Abstract:
A high temperature centrifugal pump includes non-linear fluid flow paths between the pump housing and seal housing to reduce fluid flow therebetween. In another aspect, the invention provides a drive shaft of variable diameter to compensate for different amounts of thermal expansion to maintain a more consistent gap profile between the drive shaft and thermal insulators or other components positioned around the drive shaft. In yet another aspect, a tube heat exchanger may be placed in fluid communication with the seal housing to re-circulate and cool the fluid therethrough.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to a high temperature centrifugal pump, and more particularly relates, in a first aspect thereof, to a pump having a drive shaft with a non-linear fluid channel designed to reduce the rate of fluid flow between the pump housing and the seal housing. In a second aspect thereof, the present invention relates to a thermally expandable drive shaft having a variable diameter designed to balance and compensate for different amounts of thermal expansion along its length. In yet a third aspect, the present invention relates to a tube heat exchanger operable to re-circulate and cool fluid through the seal housing interior cavity. 
         [0002]    The present invention provides improvements to the invention of commonly owned U.S. Pat. No. 5,624,245, the entire disclosure of which is incorporated herein by reference. Centrifugal pumps which operate at high temperatures (e.g., up to about and exceeding 400° F.) typically incorporate features designed to protect the motor and seals from the high temperature of the working fluid in the pump housing. FIG. 1 of the &#39;245 patent is reproduced herein and is seen to include a pump housing  16  connected to a pump adapter casing which surrounds a seal housing  34  containing a mechanical seal  42  which prevents fluid from passing along drive shaft  28  toward the motor and fan  56 . The drive shaft connects to an impeller  22  in pump housing  16  for directing fluid from pump inlet  24  to pump outlet  26 . Thermal insulators  46  are mounted about the drive shaft between the pump housing and the seal housing to reduce heat transfer from the pump housing to the seal housing. An annular passage  54  allows working fluid to travel from the pump housing to the interior cavity of the seal housing to lubricate the seal during pump operation. To aid in dissipating heat from fluid within reservoir  40 , the seal housing  34  includes external and internal fins  36 ,  38 , respectively, to help draw heat away from the reservoir and into the interior cavity  62  of the pump adapter casing  12 . The interior cavity  62  is vented to the ambient through one or more holes  64  to allow the escape of heated air. 
         [0003]    While the invention of the &#39;245 patent is effective at providing a good degree of thermal protection to the mechanical seal, some heat still reaches the mechanical seal due to the necessary lubricating fluid delivered via passage  54 . Although, as noted in the &#39;245 patent, little liquid circulates through passage  54  after pump startup (see Col. 3, Lns. 61-67), the liquid coming from the pump housing is very hot and it would therefore be desirable to further minimize and/or reduce the velocity of the free fluid transfer between the pump housing and seal housing as much as possible. It would furthermore be desirable to carry away a higher percentage of heat from seal reservoir  40  to further protect the mechanical seal from heat damage. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention addresses the above needs by providing, in a first aspect thereof a non-linear or variable diameter fluid path between the pump housing and the seal housing. In one embodiment, a variable diameter fluid path is created by providing one or more enlarged annular fluid channels along the fluid path. The enlarged fluid channels may be formed by areas of reduced diameter on either or both of the facing surfaces of the drive shaft and thermal insulators or other component defining the fluid path from the pump housing to the seal housing. The annular fluid channels operate to allow the fluid to pool in the channels which creates turbulence in the fluid path between the pump housing and seal housing. As such, movement of fluid through the fluid path is reduced as compared to a fluid path that is of constant width along a linear path. 
         [0005]    In a second aspect, which may be used separately or in combination with the first aspect of the invention described above, a variable diameter drive shaft is provided to compensate for differing amounts of thermal expansion along the length of the drive shaft. As discussed above, a fluid path or “gap” is formed between the facing surfaces of the drive shaft and thermal insulators (or other components in facing relation to the drive shaft). Other than at the enlarged areas for reducing fluid flow described above, it is generally desirable to minimize the gap as much as possible while still allowing the drive shaft to freely rotate with respect to the rotationally fixed thermal insulators. The end of the drive shaft closest to the pump housing will undergo a larger amount of thermal expansion than the end nearest the motor. As such, a thermal gradient develops along the length of the drive shaft which affects the amount of thermal expansion in a like manner. In prior art drive shaft designs of constant diameter, this thermal expansion gradient created a variable and hence out of specification gap along its length. The present invention provides a variable diameter drive shaft which increases in diameter toward the cool end of the shaft to balance and compensate for the higher thermal gradient near the hot end of the drive shaft. 
         [0006]    In a third aspect of the invention, which may be used in combination with or separately of the first and second aspects described above, a tube heat exchanger is provided in fluid communication with the seal housing interior reservoir. In one embodiment, the heat exchanger is a coil type heat exchanger having inlet and outlet ends connected to the seal housing with the coil extending exteriorly of the seal housing. Lubricating fluid in the seal housing reservoir is directed into the inlet end of the heat exchanger, travels and in the process cools through the coil, and then returns to the seal housing reservoir through the outlet end of the heat exchanger. As such, the lubricating fluid is being constantly recirculated and cooled through the seal housing reservoir, thus increasing the amount of heat carried away from the lubricating fluid which even further protects the mechanical seal from heat damage. The coil may be advantageously placed adjacent the fan located between the motor and seal housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]      FIG. 1  is a cross-sectional view of the prior art pump depicted in FIG. 1 of commonly owned U.S. Pat. No. 5,624,245; 
           [0008]      FIG. 2  is a cross-sectional view of an embodiment of the present invention; 
           [0009]      FIG. 2   a  is an enlarged view of the detail circle seen in  FIG. 2 ; 
           [0010]      FIG. 3  is a side elevational view of an embodiment of the drive shaft of the present invention; 
           [0011]      FIGS. 4   a - 4   f  are diagrammatic representations of a variety of possible fluid path profiles; 
           [0012]      FIG. 5  is an elevational view of certain components of the pump of  FIG. 2  to better illustrate the tube heat exchanger connection to the seal housing; and 
           [0013]      FIG. 6  is a perspective view of the seal housing. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0014]    Referring now to the drawing, there is seen in  FIG. 2  an embodiment of a high temperature centrifugal pump  100  having a pump housing  112  in which is contained an impeller  114  to which a first threaded end  116  of a drive shaft  118  attaches via lock nut  117 , the opposite shaft end  120  attaching to the drive output  122  of a motor  124 . To reduce heat transfer along the drive shaft, in a preferred embodiment, drive shaft  118  is in the form of a hollow sleeve as shown with output shaft  122  telescoping into sleeve  118 . Motor  124  is operable to rotate drive shaft  118  and impeller  114  to pump a working fluid from pump inlet  126  to pump outlet  128 . 
         [0015]    A seal housing  130  is provided which surrounds a mechanical seal  132  mounted on shaft  118  to prevent working fluid from reaching motor  124 . In a preferred embodiment, seal  132  is spring loaded with a spring  134  applying a biasing force toward motor  124 . A snap ring  136  may be fitted to an annular groove  138  formed in drive shaft  118  (see also  FIG. 3 ) to fix first spring end plate  140  in place. Spring seat  144  may freely translate axially along the neck of seal  132  with spring  134  biasing seat  144  against the shoulder of the seal  132  to form a tight seal between seal housing  130  and drive shaft  118 . As more fully explained below, a fluid path is provided between pump housing  112  and seal housing  130  with fluid collecting in seal housing reservoir  130 ′ to lubricate seal  132 . 
         [0016]    One or more thermal insulators  142  as described in the &#39;245 patent may be provided to extend about drive shaft  118  adjacent end  116  thereof to provide thermal insulation between pump housing  114  and seal housing  132 . As seen best in  FIG. 2   a , the inner diameter surface  142   a  is in close, facing relation to drive shaft cylindrical surface  118   a , forming a small gap G therebetween wherethrough fluid may flow from pump housing  114  to seal housing reservoir  130 ′ along the fluid path indicated by the arrows labeled FP. 
         [0017]    As seen in prior art  FIG. 1 , the fluid path  54  formed an essentially linear fluid path between the facing surfaces of the insulators  46  and drive shaft  28 , As such, fluid easily flows between pump housing  16  and seal housing reservoir  40  and is an added source of heat transfer between the pump and seal housings. Rather than the linear fluid path of the prior art, the present invention provides a non-linear fluid path FP to slow down the flow of fluid (and hence the amount of heat transfer) between the pump and seal housings. 
         [0018]    Non-linear flow paths may be formed in a variety of configurations, for example, geometries designed to create one or more areas of turbulence within the path which act to slow down the flow of fluid. Referring to  FIGS. 2   a  and  3 , one possible configuration of a non-linear fluid path FP is created by providing first and second areas of reduced diameter A 1  and A 2  on drive shaft  118 . Areas A 1  and A 2  extend in longitudinally spaced relation on drive shaft  118  and face inner diameter surface  142   a  of thermal insulators together defining non-linear flow path FP. The areas of reduced diameter A 1  and A 2  create enlarged fluid areas EFA which allow the fluid to pool and undergo turbulence in these areas, thus slowing down the advance of fluid from the pump housing toward the seal housing. 
         [0019]      FIGS. 4   a - f  illustrate other possible configurations of a non-linear flow path although it will be understood to those skilled in the art that further configurations are possible and the invention is thus not to be limited thereby. Also, although not shown in  FIGS. 4   a - f , it is understood the flow paths are created between the spaced, facing surfaces of the drive shaft and insulators (or other component) which are each configured to achieve the desired path flow profile. 
         [0020]      FIG. 4   a  illustrates a non-linear flow path FP 1  having one or more enlarged fluid areas EFA located on the same side of the fluid path. 
         [0021]      FIG. 4   b  illustrates a non-linear flow path FP 2  having one or more enlarged fluid areas EFA located on opposite sides of the fluid path. 
         [0022]      FIG. 4   c  illustrates a non-linear flow path FP 3  having one or more enlarged fluid areas EFA which are curved and located on the same side of the fluid path. 
         [0023]      FIG. 4   d  illustrates a non-linear flow path FP 4  having one or more reduced fluid areas RFA created by one or more obstructions O located on the same side of the fluid path. The obstructions O may be created by elements integral or separate to the drive shaft and insulators or other components. 
         [0024]      FIG. 4   e  illustrates a non-linear flow path FP 5  having one or more enlarged fluid areas EFA created by both an obstruction O and an area of reduced diameter A 3 . 
         [0025]      FIG. 4   f  illustrates a non-linear flow path FP 6  having one or more curved areas CA and may or may not be of constant width W 1  throughout the length of the flow path. 
         [0026]    Referring again to  FIGS. 2   a  and  3 , since drive shaft  118  must be able to freely rotate with respect to rotationally fixed insulators  142 , a gap G exists between their facing surfaces (which also forms the fluid path as described above). Thus, while gap G is necessary, it is generally desirable to maintain gap G to a very small width regardless of the presence or absence of enlarged or reduced fluid areas designed to slow the fluid flow as described above. Drive shaft  118  may be formed of a thermally expandable material and, as such, will expand more in the area near end  116  which is closest to hot pump housing  112 , and less in the area near end  120  which is closest to cooler motor  124 . In prior art embodiments where the drive shaft is of constant diameter, the difference in thermal expansion along the length of the shaft can cause unequal gap widths leading to an inconsistent and out of specification gap dimension profile along the shaft length during pump operation. To address this problem with prior art drive shafts, the present invention provides a drive shaft having a variable diameter longitudinal section such as at D 1 , D 2  and D 3  (see  FIG. 3 ) with the shaft diameter gradually increasing in the direction toward cool end  120  whereby D 1 &lt;D 2 &lt;D 3 , During pump operation, the shaft temperature gradient will start and be highest adjacent hot end  116  and gradually lower in the direction of cool end  120 . As such, thermal expansion will be greatest at diameter D 1  and lower at diameter D 2  and yet still lower at diameter D 3 . By making D 1 &lt;D 2 &lt;D 3 , the differences in thermal expansion are compensated for whereby upon full thermal expansion of the drive shaft, the variable diameter longitudinal section becomes substantially equal in diameter resulting in a uniform gap profile G. 
         [0027]    Referring to  FIGS. 2 ,  5  and  6 , as in the &#39;245 patent, seal housing  130  may include a plurality of radially extending fins  131  to carry heat away from seal housing reservoir  130 ′. To even further aid in cooling of seal housing  130 , a tube heat exchanger  146  may be provided to constantly re-circulate fluid through reservoir  130 ′. In the embodiment illustrated in the Figures, heat exchanger  146  is a coil type heat exchanger having an inlet end  146   a  and outlet end  146   b  which are attached to respective through holes  130   a  and  130   b  formed in seal housing  130  (see  FIG. 6 ). Fluid in reservoir  130 ′ enters the heat exchanger  146  at inlet end  146   a , circulates and cools through coils  146   c , and returns to reservoir  130 ′ through outlet end  146   b . As seen in  FIG. 5 , inlet end  146   a  may be axially spaced from outlet end  146   b  in a direction toward motor  124  which assists in the re-circulating flow of fluid through reservoir  130 ′ and heat exchanger  146 . 
         [0028]    A cooling fan  148  may be provided as seen in  FIG. 2  to assist in cooling seal housing  130 . Heat exchanger  146  may be strategically positioned with respect to fan  148  to benefit from the air currents created by fan  148 . In the embodiment of  FIG. 2 , heat exchanger coils  146  are positioned radially outwardly of and span the area between fan  148  and motor  124 . A shield  150  may also be provided if desired. 
         [0029]    While the present invention has been described with respect to preferred embodiments thereof, it is understood that variations will be apparent to those skilled in the art and the invention is not to be limited thereby but rather by the full spirit and scope of the claims which follow. For example, although the invention has been described in the context of a high temperature centrifugal pump, it is envisioned the advantages afforded by the various aspects of the invention may have applicability to other applications where heat transfer control is an issue.