Patent Publication Number: US-10316842-B2

Title: Air compressor

Description:
TECHNICAL FIELD 
     The present invention relates to a liquid supplying air compressor and particularly relates to an air compressor provided with an intake throttle valve and an air release valve. 
     BACKGROUND ART 
     An oil supplying screw compressor, for instance, which is one of liquid supplying air compressors, is provided with a compressor body that has a pair of male and female screw rotors and is configured to inject oil into a compression chamber for purposes of cooling the heat of compression, improving the sealability of the compression chamber, and lubrication of the screw rotors. Compressed air that has been compressed to a predetermined pressure in the compression chamber of the compressor body is delivered in a state of being mixed with the oil. After the oil has been separated from the compressed air by a separator, the resulting compressed air is supplied to a user&#39;s destination via a compressed air system. The separated oil is temporarily accumulated in a lower portion of the separator and then supplied to the compressor body via an oil system by an internal pressure of the separator. Namely, the oil circulates between the compressor body and the separator. 
     The air compressor adopts a scheme for controlling a capacity in response to a utilization state of the compressed air, with a view to power reduction. Specifically, for instance, an intake throttle valve is provided on an intake side of the compressor body, an air release system is provided to be connected to a primary side of a check valve in the compressed air system (in other words, a secondary side of the separator), and an air release valve is provided in this air release system. Furthermore, a pressure sensor is provided on a secondary side of the check valve in the compressed air system. For instance, when the quantity of the used compressed air falls and the pressure detected by the pressure sensor reaches a predetermined upper limit, a mode of the air compressor is switched to either a no-load operation mode or an automatic stop mode and the supply of the compressed air is stopped. In the no-load operation mode and the automatic stop mode, the following controls are exerted. 
     In the no-load operation mode, the intake throttle valve is closed while the compressor body continues to operate without stopping a motor. Furthermore, the air release valve is opened to release the compressed air, and the pressure on the primary side of the check valve in the compressed air system, that is, the internal pressure of the separator is reduced to some extent. Subsequently, when the pressure detected by the pressure sensor falls to a predetermined lower limit, the mode is switched to a load operation mode. In other words, the intake throttle valve is opened and the air release valve is closed. 
     In the automatic stop mode, the motor is stopped to stop the compressor body. Furthermore, the air release valve is opened to release the compressed air, and the pressure on the primary side of the check valve in the compressed air system, that is, the internal pressure of the separator is reduced nearly to an atmospheric pressure. Moreover, the intake throttle valve is closed in order to prevent the oil within the compressor body from flowing backward and then to the primary side of the intake throttle valve. Subsequently, when the pressure detected by the pressure sensor falls to the predetermined lower limit, the mode is switched to the load operation mode. In other words, the motor is driven to restart operation of the compressor body. Furthermore, the intake throttle valve is opened and the air release valve is closed. 
     In this case, if the time from stop to restart of the compressor body is short, the internal pressure of the separator falls insufficiently, and the residual pressure thereof causes starting congestion of the compressor body. Owing to this, a time limit from the stop of the compressor body until the compressor body becomes restartable is set, and the compressor body is restarted after this time limit has elapsed. 
     Note that Patent Document 1, for instance, uses an air-pressure-operated intake throttle valve and a solenoid-operated air release valve. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: JP-2011-99348-A 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In the no-load operation mode, it is preferred to increase a descending speed of the internal pressure of the separator in order to reduce power. Furthermore, in the automatic stop mode, it is preferred to increase the descending speed of the internal pressure of the separator in order to shorten the abovementioned time limit. For these reasons, the air release valve needs to have a magnitude to a certain degree, and when the solenoid-controlled air release valve is employed as disclosed in, for instance, Patent Document 1, a necessary electromagnetic force and eventually power consumption increase. There has been, thus, room for improvement in energy saving. 
     The present invention has been made in light of the abovementioned circumstances and it is one object to achieve the energy saving. 
     Means for Solving the Problems 
     To achieve the abovementioned object, the invention set forth in claims is employed. An air compressor includes: a compressor body compressing air with injecting a liquid into a compression chamber; an air-pressure-operated intake throttle valve provided on an intake side of the compressor body; a separator provided on a delivery side of the compressor body, the separator separating, from the compressed air delivered from the compressor body, the liquid contained in the compressed air; a compressed air system supplying the compressed air separated by the separator to a supply destination; a check valve provided in the compressed air system; an air release system connected to a primary side of the check valve in the compressed air system; an air-pressure-operated air release valve provided in the air release system; an air-pressure operation circuit that has at least one solenoid-operated three-way valve, selects one of the primary side of the check valve in the compressed air system and a primary side of the intake throttle valve to make the selected primary side communicate with an operation chamber of the intake throttle valve, and selects one of the primary side of the check valve in the compressed air system and the primary side of the intake throttle valve to make the selected primary side communicate with an operation chamber of the air release valve; a pressure sensor provided on a secondary side of the check valve in the compressed air system; and a control unit controlling the three-way valve by switching a mode to any one of a load operation mode, a no-load operation mode, and an automatic stop mode in response to a pressure detected by the pressure sensor. 
     Effect of the Invention 
     According to the present invention, the air compressor employs the air-pressure-operated air release valve and has the solenoid-operated three-way valve that constitutes the air-pressure operation circuit. However, this three-way valve is sufficiently smaller than the air release valve; thus, it is possible to reduce a necessary electromagnetic force and eventually power consumption. Therefore, compared with a case where the air compressor employs a solenoid-operated air release valve, it is possible to achieve energy saving. 
     Other objects and advantages of the present invention will become further obvious from the following disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram representing a configuration of an oil supplying air compressor according to a first embodiment of the present invention. 
         FIG. 2  is a cross-sectional view representing a structure of an air-pressure-operated air release valve according to the first embodiment of the present invention and shows that the air release valve is in a fully closed state. 
         FIG. 3  is a cross-sectional view representing the air-pressure-operated air release valve according to the first embodiment of the present invention and shows that the air release valve is in a fully opened state. 
         FIG. 4  is a schematic diagram representing a configuration of an oil supplying air compressor according to a second embodiment of the present invention. 
         FIG. 5  is a cross-sectional view representing a structure of an air-pressure-operated air release valve according to the second embodiment of the present invention and shows that the air release valve is in a throttled state. 
         FIG. 6  is a schematic diagram representing a configuration of an oil supplying air compressor according to a third embodiment of the present invention. 
         FIG. 7  is a flowchart representing a control processing content associated with an abnormality diagnostic function of a control unit according to the third embodiment of the present invention. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will be described hereinafter while taking as an example an oil supplying air compressor that is one of objects to which the present invention is applied. 
     A first embodiment of the present invention will be described with reference to  FIGS. 1 to 3 . 
       FIG. 1  is a schematic diagram representing a configuration of an oil supplying air compressor according to the present embodiment. Note that a broken line part in  FIG. 1  denotes electric wiring.  FIGS. 2 and 3  are cross-sectional views each representing a structure of an air-pressure-operated air release valve according to the present embodiment,  FIG. 2  shows that the air release valve is in a fully closed state, and  FIG. 3  shows that the air release valve is in a fully opened state. 
     The air compressor is provided with a compressor body  1  that compresses air, a motor  2  that drives this compressor body  1 , an air-pressure-operated intake throttle valve  3  provided on an intake side of the compressor body  1 , an intake filter (not shown) provided on an upstream side of this intake throttle valve  3 , and a separator  4  provided on a delivery side of the compressor body  1 . 
     The intake throttle valve  3  has a valve body  6  that opens/closes a valve seat  5 , a piston  7  connected to this valve body  6 , a spring  8  provided on one side, in the moving direction, of this piston  7  (lower side in  FIG. 1 ), and an operation chamber  9  formed on the other side, in the moving direction, of the piston  7  (upper side in  FIG. 1 ). 
     When an air-pressure operation circuit  10  (described later in detail) increases a pressure of the operation chamber  9  of the intake throttle valve  3 , the piston  7  and the valve body  6  surpass a force of the spring  8  to move to the one side and the valve seat  5  is opened. On the other hand, when the air-pressure operation circuit  10  reduces the pressure of the operation chamber  9  of the intake throttle valve  3 , the force of the spring  8  causes the piston  7  and the valve body  6  to move to the other side and the valve seat  5  is closed. An air intake amount of the compressor body  1  and eventually a load thereof are thereby adjusted. 
     Although not shown in detail, the compressor body  1  may have a pair of male and female screw rotors and a casing that accommodates the screw rotors therein, and compression chambers are formed between tooth grooves of the screw rotors and the casing. Oil supplied from the separator  4  is injected into the compression chamber, where the air is compressed. 
     The separator  4  is composed of a separation mechanism (more specifically, a centrifugal separation mechanism or a filter separation mechanism, for example) that separates, from the compressed air delivered from the compressor body  1 , the oil contained in the compressed air, and a tank that accumulates therein the oil separated by this separation mechanism. An oil system (not shown) is connected to the separator  4 , and also has a cooler and the like (not shown). The oil system is configured to supply the oil accumulated in the separator  4  to the compression chamber of the compressor body  1  by an internal pressure of the separator  4 . Note that in a no-load operation mode, to be described later, the internal pressure of the separator  4  falls to, for instance, about 0.2 MPa but a sufficient quantity of oil is supplied to the compression chamber of the compressor body  1 . 
     A compressed air system  11  is connected to the oil separator  4 . The compressed air system  11  is configured to supply the compressed air separated by the separator  4  to a user side. A check valve  12  is provided in the compressed air system  11 , and a pressure sensor  13  is provided on a secondary side of the check valve  12 . A drier and the like (not shown) are also provided on the secondary side of the check valve  12 . 
     Furthermore, an air release system  14  (air release flow passages  15   a ,  15   b ) is connected between a primary side of the check valve  12  in the compressed air system  11  (in other words, a secondary side of the separator  4 ) and a primary side of the intake throttle valve  3  (more specifically, a primary side of the valve seat  5 ). An air-pressure-operated air release valve  16  is provided in the air release system  14 . 
     The air release valve  16  has a body  17 , a spool (valve body)  18  slidable within this body  17 , a piston  19  connected to one side, in the moving direction, of this spool  18  (right side in  FIGS. 2 and 3 ), a spring  20  provided on one side, in the moving direction, of the piston  19 , a lid  21  that supports this spring  20 , and an operation chamber  22  formed on the other side, in the moving direction, of the spool  18  (left side in  FIGS. 2 and 3 ). 
     An inlet port  23   a  and an outlet port  23   b  are formed in the body  17 , and are connected to the air release flow passages  15   a  and  15   b , respectively. Furthermore, the ports  23   a  and  23   b  are apart from each other in the moving direction of the spool  18 , and a cross-section of a flow passage formed between the ports  23   a  and  23   b  is larger than a cross-section of each port. 
     O-rings  24   a  and  24   b  are provided on outer circumferential sides of the spool  18  and the piston  19 . Stopper parts  25   a  and  25   b  that restrict a moving range of the piston  19  (and eventually a moving range of the spool  18 ) are formed in the body  17  and the lid  21 . A hole  26  for air vent of a spring chamber is formed in the lid  21 . 
     When the air-pressure operation circuit  10  increases the pressure of the operation chamber  22  of the air release valve  16  up to, for example, 0.12 MPa, the spool  18  and the piston  19  surpass a force of the spring  20  and start moving to the one side. Furthermore, when the pressure of the operation chamber  22  of the air release valve  16  becomes, for example, equal to or higher than 0.22 MPa, the piston  19  is made into a state of contacting the stopper part  25   b , and the spool  18  closes the flow passage between the ports  23   a  and  23   b  (fully closed state) as shown in  FIG. 2 . On the other hand, when the air-pressure operation circuit  10  reduces the pressure of the operation chamber  22  of the air release valve  16  to, for example, be lower than 0.12 MPa, the force of the spring  20  causes the spool  18  and the piston  19  to move to the other side, the piston  19  is made into a state of contacting the stopper part  25   a  to open the flow passage between the ports  23   a  and  23   b  (fully opened state) as shown in  FIG. 3 . As a result, the compressed air is released from the primary side of the check valve  12  in the compressed air system  11  (in other words, the secondary side of the separator  4 ) to the primary side of the intake throttle valve  3  via the air release system  14 , thereby reducing the internal pressure of the separator  4 . 
     The air-pressure operation circuit  10  is composed of a flow passage  27   a  connected to the primary side of the check valve  12  in the compressed air system  11 , a flow passage  27   b  connected to the primary side of the intake throttle valve  3 , a flow passage  27   c  connected to the operation chamber  9  of the intake throttle valve  3  and the operation chamber  22  of the air release valve  16 , and a solenoid-operated three-way valve  28  that selects one of the flow passages  27   a  and  27   b  and makes the selected flow passage communicate with the flow passage  27   c . The three-way valve  28  is controlled by a control unit  29 . 
     The control unit  29  switches among a load operation mode, a no-load operation mode, and an automatic stop mode in response to a pressure, which has been detected by the pressure sensor  13 , on the secondary side of the check valve  12  in the compressed air system  11 . In the load operation mode, the no-load operation mode, and the automatic stop mode, the following controls are exerted. 
     In the load operation mode, the control unit  29  drives the motor  2  to cause the compressor body  1  to operate. Furthermore, the control unit  29  turns the three-way valve  28  into a current-carrying state to make the flow passage  27   a  communicate with the flow passage  27   c . As a result, the compressed air from the primary side of the check valve  12  in the compressed air system  11  is supplied to the operation chamber  9  of the intake throttle valve  3  and the operation chamber  22  of the air release valve  16 , thereby increasing the pressure in the operation chamber  9  and the operation chamber  22 . Therefore, the intake throttle valve  3  is made into a fully opened state and the air release valve  16  is made into a fully closed state. 
     The control unit  29  determines whether the pressure detected by the pressure sensor  13  has reached a predetermined upper limit during the load operation mode. For instance, when the pressure detected by the pressure sensor  13  has not reached the predetermined upper limit, the control unit  29  continues the load operation mode. On the other hand, for instance, when the pressure detected by the pressure sensor  13  has reached the predetermined upper limit, the control unit  29  switches the load operation mode to either the no-load operation mode or the automatic stop mode. Generally, the load operation mode is switched first to the no-load operation mode and after predetermined time has elapsed, the no-load operation mode is switched to the automatic stop mode. Nevertheless, when some sort of conditions is satisfied, the load operation mode may be directly switched to the automatic stop mode without via the no-load operation mode. 
     In the no-load operation mode, the control unit  29  causes the compressor body  1  to continue operating without stopping the motor  2 . Furthermore, the control unit  29  turns the three-way valve  28  into a no-current-carrying state to make the flow passage  27   b  communicate with the flow passage  27   c . As a result, the operation chamber  9  of the intake throttle valve  3  and the operation chamber  22  of the air release valve  16  release the compressed air to the primary side of the intake throttle valve  3 , thereby reducing the pressure in the operation chamber  9  and the operation chamber  22  nearly to the atmospheric pressure. Therefore, the intake throttle valve  3  is made into the fully closed state and the air release valve  16  is made into the fully opened state. Furthermore, the internal pressure of the separator  4  falls to, for example, about 0.2 MPa. 
     In the automatic stop mode, the control unit  29  stops the motor  2  to stop the compressor body  1 . Furthermore, the control unit  29  turns the three-way valve  28  into a non-current-carrying state to make the flow passage  27   b  communicate with the flow passage  27   c . As a result, the operation chamber  9  of the intake throttle valve  3  and the operation chamber  22  of the air release valve  16  release the compressed air to the primary side of the intake throttle valve  3 , thereby reducing the pressure in the operation chamber  9  and the operation chamber  22  nearly to the atmospheric pressure. Therefore, the intake throttle valve  3  is made into the fully closed state and the air release valve  16  is made into the fully opened state. Furthermore, the internal pressure of the separator  4  falls nearly to the atmospheric pressure. 
     The control unit  29  determines whether the pressure detected by the pressure sensor  13  has reached a predetermined lower limit during the no-load operation mode or the automatic stop mode. For instance, when the pressure detected by the pressure sensor  13  has not reached the predetermined lower limit, the control unit  29  continues the no-load operation mode or the automatic stop mode. On the other hand, for instance, when the pressure detected by the pressure sensor  13  has reached the predetermined lower limit, the control unit  29  switches the no-load operation mode or the automatic stop mode to the load operation mode. 
     Note that when the internal pressure of the separator  4  falls to, for instance, about 0.2 MPa in the no-load operation mode, the pressure of the air supplied by the air-pressure operation circuit  10  to the operation chamber  22  of the air release valve  16  is also about 0.2 MPa at a time of start of transition from the no-load operation mode to the load operation mode. Owing to this, an energization force of the spring  20  of the air release valve  16  is set smaller than a force of the pressure (about 0.2 MPa) of the air release valve  16  that acts on the spool  18 . 
     In the present embodiment configured as described so far, the air compressor employs the air-pressure-operated air release valve  16  and has the solenoid-operated three-way valve  28  that constitutes the air-pressure operation circuit  10 . However, this three-way valve  28  is sufficiently smaller than the air release valve; thus, it is possible to reduce a necessary electromagnetic force and eventually power consumption. Therefore, compared with a case where the air compressor employs the solenoid-operated air release valve, it is possible to achieve energy saving. 
     Meanwhile, in the automatic stop mode, it is preferred to turn the air release valve  16  into a fully opened state and to increase a flow rate of the released air in order to shorten a time limit since the compressor body  1  is stopped until the compressor body  1  becomes restartable. Furthermore, in the no-load operation mode, it is often preferred to turn the air release valve  16  into the fully opened state and to increase the flow rate of the released air in order to reduce the power. The first embodiment of the present invention is intended to deal with such a case. However, in the no-load operation mode, it is often preferred to turn the air release valve  16  into a throttled state and to reduce the flow rate of the released air in order to ensure the internal pressure of the separator  4  for supplying a sufficient quantity of oil to the compression chamber of the compressor body  1 . A second embodiment of the present invention intended to deal with such a case will be described with reference to  FIGS. 4 and 5 . 
       FIG. 4  is a schematic diagram representing a configuration of an air compressor according to the present embodiment.  FIG. 5  is a cross-sectional view representing a structure of an air release valve according to the present embodiment and shows that the air release valve is in a throttled state. Note that the same parts as those in the abovementioned first embodiment are denoted by the same reference symbols and are not described, as appropriate. 
     In the present embodiment, an air-pressure operation circuit  10 A has the flow passage  27   a  connected to the primary side of the check valve  12  in the compressed air system  11 , the flow passage  27   b  connected to the primary side of the intake throttle valve  3 , a flow passage  27   d  connected to the operation chamber  9  of the intake throttle valve  3 , and the solenoid-operated three-way valve  28  that selects one of the flow passages  27   a  and  27   b  and makes the selected flow passage communicate with the flow passage  27   d . Furthermore, the air-pressure operation circuit  10 A has a flow passage  27   e  connected to the flow passage  27   d , a flow passage  27   f  that is connected to the primary side of the check valve  12  in the compressed air system  11  and in which a decompression part  30  (more specifically, a decompression valve, for instance) is interposed, a flow passage  27   g  connected to the operation chamber  22  of the air release valve  16 , and a solenoid-operated three-way valve  31  that selects one of the flow passages  27   e  and  27   f  and makes the selected flow passage communicate with the flow passage  27   g . The three-way valves  28  and  31  are controlled by a control unit  29 A. 
     In the load operation mode, the control unit  29 A drives the motor  2  to cause the compressor body  1  to operate. Furthermore, the control unit  29 A turns the three-way valve  28  into a current-carrying state to make the flow passage  27   a  communicate with the flow passage  27   d . As a result, the compressed air from the primary side of the check valve  12  in the compressed air system  11  is supplied to the operation chamber  9  of the intake throttle valve  3  via the flow passages  27   a  and  27   d , thereby increasing the pressure in the operation chamber  9 . Therefore, the intake throttle valve  3  is made into a fully opened state. 
     At the same time, the control unit  29 A turns the three-way valve  31  into a non-current-carrying state to make the flow passage  27   e  communicate with the flow passage  27   g . As a result, the compressed air from the primary side of the check valve  12  in the compressed air system  11  is supplied to the operation chamber  22  of the air release valve  16  via the flow passages  27   a ,  27   d ,  27   e , and  27   g , thereby increasing the pressure in the operation chamber  22 . Therefore, the air release valve  16  is made into a fully closed state. 
     In the automatic stop mode, the control unit  29 A stops the motor  2  to stop the compressor body  1 . Furthermore, the control unit  29 A turns the three-way valve  28  into a non-current-carrying state to make the flow passage  27   b  communicate with the flow passage  27   d . As a result, the operation chamber  9  of the intake throttle valve  3  releases the compressed air to the primary side of the intake throttle valve  3  via the flow passages  27   b  and  27   d , thereby reducing the pressure in the operation chamber  9  nearly to the atmospheric pressure. Therefore, the intake throttle valve  3  is made into a fully closed state. 
     At the same time, the control unit  29 A turns the three-way valve  31  into a non-current-carrying state to make the flow passage  27   e  communicate with the flow passage  27   g . As a result, the operation chamber  22  of the air release valve  16  releases the compressed air to the primary side of the intake throttle valve  3  via the flow passages  27   b ,  27   d ,  27   e , and  27   g , thereby reducing the pressure in the operation chamber  22  nearly to the atmospheric pressure. Therefore, the air release valve  16  is made into a fully opened state. 
     In the no-load operation mode, the control unit  29 A causes the compressor body  1  to continue operating without stopping the motor  2 . Furthermore, the control unit  29 A turns the three-way valve  28  into a non-current-carrying state to make the flow passage  27   b  communicate with the flow passage  27   d . As a result, the operation chamber  9  of the intake throttle valve  3  releases the compressed air to the primary side of the intake throttle valve  3  via the flow passages  27   b  and  27   d , thereby reducing the pressure in the operation chamber  9  nearly to the atmospheric pressure. Therefore, the intake throttle valve  3  is made into a fully closed state. 
     At the same time, the control unit  29 A turns the three-way valve  31  into a current-carrying state to make the flow passage  27   f  communicate with the flow passage  27   g . As a result, the compressed air from the primary side of the check valve  12  in the compressed air system  11  is supplied to the operation chamber  22  of the air release valve  16  via the flow passages  27   f  and  27   g  and the decompression part  30 . At this time, the decompression part  30  reduces the pressure of the air from the primary side of the check valve  12  in the compressed air system  11  (more specifically, pressure varying, for instance, from about 0.7 MPa to about 0.2 MPa) to, for instance, about 0.13 MPa. Owing to this, as shown in  FIG. 5 , the air release valve  16  is made into the throttled state. 
     In the present embodiment configured as described so far, the number of the solenoid-operated three-way valves is larger than in the abovementioned first embodiment and, therefore, power consumption increases. Notwithstanding, compared with the case where the air compressor employs the solenoid-operated air release valve, it is possible to reduce the power consumption and achieve energy saving. 
     Furthermore, in the present embodiment, in the automatic stop mode, the air release valve  16  is made into the fully opened state and the flow rate of the released air is increased; therefore, it is possible to shorten the time limit since the compressor body  1  is stopped until the compressor body  1  becomes restartable. On the other hand, in the no-load operation mode, the air release valve  16  is made into the throttled state and the flow rate of the released air is reduced; therefore, the internal pressure of the separator  4  can be stabilized at, for instance, about 0.2 MPa and it is possible to supply a sufficient quantity of oil to the compression chamber of the compressor body  1 . It is thereby possible to suppress a temperature increase of the compressed air. As a result, it is possible to suppress an increase in a quantity of drain and suppress the degradation of lives of the members and the oil. 
     A third embodiment of the present invention will be described with reference to  FIGS. 6 and 7 . 
       FIG. 6  is a schematic diagram representing a configuration of an air compressor according to the present embodiment. Note that the same parts as those in the abovementioned first embodiment are denoted by the same reference symbols and are not described, as appropriate. 
     In the present embodiment, an emergency air release system  32  is connected between the secondary side of the check valve  12  in the compressed air system  11  and the primary side of the intake throttle valve  3 . A solenoid-operated emergency air release valve  33  is provided in the emergency air release system  32  and an orifice  34  is provided on a secondary side of the emergency air release valve  33 . The emergency air release valve  33  is normally in a non-current-carrying state and in a closed state. 
     An abnormality diagnostic pressure sensor  35  is provided on the primary side of the check valve  12  in the compressed air system  11 . A control unit  29 B has not only the same function as that of the control unit  29  in the first embodiment but also a function to diagnose whether an abnormality has occurred in the normal air release valve  16 , on the basis of a detection result of the abnormality diagnostic pressure sensor  35  during the no-load operation mode. 
       FIG. 7  is a flowchart representing a control processing content associated with the abnormality diagnostic function of the control unit  29 B according to the present embodiment. 
     First, in step  100 , it is determined whether the mode has been switched to the no-load operation mode. When the mode has not been switched to the no-load operation mode, a condition for a determination in step  100  is not satisfied and the determination is repeated. On the other hand, when the mode has been switched to the no-load operation mode, the condition for the determination in step  100  is satisfied and processing goes to step  110 . In step  110 , a decompression speed is computed on the basis of a pressure detected by the abnormality diagnostic pressure sensor  35 . The processing goes to step  120 , in which it is determined whether the normal air release valve  16  is fixedly made into a closed state by determining whether the decompression speed is lower than a preset, predetermined value. 
     For instance, when the decompression speed is higher than the predetermined value (in other words, it is determined that the normal air release valve  16  is not fixedly made into the closed state), a condition for a determination in step  120  is not satisfied and similar procedures are repeated back to step  100  mentioned above. On the other hand, for instance, when the decompression speed is lower than the predetermined value (in other words, it is determined that the normal air release valve  16  is fixedly made into the closed state), the condition for the determination in step  120  is satisfied and the processing goes to step  130 . In step  130 , the motor  2  is stopped and the compressor body  1  is stopped. Furthermore, the processing goes to step  140 , in which the emergency air release valve  33  is made into a current-carrying state to be switched to an open state. As a result, the compressed air is released from the secondary side of the check valve  12  in the compressed air system  11  to the primary side of the intake throttle valve  3  via the emergency air release system  32 . Moreover, the processing goes to step  150 , in which an error is displayed in a display part  36 . 
     In the present embodiment configured as described so far, the air compressor employs the air-pressure-operated normal air release valve  16  and does not normally use the solenoid-operated emergency air release valve  33 . Therefore, compared with the case where the air compressor employs the solenoid-operated normal air release valve, it is possible to achieve energy saving. 
     Furthermore, in the present embodiment, even when the air-pressure-operated normal air release valve  16  is fixedly made into the closed state, the solenoid-operated emergency air release valve  33  can be made into the open state. As a result, maintenance can be conducted to the air compressor. 
     The third embodiment has been described while taking as an example a case where the air compressor is provided with the air-pressure operation circuit  10  similar to that in the first embodiment; however, the present invention is not limited to this case and the air compressor may be provided with the air-pressure operation circuit  10 A similar to that in the second embodiment. 
     Furthermore, the first to third embodiments have been described while taking a case where the invention is applied to the oil supplying air compressor as an example; however, it goes without saying that the present invention is not limited to this case and may be applied to a water supplying air compressor. 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
           1 : Compressor body 
           3 : Intake throttle valve 
           4 : Separator 
           9 : Operation chamber 
           10 ,  10 A: Air-pressure operation circuit 
           11 : Compressed air system 
           12 : Check valve 
           13 : Pressure sensor 
           14 : Air release system 
           16 : Air release valve 
           22 : Operation chamber 
           27   a - 27   g : Flow passage 
           28 : Three-way valve 
           29 ,  29 A,  29 B: Control unit 
           30 : Decompression part 
           31 : Three-way valve 
           32 : Emergency air release system 
           33 : Emergency air release valve 
           35 : Abnormality diagnostic pressure sensor 
           36 : Display part