Capacity control device and capacity control method for screw compressor

Abstract

An object of the invention is to provide a capacity control device of a screw compressor capable of restraining the consumption of components such as a suction throttle valve and of extending the life of the components. According to the present invention, there is provided a capacity control device of a screw compressor which controls a suction throttle valve by a controlled pressure at which a load operation state is switched to a low pressure operation, and by a minimum operation pressure at which the low pressure operation is switched to the load operation, the capacity control device being characterized by comprising: a pressure detector 15 which detects a discharge pressure of the compressor 1; and a control device 16 which calculates a cycle time between the load operation and the low pressure operation on the basis of the measured pressure detected by the pressure detector 15, and which changes the controlled pressure to a high pressure side when the cycle time becomes not longer than a shortest period specified beforehand on the basis of the life of each component.

Description

INCORPORATION BY REFERENCE

The present application claims priority from Japanese application JP 2006-016198 filed on Jan. 25, 2006, the content of which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a capacity control device and a capacity control method of a screw compressor, and more particularly to a capacity control device and a capacity control method of an oil cooled screw compressor.

(2) Description of Related Art

In a capacity control device of a screw compressor, in order to enhance the energy saving effect by reducing operation under an unnecessary high pressure in the case where the operation load is low and where the capacity of discharge side piping is large, there is proposed a capacity control device in which when a cycle time between a minimum operation pressure and a controlled pressure in a repeated operation (load operation and unload operation) of a load operation state from the minimum operation pressure to the controlled pressure and a low pressure operation (unload operation) state from the controlled pressure to the minimum operation pressure, becomes not longer than a shortest period specified beforehand on the basis of the life of each component, the controlled pressure is lowered to a limit corresponding to the shortest period (for example, see JP-A-4-159491).

In the capacity control device of the screw compressor, in order to make the cycle time between the load operation and the unload operation of the compressor not shorter than the shortest cycle T specified on the basis of the life of each component, when the air discharge quantity of the compressor is Qs, the suction pressure is Ps, the load factor is X, and the pressure difference between the controlled pressure and the minimum operation pressure is ΔP, the required air tank capacity C can be calculated by the following general expression.

$C=\frac{T\times \mathrm{Qs}\times \mathrm{Ps}\times X\left({\hspace{0.17em}}_{1-}X\right)}{\Delta P}$

From the above general expression, it can be seen that when the pressure difference ΔP between the controlled pressure and the minimum operation pressure is reduced, the required air tank capacity C is increased. From this, it can be seen that the above described prior art has an advantage that the energy saving effect is enhanced by lowering the controlled pressure, but on the contrary, it can be also seen that the prior art device uses a method of further reducing the pressure difference ΔP between the controlled pressure and the minimum operation pressure, and thereby the required air tank capacity C is further increased.

In general, the larger air tank installed in the compressor is better in view of the energy saving performance, but in many cases, it is difficult to install the air tank with the capacity calculated on the basis of the above described expression. Therefore, in practice, the capacity of the air tank needs to be reduced. For this reason, in the above described prior art, the load operation and the unload operation are frequently repeated, which may result in a problem such as the abnormal wear of components.

The present invention has been made on the basis of the above described problems, and an object of the invention is to provide a capacity control device of a screw compressor capable of restraining the consumption of components such as a suction throttle valve and of extending the life of the components.

SUMMARY OF THE INVENTION

In order to achieve the above described object, according to a first aspect of the present invention, there is provided a capacity control device of a screw compressor which controls a suction throttle valve by a controlled pressure at which a load operation state is switched to a low pressure operation and by a minimum operation pressure at which the low pressure operation is switched to the load operation, the capacity control device being characterized by comprising: a pressure detector which detects a discharge pressure of the compressor; and a control device which calculates a cycle time between the load operation and the low pressure operation on the basis of the measured pressure detected by the pressure detector, and which changes the controlled pressure to a high pressure side when the cycle time becomes not longer than a shortest period specified beforehand on the basis of the life of each component.

Further, according to a second aspect of the present invention, there is provided a capacity control device of an oil cooled screw compressor which controls a suction throttle valve by a controlled pressure at which a load operation state is switched to a low pressure operation and by a minimum operation pressure at which the low pressure operation is switched to the load operation, the capacity control device being characterized by comprising: a pressure detector which detects a discharge pressure of the compressor; and a control device which calculates a cycle time between the load operation and the low pressure operation on the basis of the measured pressure detected by the pressure detector, which changes the controlled pressure to a high pressure side when the cycle time becomes not longer than a shortest period specified beforehand on the basis of the life of each component, and which effects a shift to suction throttle control when the cycle time cannot be prevented from becoming not longer than the shortest period by the increase of the controlled pressure.

Further, a third aspect according to the present invention is characterized in that in one of the first and second aspects of the present invention, the shortest period is specified on the basis of the life of the suction throttle valve.

Further, according to a fourth aspect of the present invention, there is provided a capacity control method of a screw compressor which controls a suction throttle valve by a controlled pressure at which a load operation state is switched to a low pressure operation, and by a minimum operation pressure at which the low pressure operation is switched to the load operation, the capacity control method being characterized by comprising: calculating a cycle time between the load operation and the low pressure operation by a control device on the basis of a measured pressure detected by a pressure detector which detects a discharge pressure of the compressor; and changing the controlled pressure to a high pressure side when the cycle time becomes not longer than a shortest period specified beforehand on the basis of the life of each component.

Further, according to a fifth aspect of the present invention, there is provided a capacity control method of an oil cooled screw compressor which controls a suction throttle valve by a controlled pressure at which a load operation state is switched to a low pressure operation, and by a minimum operation pressure at which the low pressure operation is switched to the load operation, the capacity control method being characterized by comprising: calculating a cycle time between the load operation and the low pressure operation by a control device on the basis of a measured pressure detected by a pressure detector which detects a discharge pressure of the compressor; changing the controlled pressure to a high pressure side when the cycle time becomes not longer than a shortest period specified beforehand on the basis of the life of each component; and effecting a shift to suction throttle control when the cycle time cannot be prevented from becoming not longer than the shortest period by the increase of the controlled pressure.

According to the control device of the present invention, in the case where the cycle time between the load operation and the unload operation which is measured at the time when the controlled pressure is reached, is not longer than the shortest period T specified on the basis of the life of each component, it is possible to change the controlled pressure to a high pressure side and to perform capacity control by using the changed pressure as a new controlled pressure, so that the consumption of components such as a suction throttle valve can be restrained. As a result, it is possible to extend the life of components such as a suction throttle valve, and to reduce maintenance work of the components.

According to the control method of the present invention, in the case where the cycle time between the load operation and the unload operation which is measured at the time when the controlled pressure is reached, is not longer than the shortest period T specified on the basis of the life of each component, it is possible to change the controlled pressure to a high pressure side and to perform capacity control by using the changed pressure as a new controlled pressure, so that the consumption of components such as a suction throttle valve can be restrained. As a result, it is possible to extend the life of components such as a suction throttle valve, and to reduce maintenance work of the components.

Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic diagram showing a whole configuration of an embodiment of an oil cooled screw compressor and a capacity control device of the oil cooled screw compressor according to the present invention;

FIG. 2 is a characteristic diagram showing compressor pressure fluctuation caused in the embodiment of the capacity control device of the oil cooled screw compressor according to the present invention shown in FIG. 1;

FIG. 3 is a flowchart diagram for control performed in the embodiment of the capacity control device of the oil cooled screw compressor according to the present invention shown in FIG. 1;

FIG. 4 is a characteristic diagram showing compressor pressure fluctuation caused in another embodiment of a capacity control device of an oil cooled screw compressor according to the present invention; and

FIG. 5 is a figure showing a comparison of energy-saving characteristics between another embodiment of the capacity control device of the oil cooled screw compressor according to the present invention and the other capacity control methods.

DETAILED DESCRIPTION OF THE INVENTION

In the following, an embodiment of a capacity control device of a screw compressor according to the present invention will be described with reference to the accompanying drawings. FIG. 1 to FIG. 3 show an embodiment of a capacity control device of an oil cooled screw compressor according to the present invention. In the figures, FIG. 1 is a diagrammatic view showing the whole configuration of the embodiment of the oil cooled screw compressor and the capacity control device of the oil cooled screw compressor according to the present invention, FIG. 2 is a characteristic diagram showing compressor pressure fluctuation according to the present invention, and FIG. 3 is a flow chart diagram for control in the embodiment of the capacity control device of the oil cooled screw compressor according to the present invention.

In FIG. 1, reference numeral 1 denotes an oil cooled screw compressor, 2 denotes a drive motor of the screw compressor 1, 3 denotes a suction throttle valve provided at the suction side of the screw compressor 1, 4 denotes an operation cylinder of the suction throttle valve 3, 5 denotes a solenoid valve which switches the operation cylinder to the side of a compressed air supply pipe 6 or to the outside air side 7, 8 denotes a suction filter provided on the suction side of the screw compressor 1, 9 denotes an oil tank provided on the discharge side of the screw compressor 1. In the oil tank 9, compressed air, after being subjected to compression by the screw compressor 1, is primarily separated into compressed air and lubricating oil, and the separated lubricating oil is stored in the lower part of the oil tank 9.

Reference numeral 10 denotes a separator element which takes in the compressed air separated by the oil tank 9, and in the separator element 10, the taken compressed air is secondarily separated into compressed air and lubricating oil. Reference numeral 11 denotes a pressure regulating check valve provided at a portion of the separator element 10. Reference numeral 12 denotes an after cooler which cools the compressed air from the pressure regulating check valve 11, and 13 denotes an air tank connected to the after cooler 12. Reference numeral 14 denotes an oil cooler which takes in and cools the lubricating oil stored in the lower part of the oil tank 9, and the lubricating oil cooled by the oil cooler 14 is introduced into the suction side of the screw compressor 1.

Reference numeral 15 denotes a pressure detector which detects the pressure of the compressed air, and the pressure detector 15 is provided at a portion of the separator element 10 in this example. Reference numeral 16 denotes a control device which is provided with a storage section 16A, an arithmetic section 16B, an input section 16C, and an output section 16D. In the storage section 16A, a shortest period T specified on the basis of the life of each components (for example, the frequency of opening and closing the suction throttle valve 3), a controlled pressure P1, an pressure rise value ΔPU with respect to the controlled pressure P1, and a minimum operation pressure P2 are stored. The arithmetic section 16B has a function that compares a pressure detected by the pressure detector 15 with the minimum operation pressure P2 and the controlled pressure P1, and outputs a load command or an unload command to the selector valve 5, and also has a function that calculates a cycle time t between the load operation and the unload operation and that adds the pressure rise value ΔPU to the controlled pressure P1, so as to set the resultant pressure as a new controlled pressure at the time when the cycle time t becomes not longer than the shortest period T stored beforehand.

Next, an operation of the above described embodiment of the oil cooled screw compressor according to the present invention is explained.

In FIG. 1, the air to be compressed is sucked in the main body of the compressor 1 via the suction filter 8 and the suction throttle valve 3. The compressed air, after being subjected to compression by the compressor 1, is primarily separated into compressed air and lubricating oil in the oil tank 9. The separated lubricating oil is accumulated in the lower part of the oil tank 9, and is cooled in the oil cooler 14. Thereafter, the lubricating oil again lubricates the main body of the unit of the compressor 1.

The compressed air in the oil tank 9 flows into the separator element 10, and is secondarily divided into compressed air and lubricating oil. The separated compressed air passes through the pressure regulating check valve 11, and is heat-exchanged with the outdoor air in the after cooler 12 so as to be cooled to a predetermined temperature. Thereafter, the cooled compressed air is discharged to the outside of the unit of the compressor 1.

The compressed air discharged outside the unit of the compressor 1 passes through various auxiliary apparatuses depending on use purposes, and thereafter supplied to the distal end through the air tank 13. As the air tank 13 is made larger, the pressure fluctuation in the whole compressed air passage as described above is reduced, so that the number of cycles of the load operation and the unload operation can be reduced.

However, in practice, the air tank 13 having a capacity larger than required is rarely installed, which may result in a problem of the consumption of components or the automatic shift to the suction throttle control.

The pressure fluctuation in the compressed air passage is measured by the pressure detector 15. The measured pressure is inputted into the control device 16. On the basis of the measured pressure, the control device 16 manages the capacity control of the compressor 1.

The pressure fluctuation measured by the pressure detector 15 results in a waveform as shown by a dotted line in FIG. 2. That is, when the compressor 1 shifts to the load operation, the pressure is increased, and when the pressure reaches the controlled pressure P1, the compressor 1 shifts to the unload operation.

When the compressor 1 shifts to the unload operation, the pressure is lowered, and when the pressure reaches the minimum operation pressure P2, the compressor 1 shifts to the load operation. Since the suction throttle valve 3 is closed to avoid the air intake during the unload operation, a negative pressure is produced on the intake side of the main body of the compressor 1. Thus, during the unload operation, the pressure between the main body of compressor 1 and the pressure regulating check valves 11 becomes lower than the pressure during the load operation, as a result of which the power becomes lower than that during the load operation.

The compressor 1 repeatedly performs the load operation and the unload operation, thereby enabling the energy to be saved. The pressure pulsation shown by the dotted line in FIG. 2 is a pressure waveform of the prior art. In this case, however, when the cycle time between the load operation and the unload operation in a certain load is not shorter than the shortest period T specified on the basis of the life of each component, the operation is arranged to lower the controlled pressure to a controlled pressure PD, so that the cycle time automatically becomes the shortest period T in the following cycle.

This makes it possible to save energy, amount of saving energy corresponds to the range of pressure which is made lower than the original controlled pressure. However, as a problem of the prior art as described above, the repeated operation of the load operation and the unload operation based on the shortest period T is frequently caused. Further, in the case where the load fluctuation is caused as shown in FIG. 2 after the controlled pressure is automatically adjusted to the controlled pressure PD so as to make the cycle time between the load operation and the unload operation set to the shortest period T, the cycle time becomes shorter than the shortest period T in the following cycle, the operation is shifted to the suction throttle control as shown by a two-dot chain line in FIG. 2 in the following cycle.

For this reason, the frequency of the load operation and the unload operation can be restrained by effecting the shift to the suction throttle control, but there is a problem that such operation shift is disadvantageous in terms of energy saving.

Thus, in the present invention, the fluctuation waveform of pressure is controlled as shown by the solid line in FIG. 2. That is, in the case where the control of the fluctuation waveform of pressure according to the present invention is explained as an extension of the pressure fluctuation of the prior art shown by the dotted line in FIG. 2, a case in which the shift is effected at the controlled pressure PD is described in FIG. 2, but the shift may also be effected at the controlled pressure P1. In the following cycle, the control device 16 effects the shift to the unload operation at a controlled pressure which is increased by the pressure rise value ΔPU from the controlled pressure PD (the controlled pressure P1 in this example), and counts the cycle time t.

Then, when the cycle time t is shorter than the shortest period T stored in the storage section 16A, the arithmetic section 16B of the control device 16 sets a pressure PU which is further increased by the pressure rise value ΔPU, as the controlled pressure PU. The compressor 1 shifts to the unload operation on the basis of the set controlled pressure PU.

In this way, increasing the pressure by the pressure rise value ΔPU and making a comparison between the previous cycle time t between the load operation and the unload operation and the shortest period T are repeated until the relation T>t is satisfied. Thereby, the energy saving operation can be continued, under the condition that the operation is not performed while the cycle time between the load operation and the unload operation is made shorter than the shortest period T, and that the capacity control is performed without effecting the shift to the suction throttle control.

FIG. 3 shows the flow of control as described above. It is preferred that the pressure rise value ΔPU is set to about 0.05 MPa, that the maximum value of the controlled pressure is set to a range which is not higher than the allowable pressure of each component, the temperature rise due to the pressure rise is permissible, and that the temperature is further set to be not higher than a range in which the energy saving characteristic can be exhibited in the case of the suction throttle valve control.

FIG. 4 is a characteristic diagram showing a compressor pressure fluctuation in another embodiment of a capacity control device of an oil cooled screw compressor according to the present invention. In FIGS. 3 and 4, the same reference characters denote the same or corresponding meanings.

In this embodiment, in addition to the capacity control of the above described embodiment according to the present invention, when the cycle time t between the load operation and the unload operation cannot be prevented from becoming not longer than the shortest period T by increasing the previously set controlled pressure by the pressure rise value ΔPU, that is, by comparing the previous cycle time t between the load operation and the unload operation and the shortest period T, the relation T>t is measured, and the measured pressure reaches the controlled pressure P1, the control device 16 is arranged to output a signal for effecting a shift to the suction throttle control to the solenoid valve 5, so that the suction throttle control can be performed as shown by a two-dot chain line Y in FIG. 4 is performed.

According to this embodiment, in addition to the effect that the consumption of components can be restrained similarly to the above described embodiment, the frequency of the shifts to the suction throttle control is effected can be reduced as compared with the conventional control, as a result of which the effect of energy saving can be further enhanced as compared with the conventional control.

Next, the reason why the above described embodiment according to the present invention is advantageous for the suction throttle control in terms of energy saving, is explained by using FIG. 5.

FIG. 5 is a figure showing a comparison between energy-saving characteristics of respective capacity control methods, in which figure the shaft power ratio W is represented with respect to each used air quantity ratio Q. In FIG. 5, a characteristic line A represents a general energy saving characteristic of the load/unload operation control, a characteristic line B represents an energy saving characteristic of the suction throttle control, and a characteristic line C represents an energy saving characteristic according to the present invention.

As for the energy saving, the best method is to fully secure the capacity of the air tank 13 so as to enable the general energy saving characteristic of the load/unload operation control to be exhibited. However, as described above, it is difficult to secure such air tank 13 with sufficient capacity because of restrictions on the installation space and the like. Usually, even in the present situation, in many cases, the suction throttle control is normally provided by the compressor manufacturer side.

The energy-saving characteristic according to the embodiment of the present invention shown in FIG. 5 is plotted so as to correspond to the pressure rise of 0.05 MPa. It can be seen from FIG. 5 that even in the case of this pressure rise ΔPU, in the capacity control method according to the present invention, the shaft power ratio W becomes lower than that in the suction throttle control method which is conventionally used, in almost all the region of the used air quantity ratio Q.

As described above, according to the present invention, it is possible to provide a method which has the effect of restraining the consumption of components, and which is also advantageous in terms of energy saving as compared with the suction throttle control that similarly has the effect of restraining the consumption of components.

Note that in the above described embodiment, the present invention is explained by using an example suitable for an oil cooled screw compressor, but the same effect can also be obtained when the present invention is applied to a water lubricated type screw compressor which sprays water during the compression process.

It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.

Claims

1. A capacity control device of a screw compressor which controls a suction throttle valve by a controlled pressure at which a load operation state is switched to a low pressure operation, and by a minimum operation pressure at which the low pressure operation is switched to the load operation,

the capacity control device comprising: a pressure detector which detects a discharge pressure of the compressor; and a control device which calculates a cycle time between the load operation and the low pressure operation on the basis of the measured pressure detected by the pressure detector, and which changes the controlled pressure to a high pressure side when the cycle time becomes not longer than a shortest period specified beforehand on the basis of the life of each component.

2. A capacity control device of an oil cooled screw compressor which controls a suction throttle valve by a controlled pressure at which a load operation state is switched to a low pressure operation, and by a minimum operation pressure at which the low pressure operation is switched to the load operation,

the capacity control device comprising: a pressure detector which detects a discharge pressure of the compressor; and a control device which calculates a cycle time between the load operation and the low pressure operation on the basis of the measured pressure detected by the pressure detector, which changes the controlled pressure to a high pressure side when the cycle time becomes not longer than a shortest period specified beforehand on the basis of the life of each component, and which effects a shift to suction throttle control when the cycle time cannot be prevented from becoming not longer than the shortest period by the increase of the controlled pressure.

3. The capacity control device of the screw compressor according to claim 1, wherein the shortest period is specified on the basis of the life of the suction throttle valve.

4. The capacity control device of the screw compressor according to claim 2, wherein the shortest period is specified on the basis of the life of the suction throttle valve.

5. A capacity control method of a screw compressor which controls a suction throttle valve by a controlled pressure at which a load operation state is switched to a low pressure operation, and by a minimum operation pressure at which the low pressure operation is switched to the load operation,

the capacity control method comprising: calculating a cycle time between the load operation and the low pressure operation by a control device on the basis of a measured pressure detected by a pressure detector which detects a discharge pressure of the compressor; and changing the controlled pressure to a high pressure side when the cycle time becomes not longer than a shortest period specified beforehand on the basis of the life of each component.

6. A capacity control method of an oil cooled screw compressor which controls a suction throttle valve by a controlled pressure at which a load operation state is switched to a low pressure operation, and by a minimum operation pressure at which the low pressure operation is switched to the load operation,

the capacity control method comprising: calculating a cycle time between the load operation and the low pressure operation by a control device on the basis of a measured pressure detected by a pressure detector which detects a discharge pressure of the compressor; changing the controlled pressure to a high pressure side when the cycle time becomes not longer than a shortest period specified beforehand on the basis of the life of each component; and effecting a shift to suction throttle control when the cycle time cannot be prevented from becoming not longer than the shortest period by the increase of the controlled pressure.