in the rotary oil pump 13 and the aforementioned space 31, will ensure that the rotary oil pump 13 is able to immediately pump and transfer oil 11 to the oil circuit 5, such that the compressor element 2 is immediately provided with the necessary oil 11 right from the start-up of the oil-free compressor 1.

(90) Alternatively, it is also possible that firstly a lubricant which is less volatile than the oil 11 is brought into the rotary oil pump 13 internally, before the motor 4 is started.

(91) Such method is preferably applied at the assembly of the oil-free compressor 1, such that at a first start-up of the oil-free compressor 1, the less volatile lubricant is present in the rotary oil pump 13.

(92) It is of course not excluded that both methods are combined, whereby at the first start-up a less volatile lubricant is brought in and whereby at a subsequent start-up of the oil-free compressor 1 oil 11 is brought into the oil circuit 5.

(93) From the moment that the motor 4 is started, the rotary oil pump 13 will immediately pump up oil 11 from the oil reservoir 10 via the inlet channel 23.

(94) The pumped oil 11 will then leave the rotary oil pump 13 via the outlet port 29 and come into the oil circuit 5 from where it is transferred to different nozzles at different to be lubricated and/or cooled components of the compressor element 2 and/or the motor 4.

(95) The compressor element 2 and/or the motor 4 will therefore be almost immediately provided with oil 11 from the start-up of the motor 4 and the oil-free compressor 1.

(96) It is not excluded that the oil-free compressor 1 comprises a sensor configured to register whether oil 11 is present in the space 31 between the rotary oil pump 13 and the dam 30.

(97) The aforementioned sensor may be any type of oil-level sensor, but also an oil pressure sensor or oil temperature sensor according to the invention.

(98) For the start-up of an oil-free compressor 1 with such sensor, the motor 4 is preferably only started after oil 11 has been detected in the inlet channel 23 between the rotary pump 13 and the dam 30.

(99) If no oil 11 is detected, the oil-free compressor 1 is not started, but instead a warning signal is sent out to the user.

(100) It is clear that the sensor and the aforementioned method to control the lubrication and/or cooling of the oil-free compressor 1 at start-up, may be combined with the previously described methods. This method will incorporate an additional safety feature to prevent that the oil-free compressor 1 may be started without oil 11 being present in the inlet channel 23 between the rotary oil pump 13 and the dam 30.

(101) It is also possible that the oil-free compressor 1 comprises a fluid connection between the oil reservoir 10 and the space 31 between the rotary oil pump 13 and the dam 30, whereby the fluid connection is configured to transfer oil 11 from the oil reservoir 10 to the space 31 between the rotary oil pump 13 and the dam 30.

(102) This may for example be realized by means of a small pump which is manually or electrically operated.

(103) When the oil-free compressor 1 is provided with such a fluid connection, the following method may be executed for the start-up of the oil-free compressor 1: transferring oil 11 from the oil reservoir 10 to the space 31 between the rotary oil pump 13 and the dam 30, until the space 31 is completely filled with oil 11; and then starting the motor 4.

(104) It is of course not excluded that the oil-free compressor 1 is also provided with a sensor configured to register whether oil 11 is present in the inlet channel 23 between the dam 30 and the rotary oil pump 13.

(105) In this case, when no oil 11 is detected at start-up, a signal will be sent out to the user to transfer oil 11 from the oil reservoir 10 to the space 31 between the rotary oil pump 13 and the dam 30 by operating the small pump or, when this small pump operates electrically, the small pump will be automatically started by the oil-free compressor 1 in order to ensure that oil 11 is transferred from the oil reservoir 10 to the space 31 between the rotary oil pump 13 and the dam 30, after which it is possible to start the motor 4 smoothly without problems.

(106) The present invention is by no means limited to the embodiments described as an example and shown in the drawings, but an oil circuit according to the invention and an oil-free compressor provided with such an oil circuit can be realised in all kinds of forms and dimensions without departing from the scope of the invention.