Abstract:
A high reliability water injected scroll air compressor is provided. In the scroll air compressor provided with: an orbiting scroll; a fixed scroll corresponding the orbiting scroll; a motor that generates driving force for making the orbiting scroll orbit the fixed scroll; a compressing path from a suction port to a discharge port; and a portion for injecting water into the compressing path, the operation is controlled by switching operation in which water is injected into the compressing path and operation in which no water is injected. Corrosion, the failure of activation and a touch to the wrap respectively worried when water is injected into an air end are avoided by switching the operation with water injection and the operation without water injection as described above so as to prevent water from remaining in the air end.

Description:
[0001]    This application claims the priority of Japanese Patent Application No. JP 2010-027101, filed Feb. 10, 2010, the disclosure of which is expressly incorporated by reference herein in its entirety. 
       TECHNICAL FIELD 
       [0002]    This subject matter relates to a scroll air compressor that compresses air, particularly relates to water injected scroll air compressor of a type that water is injected into the compression chamber. 
       BACKGROUND 
       [0003]    For a portion to enhance the energy efficiency of an air compressor for general industry, an oil injected type and a water injected type that mix oil or water with air sucked inside a air end and compress them together are known. 
         [0004]    The oil or the water has effect that inside leakage is reduced because it seals narrow clearance via which compression chamber connects with another space and effect that the heat of compression is absorbed and the thermic deformation of members of the compressor is prevented, reducing compressing power, and both effects enhance the energy efficiency. The oil injected type excels in reliability because the type has many achievements, however, as a component of oil remains in supplied discharged air though the component is slight, the oil injected type is often not used for application that does not allow even the existence of the minute oil component to food and a semiconductor. 
         [0005]    The prevalence of the water injected type has been retarded, compared with the oil injected type because countermeasures against rust, corrosion, the failure of lubrication and others are required, compared with oil because of characteristics of water though no oil content is mixed in supplied air as to the water injected type. However, the development of a water injected air compressor has been recently active because of a request of a market for clean air that includes no oil content and for example, Japanese Patent Application Laid-Open Publication No. 2009-180099 is disclosed. 
         [0006]    The adoption of a water injected scroll air compressor is disclosed in Japanese Patent Application Laid-Open Publication No. H8-128395 and Japanese Patent Application Laid-Open Publication No. 2002-89447. Besides, results of experiments in which the efficiency is enhanced by injecting water into the scroll air compressor are described in “Performance of oil-free scroll-type air compressors” written by T. Yanagisawa, M. Fukuta, and Y. Oqi (Shizuoka University) in Proceedings of International Conference on Compressors and Their Systems as an identification number of IMechE 1999 C542/088, issued in September 1999 and published by Institution of Mechanical Engineers (IMechE). 
       SUMMARY 
       [0007]    In the case of an oil-free water injected scroll air compressor, at least the following three problems are supposed and its product planning does not progress, compared with a screw type. 
         [0008]    (1) As an aluminum alloy the density of which is small and which is excellent in thermal conductivity is used for the material of a scroll because of a dimensional constraint of a balance weight and characteristics of heat. radiation, the corrosion of the material when water is injected is worried. 
         [0009]    (2) As compression chamber radially moves from the periphery toward the center along a scroll wrap, reducing its radius, injected water itself causes uncertain unbalance. 
         [0010]    (3) As there is a limit in thickening the wrap because of a shape of the scroll air compressor and tolerance decreases in the strength of the wrap particularly in the center, the breakage of the wrap may be caused when injected water is compressed. 
         [0011]    Besides, problems to be particularly solved by the present subject matters are as follows. 
         [0012]    (4) When water remains in the compression chamber in activation, the activation fails because of excessive torque caused by the compression of the liquid, the scroll wraps are touched because of a thermal transient state, unbalance is caused, and vibration is increased. 
         [0013]    (5) When water remains in the compression chamber in a stop, an orbiting scroll and a fixed scroll respectively made of an aluminum alloy for example may corrode. 
         [0014]    The present subject matter is made in view of the above-mentioned problems and its object is to avoid the failure of activation caused by the injection of water and a problem that the material of the scroll corrodes due to water left in compression chamber in a stop and to provide a water injected scroll air compressor that enables stable operation and has high reliability. 
         [0015]    (1) To achieve the object, the present subject matter is based upon a scroll air compressor which is provided with an orbiting scroll member equipped with a scroll wrap, a fixed scroll member equipped with a substantial scroll wrap corresponding to the wrap of the orbiting scroll member, and a driving unit that generates driving force for making the orbiting scroll member orbit the fixed scroll member. The scroll air compressor is provided with a compressing path from a suction port to a discharge port and in which water is injected into the compressing path, and has a characteristic that the operation is initiated without injecting water (hereinafter called operation without water injection) and the injection of water is initiated after certain time elapses since the initiation of the operation (hereinafter called operation with water injection). 
         [0016]    Besides, the present subject matter is provided with a portion to detect at least either of the temperature or the pressure of compressed gas discharged from the compressing path, is also provided with a portion to operate the detecting portion and operating time, and during the operation, operation with water injection may be also initiated based upon a result of operation using at least one parameter of the pressure, the temperature and the operating time 
         [0017]    (2) To achieve the object, the present subject matter is based upon the scroll air compressor which is provided with the orbiting scroll member equipped with the scroll wrap, the fixed scroll member equipped with the substantial scroll wrap corresponding to the wrap of the orbiting scroll member, and the driving unit that generates driving force for making the orbiting scroll member orbit the fixed scroll member. The scroll air compressor is provided with the compressing path from the suction port to the discharge port and in which water is injected into the compressing path, and has a characteristic that at the same time that the driving unit is stopped, the injection of water is stopped or before the driving unit is stopped, operation without water injection is executed. 
         [0018]    Besides, a portion to detect at least either of the temperature or the pressure of compressed gas discharged from the compressing path is provided, a portion to operate the detecting portion and operating time is also provided, and during the operation, the injection of water into the compressing path may be also stopped or reduced based upon a result of operation using at least one parameter of the pressure, the temperature and the operating time. 
         [0019]    For example, line pressure is detected, it is estimated based upon its value and the variation that the compressor is automatically stopped, and the injection of water is stopped before the compressor is stopped. At this time, the quantity of injected water may be also gradually reduced based upon a value of the pressure and the variation. When line pressure rapidly decreases and the compressor is not automatically stopped to the contrary to the estimate, operation with water injection is resumed based upon pressure or the elapse of time respectively separately determined. 
         [0020]    Besides, for example, when no external air vessel is provided and pressure rapidly varies, water may be also ordinarily stopped. 
         [0021]    Hereby, in the stop, water is lost in the compression chamber and the corrosion of the material of the scroll and a problem in activation can be avoided. Particularly, when the material of the scroll is made of an aluminum alloy, the protection against corrosion of the compressor is enhanced. 
         [0022]    (3) In (1) and (2) described above, it is desirable that a variable frequency drive is provided for the following reasons. 
         [0023]    For example, when the injection of water is stopped to be operation without water injection during the operation of the compressor because discharge pressure rises and the driving unit is stopped after the compression chamber is dried, it is supposed that the pressure exceeds set cut-out pressure before the compression chamber is fully dried, a relief valve is operated and a protective device such as a thermal relay is operated. Besides, to avoid this situation, the compressor is stopped before the compression chamber is fully dried. According to research by the inventors, drying operation for approximately one minute is required so as to dry the compression chamber, while in a case that compressed fluid is air, sufficient drying time cannot be secured in the currently normal combination of a compressor and an air vessel (the air vessel of approximately 0.1 to 0.2 m 3  for the compressor. of a flow amount of 1 m 3 /min. in conversion in a suction condition). Then, when a usage rate of compressed fluid is low, energy saving operation according to the usage rate of air is enabled by using the variable frequency drive, controlling so that the rotating speed of the driving unit is reduced and the compressor is not stopped as much as possible 
         [0024]    Besides, to more effectively stop the compressor in a dry condition, when the rotating speed of the driving unit decreases to some extent, the injection of water may be also stopped to be operation without water injection. 
         [0025]    (4) In (1) to (3) described above, a check valve or a minimum pressure valve is provided on the path where air of the compressor passes and as a result, after the injection of water into the compressing path is stopped during operation, the operation (hereinafter called unload operation without water injection) is continued, blow-off air on the primary side of the check valve or the minimum pressure valve into the atmosphere. Hereby, operation without water injection is enabled without operating the protective device described in (3), besides, when a compressed air flow rate is increased during operation without water injection, the supply of compressed air can be resumed by stopping the blow-off of air, and when the compressed air flow rate is further increased, the injection of water into the compression chamber can be also resumed. 
         [0026]    Further, when an air flow rate is small and an automatic stop is continued for long time, unload operation without water injection is executed for fixed time and the compression chamber is dried. 
         [0027]    (5) In (1) to (4) described above, a suction throttle valve is provided on the suction side of the compressor, as a result, inlet pressure in the compression chamber is turned negative by closing the suction throttle valve in operation without water injection before the compressor is stopped, and the compression chamber can be faster dried. When the blow-off of air is executed while the suction throttle valve is closed, compression ratio decreases, power is reduced, and the rise of discharge temperature can be reduced. 
         [0028]    (6) In (4) and (5) described above, as blow-off air may include moisture, a circumference of the compressor can be protected by utilizing a water separator before the blow-off. 
         [0029]    (7) In (1) to (6) described above, pressure when operation without water injection is initiated is set to be equal to or lower than the cut-out pressure. 
         [0030]    (8) In (1) to (7) described above, the injection of water and a stop of the driving unit are simultaneously executed in capacity control, that is, in an automatic stop according to line pressure and if operation without water injection is executed only in a stop not necessarily linked with the variation of line pressure such as a stop instruction from the field, a stop instruction depending upon multi unit control and a stop instruction depending upon scheduled operation, more energy can be saved. 
         [0031]    (9) In some described above, the driving unit that generates driving force for making the orbiting scroll member orbit shall be a motor. 
         [0032]    (10) In some described above, when an automatically stopped condition is continued, operation without water injection is executed for fixed time and blow-off air shall be brown into the atmosphere from the primary side of the check valve or the minimum pressure valve. 
         [0033]    According to the above-mentioned examples, the failure of activation caused by the injection of water and a problem that the material of the scroll is corroded by water left in the compression chamber in a stop can be avoided by suitably executing operation without water injection. 
         [0034]    According to the present subject matter, the water injected scroll air compressor that enables stable operation and has high reliability can be provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    The present subject matter will become fully understood from the detailed description given hereinafter and the accompanying drawings, wherein: 
           [0036]      FIG. 1  is a block diagram showing a compressor in an example of the present subject matter; 
           [0037]      FIG. 2  is a top sectional view showing the scroll air compressor in the example of the present subject matter; 
           [0038]      FIG. 3  is a side sectional view showing the scroll air compressor in the example of the present subject matter; 
           [0039]      FIG. 4  is a time chart in a first example of control in this example; 
           [0040]      FIG. 5  is a time chart in a second example of control in this example; 
           [0041]      FIG. 6  is a time chart in a third example of control in this example; 
           [0042]      FIG. 7  is a flowchart in the first example of control in this example; 
           [0043]      FIG. 8  is a flowchart in the first example of control in this example; 
           [0044]      FIG. 9  is a flowchart in the second example of control in this example; 
           [0045]      FIG. 10  is a flowchart in the second and third examples of control in this example; and 
           [0046]      FIG. 11  is a flowchart in the third example of control in this example. 
       
    
    
     DETAILED DESCRIPTION 
       [0047]    In the following detailed description, numerous specific details are set forth by way of examples in order to provide a thorough understanding of the relevant teachings. However, it should be apparent to those skilled in the art that the present teachings may be practiced without such details. In other instances, well known methods, procedures, components, and/or circuitry have been described at a relatively high-level, without detail, in order to avoid unnecessarily obscuring aspects of the present teachings. 
         [0048]    Hereinafter, examples of the present subject matter will he described with reference to the accompanying drawings. 
         [0049]      FIG. 1  is a system diagram showing the whole configuration of a water injected scroll air compressor equivalent to this example. As described later, the whole is not essential configuration, however, desired effects are acquired by controlling specific configuration every example. 
         [0050]      FIG. 2  is a top sectional view showing an air end of the scroll air compressor and  FIG. 3  is a side sectional view showing the air end of the scroll air compressor. 
         [0051]      FIGS. 4 to 6  show examples of an operational time chart of the water injected scroll air compressor and FIGS.  7  to  11  show examples of a control flow chart. 
         [0052]    Before the whole configuration is described, the structure of the air end  1  of the scroll air compressor will be described using  FIGS. 2 and 3 . 
         [0053]    The air end  1  of the scroll air compressor is provided with left and right two scroll mechanisms  2 ,  3  and each scroll mechanism is configured by a wrap on the orbiting side, a wrap on the fixed side and end plates equivalent to bottoms of the wraps. The left and right two wraps on the orbiting side are formed back to back with the same orbiting scroll  5  and in the center of the orbiting scroll  5  held between the end plates of both wraps, a through hole  6  for letting cooling air pass is provided. 
         [0054]    The wrap on the fixed side engaged with the wrap of the orbiting scroll  5  is formed inside a left fixed scroll  7  and inside a right fixed scroll  8  and these left and right two fixed scrolls are connected by bolts in a peripheral connecting part  9  to be a casing of the air end  1 . Each cooling fin  11 ,  12  is formed on a surface to be the reverse surface to the wrap provided inside each fixed scroll  7 ,  8 . 
         [0055]    The orbiting scroll  5  is supported by each eccentric part of a main shaft  13  and a counter shaft  14  via each bearing outside the wraps. The eccentricity of the two shafts is the same and a link mechanism configured by parallel four poles is formed. The main shaft  13  and the countershaft  14  are supported by the casing via bearings and are synchronously rotated by the effect of a timing belt  15  wound onto synchronous pulleys provided to ends of them. For a driving unit in this example, a motor  100  ( FIG. 1 ) is used and the main shaft  13  receives power from an output shaft of the motor  100  via a belt  17  wound onto a driving pulley  16 . 
         [0056]    Suction ports  18 ,  19  that pierce each wall are provided just outside the wrap of each fixed scroll  7 ,  8 . As the two suction ports are arranged on one side, the total right and left four suction ports are provided. A passage that ranges from the outside to the inside of the casing through the suction ports  18 ,  19  continues to the inside of a dust seal  20  and connects with a peripheral room  54  that surrounds the wrap. The dust seal  20  is attached to ends of a cylindrical wall that overhangs inside the left and right fixed scrolls  7 ,  8  and that surrounds the wrap and is slid in the vicinity of the periphery of the end plate of the orbiting scroll  5 . The dust seal  20  is attached to prevent foreign matters from invading compression chamber. 
         [0057]    Each discharge port  21 ,  22  that pierces each fixed scroll  7 ,  8  so as to make the compression chamber at a final stage and the outside communicate is provided in the center of each left or right wrap. To balance the left and right compression chamber, a pipe line that makes the two discharge ports  21 ,  22  communicate by piercing the center of the orbiting scroll  5  is provided. 
         [0058]    According to the above-mentioned configuration, the orbiting scroll  5  is orbited by the motor  100  and air sucked from the suction ports  18 ,  19  is compressed by the scroll mechanisms  2 ,  3 . The compressed air is discharged from the discharge ports  21 ,  22  and is supplied to the outside via a passage described later. 
         [0059]    Referring to  FIG. 1 , the whole configuration of this example will be described below. 
         [0060]    The air end  1  is configured by combining scroll members provided with the scrolled wrap and has structure that air is sucked from the suction port and water can be injected into the compression chamber together with the air for example. Besides, the air end is configured via optimum clearance to enable operation in an oil-free state. 
         [0061]    Compressed fluid flows as follows. 
         [0062]    A suction filter  101  is provided on the suction side of the air end  1  and a suction throttle valve  102  for regulating capacity may be also provided on the secondary side. 
         [0063]    Fluid compressed in the air end  1  passes a check valve  103 , is cooled by an aftercooler  104 , and afterward, is discharged via configuration in which water is removed. In this example, after the moisture of compressed air that passes the aftercooler  104  is separated in a water separator tank  105 , the compressed air passes a minimum pressure valve  106 , passes a drier  117  depending upon a specification of a required dew point, the moisture is further removed, and the compressed air is discharged. A water separator element  128  may be also provided in the water separator tank  105  or on the secondary side of the water separator tank. For the aftercooler  104 , a heat exchanger is used, for example, the heat of the compressed air is exchanged for wind sent from a cooling fan not shown, and the compressed air is cooled. 
         [0064]    In operation without water injection, the temperature of fluid discharged from the air end exceeds a boiling point of water to be approximately 200° C., however, operation without water injection is enabled by arranging the aftercooler  104  between the air end  1  and the water separator tank  105  and cooling the temperature of fluid at an entrance of the water separator tank below 100° C. equivalent to the boiling point of water. 
         [0065]    That is, according to this configuration, operation with water injection and operation without water injection are enabled with one compressor. 
         [0066]    Water injected into the air end  1  flows as follows. 
         [0067]    Water is injected into the air end  1  by opening an injection control valve  107 . The injected water passes the check valve  103  together with the compressed fluid, is cooled by the aftercooler  104 , and is separated in the water separator tank  105 . The separated moisture is purified in a strainer  108  and a water filter  109  and is injected into the air end  1  again according to an open degree of the injection control valve  107 . 
         [0068]    As described above, a water supply path (shown by a broken line in  FIG. 1 ) that makes the water separator tank  105  and the suction side of the air end  1  communicate is provided, water in the water separator tank  105  is supplied to the air end  1  via the strainer  108  and the water filter  109  through the water supply path, and water injection is enabled by controlling the injection control valve  107 . Besides, as water injected into the air end  1  reaches the water separator tank  105  via the discharge piping together with compressed air as described above, a water circulating path is configured by each passage. 
         [0069]    As for a driving system, the air end  1  is driven by the driving force of the motor  100  via the V-belt  17 . A variable frequency drive  112  may be also built in a control panel  113  and hereby, the rotating speed of the motor  100  can be adjusted. 
         [0070]    As for an air blow-off line, at least either of first one or second one has only to be provided and no air blow-off line may be also provided. The first air blow-off line is provided between the air end  1  and the aftercooler  104  and after high-temperature fluid after compression is cooled utilizing wind discharged from the aftercooler  104  so as to emit the fluid, the fluid is let to pass a water separator  114  and is brown from an air blow-off solenoid valve  115 . 
         [0071]    The second air blow-off line is provided between the water separator tank  105  and the minimum pressure valve  106  and air is brown by an air blow-off solenoid valve  125  after it passes a water separator  124 . When the air blow-off line is provided on the secondary side of the water separator, no aftercooler check valve  116  is required. Besides, when the moisture is fully removed in the water separator tank  105  or in the water separator element  128 , the water separator  124  can be omitted. The air blow off line may be also provided between the aftercooler  104  and the water separator tank  105 . 
         [0072]    A control system is configured as follows. 
         [0073]    When the variable frequency drive  122  is provided, the rotating speed of the motor  100  can be controlled. In the control panel  113 , an arithmetic unit  123  to which signals from pressure sensors  118 ,  119  and temperature sensors  120 ,  121  are input and which can operate operating time, stop time, the rotating speed directed from the variable frequency drive  122  of the motor  100  and others is built. The activation and the stop of the motor  100 , the opening and the closing of the suction throttle valve  102  and the air blow-off solenoid valves  119 ,  125 , the adjustment of an aperture of the injection control valve  107  and the rotating speed directed from the variable frequency drive  122  of the motor  100  can be adjusted by operating the operating time, the stop time, the rotating speed and others. The pressure sensors  118 ,  119  and the temperature sensors  120 ,  121  may be also respectively a pressure switch and a temperature switch. 
         [0074]    The whole configuration of this example has been described. Next, an example of control will be described. In the following control, detection information from the pressure sensors ( 118 ,  119 ) and count time are used. The detection information is input to a control unit not shown and the count time is also operated by the control unit (needless to say, an external time counter may be also used). Various instructions such as the opening and the closing of various valves, the operation and the stop of the motor and a rotating speed control instruction are also transmitted from the control unit. An operator can input an instruction to operate the compressor and an instruction to stop it from an external device, however, the input information is transmitted to the control unit, and the control unit transmits a control instruction to each control object based upon the input information. 
         [0075]    Referring to  FIGS. 4 ,  7  and  8 , first control example and operation in this example will be described below. 
         [0076]    In the description, a case that configuration is based upon  FIG. 1 , no air blow-off solenoid valve  115 ,  125  and no water separator  114 ,  124  are installed, the aftercooler check valve is not attached, no variable frequency drive is provided to the control system and the suction throttle valve  102  is also not attached is described, however, these may be also provided unless these obstruct this control. 
         [0077]    First, referring to  FIGS. 4 and 7 , the activation and the operation will be described. Line pressure shown by a full line in  FIG. 4  is detected by the pressure sensor  119  and pressure at an exit of the air end shown by a broken line with an arrow is detected by the pressure sensor  118 , however, the two sensors are not required to be always used and control based upon only line pressure as shown in the example of control is also allowed. The example will be described in detail below. 
         [0078]    First, when an instruction to initiate operation is turned on (a step S 1001  in  FIG. 7 ) while the compressor is activated, operation without water injection is initiated (S 1002 ). The operation without water injection is performed when the injection control valve  107  is closed. 
         [0079]    The operation without water injection is continued for predetermined fixed time t 1 . When the time t 1  elapses after the operation is initiated, the injection control valve  107  is opened and operation with water injection is initiated (S 1003  to S 1004 ). 
         [0080]    As for the quantity of injected water, it is clarified by verification by the inventors that the efficiency is greatly enhanced with small quantity. An object of this example is also to enhance the efficiency by injecting small quantity of water and control according to the object is made. Concretely, water is injected on the suction side (or into the compression chamber) of the air end in a range in which the ratio of the quantity of injected water that is the ratio in volume of an injected water flow rate to a sucked air flow rate is ‘5×10−5 to 40×10 31 5 ’ and in a range of the ratio of the quantity of injected water having a characteristic that the increasing width of the whole adiabatic efficiency of the compressor per the increasing width, ‘1×10 −5 ’ of the ratio of the quantity of injected water is below 2%. 
         [0081]    Besides, in this example, injected water is controlled using line pressure (or pressure at the exit of the air end). Therefore, injection stop pressure P 1  to be a pressure value between cut-out pressure P 2  and cut-in pressure P 3  that determine a range of supplied pressure is preset. 
         [0082]    In control, it is judged whether line pressure teaches the injection stop pressure P 1  or not in operation with water injection (S 1005 ), when the line pressure reach P 1 , injection is stopped, and the operation with water injection is made to proceed to operation without water injection (S 1006 ). 
         [0083]    As sealability between the scroll wraps is lost in operation without water injection, compared with operation with water injection, the quantity of discharged air decreases, a curve showing the rise of pressure is made gentle, and the rise of pressure gradually declines. When time t 2  elapses before line pressure reaches P 2  in operation without water injection, the motor  100  is stopped. Besides, when line pressure further rises and reaches the cut-out pressure P 2  before the time t 2  elapses, the motor  100  is also stopped (S 1007  to S 1009 ). 
         [0084]    Next, as no compressed air is supplied in a state in which the motor  100  is stopped, line pressure decreases when compressed air is used. When line pressure decreases and reaches the cut-in pressure P 3 , the operation is resumed. Concretely, operation without water injection is resumed (S 1010  to S 1011 ). 
         [0085]    After the operation is resumed, time is also counted (S 1012 ) and when time t 3  elapses, the operation without water injection is made to proceed to operation with water injection (S 1013 ). Afterward, control in which operation with water injection and operation without water injection are repeated is executed by contrasting the pressure P 1 , P 2 , P 3 , the time t 2 , t 3 , detected pressure and count time. 
         [0086]    Next, control in the stop will be described referring to  FIGS. 4 and 8 . When a stop instruction is issued in operation (at the timing of T 1  in  FIG. 4 , S 1501 ), it is judged whether operation with water injection is made or not (S 1502 ). As operation with water injection is made in the example shown in  FIG. 4 , the injection control valve  107  is first closed and after the operation with water injection is made to proceed to operation without water injection (S 1503 ), the motor  100  is stopped after time t 4  elapses (S 1504  to S 1505 ). 
         [0087]    When timing at which the stop instruction is issued is not in operation with water injection, the motor  100  is stopped after the time t 4  elapses (S 1507  to S 1505 ) as described above in the case of operation without water injection (S 1506 ). Further, when operation with water injection is not made (S 1508 ), operation without water injection is made and the similar control is executed (S 1509 , S 1510 , and S 1505  in this order). 
         [0088]    As operation without water injection is made before a stop by executing stop control as described above, the air end  1  can be dried by heat in compression in the stop and the reliability can be enhanced. 
         [0089]    When a stop instruction is issued in the vicinity of the cut-out pressure P 2 , operation without water injection is also made. At this time, time t 4  for operation without water injection is required to be secured. That is, pressure may rise by the operation without water injection and a case that pressure exceeds the cut-out pressure P 2  is supposed. Therefore, the cut-out pressure P 2  is required to be set to be lower than the actual cut-out pressure P 4  of the compressor, for example, the set pressure of a pressure relief valve  127  which is arranged between the minimum pressure valve  106  and the air end  1  (see  FIG. 1 ). In this example, second cut-out pressure P 4  is set as a higher pressure value than the cut-out pressure P 2  in control and control is made so that line pressure does not exceed P 4 . 
         [0090]    The time t 1  used for control in operation and the time t 3  may be also the same. Intervals shown as A 1  to A 5  in  FIG. 4  are equivalent to intervals for operation without water injection. 
         [0091]    Next, a second example of control and the operation in this example will be described referring to  FIGS. 5 and 10 . In this example, control in which no-load running by opening the air blow-off solenoid valve is adopted is made. 
         [0092]    The configuration is similar to that in the first example of control as to items which are not especially described except that the air blow-off solenoid valve  125  and the water separator  124  are added in addition to the configuration in the first example. Other configurations may also exist in a range in which it is not against this control. 
         [0093]    First, the activation and the operation will be first described referring to  FIGS. 5 and 9 . When an instruction to initiate operation is turned on (a step S 2001  in  FIG. 9 ) while the compressor is activated, operation without water injection is initiated (S 2002 ). The operation without water injection is operation in a state in which the air blow-off solenoid valve  125  and the injection control valve  107  are closed. When the time t 1  elapses after the operation is initiated, the injection control valve  107  is opened and the operation without water injection is made to proceed to operation with water injection (S 2003  to S 2004 ), 
         [0094]    When line pressure reaches the cut-out pressure P 2  in operation with water injection, water injection is stopped, the air blow-off solenoid valve  125  is further opened, air between the exit of the air end  1  and the minimum pressure valve  106  is brown, and the operation with water injection is made to proceed to unload operation without water injection (S 2005  to S 2006 ). The unload operation without water injection is operation in a state in which a load is reduced by opening the air blow-off solenoid valve  125  when the supply of compressed air is not required and in this state, control in which the injection control valve  107  is closed is made. At this time, discharge pressure of the air end  1  is pressure P 4  which is balanced by the discharged quantity of compressed fluid and the inside diameter of the air blow-off solenoid valve  125 . It need scarcely be said that the pressure P 4  is lower than the cut-out pressure P 2  and as the pressure P 4  is lower than the cut-in pressure P 3 , a load of the motor  100  is reduced by the quantity. 
         [0095]    When the unload operation without water injection. continues for predetermined time, it is judged that time in which the supply of compressed air is not required continues and the operation of the compressor is stopped. In the example of this control, the time of the unload operation without water injection is counted and when the time t 2  elapses after the unload operation without water injection is initiated, the motor  100  is stopped (S 2007  to S 20008 ). At this time, the air blow-off solenoid valve  125  is closed. 
         [0096]    When compressed air is used at a destination to which air is supplied in a state in which the motor  100  is stopped, line pressure decreases. When the line pressure decreases up to the cut-in pressure P 3 , the motor  100  is activated and operation without water injection is resumed (S 2008 , S 2009 , and S 2010  in this order). When line pressure decreases up to the cut-in pressure P 3  before the time t 2  elapses in unload operation without water injection, it is also judged that the supply of compressed air is required and operation without water injection is resumed (S 2007 , S 2009 , and S 2010  in this order). 
         [0097]    After the time t 3  elapses since operation without water injection is initiated, the operation without water injection is made to proceed to operation with water injection. After the operation with water injection, the similar control to control in the stop S 2004  and the following steps in  FIG. 9  is made, when line pressure reaches the cut-out pressure P 2 , water injection is stopped, further, the air blow-off solenoid valve  125  is opened, air between the exit of the air end  1  and the minimum pressure valve  106  is blown, and transition to unload operation without water injection is made (S 2005  to S 2006 ). 
         [0098]    When line pressure decrease up to P 3  in the unload operation without water injection, the solenoid valve  125  is closed and the unload operation without water injection is made to proceed to operation with water injection. That is, control in which operation with water injection and operation without water injection are repeated is made by contrasting the pressure P 2 , P 3 , the time t 3 , detected pressure and count time. 
         [0099]    Next, control in a stop will be described referring to  FIGS. 5 and 10 . As operation with water injection is executed in this example when a stop instruction is issued in operation (the timing of T 1  in  FIG. 5 ) (S 2501 ), the injection control valve  107  is closed, the air blow-off solenoid valve  125  is opened, and after the operation with water injection is made to proceed to unload operation without water injection (S 2502  to S 2503 ), the motor  100  is stopped after the operation without water injection continues for the time t 4  (S 2504  to S 2505 ). When a stop instruction is turned on in operation without water injection (S 2501  to S 2502 ), the injection control valve  107  is kept closed, the air blow-off solenoid valve  125  is opened, and the operation without water injection is made to proceed to unload operation without water injection (S 2503 ). After operation without water injection continues for the time t 4 , the motor  100  stopped (S 2504  to S 2505 ). 
         [0100]    In the meantime, when a stop instruction is issued in unload operation without water injection, the motor  100  is stopped after the time t 4  elapses since the stop signal (S 2506 , S 2504 , and S 2505  in this order). However, when the time of unload operation without water injection is counted and the time t 4  already elapses at the time at which the stop instruction is turned on, it is not required that the time t 4  elapses, the motor  100  may be also immediately stopped, and when a total value of elapsed time before the stop instruction is turned on and elapsed time after the stop instruction is turned on exceeds t 4 , the motor may he also stopped. If the motor is automatically stopped when the stop instruction is turned on, the motor is kept stopped as it is (S 2507 , and S 2505  in this order). 
         [0101]    Intervals shown by A 1  to A 4  in  FIG. 5  are equivalent to intervals of operation without water injection. 
         [0102]    Transition to operation without water injection is enabled by adding the air blow-off solenoid valve  125  to the configuration in the first example as described above without exceeding the cut-out pressure and the further sufficient time of operation without water injection can be secured. 
         [0103]    Next, a third example of control and the operation in this example will be described referring to  FIGS. 6 and 11 . A flow for a stop is similar to that in  FIG. 10 . As for the configuration, the variable frequency drive  122  is added to the configuration in the second example of control. That is, control over the rotating speed of the motor  100  is enabled. Items which are not especially described are similar to those in the second example of control. Other configurations may also exist in a range in which it is not against this control. 
         [0104]    When an instruction to initiate operation is turned on in activation, operation without water injection is initiated in a state in which the air blow-off solenoid valve  125  is closed and the injection control valve  107  is closed (S 3001  to S 3002 ). When the time t 1  elapses after the operation is initiated, the injection control valve  107  is opened and the operation without water injection is made to proceed to operation with water injection (S 3003  to S 3004 (. 
         [0105]    When pressure rises and line pressure reaches control pressure (equivalent to the cut-in pressure in this control) P 3 , pressure fixing control according to load fluctuation is executed according to variable frequency control (S 3006 ). That is, as the variable frequency drive  122  is mounted in this example of control, the rotating speed of the motor  100  can be controlled according to an air flow rate required by a customer and hereby, control in which pressure is fixed at the control pressure P 3  is enabled. 
         [0106]    In a case that only a small quantity of an air flow rate is required and line pressure rises even at the minimum rotating speed of the motor by the variable frequency drive  122 , when the line pressure reaches the cut-out pressure P 2 , the injection control valve  107  is closed and the air blow-off solenoid valve  125  is opened, and the operation is made to proceed to unload operation without water injection (S 3007  to S 3008 ). At this time, it is desirable that the rotating speed of the motor  100  is kept at minimum rotating speed by the variable frequency drive  122 . 
         [0107]    In the unload operation without water injection, when the time t 2  elapses in a state in which line pressure is not reduced up to P 3 , it is judged that the supply of air is not required and the motor  100  is stopped (S 3009  to S 3010 ). When compressed air is used at a destination to which air is supplied in this state, line pressure decreases. When line pressure reaches the control pressure (the cut-in pressure) P 3 , operation is resumed. In this example of control, when the injection control valve  107  is closed, operation without water injection is resumed (S 3011  to S 3012 ) and the time t 3  elapses after the operation is resumed, the operation without water injection is made to proceed to operation with water injection (S 3013  to S 3014 ). Afterward, control is returned to the step S 3006  and when line pressure reaches P 2 , the operation with water injection is made to proceed to unload operation without water injection (S 3007  to S 3008 ). 
         [0108]    Next, control when pressure decreases up to the cut-in pressure P 3  (equivalent to an interval A 3  in  FIG. 6 ) before the time t 2  elapses after the pressure reaches the pressure P 2  and transition to the unload operation without water injection is made will be described. In this control, it is desirable that a control parameter of the rotating speed of the motor  100 , that is, a rotating speed instructed value from the variable frequency drive  122  is introduced. This parameter shall be a set value determined as an instructed value between a rotating speed instructed cut-out value and a rotating speed instructed cut-in value. 
         [0109]    When pressure decreases up to P 3  before the time t 2  elapses, the rotating speed of the motor  100  further controlled by the variable frequency drive  122  and the set value are contrasted (S 3009 , S 3015 , and S 3016  in this order). When the rotating speed is slower than the set value, the air blow-off solenoid valve  125  is closed and operation is made to proceed to operation without water injection (S 3017 ). In the meantime, when pressure decreases up to P 3  and further, the rotating speed of the motor  100  is faster than the set value, water injection is initiated and pressure fixing control is made (S 3015 , S 3016 , and S 3020  in this order). 
         [0110]    As pressure fixing control is enabled at the control pressure (the cut-in pressure) P 3  by adding the variable frequency drive  122  as described above, energy can be saved. Besides, as operation without water injection is executed at the minimum rotating speed of the motor at the interval A 3 , the time of operation without water injection after a stop instruction can be minimized when the stop instruction is issued and energy can be saved. Besides, if the motor is also revolved at the minimum rotating speed in operation without water injection after the stop instruction, energy can be saved, compared with a case that no variable frequency drive is provided. Intervals shown as A 1  to A 4  in the drawing are equivalent to intervals of operation without water injection. 
         [0111]    The effects of energy saving that pressure fixing control at the cut-in pressure is enabled and the cut-out pressure P 2  can be set to be lower are acquired by comparing with the second example of control and adding the variable frequency drive  122 . 
         [0112]    While the foregoing has described what are considered to be the best mode and/or other examples, it is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that the teachings may be applied in numerous applications, only some of which have been described herein. It is intended by the following claims to claim any and all applications, modifications and variations that fall within the true scope of the present teachings.