US20140140813A1

US20140140813A1 - Number-of-compressors controlling system

Info

Publication number: US20140140813A1
Application number: US13/496,735
Authority: US
Inventors: Shigeki Ochi; Yoshihiro Sangawa; Yusuke Okamoto
Original assignee: Miura Co Ltd
Current assignee: Miura Co Ltd
Priority date: 2011-07-22
Filing date: 2011-09-02
Publication date: 2014-05-22
Also published as: JP4924855B1; WO2013014808A1; JP2013024163A

Abstract

A number-of-compressors controlling system which has a simple configuration and can produce compressed air by immediately following the load used of the compressed air is provided. The number-of-compressors controlling system has a plurality of compressors (2), a receiver tank (3) in which compressed air is supplied from these compressors (2) and is fed to a compressed air utilization device, a pressure sensor (4) provided on the receiver tank (3), and a number-of-compressors controller (5) which changes the number of compressors being operated based on the detected pressure of the pressure sensor (4). A number-of-compressors decreasing pressure as a threshold value whether or not the number of compressors being operated is decreased by the number-of-compressors controller (5) is set to be lower as the number of compressors being operated is larger.

Description

TECHNICAL FIELD

The present invention relates to a number-of-compressors controlling system which has a plurality of air compressors and changes the number of compressors being operated according to the load used of compressed air. This application claims the priority of Japanese Patent Application No. 2011-160762, filed on Jul. 22, 2011, the contents of which are herein incorporated by reference.

BACKGROUND ART

Conventionally, as disclosed in Patent Document 1, it is proposed that a pressure threshold value which increases or decreases the number of compressors being operated be changed based on a pressure and its change rate. In the invention described therein, all the compressors are on-off controlled (paragraph No. 0029, and so on).

Patent Document 1: Japanese Patent Application Laid-Open (JF-A) No. 2007-120497 (claims, paragraphs Nos. 0140-0155, and FIGS. 15 and 16)

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

However, although the pressure change rate is considered, the number of compressors being operated is not considered. In addition, a plurality of compressors are merely on-off controlled. With this, compressed air cannot be produced by immediately following the load used of the compressed air.
A problem to be solved by the present invention is to provide a number-of-compressors controlling system which has a simple configuration to consider and control the number of compressors being operated, and can produce compressed air by immediately following the load used of the compressed air.

Means for Solving the Problems

The present invention has been made to solve the above problem and a first aspect of the invention provides a number-of-compressors controlling system having a plurality of compressors, a pressure sensor which is provided in a position where compressed air is supplied from these compressors and is fed to a compressed air utilization device and detects the pressure of the compressed air, and a number-of-compressors controller which changes the number of compressors being operated based on the detected pressure of the pressure sensor, wherein a number-of-compressors decreasing pressure as a threshold value whether or not the number of compressors being operated is decreased by the number-of-compressors controller is set to be lower as the number of compressors being operated is larger.
According to the first aspect of the invention, number-of-compressors decreasing pressure as a threshold value whether or not the number of compressors being operated is decreased is set to be lower as the number of compressors being operated is larger. As the number of compressors being operated is larger, the contribution rate of each of the compressors for maintaining the detected pressure at the target pressure is lowered to reduce pressure fluctuations, so that the number-of-compressors decreasing pressure can be lowered according to the increase of the number of compressors being operated. In other words, although typically, one compressor is stopped when the detected pressure is equal to or greater than the number-of-compressors decreasing pressure and one compressor is started when the detected pressure is equal to or less than a number-of-compressors increasing pressure, the number-of-compressors decreasing pressure is lowered as the number of compressors being operated is larger, thereby enabling the pressure fluctuation width to be reduced.
A second aspect of the invention provides the number-of-compressors controlling system according to the first aspect, wherein a plurality of compressors are all capacity-controlled even when operated at the same time, wherein in the case that a load rate at stop is 0% and a load rate at full load is 100%, one compressor being operated is stopped when the load rate of each of the compressors being operated is equal to or less than a stop load rate calculated by the following equation:
the stop load rate (%)=(the number of compressors being operated−1)/the number of compressors being operated×100.
According to the second aspect of the invention, a plurality of compressors are all capacity-controlled even when operated at the same time, and stop control is performed based on the load rate of each of the compressors according to the number of compressors being operated in a manner in which one of two compressors being operated is stopped when the load rate of each of the compressors is equal to or less than 50% and one of three compressors being operated is stopped when a load rate of each of the compressors is equal to or less than 67%, so that an optimum operation can be performed by a simple configuration.
A third aspect of the invention, provides the number-of-compressors controlling system according to the first or second aspect, wherein each of the compressors is capacity-controlled so that the pressure on the discharge side thereof is maintained between a lower limit pressure PL and an upper limit pressure PH, wherein to stop one compressor being operated at the stop load rate according to the number of compressors being operated, the number-of-compressors decreasing pressure is set by the following equation based on the number of compressors being operated:
the number-of-compressors decreasing pressure=((the upper limit pressure PH−the lower limit pressure PL)÷the number of compressors being operated)+the lower limit pressure PL.
According to the third aspect of the invention, the stop load rate is converted to the pressure, thereby enabling easy control.
A fourth aspect of the invention provides the number-of-compressors controlling system according to the third aspect, wherein the number of compressors being operated is decreased after the detected pressure of the pressure sensor continues to be equal to or greater than the number-of-compressors decreasing pressure for a set time.
According to the fourth aspect of the invention, the number of compressors being operated is decreased after the detected pressure of the pressure sensor continues to be equal to or greater than the number-of-compressors decreasing pressure for a set time, so that the compressors can be prevented from being stopped excessively one after another.
A fifth aspect of the invention provides the number-of-compressors controlling system according to the third or fourth aspect, wherein compressed air from each of the compressors is supplied to a shared receiver tank and is then fed to one or a plurality of compressed air utilization devices, wherein the pressure sensor is provided on the receiver tank, wherein a number-of-compressors increasing pressure as a threshold value whether or not the number of compressors being operated is increased by the number-of-compressors controller is set to be different based on a pressure change rate ΔP of a detected pressure P of the pressure sensor, wherein in the case that the absolute value of the pressure change rate ΔP is less than a first set value ΔP1, one compressor is started when the detected pressure P of the pressure sensor is equal to or less than a second lower limit pressure PL2 as the number-of-compressors increasing pressure, wherein in the case that the absolute value of the pressure change rate ΔP is equal to or greater than the first set value ΔP1 and is less than a second set value ΔP2, one compressor is started when the detected pressure P of the pressure sensor is equal to or less than a first lower limit pressure PL1 as the number-of-compressors increasing pressure, when the detected pressure P is still maintained to be equal to or less than the first lower limit pressure PL1, one compressor is started each time a predetermined continuous start prevention time elapses, and when the detected pressure P is equal to or less than the second lower limit pressure PL2 lower than the first lower limit pressure PL1, one more compressor is started without waiting for the elapse of the continuous start prevention time, wherein in the case that the absolute value of the pressure change rate ΔP is equal to or greater than the second set value ΔP2, one compressor is started when the detected pressure P of the pressure sensor is equal to or less than the upper limit pressure PH as the number-of-compressors increasing pressure, when the detected pressure P is still maintained to be equal to or less than the upper limit pressure PH, one compressor is started each time the predetermined continuous start prevention time elapses, and when the detected pressure P is equal to or less than the first lower limit pressure PL1 lower than the upper limit pressure PH, one more compressor is started without waiting for the elapse of the continuous start prevention time.
According to the fifth aspect of the invention, one compressor is started when the detected pressure of the pressure sensor is equal to or less than the number-of-compressors increasing pressure, and when the detected pressure is still maintained to be equal to or less than the number-of-compressors increasing pressure, one compressor is started each time the predetermined continuous start prevention time elapses, but in the region in which the absolute value of the pressure change rate is equal to or greater than the set value, when the detected pressure of the pressure sensor is equal to or less than an immediately number-of-compressors increasing pressure, one compressor is started without waiting for the elapse of the predetermined time. In this way, when the pressure change rate is large and is equal to or greater than the set value so as to be greatly apart from the target pressure range, one compressor is started when the detected pressure of the pressure sensor is equal to or less than the immediately number-of-compressors increasing pressure without waiting for the elapse of the predetermined time, so that the difference between the amount of discharge of the compressors and the amount of use of the compressed air utilization device can be corrected immediately.
A sixth aspect of the invention provides the number-of-compressors controlling system according to any one of the first to fifth aspects, wherein the number-of-compressors controller changes the number of compressors being operated based on the discharge flow rate of compressed air from each of the compressors, or the rotating speed, the electric current used, or the electricity used of each of the compressors, in addition to the detected pressure of the pressure sensor.
According to the sixth aspect of the invention, the number of compressors being operated is corrected based on the discharge flow rate of compressed air from each of the compressors, or the rotating speed, the electric current used, or the electricity used of each of the compressors, so that the optimum number of compressors can be operated.
Further, a seventh aspect of the invention provides the number-of-compressors controlling system according to the sixth aspect, wherein the number-of-compressors controller monitors the electric current used or the electricity used of each of the compressors, wherein the compressor with less electric current used or electricity used is operated on a priority basis.
According to the seventh aspect of the invention, the compressor with less electric current used or electricity used is operated on a priority basis, so that the operation efficiency can be increased.

Effect of the Invention

According to the present invention, the number of compressors being operated is considered and controlled by a simple configuration, and compressed air can be produced by immediately following the load used of the compressed air.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an embodiment of a number-of-compressors controlling system of the present invention.

FIG. 2 is a diagram showing an example of a number-of-compressors controlling method of the number-of-compressors controlling system of FIG. 1 and shows the discharged pressure of each of compressors being operated, the pressure in a receiver tank, and a number-of-compressors being operated increase or decrease chart.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a specific embodiment of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic diagram showing an embodiment of a number-of-compressors controlling system of the present invention. A number-of-compressors controlling system 1 of this embodiment has a plurality of compressors 2, 2, . . . , a receiver tank 3 to which compressed air is supplied from these compressors 2, a pressure sensor 4 which detects the pressure in the receiver tank 3, and a number-of-compressors controller 5 which controls each of the compressors 2 based on the detected pressure of the pressure sensor 4.
Each of the compressors 2 is an electrically operated air compressor, has a main body driven by a motor, and sucks, compresses, and discharges outside air. Compressed air from each of the compressors 2 is fed via the shared receiver tank 3 to one or a plurality of compressed air utilization devices (not shown).
The compressors 2 may have any configuration such as a screw type, a turbo type, or a reciprocating type, but typically have the same configuration. In addition, the compressors 2 typically have the same discharge capacity.
Each of the compressors 2 of this embodiment can be capacity-controlled. Here, each of the compressors 2 can be mechanically capacity-controlled by itself. Any specific capacity control configuration can be used, but in this embodiment, capacity control is performed by regulating the opening of a capacity regulating valve (not shown) provided on the suction side of each of the compressors 2.
The capacity regulating valve regulates the opening by itself so as to maintain the pressure on the discharge side of each of the compressors 2 at the desired pressure. In other words, with the increase of the pressure on the discharge side of each of the compressors 2, the capacity regulating valve decreases the opening to decrease the amount of suction, whereby the compressor 2 decreases the amount of discharge, and with the decrease of the pressure on the discharge side of each of the compressors 2, the capacity regulating valve increases the opening to increase the amount of suction, whereby the compressor 2 increases the amount of discharge.
More specifically, in FIG. 2, the capacity regulating valve regulates the opening so as to maintain the pressure on the discharge side of each of the compressors 2 between a lower limit pressure PL and an upper limit pressure PH. In this case, the capacity regulating valve is fully opened when the pressure on the discharge side of each of the compressors 2 is equal to or less than the lower limit pressure PL, and is fully closed when the pressure on the discharge side of each of the compressors 2 is equal to or greater than the upper limit pressure PH. In addition, between the lower limit pressure PL and the upper limit pressure PH, the opening is proportionally decreased from the lower limit pressure PL toward the upper limit pressure PH. In this way, the pressure range between the lower limit pressure PL and the upper limit pressure PH is the capacity regulating valve control range. That is, each of the compressors 2 has a linear characteristic in which the discharged pressure and the discharged flow rate are in inverse proportion in the defined regulation range of PL to PH by the capacity regulating valve. In other words, the discharged pressure and the load rate of each of the compressors 2 have a linear function. In case that the pressure on the discharge side of each of the compressors 2 exceeds a predetermined stop pressure PS, the compressor 2 is forcefully stopped.
In this embodiment, typically, a plurality of compressors 2 are all capacity-controlled even when operated at the same time. Therefore, each of the compressors 2 is not required to have a full load lock function for holding the compressors 2 in the full load operation.
While the receiver tank 3 is a hollow case in which compressed air is supplied from each of the compressors 2 and is supplied to one or a plurality of compressed air utilization devices. The pressure sensor 4 is provided so that the pressure in the receiver tank 3 can be detected.
The number-of-compressors controller 5 is connected to each of the compressors 2 and the pressure sensor 4, and controls each of the compressors 2 based on the detected pressure of the pressure sensor 4. In this embodiment, the presence or absence of the operation of each of the compressors 2 (that is, the change of the number of compressors being operated) is switched. A specific controlling method is as follows.
FIG. 2 is a diagram showing an example of a number-of-compressors controlling method of the number-of-compressors controlling system 1 of this embodiment and shows the discharged pressure of each of the compressors 2 being operated, the pressure in the receiver tank 3 (that is, the detected pressure of the pressure sensor 4), and a number-of-compressors being operated increase or decrease chart.
The number-of-compressors being operated increase or decrease chart is divided into a start chart for increasing the number of compressors being operated, as shown in a chart form at the center of FIG. 2, and a stop chart for decreasing the number of compressors being operated, as shown in a bar graph form on the right side of FIG. 2. The start chart shows how each of the compressors 2 is started, in other words, how the number of compressors being operated is increased, based on a pressure P in the receiver tank 3 and its change rate ΔP. On the other hand, the stop chart shows how each of the compressors 2 is stopped, in other words, how the number of compressors being operated is decreased, based on the pressure P in the receiver tank 3 and the number of compressors being actually operated at present. These controls are performed based on the detected pressure P of the pressure sensor 4 and the pressure change rate ΔP calculated in each predetermined cycle. In this embodiment, the average value of the detected pressures P in the predetermined number of computation cycles of the CPU of the number-of-compressors controller 5 (e.g., 20 cycles) is used, and the average value of the pressure change rates ΔP in the nearest predetermined time (e.g., the nearest 20 seconds) is used.
The pressure change rate ΔP is the changed pressure in each predetermined time. When the pressure change rate ΔP is negative, the pressure in the receiver tank 3 tends to be decreased, and when the pressure change rate ΔP is positive, the pressure in the receiver tank 3 tends to be increased. When the amount of compressed air used by the compressed air utilization device is larger than the amount of compressed air discharged by the compressor 2, the pressure in the receiver tank 3 is decreased, and on the contrary, when the amount of compressed air discharged by the compressor 2 is larger than the amount of compressed air used by the compressed air utilization device, the pressure in the receiver tank 3 is increased.
The pressure in the receiver tank 3 becomes slightly lower than the discharged pressure of the compressors 2 due to the pressure loss of the piping from the compressors 2 to the receiver tank 3. Therefore, as connected by the slightly inclined dashed lines in FIG. 2, pressures PL1 and PL2 in the receiver tank 3 correspond to discharged pressures PL1′ and PL2′ of the compressors 2, respectively. In the number-of-compressors controlling system 1 of this embodiment, as is apparent from the stop chart, when the pressure in the receiver tank 3 is the upper limit pressure PH in the capacity regulating valve control range, all the compressors 2 are stopped to allow the air flow rate to be zero, so that regarding the upper limit pressure PH, the compressor discharged pressure and the receiver tank pressure are the same.
The number-of-compressors controller 5 compares the detected pressure of the pressure sensor 4 with a predetermined pressure value to increase or decrease the number of compressors 2 being operated. In this case, as shown in the start chart, the pressure value which increases the number of compressors being operated is set to be different based on the pressure change rate ΔP of the detected pressure P of the pressure sensor 4. That is, the number-of-compressors controller 5 starts one compressor 2 when the detected pressure P of the pressure sensor 4 is equal to or less than a number-of-compressors increasing pressure A, but the number-of-compressors increasing pressure A as a threshold value whether or not the number of compressors being operated is increased is set to be higher stepwise as the pressure change rate ΔP is increased toward the negative side.
In increasing the number of compressors being operated, when the detected pressure P of the pressure sensor 4 is maintained to be equal to or less than the number-of-compressors increasing pressure A, the number-of-compressors controller 5 starts one compressor 2 each time a predetermined time (continuous start prevention time) elapses, but in the region in which the pressure change rate ΔP is equal to or less than the set value (−ΔP1) (that is, ΔP≦−ΔP1), when the detected pressure P of the pressure sensor 4 is equal to or less than an immediately number-of-compressors increasing pressure B, one more compressor is started without waiting for the elapse of the predetermined time. The immediately number-of-compressors increasing pressure B should be set to be higher as the absolute value of the pressure change rate ΔP is larger.
On the other hand, as shown in the stop chart, the pressure value which decreases the number of compressors being operated is set to be different based on the number of compressors 2 being actually operated at present. That is, the number-of-compressors controller 5 stops one compressor 2 when the detected pressure P of the pressure sensor 4 is equal to or greater than a number-of-compressors decreasing pressure C, but the number-of-compressors decreasing pressure C as a threshold value whether or not the number of compressors being operated is decreased is set to be lower stepwise as the number of compressors being operated is larger.
The number-of-compressors decreasing pressure C should be determined in consideration of the load rate of each of the compressors 2. That is, in the case that the load rate at stop is 0% and the load rate at full load is 100%, one compressor being operated is stopped when the load rate of each of the compressors 2 being operated is equal to or less than a stop load rate calculated by the following equation:
the stop load rate (%)=(the number of compressors being operated−1)/the number of compressors being operated×100. [Equation 1]
To stop one compressor 2 at the stop load rate according to the number of compressors being operated, the number-of-compressors controller 5 decreases the number of compressors being operated, as needed, based on the number-of-compressors decreasing pressure C calculated by the following equation based on the number of compressors being operated:
the number-of-compressors decreasing pressure C=the upper limit pressure PH−{(the upper limit pressure PH−the lower limit pressure PL)×the stop load rate (%)/100}. [Equation 2]
Equation 2 can be rewritten as follows using Equation 1:
the number-of-compressors decreasing pressure C={(the upper limit pressure PH−the lower limit pressure PL)÷the number of compressors being operated}+the lower limit pressure PL. [Equation 3]
In this way, the number-of-compressors decreasing pressure C can be defined according to the number of compressors being operated. As described above, the upper limit pressure PH and the lower limit pressure PL in Equations 2 and 3 are the compressor discharge pressure which defines the capacity regulating valve control range, but in this embodiment, the actual stop control is performed based on the detected pressure of the pressure sensor 4 provided on the receiver tank 3, so that the value corrected in consideration of the pressure loss between each of the compressors 2 and the receiver tank 3 is preferably used. However, as described above, regarding the upper limit pressure PH, the compressor discharge pressure and the receiver tank pressure are the same. Therefore, regarding the lower limit pressure PL, the value converted to the receiver tank pressure is preferably used. Alternatively, the number-of-compressors decreasing pressure C derived from Equations 2 and 3 is strictly the compressor discharge pressure, which is preferably converted to the receiver tank pressure for control.
Hereinafter, specific control will be described with reference to FIG. 2. The second lower limit pressure PL2 is set to be lower than the first lower limit pressure PL1, and the first lower limit pressure PL1 and the second lower limit pressure PL2 are set to be lower than the lower limit value PL in the capacity regulating valve control range. In addition, the first set value ΔP1 and the second set value ΔP2 are set in consideration of the discharge capacity at the time of the full load operation for one compressor.
(1) the Increase Control of the Number of Compressors 2 being Operated
(1-1) the Case that the Absolute Value of the Pressure Change Rate ΔP is Less than the First Set Value ΔP1. Specifically, −ΔP1<ΔP<+ΔP1.
One compressor is started when the detected pressure P of the pressure sensor 4 is equal to or less than the second lower limit pressure PL2 as the number-of-compressors increasing pressure A. Thereby, typically, the pressure is above the second lower limit pressure PL2, but when the load used of compressed air continues to be increased during this, the pressure can be maintained to be equal to or less than the second lower limit pressure PL2. In that case, one compressor 2 is started each time the predetermined continuous start prevention time elapses. That is, when the detected pressure P of the pressure sensor 4 is stayed in the “start of one compressor” region in FIG. 2, as long as there are the compressors 2 being stopped, each of the compressors 2 being stopped is started each time the continuous start prevention time elapses.
(1-2) the Case that the Absolute Value of the Pressure Change Rate ΔP is Equal to or Greater than the First Set Value ΔP1 and is Less than the Second Set Value ΔP2. Specifically, −ΔP2<ΔP=−ΔP1.
One compressor is started when the detected pressure P of the pressure sensor 4 is equal to or less than the first lower limit pressure PL1 as the number-of-compressors increasing pressure A. In this case, when even with start of one compressor, the detected pressure is maintained to be equal to or less than the first lower limit pressure PL1, one compressor is started each time the predetermined continuous start prevention time elapses, and when the detected pressure is equal to or less than the second lower limit pressure PL2 as the immediately number-of-compressors increasing pressure B, one more compressor is started without waiting for the elapse of the continuous start prevention time.
That is, in FIG. 2, when the detected pressure P enters the “start of one compressor” region to start one compressor but is still stayed in the region, as long as there are the compressors 2 being stopped, each of the compressors 2 being stopped is started each time the continuous start prevention time elapses. In addition, during that, when the detected pressure P enters the “start of one more compressor” region, one more compressor is started without the elapse of the continuous start prevention time.
(1-3) The Case that the Absolute Value of the Pressure Change Rate \P is Equal to or Greater than the Second Set Value ΔP2. Specifically, ΔP≦−ΔP2.
When the pressure in the receiver tank 3 is lowered (when the pressure change rate ΔP is negative, that is, ΔP≦−ΔP2), one compressor is started even when the detected pressure P of the pressure sensor 4 is equal to or less than the upper limit value PH in the capacity regulating valve control range as the number-of-compressors increasing pressure A, that is, is in the capacity regulating valve control range of PL to PH. In this case, when even with start of one compressor, the detected pressure is maintained in the capacity regulating valve control range of PL to PH, one compressor is started each time the predetermined continuous prevention time elapses, and when the detected pressure is equal to or less than the first lower limit pressure PL1 as the immediately number-of-compressors increasing pressure B, one more compressor is started without waiting for the elapse of the continuous start prevention time.
That is, in FIG. 2, when the detected pressure P enters the “start of one compressor” region to start one compressor but is still stayed in the region, as long as there are the compressors 2 being stopped, each of the compressors 2 being stopped is started each time the continuous start prevention time elapses. In addition, during that, when the detected pressure P enters the “start of one more compressor” region, one more compressor is started without the elapse of the continuous start prevention time.
(2) The Decrease Control of the Number of Compressors 2 being Operated
An air pressure is monitored by the pressure sensor 4 to decrease the number of compressors 2 being operated in consideration of the stop load rate from Equation 1 in a manner in which one of two compressors being operated is stopped when the load rate of each of the compressors is equal to or less than 50%, one of three compressors being operated is stopped when the load rate of each of the compressors is equal to or less than 67%, and one of four compressors being operated is stopped when the load rate of each of the compressors is equal to or less than 75%.
Thereby, as the number of compressors is increased, a high load operation is performed in a manner in which when only one compressor is operated, the load rate of the compressor is 0 to 100%, when two compressors are operated, the load rate of each of the compressors is 50 to 100%, and when three compressors are operated, the load rate of each of the compressors is 67 to 100%.
To perform control based on pressure, one compressor should be stopped when the detected pressure P is equal to or greater than the number-of-compressors decreasing pressure C according to the number of compressors being operated calculated by Equation 3 (or Equation 2). For instance, at the time of the operation of two compressors, when the detected pressure P is equal to or greater than “{(the upper limit pressure PH−the lower limit pressure PL)÷2)}+the lower limit pressure PL”, one compressor 2 is stopped. In addition, at the time of the operation of three compressors, the detected pressure P is equal to or greater than “{(the upper limit pressure PH−the lower limit pressure PL)÷3}+the lower limit pressure PL”, one compressor 2 is stopped, and in such a manner, the number-of-compressors decreasing pressure C is set from Equation 3 based on the number of compressors being operated.
Here, the number of compressors 2 being operated is preferably decreased after the detected pressure P of the pressure sensor 4 continues to be equal to or greater than the number-of-compressors decreasing pressure C for a set time. Thereby, the defined time is required from the stop of one compressor to the stop of the next compressor, so that the compressors cannot be stopped excessively one after another.
Only by the pressure in the receiver tank 3, the actual load used of compressed air in the compressed air utilization device cannot be found. Accordingly, the number-of-compressors controller 5 may correct the number of compressors 2 being operated based on the discharge flow rate of compressed air from each of the compressors 2, or the rotating speed, the electric current used, or the electricity used of each of the compressors 2, in addition to the detected pressure of the pressure sensor 4.
For instance, an airflow meter is provided in the discharge piping from each of the compressors 2 to the receiver tank 3, an airflow meter is provided in the discharge piping from the receiver tank 3 to the compressed air utilization device, or the rotating speed and the electric current of each of the compressors 2 are detected, thereby grasping the load rate, that is, the air load, of each of the compressors 2. Then, the number (capacity) of compressors 2 corresponding to the air load is set, so that the excessive compressor 2 should be stopped. In particular, as found from the stop chart of FIG. 2, the difference in the number-of-compressors decreasing pressure C is smaller as the number of compressors is larger, with the result that the number of compressors being optimally operated can be deviated, whereby the power consumption in each of the compressors 2 may be monitored to correct the number-of-compressors decreasing pressure or the number of compressors being operated.
Alternatively, in the above embodiment, each of the compressors 2 is controlled according to the pressure in the receiver tank 3, but may be controlled at the air flow rate, as needed. When electric current measurement is performed, the power consumption with respect to the air load can be managed, so that the energy saving effect according to the presence or absence of the number-of-compressors controlling system 1 of the present invention can also be notified.
In addition, the number-of-compressors controller 5 may monitor the electric current used or the electricity used of each of the compressors 2 and operate the compressor 2 with less electric current used or electricity used on a priority basis. Further, there is a fear that the compressors 2 being operated cannot have the same discharge capacity due to the pressure loss in the piping, the error in the capacity regulating valves, and the interference between the devices, so that as well as the power consumption of each of the compressors 2 may be monitored to operate the compressor with less power consumption on a priority basis, a notification that there is deviation (difference) above the fixed level in the power consumption in each of the compressors 2 may be provided to promote maintenance therefor.
In the operation of a plurality of compressors at the same time, if only one compressor is capacity-controlled and the remaining compressors are operated at full load, the capacity regulation is performed by only one compressor, so that when the amount of air used is abruptly decreased, the pressure in the receiver tank 3 can be excessively increased. However, according to the configuration of this embodiment, even in the operation of a plurality of compressors 2, they are all capacity-controlled, so that even when the amount of air used is abruptly decreased, the capacity control functions of the compressors 2 act in parallel, thereby enabling the pressure in the receiver tank 3 to be prevented from being excessively increased. Even if the amount of air used is lost suddenly and completely, the air pressure cannot be excessively increased.
In the operation of a plurality of compressors 2 at the same time, if only one compressor 2 is capacity-controlled and the remaining compressors 2 are operated at full load, each of the compressors 2 needs the full load lock function for holding the compressors 2 in the full load operation. However, since this function is not provided in each of the compressors 2 as standard, each of the compressors 2 is required to be remodeled. However, according to the system 1 of this embodiment, since each of the compressors 2 does not require the full load lock function, each of the compressors 2 is not required to be remodeled.
In addition, when the number-of-compressors decreasing pressure C is lowered according to the increase of the number of compressors being operated, the load rate of each of the compressors 2 being operated is increased, as described above, and in other words, each of the compressors 2 is operated on the low discharge pressure side. Then, typically, the higher efficiency is provided as each of the compressors 2 is operated on the lower discharge pressure side, so that the operation efficiency can be increased by lowering the number-of-compressors decreasing pressure C.
The number-of-compressors controlling system 1 of the present invention is not limited to the configuration of the above embodiment and can be changed, as needed. For instance, each of the compressors 2 may have an unloader function. In that case, in the above embodiment, the start and stop of each of the compressors 2 should be performed by loading and unloading each of the compressors 2.
In addition, the capacity controlling method of each of the compressors 2 is not limited to the adoption of the capacity regulating valve provided on the suction side of each of the compressors 2, as in the above embodiment, but other conventionally known configurations can also be adopted. Further, without performing capacity control, all the compressors 2 may be simply on-off controlled or load-unload controlled.
In addition, in the above embodiment, compressed air from each of the compressors 2 is fed via the receiver tank 3 to the compressed air utilization device and the pressure sensor 4 is provided on the receiver tank 3, but the pressure sensor 4 may be provided in the position where compressed air is supplied from each of the compressors 2 or in the position where compressed air is fed to the compressed air utilization device, other than on the receiver tank 3.
Further, in the above embodiment, the compressors have the same configuration and discharge capacity, which may be different, as needed. For instance, when the compressor having a discharge capacity twice that of the remaining compressors is included, the compressor may be controlled as two compressors.

EXPLANATION OF REFERENCE NUMERALS

1 Number-of-compressors controlling system
2 Compressor
3 Receiver tank
4 Pressure sensor
5 Number-of-compressors controller
A Number-of-compressors increasing pressure
B Immediately number-of-compressors increasing pressure
C Number-of-compressors decreasing pressure

Claims

1. A number-of-compressors controlling system comprising:

a plurality of compressors;

a pressure sensor which is provided in a position where compressed air is supplied from these compressors and is fed to a compressed air utilization device and detects the pressure of the compressed air; and

a number-of-compressors controller which changes the number of compressors being operated based on the detected pressure of the pressure sensor,

wherein a number-of-compressors decreasing pressure as a threshold value whether or not the number of compressors being operated is decreased by the number-of-compressors controller is set to be lower as the number of compressors being operated is larger.

2. The number-of-compressors controlling system according to claim 1,

wherein a plurality of compressors are all capacity-controlled even when operated at the same time,

wherein in the case that a load rate at stop is 0% and a load rate at full load is 100%, one compressor being operated is stopped when the load rate of each of the compressors being operated is equal to or less than a stop load rate calculated by the following equation:

the stop load rate (%)=(the number of compressors being operated−1)/the number of compressors being operated×100.

3. The number-of-compressors controlling system according to claim 1,

wherein each of the compressors is capacity-controlled so that the pressure on the discharge side thereof is maintained between a lower limit pressure PL and an upper limit pressure PH,

wherein to stop one compressor being operated at the stop load rate according to the number of compressors being operated, the number-of-compressors decreasing pressure is set by the following equation based on the number of compressors being operated:

the number-of-compressors decreasing pressure={(the upper limit pressure PH−the lower limit pressure PL)÷the number of compressors being operated}+the lower limit pressure PL.

4. The number-of-compressors controlling system according to claim 3,

wherein the number of compressors being operated is decreased after the detected pressure of the pressure sensor continues to be equal to or greater than the number-of-compressors decreasing pressure for a set time.

5. The number-of-compressors controlling system according to claim 3,

wherein compressed air from each of the compressors is supplied to a shared receiver tank and is then fed to one or a plurality of compressed air utilization devices,

wherein the pressure sensor is provided on the receiver tank,

wherein a number-of-compressors increasing pressure as a threshold value whether or not the number of compressors being operated is increased by the number-of-compressors controller is set to be different based on a pressure change rate ΔP of a detected pressure P of the pressure sensor,

wherein in the case that the absolute value of the pressure change rate ΔP is less than a first set value ΔP1, one compressor is started when the detected pressure P of the pressure sensor is equal to or less than a second lower limit pressure PL2 as the number-of-compressors increasing pressure,

wherein in the case that the absolute value of the pressure change rate ΔP is equal to or greater than the first set value ΔP1 and is less than a second set value ΔP2, one compressor is started when the detected pressure P of the pressure sensor is equal to or less than a first lower limit pressure PL1 as the number-of-compressors increasing pressure, when the detected pressure P is still maintained to be equal to or less than the first lower limit pressure PL1, one compressor is started each time a predetermined continuous start prevention time elapses, and when the detected pressure P is equal to or less than the second lower limit pressure PL2 lower than the first lower limit pressure PL1, one more compressor is started without waiting for the elapse of the continuous start prevention time,

wherein in the case that the absolute value of the pressure change rate ΔP is equal to or greater than the second set value ΔP2, one compressor is started when the detected pressure P of the pressure sensor is equal to or less than the upper limit pressure PH as the number-of-compressors increasing pressure, when the detected pressure P is still maintained to be equal to or less than the upper limit pressure PH, one compressor is started each time the predetermined continuous start prevention time elapses, and when the detected pressure P is equal to or less than the first lower limit pressure PL1 lower than the upper limit pressure PH, one more compressor is started without waiting for the elapse of the continuous start prevention time.

6. The number-of-compressors controlling system according to claim 1,

wherein the number-of-compressors controller changes the number of compressors being operated based on the discharge flow rate of compressed air from each of the compressors, or the rotating speed, the electric current used, or the electricity used of each of the compressors, in addition to the detected pressure of the pressure sensor.

7. The number-of-compressors controlling system according to claim 6,

wherein the number-of-compressors controller monitors the electric current used or the electricity used of each of the compressors,

wherein the compressor with less electric current used or electricity used is operated on a priority basis.

8. The number-of-compressors controlling system according to claim 2,

9. The number-of-compressors controlling system according to claim 4,

wherein the pressure sensor is provided on the receiver tank,

10. The number-of-compressors controlling system according to claim 2,

11. The number-of-compressors controlling system according to claim 3,

12. The number-of-compressors controlling system according to claim 4,

13. The number-of-compressors controlling system according to claim 5,

14. The number-of-compressors controlling system according to claim 9,

15. The number-of-compressors controlling system according to claim 10,

16. The number-of-compressors controlling system according to claim 11,

17. The number-of-compressors controlling system according to claim 12,

18. The number-of-compressors controlling system according to claim 13,