Patent Publication Number: US-2005126383-A1

Title: Fluid circuit system

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a fluid circuit system, which makes it possible to efficiently operate a compressor for supplying compressed air (supply gas), for example, to pneumatic actuators such as cylinders.  
      2. Description of the Related Art  
       FIG. 18  shows an air circuit that uses a conventional technique for driving a plurality of air cylinders attached to each of solenoid-operated valve manifolds.  
      The air circuit  1  comprises a large capacity type compressor  2 , an after cooler  3 , an air tank  4 , a first filter  5 , and an air drier  6 , which are connected in series respectively. Second filters  8 , pressure-reducing valves  9 , pressure gauges  10 , and a plurality of solenoid-operated valve manifolds  11   a ,  11   b , . . . are connected in series respectively via a plurality of branched passages  7   a ,  7   b , . . . on the output side of the air drier  6 . Plural air cylinders  12   a  to  12   c  are connected in parallel to the solenoid-operated valve manifold  11   a . A silencer  14  is connected to a common discharge port  13  of the solenoid-operated valve manifold  11   a , which communicates with atmospheric air.  
      Operation of the air circuit  1  shall be explained schematically. A large volume of compressed air, derived from the compressor  2 , flows through the after cooler  3 , the air tank  4 , the first filter  5 , and the air drier  6  respectively. Accordingly, for example, temperature, humidity, and pulsation are controlled.  
      Compressed air, which has been controlled as described above, is appropriately distributed via the branched passages  7   a ,  7   b , . . . , and then the pressure of the compressed air is reduced to a predetermined pressure corresponding to each of the air cylinders  12   a  to  12   c  by the aid of the pressure-reducing valve  9 . Further, compressed air is supplied from common supply ports  15  to the solenoid-operated valve manifolds  11   a ,  11   b , . . . In the solenoid-operated valve manifold  11   a , compressed air is supplied to one cylinder chamber  16   a  of each of the air cylinders  12   a  to  12   c  via respective unillustrated ports that communicate with the common supply port  15 . Accordingly, the respective air cylinders  12   a  to  12   c  are driven.  
      When each of the air cylinders  12   a  to  12   c  is driven, air derived from the other cylinder chamber  16   b  is discharged to the atmosphere via the common discharge port  13  of the solenoid-operated valve manifolds  11   a ,  11   b , . . . and the silencer  14 .  
      As described above, in the case of the conventional air circuit  1 , when the air cylinders  12   a  to  12   c  are driven, the entire amount of air, which is discharged from the air cylinders  12   a  to  12   c , is discharged to the atmosphere without being reused, whereupon the operation proceeds to the next step.  
      In view of the above, as shown in  FIG. 19 , an exhaust air recovery circuit  20  has been proposed in order to recover and reuse exhausted air while maintaining pressure, as described in “Energy Saving for Pneumatic System,” first edition, written by Naotake ONEYAMA, and published by The Energy Conservation Center, Mar. 31, 2003, pp. 306-307.  
      The exhaust air recovery circuit  20  is constructed such that high pressure air, which is supplied to a head side cylinder chamber  22   a  during the operational stroke of an air cylinder  21 , is recovered in an accumulator  24  via a recovery valve  23  until the pressure is lowered to a predetermined pressure. Meanwhile, air is discharged to the atmosphere from the recovery valve  23  when the pressure is lowered below a predetermined pressure. The low pressure air, which is recovered by the accumulator  24 , is utilized as an air source for a rod side cylinder chamber  22   b , during its return stroke, and thus the air cylinder  21  is driven dually.  
     SUMMARY OF THE INVENTION  
      A general object of the present invention is to provide a fluid circuit system, which enables energy savings by improving the operational efficiency of a compressor for supplying a supply gas in a closed loop fluid circuit in which a gas such as air is circulated.  
      The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a circuit diagram illustrating a fluid circuit system according to a first embodiment of the present invention;  
       FIG. 2  shows characteristics for illustrating the relationship between enthalpy and entropy in the fluid circuit system according to the first embodiment, and in a fluid circuit system according to a first comparative embodiment;  
       FIG. 3  shows a circuit diagram illustrating a fluid circuit system according to a second embodiment of the present invention;  
       FIG. 4  shows a circuit diagram illustrating a fluid circuit system according to a third embodiment of the present invention;  
       FIG. 5  shows another circuit diagram illustrating a fluid circuit system according to the third embodiment of the present invention;  
       FIG. 6  shows a circuit diagram illustrating a fluid circuit system according to a fourth embodiment of the present invention;  
       FIG. 7  shows a circuit diagram illustrating a fluid circuit system according to a fifth embodiment of the present invention;  
       FIG. 8  shows a longitudinal sectional view illustrating in greater detail a manifold frame and a third double tube joint, as shown in  FIG. 7 ;  
       FIG. 9  shows a circuit diagram illustrating a fluid circuit system according to a sixth embodiment of the present invention;  
       FIG. 10  shows a partial magnified vertical sectional view illustrating in greater detail a suction unit and a double tube, as shown in  FIG. 9 ;  
       FIG. 11  shows a circuit diagram illustrating the fluid circuit system according to a first comparative embodiment;  
       FIG. 12  shows a circuit diagram illustrating a fluid circuit system according to a second comparative embodiment;  
       FIG. 13  shows a circuit diagram illustrating a fluid circuit system according to a third comparative embodiment;  
       FIG. 14  shows a schematic arrangement of a compressor provided with a plurality of air-compressing mechanisms;  
       FIG. 15  shows a circuit diagram illustrating a single operational state of the air-compressing mechanisms, which make up the compressor shown in  FIG. 14 ;  
       FIG. 16  shows a circuit diagram illustrating a serial operational state of the air-compressing mechanisms, which make up the compressor shown in  FIG. 14 ;  
       FIG. 17  shows a circuit diagram illustrating a parallel operational state of the air-compressing mechanisms, which make up the compressor shown in  FIG. 14 ;  
       FIG. 18  shows a circuit diagram of an air circuit in accordance with the conventional technique; and  
       FIG. 19  shows a circuit diagram illustrating an exhaust gas recovery circuit in accordance with the conventional technique. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      With reference to  FIG. 1 , reference numeral  100  indicates a fluid circuit system according to a first embodiment of the present invention. In the following inventive and comparative embodiments, the same constitutive components are designated with the same reference numerals, and detailed explanation thereof shall be omitted.  
      The fluid circuit system  100  according to the first embodiment is composed of a pressurized fluid-circulating circuit constructing a closed loop. The fluid circuit system  100  comprises a compressor  106  provided with an air supply port  102  and an air aspiration port  104 , a heat exchange unit (heat exchange mechanism)  108 , which is connected to the air supply port  102  and the air aspiration port  104  of the compressor  106  respectively, a tank  110  which temporarily stores a supply gas, such as compressed air and gas supplied from the compressor  106 , and which suppresses pulsations in the compressed air, a filter  112  which removes dust or the like contained in the compressed air derived from the tank  110 , and an air drier  114  which cools the compressed air having passed through the filter  112 .  
      The fluid circuit system  100  further comprises a filter  118  and a regulator  120 , to which the pressurized fluid from the air drier  114  is supplied, and which are connected to the piping passage of the factory equipment  116 , a solenoid-operated valve manifold  122  which is connected to the output side of the regulator  120 , a plurality of air cylinders  124  which are driven respectively in accordance with the supply of pressurized fluid via a plurality of branched passages connected to the solenoid-operated valve manifold  122 , and speed controllers  126  which adjust the flow rates of compressed air supplied to cylinder chambers of the air cylinders  124 .  
      The heat exchange unit  108  includes a high temperature air supply port  128  connected to the air supply port  102  of the compressor  106 , a first connecting port  130  connected to the tank  110 , a low temperature air supply port  134  connected to a discharge port of the solenoid-operated valve manifold  122  via a first circulating passage  132 , and a second connecting port  138  connected to the air aspiration port  104  of the compressor  106  via a second circulating passage  136 .  
      In this arrangement, the high temperature air supply port  128  and the first connecting port  130  communicate via a first communication passage, and the low temperature air supply port  134  and the second connecting port  138  communicate via a second communication passage.  
      As described above, the compressor  106 , the heat exchange unit  108 , and other components are connected to each other in a closed loop. The compressor  106  may also be connected to an unillustrated supply tank, which initially supplies pressurized fluid to the closed loop, and/or which replenishes the closed loop with a previously stored pressurized fluid when a decrease occurs in the pressurized fluid in the closed loop. Further, pressurized fluid, such as a gas that has been previously produced according to predetermined quality controls, may be stored in the supply tank.  
      The fluid circuit system  100  according to the first embodiment of the present invention is basically constructed as described above. Next, its operations, functions and effects shall be explained.  
      Air, which is aspirated by the compressor  106 , is compressed internally, and the temperature of the compressed air is raised. Compressed air having a high temperature is discharged from the air supply port  102 . The compressed air is then introduced into a high temperature air supply port  128  of the heat exchange unit  108 , and the compressed air is supplied to the tank  110  via the first communication passage and first connecting port  130 .  
      Compressed air derived from the tank  110  passes through the filter  112 , and the compressed air is introduced into the air drier  114 . The air drier  114  functions so that water contained in the compressed air is separated out to provide dry air, and the lowered temperature of the air is restored to a temperature in the vicinity of normal temperature.  
      Usually, in the case of blown air or the like, no problem arises concerning use thereof with compressed air having its quality adjusted to some extent. However, when an actuator system composed of, for example, the solenoid-operated valve manifold  122  and the plurality of air cylinders  124 , is used with the factory equipment  116 , use of compressed air having better quality makes it possible to prolong the service life of the equipment making up the actuator system and maintain a satisfactory operational state. Therefore, it is preferable to use a filter and mist separator.  
      Pressure of the compressed air, which has its quality controlled until arrival at a final stage, is adjusted by the regulator  120 , and the compressed air is supplied to the solenoid-operated valve manifold  122 . A desired air cylinder  124  can be driven by the aid of a solenoid-operated valve, wherein the solenoid-operated valve is energized by an energizing signal fed from an unillustrated controller.  
      During this process, the flow rate of the supplied compressed air is decreased by the speed controller  126 . Thus, the displacement speed of an unillustrated piston in the air cylinder  124  can be adjusted.  
      Air that remains in the cylinder chamber of the air cylinder  124  is discharged from the discharge port of the solenoid-operated valve manifold  122  by a displacement action of the piston. Low temperature exhaust air, which is discharged from the discharge port, is introduced into the heat exchange unit  108  via the first circulating passage  132  and the low temperature air supply port  134 .  
      Heat exchange is effected in the heat exchange unit  108  between the high temperature compressed air (supply gas) introduced from the high temperature air supply port  128  and the low temperature exhaust air (return gas) introduced from the low temperature air supply port  134 . Thermal energy of the high temperature compressed air is migrated to the low temperature exhaust air. Therefore, the temperature of the compressed air is lowered, while the temperature of the exhaust air is raised. Exhaust air, after having its temperature raised, is introduced into the compressor  106  from the air aspiration port  104  via the second circulating passage  136 .  
      Accordingly, it is unnecessary to replenish thermal energy from the outside, and the enthalpy of the air aspirated by the compressor  106  is raised. As a result, thermal energy, which would otherwise be released to the outside, is decreased, thus enabling energy savings.  
      A fluid circuit system  140  according to a first comparative embodiment is shown in  FIG. 11 .  
      The fluid circuit system  140  according to the first comparative embodiment is different from the first embodiment in that an after cooler  142 , which performs cooling using a liquid, is connected to the output side of the compressor  106 . Further, exhaust air, which is discharged from the air cylinders  124 , is discharged to atmospheric air from a silencer  144  connected to the discharge port of the solenoid-operated valve manifold  122 .  
      High temperature compressed air, which is introduced into the after cooler  142  via piping, is cooled by circulating industrial water or oil connected to an oil cooler within cooling piping  146 , and thermal energy is released to atmospheric air. Accordingly, the temperature of the compressed air is lowered.  
      Next,  FIG. 2  shows the relationship between enthalpy and entropy in the fluid circuit system  100  according to the first embodiment and the fluid circuit system  140  according to the first comparative embodiment. The characteristic curves A to F depicted in  FIG. 2  indicate respective states in which the pressures of the compressed air are 0 kgf/cm 2 , 1 kgf/cm 2 , 2 kgf/cm 2 , 3 kgf/cm 2 , 4 kgf/cm 2 , and 5 kgf/cm 2  respectively. The enthalpy (h) indicates the energy (amount of heat) sent/received when a gas, such as compressed air, is subjected to an isobaric change, and the entropy (s) indicates the quantity of state of the gas.  
      When the compressor  106  is used in an ordinary state, for example, air (atmospheric air) or N 2  gas having an enthalpy (h 1 ) is compressed on the basis of a preset compression ratio by the compressor  106  based on, for example, a scroll system, a screw system, a reciprocal system, a vane system, or a turbo system.  
      Compressed air, which has been raised in pressure by the compressor  106 , also has its temperature raised in accordance with an adiabatic change. Since the temperature of the compressed air is in a proportional relationship with respect to enthalpy, the enthalpy of the compressed air is increased to h 2 . During this process, work (power source required for compression), which is performed by the compressor  106 , is W (changing from State 1 to State 2).  
      If compressed air, after being compressed and having acquired a high enthalpy state, is used, as is, for the actuator system, it is feared that troubles may occur by exceeding the applicable temperature ranges of the fluid equipment including, for example, the tank  110 , the regulator  120 , the filter  112 , the piping, the tube joint, the solenoid-operated valve, the flow rate-adjusting valve, and the air cylinder  124 . Accordingly, in general, in order to lower the temperature of the compressed air, adjustments are made to obtain temperatures within the applicable ranges for the fluid equipment using, for example, the after cooler  142  and the refrigerating-type air drier.  
      However, the enthalpy of the compressed air is decreased from h 2  to h 3  as a result of being released to atmospheric air, in accordance with a temperature adjustment (change from State 2 to State 3). This decrease in enthalpy from h 2  to h 3  also indicates the fact that energy Q 1  is released to atmospheric air. In addition, therefore, compressed air having an appropriate temperature performs desired work using the actuator system. Enthalpy is further decreased from h 3  to h 4 , and the pressure of the compressed air is lowered (change from State 3 to State 4).  
      In the fluid circuit system  140  according to the first comparative embodiment, compressed air having an enthalpy h 4 , existing in the closed space between the air cylinders  124  and the solenoid-operated valve manifold  122  of the actuator system, is discharged to atmospheric air when the displacement direction of the piston is switched, in accordance with an energizing/deenergizing action of the solenoid-operated valve. Thus, pressure is lowered while remaining in the same temperature state (change from State 4 to State 1).  
      In contrast, in the fluid circuit system  100  according to the first embodiment, exhaust air, which would be otherwise released to atmospheric air, is not discharged, but instead the exhaust air is introduced again into the heat exchange unit  108  via the first circulating passage  132 . Accordingly, it is possible to increase the enthalpy to h 1 ′. Therefore, a reduction can be achieved, in an amount corresponding to the energy Q 2  required to cause an increase of the compressed air to a state in which the enthalpy is h 2  again (see the hatched portion shown in  FIG. 2 ).  
      Therefore, in the first comparative embodiment, the state is changed, i.e., from State 1→State 2→State 3→State 4→State 1. On the other hand, in the first embodiment, the state is changed, i.e., from State 1′→State 2→State 3→State 4→State 1′. Therefore, operations can be performed more efficiently by reducing the power for the compressor  106 , and an energy saving in an amount corresponding to the energy Q 2  can be achieved.  
      In other words, the first comparative embodiment, which is based on using the after cooler  142  and aimed only at decreasing the temperature of the high temperature compressed air, involves the following problem. That is, the amount of heat generated when the temperature of the compressed air is lowered, is thermally conducted to water or oil, which act as a circulating cooling liquid, and as a result, this amount of heat is released from the fluid circuit to the outside.  
      In contrast, in the first embodiment, a first circulating passage  132  is provided, which connects the heat exchange unit  108  and the discharge port of the solenoid-operated valve manifold  122 . Exhaust air, which has a low temperature, is aspirated into the heat exchange unit  108  during the discharge operation of the air cylinder  124 . Thus, heat exchange is effected in the heat exchange unit  108  between the high temperature compressed air from the compressor  106  and the low temperature exhaust air.  
      Therefore, in the first embodiment, heat exchange is performed such that the temperature of the high temperature compressed air is decreased, while the temperature of the low temperature exhaust air is raised. Exhaust air, which is reheated to the raised temperature, is aspirated via the air aspiration port  104  of the compressor  106 . As a result, in the first embodiment, it is unnecessary to replenish thermal energy from the outside. The enthalpy of the air aspirated by the compressor  106  is raised, and the energy released to the outside is decreased. Thus, substantial energy savings can be obtained.  
      During this procedure, all of the energy Q 1 , corresponding to the endothermic action effected until arrival at the temperature of use in relation to the increase in temperature (increase in enthalpy) caused by air compression performed by the compressor  106 , is not released to the environment (atmospheric air). Rather, the increase in enthalpy (increase in the temperature) of the air aspirated by the compressor  106  is reused. Accordingly, it is possible to decrease the required pressurizing work during the air compression step, and thus it is possible to raise the operation efficiency of the compressor  106 .  
      More specifically, in the case of an ideal gas, there is given Entropy h 1 ≈Entropy h 4 . However, by effecting the above-described heating, is possible to give Entropy h 1 ′&gt;Entropy h 4 . Assuming that W and W′ represent the work (required power) performed by the compressor  106  with respect to the compressed air, the following expressions are obtained. 
 
 W≈h   2 − h   1  
 
 W′≈h   2 − h   1 ′
 
 Therefore, there is given ∇W=W−W′&gt;0 whereby energy savings is achieved in an amount corresponding to ∇W. 
 
      Next,  FIG. 3  shows a fluid circuit system  150  according to a second embodiment, whereas  FIG. 12  shows a fluid circuit system  160  according to a second comparative embodiment. In the following embodiments, detailed explanation of any functions and effects that are the same as those of the first embodiment shall be omitted, and explanations shall be given only where such functions and effects differ.  
      The second embodiment is different from the first embodiment in that the heat exchange unit  108  includes a fan  152  therein, which effects thermal conduction by ventilating the heat from the high temperature compressed air, obtained from the compressor  106 , to the piping through which the low temperature exhaust air flows. The fan  152  is rotatably driven by the aid of an electric motor  154  or the like.  
      In the second comparative embodiment, high temperature compressed air, which is derived from the compressor  106 , is cooled by an air-cooling type after cooler  162 , also based on the use of a fan  152 . However, the second comparative embodiment involves the problem that thermal energy is released to atmospheric air, as the temperature of the compressed air is lowered during this process, and therefore it is impossible to efficiently utilize such thermal energy.  
      Next,  FIGS. 4 and 5  show fluid circuit systems  170 ,  180  according to a third embodiment.  
      The third embodiment is different from the first and second embodiments in that a pressure supply mechanism  190  is connected to the second circulating passage  136  between the heat exchange unit  108  and the compressor  106 .  
      The pressure supply mechanism  190  includes a shuttle valve  192  connected to the second connecting port  138  of the heat exchange unit  108 , a regulator  196  connected to another compressed air supply source  194 , wherein the regulator  196  reduces to a predetermined pressure the pressure of the replenished compressed air supplied to the shuttle valve  192 , and an exhaust air recovery tank  198  arranged between the shuttle valve  192  and the compressor  106 .  
      Exhaust air, which is derived from the heat exchange unit  108  via the second connecting port  138 , is introduced into the shuttle valve  192 . When the pressure is higher than the pressure of the low pressure compressed air preset by the regulator  196 , the exhaust air is supplied, as is, to the exhaust air recovery tank  198  via the shuttle valve  192 . Further, exhaust air is introduced into the compressor  106  via the air aspiration port  104  of the compressor  106 . During this process, air is not consumed from the compressed air supply line  199 , since the compressed air supply line  199  forms a separate line connected to another compressed air supply source  194 .  
      When the flow rate of the compressed exhaust air decreases, and the pressure in the exhaust air recovery tank  198  becomes lower than the preset pressure of the regulator  196 , then the shuttle valve  192  is operated. Thus, compressed air, which is fed from the compressed air supply line  199 , and which as noted above forms a separate line connected to another compressed air supply source  194 , is supplied into the exhaust air recovery tank  198  via the shuttle valve  192 .  
      By providing the pressure supply mechanism  190  and the exhaust air recovery tank  198 , a constant discharge side pressure (pressure of the exhaust air) is always retained. Therefore, it is possible to stably operate the compressor  106  for effectively utilizing the exhaust air.  
      The fluid circuit system  180  shown in  FIG. 5  is different from the fluid circuit system  170  shown in  FIG. 4  in that a fan  152  and an electric motor  154  are provided. Otherwise, the arrangement is the same.  
       FIG. 6  shows a fluid circuit system  200  according to a fourth embodiment.  
      Exhaust air, which is aspirated from an aspiration port  202  via an unillustrated circulating passage, passes through an air cleaner  204  to remove dust or the like contained in the air, and the exhaust air is introduced into an aspiration volume-adjusting valve  206 . Heat exchange piping  212 , which is immersed in lubricating oil  211  contained in an oil chamber  208 , is arranged between the air cleaner  204  and the aspiration volume-adjusting valve  206 . Low temperature air, which has passed through the air cleaner  204 , is introduced into the aspiration volume-adjusting valve  206  via the heat exchange piping  212 .  
      The aspiration volume-adjusting valve  206  operates such that, when it is detected that the pressure of air introduced into the inside of the aspiration volume-adjusting valve  206  is raised higher than a preset pressure, the aspiration passage is closed in order to avoid excessive aspiration of air.  
      Air that has passed through the aspiration volume-adjusting valve  206  is introduced into the compressor  106 , whereupon the air is compressed to provide compressed air. The compressor  106  may be based on, for example, any one of a twin screw system, a scroll system, a reciprocal system, a vane system, and a turbo system.  
      In the compressor  106 , a rotational force from a motor  209  driven by a supplied power source is transmitted from a small pulley  213  to a large pulley  214 , by means of a rotary driving force-transmitting means, such as a V-belt  210  and a timing belt, wherein the number of revolutions is decreased and the speed of rotation is decelerated. Using this arrangement, when the pulley diameter is exchanged to other sizes, it is possible to change the number of revolutions provided by an inexpensive induction motor, which rotates at a constant number of revolutions. As a result, it is possible to change the discharge flow rate of the compressed air.  
      Compressed air, having been raised in pressure by the compressor  106 , is released from the output port  216  into the oil chamber  208 . Lubricating oil  211 , to which the heat of the compressed air having a high temperature is transmitted, is accommodated in the oil chamber  208 . The high temperature compressed air is mixed with the lubricating oil  211 . The lubricating oil  211 , thus having been raised in temperature, is extruded by the internal pressure of the oil chamber  208  and the lubricating oil  211  flows into an oil cooler  218 . The temperature of the lubricating oil  211  is lowered in the oil cooler  218 . Further, when the lubricating oil  211  passes through an oil filter  220 , impurities such as dust and metal powder contained in the lubricating oil  211  are filtered out. The lubricating oil  211  is fed into the compressor  106 , and the lubricating oil  211  is returned to the oil chamber  208  along with compressed air, again through operation of the compressor  106 .  
      Compressed air, which contains the lubricating oil  211  in the oil chamber  208 , passes through an oil separator  222  for removing the oil component. The compressed air is then fed from a chamber outlet  224  to a pressure-keeping valve  226 . Lubricating oil  211 , having been separated by the oil separator  222 , is extruded as a result of the internal pressure inside the oil chamber  208 , and the lubricating oil  211  is returned to the compressor  106  via piping  228 .  
      Compressed air that is introduced into the pressure-keeping valve  226  has a pressure, which is automatically regulated so that the pressure is more than a prescribed value inherent in every compressor  106 . Accordingly, deterioration of the separating function of the oil separator  222  is avoided beforehand, along with any shortage in the lubricating amount of the lubricating oil  211  caused thereby. Further, excessive heating of the compressor  106 , which would be otherwise caused by a decrease in the pressure when the air consumption amount is larger than the discharge air flow rate, can be avoided. The pressure in the oil chamber  208  is branched at the chamber outlet  224 , so as to communicate and connect with the subsequent equipment.  
      In  FIG. 6 , reference numeral  230  indicates a discharge air pressure gauge by which the pressure can be observed, reference numeral  232  indicates a safety valve which automatically reduces the pressure in the oil chamber  208  whenever the pressure is abnormal, and reference numeral  234  indicates a manual release valve which is opened manually when the pressure in the oil chamber  208  is in a reduced state, for example, when replenishing the lubricating oil  211  after stopping the compressor  106 .  
      Reference numeral  236  indicates a solenoid-operated valve, which is designed as follows. That is, the solenoid-operated valve  236  is normally in an ON state. When compressed air, which is branched from the chamber outlet  224 , is directly introduced into the aspiration volume-adjusting valve  206  and the compressed air has a pressure not less than the preset pressure, then the solenoid-operated valve  236  is placed in an OFF state. Moreover, when it is detected, by a pressure switch  238 , that the pressure of the compressed air discharged from the secondary side of the pressure-keeping valve  226  is lower than a preset pressure, then the solenoid-operated valve  236  is placed in an OFF state on the basis of a detection signal derived from the pressure switch  238 , and the aspiration volume-adjusting valve  206  is opened again to form compressed air.  
      In the fourth embodiment, during the driving of the air cylinders, exhaust air aspirated from the aspiration port  202  passes through the air cleaner  204 , and then the exhaust air flows along the heat exchange piping  212  immersed in lubricating oil  211  in the oil chamber  208 . In the heat exchange piping  212  it is preferable for fins  240 , which increase the coefficient of thermal conductivity, to be formed on an outer circumferential surface of the piping, for example, as in an ever-fin tube.  
      When exhaust air passes through the heat exchange piping  212 , then the enthalpy of the exhaust air is increased by the high temperature lubricating oil  211 , and exhaust air having been raised in temperature is aspirated and compressed by the compressor  106 . As a result, the amount of thermal energy released to atmospheric air from the oil cooler of the compressor  106  is decreased, and work necessary for performing the compressing step by the compressor  106  is decreased. Thus, it is possible to achieve energy savings.  
       FIG. 13  shows a fluid circuit system  250  according to a third comparative embodiment.  
      The fluid circuit system  250  according to the third comparative embodiment has a structure wherein atmospheric air, at an ordinary temperature, is aspirated as is, thereby making up the air that is aspirated by the compressor  106 . Heat exchange piping  212  is not provided. With this structure, it is impossible to decrease the thermal energy during the compressing step performed by the compressor  106 .  
      Next,  FIGS. 7 and 8  show a fluid circuit system  300  according to a fifth embodiment of the invention, which is based on the use of double tubes.  
      High temperature compressed air derived from a compressor  106  passes through an inner tube of a first double tube  304 , via a first double tube joint  302  connected to the compressor  106 , and the compressed air is introduced into a manifold frame  308  via a second double tube joint  306  disposed on a primary side.  
      As shown in  FIG. 8 , the manifold frame  308  is provided with a first chamber  312  disposed on a lower side and a second chamber  314  disposed on an upper side, both of which are constructed separately by a partition wall  310  arranged at a substantially central portion. A plurality of third double tube joints  316  is connected to the secondary side of the manifold frame  308 . An inner tube passage  322  of the second double tube  320 , which is connected with the third double tube joint  316 , is connects and communicates with the first chamber  312  disposed on the lower side by means of an inner collector  318  provided for the third double tube joint  316 . An outer tube passage  324  of the second double tube  320  connects and communicates with the second chamber  314  disposed on the upper side (see  FIG. 8 ).  
      In this arrangement, high temperature compressed air, which has passed through the inner tube passage  326  of the first double tube  304 , is introduced into the first chamber  312  disposed on the lower side of the manifold frame  308 . Further, the compressed air passes through the inner collector  318  of the third double tube joint  316 , so as to flow along the inner tube of the second double tube  320 . Compressed air also is introduced into a solenoid-operated valve  332 , to which a fourth double tube joint  328  on the primary side, and a fifth double tube joint  330  on the secondary side, are connected respectively.  
      The solenoid-operated valve  332 , to which the fourth double tube joint  328  on the primary side and the fifth double tube joint  330  on the secondary side are connected respectively, has a structure that differs from that of a conventional double tube solenoid-operated valve in the following point. That is, a conventional double tube solenoid-operated valve has a compressed air supply port composed of a single tube system. Further, double tube joints are connected only to an A port and a B port, which are connected to a cylinder. The supply and discharge ports are ports for the single tube.  
      In contrast, the solenoid-operated valve  332  that is incorporated into the fluid circuit system  300  according to the fifth embodiment differs in that a supply port  334  and a discharge port  336  are integrally connected by a fourth double tube joint  328  disposed on the primary side respectively. The fifth double tube joint  330 , disposed on the secondary side of the solenoid-operated valve  332 , is constructed in the same manner as in a conventional solenoid-operated valve, having a piping structure in which the distribution is effected with an A port  338  and a B port  340 .  
      One end of a third double tube  342  is connected to the fifth double tube joint  330  disposed on the secondary side of the solenoid-operated valve  332 . A T-joint  344 , which distributes the double tube into single tubes, is connected to the other end of the third double tube  342 . A pair of speed controllers  346  is connected via single tubes on the downstream side of the T-joint  344 . Compressed air is introduced into the speed controller  346  via the inner tube of the third double tube  342  and the T-joint  344 . Compressed air, the pressure of which is reduced to a predetermined pressure by the speed controller  346 , is supplied to one cylinder chamber in which a piston is accommodated, via the A port  338  or the B port  340  of a cylinder  348 . Thus, work is performed to press the piston.  
      During the exhausting step, exhaust air, which is discharged from the other cylinder chambers after completion of the work, is gathered in the outer tube passage formed between the inner tubes and the outer tubes of the first to third double tubes  304 ,  320 ,  342 . Exhaust air that is discharged via the outer tube passage passes through an outer circumferential passage  350  of the inner collector  318  of the third double tube joint  316 , and the exhaust air is introduced into the second chamber  314  disposed on the upper side of the manifold frame  308 . In this situation, heat exchange is effected by means of heat release fins  352  formed on the partition wall  310  between the first chamber  312  disposed on the lower side to which high temperature compressed air is supplied and the second chamber  314  disposed on the upper side to which low temperature exhaust air is supplied. The temperature of the high temperature compressed air is lowered, and the temperature of the low temperature exhaust air is raised. Exhaust air, the enthalpy of which is raised by heat exchange, passes through the outer tube passage of the first double tube  304  after passing through the second double tube joint  306 . The exhaust air is aspirated into the internal aspiration port via the first double tube joint  302  connected to the compressor  106 .  
      The manifold frame  308  is produced by means of extrusion forming using a light metal such as aluminum, having an identical cross-sectional shape in the axial direction. The manifold frame  308  can be designed to have any arbitrary length in the axial direction corresponding to, for example, the number of cylinders  348  and the number of solenoid-operated valves  332 .  
      Next, a fluid circuit system  400  according to a sixth embodiment is shown in  FIGS. 9 and 10 .  
      The fluid circuit system  400  comprises a compressor  106 , a first tank  402  connected to an air supply port  102  of the compressor  106 , a solenoid-operated valve manifold  406  and air cylinders  408  connected to the output side of the first tank  402  via a first passage  404 , a vacuum pump mechanism  412  connected to the output side of the first tank  402  via a second passage  410  branched from the first passage  404 , and a second tank  416  connected to the solenoid-operated valve manifold  406  and the suction side of the vacuum pump mechanism  412  via a third passage  414 .  
      Positively pressurized air is stored in the first tank  402 , and negatively pressurized air is stored in the second tank  416 .  
      As shown in  FIG. 10 , a suction unit  424 , to which a suction pad  422  is installed via a double tube  418  and a double tube joint  420 , is connected to the vacuum pump mechanism  412 . In this arrangement, fluid at a negative pressure is supplied along an inner tube passage  426  formed by an inner tube of the double tube  418 . Thus, an unillustrated workpiece may be sucked by the suction pad  422 .  
      On the other hand, fluid having a positive pressure for breaking the vacuum, which is used to disengage the workpiece, is supplied via an outer tube passage  428  formed between the inner tube and the outer tube of the double tube  418 . When the double tube  418  is provided so that positively pressurized fluid and negatively pressurized fluid are allowed to flow in an integrated manner via the double tube  418 , it is possible to prevent the positively pressurized fluid from becoming contaminated with dust or the like.  
      Next,  FIG. 14  shows a schematic structure of a compressor  106 .  
      The compressor  106 , which is usable in the embodiments of the present invention, may be provided with a single air-compressing mechanism. However, as shown in  FIG. 14 , the compressor  106  may also be provided with plural air-compressing mechanisms  502 , including a first air-compressing mechanism  500   a  and a second air-compressing mechanism  500   b.    
      The compressor  106  comprises a servo motor  506  or an induction motor employing an inverter control in which, for example, the number of revolutions and rotational torque are controlled on the basis of a control signal supplied by a control unit  504 , a bevel gear mechanism  510  including second and third bevel gears  508   b ,  508   c  meshed with a first bevel gear  508   a  coupled to a motor shaft of the servo motor  506  (or induction motor). First and second electromagnetic clutches  512   a ,  512   b  are connected to the second and third bevel gears  508   b ,  508   c  respectively, for transmitting the rotational force of the servo motor  506  to a first air-compressing mechanism  500   a  and/or a second air-compressing mechanism  500   b , on the basis of an energizing/deenergizing signal supplied from the control unit  504 .  
      The compressor  106  further comprises a first air tank  516   a  provided in a passage communicating between an air aspiration port  104  and a directional control valve  514 , and a second air tank  516   b  provided in a passage communicating between an air supply port  102  and the directional control valve  514 . The directional control valve  514  changes the supply of air introduced from the air aspiration port  104  to any one of the first and second air-compressing mechanisms  500   a ,  500   b , and further, branches air introduced from the air aspiration port  104  to change the supply to both of the first and second air-compressing mechanisms  500   a ,  500   b . The compressor also comprises first to fourth changeover passages  518   a  to  518   d  which connect the directional control valve  514  and the first and second air-compressing mechanisms  500   a ,  500   b , along with first and second pressure sensors  520   a ,  520   b  which detect the respective pressures of air stored in the first and second air tanks  516   a ,  516   b  to supply detection signals to the control unit  504  respectively.  
       FIG. 15  shows a state in which only the second air-compressing mechanism  500   b  is operated alone, by cutting off the connection of the first electromagnetic clutch  512   a  that transmits rotational torque to the first air-compressing mechanism  500   a  on the basis of the energizing/deenergizing signal output from the control unit  504 . In this case, air introduced from the air aspiration port  104  is supplied to the second air-compressing mechanism  500   b  via the first air tank  516   a , the directional control valve  514 , and the fourth changeover passage  518   d . Compressed air, which is compressed by the second air-compressing mechanism  500   b , is discharged from the air supply port  102  via the second changeover passage  518   b , the directional control valve  514 , and the second air tank  516   b.    
      Respective pressures of the air contained in the first air tank  516   a  and the second air tank  516   b  are detected by the first and second pressure sensors  520   a ,  520   b , and detection signals therefrom are introduced into the control unit  504 . Of course, it is also allowable that only the first air-compressing mechanism  500   a  is operated alone, by cutting off connection to the second electromagnetic clutch  512   b  in contrast to the arrangement shown in  FIG. 15 .  
       FIG. 16  shows a serial operational state, in which air introduced from the air aspiration port  104  is compressed in two stages using the first and second air-compressing mechanisms  500   a ,  500   b . In this case, air introduced from the air aspiration port  104  is initially supplied to the first air-compressing mechanism  500   a  via the directional control valve  514  and the third changeover passage  518   c , and thereafter, compressed air compressed by the first air-compressing mechanism  500   a  is returned to the directional control valve  514  via the first changeover passage  518   a . Subsequently, compressed air is further compressed by the second air-compressing mechanism  500   b . Air, compressed at the two stages as described above, is discharged from the air supply port  102 .  
      As described above, air introduced from the air aspiration port  104  is compressed in two stages in accordance with serial operation of the first and second air-compressing mechanisms  500   a ,  500   b . Accordingly, the compressed air pressure can be approximately twice that of its original pressure.  
       FIG. 17  shows a parallel operational state in which air introduced from the air aspiration port  104  is distributed via the first and second air-compressing mechanisms  500   a ,  500   b . In this case, air introduced from the air aspiration port  104  is supplied to both the first air-compressing mechanism  500   a  and the second air-compressing mechanism  500   b  via the directional control valve  514 . Compressed air output from the first and second air-compressing mechanisms  500   a ,  500   b  is returned and merged in the directional control valve  514  respectively, whereupon air is discharged from the air supply port  102 .  
      As described above, the flow rate of compressed air discharged from the air supply port  102  can be approximately twice the original flow rate, by individually and simultaneously operating the first and second air-compressing mechanisms  500   a ,  500   b  and through parallel operation of the first and second air-compressing mechanisms  500   a ,  500   b.    
      As described above, the compressor  106  can arbitrarily provide switching and control operations to enable a single operational state, a serial operational state, or a parallel operational state, as shown in FIGS.  15  to  17 , through control of the switching signal from the control unit  504  to the directional control valve  514 , the energizing/deenergizing signal supplied to the first electromagnetic clutch  512   a  and/or the second electromagnetic clutch  512   b , and the control signal supplied to the servo motor  506 , corresponding to the state of air consumption.  
      While the invention has been particularly shown and described with reference to preferred embodiments, it will be understood that variations and modifications can be effected thereto by those skilled in the art without departing from the spirit and scope of the invention as defined by appended claims.