JP2004137934A - Adjustable vi type inverter screw compressor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To always enable the maximum efficiency operation corresponding to a load. <P>SOLUTION: Adjustment of ability corresponding to a load is performed by control of the number of revolution of a motor 11 by an inverter 15. When adjusting ability, performing the unload control is dispensed with to restrict deterioration of the operation efficiency. Furthermore, a capacity control valve for controlling capacity is eliminated to simplify a valve control mechanism. In order to obtain the maximum compressor efficiency corresponding to the operating condition (ability) in the variable VI, when a low VI is commanded, a compression part controller 27 moves a slide valve 19 toward a motor 11 side in the axial direction to quicken the compression process completing time, and quickly discharges the compression gas. When a high VI is commanded, the slide valve 19 is moved toward a piston 25 side in the axial direction to delay the compression process completing time, and slowly discharges the compression gas. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a variable VI (internal volume ratio) type inverter screw compressor in which an internal volume ratio VI which is a ratio between a suction volume and a discharge volume of a screw compressor is made variable.
[0002]
[Prior art]
Conventionally, there is a VI variable screw compressor in which the internal volume ratio VI is made variable as shown in FIG. 7 (for example, see Patent Document 1).
[0003]
In the VI variable screw compressor, when the internal volume ratio VI needs to be changed, the rod 2 is rotated by the stepping motor 1 to move the VI variable valve 3 backward, for example. At that time, the displacement control valve 4 is retracted with the retraction of the VI variable valve 3, and when the VI variable valve 3 is fixed at the new set position, it is fixed again in a state of contact with the VI variable valve 3. In this way, the tip of the displacement control valve 4 retreats to a position corresponding to the changed internal volume ratio VI, and newly defines the opening degree of the discharge port 5.
[0004]
In this case, the internal volume ratio VI detects the pressure P _d1 immediately before the space formed by the rotor and the inner wall of the casing 7 communicates with the discharge space during operation, and detects the detected pressure P _d1 and the discharge pressure P _d2 . Is specified by giving a signal to the step motor 1 so as to minimize the difference ΔP. Alternatively, the optimum internal volume ratio VI is predicted by performing trend analysis of parameters such as suction pressure and discharge pressure during operation by the control device 10, and designated by giving a signal representing the value of the optimum VI to the step motor 1. You.
[0005]
In the above configuration, the fluid sucked from the suction hole 6 is compressed by the male and female rotors (not shown) in the casing 7 and then discharged to the discharge hole 8 through the discharge port 5.
[0006]
In this state, when the load applied to the VI variable screw type compressor fluctuates and capacity control becomes necessary, the hydraulic piston 9 moves forward based on the control command to move the capacity control valve 4 by the required amount. Since the valve is advanced, a gap is generated between the VI variable valve 3 and the capacity control valve 4. Then, the fluid during compression is bypassed to the suction side from the gap between the VI variable valve 3 and the capacity control valve 4.
[0007]
That is, in the above Patent Document 1, the internal volume of the compressed gas discharged from the displacement control valve 4 is adjusted so that the maximum compressor efficiency is obtained in accordance with the high and low pressure conditions during operation at full load capacity (100% load). The ratio VI is made variable.
[0008]
[Patent Document 1]
Japanese Patent No. 3159762
[Problems to be solved by the invention]
However, the VI variable screw compressor disclosed in Patent Document 1 has the following problems.
[0010]
That is, the variable VI technology in the above-mentioned conventional VI variable screw compressor uses the internal volume ratio VI of the compressed gas discharged from the discharge port 5 so that the highest compressor efficiency is obtained in accordance with the high and low pressure conditions during operation. Although it is variable, it is set according to the full load capacity (100% load). Then, during partial load capacity (part load), capacity adjustment (unload control) is performed by bypassing the fluid under compression from the gap between the VI variable valve 3 and the capacity control valve 4 to the suction side. There is a problem that it is inefficient because it is.
[0011]
Further, since it has the VI variable valve 3 for changing the internal volume ratio VI and the capacity control valve 4 for controlling the capacity, the VI variable valve 3 control mechanism for changing the internal volume ratio VI and the capacity control valve for controlling the capacity are provided. It is necessary to separately provide four control mechanisms, and there is a problem that the valve control mechanism is complicated.
[0012]
Therefore, an object of the present invention is to provide a VI variable screw compressor capable of always operating at maximum efficiency according to a load (operating condition).
[0013]
[Means for Solving the Problems]
In order to achieve the above object, a variable VI inverter screw compressor according to the first aspect of the present invention includes a variable VI valve that changes an internal volume ratio VI by changing a time point at which a compression step in a screw compression section ends, It is characterized by comprising an electric motor that rotationally drives the screw compression section, and an inverter that controls the rotational frequency of the electric motor in accordance with the load.
[0014]
According to the above configuration, when adjusting the compression capacity according to the load, the rotation frequency of the electric motor is controlled by the inverter. Thus, the capacity adjustment is performed without performing the unload control. Then, the opening of the variable VI valve is controlled so that the highest compressor efficiency according to the adjusted rotation frequency of the electric motor is set, and the end point of the compression step in the screw compression section is set. As a result, maximum efficiency operation is always possible according to the load.
[0015]
Further, the invention according to claim 2 is the variable VI inverter screw compressor according to claim 1, wherein the pressure based on the suction side pressure and the discharge side pressure in the screw compression section and the rotation frequency of the electric motor are used. It is characterized by including a control unit for controlling the opening of the variable VI valve.
[0016]
According to the configuration, at the time of the variable VI, the control unit controls the opening of the variable VI valve based on the suction side pressure and the discharge side pressure in the screw compression unit and the rotation frequency of the electric motor. Therefore, by using a predetermined relationship between the compression ratio, the rotation frequency of the motor, and the optimum internal volume ratio VI, the internal volume ratio VI can be adjusted to the highest value according to the rotation frequency of the motor adjusted by the inverter. It is controlled precisely and easily to achieve compressor efficiency.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments. FIG. 1 is a schematic configuration diagram of a variable VI inverter screw compressor according to the present embodiment. FIG. 1A shows a low VI state, and FIG. 1B shows a high VI state.
[0018]
In FIG. 1, reference numeral 11 denotes an electric motor having a stator 12 fixed to a casing (not shown) and a rotor 13 fixed to one end of a main shaft 14 and rotating. The electric motor 11 is inverter-driven by an inverter 15. Both ends of the main shaft 14 are supported by bearings 16 and 17, and a screw rotor 18 is attached to the other end of the main shaft 14. When the main shaft 14 is rotated by the electric motor 11, the screw rotor 18 is rotated, and the suction gas is compressed by a screw groove (not shown) on the outer peripheral surface. A discharge port 20 having a predetermined length in the axial direction is provided, and a cylindrical slide valve 19 facing the outer peripheral surface of the screw rotor 18 is provided. Gas compressed by the screw rotor 18 is discharged from the discharge port 20. .
[0019]
One end of a plurality of rods 22 slidably supported by a support plate 21 is attached to an end surface of the slide valve 19 on the side opposite to the electric motor 11. The other end of each rod 22 is attached to one connecting plate 23. A cylinder 24 is provided at the center of the surface of the support plate 21 on the side opposite to the screw rotor 18, and a connecting plate 23 is attached to the tip of a piston rod 26 attached to the side of the piston 25 accommodated on the side opposite to the screw rotor 18. Is attached. Thus, as the piston 25 moves in the axial direction, the slide valve 19 moves in the axial direction via the piston rod 26, the connecting plate 23, and the rod 22.
[0020]
The working fluid supplied to and discharged from the working chambers on both sides of the piston 25 in the cylinder 24 is controlled by a fluid control device 28 based on a control signal from a compression unit controller 27. The specific configuration of the fluid control device 28 is such that when the internal volume ratio VI is reduced, the piston 25 is moved to the screw rotor 18 side as shown in FIG. In this case, there is no particular limitation as long as the piston 25 is moved to the side opposite to the screw rotor 18 as shown in FIG.
[0021]
In the variable VI inverter screw compressor having the above configuration, the capacity adjustment for the load is performed by controlling the rotation speed of the electric motor 11 by the inverter 15. By doing so, there is no need to perform unload control at the time of capacity adjustment, and a decrease in operating efficiency can be suppressed. Further, the capacity control valve for performing the capacity control can be eliminated, and the valve control mechanism can be simplified.
[0022]
On the other hand, in the variable VI, the position of the slide valve 19 is controlled by the compression unit controller 27 so as to have the highest efficiency according to the operation state. Then, at the time of the low VI command, by moving the slide valve 19 (that is, the start position of the discharge port 20) toward the axial motor 11, the end point of the compression step in the compression section is expedited to discharge the compressed gas earlier. On the other hand, at the time of the high VI command, the slide valve 19 (that is, the start position of the discharge port 20) is moved toward the piston 25 in the axial direction, so that the end point of the compression step in the compression section is delayed to discharge the compressed gas later. It is. That is, in the present embodiment, the variable VI valve is constituted by the slide valve 19.
[0023]
Then, as described above, when the rotation speed of the electric motor 11 is set by the inverter 15 and the position of the slide valve 19 is set by the compression unit controller 27, the suction gas sucked from the suction port flows into the motor 11. And is guided to the screw rotor 18. Then, the fluid is compressed by the screw groove formed on the outer peripheral surface of the screw rotor 18 and is discharged from the discharge port 20 of the slide valve 19.
[0024]
Hereinafter, the rotation speed control of the electric motor 11 and the position control of the slide valve 19 in the present embodiment will be described.
[0025]
FIG. 2 is a diagram showing a capacity / VI control system in the variable VI inverter screw compressor. In FIG. 2, a screw compressor 31 mounted on a refrigerator to compress and heat a refrigerant will be described as an example.
[0026]
The refrigerator is configured such that a screw compressor 31, a condenser 32, an expansion valve 33, and an evaporator 34 are sequentially connected in a ring shape. The high-temperature and high-pressure refrigerant discharged from the screw compressor 31 is condensed in the condenser 32 by heat exchange with cooling water or air, and is supplied to the expansion valve 33 as a low-temperature and high-pressure liquid refrigerant. Then, the low-temperature and low-pressure liquid refrigerant decompressed by the expansion valve 33 evaporates by heat exchange with water in the evaporator 34 and returns to the screw compressor 31 as a low-pressure gas. Then, the cold water cooled by the evaporator 34 is used for cooling.
[0027]
A temperature sensor 35 is attached to the refrigerant pipe of the evaporator 34, and a detection signal indicating the cooling water temperature Tw from the temperature sensor 35 is input to a rotation speed output unit 37 of a control device 36. Then, the rotation speed output unit 37 uses the cooling water temperature Tw based on the input detection signal as information on the load side, for example, based on the difference from the set temperature, based on the rotation frequency of the electric motor 11 for obtaining the required refrigerating capacity. Hz is calculated and output to the optimum VI output unit 38 of the control device 36 and the inverter 15. The inverter 15 controls the rotation speed of the electric motor 11 based on the received rotation frequency Hz. Thus, the capacity adjustment for the load is performed.
[0028]
On the other hand, a low pressure side pressure sensor 40 is mounted on the suction side of the screw compression section 39 including the screw rotor 18 and the slide valve 19, and a high pressure side pressure sensor 41 is mounted on the discharge side. Then, the detection signal indicating the low pressure LP from the low pressure side pressure sensor 40 and the detection signal indicating the high pressure HP from the high pressure side pressure sensor 41 are input to the optimum VI output unit 38. Then, the optimum VI output unit 38 detects an operation state after the rotation speed of the electric motor 11 is set, based on the suction-side low pressure LP and the discharge-side high pressure HP based on the input detection signal. Then, arithmetic processing is performed based on the low pressure LP, the high pressure HP, and the rotation frequency Hz from the rotation speed output unit 37, and the optimum VI at the current rotation frequency Hz is calculated and output to the compression unit controller 27. Then, the compression unit controller 27 controls the operation of the fluid control device 28 based on the received internal volume ratio VI. Thus, the VI control according to the driving situation is performed.
[0029]
By the way, when the configuration of the fluid control device 28 includes an element (such as an external drive motor for operating a pilot valve) that performs an operation proportional to the axial movement of the slide valve 19, The position of the slide valve 19 can be detected by the operating position. In that case, a detection signal indicating the position SV of the slide valve 19 from the fluid control device 28 is input to the optimum VI output unit 38 via the compression unit controller 27 or directly. Then, the optimum VI output section 38 obtains the current VI value based on the received position SV of the slide valve 19 and performs feedback control of the optimum VI value. By doing so, the variable VI control can be performed with high accuracy.
[0030]
If the configuration of the fluid control device 28 is such that the position of the slide valve 19 cannot be detected (for example, a configuration including a pipe and an electromagnetic valve), the optimum VI output unit 38 outputs the output VI from the start. Add up the values. Then, the feedback control can be performed by calculating the control amount ΔVI to the optimum VI value using the integrated VI value as the current VI value.
[0031]
FIG. 3 is a diagram showing a capability / VI control system different from FIG. Also in FIG. 3, the screw compressor 31 is mounted on the refrigerator. The control device 51 and the inverter 54 have configurations different from those in FIG. Hereinafter, the same members as those in FIG.
[0032]
As in the case of FIG. 2, a detection signal indicating the cooling water temperature Tw from the temperature sensor 35 is input to the rotation speed output unit 52 of the control device 51. Further, a detection signal indicating the low pressure LP from the low pressure side pressure sensor 40 and a detection signal indicating the high pressure HP from the high pressure side pressure sensor 41 are input to the optimum VI output unit 53 of the control device 51. Then, the rotation frequency output section 52 calculates the rotation frequency Hz of the electric motor 11 for obtaining the required refrigerating capacity based on the cooling water temperature Tw, and the inverter 54 controls the rotation number of the electric motor 11. Thus, the capacity adjustment for the load is performed.
[0033]
The inverter 54 in the present embodiment is capable of detecting the drive voltage V and the drive current A (or the drive power W) of the electric motor 11 and detects the detected drive voltage V and the drive current A (or the drive power W). W is returned to the rotation speed output unit 52). Then, the rotation frequency output section 52 sends the calculated rotation frequency Hz and the received drive voltage V and drive current A (or the drive power W) to the optimum VI output section 53.
[0034]
Then, the optimum VI output unit 53 outputs the low pressure LP and the high pressure HP from the pressure sensors 40 and 41, the rotation frequency Hz from the rotation speed output unit 52, and the output from the fluid control device 28 as in the case of FIG. An arithmetic process is performed based on the position SV of the slide valve 19 to calculate a control amount ΔVI to the optimum VI, and outputs the control amount ΔVI to the compression unit controller 27.
[0035]
Further, in the present embodiment, the optimum VI output unit 53 stores the change transition of the drive voltage V and the drive current A (or the drive power W) from the rotation speed output unit 52. Then, while repeating the above-described VI operation, VI control is performed so that the drive voltage V and the drive current A (or the drive power W) are minimized.
[0036]
Thereafter, as in the case of FIG. 2, the operation of the fluid control device 28 is controlled by the compression unit controller 27 based on the received control amount ΔVI, and the internal volume ratio VI according to the operation state is feedback-controlled. You.
[0037]
In this case, as in the case of FIG. 2, if the configuration of the fluid control device 28 is such that the position of the slide valve 19 cannot be detected, the optimum VI output unit 53 outputs the output VI from the start. The control amount ΔVI of the optimum VI value is calculated using the integrated VI value obtained by integrating the values as the current VI value.
[0038]
Meanwhile, the optimum VI output units 38 and 53 of the control devices 36 and 51 shown in FIGS. 2 and 3 perform arithmetic processing to calculate the control amount ΔVI to the optimum VI. However, the low pressure LP from the low pressure side pressure sensor 40, the high pressure HP from the high pressure side pressure sensor 41, and the rotation frequency Hz from the rotation speed output units 37 and 52 are sequentially stored in the memory. Then, the low pressure LP, the high pressure HP, and the rotation frequency Hz are compared with the low pressure LP, the high pressure HP, and the rotation frequency Hz at the time of the previous VI operation, and the control amount to the optimum VI is determined based on the transition of those changes. It is also possible to obtain ΔVI.
[0039]
FIG. 4 shows the operation of the compression ratio represented by the ratio (HP / LP) between the high pressure HP from the high pressure side pressure sensor 41 and the low pressure LP from the low pressure side pressure sensor 40, and the optimum internal volume ratio VI. The relationship for each frequency Hz (= 30 Hz, 60 Hz, 90 Hz) is shown. The straight line in FIG. 4 is a theoretical value represented by VI = (HP / LP) ^{1 / k} (k: refrigerant specific heat ratio). The relationship between the compression ratio, the optimum internal volume ratio VI, and the operating frequency Hz is determined for each refrigerant, and the above relationship is used in the arithmetic expression when the arithmetic processing is performed by the optimum VI output units 38 and 53 shown in FIGS. Incorporate it.
[0040]
In this way, the control amount ΔVI to the optimum VI at the current rotational frequency Hz can be accurately calculated by the calculation processing by the optimum VI output units 38 and 53.
[0041]
As described above, in the present embodiment, the electric motor 11 in the screw compressor is driven by the inverter 15 by the inverter. Further, the axial position of the slide valve 19 that defines the discharge start position is controlled by the fluid control device 28 based on a control signal from the compression unit controller 27 to control the working fluid supplied to and discharged from the working chamber in the cylinder 24. By doing so, it is controlled.
[0042]
The capacity adjustment for the load is performed by calculating the rotation frequency Hz for obtaining the refrigerating capacity that requires the cooling water temperature Tw as information on the load side by the rotation speed output units 37 and 52 configuring the control devices 36 and 51, The control is performed by controlling the number of rotations of the electric motor 11 by the inverters 15 and 54 so as to be at this rotation frequency Hz. Therefore, it is possible to eliminate the necessity of the unload control at the time of the capacity adjustment and to suppress a decrease in the operation efficiency. Further, the capacity control valve for performing the capacity control can be eliminated, and the valve control mechanism can be simplified.
[0043]
Further, the variable VI performs an arithmetic process based on the suction-side low pressure LP, the discharge-side high pressure HP, and the rotation frequency Hz by the optimum VI output units 38, 53 of the control devices 36, 51. Is calculated by calculating the optimum VI (or ΔVI) at the rotation frequency Hz, and setting the axial position of the slide valve 19 by the compression unit controller 27 and the fluid control device 28 to define the discharge start position. . Therefore, the internal volume ratio VI can be set so that the highest compressor efficiency according to the rotation frequency Hz of the electric motor 11 is obtained.
[0044]
Therefore, according to this embodiment, it is possible to minimize a decrease in the compressor efficiency when the rotation frequency Hz of the screw compressor 31 is controlled in order to adjust the capacity with respect to the load.
[0045]
FIG. 5 shows the relationship between refrigeration capacity and compressor efficiency. The horizontal axis indicates the refrigerating capacity Q, which is expressed as a percentage with the refrigerating capacity at 60 Hz in a VI variable screw compressor using both the conventional variable VI and unload control as 100%. On the other hand, the vertical axis indicates the compressor efficiency. Further, the compression ratio is changed to 2.1, 3.9, 5.5, 7.9.
[0046]
According to the figure, in the case of the variable VI type inverter screw compressor using the variable VI and the inverter control according to the present embodiment, the case of the conventional VI variable screw compressor using the variable VI and the unload control together is used. In comparison, at a refrigerating capacity Q of 100% or less, the compressor efficiency can be increased regardless of the compression ratio. Of course, the lower the refrigerating capacity, the greater the compressor efficiency can be increased, and a greater effect can be obtained. Further, in the case of the variable VI inverter screw compressor, the capacity adjustment for the load is performed by inverter control. Therefore, the capability adjustment of 100% or more can be performed. Incidentally, in the case of the conventional screw compressor in which the capacity adjustment for the load is performed by the unload control, the capacity adjustment of 100% or more cannot be naturally performed.
[0047]
By the way, in the case of the screw compressor, even under the same pressure condition, there is a difference in the internal pressure depending on the rotation frequency, so that there is an optimum value of the internal volume ratio VI corresponding to each frequency. FIG. 6 shows the relationship between the internal volume and the pressure when the frequency is 30 Hz (FIG. 6A) and when the frequency is 90 Hz (FIG. 6B). The dotted line in the figure is a curve showing the relationship between the internal volume and the pressure in the case of a fixed VI in which the internal volume ratio VI is fixed to the optimum VI value at a frequency of 60 Hz. The dashed line is a curve showing the relationship between the internal volume and the pressure during theoretical adiabatic compression. In the above fixed VI, when the frequency is 30 Hz, the compression is insufficient at the time (A), and the pressure is rapidly reduced. In the case of a frequency of 90 Hz, overcompression occurs at the time (B), and the pressure is considerably higher than the theoretical value. From the above, the inverter cannot be simply applied to the capacity control of the screw compressor.
[0048]
However, by using a variable VI as in the present embodiment, as shown by a solid line, it is possible to eliminate insufficient compression occurring at a frequency of 30 Hz at the time of fixed VI and reduce the width of pressure fluctuation. . Further, when the frequency is 90 Hz at the time of the fixed VI, it is possible to eliminate the over-compression that has occurred and to reduce the width of the pressure fluctuation.
[0049]
In the above embodiment, the description of the capacity / VI control system in the variable VI inverter screw compressor is applied to a refrigerator as an example, but the present invention is not limited to this. is not. In short, in FIGS. 2 and 3, the detection signals input to the rotation speed output units 37 and 52 of the control devices 36 and 51 only need to be signals indicating the state of the load.
[0050]
【The invention's effect】
As is apparent from the above description, the variable VI inverter screw compressor according to the first aspect of the present invention controls the rotation frequency of the electric motor with the inverter to adjust the compression capacity according to the load. Ability adjustment can be performed without performing. Therefore, it is possible to suppress a decrease in operation efficiency during the capacity adjustment. Furthermore, the capacity control valve for performing the unload control is eliminated, and the valve control mechanism can be simplified to reduce the cost.
[0051]
Since the internal volume ratio VI is set by changing the end point of the compression process with the variable VI valve, the compressor efficiency is set to be the highest efficiency according to the adjusted rotation frequency of the electric motor. Becomes possible. Therefore, it is possible to always perform the maximum efficiency operation according to the load.
[0052]
Further, in the variable VI inverter screw compressor according to the second aspect of the present invention, at the time of the variable VI, the control unit controls the suction-side pressure and the discharge-side pressure in the screw compression unit and the rotation frequency of the electric motor based on the rotation frequency. Since the degree of opening of the variable VI valve is controlled, the relationship between the compression ratio, the rotation frequency of the electric motor, and the optimum internal volume ratio VI is determined in advance, so that the internal volume ratio VI is adjusted by the inverter. The control can be performed accurately and easily so that the highest compressor efficiency according to the rotation frequency of the electric motor is obtained.
[0053]
Further, by performing the above-described variable VI, it is possible to eliminate insufficient compression or overcompression that occurs in the inverter screw compressor according to the rotation frequency.
[Brief description of the drawings]
FIG. 1 is a main part configuration diagram of a variable VI inverter screw compressor of the present invention.
FIG. 2 is a diagram showing a capacity / VI control system in the variable VI inverter screw compressor shown in FIG.
FIG. 3 is a diagram showing a capability / VI control system different from FIG. 2;
FIG. 4 is a diagram showing a relationship between a compression ratio and an optimum internal volume ratio for each operation frequency.
FIG. 5 is a diagram showing a relationship between a refrigerating capacity and a compressor efficiency for each compression ratio.
FIG. 6 is a diagram showing the relationship between the internal volume and the pressure in the screw compressor.
FIG. 7 is a cross-sectional view of a conventional VI variable screw compressor.
[Explanation of symbols]
11 ... electric motor,
14 ... spindle,
15, 54 ... inverter,
18 ... Screw rotor,
19 ... Slide valve,
20 ... Discharge port,
24 ... cylinder,
25 ... piston,
27 ... Compression controller
28 ... fluid control device,
31 ... screw compressor,
32 ... condenser
33 ... expansion valve,
34 ... Evaporator,
36, 51 ... control device,
37, 52 ... rotation number output part,
38, 53 ... Optimal VI output unit,
39: Screw compression section.

Claims (2)

A variable VI valve (19) for varying the internal volume ratio VI by changing the end point of the compression step in the screw compression section (39);
An electric motor (11) that rotationally drives the screw compression section (39);
An inverter (15) for controlling a rotation frequency of the electric motor (11) according to a load;
A variable VI inverter screw compressor comprising: The variable VI inverter screw compressor according to claim 1,
A control unit (36, 51, which controls an opening degree of the variable VI valve (19) based on a suction side pressure and a discharge side pressure in the screw compression unit (39) and a rotation frequency of the electric motor (11). 27, 28), wherein a variable VI inverter screw compressor is provided.

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