Control of a two-stage vacuum pump

Abstract: An oil-free two-stage vacuum pump is disclosed, which comprises a first and a second pump stage 200A, 200 B coupled for series driving. The first and second pump stages have their discharge spaces 56, 57 capable of being communicated with each other via a bypass passage. The bypass passage is provided with a pressure control valve 125 to be closed when the pressure in it becomes lower than a predetermined pressure. This permits scroll size reduction. In addition, the pump is free from such problems posed in the case of large scroll size as vibrations of drive shaft due to warping thereof in high speed rotation and noise and heat generation or durability reduction due to such cause as non-uniform contact between stationary and revolving scrolls. Moreover, in the compression step in the first pump stage 200A, gas withdrawn is under high pressure because the gas pressure in a sealed vessel to be evacuated is close to the atmospheric pressure in an initial stage of driving. With the pressure increase beyond a predetermined pressure, a pressure control valve 125 is opened, so that the compressed gas under high pressure is no longer supplied to the second pump stage 200B but is exhausted to the outside 58. The second pump stage thus does not withdraw compressed gas under pressure higher than the atmospheric pressure, and is free from heat generation that might result from excessive compression.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an oil-free vacuum pump comprising a plurality of vacuum pumps for evacuating vessels, and also to a method of controlling the same pump (e.g. PATENT ABSTRACT OF JAPAN, Vol. 18, no 465 (m-1665), 30 August 1984 and JP-A-06 147 166-27 May 1994) Description of the Related Art Techniques for evacuating vessels have been finding extensive applications in various fields from general life to low temperature techniques. Among these applications are vacuum packs, such as polyvinyl packs, of foods for preventing attachment of bacteria floating air to the foods to prevent corrosion thereof, vacuum cars, blood extraction tubes, magic bottles for preventing heat conduction by air convection, and covers of vessels accommodating cooling media for medical, industrial or experimental purposes. A sealed vessel is evacuated by withdrawing the contained air or other gases using a vacuum pump which is coupled to a withdrawal port of the vessel. Among vacuum pumps are wet type oil rotation pumps using oil, dry roof or scroll pumps not using oil, molecular pumps or like mechanical pumps which exhaust gas to atmosphere through mechanical compression, oil dispersion pumps or like vapor jet pumps for exhausting gas with the force of jet vapor, and spatter ion pumps or like dry pumps for withdrawing and exhausting gas by forming a getter film through sublimation or spattering. These pumps are suitably selected or a plurality of these pumps are combined to construct an exhausting system in dependence on the desired operating pressure range of vacuum. A low vacuum exhausting system uses two parallel-connected oil rotation pumps accommodated in a housing, while a high vacuum exhausting system has resort to a wet vacuum pump unit comprising a combination of an oil dispersion pump and an oil rotation pump. In the latter exhausting system, vapor of oil evaporated in a boiler by heating with a heater, is blown to compress dispersed gas, which is then compressed by the oil rotation pump up to the atmospheric pressure to be exhausted to the outside. This wet type exhausting system, however, has a problem that oil having been attached to the system interior from oil vapor is re-evaporated to flow reversely into the vessel being evacuated. Another problem is that the system structure is complicated because of the use of cold trap and baffle trap for cooling. A further problem is that oil is subject to reaction with such gas as chlorine or fluorine gas to be denatured so as to increase the resistance offered to the rotation, thus reducing the pump capacity and making the maintenance and inspection correspondingly cumbersome. The dry type vacuum pumps are free from the above problems and are thus desired, and the oil-free scroll vacuum pumps are attracting attentions. The oil-free scroll vacuum pumps are roughly classified into stationary/revolving type, which comprises a stationary scroll having a first lap and a revolving scroll having a second lap capable of engagement with the first lap, and drive/driven scroll type, which comprises a drive scroll having a first lap and a driven scroll having a second lap capable of engagement with the first lap. In the stationary/revolving scroll type, the revolving scroll can be caused to undergo revolution about the stationary scroll without being caused to undergo rotation, thus varying the volume of a closed space formed between the two laps. The revolving scroll is caused to undergo revolution with a fixed radius about the center of the lap of the stationary scroll such that the point of contact between the two laps defining the closed space noted above, which functions as a compression chamber, is gradually shifted toward the center of the system. Gas which is withdrawn from a withdrawal port, is led around the winding end of the second lap to enter the closed space between the two laps. With the revolution of the revolving scroll, the withdrawn gas is pressurized as it is shifted toward the system center while reducing its volume and, when the closed is brought into communication with a discharge port, is exhausted to the outside. In the drive/driven scroll type, the withdrawn gas is pressurized as it is shifted toward the system center with gradual volume reduction of a closed space defined by the drive and driven scrolls and, when the closed space is brought into communication with a discharge port, is exhausted to the outside. Nowadays, along with a demand for vacuum degree increase, it is demanded the reduction of the time of operation until a desired vacuum degree is obtained. Low compression ratio vacuum pumps require considerable time for the evacuation, and therefore high compression ratio vacuum pumps are desired. The high compression ratio can be increased by increasing the turns number of the spiral scrolls. Increasing the turns number of scroll, however, increases the outer size of the scroll, thus giving rise to such problems as vibration of shaft due to sagging thereof in the shaft is rotated at high speeds and also generation of noise and heat and reduction of durability due to such causes as non-uniform contact between the stationary and revolving scrolls. To solve these problems, it is conceivable to use two vacuum pumps, which has a small scroll turns number and thus has a small scroll size, and drive these pumps by coupling the withdrawal port of the second stage pump to the discharge port of the first one. When this method of driving is adopted, however, in an initial stage of driving in which the pressure in the sealed vessel connected to the system is close to the atmospheric pressure, a high pressure is built up in the inter-scroll space due to the high compression ratio, thus resulting in the generation of high heat. In this case, it is necessary to cause the compressed gas under high pressure to escape to the outside. As a related technique, Japanese Laid-Open Patent Publication No. 62-48979 discloses a structure for reducing load at the pump load at the time of the start of the pump. Specifically, in the disclosed system, when the pressure in a first space defined by a stationary scroll and a revolving scroll becomes higher than the pressure in the next, i.e., a second, space, the gas in the first space is exhausted through a valve means into the second space, so that it is exhausted to the outside when the second space is brought into communication with a discharge port in communication with the outside. In this technique, a discharge port for exhausting compressed gas to the outside is provided in a central part of a polished member of stationary scroll, and a valve chamber is provided near the discharge port. The valve chamber is communicated with a first communication hole, which is open to a first closed space or gas pocket defined by stationary and revolving scrolls is led from the end of the revolving scroll into the first gas pocket. The valve chamber is also in communication with a second communication hole, which is formed near the discharge port and is open to a second closed space or gas pocket defined by stationary and revoiving scrolls during compression of gas before compressed gas is exhausted to the outside and also when compressed gas is exhausted from the discharge port to the outside. Valve means is provided in the opening of the first communication hole in the valve chamber. In this structure, when the pressure in the first gas pocket becomes higher than that in the second gas pocket, the valve means is opened to cause the gas in the first gas pocket to be exhausted into the second gas pocket. It is conceivable to apply this technique to the above method of driving two small scroll size, small scroll turns number vacuum pumps by coupling the withdrawal port of the second stage pump to the discharge port of the first stage pump. In this case, the valve means may be provided on the first stage pump, so that an increase of the pressure in the first gas pocket beyond a predetermined level causes the first communication hole to be opened by the valve means to exhaust the compressed gas in the first gas pocket into the second gas pocket. With the revolution of the revolving scroll, however, the second gas pocket is communicated with the discharge port, which is in communication with the withdrawal port of the second stage pump. Consequently, gas that has been compressed in the first stage pump is entirely led to the second stage pump. Therefore, like the first stage pump, high pressure is also built up in the second stage pump gas pocket defined by the stationary and revolving scrolls, thus resulting in high heat generation. As a pump system with a combination of two pumps, one as shown in Fig. 19 is used, in which a turbo molecular pump and a dry pump, i.e., a mechanical pump, are used in combination. In this system, compressed gas is collected in the discharge port of the turbo molecular pump by rotating a multiple stage blade therein at a high speed and exhausted from the discharge port through the dry pump which serves as an auxiliary pump. However, since the multiple stage blade is rotated at a high speed, it is broken when the turbo molecular pump is operated from state in which the atmospheric pressure prevails in the sealed vessel. Accordingly, the turbo molecular pump is started after the gas in the sealed vessel has been exhausted through compression by the auxiliary roughing pump up to about 10 -2 Torr. In serial coupling of the sealed vessel, turbo molecular pump and auxiliary pump in the mentioned order, the auxiliary pump, when driven with the turbo molecular pump held stationary, withdraws gas via the obstacle of the multiple stage blade of the turbo molecular pump. In this case, therefore, the load is increased, the mechanical loss is increased, and the efficiency is reduced. To overcome these drawbacks, a valve is coupled to the sealed vessel for switching the turbo molecular pump and the auxiliary pump one over to the other. Specifically, in Fig. 19, a three-way valve 438 is provided between the discharge port 432a of the sealed vessel 432 and the withdrawal port 434a of the turbo molecular pump 434. The remaining inlet/outlet port of the three-way valve 438 is coupled to the withdrawal port 435a of the dry pump 435 by bypassing the turbo molecular pump 434. The turbo molecular pump 434 and the dry pump 435 are thus switched one over to the other to be coupled to the sealed vessel 432 under control of an electronic controller 433. Initially, the electronic controller 433 provides a command for coupling the three-way valve 438 to the dry pump 435 to drive this pump 435 for exhausting the gas in the sealed vessel 432 through compression while holding the turbo molecular pump 434 inoperative. Since the discharge port 434b of the turbo molecular pump 434 is also coupled to the withdrawal port 435a of the dry pump 435, the driving thereof also has an effect of compressing and exhausting the gas in the turbo molecular pump 434. After the lapse of a predetermined period of time, which is determined by such factors as the volumes of the sealed vessel and turbo molecular pump, the compressing/exhausting capacity of the dry pump 435, etc. into considerations, the electronic controller 433 issues a drive signal to the turbo molecular pump 434 while driving the electromagnetic valve of the three-way valve 438 to switch coupling thereof to the withdrawal port 434a of the turbo molecular pump 434. Now, the turbo molecular pump 434 is rotated at a high speed for withdrawing the gas in the sealed vessel 432 for compressing and exhausting by the dry pump 435. In order to reduce the time necessary for evacuating the sealed vessel with the above technique, it is conceivable to increase the process volume by increasing the volume of the compression chamber of the dry pump. With an increased volume of the compression chamber, a greater volume of gas can be compressed and exhausted to reduce the evacuating time when the vacuum degree of the sealed vessel is low. When the vacuum degree of the vessel is high, however, the compression to atmospheric pressure has to be done a number of times because of the large volume of the compression chamber while the quantity of gas from the turbo molecular pump is little. This rather requires a prolonged evacuating time. As an alternative for the process time reduction, it is conceivable to increase the rotation number of the dry pump instead of increasing the volume of the compression chamber. Doing so under a low vacuum degree condition, however, has influence on the durability of the dry pump due to increase the temperature in the pump. Systems having two pumps in similar arrangements as shown in Figure 19 are disclosed in the following prior art: JP-A-61-47166 discloses a pump having two stages interconnected by a three-way valve such that selectively only the first stage or a series connection of the two stages is used to evacuate a container. DE-A-41 29 897 discloses an'arrangement of two vacuum pumps interconnected by a system of conduits and spring-biased valves so as to operate either in parallel or in series to evacuate a container. GB-A-2 214 572 discloses a twin-pump having two stages which can be operated in a parallel mode or in a mode where either one of the stages is effective. SUMMARY OF THE INVENTION It is an object of the invention to provide a pump system which can evacuate a sealed space in a short time. This object is solved by the system set forth in claim 1. The subclaims are directed to preferred embodiments of the invention. According to an embodiment of the invention, an oil-free vacuum pump system for withdrawing and exhausting gas in a sealed vessel through a plurality of oil-free vacuum pumps is controlled so that the plurality of oil-free vacuum pumps are driven in parallel while the vacuum degree of the sealed vessel is in a low vacuum range and driven in series while the vacuum degree of the sealed vessel is in a high vacuum range. According to another embodiment of the invention there is provided an oil-free vacuum pump system comprising a plurality of oil-free vacuum pumps, these pumps being driven as respective pump stages in parallel while the vacuum degree of the sealed vessel is in a low vacuum range and driven in series while the vacuum degree is in a high vacuum range, the pump stages being switched by a valve means, which selectively couples the withdrawal port of a succeeding one of the pump stages to the sealed vessel or to the discharge port of a preceding pump stage so that gas in the sealed vessel or gas exhausted from the preceding pump stage is selectively supplied to the succeeding pump stage. Suitably, the preceding pump stage that is coupled to the sealed vessel is coupled in series to the succeeding pump stage via a first three-way valve while the succeeding pump stage is coupled to the sealed vessel via a second three-way valve coupled to one port of the first three-way valve, the succeeding and preceding pump stages being thereby selectively coupled to the sealed vessel. Suitably, the pump system further comprises a controller for controlling the rotation number of the preceding and succeeding pump stages and also controlling the first and second three-way valves to change the state of coupling of the succeeding pump stage to the preceding one such that the two pump stages are coupled in parallel while the vacuum degree of the sealed vessel is in a low vacuum range and that the two pump stages are coupled in series while the vacuum degree is in a high vacuum range. According to these embodiments of the invention, while the vacuum degree of the sealed vessel is in the low vacuum range, the plurality of oil-free vacuum pumps are driven in parallel for evacuation to a predetermined vacuum degree, for instance about 10 -2 Torr. With the parallel driving of the plurality of pumps, the sealed vessel can be evacuated to a predetermined vacuum degree in a short period of time. While the vacuum degree of the sealed vessel is in the high vacuum degree, the pumps are driven in series. This permits a high compression ratio to be obtained compared to the case of driving a single pump, permitting the sealed vessel to be brought to high vacuum in a short period of time. The selective parallel or series driving of the plurality of oil-free vacuum pumps is brought about by valve means. Specifically, the first three-way valve is coupled between the sealed vessel and the withdrawal port of a succeeding one of the plurality of pumps, the second three-way valve is coupled to the discharge port of a preceding one -of the pumps, and remaining inlet/outlet ports of the two three-way valves are coupled to each other. Initially, the first and second three-way valves are controlled to let gas exhausted from the preceding pump not to the succeeding pump but to the outside, while permitting the gas in the sealed vessel to be supplied to the preceding and succeeding pumps in parallel. At this time, the preceding and succeeding pumps are driven simultaneously, i.e., in parallel, to withdraw, compress and exhaust the gas in the sealed vessel. When the preceding and succeeding pumps have been driven until the sealed vessel is in a predetermined vacuum degree, the first and second three-way valves are controlled to switch the coupling of the pumps to the serial driving to let gas exhausted from the preceding pump to be supplied to the succeeding pump. At this time, a controller increases the rotation number of the preceding pump to be higher than that in the parallel driving. The increase of the rotation number of the preceding pump increases the inner temperature thereof. However, the succeeding pump robs latent heat of the preceding pump, while the amount of exhausted gas is increased. It is thus possible to evacuate the sealed vessel in a shorter period of time without having adverse effects on the pump system due to heat generation. Suitably, the plurality of oil-free vacuum pumps are alike. In this case, the maintenance and inspection of the individual pumps may be made by using the same instruction manual. This economically precludes cumbersomeness that might otherwise be involved. The drawings show preferred embodiments of the invention and examples useful to understand the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing an oil-free vacuum pump; Fig. 2(a) is a sectional view taken along line A-A in Fig. 1; Fig. 2(b) is a sectional view taken along line B-B in Fig. 1; Fig. 3 is a sectional view taken along line C-C in Fig. 1; Figs. 4(a) to 4(b) are views referred to in the description of the operation of a first pump stage; Figs. 5(a) to 5(d) are views referred to in he description of the operation of a second pump stage; Fig. 6 is a sectional view showing an oil-free vacuum pump; Fig. 7 is a sectional view showing an oil-free vacuum pump; Fig. 8(a) is a schematic view showing an oil-free vacuum pump; Fig. 8(b) is a schematic view showing an oil-free vacuum pump; Fig. 9 is a sectional view showing a first pump stage side of a scroll mechanism shown in Fig. 8(a); Fig. 10 is a sectional view showing a second pump stage side of a the scroll mechanism shown in Fig. 8(a); Figs. 11(a) and 11(b) are block diagrams referred to in the description of controllers for driving the oil-free vacuum pumps of Fig. 1 to 7; Figs. 12(a) and 12(b) are block diagrams referred to in the description of controllers for driving the oil-free vacuum pumps of Fig. 8 to 10; Fig. 13 is a schematic showing a twin scroll vacuum pump as an embodiment of the invention; Fig. 14 is a schematic showing an embodiment of the invention; Fig. 15 is a schematic showing an embodiment of the invention; Fig. 16 is a sectional view showing an oil-free scroll vacuum pump used in the embodiments of the invention shown in Fig. 14 and 15; Fig. 17 is an exploded perspective view showing scroll blade and a seal; Figs. 18A and 18B are views referred to in the description of the function of scrolls in the embodiments of Fig. 14 and 15, and Fig. 19 is a schematic showing a prior art vacuum pump system. DETAILED DESCRIPTION OF THE DRAWINGS Fig. 1 shows, in a sectional view, an oil-free two-stage vacuum pump. Referring to the Figure, the oil-free two-stage vacuum pump is shown as designated generally at 1, which basically comprises housing parts 3 and 11 defining a housing space, two stationary scroll laps 4 and 5 disposed in a housing space defined by housing parts 3 and 11, two revolving scroll laps 8 and 9 embedded in revolving scroll blades 6 and 7 also disposed in the housing space in correspondence to the respective stationary scroll laps 4 and 5, a drive shaft 28 extending into the housing space for driving the revolving scroll blades 6 and 7, and a fan 22 mounted on the drive shaft 28 and for cooling the housing part 3. The housing part 3 has its end wall 3e formed with a central hole 3a with a right part thereof having a greater diameter spot facing 3f. The drive shaft 28, which is coupled to a motor (not shown), is rotatably fitted in the hole 3a and supported in a bearing provided in the spot facing 3f. The outer surface of the end wall of the housing part 3 has a plurality of radially spaced-apart ribs 39 extending from its center toward its edge, and a cover 36 having a plurality of vent holes 36a is mounted on the ribs 39. With the rotation of the fan, cooling air entering from above in Fig. 1 flows to the right as shown by arrows. The second pump stage scroll lap 5 which has a spiral shape, is embedded in an end wall 3e of the housing part 3. A tip seal 23 having a self-lubricating property and being elastic in the thrust direction, is fitted in the tip face of the scroll lap 5. Near the hole 3a, a hole 3b for exhausting compressed gas is provided, which can be coupled by a check valve 24 to a discharge port 3c communicated with the outside. When the pressure of compressed gas in the hole 3b exceeds the atmospheric pressure in the outside, the check valve 24 is opened to communicate the hole 3b with the discharge port 3c so as to exhaust the compressed gas to the outside. When the pressure of compressed gas in the hole 3b becomes lower than the atmospheric pressure, the check valve 24 is closed to allow reverse flow of external gas into the hole 3b. In this way, no extra drive load is given at the time of the start of the pump. The housing part 3 has an independent peripheral wall 3h surrounding its end wall 3e in order to maintain its gas tightness on the side of the end wall 3e. The end wall 3e has another hole 3d, which is formed adjacent the outer periphery of the second pump stage stationary scroll lap 5 and also adjacent the inner surface of the peripheral wall 3h. The hole 3d can be coupled by a pressure control valve 25 to the discharge port 3c in communication with the outside. When the pressure of compressed gas in a closed space or gas pocket 3g defined by the peripheral wall 3h and the second pump stage stationary scroll lap 5 exceeds the atmospheric pressure in the outside, the pressure control valve 25 is opened to communicate the hole 3d with the discharge port 3c so as to exhaust the compressed gas to the outside. When the pressure in the gas pocket 3g becomes lower than the atmospheric pressure, the pressure control valve 25 is closed so that the second pump stage withdraws the compressed gas under high pressure. The temperature inside the second pump stage is thus controlled such that it is not elevated beyond a predetermined temperature. The second pump stage revolving scroll lap 9, which has substantially the same spiral shape as the second pump stage stationary scroll lap 5 noted above, is embedded in the second pump stage scroll blade 7 disposed in the housing part 3. The laps 5 and 9 engage each other in a 180-degree out-of-phase relation to each other. In a preferred case, the maximum and minimum volumes of the gas pocket defined by the stationary and revolving scroll laps 5 and 9 of the second pump stage are set to 56.6 cc and 19.1 cc, respectively, and the volume ratio (i.e., the maximum volume divided by the minimum volume, which is the compression ratio) is set to 2.96. The revolving scroll blade 7 has a central cylindrical boss 7b having a central bore 7a with a left part thereof having a greater diameter spot facing 7f, in which a bearing is supported. The drive shaft 28 coupled to the motor (not shown), has an eccentric extension 28a which is rotatably supported in the bearing provided in the spot facing 7f. The end face of the cylindrical boss 7b has a plurality of positioning pins 7c projecting form it for being engaged in positioning holes of and positioning the first pump stage revolving scroll blade 6 to be described later in detail, and also has a plurality of threaded holes for securing the scroll blade 6 to the boss 7b. A tip seal 23 which has a self-lubricating property and is elastic in the thrust direction like the one fitted in the tip face of the scroll lap 5, is fitted in the tip face of the second pump stage revolving scroll lap 9 provided in the scroll blade 7 noted above. Specifically, the tip faces of the scroll laps 5 and 9, which are in contact with the scroll blades 9 and 5 respectively, have seal grooves, in which the self-lubricating tip seals 23 are fitted for lubricant-free sliding over the corresponding scroll blades. The tip seals 23 thus maintain the gas tightness of the gas pocket defined by the scroll laps 5 and 9 with respect to the outside. The surface of the second pump stage revolving scroll blade 7 on the side thereof opposite the lap 9 is provided adjacent its edge with three revolving mechanism couplers, which are disposed with a radial spacing angle of 120 degrees and coupled to respective revolving mechanisms 37 with crankshafts coupled to a housing part 2 of the first pump stage to be described later. With rotation of the drive shaft 28, the revolving scroll blade 7 thus is reciprocated vertically in Fig. 1, i.e., undergoes revolution in correspondence to the length of the crankshafts of the revolving mechanisms 37. That is, the revolving scroll blade 7 can revolve about the center of the stationary scroll lap 5 with a predetermined radius without being rotated. The housing 2 is secured via a packing 38 to the housing part 3 by bolts or the like. The inner wall 2e of the housing part 2 has a central hole 2a, in which the cylindrical boss 7b of the second pump stage revolving scroll blade 7 is rotationally slidably fitted. The peripheral wall of the housing part 2 has a withdrawal hole 2b, which is coupled to a sealed vessel (not shown) for withdrawing gas therefrom. The first pump stage scroll lap 4 which also has a spiral shape, is embedded in the surface of the inner wall 2e of the housing part 2. A tip seal 23 having a self-lubricating property and elastic in the thrust direction is again fitted in the tip face of the lap 4. The first pump stage revolving scroll lap 8 which has substantially the same spiral shape as the stationary scroll lap 4 of this pump stage, is embedded in the first pump stage revolving scroll blade 6. The laps 4 and 8 are disposed in the housing part 2 in the 180-degree out-of-phase relation to each other. In a preferred case, the maximum and minimum volumes Vmax and Vmin of the gas Pocket defined by the stationary and revolving laps 4 and 8 of the first pump stage are set to 189.7 and 82.7 cc, respectively, and the volume ratio is set to 2.29. The first pump stage scroll blade 6 has a central cylindrical portion 6b extending in the direction of embedding of the lap 8, and near the cylindrical portion 6b it has positioning holes 6c which are fitted on the pins 7c provided on the cylindrical boss 7b of the second pump stage revolving scroll blade 7. The first pump stage scroll blade 6 is secured to the second pump stage one 7 by bolts 27 inserted through bolt holes provided in it in a row near the positioning holes 6c. Like the tip seal 23 fitted in the tip face of scroll lap 4, a tip seal 23 having a self-lubricating property and elastic in the thrust direction is fitted in the tip face of the first pump stage revolving scroll lap 8. As described before, the tip faces of the scroll laps 4 and 8, which are in contact with the corresponding scroll blades have seal grooves, in which the tip seals 23 are fitted for lubricant-free sliding over the corresponding scroll blade, so the seal tips 23 maintain the gas tightness of the gas pocket defined by the laps 4 and 8 with respect to the outside. The housing part 11 is secured via a packing 38 to the housing part 2. Figs. 11(a) and 11(b) are block diagrams showing controllers for controlling vacuum pumps with scroll mechanisms each formed by a combination of a stationary scroll and a revolving scroll. In the case of Fig. 11(a), to the withdrawal port of a sealed vessel 35 is connected the withdrawal port of the vacuum pump body 1 driven by a motor 32, which is in turn controlled by an electronic controller 34A. The electronic controller 34A includes measuring means for measuring the gas pressure in the sealed vessel 35, and the rotation number of the motor 35 is controlled according to the measurement value obtained by the measuring means. In the case of Fig. 11(b), again to the withdrawal port of a sealed vessel 35 the withdrawal port of the vacuum pump body 10 is connected. In this case, however, the vacuum pump body 10 has a first scroll mechanism stage driven by a motor 33 and a second scroll mechanism stage driven by a motor 32, and the motors 32 and 33 are controlled by an electronic controller 34A. Like the case of Fig. 11(a), the electronic controller 34A includes measuring means for measuring the gas pressure in the sealed vessel 35, and the rotation number of the motors 32 and 33 is controlled according to the measurement value obtained by the electronic controller 34A. The operation of the pump shown in Fig. 1 will now be described. As shown in Fig. 1 and 11(a), the withdrawal hole 2b of the vacuum pump body 1 is coupled by piping to the withdrawal port of the sealed vessel 35, and the drive shaft 28 of the vacuum pump body 1 is coupled to the motor 32 which is in turn coupled to the electronic controller 34A. When the motor 32 is driven by the electronic controller 34A, the first and second pump stage scroll blades 6 and 7 start rotation. With the rotation of the drive shaft 28, the cylindrical boss 7b of the second pump stage scroll blade 7 that is eccentric with the drive shaft 28, undergoes revolution in correspondence to the crankshaft length of the revolving mechanisms 37 (Fig. 3) and thus undergoes vertical reciprocation in the hole 2a of the housing part 2 in frictional contact with the surface of the hole 2a as shown in Fig. 2(a). That is, the revolving scroll blade 7 is caused to undergo counterclockwise revolution with a predetermined radius thereof about the center of the stationary scroll lap 4 without being rotated. The first pump stage revolving scroll lap 8 thus undergoes revolution in the counterclockwise direction in Fig. 2(a) in frictional contact with wall surface of the first pump stage stationary scroll lap 4, and the end 8a of the lap 8 undergoes revolution under restriction of and along an R-shaped wall surface 2h extending from the end of the lap 4 at the center of the housing part 2, whereby compressed gas is exhausted through the hole 2a. On the other hand, the second pump stage revolving scroll lap 9 which is integral with the bearing 7b, undergoes revolution in the counterclockwise direction in Fig. 2(b) in frictional contact with the wall surface of the second pump stage stationary scroll lap 5, and the end 9a of the lap 9 undergoes revolution under restriction of and along an R-shaped wall surface 3h extending from the end of the lap 5 at the center of the housing part 3, whereby compressed air is exhausted from the discharge port 3b. The operation of this pump will now be described in greater detail. When the withdrawal port 2b and the sealed vessel 35 are coupled together by a piping, the space 2g (Figs. 4(a) to 4(d)) in communication with the port 2b, in the housing part 2 constituting the first pump stage, is filled with gas under the same pressure as in the sealed vessel 35. With the rotation of the first pump stage revolving scroll, the gas in the space 2g is withdrawn into the maximum volume gas pocket Tmax, which has its outer side defined by the stationary scroll lap 4 and its inner side defined by the revolving scroll lap 8, and also into the maximum volume gas pocket Smax, which has its outer side defined by the revolving scroll lap 8 and its inner side defined by the stationary scroll lap 4, as shown in Figs. 4(a) and 4(d). With the revolution of the revolving scroll lap 8,-of the gas withdrawn into the maximum volume gas pockets Tmax and Smax, the gas in the gas pocket Tmax is compressed into a minimum volume gas pocket Tmin, as shown in Fig. 4(b). When clearance is formed between the end 8a of the lap 8 and the R-shaped wall surface 2h with further revolution of the lap 8, as shown in Fig. 4(c), the compressed gas is exhausted through the clearance into the hole 2a. The gas withdrawn into the gas pocket Smax, on the other hand, is compressed into a minimum volume gas pocket Smin as shown in Fig. 4(c). When the clearance between end 4a of the lap 4 at the center thereof and the inner wall surface of the revolving scroll lap 8 is opened with further rotation of the revolving scroll as shown in Fig. 4(d), compressed gas is exhausted through the clearance into the hole 2a. The exhausted compressed gas flows from the hole 2a toward the space 3g formed in the housing 3 from the central part to the outer periphery part of the second pump stage scroll blade 7 to fill a space on the back side of the scroll blade 7 and the space 3g. In an initial stage of driving the pump, the pressure in the sealed vessel 35 is the same as the atmospheric pressure, and the gas that is withdrawn by the first pump stage scrolls fills the space 3g under double the atmospheric pressure. Since the pressure in the space 3g is higher than the atmospheric pressure, the pressure control valve 25 disposed in the hole 3d in communication with the discharge port 3c communicated with the outside, is open, and the compressed gas is exhausted to the outside. Meanwhile, in the initial stage of driving, in the second scroll mechanism stage not only the space 3g but also the gas pocket defined by the stationary and revolving scroll laps 5 and 9 is filled by gas substantially under the same pressure as the atmospheric pressure. This is due to leakage of gas through a slight clearance between the stationary and revolving scroll laps. While the gas leakage can be ignored during driving, when the system is left under atmospheric pressure for long time, the pressure becomes substantially the same as the atmospheric pressure due to gas entering through the clearance noted above. In the initial stage of driving, the second pump stage scroll mechanism withdraws gas substantially under the atmospheric pressure, and it withdraws and compresses atmospheric pressure gas until the pressure of the mixture of the gas exhausted from the first pump stage scroll mechanism and the gas present in the space 3g becomes lower than the atmospheric pressure. Accordingly, its shape and dimensions are designed from considerations of the temperature characteristics of the tip seals 23 fitted in the lap tip faces, rotational speed of the revolving scroll, the maximum volume of gas withdrawn by the revolving scroll, compression ratio, cooling performance of the fan 22, time until the gas pressure in the space 3g becomes lower than the atmospheric pressure, etc., and it is operated within these design basis ranges. With the rotation of the second pump stage revolving scroll, the gas in the space 3g is withdrawn into the maximum volume gas pocket Wmax, which has its outer side defined by the stationary scroll lap 5 and its inner side defined by the revolving scroll Lap 9, and also into the maximum volume gas pocket Xmax, which has its outer side defined by the revolving scroll lap 9 and its inner side defined by the stationary scroll lap 5, as shown in Figs. 5(a) and 5(d). With the revolution of the revolving scroll lap 9, of the gas withdrawn into the maximum volume gas pockets Wmax and Xmax, the gas in the gas pocket Xmax is compressed into a minimum volume gas pocket Xmin as shown in Fig. 5(b). When the clearance between the end 9a of the lap 9 and the wall surface 3j of the central part of the stationary scroll lap 5 is opened with further rotation of the revolution of the lap 9, as shown in Fig. 5(c), the compressed gas is exhausted through the clearance into the hole 3b. The gas withdrawn into the gas pocket Wmax, on the other hand, is compressed into a minimum gas pocket Wmin as shown in Fig. 5(d). When a clearance is formed between the R-shaped wall surface 3i at the center of the lap 5 and the end 9a of the revolving scroll 9, the compressed gas is exhausted through the clearance into the hole 3b. As the pressure in the sealed vessel 35 is reduced with the progress of the evacuation of the vessel, the amount of gas withdrawn is reduced. By detecting this pressure reduction, the electronic controller 34A increases the rotation number of the motor 32 to make up for the reduction of the amount of withdrawn gas. The rotation number of the motor may be controlled as well after the lapse of a predetermined period of time with such parameters as the volume of the sealed vessel, performance of the vacuum pump, etc. inputted in advance to the electronic controller 34A. As shown above, while the second scroll mechanism stage can compresses gas substantially under the atmospheric pressure for exhausting to the outside, compressed gas under pressure in excess of the atmospheric pressure, supplied from the first scroll mechanism stage, is bypassed by the pressure control valve to be exhausted to the outside. Thus, the second scroll mechanism stage neither withdraws nor compresses excess pressure gas, so that it is free from its durability reduction or breakage that might otherwise result form high heat generation. Fig. 6 shows, in a sectional view, an oil-free two-stage vacuum pump. Referring to the Figure, the illustrated oil-free two-stage vacuum pump generally designated at 10, basically comprises two stationary scroll laps 14 and 15 disposed in a housing space defined by housing parts 13 and 20, two revolving scroll laps 18 and 19 embedded in revolving scroll blades 16 and 17 also disposed in the housing space in correspondence to the respective stationary scroll laps 14 and 15, drive shafts 29 and 30 extending into the housing space for driving the revolving scrolls, and fans 22 mounted on the drive shafts 29 and 30 for cooling the housing parts 13 and 20. The housing part 13 has its end wall 13e formed with a central hole 13a with a right part thereof having a greater diameter spot facing 13f. The drive shaft 29, which is coupled to a motor (not shown), is rotatably fitted in the hole 13a such that it is supported in a bearing provided in the spot facing 13f. The outer surface of the end wall of the housing part 13 has a plurality of radially spaced-apart ribs 41 extending from its center toward its edge, and a cover 36 having a plurality of vent holes 36a is mounted on the ribs 41. With the rotation of the fan 22, cooling air entering the space defined by the housing part 13 and the cover 36 from above in Fig. 6 flows to the right as shown by arrows. The second pump stage scroll lap 15, having a spiral shape, is embedded in the inner wall 13e of the housing part 13, and a tip seal having a self-lubricating property and elastic in the thrust direction is fitted in the tip face of the lap 15. Near the hole 13a, a hole 13b for exhausting compressed gas is provided, which can be coupled by a check valve 24 to a discharge port 13c communicating with the outside. When the pressure of compressed gas in the hole 13b exceeds the atmospheric pressure of the outside, the check valve 24 is opened to communicate the hole 13b with the discharge port 13c so as to exhaust the compressed gas to the outside. When the pressure in the hole 13b becomes lower than the atmospheric pressure, the check valve 24 is closed to cause reverse flow of external gas into the hole 13b. In this way, no extra drive load is given at the time of the start of the pump. The housing part 13 has an independent peripheral wall 13h surrounding its end wall 13e in order to maintain its gas tightness on the side of the end wall 13e. The end wall 13a has another hole 13d, which is formed adjacent the outer periphery of the second pump stage stationary scroll lap 15 and also adjacent the inner surface of the peripheral wall 13h. The hole 13d can be coupled by a pressure control valve 25 to the discharge port 13c in communication with the outside. When the pressure of compressed gas in a closed space or gas pocket 13g defined by the peripheral wall 13h and the second pump stage scroll lap 15 exceeds the atmospheric pressure of the outside, the pressure control valve 25 is opened to communicate the hole 13d with the discharge port 13c so as to exhaust the compressed gas to the outside. When the pressure in the gas pocket 13g becomes lower than the atmospheric pressure of the outside, the pressure control valve 25 is closed, so that the second pump stage withdraws the compressed gas under high pressure. The temperature inside the second pump stage is thus controlled such that it is not elevated beyond a predetermined temperature. The second pump stage revolving scroll lap 19, having substantially the same shape as the second pump stage scroll lap 15 noted above, is embedded in the second pump stage scroll blade 17 which is disposed in the housing part 13. The laps 15 and 19 engage each other in a 180-degree out-of-phase relation to each other. In a preferred case, the maximum and minimum volumes of the gas pocket defined by the second pump stage stationary and revolving scroll laps 15 and 19 are set to 56.6 and 19.1 cc, respectively, and the volume ratio is set to 2.06. The revolving scroll blade 17 has a central cylindrical boss 17b having a central bore 17c with a left part thereof having a greater diameter spot facing 17f, in which a bearing is supported. The drive shaft 29 coupled to a motor (not shown), has an eccentric extension 29a which is rotatably supported in the bearing provided in the spot facing 17f. A tip seal 23 which has a self-lubricating property and is elastic in the thrust direction, is fitted in the tip face of the second pump stage revolving scroll lap 19 provided in the scroll blade 17 noted above. Like tip seal 23 is also fitted in the tip face of the second pump stage stationary scroll lap 15. Specifically, the tip faces of the scroll laps 15 and 19, which are in contact with the scroll blades 19 and 15 respectively, have seal grooves, in which the self-lubricating tip seals 23 are fitted for lubricant-free sliding over the corresponding scroll blades. The tip seals 23 thus maintain the gas tightness of the gas pocket defined by the scroll laps 5 and 9 with respect to the outside. The surface of the second pump stage revolving scroll blade 17 on the side thereof opposite the lap 18 is provided adjacent its edge with three revolving mechanism couplers, which are disposed with a radial spacing angle of 120 degrees and coupled to respective revolving mechanism 47 with crankshafts coupled to a housing part 12 of the first pump stage to e described later. With the rotation of the drive shaft 29, the revolving scroll blade 17 thus is reciprocated vertically in Fig. 6, i.e., undergoes revolution in correspondence to the length or the crankshaft of the revolving mechanism 47. That is, the revolving scroll blade 17 can revolve about the center of the stationary scroll lap 15 with a predetermined radius without being rotated. The housing part 12 is secured via a packing 38 to the housing part 13 by bolts or the like. The peripheral wall of the housing 12 has a withdrawal port 12b, which is coupled to a sealed vessel (not shown) for withdrawing gas therefrom. The first pump stage scroll Lap 14 having a spiral shape is embedded in the inner wall 12e of the housing 12, and a tip seal 23 having a self-lubricating property and elastic in the thrust direction is fitted in the tip face of the lap 14. The inner wall 12e of the housing 12 has a central recess 12f formed on its side opposite the lap 14. The depth of the recess 12f from the tip face of the lap 14 is smaller than the thickness of the inner wall 12e. A hole 12a is open to an edge portion of the recess 12f for supplying compressed gas to the second scroll mechanism stage. Three revolving mechanisms 37 having one end coupled to the second pump stage revolving scroll blade 17, have their stem provided on the outer periphery of the housing part 12 at a 120-degree angle interval. The first pump stage revolving scroll lap 18 which has substantially the same spiral shape as the stationary scroll lap 14 of this pump stage, is embedded in the first pump stage revolving scroll blade 16. The laps 14 and 18 are disposed in the housing part 12 in the 180-degree out-of-phase relation to each other. Three revolving mechanisms 47 having one end coupled to the second pump stage revolving scroll blade 17, have their stem provided on the first pump stage revolving scroll blade 16 adjacent the edge thereof at a 120-degree angle interval. The first pump stage revolving scroll blade 16 has a central cylindrical portion 16b, which extends in the direction of embedding of the lap 18 and has an end rotatably provided on an eccentric extension 30a of the drive shaft 30 with its end in contact via a tip seal 23 with the surface of the recess 12f of the housing part 12. In a preferred case, the maximum and minimum volumes Vmax and Vmin of the gas pocket defined by the stationary and revolving laps 14 and 18 of the first pump stage are set to 189.7 and 82.7 cc, respectively, and the volume ratio is set to 2.29. Like the tip seals 23 fitted in the tip face of the scroll lap 14, a tip seal 23 having a self-lubricating property and elastic in the thrust direction is fitted in the tip face of the first pump stage revolving scroll lap 18. As described before, the tip faces of the scroll laps 14 and 18 which are in contact with the corresponding scroll blades have seal grooves, in which the tip seals 23 are fitted for lubrication-free sliding over the corresponding scroll blades, so the seal tips 23 maintain the gas tightness of the gas pocket defined by the laps 14 and 18 with respect to the outside. The housing part 20 is secured via a packing 38 to the housing part 12. The inner wall 20e of the housing 20 has a central bore 20a with a left part thereof having a greater diameter spot facing 20f, in which a bearing is provided. The drive shaft 30 coupled to a motor (not shown), is rotatably fitted in the bore 30a such that it is supported in the bearing provided in the spot facing 20f. The outer wall surface of the housing part 20 has a plurality of radially spaced-apart ribs 40 extending from the center toward the periphery of it, and a cover 36 having a plurality of vent holes 36a is mounted on the ribs 40. With the rotation of the fan 22, cooling air entering the space defined by the housing part 20 and cover 36 from above in Fig. 6 flows to the left as shown by arrows. Now, the operation of the pump having the construction shown in Fig. 6 and described above, will be described with reference to Fig. 11(b) as well. Referring to Fig. 6, the electric controller 34A drives the motor 33 to drive the first scroll mechanism stage. Referring to Fig. 6, gas under substantially the same pressure as the atmospheric pressure is withdrawn through the withdrawal port 12b of the housing part 12 into the first scroll mechanism stage, and compressed gas is exhausted from the discharge port 12a into the space 13g in the housing part 13. In an initial stage of driving of the pump, the exhausted gas is under a pressure higher than the atmospheric pressure, and the compressed as is exhausted by the pressure control valve 25 to the outside. After the lapse of time which is calculated from the considerations of the volume of the sealed vessel 35, withdrawal volume and rotational speed of the first pump stage revolving scroll, etc., the electric controller 34A drives the motor 32. Around this time, the pressure of gas compressed by the first pump stage scrolls and exhausted into the space 13g becomes lower than the atmospheric pressure, so that the pressure control valve 25 is closed. Thereafter, the compressed gas exhausted from the first scroll mechanism stage is compressed in the second scroll mechanism stage to be exhausted from the hole 13b. As the pressure in the sealed vessel 35 being evacuated by the vacuum pump is reduced, the rotation number of the motors 33 and 32 is increased by the electric controller 34A. This has an effect of making up for the reduction of the rate of gas exhausting form the sealed vessel and thus reducing the process time. Fig. 7 shows an oil-free two-stage vacuum pump. Referring to the Figure, the oil-free two-stage vacuum pump is shown as designated generally at 100, which basically comprises housing parts 102 and 103 defining a housing space, two stationary scroll laps 104 and 105 disposed in the housing space, two revolving scroll laps 108 and 109 embedded in revolving scroll blades 106 and 107, also disposed in the housing space in correspondence to the respective stationary scroll laps 104 and 105, a drive shaft 31 extending into the housing space for driving the revolving scroll, and a fan 22 mounted on the drive shaft 31 for cooling the housing parts 103 and 102. The housing part 103 has its end wall 103e formed with a central hole 103a with a right part thereof having a greater diameter spot facing 103f for supporting a bearing. The drive shaft 31, which is coupled to a motor (not shown), is rotatably fitted in the hole 103a such that it is supported in the bearing fitted in the spot facing 103f. The outer surface of the end wall of the housing part 103 has a plurality of radially spaced-apart ribs 42 extending from its center toward its edge, and a cover 36 having a plurality of vent holes 36a is mounted on the ribs 42. With the rotation of the fan 22, cooling air entering the space defined by the housing part 3 and cover 36 from above in Fig. 7 flows to the right as shown by arrows. The second pump stage scroll lap 15 which has a spiral shape, is embedded in an end wall 103e of the housing part 103. A tip seal 23 having a self-lubricating property and being elastic in the thrust direction, is fitted in the tip face of the scroll lap 105. Near the hole 103a, a hole 103b for exhausting compressed gas is provided, which can be coupled by a check valve 24 to a discharge port 103c communicated with the outside. When the pressure of compressed gas in the hole 103b exceeds the atmospheric pressure in the outside, the check valve 24 is opened to communicate the hole 103 with the discharge port 103c so as to exhaust the compressed gas to the outside. When the pressure of compressed gas in the hole 103 becomes lower than the atmospheric pressure, the check valve 24 is closed to allow reverse flow of external gas into the hole 103b. In this way, no extra drive load is given at the time of the start of the pump. The housing part 103 has an independent peripheral wall 3h surrounding its end wall 3e in order to maintain its gas tightness on the side of the end wall 103e. The end wall 103e has another hole 103d, which is formed adjacent the outer periphery of the second pump stage stationary scroll lap 105 and also adjacent the inner surface of the peripheral wall 103h. The hole 103d can be coupled by a pressure control valve 25 to the discharge port 103c in communication with the outside. When the pressure of compressed gas in a closed space or gas pocket 103g defined by the peripheral wall 103h and the second pump stage stationary scroll lap 105 exceeds the atmospheric pressure in the outside, the pressure control valve 25 is opened to communicate the hole 3d with the discharge port 103c so as to exhaust the compressed gas to the outside. When the pressure in the gas pocket 103g becomes lower than the atmospheric pressure, the pressure control valve 25 is closed so that the second pump stage withdraws the compressed gas under high pressure. The temperature inside the second pump stage is thus controlled such that it is not elevated beyond a predetermined temperature. The housing part 102 is secured via a packing 38 and by bolts to the housing part 103. The outer periphery of the housing part 102 has a hole 102b coupled to a sealed vessel (not shown) for withdrawing gas therefrom. The first pump stage scroll lap 104 which has a spiral shape, is embedded in the inner wall 102e of the housing 102. A tip seal 23 having a self-lubricating property and elastic in the thrust direction is fitted in the tip face of the lap 104. The inner wall 102e of the housing part 102 has a central bore 102a with a left part thereof formed with a greater diameter spot facing 102f for supporting a bearing. The drive shaft 31 coupled to a motor (not shown) is rotatably fitted in the bore 102a such that it is supported in the bearing fitted in the spot facing 102f. The outer surface of the end wall of the housing part 102 has a plurality of radially spaced-apart ribs 43 extending from the center toward the periphery of it. A cover 36 having a plurality of vent holes 36a is mounted on the ribs 43. With the rotation of the fan 22, cooling air entering the space defined by the housing part 102 and the cover 36 flows to the left as shown by arrows in Fig. 7. The inner wall of the housing part 102 is formed near its center with a hole 102a for exhausting compressed gas therethrough, compressed gas being thence supplied through a discharge passage 102c to the second pump stage scrolls. Three revolving mechanisms 37 have their stem provided at a 120-degree angle interval on the housing part 102 adjacent the periphery thereof and have one end coupled to the revolving scroll blade 106. The first pump stage revolving scroll lap 108 which has substantially the same spiral shape as the first pump stage stationary scroll lap 104, is embedded in the revolving scroll blade 106 provided in the housing space 102. The laps 104 and 108 engage each other in a 180-degree out-of-phase relation to each other. In a preferred case, the maximum and minimum volumes Vmax and Vmin of the gas pocket defined by the stationary and revolving scroll laps 4 and 8 of the first pump stage, are set to 189.7 and 82.7 cc, respectively, and the volume ratio is set to 2.29. The second pump stage revolving scroll lap 107 which has substantially the same spiral shape as the second pump stage stationary scroll lap 105, is embedded in the surface 106g of the revolving scroll blade 106. The laps 105 and 107 engage each other in a 180-degree out-of-phase relation to each other. In a preferred case, the maximum and minimum volumes of the gas pocket defined by the stationary and revolving scroll laps 15 and 19 of the second pump stage, are set to 56.6 and 19.1 cc, respectively, and the volume ratio is set to 2.96. Three pin crankshaft mechanisms 37 have their stem provided at a 120-degree angle interval on the revolving scroll blade 106 adjacent the periphery thereof and have their stem coupled to the housing part 102. The revolving scroll blade 106 has a central eccentric cylindrical boss 106b, which extends in the direction of embedding of the lap 108 and is rotatably coupled to an extension 31a of the drive shaft 31 with an end of it in contact via a tip seal 23 with a polished surface 102e of the housing part 102. The central cylindrical boss 106b of the blade 106 has a central bore 106a with a left part thereof formed with a greater diameter spot facing 106f for supporting a bearing. The eccentric extension 31a of the drive shaft 31 coupled to a motor (not shown), is rotatably supported in the bearing provided in the spot facing 106f. The operation of the pump shown in Fig. 7 and having the above construction, will now be described with reference to Fig. 11(a). Referring to Fig. 11(a), the electric controller 34A drives the motor 32 to drive the revolving scroll blade 106. Referring to Fig. 7, gas substantially under the same pressure as the atmospheric pressure is withdrawn into the withdrawal port 102b provided in the housing part 102. The withdrawn gas is taken and compressed by the revolving and stationary scroll laps 108 and 104 of the first pump stage, and compressed gas is withdrawn through the hole 102a into the space 103g in the housing part 103. In an initial stage of pump driving, the pressure in the sealed vessel 35 is the same as the atmospheric pressure, and the gas taken by the first pump stage scrolls is compressed to about double the atmospheric pressure to fill the space 103g. Since the space 103g is under a pressure higher than the atmospheric pressure, the pressure control valve 25 which is disposed in the hole 103d communicating with the discharge passage 103c which in turn communicates with the outside, is held opened, and the compressed gas is exhausted to the outside. Meanwhile, in the initial pump drive stage, in the second scroll mechanism stage not only the space 103g but also the gas pocket defined by the stationary and revolving scroll laps 105 and 107 is filled with gas which is substantially under the same pressure as the atmospheric pressure. Thus, the second scroll mechanism stage, in the initial pump driving stage, takes and compresses the atmospheric pressure gas to exhaust the compressed gas into the hole 103b until the pressure of the mixture of the gas exhausted by the first scroll mechanism stage and the gas in the space 103g becomes lower than the atmospheric pressure. With the progress of evacuation of the sealed vessel 35, the pressure therein is reduced to reduce the gas withdrawal rate. The electric controller 34A detects this pressure detection for increasing the rotation number of the motor 32 and making up for the gas withdrawal rate reduction. As an alternative arrangement, the rotation number of the motor may be controlled after the lapse of a predetermined period of time with such parameters as the volume of the sealed vessel, performance of the vacuum pump, etc. inputted in advance to the electric controller 34A. Figs. 8(a) and 8(b) schematically show oil-free two-stage vacuum pumps using a drive scroll and a driven scroll. Referring to the Figures, the oil-free two-stage vacuum pump 200 comprises a first vacuum pump stage 200A and a second vacuum pump stage 200B, these pump stages 200A and 200B being coupled to the opposite ends of a drive shaft 53 of a motor 50. The discharge section of the second pump stage 200B can be coupled by a check valve 124 to a discharge passage 57 in communication with the outside. The discharge section of the first pump stage 200A is coupled by a piping 56 to the withdrawal section of the second pump stage 200B, and the piping 56 can be bypassed to the discharge passage 57 by a pressure control valve 125, which is opened to exhaust gas when the pressure in the piping 56 exceeds a predetermined pressure. The first and second vacuum pump stages 200A and 200B will now be described in detail. Fig. 9 is a sectional showing first vacuum pump stage 200A in detail. Referring to the Figure, housing parts 60A and 60B are made integral together with a doughnut-like intermediate housing part 61 disposed between them by mounting members (not shown). The housing part 60A has its outer wall 60Ad formed with a central hole 60Ac, which is open to an inner wall surface 60Ab and is rotatably penetrated by a drive shaft 53A of the motor 50. The housing part 60B has its outer wall 60Bd formed with a central hole 60Bc, which is open to an inner wall surface 60Bd and is rotatably penetrated by a shaft portion of a mounting seat 67. A mounting seat 66 rotatably extends in the housing part 60A such that it is secured to the drive shaft 53A. The mounting seat 66 is like a mushroom and has a stem portion and a disc-like portion. It has a bore extending through its stem and disc-like portion and fitted on the drive shaft 53A. The disc-like portion has three radially spaced-apart mounting portions 66b, and the stem portion has three holes 66a, through which cooling air is caused to flow. A bearing is fitted on the stem portion of the mounting seat 66, and it is received in a recess 60Aa formed in the housing part 60A. The mounting seat 66 is secured to the drive shaft 53A and, in this state, rotatably disposed in the housing part 60A. The peripheral wall of the housing part 60A has a plurality of holes 60Ag, through which cooling air for cooling a drive scroll 62 enters, and a plurality of holes 60Ai, through which the cooling air gets out. The drive scroll 62 basically includes a scroll blade, a plurality of radially spaced-apart fan members 62a provided on the back surface of the scroll blade and extending from the center toward the periphery, and a scroll lap 63 having a spiral shape. The drive scroll 62 has its back surface provided with three fan blades 62c radially spaced-apart at a 120-degree angle interval, and the mounting seat 66 is mounted by the mounting portion 66b on upper, large thickness portions the mounting blades 62c. The scroll lap 63 is embedded in the scroll blade part 62, which has its outer periphery provided with three circumferentially spaced-apart revolving mechanisms 68 at a 120-degree angle interval. A driven scroll 64 with a scroll lap 65, which has a lap surface facing the lap surface of the lap 63, is coupled to the revolving mechanisms 68. The driven scroll 64 has a cylindrical boss 64b provided on its side opposite the lap. The cylindrical boss 64b has a central thorough bore 64a, which extends form the surface with the lap embedded therein to the end face of the cylindrical boss 64b for exhausting compressed gas to the outside. The driven scroll 64 has its back surface provided with three fan blades radially spaced-apart at a 120-degree angle interval, and mounting portions 67b of the mounting seat 67 are mounted on the fan members 64a. A packing 69 is interposed between the end face of the cylindrical boss 64b and the mounting seat 67 to maintain gas tightness. The mounting seat 67 is like a mushroom, having a stem portion and a disc-like portion, and has a bore 67c extending through these portions for exhausting compressed gas from the bore 64a of the driven scroll 64 to the out side. The disc-like portion has three radially spaced-apart mounting portions 67b, and the stem portion has three holes 67a, through which cooling air is caused to flow. The stem portion of the mounting seat 67 is received in a bearing, which is in turn received in a hole 60Ba of the housing part 60B and secured to the same. The stem portion has a cylindrical extension rotatably fitted in a bore 60Bc of the housing part 60B. The mounting seat 67 is rotatably disposed with the driven scroll 64 secured to it in the housing part 60B. The peripheral wall of the housing part 60B has a plurality of holes 60Bg, through which cooling air for cooling the driven scroll 64 enters, and a plurality of holes 60Bi, through which the cooling air gets out. In a preferred case, the maximum and minimum volumes Vmax and Vmin of the gas pocket defined by the drive and driven scroll laps 63 and 65 of the first vacuum pump stage are set to 189.7 cc and 82.7 cc, respectively, and the volume ratio is set to 2.29. Fig. 10 shows, in a sectional view, the second vacuum pump stage 200B in detail. Parts like those in Fig. 9 are designated by like reference numerals and symbols. Referring to the Figure, the housing parts 60A and 60B are made integral with the doughnut-like intermediate housing part 61 interposed between them by mounting members (not shown). The housing part 60A has its outer wall 60Ad formed with a central hole 60Ac, which is open to an inner wall surface 60Ab and is rotatably penetrated by a drive shaft 53B of the motor 50. The housing part 60B has its outer wall 60Bd formed with a central hole 60Bc, which is open ton an inner wall surface 60Bb and is rotatably penetrated by a shaft portion of a mounting seat 67. A mounting seat 66 rotatably extends in the housing part 60A such that it is secured to the drive shaft 53B. The mounting seat 66 is like a mushroom and has a stem portion and a disc-like portion. It has a bore extending through its stem and disc-Like portion and fitted on the drive shaft 53B. The disc-like portion has three radially spaced-apart mounting portions 66b, and the stem portion has three holes 66a, through which cooling air is caused to flow. A bearing is fitted on the stem portion of the mounting seat 66, and it is received in a recess 60Aa formed in the housing part 60A. The peripheral wall of the housing part 60A has a plurality of holes 60Ag, through which cooling air cooling a drive screw 62 enters, and a plurality of holes 60Ai, through which the cooling air gets out. The drive scroll 62 basically includes a scroll blade, a plurality of radially spaced-apart fan blades 62a provided on the back surface of the scroll blade and extending from the center toward the periphery, and a scroll lap 63 having a spiral shape. The drive scroll 62 has its back surface provided with three fan blades 62a radially spaced-apart at a 120-degree angle interval, and the mounting seat 66 are mounted by the mounting portions 66b on the mounting portions 62a. The scroll lap 63 is embedded in the drive scroll 62, which has its outer periphery provided with the three revolving mechanisms 68 circumferentially spaced-apart at a 120-degree angle interval. A driven scroll 64 with a scroll lap 65, which has a lap surface facing the lap surface of the lap 63, is coupled to the revolving mechanism 68. The driven scroll 64 has a cylindrical boss 64b provided on its side opposite the lap. The cylindrical boss 64b has a central thorough bore 64a, which extends from the surface with the lap embedded therein to the end face of the cylindrical boss 64b for exhausting compressed gas to the outside. The driven scroll 64 has its back surface provided with three fan blades 64c radially spaced apart at a 120-degree angle interval, and mounting portions 67b of the mounting seat 67 are mounted on the fan blades 64c. A packing 69 is interposed between the end face of the cylindrical boss 64b and the mounting seat 67 to maintain gas tightness. The mounting seat 67 is like a mushroom, having a stem portion and a disc-like portion, and has a bore 67c extending through these portions. The disc-like portion has three radially spaced-apart mounting portions 67b, and the stem portion has three holes 67a, through which cooling air is caused to flow. The stem portion of the mounting seat 67 is received in a bearing, which is in turn received in a hole 69Ba of the housing part 60B and secured to the same. The stem portion has a cylindrical extension rotatably fitted in a bore 60Bc of the housing part 60B. The mounting seat 67 is rotatably disposed with the driven scroll 64 secured to it in the housing part 60B. The peripheral wall of the housing part 60B has a plurality of holes 60Bg, through which cooling air for cooling the driven scroll 64 enters, and a plurality of holes 60Bi, through the cooling air gets out, In a preferred case, the maximum and minimum volume Vmax and Vmin of the gas pocket defined by the drive and driven scroll laps 63 and 65 of the second vacuum stage are set to 56.6 cc and 19.1 cc, respectively, and the volume ratio is set to 2.96. Figs. 12(a) and 12(b) schematically show a control system for driving a vacuum pump with drive and driven scrolls. In the case of Fig. 12(a), a sealed vessel 35 has its withdrawal port coupled by a duct 59 to a withdrawal section of the first vacuum pump stage 200A, which in turn has the discharge section coupled by a duct 56 to the withdrawal section of the second vacuum pump stage 200B. The withdrawal and discharge sections of the second vacuum pump stage 200B are bypassed to each other by a duct 57. The first pump stage 200A is coupled to the drive shaft 53A of the motor 50, while the second pump stage 200B is coupled to the drive shaft 53B of the motor 50. The motor 50 is controlled by the electric controller 34A. The electric controller 34A includes measuring means for measuring the gas pressure in the sealed vessel 35. The rotation number of the motor 50 is controlled according to the measurement obtained by the measuring means. In the case of Fig. 12(b), the sealed vessel 35 again has its withdrawal port coupled by a duct 59 to the withdrawal section of the first pump stage 300A, which in turn has the discharge section coupled by a duct 56 to the withdrawal section of the second pump stage 300B. The discharge and withdrawal sections of the second pump stage 300B are again bypassed to each other by a duct 57. The first and second pump stages 300A and 300B are coupled to drive shafts 54 and 55 of respective motors 51 and 52, which are wired to the electronic controller 34A for rotation control thereby. The electronic controller 34A includes measuring means for measuring the gas pressure in the sealed vessel 35, and the rotation number of the motors 51 and 52 is controlled according to the measurement obtained by the measuring means. The operation of the pump having the above construction, will now be described with reference to Figs. 8(a), 9, 10 and 12(a). By coupling the first vacuum pump stage 200A to the sealed vessel 35, the motor 50 is driven by the electric controller 34A. The drive torque is transmitted by the revolving mechanisms 68 to the driven scroll 64 to drive the same. Gas compressed by the drive and driven scrolls is supplied through the discharge passage 67c in Fig. 9 from the duct 56 to the withdrawal section 61a of the second vacuum pump stage 200B. At this time, the duct 56 is filled by the gas which is exhausted from the first vacuum pump stage and under a pressure higher than the atmospheric pressure. The pressure control valve 125 is thus opened by this pressure to exhaust the inner compressed gas to the outside. When the gas pressure in the duct 56 becomes lower than the atmospheric pressure, the pressure control valve 125 is closed. Meanwhile, the second vacuum pump stage 200B is driven simultaneously with the start of operation of the first vacuum pump stage 200A caused with the rotation of the drive shaft 53B, and gas compressed by the drive and driven scrolls 62 and 64 is exhausted through the discharge passage 67c and the check valve 124 to the outside. As the pressure in the sealed vessel 35 is reduced, the electric controller 34A increases the rotation number of the motor 50 to make up for the gas withdrawal rate reduction. Fig. 8(b) schematically shows an oil-free two-stage vacuum pump using drive and driven scrolls. Parts like those in the preceding pump of Figure 8(a) are designated by like reference numerals and symbols. Referring to the Figure, in this oil-free two-stage vacuum pump 300, first and second vacuum pump stages 300A and 300B are coupled to respective drive shafts 54 and 55 of the motors 51 and 52. The second vacuum pump stage 300B has its discharge section adapted to be coupled by a check valve 124 to a discharge passage 57 in communication with the outside. The discharge section of the first vacuum pump stage 300A and the withdrawal section of the second vacuum pump stage 300B are coupled to each other by a duct 56. The discharge passage 57 is bypassed by a discharge valve 125, which is opened to exhaust gas to the outside when the pressure in the duct 56 exceeds a predetermined pressure. The illustrated first vacuum pump stage 300A is the same in structure as the first vacuum pump stage 200A shown in Fig. 9, and the second vacuum pump stage 300B is the same in structure as the second vacuum pump stage 200B shown in Fig. 10. This pump is different from the previous pump of Fig 8(a) in which the first and second vacuum pump stages are driven from the same motor in that, in this pump, these pump stages are driven from separate motors. The operation of this pump will now be described with reference to Fig. 8(b), 9, 10 and 12(b). By coupling the first vacuum pump stage 300A to the sealed vessel 35, the motor 51 is driven by the electric controller 34A. As a result, the drive shaft 54 causes rotation of the drive scroll 62, and the rotational torque is transmitted by the revolving mechanisms 68 to the driven scroll 64 to drive the same. Gas compressed by the drive and driven scrolls is supplied through the duct 56 to the second vacuum pump stage 300B. At this time, the duct 56 is filled with gas which is exhausted form the first vacuum stage pump and under a pressure higher than the atmospheric pressure, and the discharge valve 125 is opened by this pressure to exhaust the inner compressed gas to the outside. This operation is continued until the gas pressure in the duct 56 becomes lower than the atmospheric pressure. After the lapse of time calculated from the considerations of the volume of the sealed vessel 35, the take-in volume and rotation speed of the first scroll mechanism stage, etc., the electric controller 34A starts the motor 52. Around this time, the pressure of gas compressed by the first scroll mechanism stage and exhausted to the duct 56 has become Lower than the atmospheric pressure, so that the pressure control valve 125 is closed. Subsequently, the compressed gas exhausted from the first scroll mechanism stage is compressed by the second scroll mechanism stage to close the check valve 124 and be exhausted to the outside. As the pressure in the sealed vessel 35 being evacuated by the vacuum pump is reduced, the electric controller 34A increases the rotation numbers of the motors 51 and 52 to make up for the reduction of the rate of exhausting of gas from the sealed vessel and thus curtailing the process time. While in the pumps of Fig. 6 and 8b the timing of starting the second vacuum pump stage drive motor is determined by calculation from the considerations of the volume of the sealed vessel and performance of the first vacuum pump stage, this is not limitative; for example, a movable piece or a sensor which is operable in an interlocked relation to the on-off operation of the pressure control valve, may be provided, and the second vacuum pump stage may be driven according to the detection output of the movable piece or the sensor. While in the pumps of Fig. 8(a) and (b) the first and second vacuum pump stages were a combination of the stationary and revolving scrolls or a combination of the drive and driven scrolls, it is of course possible as well to use the former combination for the first vacuum pump stage and the latter combination for the second vacuum pump stage or use the latter combination for the second stage and the former combination for the first stage. As shown above, the oil-free two-stage vacuum pump in which the first and second pump stages are coupled and driven in series, permits the scroll size reduction. The vacuum pump is thus free from problems posed by the large scroll size, such as vibrations of shaft due to warping thereof in high speed rotation and generation of noise and heat or durability reduction due to such causes as non-uniform contact between the stationary and revolving scrolls. In addition, the discharge space of the first pump stage is communicated with the discharge space of the second pump stage via the bypass passage, on which the pressure control valve is provided which is closed by pressure reduction to be lower than a predetermined pressure. Thus, in the compression step in the first pump stage, the withdrawal port of which the sealed vessel to be evacuated is connected to, gas that is withdrawn into the first pump stage is under high pressure because the pressure in the sealed vessel is close to the atmospheric pressure in an initial stage from the start of the pump. When the pressure in the first pump stage exceeds a predetermined pressure, for instance the outside pressure, i.e., the pressure in the second pump stage discharge space, the pressure control valve is opened, so that the compressed gas under high pressure from the first pump stage is exhausted to the outside. The second pump stage thus has no possibility of withdrawing compressed gas under a pressure above the atmospheric pressure, and it is free from heat generation due to otherwise possible excessive compression. That is, the second pump stage is free from the possibility of its durability reduction or its seizure and breakage due to heat generated by high pressure. The first and second pump stages may be mounted on a common shaft such that they are integral with each other and driven from a common drive source via the common shaft. This permits a compact vacuum pump to be provided, which has a reduced number of components. The sealed vessel may be coupled as a load to the withdrawal port side of the first pump stage, and the rotation number of the pump may be controlled by control means according to the vacuum degree of the sealed vessel, the control means controlling the rotation of the common drive source. In this case, with reducing pressure in the sealed vessel as the load the rotation number of the first and second pump stages can be increased to increase the number of operating cycles of exhausting of gas in the sealed vessel per unit time. This permits reduction of the process time. The first and second pump stages may be driven from separate drive sources. In this case, it is possible to adopt optimum drive sources for the respective first and second pump stages from the considerations of the compressed gas as loads corresponding to the compression ratio of the first and second pump stages. In addition, in an initial sealed vessel gas withdrawal state, in which the pressure of compression gas in the first pump stage is above the atmospheric pressure, i.e., in a viscose flow range in which the sealed vessel is in a low vacuum degree, the sole first pump stage may be driven to exhaust gas through an exhaust valve to the outside, and the second pump stage may be driven when the pressure of the compressed gas in the first pump stage has become lower than the atmospheric pressure. Such operation of the pump is more economical. The revolving scrolls of the two pump stages can be driven from the opposite sides of the pump body, respectively. This means that compared to the case of driving of the scrolls of the two pump stages from the common drive source, the position at which each revolving scroll is secured to the shaft extending each drive source, can be at a reduced distance from the drive source, thus reducing the vibrations of the shaft due to warping thereof or like causes. Where each pump stage comprises a combination of a stationary scroll and a revolving scroll, the stationary scroll has a bottom wall having a bypass hole constituting a bypass passage. With this structure, the bypass passage may be formed by merely forming a hole in the stationary scroll which is not driven, and it is possible to obtain a simplified structure. Particularly, the first and second pump stages may be disposed such that the stationary scroll of the former and the revolving scroll of the latter face each other to supply compressed gas from the first pump stage through the discharge port thereof provided in the stationary scroll to the revolving scroll of the second pump stage. This structure permits providing a reduced distance between the final closed space that is defined by the stationary and revolving scroll laps of the first pump stage and the initial closed space defined by the stationary and revolving scrolls of the second pump stage. It is thus possible to provide an efficient vacuum pump, in which less gas left between the two spaces without being immediately taken into the closed space of the second pump stage. It is to be appreciated that a vacuum pump can be provided, which can reduce heat generation even in the low vacuum viscose range and is economical. Fig. 13 shows, in a schematic, an embodiment of a twin type oil-free scroll vacuum pump. This vacuum pump comprises a twin scroll blade, which is interposed between two stationary scrolls and has two revolving scroll laps each engaging with each of the stationary scroll lap of each stationary scroll for movement in the thrust direction. In this embodiment, the polished surface of each stationary scroll and the tip face of each revolving scroll lap is elastically sealed together by providing a involute tip seal, which has a self-lubricating property and is elastic in the thrust direction, between the polished surface of each stationary scroll and the tip face of the corresponding revolving scroll lap and also between the polished surface of each revolving scroll and the tip face of the corresponding stationary scroll lap. With this arrangement, revolving scroll thrust force non-uniformity that may result from errors in the assembling or machining of the scrolls can be made up for by the elastic force of the seal, thus providing automatic position correction and permitting ready absorption of vibrations of the shaft of the revolving scrolls. The structure of this embodiment will now be described in detail. Referring to Fig. 13, a twin type oil-free scroll vacuum pump 410 is shown, which comprises a twin revolving scroll 128 disposed in an enclosed space defined by two stationary scrolls 127A and 127B. The stationary scrolls 127A and 127B have respective embedded laps 137 and 138 having a spiral shape. The twin revolving scroll 128 has two revolving scroll laps 139, which are embedded in the opposite surfaces of its blade and engage with the respective stationary scroll laps 137 and 138 in a 180-degree out-of-phase relation thereto. Involute tip seals 131 having self-lubricating property, are each fitted in a groove formed the tip face of each lap 139 of the twin revolving scroll 128 in contact with each stationary scroll blade and also in a groove formed in the tip face of each of the laps 137 and 138 of the stationary scrolls 127 in contact with the revolving scroll blade, thus maintaining the gas tightness between the sealed space for compressing gas therein and the adjacent sealed space. The stationary scrolls each have an edge wall in contact with the corresponding surface of the twin revolving scroll 128 and surrounding the corresponding lap thereof. A ring-like tip seal 132 having a self-Lubricating property is fitted in a groove formed in each edge wall noted above, thus maintaining gas tightness between the sealed space enclosing the laps and the outside and also preventing dust or the like from entering the sealed valve. The stationary scroll 127A has a withdrawal port 129 formed in its outer peripheral surface for withdrawing gas and also has a discharge port 135 formed near its center for exhausting compressed gas. Likewise, the stationary scroll 127B has a withdrawal port 130 formed in its outer peripheral surface for withdrawing gas and also has a discharge port 136 for exhausting compressed gas. The twin revolving scroll 128 has a shaft 145 eccentrically coupled to the rotor of a motor 144, and also has three crankshaft pins 143' disposed at a 120-degree angle interval with respect to the center of the shaft 145. With the rotation of the shaft 145, the twin revolving scroll 128 is caused to undergo revolution with a fixed radius about the center of the laps of the stationary scrolls 127A and 127B without being rotated. The shaft 145 has a fan 146 for cooling the stationary scroll 127A via cooling fins 127Aa provided thereon, and also has a fan 147 for cooling the stationary scroll 127B via cooling fins 127Ba provided thereon. With the above construction of the twin type oil-free scroll pump 410, by driving the motor 144 to drive the shaft 145 gas is withdrawn from the withdrawal ports 129 and 130. The gas that is withdrawn from the withdrawal port 129 is progressively compressed in the sealed space defined by the stationary scroll 127A and the corresponding lap 139 of the twin revolving scroll 128 to be exhausted from the discharge port 135. The gas that is withdrawn from the withdrawal port 130 is progressively compressed in the sealed space defined by the other stationary scroll 127B and the corresponding lap 139 of the twin revolving scroll 128 to be exhausted from the discharge port 136. Since the left and right scroll mechanisms which are driven in parallel have the same compression ratio, their thrust direction forces cancel each other. A duct 75 is fitted in the withdrawal port 129 of the stationary scroll 127A, and it is coupled via a duct 74 in communication with it to the sealed vessel 35. A duct 77 is fitted in the withdrawal port 130 of the stationary scroll 127B, and it is coupled to a three-way valve 78 which is coupled via ducts 76 and 74 to the sealed vessel 35. The discharge port 136 of the stationary scroll 127B is coupled to a duct 121 for exhausting compressed gas to the outside. The discharge port 135 of the stationary scroll 127A is coupled to a duct 119 which is in turn coupled to a three-way valve 79 for exhausting compressed gas to the outsider. The other inlet/outlet ports of the three-way valves 78 and 79 are communicated with each other by a duct 120. An electric controller 34B has its output terminal coupled via a duct 112 to the electronic valve of the three-way valve 78, also coupled via a duct 113 to the electromagnetic valve of the three-way valve 79, and further coupled via a duct 110' to the motor 144, and thus it can control the on-off operation of the three-way valves 78 and 79 and also the operation of the motor 144. The operation of this embodiment of the twin type oil-free scroll pump 410 will now be described in detail. Referring to Fig. 13, the electronic controller 34B controls the three-way 79 to communicate the discharge port 135 with the outside and also controls the three-way valve 78 to communicate the discharge port 35a of the sealed vessel 35 with the withdrawal port 129 of the stationary scroll 127A. The motor 144 is then driven with a predetermined rotation number, whereby the first vacuum pump stage constituted by the twin revolving scroll 128 and the stationary scroll 127A and the second vacuum pump stage constituted by the twin revolving scroll 128 and the stationary scroll 127B are driven in parallel. The pump 410 thus withdraws gas directly from the withdrawal port 35a of the sealed vessel 35 through the ducts 74 and 75 and the withdrawal port 129 and exhausts compressed gas through the discharge port 135 and the three-way valve 79 to the outside. In addition, it withdraws gas from the discharge port 35a of the sealed vessel 35 through the ducts 74, 76 and 77, three-way valve 78 and withdrawal port 130 and exhausts compressed gas through the discharge port 136 and duct 121 to the outside. After the lapse of a predetermined period of time, during which roughening is made in a vacuum range up to about 10 -2 Torr, the electric controller 34B supplies an electric signal to the three-way valve 79 to switch the communication route of the pump 410 to the outside over to the one through the three-way valves 78 and 79, while supplying an electric signal to the three-way valve 78 to block communication between the sealed vessel 35 and the withdrawal port 130. Thus, the first vacuum pump stage constituted by the twin revolving scroll 128 and the stationary scroll 127A and the second vacuum pump stage constituted by the twin revolving scroll 128 and the stationary scroll 127B are coupled in series. With reducing pressure in the sealed vessel, i.e., with increasing vacuum degree thereof, the pressure of gas that is taken into the sealed space of the pump is reduced, so that an increased compression ratio is required to compress the gas to the atmospheric pressure for exhausting the gas to the outside. With the first and second vacuum pump stages coupled in series as described above, the compression ratio is doubled, thus permitting compression of the gas for exhausting to the outside in a reduced period of time. Also, in an initial stage of operation of the pump 410 after the switching of the first and second vacuum pump stages over to the series coupling, both the stages are by the shaft 145 of the motor 144, that is, they are driven at a constant speed, so that the problem of heat generation due to speed increase of the first pump stage is not posed. The process time for obtaining the desired state of vacuum can be further reduced by increasing the speed of the motor 144 at the time of switching over to the series coupling from the consideration of the vacuum degree of the sealed vessel in a range free from durability reduction due to heat generation. While this embodiment concerned the twin type pump comprising the twin revolving scroll provided between the opposite side stationary scrolls, the principle is also applicable to a type, in which separate revolving scrolls are provided on the opposite ends of motor shaft and engaged with corresponding stationary or driven scrolls. Fig. 14 is a schematic showing the basic structure of an embodiment of the invention, and Fig. 15 is a schematic showing the basic structure of another embodiment of the invention. These embodiments may concern a dry vacuum pump of any type. As a typical example, a single type oil-free scroll vacuum pump will be described in connection with its structure and operation. Fig. 16 shows an embodiment of a single type oil-free scroll vacuum pump. The oil-free scroll vacuum pump 400 as shown, comprises a stationary scroll 210 a revolving scroll 220 and a housing 140 with the scrolls 210 and 220 secured thereto at a predetermined position and supported for revolution, respectively. The stationary scroll 210 has an embedded spiral lap 213, which is disposed in a recess formed in the peripheral wall 211 which is secured to the end face of the housing 140 and has a withdrawal port 216 for withdrawing gas thereinto from a sealed vessel (not shown) through a duct 144. The stationary scroll 210 has a discharge port 217 formed substantially in its central portion for exhausting compressed gas. The revolving scroll lap 220 is accommodated in a recess formed in the housing 140. A lap 221 having substantially the same spiral shape has the lap 213 of the stationary scroll 210, is embedded in the surface of the blade of the scroll 220 which is in contact with the end face of the peripheral wall 211. The laps 213 and 221 engage each other in a 180-degree out-of-phase relation to each other. The scrolls 210 and 220 have their back surfaces provided with cooling fins 230 and 224 for air cooling their inside. The scroll laps 213 and 221 have their tin faces facing their counterpart scrolls with grooves 213a and 221a, in which self-lubricating tip seals 131 are fitted for the tip faces can undergo lubrication-free sliding. A ring-like seal 232 having a self-lubricating property is fitted in a groove formed in the end face of the peripheral wall 211 in contact with the corresponding surface of the revolving scroll 220 to maintain the gas tightness between the recess in the peripheral wall 211 and the outside. The housing 140 supports a main drive crankshaft 141 penetrating through its center and having a pulley 142 mounted at one end, and it also rotatably supports three driven crankshafts 143 disposed at a 120-degree angle interval with respect to the main drive crank shaft 141. The crankshafts 141 and 143 are rotatably supported in a housing part 225 which is integral with the revolving scroll 220. The main driver crankshaft 141 can cause revolution of the revolving scroll 220 about the lap of the stationary scroll 210 with a predetermined radius of revolution while the revolving scroll 220 is not rotated. As shown, the oil-free scroll vacuum pump 400 comprises the stationary scroll 210, which is accommodated in the recess formed in the peripheral wall 211 and has the first lap 213, and the revolving scroll 220, which is the second lap 221 capable of engagement with the first lap 213. As the revolving scroll 220 is caused to undergo revolution with respect to the stationary scroll 210 without being rotated, the volume of the sealed space 222 defined by the two laps 213 and 221 can be varied. When the revolving scroll 220 is caused to undergo revolution with a predetermined radius of revolution about the lap 213 of the stationary scroll 210 such that the point of contact between the laps defining the sealed space 222 serving as a compression chamber is gradually shifted toward the center of the laps, gas withdrawn from the withdrawal port is led around the outer end of the second lap 221 into the sealed space 222 defined by the laps 213 and 222, and with the revolution of the revolving scroll 220 it is pressurized with its volume reduced progressively while it is shifted toward the center of the laps. The compressed gas is exhausted to the outside when the sealed space 222 is brought into communication with the discharge port 217. In this embodiment, it is very important from the standpoints of increasing the compression efficiency and increasing the vacuum degree to ensure the sealed state of the space 222 defined by the two laps 213 and 221. As shown in Fig. 17, between the tip face, i.e., axial end face, of each lap and the corresponding frictional contact surface is provided a tip seal 131A (or 131B), which is made of a carbon type resin material, called the thermosetting condensed polycyclic polynuclear aromatic resin (COPNA resin), which has low thermal expansion coefficient and is excellent in the heat resistance and wear resistance. More specifically, as shown in Fig. 17, the lap 213 (or 221) having an involute shape is embedded in the front surface of a disc-like scroll blade 210 (or 220) serving as the stationary or revolving scroll. The tip face of the lap is formed with a tip groove 213a or 221a, which extends from the center to the periphery of the lap, and the tip seal 131A (or 131B) is fitted in the tip groove. In the oil-free scroll vacuum pump, gas that is taken into the space a shown in Fig. 18(A) is exhausted to the outside when the pressure Pi of gas in the space i, which is provided with the discharge port 217, exceeds the external pressure Po. By closing the power source (not shown) of the vacuum pump 400, the driving of the revolving scroll 220 is started. As the Lap 221 of the revolving scroll 220 is driven, gas in the space a in Fig. 18(A) is taken into the closed space b in Fig. 18(B) to be successively taken into the closed spaces c to h as shown in Figs. 18(A) and 18(B) and be finally taken into the space i in which the discharge port 217 is open, and the compressed gas is exhausted through the discharge port 217 to the outside. Now, an embodiment using the above oil-free vacuum pump will be described. Fig. 14 shows the basic structure of the embodiment. Referring to the Figure, an oil-free vacuum pump 400 has its withdrawal port 400a coupled via gas-tight ducts 75 and 74 to the discharge port of 35a of the sealed vessel 35. Another vacuum pump 400' has its withdrawal port 400'a coupled through an electromagnetic three-way valve 78 and ducts 74, 76 and 77 to the discharge port 35a of the sealed vessel 35. The vacuum pump 400 can exhaust compressed gas from its compressed gas discharge terminal 400b through a three-way valve 79 to the outside. The other inlet-outlet port of the three-way valve 79 is coupled to the other inlet/outlet port of the other three-way valve 78. The three-way valves 78 and 79, which are electromagnetic valves, can be switched such that compressed gas is supplied from the vacuum pump 400 to the withdrawal terminal 400'a of the vacuum pump 400 to be exhausted from the discharge terminal 400'b thereof to the outside. An electric controller 34C is coupled via leads 110 and 111 to the respective vacuum pumps 400 and 400' and also coupled via leads 112 and 113 to the three-way valves 79 and 78. The electric controller 34C controls the electromagnetic valves of the three-way valves to control the direction of flow of gas and also the rotation numbers and driving of the vacuum pumps 400 and 440', etc., by calculating the time until reaching of a predetermined vacuum pressure range from such parameters as the volume of the sealed vessel 35, the volumes and rotation numbers of the vacuum pumps 400 and 400', etc. It is possible to provide a pressure gauge in the sealed vessel to measure the pressure therein for the rotation number control, driving control, etc. and also for controlling the three-way valves. In operation, the electric controller 34C controls the three-way valve 79 to communicate the discharge terminal 400b of the vacuum pump 400 with the outside and also controls the three-way valve 78 to communicate the discharge terminal 35a of the sealed vessel 35 with the withdrawal terminal 400'a of the vacuum pump 400'. Then, by driving the vacuum pumps 400 and 400' with a predetermined rotation number, these pumps are coupled in parallel. In this state, the vacuum pump 400 directly withdraws gas in the sealed vessel from the discharge terminal 35a thereof through the ducts 74 and 75 and its withdraw terminal 400a, and exhausts compressed gas from its discharge terminal 400b through the three-way valve 79 to the outside. The other vacuum pump 400', on the other hand, withdraws gas from the discharge terminal 35a of the sealed vessel 35 through the through the ducts 74, 76 and 77, the three-way valve 78 and its withdrawal terminal 400'a and exhausts compressed gas from its withdrawal terminal 400'b. After the lapse of a predetermined period of time, during which time roughening is made up to a vacuum degree of about 10 -2 Torr, the electric controller 34C provides an electric signal to the three-way valve 79 to switch the communication of the vacuum pump 400b with the outside over to that with the three-way valve 78, and it also provides an electric signal to the three-way valve 78 to block communication between the sealed vessel 35 and the withdrawal terminal 400'a and provide for communication from the three-way valve 79. As a result, the vacuum pumps 400 and 400' are coupled in series. With reducing pressure in the sealed vessel, i.e., with increasing vacuum degree thereof, the pressure of gas taken into the vacuum pump sealed spaces is reduced, so that it becomes necessary to prolong the time until compression of the gas to the atmospheric pressure for exhausting to the outside. At this time instant, the rotation number of the vacuum pump 400 which is directly coupled to the sealed vessel is doubled to supply the compressed gas to the other vacuum pump 400'. In this situation, in the vacuum pump 400 operated with the increased rotation number, gas to be exhausted to the side of the vacuum pump 400' is highly compressed and elevated in temperature by heat generation. However, in the withdrawal port of the vacuum pump 400', low pressure gas taken out of the sealed vessel 36 is present in an initial stage after the switching over to the serial coupling of the pumps. This means that in this stage low pressure gas is present in the discharge port of the vacuum pump 400 in communication with the withdrawal port thereof. Thus, the gas that has been highly compressed due to the rotation number increase, is inflated when it is exhausted into the discharge port, and latent heat is robbed from it. Consequently, the temperature is not increased continuously. That is, the rate of exhausting of gas is increased without any heat generation problem, thus permitting the sealed vessel 35 to be evacuated to a high vacuum degree. The process time for evacuation can be reduced by controlling the rotation numbers of the vacuum pumps 400 and 400' in a range free from durability reduction problem due to heat generation by taking the vacuum state of the sealed vessel into considerations. Fig. 15 shows the basic structure of another embodiment. Parts like those in Fig. 14 are designated by like reference numerals or symbols. This embodiment is different from the preceding embodiment shown in Fig. 14 in that it comprises three vacuum pumps and four three-way valves. Referring to the Figure, an oil-free vacuum pump 400 has its withdrawal port 400a coupled via gas-tight ducts 74 and 75 to the discharge port 35a of the sealed vessel 35. Another vacuum pump 400' has its withdrawal port 400'a coupled through an electromagnetic three-way valve 78 and ducts 74, 76 and 77 to the discharge port 35a of the sealed vessel 35. The remaining vacuum pump 400" has its withdrawal port 400"a coupled through an electromagnetic three-way valve 78' and ducts 118, 117 and 74 to the discharge port 35a of the sealed vessel 35. The vacuum pump 400 can exhaust compressed gas from its compressed gas discharge terminal 400b through the three-way valve 79 to the outside. The other inlet/outlet port of the three-way valve 79 is coupled to the other inlet/outlet port of the three-way valve 78. These three-way valves 78 and 79, which are electromagnetic valves, can be switched such that compressed gas is supplied from the vacuum pump 400 to the withdrawal terminal 400'a of the vacuum pump 400' to be exhausted from the discharge terminal 400'b thereof to the outside. The vacuum pump 400' can exhaust compressed gas from its compressed gas discharge terminal 400'b through a three-way valve 79' to the outside. The other inlet/outlet port of the three-way valve 79' is coupled to the other inlet/outlet port of the three-way valve 78'. These tree-way valves 78' and 79', which are electromagnetic valves, can be switched such that compressed gas is supplied from the vacuum pump 400' to the withdraw terminal 400''a of the vacuum pump 400'' to be exhausted from the discharge port 400"b thereof to the outside. An electronic controller 34D is coupled via leads 110, 111 and 116 to the respective vacuum pumps 400, 400' and 400" and also coupled via leads 112, 113, 114 and 115 to the three-way valves 78, 78', 79 and 79'. The electric controller 34D controls the electromagnetic valves of the three-way valves to control the direction of flow of gas and also the rotation numbers and driving of the vacuum pumps 400, 400' and 400", etc., by calculating the time until reaching of a predetermined vacuum pressure range from such parameters as the volume of the sealed vessel 35, the volumes and rotation numbers of the vacuum pumps 400, 400' and 400", etc. It is possible to provide a pressure gauge in the sealed vessel to measure the pressure therein for the rotation number control, driving control, etc. and also for controlling the three-way valves. In operation, the electric controller 34D controls the three-way valve 79 to communicate the discharge terminal 400b of the vacuum pump 400 with the outside and also controls the three-way valve 78 to communicate the discharge terminal 35a of the sealed vessel 35 with the withdrawal terminal 400'a of the vacuum pump 400'. The electric controller 34D controls the three-way valve 79' to communicate the discharge terminal 400'b of the vacuum pump 400' with the outside and also controls the three-way valve 78' to communicate the discharge terminal 35a of the sealed vessel 35 with the withdrawal terminal 400"a of the vacuum pump 400". Then, by driving the vacuum pumps 400, 400' and 400" with a predetermined rotation number, these pumps are coupled in parallel. In this state, the vacuum pump 400 directly withdraws gas in the sealed vessel from the discharge terminal 35a thereof through the ducts 74 and 75 and its withdrawal terminal 400a, and exhausts compressed gas from its discharge terminal 400b through the three-way valve 79 to the outside. The pump 400' withdraws gas from the discharge terminal 35a of the sealed vessel 35 through the ducts 74, 76 and 77, the three-way valve 78 and its withdrawal terminal 400'a, and exhausts compressed gas from its discharge terminal 400'b to the outside. The vacuum pump 400" further withdraws gas from the discharge terminal 35a of the sealed vessel 35 through the ducts 74, 117 and 118, the three-way valve 78' and its withdraw terminal 400''a, and exhausts compressed gas from its discharge terminal 400"b to the outside. After the lapse of a predetermined period of time, during which time roughening is made up to a vacuum degree of about 10 -2 Torr, the electric controller 34D provides an electric signal to the three-way valve 79 to switch the communication of the vacuum pump 400 with the outside over to that with the three-way valve 78, and it also provides an electric signal to the three-way valve 78 to block communication between the sealed vessel 35 and the withdrawal terminal 400'a and provide for communication from three-way valve 79. The electric controller 34D further provides an electric signal to the three-way valve 79' to switch the communication of the vacuum pomp 400'b with the outside over to that with the three-way valve 78', and it also provides an electric signal to the three-way valve 78' to block communication between the sealed vessel 35 and the withdrawal terminal 400'a and provide for communication from the three-way valve 79'. Consequently, the vacuum pumps 400, 40' and 400" are coupled in parallel. With reducing pressure in the sealed vessel, i.e., with increasing vacuum degree thereof, the pressure of gas taken into the vacuum pump sealed spaces is reduced, so that it becomes necessary to prolong the time until compression of the gas to the atmospheric pressure for exhausting to the outside. At this time, the rotation number of the vacuum pump 400 which is directly coupled to the sealed vessel is doubled to supply the compressed gas to the other vacuum pump 400'. In this situation, in the vacuum pump 400 operated with the increased rotation number, gas to be exhausted to the side of the vacuum pump 400' is highly compressed and elevated in temperature by heat generation. However, in the withdrawal port of the vacuum pump 400', low pressure gas taken out of the sealed vessel 35 is present in an initial stage after the switching over to the serial coupling of the pumps. This means that in this stage low pressure gas is present in the discharge port of the vacuum pump 400 in communication with the withdrawal Port thereof. Thus, the gas that has been highly compressed due to the rotation number increase, is inflated when it is exhausted into the discharge port, and latent heat is robbed from it. Consequently, the temperature is not increased continuously. That is, the rate of exhausting of gas is increased without any heat generation Problem, thus permitting the sealed vessel 35 to be evacuated to a high vacuum degree. Gas exhausted from the vacuum pump 400' is then withdrawn into the vacuum pump 400" to be compressed and exhausted from the discharge terminal 400"b to the outside. The rotation number of the second vacuum pump stage 400' need not be made greater than the rotation number of the preceding vacuum pump stage because the pressure in the sealed vessel 35 is caused progressively proceeds to higher vacuum range by the operation of the preceding vacuum pump stage 400. Thus it can be set to be within the rotation number of the preceding vacuum pump stage. It is possible to drive the second pump stage at a lower speed than the preceding first pump stage and at a higher speed than the third pump stage within the range, in which it is possible to prevent heat generation in the preceding first pump stage as descried before, or it is possible to drive the second and third pump stages at the same rotation number less than the rotation number of the first pump stage. The process time for evacuation can be reduced by controlling the rotation numbers of the first to third vacuum pump stages in a range free from durability reduction problem due to heat generation by taking the vacuum state of the sealed vessel into considerations. While the above embodiments of vacuum pump respectively used two and three single type dry vacuum pumps each with a stationary scroll and a revolving scroll, it is possible as well to permit four or more vacuum pump stages to be switched for driving in parallel and driving in series. The driving of a plurality of oil-free vacuum pumps by switching them between parallel driving and series driving, permits evacuation of the sealed vessel in a reduced period of time. Further process time reduction is possible with rotation number control of the plurality of pump stages after the switching over to the driving in series. Moreover, the speed of the preceding pump stage can be increased while suppressing the heat generation in the succeeding pump stage, and it is thus possible to prevent durability reduction of the oil-free vacuum pump. As has been described in the foregoing, a plurality of oil-free vacuum pumps are used for parallel driving in a low vacuum range and series driving in a high vacuum range, and it is possible to provide an oil-free vacuum pump, which permits reducing the process time for evacuating the sealed vessel.

Claims: A pump system for evacuating gas from a sealed space (35), comprising: a preceding stage vacuum pump (400) connected to the sealed space (35), and a succeeding stage vacuum pump (400') connected to the preceding stage vacuum pump (400), characterised by : a first three-way valve (79) connected to a discharge port of the preceding stage vacuum pump (400) and a second three-way valve (78) connected to a suction port of the succeeding stage vacuum pump (400'), to the first three-way valve (79) and to the sealed space (35) wherein the suction port of the succeeding stage vacuum pump (400') can be selectively coupled to the sealed space (35) or to the discharge port of the preceding stage vacuum pump (400) for either parallel or series connection of the preceding and succeeding stage vacuum pumps (400, 400'). A system according to claim 1, wherein the first and second three-way valves (78, 79) are electromagnetic three-way valves and a controller (34B, C, D) is provided for controlling the rotation of the vacuum pumps (400, 400') and for controlling the first and second three-way valves (78, 79), so that they are coupled in parallel while the sealed space (35) is in a low vacuum range and are coupled in series while it is in a high vacuum range. A system according to claim 1 or 2, wherein said preceding and succeeding pump stages (400, 400') are alike. A system according to any of claims 1 to 3, wherein said preceding and succeeding stage vacuum pumps (400, 400') comprise a pair of stationary scrolls (127A, B) each having a stationary scroll lap (137, 138), and a revolving scroll (128) having revolving scroll laps (139) meshing with the stationary scroll laps (137, 138), wherein a discharge port (135) near the centre of a first one (127A) of the stationary scrolls is connected to the first three-way valve (79), a suction port (129) at the periphery of the first stationary scroll (127A) is connected to the sealed space (35) and a suction port (130) at the periphery of the second one (127B) of the stationary scrolls is connected to the second three-way valve (78). A system according to any of claims 1 to 4, comprising a further succeeding stage vacuum pump (400") connected to the sealed space (35), a further first three-way valve (79') connected to the discharge port of the succeeding stage vacuum pump (400'), and a further second three-way valve (78') connected to the suction port of the further succeeding stage vacuum pump (400"), to the further first three-way valve (79') and to the sealed space (35) so that said vacuum pumps (400, 400', 400") can be selectively operated in parallel or in series. A system according to any of claims 1 to 5, wherein said vacuum pumps are oil free vacuum pumps (400, 400', 400").

Inventors: HAGA SHUJI, TSUCHIYA MASARU, HAGA, SHUJI, TSUCHIYA, MASARU
Applicants: ANEST IWATA CORP, ANEST IWATA CORPORATION
Application year: 1996
Main classification: F04C23
IPC codes: F04C28, F04C23