METHOD FOR CONTROLLING A SCREW COMPRESSOR

Description

[0001] The invention relates to a method for controlling a screw compressor, in particular a twin-screw compressor, during idle operation. Such a screw compressor has at least a first and a second compressor stage, wherein the first compressor stage compresses a gaseous medium, usually air, and feeds it to the second compressor stage, which further compresses the medium and delivers it to a downstream system. The method according to the invention is suitable for controlling directly driven screw compressors in which both compressor stages are driven separately and with variable speed control. The invention also relates to a compressor with a twin-screw compressor, which is controlled by this method during idle operation.

[0002] Various compressor designs are known for compressing gaseous media, particularly for generating compressed air. For example, DE 601 17 821 T2 discloses a multi-stage screw compressor with two or more compressor stages, each comprising a pair of rotors for compressing a gas. Furthermore, two or more variable-speed drive elements are provided, each driving a specific compressor stage. A control unit regulates the speeds of the drive elements, monitoring the torque and speed of each so that the screw compressor delivers gas at a required flow rate and pressure while simultaneously minimizing the compressor's energy consumption.

[0003] In practical applications of such multi-stage screw compressors, the so-called idle mode occurs as an operating state. During this mode, no compressed air is drawn from the downstream system, so the delivery of further medium must be stopped to prevent pressure overshoot. However, the compressor should not be completely switched off during idle operation if a short-term replenishment of compressed air supply is anticipated. To enable this idle operation, a throttle valve in the suction line is usually closed, and only a partial flow is supplied to the first compressor stage via a bypass. This function is typically performed by a so-called intake regulator, which is located at the inlet of the first compressor stage. Simultaneously, a blow-off valve opens to the atmosphere on the outlet side, i.e., at the outlet of the second compressor stage, so that the second compressor stage delivers against atmospheric pressure.The pressure conditions in both compressor stages remain constant, which also means that the outlet temperatures of both stages remain almost identical. A disadvantage of this idle speed control is the relatively high energy consumption of the compressor. Furthermore, the intake regulator and its control system require significant design effort. (cf. Konka, K.-H., Screw Compressors: Technology and Practice, VDI-Verlag 1988, ISBN 3-18-400819-3, page 332 ff.).

[0004] German patent DE 100 03 869 C5 describes a method for compressing fluid media in a screw compressor system with two screw compressor units. The outlet of the upstream screw compressor unit is connected to the inlet of the downstream screw compressor unit, and each screw compressor unit is driven by its own drive unit. At least some of the operating parameters of the two screw compressor units are recorded and processed, and the drive units are controlled by these recorded parameters.By changing the operating parameters of the drive units, in particular current consumption, voltage consumption, or fuel supply, the rotational speed of the upstream screw compressor unit is correlated with the rotational speed of the downstream screw compressor unit in such a way that the final outlet pressure or final delivery volume of the screw compressor system is kept constant, and / or the total power consumption of the screw compressor system is minimized, or, at a given total power consumption, a maximum final outlet pressure or a maximum final delivery volume is achieved. However, this control method provides no information on optimizing the system's idling operation and the resulting energy savings.

[0005] WO 2011 / 130807 A2 concerns a method for controlling a compressor having one or more compressor elements, each compressor element being equipped with an inlet and an outlet.

[0006] US 2007 / 189905 A1 relates to a system and a method for controlling a centrifugal compression system.

[0007] One object of the present invention is therefore to provide an improved method for controlling a twin-screw compressor, which allows for safe idling operation while simultaneously reducing the compressor's energy consumption. Furthermore, the design complexity of the complete screw compressor should be reduced, resulting in cost reductions in its manufacture.

[0008] These and other problems are solved by a method for controlling a screw compressor according to claim 1. The dependent claims specify some preferred embodiments. Furthermore, the invention provides a compressor in the form of a twin-screw compressor that can be operated using this method.

[0009] Surprisingly, it has been shown that by changing the control of the directly driven compressor stages of the screw compressor during idle operation, both a significant reduction in energy consumption and a simplification of the overall system design can be achieved.

[0010] The method according to the invention serves to control a screw compressor with a first and a second compressor stage, wherein the first compressor stage compresses a gaseous medium and feeds it to the second compressor stage, which further compresses the medium. The first compressor stage is thus located upstream of the second compressor stage in the direction of flow of the medium. In most cases, such screw compressors have exactly two compressor stages; however, designs with more than two stages are also possible. Furthermore, for the implementation of the method, it is necessary that both compressor stages are driven separately and with variable speed control, i.e., each compressor stage is driven by a variable-speed drive, in particular a direct drive, so that a transfer case can be dispensed with.

[0011] In the first step of the process, the volume flow rate of the compressed gaseous medium, which is drawn off at the outlet of the second compressor stage or delivered to subsequent units, is measured using a suitable sensor. This can be done using direct volume flow measurement, or the drawn volume flow rate can be determined indirectly, e.g., from the pressure conditions prevailing at the outlet of the second compressor stage or from the torque / drive current occurring at the drive of the second compressor stage.

[0012] Under normal load operation, a volume flow rate is drawn that can fluctuate between a maximum value, for which the screw compressor is designed, and a predetermined minimum value. Under this load, the screw compressor is controlled in a manner known per se, which includes the ability to vary the rotational speed of the drives of the two compressor stages within a predetermined range. If, during load operation, the drawn volume flow rate falls within a range between a maximum value and a predetermined minimum value, the control system reduces the rotational speed of both compressor stages. Conversely, if the volume flow rate rises again within this range, the control system increases the rotational speed of the compressor stages again, thus maintaining a predetermined outlet pressure under normal load operation.

[0013] If, however, the volume flow falls below the predetermined minimum value, i.e., no or only a very small volume flow is drawn, the operating state of the screw compressor changes from load operation to idle operation. In the next step of the process, a blow-off valve is opened to allow at least some of the volume flow, which continues to be supplied by the second compressor stage, to escape through the blow-off valve. This prevents the pressure at the outlet of the screw compressor from exceeding a maximum permissible value. The blow-off valve can, for example, be a controlled solenoid valve.

[0014] In a further step, preferably performed with only a slight delay or essentially simultaneously with the opening of the blow-off valve, the rotational speed of at least the first compressor stage is reduced to a predetermined idle speed V1L in order to reduce the volume flow supplied from the first to the second compressor stage. In contrast to the prior art, a throttle valve or intake regulator is not closed for this purpose. Rather, the inlet of the first compressor stage remains fully open. A throttle valve or intake regulator and its control mechanism can be completely omitted. The reduction of the volume flow delivered by the first compressor stage is preferably achieved solely by reducing the rotational speed of the first compressor stage to the idle speed V1L.

[0015] According to a preferred embodiment, in a next step the speed of the second compressor stage is also reduced to an idle speed V2 L. Preferably, the speeds of both compressor stages are reduced, essentially in parallel, to their respective idle speeds V1 L and V2 L.

[0016] The idle speed V1 L of the first compressor stage (low pressure - LP) is selected in conjunction with the idle speed V2 L of the second compressor stage (high pressure - HP) to ensure that the outlet temperature of the medium at the second stage does not fall below the inlet temperature of that stage. Such an undesirable operating condition can occur if the pressure ratio at the second compressor stage drops below 0.6. Therefore, the idle speeds must be carefully selected to ensure that the second stage does not operate as an "expander," which would cause the medium temperature to drop. Otherwise, undesirable condensation may occur within the compressor.Furthermore, when selecting the idle speeds, it must be ensured that the second compressor stage is not driven by the transported medium from the first compressor stage, as otherwise the drive of the second stage would switch to generator operation, which could lead to damage to the frequency converter controlling it.

[0017] The minimum idle speed is also determined by the acceptable delay when returning to load. The shorter this return time needs to be, the higher the idle speed must be.

[0018] The no-load speed ratio between the second and first stages is in the range of 2 to 3, particularly preferably about 2.5. The pressure ratio of the first stage is about 1.5, and the pressure ratio of the second stage is about 0.6 to 0.75. Preferably, the no-load speed V2L of the second compressor stage is about 1 / 2 to 1 / 4 of the load speed of this stage. Preferably, the no-load speed V1L of the first compressor stage is about 1 / 5 to 1 / 8 of the load speed of this stage.

[0019] One advantage of this control method is that both compressor stages can operate at significantly lower speeds during idle operation. This reduces energy consumption and wear. Furthermore, the temperature of the compressed medium at the outlet of each compressor stage decreases, which is also beneficial. Nevertheless, the screw compressor can be brought back into load operation very quickly when a new volume flow is required by increasing the speeds of the compressor stages.

[0020] The compressor provided by the invention for compressing gaseous media comprises a screw compressor having a first and a second compressor stage, wherein the first compressor stage compresses the gaseous medium and feeds it to the second compressor stage, which further compresses the medium, and wherein both compressor stages are driven independently of each other and their speed is controllable. The compressor further comprises a control unit configured to carry out the method described above.

[0021] The compressor is particularly distinguished by the fact that the inlet of the flow-wise front, first compressor stage is connected to the ambient atmosphere without a controllable throttling element limiting the volume flow or without an intake regulator. The compressor has a blow-off valve at the outlet of the flow-wise rear, second compressor stage, which is opened by the control unit when the extracted volume flow falls below a predetermined minimum value.

[0022] Further advantages and details will become apparent from the following description of a preferred embodiment with reference to the drawing.

[0023] They show: Fig. 1 a simplified representation of the operating parameters in a screw compressor with two compressor stages during load operation; Fig. 2A simplified representation of the operating parameters in the screw compressor during idle operation.

[0024] Fig. 1 This shows the basic structure of a compressor designed as a twin-screw compressor 200. In addition to the individual components of the twin-screw compressor, typical parameters are also given, as they occur during load operation when compressed air is required with a volume flow rate above a predetermined minimum value and not exceeding a system-specific maximum value.

[0025] A first compressor stage 201 has a first direct drive 202, which is speed-controlled. The inlet of the first compressor stage 201, through which ambient air is drawn in, is directly coupled to an intake port 203 without an intermediate intake regulator. Ambient atmosphere at a pressure of 1.0 bar and a temperature of, for example, 20°C is present at this intake port. Thus, a pressure of 1.0 bar is present at the inlet of the first compressor stage 201.

[0026] The first compressor stage 201 is operated, for example, at a speed of 15,500 min⁻¹ to compress the air. At the outlet of the first compressor stage 201, the pressure is then 3.2 bar, so that the first compressor stage has a compression ratio of 3.2 under load. Due to the compression, the temperature of the medium (compressed air) increases to 170°C. The compressed air is routed from the outlet of the first compressor stage 201 via an intercooler 204 to the inlet of a second compressor stage 206, which has a second, speed-controlled direct drive 207. After the intercooler 204, at the inlet of the second compressor stage 206, the compressed air has a temperature of, for example, 30°C and still a pressure of 3.2 bar. Under load, the second compressor stage 206 is operated at a speed of, for example, 15,500 min⁻¹. B. operated at 22,000 min -1<, resulting in further compression.The compressed air therefore has a pressure of 10.2 bar and a temperature of 180°C at the outlet of the second compressor stage 206. The second compressor stage thus also has a compression ratio of approximately 3.2. The compressed air is passed from the outlet of the second compressor stage 206 through an aftercooler 208 and cooled there to approximately 35°C. Finally, a blow-off valve 209 is arranged at the outlet of the twin-screw compressor 200, which is controlled by a control unit (not shown).

[0027] The twin-screw compressor 200 described as an example exhibits a power input of 150 kW at the maximum speed of the direct drives 202 and 207 and delivers compressed air with a maximum pressure of 12 bar and a minimum pressure of 6 bar. The speed ratio between the compressor stages is approximately 1.4 under load.

[0028] Fig. 2Figure 200 shows the twin-screw compressor in idle mode, i.e., when essentially no compressed air is being drawn. Typical parameters as they occur during idle operation are shown alongside the components of the twin-screw compressor. To enter idle mode, the blow-off valve is opened and the speed of both compressor stages is reduced. The inlet of the first compressor stage 201, through which ambient air continues to be drawn in, albeit in a reduced quantity, remains directly connected to the intake port 203, without an intermediate intake regulator. Ambient atmosphere at a pressure of 1.0 bar and a temperature of 20°C is present at the intake port 203. Thus, the pressure at the inlet of the first compressor stage 201 remains unchanged at 1.0 bar.

[0029] The first compressor stage 201 is now operated at an idle speed V1 L = 2,500 min⁻¹ to compress the air. At the outlet of the first compressor stage 201, the pressure is then 1.5 bar, so that the first compressor stage has a compression ratio of 1.5 in idle operation. Due to the reduced compression, the temperature of the medium (compressed air) only increases to 90°C. The compressed air is routed from the outlet of the first compressor stage 201 via the intercooler 204 to the inlet of the second compressor stage 206. After the intercooler 204, at the inlet of the second compressor stage 206, the compressed air has a temperature of, for example, 30°C and still a pressure of 1.5 bar (intermediate pressure) in idle operation. The cooling capacity required for intercooling is therefore reduced in idle operation. In idle mode, the second compressor stage 206 is operated at an idle speed V2 L of 7,500 min -1<.The compressed air at the outlet of the second compressor stage 206 has a reduced pressure of approximately 1.2 bar and a temperature of 70°C compared to the intermediate pressure. The second compressor stage thus has a compression ratio of approximately 0.8 (expansion). The compressed air is passed from the outlet of the second compressor stage 206 through the aftercooler 208 and cooled there to approximately 30°C.

[0030] The twin-screw compressor 200 described as an example has a power consumption of 7 kW in idle mode and delivers a maximum pressure of 1.2 bar. The speed ratio between the compressor stages is approximately 3. Reference symbol list

[0031] 200 Twin screw compressor 201 First compressor stage 202 First direct drive 203 Intake port 204 Intercooler 205 - 206 Second compressor stage 207 Second direct drive 208 Aftercooler 209 Blow-off valve

Claims

1. A method for controlling a screw compressor (200) having a first and a second compressor stage (201, 206), wherein the first compressor stage (201) compresses a gaseous medium and conducts it to the second compressor stage (206), which further compresses the medium, and wherein both compressor stages (201, 206) are driven separately from one another and in a speed-regulatable manner, including the following steps: - detecting a volume flow of the compressed medium, which volume flow is taken-down at the outlet of the second compressor stage (206); - adapting the speed of both compressor stages (201, 206), if the taken-down volume flow decreases in a range between a maximum value and a predetermined minimum value, the control reduces the speed of both compressor stages (201, 206), and if the volume flow increases again in this range, the control raises the speed of the compressor stages (201, 206) again, while maintaining a predetermined outlet pressure; - opening a blow-off valve (209) if the volume flow falls below the predetermined minimum value, in order to allow the volume flow delivered by the second compressor stage (206) to at least partially exit via the blow-off valve (209); - reducing the speed of the first compressor stage (201) to a predetermined idling speed (V1L), in order to reduce the volume flow delivered by the first compressor stage (201) to the second compressor stage (206), characterized in that a ratio of a predetermined idling speed (V2L) of the second compressor stage (206) : idling speed (V1L) of the first compressor stage (201) lies in the range 2 to 3.

2. The method according to claim 1, characterized in that the reduction of the speed of the first compressor stage (201) takes place at the idling speed (V1L) simultaneously with the opening of the blow-off valve (209).

3. The method according to claim 1 or 2, characterized in that, in a further step, the speed of the second compressor stage is reduced to its predetermined idling speed (V2L) as long as the taken-down volume flow falls below the predetermined minimum value.

4. The method according to one of claims 1 to 3, characterized in that the control of the speed of the compressor stages (201, 206) takes place by speed regulation of two direct drives (202, 207), which drive the respective compressor stages (201, 206).

5. The method according to one of claims 1 to 4, characterized in that the taken-down volume flow is determined indirectly from the power consumption of at least one of the two compressor stages (201, 206).

6. The method according to one of claims 1 to 5, characterized in that the speeds of the two compressor stages (201, 206) are raised as soon as the taken-down volume flow of the compressed medium lies above the predetermined minimum value.

7. A compressor having a screw compressor (200), which includes a first and a second compressor stage (201, 206), wherein the first compressor stage (201) compresses a gaseous medium and conducts it to the second compressor stage (206), which further compresses the medium, and wherein both compressor stages (201, 206) are driven separately from one another and in a speed-regulatable manner, characterized in that the compressor further includes a control unit, which is configured to carry out a method according to one of claims 1 to 6.

8. The compressor according to claim 7, characterized in that the inlet of the first compressor stage (201), which is at the front in terms of flow, is conducted to the ambient atmosphere without a controllable throttle element which limits the volume flow.

9. The compressor according to claim 7 or 8, characterized in that a blow-off valve (209) is arranged at the outlet of the second compressor stage (206), which is at the rear in terms of flow, this blow-off valve being caused to open by the control unit if the taken-down volume flow falls below a predetermined minimum value.