Patent Publication Number: US-2005118035-A1

Title: Multistage dry vacuum pump

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is based on and claims priority under 35 U.S.C. § 119 to Japanese Patent Application 2003-397520, filed on Nov. 27, 2003, the entire content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      This invention generally relates to a vacuum pump. More particularly, this invention generally relates to a multistage dry vacuum pump with low electricity consumption and large exhaust rate in wide range of degree of vacuum.  
     BACKGROUND  
      Conventionally, a multistage dry vacuum pump having a pair of shafts supporting plurality of rotors provided in a housing is known. The multistage dry vacuum pump includes a plurality of pump chambers each accommodating a pair of rotors. There are slight clearance between the pair of rotors in each pump chambers and between the rotor and an inner wall of the housing. The pair of rotors are rotated in opposite direction at high speed to compress fluid sucked from a main inlet of the housing and transport from an earlier pump chamber to a later pump chamber serially and exhaust the fluid from the main inlet of the housing to atmosphere.  
      This kind of the multistage dry vacuum pump compresses and exhausts fluid sucked from the inlet of the pump chamber against pressure applied from downstream. A compressing work is defined as the amount of work to exhaust the fluid to downstream from the outlet against the pressure. Particularly, the amount of compressing work in proportion to pressure and a scavenging volume becomes maximum at the last pump chamber because pressure of an outlet of the last pump chamber is the same as atmospheric pressure. In this case, because compressing work is in proportion with the scavenging volume of the pump chamber as described above, the smaller the scavenging volume of the last pump chamber becomes, the smaller the compressing work becomes. Accordingly, by reducing the scavenging volume of the pump chamber of downstream side, the compressing work can be reduced. Therefore, electricity consumption can also be reduced.  
      In order to reduce the scavenging volume of a downstream pump chamber, a known pump is structured that a thickness of each rotors supported by each of a pair of shafts and accommodated in each pumps becomes thinner in the downstream pump chamber to reduce its scavenging volume of the downstream pump chamber. Further, JP2002-364569A describes that the number of blades of the rotor for the Roots pump becomes larger in a later pump chamber to reduce the scavenging volume thereof. Further, JP2003-155988A describes a multistage dry vacuum pump with an assistant pump connected to an outlet of a later pump chamber of the multistage dry vacuum pump to make combination of two exhaust systems.  
      In the multistage Roots vacuum pump according to JP2002-364569A, thickness of the rotors for the Roots pump are reduced and the number of the blades of the rotors for the Roots pump is increased in the later pump chamber to reduce electricity consumption, for example, by making the scavenging volume of the last pump chamber provided at the main outlet side about 25% of the scavenging volume of the first pump chamber provided at the main inlet side, or the like. In case that the scavenging volume becomes smaller in the later pump chamber as mentioned above, when sucking pressure is such high as equal to or higher than 10000 Pa, the sucking pressure of the later pump chamber can exceed pressure of the outside of the multistage dry vacuum pump (atmospheric pressure, or the like). On the other hand, pressure of the outlet of the last pump is constant and the same as pressure of the outside of the multistage dry vacuum pump (atmospheric pressure). Therefore, these pump chambers become resistance for fluid flow. These causes increase in electricity consumption and rapid decrease in exhaust rate.  
      In addition, the pump described in JP2003-155988A includes the assistant pump with a small scavenging volume provided at the pump body and an exhaust conduit having a one-way valve letting fluid flow to atmosphere provided in parallel with the assistant pump. Therefore, decrease in exhaust rate and increase in electricity consumption at the high sucking pressure area described in JP 2002-364569A can be solved in some degree. However, the pump has two exhaust systems, which causes more complicated structure, lower reliability, and high manufacturing cost of the pump caused by increase of the number of parts for the pump including piping system, and causes lower efficiency and larger installation space of the pump caused by combination of pumps.  
      A need thus exists for a multistage dry vacuum pump having high exhaust rate and low electricity consumption at wide vacuum range from high pressure to low pressure of sucking pressure of exhausted fluid and good operationality and simple structure to make it compact at lower manufacturing cost.  
     SUMMARY OF THE INVENTION  
      According to an aspect of the present invention, a multistage dry vacuum pump includes a housing having a main inlet and a main outlet, a plurality of pump chambers formed in the housing in series and spatially connecting the main inlet with the main outlet, each pump chamber having an inlet connected to upstream and an outlet connected to downstream, a plurality of rotors each rotatably provided in the each pump chamber for transporting fluid in a scavenging space defined in the pump chamber through the outlet of the pump chamber to the downstream by rotating, a shaft connected to each rotor provided in the each pump chamber for rotating the each rotor synchronously, a rotation driving means connected to the shaft, an intermediate exhaust conduit, one end thereof being connected to the outlet of the pump chamber located at other than the most downstream side and the other end thereof being opened to outside, and a first fluid flow control means provided in the intermediate exhaust conduit for closing the intermediate exhaust conduit when the fluid pressure in the one end of the intermediate exhaust conduit is lower than that in the other end of the intermediate exhaust conduit and for opening the intermediate exhaust conduit when the fluid pressure in the one end of the intermediate exhaust conduit is equal to or higher than that in the other end of the intermediate exhaust conduit.  
      According to another aspect of the present invention, a multistage dry vacuum pump includes a housing having a main inlet and a main outlet, a plurality of pump chambers formed in the housing in series and spatially connecting the main inlet with the main outlet, each pump chamber having an inlet connected to upstream side and an outlet connected to downstream side, drive and dependent rotors provided rotatably in the each pump chamber, the drive rotor and the dependent rotor transporting fluid in a scavenging space defined in the pump chamber through the outlet of the pump chamber to the downstream by synchronously rotating, a drive shaft connected to each drive rotor provided in the each pump chamber for synchronously rotating the each drive rotor, a dependent shaft connected to each dependent rotor provided in the each pump chamber for synchronously rotating the each dependent rotor, a rotation driving means connected to the drive shaft, a transmitting means for transmitting rotation of the drive shaft to the dependent shaft, an intermediate exhaust conduit, one end thereof being connected to the outlet of the pump chamber located at other than the most downstream side and the other end thereof being opened to outside, and a first fluid flow control means provided in the intermediate exhaust conduit for closing the intermediate exhaust conduit when the fluid pressure in the one end of the intermediate exhaust conduit is lower than that in the other end of the intermediate exhaust conduit and for opening the intermediate exhaust conduit when the fluid pressure in the one end of the intermediate exhaust conduit is equal to or higher than that in the other end of the intermediate exhaust conduit.  
      According to another aspect of the present invention, a multistage dry vacuum pump includes a housing having a main inlet and a main outlet, a plurality of pump chambers formed in the housing in series and spatially connecting the main inlet with the main outlet, each pump chamber having an inlet connected to upstream side and an outlet connected to downstream side, a plurality of rotors each rotatably provided in the each pump chamber for transporting fluid in a scavenging space defined in the pump chamber from the outlet of the pump chamber to the downstream by rotating, a shaft connected to each rotor provided in the each pump chamber for rotating the each rotor synchronously, a rotation driving means connected to the shaft, and a pressure adjusting means for adjusting the fluid pressure in the outlet side of the pump chamber to be equal to or lower than atmospheric pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and additional features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawings, wherein:  
       FIG. 1  shows a vertical cross-sectional view of a multistage dry vacuum pump according to a first embodiment of the present invention.  
       FIG. 2  shows a transverse cross-sectional view taken on line II-II of  FIG. 1  according to the first embodiment of the present invention.  
       FIG. 3  shows a vertical cross-sectional view of the multistage dry vacuum pump according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      Embodiments of the present invention will be explained with reference to the illustrations of the drawing figures as follows.  
      A first embodiment of the present invention will be explained with reference to the illustrations of the drawing figures as follows.  FIG. 1  shows a vertical cross-sectional view of a multistage dry vacuum pump according to the first embodiment of the present invention.  
       FIG. 2  shows a transverse cross-sectional view taken on line II-II of  FIG. 1 . Generally, a multistage dry vacuum pump has 4-6 compression steps. In the following example, a multistage dry vacuum pump having four compression steps will be explained.  
      As shown in the figures, the multistage dry vacuum pump includes a housing  2 , a plurality of rotors (first rotors  12   a  and  12   b , second rotors  13   a  and  13   b , third rotors  14   a  and  14   b , and fourth rotors  15   a  and  15   b ), each pair provided in each pump chamber  8 ,  9 ,  10 ,  11  formed in the housing  2 , a pair of shaft (a first shaft  16   a  and a second shaft  16   b ) rotatably supported in the housing  2  having the four-steps pump chambers  8 ,  9 ,  10 ,  11 , and a shaft driving means  20  serving as a rotation driving means connected to the shaft  16   a.    
      As shown in  FIG. 1 , the housing  2  is made of metallic material such as iron, aluminum in cylindrical shape. The housing  2  includes a main inlet  3  and a main outlet  4 . The housing  2  further includes the plurality of pump chambers  8 ,  9 ,  10 ,  11  (in following example, four-steps) at the inside thereof as described above. These four pump chambers are divided by walls  5 ,  6 ,  7  each other. These four pump chambers, that is, a first pump chamber  8 , a second pump chamber  9 , a third pump chamber  10 , a fourth pump chamber  11  are connected in series and in this order from the main inlet  3  to the main outlet  4 . An inlet of the first pump chamber  8  is serving as the main inlet  3 . An outlet  29  of the fourth pump chamber  10  is connected to the main outlet  4  via a main exhaust conduit  31 .  
      As shown in  FIG. 1 , a width (thickness) of the each pump chamber is set to become smaller in order of the pump chambers  8 ,  9 ,  10 ,  11 . In other words, as shown in  FIG. 1 , the each pump chamber is formed to fill a relation T 1 &gt;T 2 &gt;T 3 &gt;T 4 , where T 1  is a width of the first pump chamber  8 , T 2  is a width of the second pump chamber  9 , T 3  is a width of the third pump chamber  10 , and T 4  is a width of the fourth pump chamber  11 . Further, the each pump chamber  8 ,  9 ,  10 ,  11  accommodates the each pair of the rotor. Because the each pump chamber fulfills the relation as described above, a thickness of the each pair of rotor  12   a  and  12   b ,  13   a  and  13   b ,  14   a  and  14   b ,  15   a  and  15   b  is also determined by the width of the each pump chamber described above.  
      As representatively shown in  FIG. 2 , the pair of cocoon-shaped second rotor  13   a  and  13   b  is rotatably provided in the second pump chamber  9 . Similarly, the pair of cocoon-shaped first rotor  12   a  and  12   b  (third rotor  14   a  and  14   b , fourth rotor  15   a  and  15   b ) is rotatably provided in the first pump chamber  8  (the third pump chamber  10 , the fourth pump chamber  11 ).  
      Further, the pair of the shaft  16   a  and  16   b  is penetrating the pump chambers and rotatably supported in the housing  2 . One of the pair of first rotor  12   a , one of the pair of the second rotor  13   a , one of the pair of the third rotor  14   a , one of the pair of the fourth rotor  15   a  are serially connected to the same first shaft  16   a . The other of the first rotor  12   b , the other of the second rotor  13   b , the other of the third rotor  14   b , the other of the fourth rotor  15   b  are serially connected to the same second shaft  16   b . Accordingly, the one of the first rotor  12   a , the one of the second rotor  13   a , the one of the third rotor  14   a , the one of the fourth rotor  15   a  are synchronously rotated in accordance with a rotation of the first shaft  16   a . Similarly, the other of the first rotor  12   b , the other of the second rotor  13   b , the other of the third rotor  14   b , the other of the fourth rotor  15   b  are synchronously rotated in accordance with rotation of the second shaft  16   b.    
      The pump chambers  8  and  9  being adjacent each other provided in the housing  2  are connected by a first fluid transport conduit  17 . Similarly, the pump chambers  9  and  10  being adjacent each other provided in the housing  2  are connected by a second fluid transport conduit  18 . The pump chambers  10  and  11  being adjacent each other provided in the housing  2  are connected by a third fluid transport conduit  19 . The main inlet  3  and the main outlet  4  are spatially connected via these pump chambers and these fluid transport conduits to compress fluid sucked from the main inlet  3  in the four pump chambers and to transport it through the fluid transport conduits serially and to exhaust it from the main outlet  4  to atmosphere.  
      Further, the main exhaust conduit  31  is provided in the housing  2 . One end of the main exhaust conduit  31  is connected to the outlet  29  of the fourth pump chamber  11 . The other end of the main exhaust conduit  31  is connected to the main outlet  4  via a confluent chamber  40  of the exhaust conduits. Further, an intermediate exhaust conduit  30  is provided in parallel with the main exhaust conduit  31 . One end of the intermediate exhaust conduit  30  is connected to an outlet  28  of the third pump chamber  10 . The other end of the intermediate exhaust conduit  30  is connected to the main outlet  4  via the confluent chamber  40  of the exhaust conduits. Accordingly, the other end of the main exhaust conduit  31  and the other end of the intermediate exhaust conduit  30  are connected to the main outlet  4  via the confluent chamber  40  of the exhaust conduits, in other words, the other ends of the both conduits  31 ,  30  are connected to the outside (atmosphere) through the main outlet  4 .  
      A one-way valve  32  letting fluid flow to the outside (atmosphere) serving as a first fluid flow control means is provided in the intermediate exhaust conduit  30 . The one-way valve  32  includes a valve seat  32   b , a sphere  32   c , and a spring  32   d  in a valve chest  32   a . The sphere  32   c  contacts with the valve seat  32   b  to close the intermediate exhaust conduit  30  by biasing force of the spring  32   d.    
      As shown in  FIG. 1 , biasing force of the spring  32   d  is applied from the outside (atmosphere). When the fluid pressure in the outlet  28  side of the third pump chamber  10  is lower than that of the outside (atmosphere), the pressure difference therebetween is applied to the spring  32   d  in the same direction as biasing force thereof. Therefore, the sphere  32   c  contacts with the valve seat  32   d  to close the intermediate exhaust conduit  30  more firmly.  
      On the other hand, when the fluid pressure of the outlet  28  side of the third pump chamber  10  is higher than that of the outside (atmosphere), force generated from the pressure difference therebetween is applied to the spring  32   d  against the biasing force thereof. When the force is larger than the biasing force of the spring  32   d , the sphere  32   c  is separated from the valve seat  32   d  to open the intermediate exhaust conduit  30 . Accordingly, When the fluid pressure of the fluid sucked from the main inlet  3  is high and sucking pressure of the forth pump chamber  11  (exhaust pressure of the third pump chamber  10 ) is higher than atmospheric pressure, part of the sucked fluid is exhausted from the outlet  28  connected to the fluid transport conduit  19  and the inlet of the fourth pump chamber  11  via the intermediate exhaust conduit  30  and the one-way valve  32  to atmosphere. Here, it is preferable that the biasing force of the spring  32   d  is as small as possible for energy saving.  
      Here, it is needless to say that when the sucking pressure of the fourth pump chamber  11 , in other words, the exhaust pressure of the third pump chamber  10 , is lower than the atmospheric pressure, atmospheric air does not flow back from the one-way valve  32  letting fluid flow to atmosphere into the multistage dry vacuum pump  1  via the intermediate exhaust conduit  30 .  
      The main inlet  3  side of the housing  2  is integral with a side cover  22  of the main inlet  3  side. The main outlet  4  side of the housing  2  is integral with a side cover  23  of the main outlet  4  side. Two bearings  24   a  and  24   b  of the main inlet  3  side is provided at the side cover  22  of the main inlet  3  side. Two bearings  25   a  and  25   b  of the main outlet  4  side is provided at the side cover  23  of the main outlet  4  side. The bearings  24   a  and  25   a  rotatably support the first shaft  16   a . The bearings  24   b  and  25   b  rotatably support the second shaft  16   b.    
      As shown in  FIG. 1 , timing gears  21   a  and  21   b  are engaged to one ends of the shafts  16   a  and  16   b  respectively to rotate the pair of shaft  16   a  and  16   b  synchronously and in opposite direction each other. A motor  20  serving as the rotation driving means is connected to the other end of the first shaft  16   a , the other end not engaged to the timing gear  21   a . Accordingly, the shaft  16   a  is serving as a drive shaft, and the shaft  16   b  is serving as a driven shaft. Further, the rotors  12   a ,  13   a ,  14   a  and  15   a  connected to the shaft  16   a  are serving as drive rotors, and the rotors  12   b ,  13   b ,  14   b  and  15   b  connected to the shaft  16   b  are serving as driven rotors.  
      A gear cover  26  is provided around the timing gears  21   a  and  21   b . As shown in  FIG. 1 , the gear cover  26  is attached to opposing side of the side cover  23  of the main outlet  3  side to the housing  2 . The gear cover  26  accommodates the timing gears  21   a ,  21   b  and oil  27  for lubricating the timing gears  21   a ,  21   b  and the bearings  25   a ,  25   b . Meanwhile, the bearing  24   a  and  24   b  are lubricated by grease.  
      The pair of the rotor  13   a  and  13   b  are rotated by the timing gears  21   a  and  21   b  engaged with the shafts  16   a  and  16   b  with a phase difference and in opposite direction each other indicated by arrows shown in  FIG. 2  to suck the fluid from upper part and to exhaust fluid to lower part of the pump chamber  9  accommodating the rotors  13   a  and  13   b  as shown in the  FIG. 1 . Similarly, the pair of the rotor  12   a  and  12   b  engaged with the shafts  16   a  and  16   b  respectively are rotated in opposite direction each other to suck and exhaust the fluid in the pump chamber  8  accommodating the rotors  12   a  and  12   b . Similarly, the pair of the rotor  14   a  and  14   b  engaged with the shafts  16   a  and  16   b  respectively are rotated in opposite direction each other to suck and exhaust the fluid in the pump chamber  10  accommodating the rotors  14   a  and  14   b . Further, the pair of the rotor  15   a  and  15   b  engaged with the shafts  16   a  and  16   b  respectively are rotated in opposite direction each other to suck and exhaust the fluid at the pump chamber  11  accommodating the rotors  15   a  and  15   b.    
      As shown in  FIG. 2 , there is slight clearance between the pair of the rotor  13   a  and  13   b . The rotors  13   a  and  13   b  are provided not to contact with each other by the timing gears  21   a  and  21   b . Further, there is slight clearance between an outer surface of the rotors  13   a ,  13   b  and an inner surface of the second pump chamber  9  not to contact with each other. The other pair of the rotor  12   a  and  12   b , the pair of the rotor  14   a  and  14   b , the pair of the rotor  15   a  and  15   b  are similarly structured.  
      As shown in  FIG. 2 , a space S surrounded by the rotors  13   a ,  13   b  and the inner surface of the second pump chamber  9  is a scavenging space. A cross-sectional shape of the each pump chamber and a cross-sectional shape of each rotors accommodated in the each pump chamber are identical with that shown in  FIG. 2 . On the other hand, as shown in  FIG. 1 , the width of the each pump chamber is designed to be smaller from the pump chamber provided at upstream side to the pump chamber provided at downstream side. Accordingly, a scavenging volume of the each pump chamber is designed to become smaller from the pump chamber provided at the upstream side to the pump chamber provided at the downstream side.  
      An operation of the multistage dry vacuum pump will be explained as follows.  
      At first, the motor  20  serving as the rotation driving means drives and the first shaft  16   a  connected to the motor  20  is driven. The rotors  12   a ,  13   a ,  14   a  and  15   a  connected to the shaft  16   a  rotates in the each pump chamber with the rotation of the first shaft  16   a . Meanwhile, the first shaft  16   a  is connected to the second shaft  16   b  by the timing gears  21   a  and  21   b . Therefore, the rotation of the motor  20  is transmitted to the second shaft  16   b  to be inversely rotated. Accordingly, the rotors  12   b ,  13   b ,  14   b  and  15   b  connected to the second shaft  16   b  are rotated synchronously and at the same speed with the rotors  12   a ,  13   a ,  14   a  and  15   a  and in inverse direction with the rotation of the rotors  12   a ,  13   a ,  14   a  and  15   a.    
      By the rotation of the each rotor, the fluid sucked from the main inlet  3  is at first compressed in the first pump chamber  8  and transported to the second pump chamber  9  via the first fluid transport conduit  17 . Further, the fluid compressed in the second pump chamber  9  is transported to the third pump chamber  10  via the second fluid transport conduit  18 . Further, the fluid compressed in the third pump chamber  10  is transported to the fourth pump chamber  11  via the third fluid transport conduit  19 . Thus the fluid sucked from the main inlet  3  is compressed in and transported to the each pump chamber provided in a descending order of the scavenging volume of the pump chambers.  
      The outlet  28  of the third pump chamber  10  is connected to the intermediate exhaust conduit  30  connected to the main outlet  4  via the one-way valve  32  letting fluid flow to atmosphere and the confluent chamber  40  of the exhaust conduits. Further, the outlet  29  of the fourth pump chamber  11  is connected to the main exhaust conduit  31  connected to the main outlet  4  via the confluent chamber  40  of the exhaust conduits. Therefore, the transported fluid sucked from the main inlet  3  and compressed in each pump chamber  8 ,  9 ,  10  and  11  is transported through the intermediate exhaust conduit  30  or the main exhaust conduit  31  and finally exhausted through the main outlet  4  to the outside (atmosphere) via the confluent chamber  40  of the exhaust conduits.  
      In other words, when pressure of the fluid sucked from the main inlet  3  is relatively low, for example, equal to or lower than a certain 100 Pa, because a mass flow rate of the fluid is small, exhaust pressure of the each pump chamber from the first pump chamber  8  to the third pump chamber  10  does not become equal to or higher than atmospheric pressure. Accordingly, the sucked fluid is not exhausted to atmosphere via the intermediate exhaust conduit  30  connecting the fluid transport conduit  19  with the main outlet  4  and having the one-way valve  32  letting fluid flow to atmosphere. The fluid sucked from the main inlet  3  is exhausted to the outside (atmosphere) through the outlet  29  of the fourth pump chamber  11  via the main exhaust conduit  31  and through the main outlet  4 .  
      On the other hand, when pressure of the fluid sucked from the main inlet  3  is relatively high, for example, equal to or higher than 10000 Pa, exhaust pressure of the outlet  28  of the third pump chamber  10 , that is, sucking pressure of the fourth pump chamber  11  connected via the third fluid transport conduit  19  may be higher than the atmospheric pressure (It depends on a volume of each pump chamber). In this case, part of the sucked fluid is exhausted to atmosphere via the intermediate exhaust conduit  30  having the one-way valve  32  letting fluid flow to atmosphere. Therefore, the sucking pressure of the fourth pump chamber  11  may not be higher than the atmospheric pressure. Thus, the fourth pump chamber  11  may not be resistance for exhaust performance of the each pump chamber provided before the fourth pump chamber  11 .  
      As explained above, the multistage dry vacuum pump  1  includes the intermediate exhaust conduit  30 , the one end thereof connected to the outlet (the outlet  28  in this embodiment) of the pump chamber other than the fourth pump chamber  11  provided at the most downstream (the third pump chamber  10  in this embodiment), the other end thereof being opened to outside, and the one-way valve  32  provided in the intermediate exhaust conduit  30  for closing the intermediate exhaust conduit  30  when the fluid pressure in the one end of the intermediate exhaust conduit  30  (pressure in the outlet  28  side of the third pump chamber  10 ) is lower than that of the other end thereof (atmospheric pressure) and for opening the intermediate exhaust conduit  30  when the fluid pressure in one end of the intermediate exhaust conduit  30  (pressure in the outlet  28  of the third pump chamber  10 ) is equal to or higher than pressure in the other end of the intermediate exhaust conduit  30  (atmospheric pressure). Accordingly, when the pressure of the fluid sucked from the main inlet  3  is relatively low, the sucked fluid is exhausted through the outlet  29  of the fourth pump chamber  11  and the main exhaust conduit  31 , and finally exhausted through the main outlet  4  to the outside (atmosphere). On the other hand, when sucking pressure of the fluid sucked from the main inlet  3  is relatively high, for example, exhaust pressure of the third pump chamber  10  is higher than the atmospheric pressure, part of the sucked fluid is exhausted to atmosphere via the intermediate exhaust conduit  30  connected with the main outlet  4  having the one-way valve  32  letting fluid flow to atmosphere. Therefore, the sucking pressure of the fourth pump chamber  11  may not be higher than the atmospheric pressure.  
      Therefore, an exhaust rate of the multistage dry vacuum pump having the pump chambers becoming serially smaller from the main inlet  3  to the main outlet  4  for reducing electricity consumption is not decreased even when the multistage dry vacuum pump sucks and exhausts the fluid at relatively high sucking pressure. In addition, not like a conventional exhaust system for reducing low electric consumption without reducing an exhaust rate including an assistant pump having a small scavenging volume connected to the multistage dry vacuum pump and an exhaust conduit with a one-way valve letting fluid flow to atmosphere in parallel with the assist pump, the multistage dry vacuum pump according to the embodiment of the present invention does not have problems such as complex structure and lowering of reliability caused by increase of the number of parts of the pump including piping system, increase in manufacturing cost, efficiency lowering caused by combination of the pumps, and increase in installing space.  
      In the embodiment, the multistage dry vacuum pump includes the intermediate exhaust conduit  30  having the one-way valve  32  letting fluid flow to atmosphere and provided at the outlet  28  of the third pump chamber  10  and connected to the main outlet  4 . However, the multistage dry vacuum pump may include an intermediate exhaust conduit connected to the main outlet  4  with a one-way valve letting fluid flow to atmosphere provided at the outlet of the other plurality of the pump chambers such as the second pump chamber  9  and the third pump chamber  10 . In addition, the multistage dry vacuum pump may include an opening/closing member such as a shutter and a pressure sensor for detecting pressure of the one end and the other end of the intermediate exhaust conduit  30  for opening/closing the shutter based on a detection result of the pressure sensor.  
      A second embodiment of the present invention will be explained with reference to  FIG. 3  as follows.  FIG. 3  shows a multistage dry vacuum pump according to this embodiment.  
      Pump chambers  8 ,  9  being adjacent each other in length direction of shafts  16   a  and  16   b  are connected by a first fluid transport conduit  17  in a housing  2 . Similarly, pump chambers  9 ,  10  adjacent each other in length direction of the shafts  16   a  and  16   b  is connected by a second fluid transport conduit  18 . Similarly, pump chambers  10 ,  11  being adjacent each other in length direction of the shafts  16   a  and  16   b  are connected by a third fluid transport conduit  19 . Thus, fluid sucked from a main inlet  3  of the multistage dry vacuum pump  1  is compressed by four steps and exhausted from a main outlet  4  of the multistage dry vacuum pump  1  to atmosphere.  
      A main exhaust conduit  31  connecting an outlet  29  of the fourth pump chamber  11  with the main outlet  4  and an intermediate exhaust conduit  30  connecting an outlet  28  of the third pump chamber  10  with the main outlet  4  in parallel with the main exhaust conduit  31  are provided in the housing  2 . A one-way valve  33  letting fluid flow to atmosphere serving as a second fluid flow control means is provided in the main exhaust conduit  31 . The fluid sucked from the main inlet  3  is exhausted to atmosphere via the outlet  29  and the one-way valve  33  letting fluid flow to atmosphere and the main exhaust conduit  31 . According to this structure, it can prevent that atmospheric air flows back through the one-way valve  33  from the main outlet  4  via the main exhaust conduit and atmospheric air flows into the vacuum processing chamber even when the multistage dry vacuum pump  1  is stopped.  
      Further, a one-way valve  32  letting fluid flow to atmosphere is provided in the intermediate exhaust conduit  30  connecting the outlet  30  of the third pump chamber  10  with the main outlet  4  to exhaust the sucked fluid to atmosphere when the pressure of the sucked fluid from the main inlet  3  is high and sucking pressure of the fourth pump chamber  11  becomes higher than atmospheric pressure. Here, the atmospheric air does not flow back into the third pump chamber  10  via the intermediate exhaust conduit  30  because of the one-way valve  32  letting fluid flow to atmosphere when the sucking pressure of the fourth pump chamber  11 , in other words, exhaust pressure of the third pump chamber  10 , is lower than the atmospheric pressure.  
      Further, each other end of the intermediate exhaust conduit  30  and the main exhaust conduit  31  is connected to the main outlet  4  via a confluent chamber  40  of the exhaust conduits.  
      A detailed structure of the multistage dry vacuum pump will not be repeated because the structure thereof is the same as that of the multistage dry vacuum pump according to the first embodiment already explained above. Operation of the multistage dry vacuum pump structured as described above will be explained as follows.  
      With rotation of each rotor, the fluid sucked from the main inlet  3  is compressed in the first pump chamber  8  at first and transported into the second pump chamber  9  via the first fluid transport conduit  17 . Next, the fluid compressed in the second pump chamber  9  is transported into the third pump chamber  10  via the second fluid transport conduit  18 . Then, the fluid compressed in the third pump chamber  10  is transported into the fourth pump chamber  11  via the third fluid transport conduit  19 . Thus, the fluid sucked from the main inlet  3  is compressed in each pump chamber  8 ,  9 ,  10  and  11  provided in series and having a scavenging volume of the each pump chamber becoming smaller in this order.  
      Here, the intermediate exhaust conduit  30  having the one-way valve  32  letting fluid flow to atmosphere connected to the main outlet  4  is connected to the outlet  28  of the third pump chamber  10 . Further, the main exhaust conduit  31  having the one-way valve  33  letting fluid flow to atmosphere connected to the main outlet  4  is connected to the outlet  29  of the fourth pump chamber  11 . Accordingly, the fluid sucked from the main inlet  3  and serially compressed in the each pump chamber in series are exhausted from the outlet  28  or the outlet  29  and finally exhausted from the main outlet  4  to the outside (atmosphere).  
      In other words, when the pressure of the fluid sucked from the main inlet  3  is relatively low, for example, when exhausting fluid at the pressure equal to or lower than several 100 Pa, exhaust pressure of the pump chambers from the first pump chamber  8  to the third pump chamber  10  does not become the atmospheric pressure normally because a mass flow rate of the fluid is small. Accordingly, part of the sucked fluid is not exhausted via the intermediate exhaust conduit  30  having the one-way valve  32  letting fluid flow to atmosphere and the main outlet  4 . The fluid sucked from the main inlet  3  is transported to the outlet  29  of the fourth pump chamber  11  and the main exhaust conduit  31  and exhausted from the main outlet  4  to the outside (atmosphere).  
      On the other hand, when the pressure of fluid sucked from the main inlet  3  is relatively high, for example equal to or higher than 10000 Pa, exhaust pressure of the third pump chamber  10 , in other words, sucking pressure of the fourth pump chamber  11  connected with the third fluid transport conduit  19  sometimes can be higher than the atmospheric pressure (It depends on the pump chambers). In this case, part of the sucked fluid is transported via the intermediate exhaust conduit  30  connecting the outlet  28  with the main outlet  4  and having the one-way valve  32  letting fluid flow to atmosphere to the outside (atmosphere). Therefore, the sucking pressure of the fourth pump chamber  11  does not become higher than the atmospheric pressure, and the fourth pump chamber  11  does not become resistance for exhaust performance of the pump chambers before the fourth pump chamber  11 .  
      Further, the intermediate exhaust conduit  30  connecting the outlet  28  of the third pump chamber  10  with the main outlet  4  and the main exhaust conduit  31  connecting the outlet  29  of the fourth pump chamber  11  with the main outlet  4  have the one-way valves  32  and  33  letting fluid flow to atmosphere respectively. Therefore, the atmospheric air does not flow back from the main outlet  4  via the exhaust conduits  30  and  31  even when the multistage dry vacuum pump  1  stops operation when the vacuum processing chamber connected to the multistage dry vacuum pump and the multistage dry vacuum pump via the main inlet  3  is in vacuo or decompressed. Accordingly, rapid worsening of degree of vacuum of the vacuum processing chamber and the multistage dry vacuum pump can be prevented. Further, contamination of the vacuum processing chamber and the multistage dry vacuum pump caused by flowing back of contaminated atmospheric air can be prevented. In addition, noise generated by the multistage dry vacuum pump when the fluid is compressed can partially be reduced by the one-way valves  32  and  33 .  
      Further, each outlet of the multistage dry vacuum pump to the outside (atmosphere) is gathered together to single main outlet  4  via the confluent chamber  40  of the exhaust conduits. Therefore, all of the sucked fluid is exhausted from the main outlet  4 . Accordingly, when connecting the outlet of the multistage dry vacuum pump with an exhaust duct or an exhausting device for exhaust fluid, the number of the required joints and pipes can be reduced and the installation of the multistage dry vacuum pump becomes easy.  
      According to an aspect of the present invention, the fluid sucked from the inlet is compressed in and transported into the each pump chamber connected in series from the upstream to the downstream by rotating the shaft connected with the plurality of the rotors at high speed and exhausted to the outside via the most downstream pump chamber and the main outlet. In this case, vacuum about from 1 to 100 Pa is generally required for pressure of the sucking. Therefore, the number of compressing steps (pumps connected in series) is usually 4-6 steps. As described above, in order to reduce compressing work, the scavenge volume of the each pump chamber is reduced in accordance with compression of the sucked fluid from the upstream to the downstream. However, when the sucking pressure of the first pump chamber (most upstream pump chamber) is relatively high, for example, at the pressure range exceeding 10000 Pa, the sucking pressure of the last pump chamber or the pump chamber before the last pump chamber, or the like, exceeds pressure of the outside (atmospheric pressure). Therefore, these pump chambers become just resistance for the fluid flow. As a result, the exhaust rate becomes low rapidly and electricity consumption becomes high.  
      On the other hand, according to the embodiments of the present invention, the multistage dry vacuum pump has the intermediate exhaust conduit, one end thereof connected with one or the plurality of pump chambers other than the last pump chamber (the pump chamber provided at the most downstream) and the fluid flow control means provided in the intermediate exhaust conduit for closing the intermediate exhaust conduit when the exhaust pressure of the multistage dry vacuum pump side is lower than pressure of the outside and for opening the intermediate exhaust conduit when the exhaust pressure of the multistage dry vacuum pump side is higher than pressure of the outside. Therefore, when the sucking pressure of the fluid of the first pump chamber (the pump chamber provided at the most upstream) is such high as equal to or higher than 10000 Pa and the sucking pressure of the later pump chamber exceeds pressure of the outside (atmospheric pressure), the sucked fluid is exhausted via the fluid flow control means. Therefore, the later pump chamber does not become resistance for the fluid flow. Accordingly, decrease of the exhaust rate becomes small and electricity consumption can be low.  
      In addition, the fluid flow control means described above can generally be a one-way valve letting fluid flow to atmosphere. The fluid flow control means can be an open/close valve opened and closed mechanically based on detected pressure.  
      According to another aspect of the present invention, flowing back of surrounding air (atmospheric air) through the exhaust conduit into the multistage dry vacuum pump and the vacuum processing chamber connected with the multistage dry vacuum pump can be prevented when the multistage dry vacuum pump is stopped. Therefore, vacuum break and contamination of the vacuum processing chamber can be prevented. Further, noise generated in the multistage dry vacuum pump by compressing the fluid can be shut and noise thereof can be reduced.  
      According to another aspect of the present invention, the multistage dry vacuum pump has one outlet to the outside thereof (atmosphere). Therefore, the number of joints and pipes used for connecting the outlet of the multistage dry vacuum pump with the exhaust duct can be reduced, which is advantageous for installing the multistage dry vacuum pump.  
      The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the sprit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.