Patent Description:
The invention relates more specifically to a method for controlling a compressor towards an unloaded state, which compressor comprises a compressor element featuring an inlet and an inlet valve, in which in the unloaded state, a residual flow is suctioned via the inlet towards and into the compressor element and vented via a blow-off valve to an outlet of the compressor, and in which for a transition from a loaded state of the compressor to the unloaded state, the inlet of the compressor element is partially closed in successive discrete transitional steps and the residual flow corresponds with a minimum gas flow required to maintain a minimum equilibrium pressure in a pressure tank connected to the pressure line.

In the unloaded state, the compressor element is not halted, and it continues to be driven at a certain rotational speed. Due to the fact that in that case, apart from some calibrated passage in the inlet valve, the inlet is closed, only a limited amount of gas is suctioned with the residual flow, and no pressure can build up in a pressure tank of the compressor, since the suctioned gas is immediately vented from the outlet into the atmosphere.

Thus, only a minimum of energy is needed to keep the compressor element running in the unloaded state.

After a transition period, a state of equilibrium is reached, in which a certain equilibrium pressure is reached in the pressure tank. The "unloaded state" refers to this state of equilibrium.

The aforementioned calibrated passages are calculated to keep the reached equilibrium pressure in the unloaded state as low as possible for purposes of a low energy use, yet high enough to guarantee, for instance, a sufficient fluid injection via a fluid circuit from the pressure tank to the compressor element in the compressor element of fluid removed from the compressed gas that is needed, among other things, for sufficient cooling and lubrication of the compressor element.

A transition from the unloaded to the loaded state is initiated when an operating pressure in the consumer network falls below a minimum value chosen and set by a user.

In most conventional compressors, the inlet valve is immediately opened entirely as soon as the operating pressure reaches the aforementioned set value, and simultaneously the blow-off valve is entirely closed.

This may cause sudden undesirable temperature peaks in the outlet of the compressor element, which may lead to compressor failure.

A solution for this was described in <CIT>, in which the inlet valve is not immediately opened, but opened only after a certain delay during the transition from the unloaded to the loaded state. This international patent application <CIT> is then therefore considered to be incorporated by reference in the present description, in the sense that a solution in this international patent application can be combined with the present invention.

<CIT> discloses a device for adjusting the operating pressure of an oil-injected compressor installation with a compressor element driven by a motor with an adjustable rotational speed and a control module, whereby the device is provided with a controlled inlet valve which is connected to the air inlet and a blow-off mechanism which can be closed by means of a blow-off valve. In order to adjust the position of the inlet valve, use is made of a stepping motor.

<CIT> discloses a method of controlling a compressor provided with a compressor element to a no-load state. A method is described to partially close the inlet of the compressor element by the inlet valve (throttle valve) and to partially close the inlet of the compressor element during the transition period, in a continuous discrete transition step, in order to avoid fluctuations of the throttle valve.

A problem that was not yet resolved, however, is a problem that occurs during the opposite transition from the loaded to the unloaded state, which is where the present invention comes in.

In this transition from the loaded to the unloaded state, in conventional compressors, as soon as the desired operating pressure in the consumer network is reached, the inlet valve is suddenly closed, and simultaneously the blow-off valve is opened. At that moment, a pressure at the outlet of the compressor element is at a maximum, and approximately equal to the set operating pressure (except for the pressure drop between the outlet of the compressor element and an outlet of the compressor), and the pressure at the inlet of the compressor element is at a minimum and equal to a negative pressure that is caused because the compressor element continues to suction a small gas flow via aforementioned calibrated openings in the inlet valve.

This means that at the time of the transition from the loaded to the unloaded state, when the inlet valve is suddenly closed and the blow-off valve is opened, the value of the pressure ratio over the compressor element, in other words: the value of the pressure ratio between the pressure at the outlet and the pressure at the inlet of the compressor element, reaches a peak.

This may lead to high vibration levels that can be attributed to periodic pulses of pressure, generated by the compression of the gas at the outlet of the compressor element and which, directly or via an elastic coupling, are conducted to rotating parts of the compressor element and a drive and possibly of a gear enclosure between the drive and the compressor element, in particular when the frequency of the vibrations coincides with the own frequency of the rotating parts or of a structure of the compressor. This negative effect is typically even more pronounced when the aforementioned pressure ratio over the compressor element is higher and might lead to undesirable damage.

The risk of undesirable damage is even larger when there is no elastic coupling between the drive and the compressor element. This is the case, for instance, when the elastic coupling is omitted in order to limit the length of the compressor, in order to save costs, or for easier maintenance.

The task of the present invention is to offer a solution to one or more of the aforementioned and/or other disadvantages, and more specifically, for the problems relating to the transition from the loaded to the unloaded state.

For these purposes, the invention relates to a method for controlling a compressor towards an unloaded state, in which the compressor comprises a compressor element, which compressor element is equipped with:.

One advantage of the method according to the invention is that by the partial closing of the inlet during the transition period, in a number of successive discrete transitional steps, and consequently the suctioning of a flow greater than the residual flow during the transition period, a lower negative pressure is realized via the inlet of the compressor element, or therefore, a greater absolute pressure in the inlet as compared with a situation in which during the transition period only a residual flow would be suctioned towards and into the compressor element immediately via the inlet.

With the transition from the loaded to the unloaded state, the pressure in the outlet of the compressor element is approximately equal to the set maximum operating pressure in the consumer network, since this transition is initiated when this set maximum operating pressure is reached. At that same time, as a result of the invention, the absolute pressure in the inlet is increased, as a result of which a peak of the pressure ratio between the pressure in the outlet and the pressure in the inlet at that moment is decreased, the advantageous result being that hazardous vibration levels resulting from excessively high peaks of the aforementioned pressure ratio can be prevented.

Due to the greater suctioned flow as compared to the residual flow that is suctioned in a normal unloaded state, an equilibrium pressure in a pressure tank connected to the pressure line will be higher than the normal equilibrium pressure in the unloaded state, and it is therefore necessary to reduce the suctioned flow in one or more transitional steps back to the normal unloaded residual flow in order to restore the equilibrium pressure in the pressure tank to its normal equilibrium value in the unloaded state for the purpose of needing as little as possible energy for the unloaded driving of the compressor element.

In order to determine the time of a subsequent transitional step, the method may also include the following steps:.

The preset initialization pressure may be chosen in advance such that immediately after performing the subsequent transitional step, a realized pressure ratio over the compressor element is smaller than a preset maximum pressure ratio.

In the alternative, a simplified method may be used in order to determine an aforementioned time of a subsequent transitional step, which method provides that:.

According to a preferred embodiment of the method according to the invention, an extra gas flow suctioned in the compressor element is determined in a first transitional step by a pressure that is needed in the inlet of the compressor element in order to obtain a realized pressure ratio immediately after performing the first transitional step that is smaller than the preset maximum pressure ratio, and this for a pressure at the outlet that is equal to the set maximum operating pressure of the consumer network.

This additional gas that was suctioned into the compressor element may preferably be determined in advance, theoretically or experimentally, as a function of a set maximum operating pressure in the consumer network.

The extra gas flow suctioned in the compressor element in the first step will then be variable, and it is the gas flow that had been determined in advance for the set maximum operating pressure at the time of the transition from the loaded to the unloaded state.

For low values of the set maximum operating pressure in the consumer network, the extra suctioned flow may be zero.

The extra gas flow suctioned in the first transitional step will then be variable, and it is the gas flow that had been determined in advance for the set maximum operating pressure at the time of the transition from the loaded to the unloaded state.

In the alternative, the extra gas flow suctioned in the first step may have a fixed value that was determined in advance, theoretically or experimentally, as a function of a safe maximum value of the operating pressure in the consumer network that must be set, which makes the controlling easier.

Preferably, the method is limited to two successive discrete steps for transitioning from loaded to unloaded.

The invention also relates to a compressor comprising a compressor element, which compressor element is equipped with:.

It goes without saying that such a compressor according to the invention has the same benefits as the previously described method according to the invention.

With the understanding [sic] to better demonstrate the features of the invention, in the following, without these descriptions having any restrictive character, some examples of preferred applications are described of a compressor and of a method according to the invention for controlling such a compressor for the transition from the loaded to the unloaded state, with reference to the enclosed drawings, in which:.

The installation shown in <FIG> relates to a compressor according to the invention, in this case, a fluid-injected screw compressor <NUM>, which compressor comprises a compressor element <NUM> of a conventional screw type with an enclosure <NUM> in which two cooperating helical rotors <NUM> are driven by means of a motor or something similar, not shown in the figure.

The compressor element <NUM> features an inlet <NUM> that is sealable by means of a controllable inlet valve <NUM> with a valve inlet <NUM>, connected by means of an suction line <NUM> with an inlet filter <NUM> in order to suction a gas, in this case, air, from the environment.

The compressor element <NUM> is also equipped with an outlet <NUM> with thereto connected a pressure line <NUM>, which is connected via a pressure tank <NUM> containing a fluid separator <NUM> and via a cooler <NUM> with a consumer network <NUM> downstream for feeding various pneumatic tools or something similar, not shown here.

In this case, a check valve <NUM> is provided at the outlet <NUM> of the compressor element <NUM>, and a minimum pressure valve <NUM> is arranged on the outlet of the pressure tank <NUM>.

In the pressure tank <NUM>, an exhaust branch <NUM> is provided, which culminates at the location of the valve inlet <NUM> of the inlet valve <NUM>, and which is sealable by means of a blow-off valve <NUM> in the form of a controllable electric valve.

The screw compressor <NUM> is equipped with a fluid circuit <NUM> in order to inject a fluid <NUM>, for instance oil, under the influence of a pressure p<NUM> in the pressure tank <NUM> from this pressure tank <NUM> into the compressor element <NUM> in order to lubricate and/or cool and/or for providing a seal between the various rotors <NUM> mutually and between the rotors <NUM> and the enclosure <NUM>.

This fluid circuit <NUM> comprises an injector <NUM> or something similar that is connected via an injection line <NUM> containing a fluid filter <NUM> with the pressurized fluid <NUM> in the pressure tank <NUM>.

The fluid <NUM> flowing from the pressure tank <NUM> to the injector <NUM> may be diverted via a thermostatic faucet <NUM> via a branch line <NUM> through a fluid cooler <NUM> in order to regulate a temperature in the injection line <NUM>.

In the example shown in the figures, a controlled shut-off valve <NUM> is provided on the injector <NUM>, which prevents fluid from flowing back from the compressor element <NUM> to the pressure tank <NUM>, and from flowing from the pressure tank <NUM> to the compressor element <NUM> while this compressor element <NUM> is at rest.

Alternatively, the functions of the check valve <NUM> and of the shut-off valve <NUM> may also be incorporated in the operations of the inlet valve <NUM>, in which case no physical check valve <NUM> and no physical shut-off valve <NUM> have to be provided.

The inlet valve <NUM> is shown in more detail in <FIG>, and comprises an enclosure <NUM>, in which a poppet valve <NUM> is movably arranged between a position as shown in <FIG>, corresponds with a loaded state, in which the inlet <NUM> of the compressor element <NUM> is set to be open to a maximum, and a position corresponds with the unloaded state, in which the inlet <NUM> is closed to a maximum as shown in <FIG>, with the exception of some calibrated passage <NUM> and <NUM> for letting through a residual flow QD.

The opening and closing of the inlet valve <NUM> is done in this case in a conventional manner under the influence of a pilot pressure that is branched off via a control line <NUM> from a lid of the pressure tank <NUM>, for instance, and is let through by means of a control valve <NUM> or something similar in order to close the inlet valve <NUM>, or which is closed in order to open the inlet valve <NUM>.

In the poppet valve <NUM> itself and in the enclosure <NUM> of the inlet valve <NUM>, the aforementioned calibrated passages are provided, respective <NUM> and <NUM>, which provide for a permanent connection between the valve inlet <NUM> of the inlet valve <NUM> and the inlet <NUM> of the compressor element <NUM> in order to be able to suction a residual flow QD in a controlled manner when the inlet valve <NUM> is closed, as in the unloaded state of <FIG>.

In addition, an electrical or electronic controller <NUM> is provided in order to regulate an operating pressure p<NUM> in the consumer network <NUM> within an pressure interval that is delimited by a minimum operating pressure p<NUM> and a maximum operating pressure p15max, which may be selected by the user of the screw compressor <NUM> and can be chosen and entered in the controller <NUM>, and which is connected for that purpose with a pressure sensor <NUM> for measuring or determining the operating pressure p<NUM> in the consumer network <NUM>.

The controller <NUM> is furthermore equipped with a program or something similar in order to control the inlet valve <NUM> via the control valve <NUM> and the blow-off valve <NUM>, such that when the operating pressure p<NUM> in the consumer network <NUM> drops below the minimum operating pressure pismin due to a decrease of air, the screw compressor <NUM> enters into a loaded state, in which the inlet valve <NUM> is open and the blow-off valve <NUM> is closed, as shown in <FIG> and <FIG>, until no further compressed air or gas can be removed, is extracted, this causing the pressure p<NUM> in the consumer network <NUM> to rise.

As of the moment that the pressure p<NUM> reaches the maximum operating pressure p15max, the controller switched from the loaded state to an unloaded state, in which the inlet valve <NUM> is closed is and the blow-off valve <NUM> is opened, as shown in <FIG> and <FIG>.

As a result, no gas is suctioned by the compressor element <NUM>, which is still being powered, except for a residual flow QD, which is suctioned and compressed via the calibrated passages <NUM> and <NUM>.

As a result, after a transition period, an equilibrium with a constant minimum equilibrium pressure p12u is generated in the pressure tank <NUM>, the value of which is dependent on the chosen calibrated passages <NUM> and <NUM>, which are preferably chosen such that in the unloaded state, this minimum equilibrium pressure p12u is as low as possible in order to limit the energy required for driving the compressor element <NUM> in the unloaded state to a minimum.

This minimum equilibrium pressure p12u is measured, for instance, by way of a. pressure sensor <NUM>, of which the signal is linked back to the controller <NUM>.

Specifically, according to the invention, the screw compressor <NUM> is equipped with means <NUM> for closing the inlet <NUM> of the compressor element <NUM> only partially at when the set operating pressure p15max is reached in a first transitional step, using the controller <NUM>, in order to suction an extra flow ΔQ, relative to the residual flow QD of the unloaded state of <FIG> and <FIG>, via the inlet <NUM> towards and into the compressor element <NUM>, thus suctioning a total flow into the compressor element <NUM> that is larger than the residual flow QD that is suctioned via the calibrated passages <NUM> and <NUM> in the unloaded state.

In the case of <FIG>, the means <NUM> are formed by an additional bypass <NUM> with a calibrated opening for bridging the poppet valve <NUM> of the inlet valve <NUM> for suctioning air when the inlet valve <NUM> is closed, in which in this additional bypass <NUM>, a controllable shutter <NUM> is provided, in this case, in the form of an electric valve connected with the controller <NUM>.

This is shown in the graphs of <FIG>, which show the transition from the loaded to the unloaded state, in which the additional bypass <NUM> is not opened, for which reason no extra flow is suctioned according to a method traditionally used for the transition from the loaded to the unloaded state and as described, for instance, in <CIT>.

In this <FIG>, the following graphs are shown, respectively one after the other: the operating pressure p<NUM> in the consumer network, the mass flow gas Q suctioned by the compressor element <NUM>, the pressure p<NUM> in the pressure tank <NUM>, an (under) pressure p<NUM> in the inlet <NUM> of the compressor element <NUM>, a pressure ratio pr = p<NUM>/p<NUM> between the two previous absolute pressures p<NUM> and p<NUM>, all these on the same time scale t.

This <FIG> illustrates a loaded state C prior before time tE and an unloaded state D, which is reached after a transition period E at a time to in which a state of equilibrium is reached.

At the aforementioned time tE, the inlet valve <NUM> is moved from an open position as in <FIG> to a closed position as in <FIG>, and simultaneously, the blow-off valve <NUM> is opened.

After closing the inlet valve <NUM>, the suctioned flow is limited to the residual flow QD that is suctioned via the calibrated passages <NUM> and <NUM>.

This generates a negative pressure in the inlet <NUM> of the compressor element <NUM>.

By opening the blow-off valve <NUM>, during the transition period E, gas is vented from the pressure tank <NUM>, as a result of which the pressure p<NUM> in the pressure tank <NUM> decreases gradually from a pressure p<NUM> which at the time tE had been approximately equal to the set maximum pressure p15max in the consumer network <NUM>, to the minimum equilibrium pressure p12u of the unloaded state D.

It therefore transpires from the graphs that at the time tE, the pressure p<NUM> in the pressure tank is at a maximum, and that therefore a pressure p<NUM> in the outlet <NUM> of the compressor element <NUM> and at the same time also the pressure p<NUM> in the inlet <NUM> of the compressor element <NUM> are at a minimum, as a result of which the resulting pressure ratio pr reaches a peak prE at the time tE.

When this peak prE of the pressure ratio pr is too high, for instance when it is greater than a maximum pressure ratio prmax as indicated in <FIG>, this may pose a problem in terms of undesirable vibrations, as explained in the introduction. A safe value prmax may be experimentally or theoretically determined, for instance, for a specific screw compressor <NUM>.

The value of the peak prE may, for instance, be determined or derived from measurements of the pressures p<NUM> and p<NUM> or similar related pressures.

To the extent that the peak prE remains below the maximum pressure ratio prmax, there is no risk of vibrations and no further action needs to be undertaken to lower that peak prE.

In the event that the measured peak prE indeed turns out to be higher than prmax, the method according to the invention provides for an additional first transitional step, in which at time tE, the inlet <NUM> of the compressor element <NUM> is opened further, for instance by opening the additional bypass <NUM> as shown in <FIG>.

As a result, an extra flow ΔQ is suctioned by the compressor element <NUM> via the additional bypass <NUM> in addition to the residual flow QD that is already suctioned via the calibrated passages <NUM> and <NUM>, as in the unloaded state D, which leads to a resulting flow QE'.

The effect of this is shown in the graphs of <FIG>.

Because more compressed gas arrives in the pressure tank <NUM>, the venting of the pressure tank <NUM> in the transition period E' will cause the pressure p<NUM> in the pressure tank <NUM> to decrease less and to evolve towards an equilibrium pressure p12u', which is higher than the aforementioned minimum equilibrium pressure p12u in <FIG> of the unloaded state of the screw compressor <NUM>.

At the same time, in the inlet <NUM> of the compressor element <NUM>, less of a vacuum will be generated, are the absolute pressure p<NUM> will therefore be greater in the transition period E'.

This results in a reduced peak of the pressure ratio pr, which is now reduced to a value prE' which, as shown in <FIG>, is smaller than the peak prE and lower than the aforementioned maximum pressure ratio prmax.

The value prE' of the pressure ratio immediately after the first transitional step is equal to the ratio of:.

The extra flow ΔQ needed for restricting the pressure ratio pr to the maximum pressure ratio prmax is therefore a function of the set maximum operating pressure p15max and may be determined theoretically or experimentally, for instance, as a function of the set maximum operating pressure p15max.

The restriction in the additional bypass <NUM> can then be controllable for instance as a function of the set maximum operating pressure p15max.

Alternatively, a fixed restriction for the additional bypass <NUM> may be chosen, which would then be chosen for safety reasons as a function of the highest possible maximum operating pressure p15max in the consumer network <NUM> that can be set.

It is clear that when a low set maximum operating pressure p15max does not pose a risk, which means that in the first transitional step, the maximum pressure ratio prmax is not exceeded without letting through an extra flow ΔQ in this transitional step, this extra step of opening the additional bypass <NUM> according to the invention may be omitted.

The higher equilibrium pressure p12u' after the first transitional step requires that the energy required to keep the screw compressor <NUM> running in this unloaded transition period E' is high.

In an additional second transitional step, the method according to the invention therefore provides for the reduction of the flow to the residual flow QD of the unloaded state D by removing the extra flow ΔQ after a first transition period E', for instance by the closing the additional bypass <NUM> again at a time tE".

After a second transition period E", this leads to a new equilibrium pressure, which is equal to the equilibrium pressure p12u of the unloaded state D.

At time tE", the closing of the additional bypass <NUM> creates a new peak prE" of the pressure ratio pr, which again may not be higher than the maximum pressure ratio prmax. If this is not the case, a third transitional step or further transitional steps may be inserted as needed, in which the flow suctioned via the inlet <NUM> is reduces with each transitional step, for instance by closing the additional bypass <NUM> or by providing multiple additional bypasses <NUM>, of which in each transitional step, one or more are at least partially closed.

In the case of <FIG>, two transitional steps are sufficient, effectively splitting the transition period E into two shorter transition period E' and E".

The time tE" of the second transitional step may be determined, for instance, by measuring the pressure p<NUM> in the pressure tank <NUM> or an injection pressure p<NUM> at the injector <NUM> or the pressure p<NUM> at the outlet <NUM> of the compressor element <NUM>, such that the second transitional step is performed at time tE", when this measured pressure has dropped to a preset safe initialization pressure p12max or p22max, as shown in <FIG>.

At time tE", the closing of the additional bypass <NUM> causes the pressure p<NUM> in the inlet <NUM> to drop suddenly, as a result of which the pressure ratio pr suddenly increases to the new peak prE".

The preset initialization pressure p12max is chosen such that immediately after performing the second transitional step, at time tE", the new peak prE" is smaller than the aforementioned preset maximum pressure ratio prmax.

If no pressures are measured, alternatively, the time tE" may be determined by means of a timer with a programmed time interval tE"-tE between the first transitional step and the subsequent transitional step. The time interval to be set may be determined experimentally, for instance.

During the transition period from the loaded to the unloaded state, it is preferable for the pressure tank <NUM> to be vented as soon as possible in order to keep the total resulting transition period E' and E" as short as possible for reasons of energy saving. In this transition period, the pressure p<NUM> in the pressure tank <NUM> is greater than the minimum equilibrium pressure p12u of the unloaded state D.

By keeping this transition period as short as possible, there will only be a small difference between the energy use in the case of the invention with a transition in two transitional steps, as compared to the energy use without the application of the invention and a transition in a single transitional step.

The additional bypass <NUM> can also be used for applying the invention described in <CIT> for the transition from the unloaded to the loaded state when the operating pressure pis in the consumer network drops below a set minimum operating pressure p<NUM>.

In this case, the controller <NUM> must be provided with an algorithm in order to close the blow-off valve <NUM> during a transition from the unloaded to the loaded state and to keep the inlet valve <NUM> closed initially and to open it only after a certain delay, and during this delay, to open the bypass <NUM> in order to allow the pressure p<NUM> in the pressure tank <NUM> to increase gradually and to open the inlet valve <NUM> only when the pressure p<NUM> in the pressure tank <NUM> has reached a set minimum threshold value p<NUM> which is sufficient for avoiding temperature peaks due to an insufficient fluid injection.

This means that the same device can be used for preventing temperature peaks during the transition from the unloaded to the loaded state and for preventing peaks of the pressure ratio pr during the transition from the loaded to the unloaded state. This only requires a control adjustment.

An alternative embodiment of a screw compressor <NUM> according to the invention is shown in <FIG>, which differs from the embodiment of <FIG> and <FIG> in that the additional bypass <NUM> in this case connects the inlet <NUM> of the compressor element <NUM> with the pressure tank <NUM>, instead of with the inlet <NUM> of the inlet valve <NUM>.

The controllable shutter <NUM> in this bypass <NUM> allows for receiving the extra flow ΔQ, in this case during the transition from the loaded to the unloaded state, from the pressure tank <NUM>.

In this case, the peak prE of the pressure ratio pr will be lower than in <FIG>, but a curve for pressure p<NUM> in the pressure tank <NUM> as a function of the time t swill drop less fast towards the equilibrium pressure p12u'.

The extra flow ΔQ may also be realized without an additional physical bypass <NUM>, but by not entirely closing the inlet valve <NUM> during the first transitional step, as shown in <FIG>, in order to suction the extra flow ΔQ via the inlet <NUM> in the compressor element <NUM> during the first transition period E' and to close it entirely only at time tE" of the second transitional step.

It is self-evident that the invention is not limited to inlet valves <NUM> as shown, but can also be expanded to other valve types such as butterfly valves, or something similar.

It is clear that depending on the type of inlet valve <NUM> and blow-off valve <NUM>, different means <NUM> may be used for allowing for an extra flow ΔQ, initially temporarily, during the transition from the loaded to the unloaded state.

Due to the invention, possible vibration peaks are prevented or the vibration image is adjusted, which may allow for the compressor element <NUM> to be driven by motor via a rigid connection, without an intermediary flexible coupling.

Claim 1:
A method for controlling a compressor towards an unloaded state, in which the compressor comprises a compressor element (<NUM>), the compressor element (<NUM>) being equipped with:
- an inlet (<NUM>) and a controllable inlet valve (<NUM>) with a valve inlet (<NUM>), in which the inlet valve (<NUM>) is configured to be able to at least partially close the inlet (<NUM>) of the compressor element (<NUM>); and
- an outlet (<NUM>) with thereto connected a pressure line (<NUM>) which is connected with a downstream consumer network (<NUM>),
in which the compressor further comprises a controllable blow-off valve (<NUM>) that is connected to the pressure line (<NUM>), in which in a loaded state of the compressor, the blow-off valve (<NUM>) is closed and the inlet valve (<NUM>) is entirely open, and
in which for a transition from the loaded state towards the unloaded state, the method provides for the following steps:
- determining an operating pressure (pis) in the consumer network (<NUM>);
- when this operating pressure (pis) reaches a set maximum operating pressure (p15max), the opening of the blow-off valve (<NUM>) and the partial closing by the inlet valve (<NUM>) of the inlet (<NUM>) of the compressor element (<NUM>), such that after a transition period from the loaded state of the compressor to the unloaded state, in the unloaded state, a residual flow (QD) is suctioned via the inlet (<NUM>) towards and into the compressor element (<NUM>),
characterized in that
the partial closing of the inlet (<NUM>) during the transition period is performed in successive discrete transitional steps; and
the residual flow (QD) corresponds with a minimum gas flow required to maintain a minimum equilibrium pressure (p12u) in a pressure tank (<NUM>) connected to the pressure line (<NUM>).