Compressed air manufacturing facility

In order to provide a compressed air manufacturing facility which can increase a stability of a supply pressure while obtaining an energy saving effect, in a compressed air manufacturing facility provided with a compressor compressing an air, an electric motor driving the compressor, and an inverter variably controlling a rotating speed of the electric motor, the compressed air manufacturing facility is provided with a pressure sensor detecting a discharge pressure of the compressor at an upstream side position of a discharge air system connected to a discharge side of the compressor, and a control apparatus computing a pressure loss of the discharge air system in correspondence to a rotating speed of the electric motor, and changing a control range of the discharge pressure of the compressor at the upstream side position of the discharge air system on the basis of the computation in such a manner that a terminal pressure at a downstream side position of the discharge air system comes to a predetermined range, and variably controlling the rotating speed of the electric motor via the inverter in such a manner that the discharge pressure of the compressor detected by the pressure sensor comes to the changed control range.

The present application claims priority from Japanese application JP2006-190728 filed on Jul. 11, 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 compressed air manufacturing facility provided with a compressor driven by an electric motor in which a rotating speed is variably controlled by an inverter.

(2) Description of Related Art

The compressed air manufacturing facility is provided with a compressor compressing an air, for example, serving as a variable speed compressor unit executing a capacity control, an electric motor driving the compressor, an inverter variably controlling a rotating speed of the electric motor, a pressure sensor detecting a discharge pressure of the compressor, and a control apparatus variably controlling the rotating speed of the electric motor via the inverter on the basis of a deviation between the discharge pressure detected by the pressure sensor and a control pressure. Further, as a structure provided with a plurality of variable speed compressor units, there have been know a structure in which the units are operated in parallel, and a structure in which the units are operated alternately and in a following manner. Further, for example, in a structure provided with a plurality of compressor units including at least one variable speed compressor unit, there has been known a structure in which one variable speed compressor unit is operated by variably controlling a rotating speed of a corresponding electric motor via an inverter, and the other compressor units are switched to a full-load operation state at a rotating speed which has an upper limit of the rotating speed of the corresponding electric motor or a stop state, thereby controlling a number of the units.

In this case, a pressure loss of a discharge air system supplying the compressed air discharged from the compressor to a supply end is changed in correspondence to a change of an amount of a discharge air of the compressor and an amount of a used air of the supply end. Accordingly, in general, a control range of the discharge pressure of the compressor at an upstream side position of the discharge air system is set by anticipating a maximum pressure loss of the discharge air system in such a manner that a terminal pressure (a supply pressure) at a downstream side position of the discharge air system comes to a desired pressure value or more. In the compressed air manufacturing facility mentioned above, it is possible to obtain a desired compressed air, however, for example, in the case that the amount of the used air is small (that is, the amount of the discharge air of the compressor becomes smaller), the control range of the discharge pressure of the compressor is kept high in spite that the pressure loss of the discharge air system becomes smaller. Accordingly, the compressor is driven more than necessary, and an extra power is consumed.

Accordingly, in order to correspond to this problem, for example, there has been proposed a control apparatus variably controlling a rotating speed of the electric motor in such a manner that the terminal pressure at the downstream side position of the discharge air system comes to a predetermined range in correspondence to the discharge pressure of the compressor at the upstream side position of the discharge air system detected by the pressure sensor (for example, refer to JP-A-2004-190583). Describing in detail, the control apparatus previously stores a pressure loss (=discharge pressure of the compressor at the upstream side position−terminal pressure at the downstream side position) of the discharge air system at a time of a specification pressure, and is structured such as to compute a pressure loss of the discharge air system on the basis of a ratio between the discharge pressure of the compressor detected by the pressure sensor and the specification pressure. Further, the structure is made such as to compute the control range of the discharge pressure of the compressor obtained by adding a computed value of the pressure loss of the discharge air system to a predetermined range of the terminal pressure at the downstream side position of the discharge air system, and variably control the rotating speed of the electric motor on the basis of the computation.

BRIEF SUMMARY OF THE INVENTION

However, there is the following room for improvement in the prior art mentioned above.

In other words, the control apparatus mentioned above is provided with a first function of computing the pressure loss of the discharge air system in correspondence to the discharge pressure of the compressor detected by the pressure sensor and changing the control range of the discharge pressure of the compressor on the basis of this computation in such a manner that the terminal pressure at the downstream side position of the discharge air system comes to the predetermined range, and a second function of variably controlling the rotating speed of the electric motor via the inverter in such a manner that the discharge pressure of the compressor detected by the pressure sensor comes to the control range changed by the first function. However, the first function is based on an assumption that a relation between the control amount (the discharge pressure of the compressor) in accordance with the second function and the operation amount (the rotating speed of the electric motor) is sufficiently kept, and the structure is made such that a convergence characteristic of the discharge pressure of the compressor in accordance with the first function and a convergence characteristic of the rotating speed of the electric motor in accordance with the second function are affected by each other. Accordingly, for example, in the case that the amount of the used air is largely changed, the discharge pressure of the compressor and the rotating speed of the electric motor generate a hunting, and the terminal pressure at the downstream side position of the discharge air system, that is, the supply pressure becomes unstable.

The present invention is made by taking the problem of the prior art mentioned above into consideration, and an object of the present invention is to provide a compressed air manufacturing facility which can increase a stability of a supply pressure while obtaining an energy saving effect.(1) In order to achieve the object mentioned above, in accordance with the present invention, there is provided a compressed air manufacturing facility comprising:a compressor compressing an air;an electric motor driving the compressor; andan inverter variably controlling a rotating speed of the electric motor,wherein the compressed air manufacturing facility comprises:a discharge pressure changing means computing a pressure loss of a discharge air system connected to a discharge side of the compressor in correspondence to the rotating speed of the electric motor, and changing a control range of a discharge pressure of the compressor at an upstream side position of the discharge air system on the basis of the computation in such a manner that a terminal pressure at a downstream side position of the discharge air system comes to a predetermined range;a pressure detecting means detecting the discharge pressure of the compressor at the upstream side position of the discharge air system; anda rotating speed control means variably controlling the rotating speed of the electric motor via the inverter in such a manner that the discharge pressure of the compressor detected by the pressure detecting means comes to the control range changed by the discharge pressure changing means.

In the present invention, the discharge pressure changing means computes the pressure loss of the discharge air system in correspondence to the rotating speed of the electric motor, and changes the control range of the discharge pressure of the compressor at the upstream side position of the discharge air system on the basis of the computation in such a manner that the terminal pressure at the downstream side position of the discharge air system comes to the predetermined range. Further, the rotating speed control means variably controls the rotating speed of the electric motor via the inverter in such a manner that the discharge pressure of the compressor detected by the pressure detecting means comes to the control range changed by the discharge pressure changing means. Accordingly, it is possible to hold the power of the compressor to a minimum, and it is possible to obtain an energy saving effect. Further, in the present invention, since there are provided with the discharge pressure changing means changing the control range of the discharge pressure of the compressor in correspondence to the rotating speed of the electric motor, and the rotating speed control means variably controlling the rotating speed of the electric motor in correspondence to the discharge pressure of the compressor, and the discharge pressure changing means and the rotating speed control means operate as the feedback control functions with each other, it is possible to increase a convergence characteristic of the discharge pressure of the compressor and the rotating speed of the electric motor. As a result, it is possible to stabilize the terminal pressure of the discharge air system, that is, the supply pressure. Accordingly, in the present invention, it is possible to increase a stability of the supply pressure while obtaining the energy saving effect.(2) In order to achieve the object mentioned above, in accordance with the present invention, there is further provided a compressed air manufacturing facility comprising:a plurality of compressors compressing an air;

a plurality of electric motors respectively driving a plurality of compressors; anda number control means operating a first compressor corresponding to one of a plurality of compressors by variously controlling a rotating speed of the electric motor corresponding thereto via an inverter, and switching the other second compressor to a full-load operation state of operating by setting the rotating speed of the electric motor corresponding thereto to an upper limit value or a stop state,wherein the compressed air manufacturing facility comprises:a discharge pressure changing means computing a pressure loss of a discharge air system connected to a discharge side of the first and second compressors in correspondence to the rotating speed of the electric motor corresponding to the first compressor and the rotating speed of the electric motor corresponding to the second compressor, and changing a control range of a discharge pressure of the first compressor at an upstream side position of the discharge air system on the basis of the computation in such a manner that a terminal pressure at a downstream side position of the discharge air system comes to a predetermined range;a pressure detecting means detecting the discharge pressure of the first compressor at the upstream side position of the discharge air system; anda rotating speed control means variably controlling the rotating speed of the electric motor corresponding to the first compressor via the inverter in such a manner that the discharge pressure of the first compressor detected by the pressure detecting means comes to the control range changed by the discharge pressure changing means.(3) In the item (1) or (2) mentioned above, it is preferable that the discharge air system has an auxiliary machinery in which a pressure loss characteristic is varied with age, and the discharge pressure changing means corrects the pressure loss of the discharge air system in correspondence to the variation with age of the pressure loss characteristic of the auxiliary machinery.(4) In the item (2) mentioned above, it is preferable that the discharge air system has a plurality of supply systems capable of supplying the compressed air discharged from each of the compressors to each of supply ends, a communication piping communicated with a plurality of supply systems, and an opening and closing valve capable of shutting off the communication piping.

In accordance with the present invention, it is possible to increase a stability of the supply pressure while obtaining the energy saving effect.

DESCRIPTION OF REFERENCE NUMERALS

DETAILED DESCRIPTION OF THE INVENTION

A description will be given below of embodiments in accordance with the present invention with reference to the accompanying drawings.

A first embodiment in accordance with the present invention will be described with reference toFIGS. 1 and 2.

FIG. 1is a schematic view showing an entire structure of a compressed air manufacturing facility in accordance with the present embodiment. In this case, a solid arrow indicates an air flow, and a dotted arrow indicates a flow of an electric signal.

In thisFIG. 1, the compressed air manufacturing facility is provided, for example, with an oil free type screw compressor1, an electric motor2driving the compressor1, an inverter3variably controlling a rotating speed of the electric motor2, a control apparatus4controlling the inverter3, a suction throttle valve5provided in a suction side of the compressor1, a suction filter6provided in an upstream side of the suction throttle valve5, and removing a powder dust or the like in the atmospheric air, and a discharge air system7connected to a discharge side of the compressor1, and supplying a compressed air discharged from the compressor1to a supply end.

The discharge air system7is provided with a check valve8, a pressure sensor9(a pressure detecting means) arranged in a downstream side of the check valve8and detecting a discharge pressure of the compressor1, an air tank10arranged in a downstream side of the pressure sensor9and having a sufficient capacity, and an air filter11arranged in a downstream side of the air tank10and removing a powder dust or the like in the compressed air.

Further, in an upstream side of the check valve8of the discharge air system7, there is connected a piping12for introducing a part of the compressed air discharged from the compressor1as an air for operating the suction throttle valve5, and the piping12is provided with a control valve13which can be switched to a communication state and a shut-off state in correspondence to a control signal from the control apparatus4. Further, for example, in the case that the control valve13is switched to the communication state from the shut-off state, the suction throttle valve5is driven so as to shut off an intake air of the compressor1, thereby switching the compressor1from a load operation to an unload operation.

In this case, the compressor1, the electric motor2, the inverter3, the control apparatus4, the suction throttle valve5, the suction filter5, a part of the discharge air system7including the check valve8and the pressure sensor9, the piping12, the control valve13and the like are stored within a casing, and are structured as a compressor unit14.

The control apparatus4corresponding to a main portion of the present embodiment is structured such as to compute a pressure loss ΔP of the discharge air system7(in detail, a pressure loss from a detection position15a(an upstream side position) of the pressure sensor9in the discharge air system7to a downstream side position15b) in correspondence to a rotating speed N of the electric motor2, and change a control range of a discharge pressure of the compressor1at the upstream side position15aof the discharge air system7on the basis of this computation in such a manner that a terminal pressure at the downstream side position15bof the discharge air system7comes to a predetermined range, first as a first function (a discharge pressure changing means). A description will be given below of details thereof.

The pressure loss ΔP of the discharge air system7is in proportion to a square of the discharge air amount of the compressor1. The control apparatus4previously sets and stores a maximum pressure loss ΔPmax of the discharge air system7, for example, at a time of a maximum discharge air amount of the compressor1(in other words, a maximum rotating speed Nmax of the electric motor2), and is structured such as to calculate the pressure loss ΔP of the discharge air system7by multiplying the maximum pressure loss ΔPmax of the discharge air system7by a square of a ratio of the rotating speed N/Nmax of the electric motor2(for example, corresponding to a rotating speed command from the control apparatus4to the electric motor2) corresponding to a ratio of the discharge air amount of the compressor1, as shown in Expression (1).
ΔP=ΔPmax×(N/Nmax)2  (1)

Further, a control value P1of the discharge pressure of the compressor1is changed to a value obtained by adding the pressure loss ΔP to a predetermined value P2of the terminal pressure (a value obtained by subtracting the maximum pressure loss ΔPmax from a predetermined control set value P1_0of the discharge pressure of the compressor1which is previously set in anticipation of the maximum pressure loss ΔPmax of the discharge air system7, in the present embodiment) (refer to Expression (2)). Further, an upper limit value P1uof the discharge pressure of the compressor1is changed to a value obtained by adding the pressure loss ΔP to a predetermined upper limit value P2uof the terminal pressure (a value obtained by subtracting the maximum pressure loss ΔPmax from a predetermined upper limit set value P1u_0of the discharge pressure of the compressor1which is previously set in anticipation of the maximum pressure loss ΔPmax of the discharge air system7, in the present embodiment) (refer to Expression (3)). Further, a lower limit value P1dof the discharge pressure of the compressor1is changed to a value obtained by adding the pressure loss ΔP mentioned above to a predetermined lower limit value P2dof the terminal pressure (a value obtained by subtracting the maximum pressure loss ΔPmax from a predetermined lower limit set value P1d_0of the discharge pressure of the compressor1which is previously set in anticipation of the maximum pressure loss ΔPmax of the discharge air system7, in the present embodiment) (refer to Expression (4)). In this case, the predetermined control set value P1_0, the predetermined upper limit set value P1u_0and the predetermined lower limit set value P1d_0of the discharge pressure of the compressor1are previously set and stored in the control apparatus4.

FIG. 2is a characteristic view showing a relation between a ratio of the rotating speed N/Nmax of the electric motor2and the control value P1of the discharge pressure of the compressor1which are obtained on the basis of a result of computation of the expressions (1) and (2) mentioned above. In this case, a solid line indicates the control value P1of the discharge pressure of the compressor1, and a dotted line indicates a terminal pressure of the discharge air system7.

In thisFIG. 2, the predetermined control set value P1_0of the discharge pressure of the compressor1is set to 0.69 MPa, and the maximum pressure loss ΔPmax of the discharge air system7is set to 0.2 MPa (that is, predetermined value of the terminal pressure P2=0.49 MPa). Further, the predetermined upper limit set value P1u_0of the discharge pressure of the compressor1is set to 0.72 MPa, and the predetermined lower limit set value P1d_0of the discharge pressure of the compressor1is set to 0.66 MPa.

Further, for example, in the case that the ratio of the rotating speed N/Nmax of the electric motor is equal to 0.5, the pressure loss ΔP of the discharge air system7is equal to 0.05 MPa, and the control value P1of the discharge pressure of the compressor1is equal to 0.54. At this time, although an illustration is omitted, the upper limit value P1uof the discharge pressure of the compressor1is equal to 0.57 MPa in accordance with the computation of the expressions (3) and (4) mentioned above, and the lower limit value P1dis equal to 0.51 MPa. Further, for example, in the case that the ratio of the rotating speed N/Nmax of the electric motor2is equal to 0.2, the pressure loss ΔP of the discharge air system7is equal to 0.008 MPa, and the control value P1of the discharge pressure of the compressor1is equal to 0.498. At this time, although an illustration is omitted, the upper limit value P1uof the discharge pressure of the compressor1is equal to 0.528 MPa and the lower limit value P1dis equal to 0.468 MPa in accordance with the computation of the expressions (3) and (4) mentioned above. Further, for example, in the case that the ratio of the rotating speed N/Nmax of the electric motor2is equal to 0, the pressure loss ΔP of the discharge air system7is equal to 0 MPa, and the control value P1of the discharge pressure of the compressor1is equal to 0.49. At this time, although an illustration is omitted, the upper limit value P1uof the discharge pressure of the compressor1is equal to 0.52 MPa and the lower limit value P1dis equal to 0.46 MPa in accordance with the computation of the expressions (3) and (4) mentioned above.

Turning back toFIG. 1, the control apparatus4is structured such as to variably control the rotating speed N of the electric motor2via the inverter3in such a manner that the discharge pressure of the compressor1detected by the pressure sensor9comes to the computed control range mentioned above, as a second function (a rotating speed control means). In other words, the control apparatus4is structured, for example, such as to execute a PID computation on the basis of a deviation between the discharge pressure of the compressor1input from the pressure sensor9and the computed control value P1mentioned above, and output a computed value (a rotating speed command 0 to 1 to the electric motor2) to the inverter3, and the inverter3is structured such as to output a frequency corresponding to the computed value from the control apparatus4to the motor2, and variably control the rotating speed of the motor2.

A description will be given of a motion, and an operation and effect of the compressed air manufacturing facility in accordance with the present embodiment. In this case, the ratio of the used air amount of the supply end and the ratio of the discharge air amount of the compressor1are expressed on the basis of the maximum amount of the discharge air of the compressor1(100%).

For example, in the case that the ratio of the used air amount is 100%, the ratio of the rotating speed N/Nmax of the electric motor2comes to 100%, and the ratio of the discharge air amount of the compressor1comes to 100%. At this time, the pressure loss ΔP of the discharge air system7becomes equal to ΔPmax=0.2 MPa, and the discharge pressure of the compressor1is maintained to the predetermined control set value P1_0=0.69 MPa. As a result, the terminal pressure of the discharge air system7is maintained to 0.49 MPa.

Further, for example, the ratio of the used air amount is changed to 20% from 100%, the discharge pressure of the compressor1tries to ascend because the ratio of the discharge air amount of the compressor1is first 100%. The control apparatus4first executes the PID computation on the basis of the deviation between the discharge pressure of the compressor1detected by the pressure sensor9and the control set value P1_0, outputs the computed value to the inverter3, and reduces the rotating speed N of the electric motor2. Thereafter, the control apparatus4computes the pressure loss ΔP of the discharge air system7in correspondence to the reduced rotating speed N of the electric motor2in accordance with the expression (1) mentioned above, and computes the control range (the control value P1, the upper limit value P1uand the lower limit value P1d) of the discharge pressure of the compressor1in accordance with the expressions (2) to (4) mentioned above. Further, the control apparatus4executes the PID computation on the basis of the deviation between the discharge pressure of the compressor1detected by the pressure sensor9and the computed control value P1mentioned above, outputs the computed value to the inverter3, and further reduces, for example, the rotating speed of the electric motor2. As mentioned above, the control apparatus4repeatedly executes the variable control of the rotating speed N of the electric motor2, and the computation of the control range of the discharge pressure of the compressor1. As a result, the ratio of the rotating speed N/Nmax of the electric motor2is reduced to 20%, and the discharge pressure of the compressor1comes to the control value P1=0.498 MPa. At this time, the pressure loss ΔP of the discharge air system7is equal to 0.008 MPa, and the terminal pressure of the discharge air system7is maintained to 0.49 MPa.

Thereafter, for example, if the ratio of the used air amount is changed in a range from 20% to 0%, the ratio of the rotating speed N/Nmax of the electric motor2reaches the lower limit value 20%, and the discharge pressure of the compressor1is increased up to 0.528 MPa because the ratio of the discharge air amount of the compressor1is 20%. At this time, the terminal pressure of the discharge air system7is increased up to 0.52 MPa. The control apparatus4determines that the discharge pressure of the compressor1detected by the pressure sensor9is equal to or more than an unload start pressure (the upper limit value P1u=0.528 MPa of the discharge pressure of the compressor1computed in correspondence to the ratio of the rotating speed N/Nmax=0.2 of the electric motor2, in the present embodiment), controls the control valve13so as to drive the suction throttle valve5, and switches to the unload operation of the compressor1.

Further, if the unload operation of the compressor1is continued, the discharge pressure of the compressor1descends to the 0.498 MPa because the ratio of the discharge air amount of the compressor1is 0%. The control apparatus4determines that the discharge pressure of the compressor1detected by the pressure sensor9is equal to or less than a load return pressure (the control value P1=0.498 MPa of the discharge pressure of the compressor computed in correspondence to the ratio of the rotating speed N/Nmax=0.2 of the electric motor2, in the present embodiment), controls the control valve13so as to disconnect the suction throttle valve5, and switches to the load operation of the compressor1.

Further, for example, in the case that the compressor1is stopped in spite that the ratio of the used air amount is not 0%, the discharge pressure of the compressor1descends to 0.46 MPa because the ratio of the discharge air amount of the compressor1is 0%. At this time, the terminal pressure of the discharge air system7descends to 0.46 MPa. The control apparatus4determines that the discharge pressure of the compressor1detected by the pressure sensor9is equal to or less than the operation return pressure (the lower limit value P1d=0.46 MPa of the discharge pressure of the compressor1computed in correspondence to the ratio of the rotating speed N/Nmax=0 of the electric motor2, in the present embodiment), and restarts the operation of the compressor1.

As mentioned above, in the present embodiment, the control apparatus4computes the pressure loss ΔP of the discharge air system7in correspondence to the rotating speed of the electric motor2, and changes the control range of the discharge pressure of the compressor1on the basis of the computation in such a manner that the terminal pressure in a downstream side position15bof the discharge air system7comes to a predetermined range (0.46 MPa to 0.52 MPa in the present embodiment). Further, the control apparatus4variably controls the rotating speed of the electric motor2via the inverter3in such a manner that the discharge pressure of the compressor1detected by the pressure sensor9comes to the changed control range. Accordingly, it is possible to hold the power of the compressor1to a minimum while keeping the terminal pressure of the discharge air system7in the predetermined range, and it is possible to obtain an energy saving effect. Further, in the present embodiment, since there are provided with the function of changing the control range of the discharge pressure of the compressor1in correspondence to the rotating speed of the electric motor2, and the function of variably controlling the rotating speed of the electric motor2in correspondence to the discharge pressure of the compressor1, and the these two functions operate as feedback control functions with each other, it is possible to increase a convergence characteristic of the discharge pressure of the compressor1and the rotating speed of the electric motor2. As a result, it is possible to stabilize the terminal pressure of the discharge air system7, that is, the supply pressure. Accordingly, in the present invention, it is possible to increase a stability of the supply pressure while obtaining the energy saving effect.

In this case, in the first embodiment mentioned above, the control apparatus4is explained by exemplifying the case that the pressure loss ΔP of the discharge air system7is computed by substituting the ratio of the rotating speed N/Nmax=0.2 of the electric motor2for the expression (1) mentioned above, at a time of the unload operation of the compressor1, however, is not limited to this. In other words, for example, the control apparatus4may compute by replacing the ratio of the rotating speed N/Nmax=0.2 of the electric motor2substituted for the expression (1) mentioned above by zero. In the case mentioned above, it is possible to obtain the same effect as mentioned above.

A description will be given of a second embodiment in accordance with the present invention with reference toFIGS. 3 and 4. The present embodiment corresponds to an embodiment in which a plurality of compressor units are provided.

FIG. 3is a schematic view showing an entire structure of a compressed air manufacturing facility in accordance with the present embodiment. In this case, the same reference numerals are attached to the same parts as those of the first embodiment mentioned above, and a description thereof will be appropriately omitted.

In thisFIG. 3, the compressed air manufacturing facility in accordance with the present embodiment is provided, for example, with two compressor units14A and14B, and each of the compressor units14A and14B is provided with a compressor1compressing the air, an electric motor2driving the compressor1, an inverter3variably controlling a rotating speed of the electric motor2, a control apparatus4controlling the inverter3, a suction throttle valve5provided in a suction side of the compressor1, and a suction filter6provided in an upstream side of the suction throttle valve5, and removing a powder dust or the like in the atmospheric air, in the same manner as the compressor14mentioned above.

Discharge pipings16A and16B are respectively connected to a discharge side of the compressor1in the compressor units14A and14B, and each of the discharge pipings16A and16B is provided with a check valve8, a pressure sensor9(a pressure detecting means) arranged in a downstream side of the check valve8and detecting a discharge pressure of the compressor1. The discharge pipings16A and16B are connected in such a manner as to flow together with a supply piping17, and the supply piping17is provided with an air tank10having a sufficient capacity, and an air filter11arranged in a downstream side of the air tank10and removing the powder dust or the like in the compressed air. Further, the discharge pipings16A and16B and the supply piping17construct a discharge air system18. In this case, in the present embodiment, a pressure loss from a detection position19a(an upstream side position) of the pressure sensor9of the compressor unit14A in the discharge air system18to a downstream side position19bis approximately equal to a pressure loss from a detection position19c(an upstream side position) of the pressure sensor9of the compressor unit14B to the downstream side position19b, and these pressure losses are collectively called as a pressure loss ΔP of the discharge air system18.

Further, there is provided an external control apparatus20concentrically controlling the control apparatus4of the compressor units14A and14B. The external control apparatus20is structured such as to operate any one compressor unit (hereinafter, refer to as a variable speed side compressor unit) of the compressor units14A and14B by variably controlling the rotating speed of the electric motor2, and operate the other compressor unit (hereinafter, refer to a constant speed side compressor unit) by switching to a full-load operation state in which the rotating speed of the electric motor2is set to an upper limit value, in the case that it is impossible to compensate only by the discharge air amount of the variable speed side compressor unit, and switching to a stop state in the case that it is possible to compensate only by the discharge air amount of the variable speed side compressor unit. Further, the external control apparatus20controls the variable speed side compressor unit and the constant speed side compressor unit so as to alternate per a predetermined cycle. As a result, for example, even in the case that the variable speed side compressor unit is operated frequently, working times of the compressor units14A and14B are leveled. Further, for example, in the case that any one of the compressor units14A and14B get out of order for some reason, the external control apparatus20controls in such a manner as to switch the compressor unit which is not out of order to an individual operation.

Further, as a great feature of the present embodiment, the external control apparatus20is structured such as to compute the pressure loss ΔP of the discharge air system18in correspondence to a rotating speed Na of the electric motor2of the compressor unit14A and a rotating speed Nb of the electric motor2of the compressor unit14B, and change a control range of a discharge pressure of the compressor1in the variable speed side compressor unit on the basis of this computation in such a manner that a terminal pressure at the downstream side position19bof the discharge air system18comes to a predetermined range. A description will be given below of details thereof.

The pressure loss ΔP of the discharge air system18is in proportion to a square of a total amount of the discharge air of the compressor units14A and14B. The external control apparatus20previously sets and stores a maximum pressure loss ΔPmax of the discharge air system18, for example, at a time of a maximum total discharge air amount of the compressor units14A and14B (in other words, a maximum rotating speed Na_max of the electric motor2of the compressor unit14A and a maximum rotating speed Nb_max of the electric motor2of the compressor unit14B), and is structured such as to calculate the pressure loss ΔP of the discharge air system18by multiplying the maximum pressure loss ΔPmax of the discharge air system18by a square of an average value of ratios of the rotating speed Na/Na_max and Nb/Nb_max of the electric motor2respectively corresponding to the ratios of the discharge air amount of the compressor units14A and14B, as shown in Expression (5).
ΔP=ΔPmax×{(Na/Na_max+Nb/Nb_max)/2}2  (5)

Further, for example, in the case of variably controlling the rotating speed Na of the electric motor2of the compressor unit14A, the control value P1of the discharge pressure of the compressor unit14A is changed to a value obtained by adding the pressure loss ΔP mentioned above to a predetermined control value P2of the terminal pressure (refer to Expression (2)). Further, an upper limit value P1uof the discharge pressure of the compressor unit14A is changed to a value obtained by adding the pressure loss ΔP mentioned above to a predetermined upper limit value P2uof the terminal pressure (refer to Expression (3)). Further, a lower limit value P1dof the discharge pressure of the compressor unit14A is changed to a value obtained by adding the pressure loss ΔP mentioned above to a predetermined lower limit value P2dof the terminal pressure (refer to Expression (4)).

In the same manner, for example, in the case of variably controlling the rotating speed of the electric motor2of the compressor unit14B, the control value P1of the discharge pressure of the compressor unit14B is changed to the value obtained by adding the pressure loss ΔP to the predetermined control value P2of the terminal pressure (refer to Expression (2)). Further, an upper limit value P1uof the discharge pressure of the compressor unit14B is changed to the value obtained by adding the pressure loss ΔP to the predetermined upper limit value P2uof the terminal pressure (refer to Expression (3)). Further, a lower limit value P1dof the discharge pressure of the compressor unit14B is changed to the value obtained by adding the pressure loss ΔP mentioned above to the predetermined lower limit value P2dof the terminal pressure (refer to Expression (4)).

Further, the control apparatus4of the variable speed side compressor is structured such as to unit variably control the rotating speed N of the electric motor2via the inverter3in such a manner that the discharge pressure of the compressor1detected by the pressure sensor9comes to the control range computed by the external control apparatus20.

A description will be given of a motion, and an operation and effect of the compressed air manufacturing facility in accordance with the present embodiment.FIG. 4is a time chart showing a variation with age of the ratio of the used air amount in the present embodiment, the discharge pressure of the compressor1in the compressor units14A and14B, the ratio of the rotating speed Na/Na_max of the electric motor2of the compressor unit14A, and the ratio of the rotating speed Nb/Nb_max of the electric motor2of the compressor unit14B. In this case, the discharge pressure of the compressor1in the compressor unit14A is shown in blocks A to G, and the discharge pressure of the compressor1in the compressor unit14B is shown in blocks H to M.

In thisFIG. 4, the predetermined control set value P1of the discharge pressure of the compressor1in the compressor units14A and14B is set to be equal to 0.69 MPa, the predetermined upper limit set value P1u_0is set to be equal to 0.72 MPa, the predetermined lower limit set value P1d_0is set to be equal to 0.66 MPa, and the maximum pressure loss ΔPmax of the discharge air system18is set to be equal to 0.2 MPa. Further, the ratio of the used air amount of the supply end and the ratio of the total discharge air amount of the compressor units14A and14B are expressed on the basis of the maximum amount of the discharge air of each of the compressor units (100%).

First, a description will be given of a case of variably changing the electric motor2of the compressor unit14A at a time when the ratio of the used air amount is changed from 200% to 0%.

In the case that the ratio of the used air amount is 200%, each of the ratios of the rotating speed Na/Na_max and Nb/Nb_max of the electric motors2of the compressor units14A and14B comes to 100%, and each of the ratios of the discharge air amount of the compressor units14A and14B comes to 100%. At this time, the pressure loss ΔP of the discharge air system18is equal to ΔPmax=0.2 MPa. Further, in the compressor unit14A, the discharge pressure of the compressor1is maintained to the predetermined control set value P1_0=0.69 MPa, and the terminal pressure of the discharge air system18is maintained to 0.49 MPa.

If the ratio of the used air amount is changed to 120% from 200% (a block A inFIG. 4), the discharge pressure of the compressor1in the compressor unit14A tries to ascend because the ratio of the total discharge air amount of the compressor units14A and14B is first 200%. Accordingly, the control apparatus4of the compressor unit14A first executes the PID computation on the basis of the deviation between the discharge pressure of the compressor1detected by the pressure sensor9and the control set value P1_0, outputs the computed value to the inverter3, and reduces the rotating speed Na of the electric motor2. Further, the external control apparatus20acquires the rotating speeds Na and Nb of the electric motor2from the control apparatuses4of the compressor units14A and14B, computes the pressure loss ΔP of the discharge air system18in correspondence to the rotating speeds Na and Nb of the electric motors2in accordance with the expression (5) mentioned above, and computes the control range (the control value P1, the upper limit value P1uand the lower limit value P1d) of the discharge pressure of the compressor1in the compressor unit14A in accordance with the expressions (2) to (4) mentioned above. Thereafter, the control apparatus4of the compressor unit14A executes the PID computation on the basis of the deviation between the discharge pressure of the compressor1detected by the pressure sensor9and the control value P1computed by the external control apparatus20, outputs the computed value to the inverter3, and reduces, for example, the rotating speed Na of the electric motor2. As mentioned above, there are repeatedly executed the variable control of the rotating speed Na of the electric motor2by the control apparatus4of the compressor unit14A, and the computation of the control range of the discharge pressure of the compressor1by the external control apparatus20. As a result, the ratio of the rotating speed Na/Na_max of the electric motor2of the compressor unit14A is reduced to 20%, and the discharge pressure of the compressor1in the compressor unit14A comes to the control value P1=0.562 MPa. At this time, the pressure loss ΔP of the discharge air system18is equal to 0.072 MPa, and the terminal pressure of the discharge air system18is maintained to 0.49 MPa.

If the ratio of the used air amount is changed in a range from 120% to 100% (a block B in FIG.4), the ratio of the rotating speed Na/Na_max of the electric motor2of the compressor unit14A reaches the lower limit value 20%, and the discharge pressure of the compressor1in the compressor unit14A is increased up to 0.592 MPa because the ratio of the total discharge air amount of the compressor units14A and14B is 120%. At this time, the terminal pressure of the discharge air system18is increased up to 0.52 MPa. The external control apparatus20determines that the discharge pressure of the compressor1in the compressor unit14A is equal to or more than an unload start pressure (the upper limit value P1u=0.592 MPa of the discharge pressure of the compressor1in the compressor unit14A computed in correspondence to the ratio of the rotating speed Na/Na_max=0.2 of the electric motor2of the compressor unit14A and the ratio of the rotating speed Nb/Nb_max=1 of the electric motor2of the compressor unit14B, in the present embodiment), and switches the compressor unit14A to the unload operation.

If the unload operation of the compressor unit14A is continued (a block C inFIG. 4), the discharge pressure of the compressor1in the compressor unit14A descends to the 0.562 MPa because the ratio of the total discharge air amount of the compressor units14A and14B is 100%. The external control apparatus20measures an unload operation time until the discharge pressure of the compressor1in the compressor unit14A reaches a load return pressure (the control value P1=0.562 MPa of the discharge pressure of the compressor1in the compressor unit14A computed in correspondence to the ratio of the rotating speed Na/Na_max=0.2 of the electric motor2of the compressor unit14A and the ratio of the rotating speed Nb/Nb_max=1 of the electric motor2of the compressor unit14B, in the present embodiment), and switches the ratio of the rotating speed Na/Na_max of the electric motor2of the compressor unit14A to 100% as well as stopping the electric motor of the compressor unit14B, in the case that the unload operation time gets over a predetermined time.

If the operation is continued at the ratio of the used air amount of 100% (a block D inFIG. 4), there are repeatedly executed the variable control of the rotating speed Na of the electric motor2by the control apparatus4of the compressor unit14A and the computation of the control range of the discharge pressure of the compressor1by the external control apparatus20, and the discharge pressure of the compressor1in the compressor unit14A comes to the control value P1=0.54 MPa. At this time, the pressure loss ΔP of the discharge air system18is equal to 0.05 MPa, and the terminal pressure of the discharge air system18is maintained to 0.49 MPa.

If the ratio of the used air amount is changed to 20% from 100% (a block E inFIG. 4), there are repeatedly executed the variable control of the rotating speed Na of the electric motor2by the control apparatus4of the compressor unit14A and the computation of the control range of the discharge pressure of the compressor1by the external control apparatus20, the ratio of the rotating speed Na/Na_max of the electric motor2of the compressor unit14A is reduced to 20%, and the discharge pressure of the compressor1in the compressor unit14A comes to the control value P1=0.492 MPa. At this time, the pressure loss ΔP of the discharge air system18is equal to 0.002 MPa, and the terminal pressure of the discharge air system18is maintained to 0.49 MPa.

If the ratio of the used air amount is changed in a range from 20% to 0% (a block F inFIG. 4), the ratio of the rotating speed Na/Na_max of the electric motor2of the compressor unit14A reaches the lower limit value 20%, and the discharge pressure of the compressor1in the compressor unit14A is increased up to 0.522 MPa because the ratio of the total discharge air amount of the compressor units14A and14B is 20%. At this time, the terminal pressure of the discharge air system18is increased up to 0.52 MPa. The external control apparatus20determines that the discharge pressure of the compressor1detected by the pressure sensor of the compressor unit14A is equal to or more than an unload start pressure (the upper limit value P1u=0.522 MPa of the discharge pressure of the compressor1in the compressor unit14A computed in correspondence to the ratio of the rotating speed Na/Na_max=0.2 of the electric motor2of the compressor unit14A and the ratio of the rotating speed Nb/Nb_max=0 of the electric motor2of the compressor unit14B, in the present embodiment), and switches the compressor unit14A to the unload operation.

If the unload operation of the compressor unit14A is continued (a block G inFIG. 4), the discharge pressure of the compressor1in the compressor unit14A is decreased to 0.492 MPa because the ratio of the total discharge air amount of the compressor units14A and14B is 0%. The external control apparatus20measures an unload operation time until the discharge pressure of the compressor1detected by the pressure sensor9of the compressor unit14A reaches a load return pressure (the control value P1=0.492 MPa of the discharge pressure of the compressor1in the compressor unit14A computed in correspondence to the ratio of the rotating speed Na/Na_max=0.2 of the electric motor2of the compressor unit14A and the ratio of the rotating speed Nb/Nb_max=1 of the electric motor2of the compressor unit14B, in the present embodiment), and stops the electric motor2of the compressor unit14A, in the case that the unload operation time gets over a predetermined time.

Next, a description will be given of a case of variably changing the electric motor2of the compressor unit14B at a time when the ratio of the used air amount is changed from 0% to 200%.

If the ratio of the used air amount is changed to 20% from 0% (a block H inFIG. 4), the discharge pressure of the compressor1in the compressor unit14B descends to 0.46 MPa because the ratio of the total discharge air amount of the compressor units14A and14B is 0%. At this time, the terminal pressure of the discharge air system18is decreased to 0.46 MPa. The external control apparatus20determines that the discharge pressure of the compressor1in the compressor unit14B is equal to or less than an operation return pressure (the lower limit value P1d=0.46 MPa of the discharge pressure of the compressor1in the compressor unit14B computed in correspondence to the ratio of the rotating speed Na/Na_max=0 of the electric motor2of the compressor unit14A and the ratio of the rotating speed Nb/Nb_max=0 of the electric motor2of the compressor unit14B, in the present embodiment), and sets the ratio of the rotating speed Nb/Nb_max of the electric motor2of the compressor unit14B to 20% so as to drive.

If the ratio of the used air amount is continued at 20% (a block I inFIG. 4), there are repeatedly executed the variable control of the rotating speed Nb of the electric motor2by the control apparatus4of the compressor unit14B and the computation of the control range of the discharge pressure of the compressor1by the external control apparatus20, and the discharge pressure of the compressor1in the compressor unit14B comes to the control value P1=0.492 MPa. At this time, the pressure loss ΔP of the discharge air system18is equal to 0.002 MPa, and the terminal pressure of the discharge air system18is maintained to 0.49 MPa.

If the ratio of the used air amount is changed to 100% from 20% (a block J inFIG. 4), there are repeatedly executed the variable control of the rotating speed Nb of the electric motor2by the control apparatus4of the compressor unit14B and the computation of the control range of the discharge pressure of the compressor1by the external control apparatus20, the ratio of the rotating speed Nb/Nb_max of the electric motor2of the compressor unit14B is increased up to 100%, and the discharge pressure of the compressor1in the compressor unit14B comes to the control value P1=0.54 MPa. At this time, the pressure loss ΔP of the discharge air system18is equal to 0.05 MPa, and the terminal pressure of the discharge air system18is maintained to 0.49 MPa.

If the ratio of the used air amount is changed from 100% to 120% (a block K inFIG. 4), the ratio of the rotating speed Nb/Nb_max of the electric motor2of the compressor unit14B reaches the upper limit value 100%, and the discharge pressure of the compressor1in the compressor unit14B is decreased to 0.51 MPa because the ratio of the total discharge air amount of the compressor units14A and14B is 100%. At this time, the terminal pressure of the discharge air system18is decreased to 0.46 MPa. The external control apparatus20determines that the discharge pressure of the compressor1detected by the pressure sensor9of the compressor unit14B is equal to or less than an operation return pressure (the lower limit value P1d=0.51 MPa of the discharge pressure of the compressor1in the compressor unit14B computed in correspondence to the ratio of the rotating speed Na/Na_max=0 of the electric motor2of the compressor unit14A and the ratio of the rotating speed Nb/Nb_max=1 of the electric motor2of the compressor unit14B, in the present embodiment), sets the ratio of the rotating speed Na/Na_max of the electric motor2of the compressor unit14A to 100% so as to drive, and switches the ratio of the rotating speed Nb/Nb_max of the electric motor2of the compressor unit14B to 20%.

If the ratio of the used air amount is continued at 120% (a block L inFIG. 4), there are repeatedly executed the variable control of the rotating speed Nb of the electric motor2by the control apparatus4of the compressor unit14B and the computation of the control range of the discharge pressure of the compressor1by the external control apparatus20, and the discharge pressure of the compressor1in the compressor unit14B comes to the control value P1=0.562 MPa. At this time, the pressure loss ΔP of the discharge air system18is equal to 0.072 MPa, and the terminal pressure of the discharge air system18is maintained to 0.49 MPa.

If the ratio of the used air amount is changed to 200% from 120% (a block M inFIG. 4), there are repeatedly executed the variable control of the rotating speed Nb of the electric motor2by the control apparatus4of the compressor unit14B and the computation of the control range of the discharge pressure of the compressor1by the external control apparatus20, the ratio of the rotating speed Nb/Nb_max of the electric motor2of the compressor unit14B is increased up to 100%, and the discharge pressure of the compressor1in the compressor unit14B comes to the control value P1=0.69 MPa. At this time, the pressure loss ΔP of the discharge air system18is equal to 0.2 MPa, and the terminal pressure of the discharge air system18is maintained to 0.49 MPa.

As mentioned above, in the present embodiment, the external control apparatus20computes the pressure loss ΔP of the discharge air system7in correspondence to the rotating speed of the electric motors2of the compressor units14A and14B, and changes the control range of the discharge pressure of the compressor1in the variable speed side compressor unit on the basis of the computation in such a manner that the terminal pressure in the downstream side position18bof the discharge air system7comes to a predetermined range (0.46 MPa to 0.52 MPa in the present embodiment). Further, the control apparatus4of the variable speed side compressor unit variably controls the rotating speed of the electric motor2via the inverter3in such a manner that the discharge pressure of the compressor1detected by the pressure sensor9comes to the control range changed by the external control apparatus20. Accordingly, it is possible to hold the power of the compressor1to a minimum while keeping the terminal pressure of the discharge air system7in the predetermined range, and it is possible to obtain an energy saving effect. Further, in the present embodiment, since there are provided with the function of changing the control range of the discharge pressure of the compressor1in the variable speed side compressor unit in correspondence to the rotating speed of the electric motors2in the compressor units14A and14B, and the function of variably controlling the rotating speed of the electric motor2in correspondence to the discharge pressure of the compressor1in the variable speed side compressor unit, and the these two functions operate as feedback control functions with each other, it is possible to increase a convergence characteristic of the discharge pressure of the compressor1and the rotating speed of the electric motor2in the variable speed side compressor unit. As a result, it is possible to stabilize the terminal pressure of the discharge air system7, that is, the supply pressure. Accordingly, even in the present invention, it is possible to increase a stability of the supply pressure while obtaining the energy saving effect, in the same manner as the first embodiment mentioned above.

In this case, in the second embodiment mentioned above, the description is given by exemplifying the case that two compressor units14A and14B are provided, and both of the compressor units14A and14B can variably control the rotating speed of the electric motor2via the inverter3, however, the structure is not limited to this. In other words, the structure may be made, for example such that three of more compressor units are provided. Further, for example, at least one compressor unit of a plurality of compressor units may be structured such as to variably control the rotating speed of the electric motor2via the inverter3in the same manner as the compressor units14A and14B mentioned above, and the other compressor units may be structured such that the rotating speed of the electric motor2is fixed. Even in the case mentioned above, it is possible to obtain the same effect as the second embodiment mentioned above.

Further, although no particular description is given in the first and second embodiment mentioned above, the pressure loss characteristic varies with age due to the influence of the attachment of the powder dust or the like, in the air filter11provided in the discharge air systems7and18. Accordingly, in order to correspond to this, the pressure loss ΔP of the discharge air systems7and18may be corrected in correspondence to the variation with age of the pressure loss characteristic of the air filter11. Describing in detail, for example, in the case that the pressure loss of the air filter11at a time of the maximum amount of the discharge air is increased, for example, in increments of 0.01 MPa per 30 days, the correction is executed by computing a pressure loss increment value ΔPf of the air filter11on the basis of a timer function, and adding the pressure loss increment value ΔPf to the maximum pressure loss ΔPmax in the expressions (1) to (5) mentioned above. Further, for example, in the case that the air filter11is replaced by a new one, the pressure loss increment value ΔPf of the air filter11is initialized to 0. In this case, as an auxiliary machinery of the discharge air system in which the pressure loss characteristic varies with age, for example, there is an oil filter or the like in the case of employing an oil lubricated type compressor, in addition to the air filter11, and it is possible to apply to this case. In the modified example mentioned above, it is possible to obtain the same effect as the first and second embodiments mentioned above.

A description will be given of a third embodiment in accordance with the present invention with reference toFIG. 5. The present embodiment corresponds to an embodiment structured such that a discharge air system connected to a plurality of compressor units can be separated.

FIG. 5is a schematic view showing an entire structure of a compressed air manufacturing facility in accordance with the present embodiment. In thisFIG. 5, the same reference numerals are attached to the same parts as those of the second embodiment mentioned above, and a description thereof will be appropriately omitted.

In the present embodiment, a discharge air system21has a supply system22A which is connected to the discharge side of the compressor1of the compressor unit14A and supplies the compressed air discharged from the compressor1to one supply end, and a supply system22B which is connected to the discharge side of the compressor1of the compressor unit14B and supplies the compressed air discharged from the compressor1to the other supply end, and each of these supply systems22A and22B is provided with the check valve8, the pressure sensor9, the air tank10and the air filter11in the order directed to a downward side. Further, a communication piping23A is connected to a portion between an upstream side of the air tank10of the supply piping system22A and an upstream side of the air tank10of the supply piping system22B, a communication piping23B is connected to a portion between a downstream side of the air filter11of the supply piping system22A and a downstream side of the air filter11of the supply piping system22B, and an opening and closing valve24is provided in each of the communication pipings23A and23B. In this case, in the present embodiment, a pressure loss from a detection position25a(an upstream side position) of the pressure sensor9of the compressor unit14A in the discharge air system21to a downstream side position25bis approximately equal to a pressure loss from a detection position25c(an upstream side position) of the pressure sensor9of the compressor unit14B to a downstream side position25b, and these pressure losses are collectively called as a pressure loss ΔP of the discharge air system21.

The external control apparatus20is structured such as to output the control signal corresponding to a command signal from an input apparatus (not shown) to the opening and closing valve23, and switch the opening and closing valve23to communication and shut-off states. Further, for example, in the case of switching the opening and closing valve23to the communication state, there is achieved a number control operation of combing the compressed airs from the compressor units14A and14B so as to supply to the supply end, whereby the same structure as the second embodiment mentioned above is achieved. In other words, the external control apparatus20changes the control range of the discharge pressure of the compressor1in the variable speed side compressor unit in correspondence to the rotating speeds Na and Nb of the electric motors2of the compressor units14A and14B. Further, the control apparatus4of the variable speed side compressor unit variably controls the rotating speed of the electric motor2via the inverter3in such a manner that the discharge pressure of the compressor1detected by the compressor sensor9comes to the control range changed by the external control apparatus20.

On the other hand, for example, in the case of switching the opening and closing valve23to the shut-off state, there is achieved a parallel operation of respectively supplying the compressed airs from the compressor units to the supply ends, whereby each of the compressor units has the same structure as the first embodiment mentioned above. In other words, each of the compressor units14A and14B, the control apparatus4changes the control range of the discharge pressure of the compressor1in correspondence to the rotating speed of the electric motor2, and variably controls the rotating speed of the electric motor2in such a manner that the discharge pressure of the compressor1detected by the pressure sensor9comes to the changed control range.

Even in the present embodiment structured as mentioned above, in the same manner as the first and second embodiments mentioned above, it is possible to increase a stability of the supply pressure while obtaining the energy saving effect. Further, in the present embodiment, since it is possible to divide the discharge air system21into the supply systems22A and22B, it is possible to easily correspond to the used condition of the compressed air.