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

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.

The present invention relates to an oil circuit, an 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.

BACKGROUND OF THE INVENTION

More specifically, the invention is intended to provide an improved oil circuit and an improved method to control lubrication and/or cooling of an oil-free compressor comprising a motor with a variable rpm or speed, i.e. with a variable speed drive (VSD) control, via this improved oil circuit.

It is known that an oil circuit is used to lubricate and cool components in such a motor.

These components are for example, but not limited to, bearings and gears of the motor.

At high motor rpms these bearings and gears need a precisely dosed oil lubrication: neither too much oil, which may lead to hydraulic losses and even overheating; nor too little oil, which may result in excessive friction and overheating.

Therefore, oil jet lubrication is applied, whereby oil is targeted precisely to a location where the oil is needed by means of nozzles with a very precise configuration.

This location may be a raceway of the bearings or the location where teeth of the gears engage with each other.

The oil in the oil circuit needs to be cooled, in order to avoid overheating of the oil in the oil circuit and concomitant changes in lubricating properties of the oil.

The oil circuit which provides the nozzles with filtered and cooled oil at a preset pressure level, typically comprise an oil reservoir, a rotary oil pump, an oil cooler, an oil filter, and connecting pipes, which may be integrated in other components of the oil-free compressor. Furthermore, there are often minimum pressure valves, bypass pipes, oil pressure sensors and oil temperature sensors.

Traditionally an oil circuit for such an oil-free compressor is arranged as follows.

Oil is pumped from an oil reservoir using a rotary oil pump, after which the oil is guided to an oil cooler. The cooler will cool the oil before it is brought to any components to be lubricated and any components to be cooled of the oil-free compressor.

During lubrication and cooling, the temperature of the oil will rise.

After the oil has flown through the components of the oil-free compressor to be lubricated and/or cooled, it will be guided back to the oil reservoir via a return pipe. The hot oil will be guided by the rotary oil pump from the oil reservoir to the oil cooler, where the oil will be cooled before being guided to the components of the oil-free compressor again.

The aforementioned rotary oil pump has an important role: if not enough oil is delivered in time to the nozzles, an insufficient lubrication may result in damage or failure of the bearings and/or gears.

It is possible to make use of a rotary oil pump which is driven by a separate motor.

This has the advantage that the rotary oil pump may be controlled, but the disadvantage that a separate motor and control unit for this motor are needed. As a result, the oil-free compressor will not only be more expensive, but also larger and furthermore the oil-free compressor will comprise additional components which need to be maintained and are prone to failure.

For this reason, it is interesting to drive the rotary oil pump by the same motor as a compressor element of the oil-free compressor. This will ensure that the rotary oil pump is working when the compressor element is in operation. This also means that at a higher speed or rpm of the motor and the compressor element of the oil-free compressor, when more oil is required for lubrication and cooling of the oil-free compressor, more oil is pumped and guided to the oil cooler and then the motor and/or the compressor element.

However, the oil pressure may not rise too high, and at higher speeds or rpm of the motor and the compressor element, the rotary oil pump will pump so much oil that the pressure becomes too high. Too high an oil pressure is not allowed, for example because too much oil is then used for the bearing lubrication such that the losses in the bearings rise.

That is why a bypass pipe with a valve is affixed in the oil circuit downstream the oil cooler, which as of a certain speed will drive a portion of the pumped oil back to the oil reservoir.

The higher the speed of the motor, and thus the rotary oil pump, the more oil the valve will guide back to the oil reservoir via the bypass pipe.

In this way the oil pressure in the oil circuit will not rise too high.

According to a conventional oil circuit, all oil that is driven to the motor and/or the compressor element will pass via the oil cooler.

Such known oil circuits thus also present the disadvantage that at low speeds of the machine, the oil is cooled too much as the oil cooler is designed to cool the oil at the maximum speed of the machine when the oil heats up the most due to losses in the rotating parts.

As a result, at these low speeds the oil will have a high viscosity, which will lead to oil losses in the bearings.

Moreover, a large temperature difference will occur in the oil at low and high speeds.

These large temperature differences are detrimental for the motor of the oil-free compressor.

As a result of this, an oil cooler will often be chosen whose cooling capacity is adjustable, which of course is more expensive and more complex.

Moreover, it will be necessary to use a large cooler designed for the entire oil flow at maximum speed.

Suitable rotary oil pumps for the oil circuit are gear pumps, internal gear pumps, such as gerotor pumps and vane pumps.

Such pumps may be designed to pump up a precise amount of oil when they are driven at the same rpm as the motor of the compressor element, through an appropriate selection of the pump width and/or the number of gear teeth or vanes, which allows to mount the rotary oil pump directly on the axis of the motor which will result in a very compact, robust, efficient and inexpensive machine.

However, a disadvantage of this kind of configuration whereby the rotary oil pump is directly mounted on the axis of the motor of the compressor element, is that the rotary oil pump needs to be mounted in a relatively high position in the oil-free compressor and, consequently, that it is in a relatively high position with respect to the oil reservoir.

This means that at start-up of the oil-free compressor, the rotary oil pump first needs to suck air from the suction pipe which is fluidly connected to the oil reservoir, and subsequently needs to suck and pump oil from the oil reservoir.

This start-up is easier if there is already some oil in the rotary oil pump, such that when the rotary oil pump is starting, this oil is spread and provides for sealing in the rotary oil pump, such that the suction power of the rotary oil pump is immediately optimal.

For this reason, during assembly of the rotary oil pump, a small volume of oil is often applied in the rotary oil pump, i.e. a volume which is small with respect to total volume of oil in the oil circuit.

When the pump is however started for the first time only after a long time after its assembly, this initial volume of oil is already partly or completely evaporated and, consequently, not sufficient anymore to start the rotary oil pump in a proper way.

U.S. Pat. No. 3,859,013 describes a rotary oil pump, whereby in an inlet channel between the rotary oil pump and the oil reservoir a kind of siphon-like structure is provided, which is configured such that a small volume of oil is kept in the inlet channel near the oil reservoir. However, at start-up of the oil-free compressor, the rotary oil pump still needs to suck a considerable volume of air before the oil is sucked from the siphon-like structure.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a solution to at least one of the aforementioned and other disadvantages.

The object of the present invention is 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; andwhereby the oil circuit is further provided with an oil cooler,
with the characteristic that the oil cooler is placed in the bypass pipe and that the bypass valve is placed in the oil pipe.

An advantage is that at low speeds of the compressor element, when little cooling is required, a small portion of the oil in the oil circuit will be guided via the bypass pipe and thus cooled; while at high speeds when more cooling is required, a relatively larger portion of the oil in the oil circuit will be guided via the bypass pipe and thus will be cooled more.

By cooling less at low speeds and cooling more at high speeds, the temperature of the oil will remain more constant and thus the temperature differences smaller, compared to the known cooling circuits.

Moreover, the average oil temperature will also be higher, so that the oil will have a lower viscosity, which will lead to fewer oil losses in the bearings and at other locations in the oil-free compressor where the oil is used for lubrication.

Another advantage is that at low speeds the oil will not be cooled as no oil will be guided via the bypass pipe and the oil cooler. In this way the oil will not have too great a viscosity at low speeds.

Moreover, at high speeds the oil will not get too hot, because more oil is then guided via the cooler.

Another advantage is that the oil cooler can have smaller dimensions, i.e. in the bypass pipe a smaller oil cooler can be chosen for a smaller oil flow compared to the known oil circuits where the oil cooler is in the oil pipe upstream the bypass valve.

In a preferred embodiment of the invention, 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.

An advantage of this preferred embodiment is that it is ensured that after stoppage of the oil-free compressor a considerable volume of oil remains in the rotary oil pump and in the inlet channel between the rotary oil pump and the dam, such that at a restart of the oil-free compressor the rotary oil pump is internally completely wetted with oil and that the suction power of the rotary oil pump will immediately be very high.

In this way, oil flow is started up swiftly and smoothly in the oil circuit at the (re)start of the oil-free compressor.

Preferably, 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.

This will prevent that oil will leak via the rotation shaft of the rotary oil pump and/or will avoid the need for additional sealings of said shaft.

The invention also concerns an oil-free compressor provided with 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; andwhereby the oil circuit is further provided with an oil cooler,
with the characteristic that the oil-free compressor is configured such that the oil cooler is placed in the bypass pipe and that the bypass valve is placed in the oil pipe.

Finally, the invention concerns 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 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 the motor during its way back to the oil reservoir; andwhereby the oil circuit is further provided with an oil cooler,
with the characteristic that the portion of the pumped oil which is guided back to oil reservoir through the bypass pipe and the bypass valve, passes through the oil cooler which is placed in the bypass pipe, and that the bypass valve is controlled such that a preset pressure is reached in the oil pipe between the bypass valve and the compressor element and/or the motor.

Preferably, the motor of the compressor element is started only after oil or a lubricant with a higher volatility than the oil has been brought into the oil circuit at a position downstream and higher than the rotary oil pump.

DETAILED DESCRIPTION OF THE INVENTION

In this case the oil-free compressor1shown inFIG. 1is a screw compressor device with a screw compressor element2, a transmission3(or ‘gearbox’) and a motor4with variable speed, whereby the oil-free compressor1is provided with an oil circuit5according to the invention.

According to the invention, it is not necessary for the oil-free compressor1to be a screw compressor1, as the compressor element2could also be of a different type, e.g. a tooth compressor element, scroll compressor element, vane compressor element, etc.

The compressor element2is provided with a housing6with an inlet7to draw in a gas and an outlet8for compressed gas. Two mating helical rotors9are mounted on bearings in the housing6.

The oil circuit5will supply the oil-free compressor1with oil11to lubricate and if need be cool the components of the oil-free compressor1.

These components are for example the gears in the transmission3, the bearings on which the helical rotors9are mounted in the compressor element2, etc.

The oil circuit5comprises an oil reservoir10with oil11and an oil pipe12to bring the oil11to the components of the oil-free compressor1to be lubricated and/or cooled.

A rotary oil pump13is provided in the oil pipe12to be able to pump oil11from the oil reservoir10.

The rotary oil pump13is driven by the motor4of the compressor element2.

The rotary oil pump13can be connected directly to the shaft of the motor4or to a drive shaft. This drive shaft is then connected to the motor4via a coupling. Then the gear is mounted on the driveshaft that is driven by the gearbox. One or more compressor elements2can be driven via the gearbox.

A bypass valve14and a bypass pipe15, that leads from the oil pipe12back to the oil reservoir10, are provided in the oil pipe12downstream from the rotary oil pump13.

Although in the example shown the bypass valve14is affixed in the oil pipe12, it is not excluded that the bypass valve is affixed in the bypass pipe15. It is not excluded either that a three-way valve is used that is affixed at the location of the connection of the oil pipe12to the bypass pipe15.

The bypass valve14will distribute the oil11that is pumped by the rotary oil pump13: a part will be driven to the components of the oil-free compressor1to be lubricated and/or cooled via the oil pipe12, the other part will be driven back to the oil reservoir10via the bypass pipe15.

In this case, but not necessarily, the bypass valve14is a mechanical valve14.

In a preferred embodiment, the valve14is a spring-loaded valve, i.e. the valve14comprises a spring or spring element, whereby the spring will open the valve14more or less depending on a pressure p upstream or downstream the valve14.

In this case the valve will be a spring-loaded valve14that will close and open the bypass pipe15depending on the pressure p downstream of the valve14. When a certain threshold value of the pressure p is exceeded, the valve14will open the bypass pipe14so that a portion of the pumped oil11will flow via the bypass pipe15to the oil reservoir10.

According to the invention an oil cooler16is placed in the bypass pipe15. This means that the oil11that flows via the bypass pipe15can be cooled, but that the oil11that flows via the oil pipe12to the components to be lubricated and/or cooled will not be cooled.

In other words: cooled cold oil11will be guided to the oil reservoir10via the bypass pipe15.

In this case the aforementioned oil cooler16forms part of a heat exchanger17. The oil cooler16could be a plate cooler for example, but any type of cooler that is suitable for cooling the oil11can be used in this invention.

In this case the oil cooler16has 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.

From the bypass valve14, the oil pipe12runs to the components of the oil-free compressor1to be lubricated and cooled if need be. Here the oil pipe12will be divided into subpipes18that may be partly integrated in the compressor element2.

Furthermore, the oil circuit5is provided with a return pipe19to carry the oil11from the compressor element2back to the oil reservoir10, after it has lubricated and if need be cooled the components.

This oil11will have a higher temperature.

In the oil reservoir10this hot oil11will be mixed with the cooled cold oil11that is guided to the oil reservoir10via the bypass pipe15.

The operation of the oil-free compressor1with the oil circuit5is very simple and as follows.

When the compressor element2is driven by the motor4, the mating rotating helical rotors9will draw in and compress air.

During the operation, the different components of the compressor element2, the transmission3and the motor4will be lubricated and cooled.

As the rotary oil pump13is driven by the motor4of the compressor element2, as of the start-up of the oil-free compressor1it will pump oil11and drive it to the components of the oil-free compressor1to be lubricated and cooled via the oil pipe12and subpipes18.

The change of the flow rate Q of the rotary oil pump13as a function of the speed n of the motor4is shown inFIG. 2.

As can be seen from this drawing, at low speeds n the rotary oil pump13will pump less oil11compared 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.

At low speeds n, all oil11that is pumped will be driven to the compressor element2and the motor4, i.e. the bypass valve14will close the bypass pipe15so that no oil11can flow back to the oil reservoir10along the bypass pipe15and the oil cooler16. As at low speeds n no cooling is required as the oil11will barely warm up, this is not a problem and this will ensure that the oil11does not get too cold.

The change of the pressure p in the oil pipe12downstream from the bypass valve14is shown inFIG. 3.

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

As of this speed n′ a pressure p′ is reached such that the bypass valve14will partially be opened to the bypass pipe15.

As a result, at higher speeds than n′, a portion of the pumped oil11will be driven through the bypass valve14via the bypass pipe15.

This is schematically shown inFIG. 2whereby the curve is divided into two branches: a portion of the oil flow Q corresponding to zone I will be driven via the oil pipe12to the components of the oil-free compressor1to be lubricated and cooled, while the other portion of the oil flow Q corresponding to zone II will be driven back to the oil reservoir10via the bypass pipe15.

Because the bypass valve14will open, as of the speed n′ the pressure p will no longer rise in proportion to the speed n of the motor4, but the curve flattens out, as shown inFIG. 3.

The higher the speed n, the more the bypass valve15will be pushed open by the higher pressure p downstream from the bypass valve15in the oil pipe12. Indeed, at a higher speed n, the flow rate Q of the rotary oil pump13will be greater, so that this pressure p will also rise such that the bypass valve14will open more.

The spring characteristics of the spring-loaded bypass valve14are chosen such that the bypass valve14is controlled by the spring such that a preset pressure p is reached in the oil pipe12between the bypass valve14and the compressor element2and/or the motor4according to the curve ofFIG. 3.

The oil11that is guided via the bypass pipe15will pass through and be cooled by the oil cooler16.

Because the cooled oil11that is guided via the bypass pipe15comes to the oil reservoir10, the temperature of the oil11in the oil reservoir10will fall. This cold(er) oil11is then pumped by the rotary oil pump13and brought to the compressor element2and/or motor4.

As at high speeds n more heat is generated in the oil-free compressor1, more cooling will be required which is taken care of precisely by the above method.

At increasing speeds n, the rotary oil pump13will always pump more oil11from the oil reservoir10. As the pressure p downstream of the bypass valve14will always be higher as a result, this bypass valve14will respond to this by always guiding more oil11via the bypass pipe15, so that the pressure p does not rise too high and continues to follow the curve ofFIG. 3.

As a result, with increasing speeds n, ever more oil11will be cooled, so that the rising temperature of the oil-free compressor1can be accommodated at these increasing speeds n.

This is shown inFIG. 2, whereby the zone II always becomes greater at higher speeds n.

The above clearly shows that at low speeds n little or no oil11is cooled, while at increasing speeds n ever more oil11is cooled.

As a result of this, the oil temperature will be more constant and higher on average, which ensures that the viscosity of the oil11will be lower on average so that there are fewer oil losses in the rotary oil pump13and at the lubrication locations.

As can be further seen fromFIG. 2, at all speeds n the oil flow Q that goes via the bypass pipe15and the oil cooler16(zone II) will be smaller than the oil flow Q that is driven to the compressor element2and/or the motor4(zone I).

This means that the oil cooler16can have smaller dimensions compared to the known cooling circuits.

The oil11of the compressor element2and/or the motor4will be driven back to the oil reservoir10via the return pipe19.

This oil11will have a higher temperature than the oil11in the oil reservoir10.

In addition to this hot oil11, the cooled oil11will also come to the oil reservoir10via the bypass pipe15.

The two will be mixed together in the oil reservoir10, which will result in an oil11at a certain temperature between the temperature of the cooled oil11and the hot oil11.

As of the oil reservoir10, the rotary oil pump13will again pump the oil11and the method and control set out above will be followed.

Although in the example shown, a spring-loaded mechanical valve is used as a bypass valve14, it is possible to use an electronic bypass valve14that is controlled by a controller20.

InFIG. 1, this controller20is shown by a dotted line by way of an example. This controller20will control the bypass valve14, for example on the basis of a signal from a pressure sensor21that is placed downstream from the bypass valve14in the oil pipe12. The controller20will control the bypass valve14so that the pressure p, as registered by the pressure sensor21, will follow the path of the curve ofFIG. 3. In other words: the bypass valve14is controlled such that a preset pressure p is reached in the oil pipe12between the bypass valve14and the compressor element2and/or the motor4.

Although in the examples shown and described, the oil circuit5is shown separate from the compressor element2and the motor4, it is of course not excluded that the oil circuit5is integrated in or physically forms part of the compressor element2and/or the motor4.

In all embodiments shown and described above it is possible that the oil circuit5also comprises an oil filter. This oil filter can for example, but not necessarily, be affixed in the oil pipe12downstream from the bypass valve14. The oil filter will collect any contaminants from the oil11before sending it to the compressor element2and/or the motor4.

The motor4will directly drive the compressor element2as well as the rotary oil pump13. InFIG. 4, it is shown that a rotation shaft22of the motor4is directly driving the rotary oil pump13.

The oil circuit5will allow that the rotary oil pump13pumps up oil11from the oil reservoir10through an inlet channel23before the rotary oil pump13, after which the oil11may be guided through the pipe12and the subpipes18to the nozzles that are positioned on specific locations in the motor4and/or the compressor element2for the lubrication and/or cooling of one or more bearings and other components of the oil-free compressor1.

As the rotary oil pump13is driven by the motor4of the compressor element2, it will be at a considerably higher position level than the oil reservoir10. This means that the inlet channel23, which is running from the oil reservoir10to the rotary oil pump13, is relatively long.

The rotary oil pump13comprises a housing24wherein a stator25and a rotor26are mounted. The rotor26is mounted on a rotation shaft27, which is driven by the rotation shaft22of the motor4.

The rotary oil pump13is a gerotor pump, however this is not a prerequisite of the invention.

The housing24is provided with an inlet port28for oil11, to which the inlet channel23is connected, and with an outlet port29for the pumped oil11.

As shown inFIG. 6, the inlet channel23is provided with a dam30near the rotary oil pump13.

By ‘dam30’ is meant a structure which ensures that, when the motor4has stopped, a certain volume of oil11will remain in a space31in the inlet channel23which is dammed by the dam30.

By ‘near the rotary oil pump13’ is meant that the aforementioned remaining volume of oil11will remain at a location such that the rotary oil pump13is able to pump up the oil11immediately at the start-up of the rotary oil pump13.

This means that the aforementioned remaining volume of oil11will for example at least partly be present in the rotary oil pump13or that the remaining volume of oil11will be located in the inlet channel23right next to inlet port28of the rotary oil pump13.

InFIG. 6, it is clearly visible that the dam30has a minimal height equal to the height A of the centreline32of the rotation shaft27of the rotary oil pump13reduced with half a smallest diameter B of the rotor26of the rotary oil pump13.

By making the dam30at least as high as this minimal height, indicated by the line C, enough oil11will remain in the by the dam30dammed space31in the inlet channel23between the dam30and the rotary oil pump13, whereby the rotary oil pump13is completely wetted internally at start-up of the oil-free compressor1. Due to this immediate internal wetting of the rotary oil pump13with oil11, the rotor26and the stator25will be immediately sealed by this oil11such that the suction power of the rotary oil pump13is immediately maximal.

In this case, and preferably, a height D of the dam30is smaller than a maximal height equal to the height A of the centreline32of the rotation shaft27of the rotary oil pump13reduced with half a diameter E of the rotation shaft27of the rotary oil pump13.

If the dam30would be higher than this maximal height, indicated by the line F, the level of the remaining oil11would be higher than a lowest point of the rotation shaft27of the rotary oil pump13. Because of this, oil11would possibly leak via the rotation shaft27of the rotary oil pump13and/or sealings would need to be provided on the rotation shaft27of the rotary oil pump13to avoid this.

Next to the minimum height C and maximum height F of the dam30, the configuration of the dam30is such that in this case, and preferably, the volume of the oil11which might be present between the rotary oil pump13and the dam30in the rotary oil pump13and the inlet channel23, is at least twice a swept volume of the rotary oil pump13.

This has the advantage that immediately enough oil11is available in the rotary oil pump13and the inlet channel23at start-up of the rotary oil pump13, such that it is not only possible to immediately wet the rotary pump13internally, but also to immediately pump up or pump through a volume of oil11via the outlet port29to the oil circuit5and so further to the components of the oil-free compressor1to be lubricated and/or cooled.

Despite the dam30inFIGS. 5 and 6being designed as a slanting slope towards the rotor26and the stator25of the rotary oil pump13, it is not excluded that the dam30has another configuration.

InFIG. 7an alternative configuration is shown, whereby the dam30has a stepped form, whereby the inlet channel23is as it were provided with a step33.

Although this embodiment has the advantage that more oil11will remain in the space31between the dam30and the rotary oil pump13, it does have the disadvantage that at the suction of the oil11, the oil11so to speak flows down via the step33, which may result in undesired turbulences. In the embodiments ofFIGS. 5 and 6, the oil11will so to speak flow down from the dam30.

The operation of the oil-free compressor1is very straightforward and as follows.

For the start-up of the oil-free compressor1, preferably the following steps are taken:bringing oil11into the oil circuit5at a position downstream and higher than the rotary oil pump13until the space31is completely filled with oil11; andthen starting the motor4.

The oil11that is brought into the oil circuit5may flow down to the rotary oil pump13and fill both the rotary oil pump13and the inlet channel23in the space31between the dam30and the rotary oil pump13to a level equal to the height D of the dam30.

When the motor4is then started, the compressor element2and the rotary oil pump13will be driven and the oil11that is brought into the oil circuit5and is now located in the rotary oil pump13and the aforementioned space31, will ensure that the rotary oil pump13is able to immediately pump and transfer oil11to the oil circuit5, such that the compressor element2is immediately provided with the necessary oil11right from the start-up of the oil-free compressor1.

Alternatively, it is also possible that firstly a lubricant which is less volatile than the oil11is brought into the rotary oil pump13internally, before the motor4is started.

Such method is preferably applied at the assembly of the oil-free compressor1, such that at a first start-up of the oil-free compressor1, the less volatile lubricant is present in the rotary oil pump13.

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 compressor1oil11is brought into the oil circuit5.

From the moment that the motor4is started, the rotary oil pump13will immediately pump up oil11from the oil reservoir10via the inlet channel23.

The pumped oil11will then leave the rotary oil pump13via the outlet port29and come into the oil circuit5from where it is transferred to different nozzles at different to be lubricated and/or cooled components of the compressor element2and/or the motor4.

The compressor element2and/or the motor4will therefore be almost immediately provided with oil11from the start-up of the motor4and the oil-free compressor1.

It is not excluded that the oil-free compressor1comprises a sensor configured to register whether oil11is present in the space31between the rotary oil pump13and the dam30.

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.

For the start-up of an oil-free compressor1with such sensor, the motor4is preferably only started after oil11has been detected in the inlet channel23between the rotary pump13and the dam30.

If no oil11is detected, the oil-free compressor1is not started, but instead a warning signal is sent out to the user.

It is clear that the sensor and the aforementioned method to control the lubrication and/or cooling of the oil-free compressor1at start-up, may be combined with the previously described methods. This method will incorporate an additional safety feature to prevent that the oil-free compressor1may be started without oil11being present in the inlet channel23between the rotary oil pump13and the dam30.

It is also possible that the oil-free compressor1comprises a fluid connection between the oil reservoir10and the space31between the rotary oil pump13and the dam30, whereby the fluid connection is configured to transfer oil11from the oil reservoir10to the space31between the rotary oil pump13and the dam30.

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

When the oil-free compressor1is provided with such a fluid connection, the following method may be executed for the start-up of the oil-free compressor1:transferring oil11from the oil reservoir10to the space31between the rotary oil pump13and the dam30, until the space31is completely filled with oil11; andthen starting the motor4.

It is of course not excluded that the oil-free compressor1is also provided with a sensor configured to register whether oil11is present in the inlet channel23between the dam30and the rotary oil pump13.

In this case, when no oil11is detected at start-up, a signal will be sent out to the user to transfer oil11from the oil reservoir10to the space31between the rotary oil pump13and the dam30by operating the small pump or, when this small pump operates electrically, the small pump will be automatically started by the oil-free compressor1in order to ensure that oil11is transferred from the oil reservoir10to the space31between the rotary oil pump13and the dam30, after which it is possible to start the motor4smoothly without problems.

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.