Patent Publication Number: US-2015078927-A1

Title: Multi-Stage Pump Having Reverse Bypass Circuit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multi-stage positive displacement pump, including a multi-stage scroll type of vacuum pump. In particular, the present invention relates to multi-stage positive displacement pumps, including vacuum pumps, having bypass systems for allowing fluid exiting a stage to be diverted from its normal flow path through the pump. 
     2. Description of the Related Art 
     Various types of positive displacement pumps, screw, vane, root&#39;s claw, scroll type of pumps, may be configured as multi-stage pumps. A multi-stage pump provides multiple stages of compression for the purpose of providing a greater capacity (displacement) and/or pressure ratio capability for the pump. Examples of multi-stage scroll type of pumps are disclosed in U.S. Pat. Nos. 6,068,459, 5,855,473, 5,616,015 and 6,884,047, the disclosures of which are hereby incorporated by reference in their entirety. 
     In a multi-stage pump, fluid is introduced through an inlet of the pump into a first (upstream) stage of the pump where the fluid is compressed, the compressed fluid exits the first stage and is directed into a second (downstream) stage where the fluid is again compressed, and the fluid compressed in the second stage flows out of the second stage and then to an outlet of the pump (either directly or via an additional downstream stage(s) of the pump for further compression). A multi-stage pump may be provided with a forward bypass circuit which under certain abnormal operating conditions of the pump causes most of the fluid exiting an upstream one of the stages to bypass one or more of the downstream stages of the pump. 
     For example, a forward bypass circuit is typically provided in a multi-stage vacuum pump in which the inlet displacement of the first stage is much larger than the inlet displacement the second stage. When the pump is first started or the pump is vented to atmosphere, a high inlet pressure (i.e., atmospheric pressure) is encountered in the first stage. In this case, the forward bypass circuit will bypass most of the fluid around the second stage of the pump. 
     Such a bypass circuit can prevent excessively high pressures from being produced in the fluid between an upstream stage and one or more downstream stages and hence, can prevent excessive power draw and excessive temperatures in the downstream stage(s). An excessive power draw can lead to an overloading of the bearings and/or motor of the pump and a reduction of grease viscosity in the bearings caused by an excessive temperature. 
     However, such a forward bypass circuit which bypasses the fluid around one or more than one downstream stage does not address all of the potential problems resulting in an excessive power draw and excessive temperatures in a multi-stage pump. In addition, such a forward bypass circuit may create additional problems when applied to a multi-stage scroll type of vacuum pump. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a multi-stage positive displacement pump that prevents both an upstream and the next downstream stage of the pump from being the source of an excessive power draw by the pump. 
     It is another object of the present invention to provide a multi-stage positive displacement pump that is ensured of reaching its design speed. 
     It is yet another object of the present invention to provide a multi-stage scroll type of vacuum pump having tips seals for each of its compression stages and means to ensure that the tip seals of the downstream stage(s) are energized. 
     According to one aspect of the present invention, there is provided a multi-stage pump which has an inlet portion and an exhaust portion, and which includes a pump head having at least two compression stages disposed in series with respect to the direction of flow of fluid through the pump, and a reverse fluid bypass circuit. 
     The inlet portion has a pump inlet and constitutes a low pressure side of the pump where fluid is drawn into the pump. The exhaust portion has a pump outlet and constitutes a compression side where fluid is discharged from the pump at pressure greater than that of the fluid at the low pressure side. The pump head also has an inlet opening to which the pump inlet extends, and an exhaust opening leading to the pump outlet. 
     Each of the compression stages of the compression mechanism has an ingress through which the fluid enters the stage and an egress through which the fluid exits the stage. A flow path along which fluid flows from the pump inlet to the pump outlet extends in the pump between the pump inlet and an upstream one of the compression stages, between the egress of the upstream one of the compression stages and the ingress of a downstream one of the compression stages, and between the egress of the downstream one of the compression stages and the pump outlet. The reverse fluid bypass circuit defines a fluid passageway discrete from the flow path and connected to the flow path at a first location between the egress of one of the compression stages and the ingress of the next compression stage downstream therefrom and at a second location upstream of the ingress of said one of the compression stages. 
     According to another aspect of the present invention, the reverse fluid bypass circuit comprises fluid flow control means for restricting or stopping the flow of fluid through the fluid passageway. 
     According to still another aspect of the present invention, the reverse fluid bypass circuit comprises fluid flow control means for restricting the flow of fluid while the fluid is in its transitional or molecular flow regime, while allowing the flow of fluid while the fluid is in its viscous flow regime. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects, features and advantages of the present invention will become more clearly understood from the following detailed description of the preferred embodiments of the invention made with reference to the attached drawings, in which: 
         FIG. 1  is a schematic longitudinal sectional view of a multi-stage pump including one example of a reverse bypass circuit, according to the present invention; 
         FIG. 2  is a schematic longitudinal sectional view of a pump head assembly of a multi-stage type of scroll pump having a reverse bypass circuit, according to the present invention; 
         FIG. 3A  is a enlarged view of part of the pump head assembly of the multi-stage type of scroll pump of  FIG. 2 ; 
         FIG. 3B  is a enlarged view of another part of the pump head assembly of the multi-stage type of scroll pump of  FIG. 2 ; 
         FIG. 4A  is a schematic diagram of the layout of a reverse fluid bypass circuit of a two stage pump, according to the present invention; 
         FIG. 4B  is a schematic diagram of the layout of a reverse fluid bypass circuit of a three stage pump, according to the present invention; 
         FIG. 4C  is a schematic diagram of the layout of a reverse fluid bypass circuit of another three stage pump, according to the present invention; 
         FIG. 5A  is a schematic diagram of a portion of a multi-stage pump having another version of a reverse fluid bypass circuit according to the present invention; 
         FIG. 5B  is a schematic diagram of a portion of a multi-stage pump having still another version of a reverse fluid bypass circuit according to the present invention; 
         FIG. 5C  is a schematic diagram of a portion of a multi-stage pump having still another version of a reverse fluid bypass circuit according to the present invention; and 
         FIG. 5D  is a schematic diagram of a portion of a multi-stage pump having yet another version of a reverse fluid bypass circuit according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Various embodiments and examples of embodiments of the inventive concept will be described more fully hereinafter with reference to the accompanying drawings. In the drawings, the sizes and relative sizes of elements may be exaggerated for clarity. Likewise, the shapes of elements may be exaggerated and/or simplified for clarity and ease of understanding. Also, like numerals and reference characters are used to designate like elements throughout the drawings. 
     Furthermore, terminology used herein for the purpose of describing particular examples or embodiments of the inventive concept is to be taken in context. For example, the terms “comprises” or “comprising” when used in this specification indicates the presence of stated features or processes but does not preclude the presence of additional features or processes. The term “pump” may refer to apparatus that drives, or raises or decreases the pressure of a fluid, etc. The term “upstream stage” as used in connection with the compression mechanism of the pump will refer to any compression stage of the pump that is upstream of at least one other compression stage of the pump with respect to the direction of flow of fluid through the compression mechanism, i.e., the term “upstream stage” may not necessarily refer to the first stage of the compression mechanism if the pump has three or more compression stages. Likewise, the term “downstream stage” will refer to any compression stage of the pump that is downstream of at least one other stage. Therefore, unless otherwise stated, the terms “upstream” and “downstream” stages when used together will not imply that the stages are directly in series. 
     Referring now to  FIG. 1 , a multi-stage pump  1  to which the present invention can be applied generally includes a housing  100 , and a pump head assembly  200 , and a pump motor  300  disposed in the housing  100 . The pump may also include a cooling fan  400  for cooling the pump head assembly  200  and/or the pump motor  300 . In this example, the housing  100  defines an air inlet  100 A and an air outlet  100 B at opposite ends thereof, respectively, and the cooling fan  400  is provided in the air inlet  100 A to force cooling air through the housing and out the air outlet  100 B. 
     The housing  100  may also include a cowling  110  that covers the pump head assembly  200  and pump motor  300 , and a base  120  that supports the pump head assembly  200  and pump motor  300 . The cowling  110  may be of one or more parts and is detachably connected to the base  120  such that the cowling  110  can be removed from the base  120  to access the pump head assembly  200  for maintenance, repairs, etc. 
     The scroll vacuum pump  1  also has an inlet portion having a pump inlet  140  and constituting a low pressure side (e.g., a vacuum side) of the pump where fluid is drawn into the pump, and an exhaust portion having a pump outlet  150  and constituting a compression side where fluid is discharged from the pump at pressure greater than that of the fluid at the low pressure side. The pump head assembly  200  also has an inlet opening  260  at the inlet side of the pump, and a compression mechanism  270  that compresses the fluid, and an exhaust opening  280 . The inlet opening  200  connects the inlet  140  of the pump to the compression mechanism  270  of the pump. The exhaust opening  280  connects the compression mechanism  270  of the pump to the pump outlet  150 . 
     The compression mechanism  270  has a plurality of compression stages, including at least two compression stages disposed in series with respect to the direction of flow of fluid along a flow path defined by the pump. Each of the compression stages has an ingress through which the fluid enters the stage and an egress through which the fluid exits the stage. In the example shown  FIG. 1 , the pump I is a two stage pump having an upstream compression stage  270   a  and a downstream compression stage  270   b.  The flow path, as shown by the solid line with arrow heads, extends between the pump inlet  140  and the upstream compression stage  270   a,  between the egress of the upstream one of the compression stages  270   a  and the ingress of a downstream compression stage  270   b , and between the egress of the downstream compression stage  270   b  and the pump outlet  150 . 
     The pump  1  also has a reverse fluid bypass circuit  290  integrated with the pump head assembly  200 . The reverse fluid bypass circuit  290  defines a fluid passageway  295  discrete from the flow path and connected to the flow path at a first location ‘A’ between the egress of one (an upstream) compression stage  270   a  and the ingress of the next downstream compression stage  270   b,  and at a second location ‘B’ upstream of the ingress of the one (upstream) compression stage  270   a . The reverse fluid bypass circuit  290  also has fluid flow control means  299 . The reverse fluid bypass circuit  290  will be described in more detail below. 
     First, however, an example of the pump head assembly  200  of a multi-stage scroll type of pump according to the present invention will be described with reference to  FIG. 2   
     The pump head assembly  200  includes a frame  210 , an inner (first) stationary plate scroll  220 A, an orbiting plate scroll  230 , an outer (second) stationary plate scroll  220 B, an eccentric drive mechanism  240  driven as a result of a rotary output by the motor  300 , a tubular member  250  and fasteners (not shown) fixing the stationary plate scrolls  220 A and  220 B to the frame  210  and the tubular member  250  to both the frame  210  and the orbiting plate scroll  230 . As shown in the figure, the outer stationary plate scroll  220 B may be fixed to the frame  210  through the intermediary of the inner stationary plate scroll  220 A. 
     The inner stationary plate scroll  220 A includes a first stationary scroll blade  221  of the pump and a first stationary plate  222  having an outer (front) side and an inner (back) side. The first stationary scroll blade  221  projects axially (parallel to a longitudinal axis of the pump) in a first direction from the outer side of the first stationary plate  222 . The outer stationary plate scroll  220 B includes a second stationary scroll blade  223  of the pump and a second stationary plate  224  having an outer (back) side and an inner (front) side. The second stationary scroll blade  223  projects axially in a second direction, opposite the first direction, from the inner side of the second stationary plate  224 . 
     The orbiting plate scroll  230  is interposed between the inner and outer stationary plate scrolls  220 A,  220 B in the axial direction of the pump and is coupled to the eccentric drive mechanism  240  so as to be driven by the eccentric drive mechanism  240  in an orbit about the longitudinal axis of the pump. The orbiting plate scroll  230  includes an orbiting plate  231  having an outer side and an inner side, a first orbiting scroll blade  232  projecting axially in the second direction from the inner side of the orbiting plate  231 , and a second orbiting scroll blade  233  projecting axially in the first direction from the outer side of the orbiting plate  231 . The first orbiting scroll blade  232  is juxtaposed with the first stationary scroll blade  221  in the radial direction of the pump such that the first stationary scroll blade  221  and the first orbiting scroll blade  232  are nested, The second orbiting scroll blade  233  is juxtaposed with the second stationary scroll blade  223  of the pump in the radial direction of the pump such that the second stationary scroll blade  223  and the second orbiting scroll blade  233  are nested. In these respects, the stationary and orbiting scroll blades  223 ,  233  and  221 ,  232  are nested with a clearance and predetermined relative angular positioning such that a pocket for pockets) is delimited by and between the nested scroll blades. 
     The eccentric drive mechanism  240  includes a drive shaft and bearings  246 . In this example, the drive shaft is a crank shaft having a main portion  242  connected to and rotated by the motor  300  about the longitudinal axis of the pump  100 , and a crank  243  whose central longitudinal axis is offset in a radial direction from the longitudinal axis. The bearings  246  may comprise a plurality of sets of rolling elements. 
     Also, in this example, the main portion  242  of the crank shaft is supported by the frame  210  via one or more sets of the bearings  246  so as to be rotatable relative to the frame  210 . The orbiting plate scroll  230  is mounted to the crank  243  via another set or sets of the bearings  246 . Thus, the orbiting plate scroll  230  is carried by crank  243  so as to orbit about the longitudinal axis of the pump when the main shaft  242  is rotated by the motor  300 , and the orbiting plate scroll  230  is supported by the crank  243  so as to be rotatable about the central longitudinal axis of the crank  243 . 
     The volume of the pocket(s) delimited by the sets of nested scroll blades  221 ,  232  and  223 ,  233  is varied as the orbiting scroll blades  232 ,  233  move relative to the stationary scroll blades  221 ,  223 , respectively. The motion of the orbiting scroll blades  232 ,  233  also causes the pocket(s) defined between the sets of nested scroll blades  221 ,  232  and  223 ,  233  to move within the pump head assembly  200  such that the pocket(s) is/are selectively placed in open communication with the pump inlet (via inlet opening  260 ) and the pump outlet (via exhaust opening  280 ). As a result, fluid is drawn along a flow path extending through the pump head assembly  200  as indicated by the arrows in the figure. 
     In this example, there are at least two compression stages where pumping occurs. For example, a downstream compression stage of the pump, corresponding to stage  270   b  in  FIG. 1 , may be defined between the orbiting plane scroll  230  and the inner stationary plate scroll  220 A. The upstream compression stage of the pump, corresponding to compression stage  270   a  in  FIG. 1 , may be defined between the orbiting plate scroll  230  and the outer stationary plate scroll  220 B. 
     Alternatively, the same pump may be configured as a three stage pump. For example, an upstream (or first) compression stage of the pump and an intermediate (or second) compression stage may be defined between the orbiting plate scroll  230  and the outer stationary plate scroll  220 B. In this respect, reference may be made to the aforementioned U.S. Pat. No. 6,884,047 showing the provision of two stages between an orbiting plate scroll and a stationary plate scroll. In the first stage, the fluid is pumped by the action of the radially outer parts of the stationary y and orbiting scroll blades. In the second stage, the fluid is pumped by the action of the radially inner parts of the stationary and orbiting scroll blades. The radially inner parts of the stationary and orbiting scroll blades have heights that are less than those of radially outer parts of the stationary and orbiting scroll blades. Thus, the first stage has an axial depth that s greater than that of the second stage. 
     On the other hand, a downstream (or third) compression stage of the pump may be defined between the orbiting plate scroll  230  and the inner stationary plate scroll  220 A. In this downstream compression stage, therefore, the fluid is pumped by the action of the stationary scroll blade  221  of the inner stationary plate scroll  220 A and the first orbiting scroll blade  232  of the orbiting plate scroll  230 . 
     Referring still to  FIG. 2 , the tubular member  250  has a first end at which it is fixed to the orbiting plate scroll  230 , and a second end at which it is fixed to the frame  210 . The tubular member  250  also extends around a portion of the crank shaft  243  and the bearings  246  of the eccentric drive mechanism  240 . In this way, the tubular member  250  may also seal the bearings  246  and bearing surfaces. Accordingly, lubricant employed by the bearings  246  and/or particulate matter generated by the bearings surfaces can be prevented from passing into the flow path. The tubular member  250  is radially flexible enough to allow the first end thereof to follow along with the orbiting plate scroll  230  while the second end thereof remains fixed to the frame  210 . 
     In the illustrated example, the tubular member  250  is a metallic bellows whose torsional stiffness prevents the first end thereof from rotating significantly about the central longitudinal axis of the bellows, i.e., from rotating significantly in its circumferential direction, while the second end of the bellows remains fixed to the frame  210 . Accordingly, the metallic bellows  250  may provide the angular synchronization between the stationary scroll blades  221  and  223  and the first and second orbiting scroll blades  232  and  233 , respectively, during the operation of the pump. 
     In addition, and although not shown in  FIG. 2  for the sake of simplicity, the scroll pump is a dry scroll pump including tip seals each seated in a groove extending in and along the length of the tip (axial end) of a respective one of the scroll blades. 
       FIG. 3A  shows the tip seals  220 AS,  230 S associated with the first stationary plate scroll  220 A and the orbiting plate scroll  230 , respectively.  FIG. 3B  shows the tip seals  220 BS associated with the second stationary plate scroll  220 B and the orbiting plate scroll  230 , respectively. Each tip seal in this example is a plastic member interposed between the tip of the scroll blade  221 ,  232  of one of the first stationary and orbiting plate scrolls  220 A,  230  and the plate  231 ,  222  of the other of the first stationary and orbiting plate scrolls  220 A,  230 . 
     The multi-stage scroll type of pump also has a reverse fluid bypass circuit corresponding to reverse fluid bypass circuit  299  shown in  FIG. 1 . The reverse fluid bypass circuit functions as follows. Here, reference will be made to the pump  1  of  FIG. 1 . 
     In general, the reverse bypass fluid circuit  290  circulates fluid flowing along the leg of the flow path directly connecting first and second ones of the compression stages back upstream of the ingress of the first of the compression stages when the pressure difference of the fluid between the compression stages (referred to as the inter-stage pressure) and the pressure at the ingress of the first stage exceeds a predetermined value. In the case of the two stage pump  1  of FIG.  1 , for example, the reverse bypass fluid circuit  290  circulates fluid flowing along the leg of the flow path connecting the first and second compression stages  270   a ,  270   h  back upstream of the ingress of the first compression stage  270   a  and therefore, essentially back to the inlet  140  of the pump, when the inter-stage pressure compared to the inlet pressure is higher by a predetermined amount, e.g., is higher by ˜1-2 psi. 
     To this end, the fluid flow control means  299  may comprise a valve, such as a spring-loaded check valve whose cracking pressure is set to open When the inter-stage pressure is higher than the inlet pressure by the predetermined amount. In the example given above in which the inter-stage pressure is ˜1-2 psi higher than the inlet pressure, the cracking pressure of the check valve may be 1-1.5 psi. Other types of valves, such as pneumatically- or solenoid-operated valves may be employed instead. In these cases, the reverse fluid bypass circuit may also include one or more pressure sensors connected to the fluid passageway  295  and operatively connected to the valve so as to open the valve when a predetermined pressure or pressure difference is sensed in passageway  295 . 
     This provides the following advantages. Maximum power consumption in both the first and second stages  270   a,    270   b  is constrained by the operation of the reverse fluid bypass circuit  290 . Constraining the maximum power consumption in the first stage  270   a,  in addition to in the second stage  270   b,  results in a dramatic reduction in driving torque, allowing the pump  1  to start as well as protecting the bearings and motor  300  from excessive loads. 
     Secondly, a tip seal that is not spring loaded, as shown in and described with reference to  FIGS. 2 ,  3 A and  3 B, must remain “energized” meaning that it must remain in engagement with the opposing plate to provide a sufficient seal even, for example, when the tip seal Nears by a certain amount during use. If the tip seals of a second and any additional downstream compression stages are not “energized”, the large pressure differential across the first stage combined with a relatively large first stage displacement could potential overload the motor. This, in turn, could prevent the pump from starting or damage other components in the pump such as bearings. 
     In order to energize each of the tip seals, a pressure differential across the tip seal is typically required to force the tip seal against the opposing plate which will then provide the required sealing. However, in order to generate the necessary pressure to “energize” the tip seals, the tip seals must first move forward against the opposing plate to create a sufficient seal. Accordingly, it is possible for the tip seals to not “energize” properly when the pump is starting, especially if the pump is equipped with a forward fluid bypass circuit which prevents pressure from being developed in the space between the first and second stages that would otherwise serve to energize the tip seals. 
     By circulating the fluid back upstream of the ingress of an upstream compression stage when a certain inter-stage pressure differential exists between the ingress of the upstream compression stage and the ingress of the next downstream compression stage, the reverse fluid bypass circuit  290  of the present invention helps ensure that the tip seals of the next downstream compression stage and those of any additional downstream compression stages are “energized”. 
       FIGS. 4A ,  4 B and  4 C illustrate different layouts of the reverse fluid bypass circuit  290 . Any of these layouts may be applied to the multi-stage scroll type of pump shown in and described with reference to  FIG. 2 , as well as to any other type of multi-stage positive displacement pump. 
       FIG. 4A  shows a two stage pump in which the compression mechanism consists of a 1 st  (upstream) compression stage and a 2 nd  (downstream) compression stage. Here, the inter-stage pressure differential that the reverse fluid bypass circuit  290  regulates is that between the ingress of the 1 st  stage and ingress of the 2 nd  stage. 
       FIG. 4B  shows a three stage pump in which the 1 st , 2 nd , and 3 rd  compression stages are connected in series by a contiguous flow path of the pump extending from the pump inlet to the pump outlet. In this example, the 2 nd  compression stage is an upstream compression stage, the 3 rd  compression stage is the next downstream compression stage and the reverse fluid bypass circuit circulates the fluid back to location B upstream of the ingress of the 1 st  compression stage and hence, upstream of the ingress of the 2 nd  compression stage. Here, the inter-stage pressure differential that the reverse fluid bypass  290  regulates is that between the egress of the 2 nd  stage and the ingress of the 1 st  stage. That is, this example shows that the upstream compression stage does not have to be the first compression stage along the flow path. 
       FIG. 4C  shows a three stage pump in which the 1 st , 2 nd , and 3 rd  compression stages have a parallel arrangement. In this case, a contiguous flow path of the pump extends from the pump inlet directly to the 2 nd  compression stage, from the 2 nd  compression to the 3 rd  compression stage, and from the 3 rd  compression stage to the pump outlet. In this example, the 2 nd  compression stage is an upstream compression stage, the 3 rd  compression stage is the next downstream compression stage and the reverse fluid bypass circuit circulates the fluid back to location B upstream of the ingress of the 2 nd  compression stage. Here, the inter-stage pressure differential that the reverse fluid bypass circuit  290  regulates is between the egress and ingress of the 2 nd  stage. 
     Of course, these layouts may be extended to apply to multi-stage pumps having more than three compression stages. 
     Furthermore, although the reverse fluid bypass circuit  290  has been described above as having flow control means in the form of a valve such as a spring-loaded check valve,  FIGS. 5A-5D  show alternative forms of the flow control means. 
       FIG. 5A  shows the reverse fluid bypass circuit  290  as including a capillary tube  299   a  as flow control means. 
       FIG. 5B  shows the reverse fluid bypass circuit  290  as including an orifice  299   b  disposed in-line with the fluid passageway  295 . 
       FIG. 5C  shows the reverse fluid bypass circuit  290  as including a flexible tube which collapses when the pressure inside the tube is less than the pressure outside the tube by a certain amount. Atmospheric or another reference pressure provides the force to collapse the flexible tube when the pressure inside drops below a predetermined value. 
       FIG. 5D  shows an example of fluid flow control means for restricting the flow of fluid while the fluid is in its transitional or molecular flow regime, while allowing the flow of fluid while the fluid is in its viscous flow regime. In this example, the reverse fluid bypass circuit  290  includes a labyrinth seal  299   d . However, other fluid flow control means may be used to restrict flow in the transitional or molecular regime, while allowing flow in the viscous regime. 
     Finally, embodiments of the inventive concept and examples thereof have been described above in detail. The inventive concept may, however, be embodied in many different forms and should not be construed as being limited to the embodiments described above. Rather, these embodiments were described so that this disclosure is thorough and complete, and fully conveys the inventive concept to those skilled in the art. Thus, the true spirit and scope of the inventive concept is not limited by the embodiment and examples described above but by the following claims.