Abstract:
A seal assembly for a mechanical seal includes a rotatable shaft with at least one mechanical seal disposed about the shaft, the seal having a rotatable face coupled to the shaft and a stationary face, wherein the respective faces of the seal are in contact with one another, and a chamber for holding a cooling fluid, disposed about the shaft and in communication with the faces of the seal, and a closed loop fluid path disposed about the outer diameter of the shaft, preferably or in a non linear manner, in fluid communication with the chamber, for circulating fluid about the seal faces.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to mechanical seals, and more particularly, to a fluid-cooled mechanical seal on a shaft. 
     A variety of mechanical seals have been developed for use along a shaft, often in the context of pumps. One typical configuration is a mechanical seal with one stationary face and one rotating face. The rotating face of the seal rotates with the shaft of the pump, while the stationary face of the seal is generally coupled to the housing of the pump. In order to provide a tight seal, the two faces are typically in contact with each other. The frictional contact between the faces generates heat. 
     In order to dissipate heat, a fluid may be added to help transfer the heat away from the seal faces. Typically, a small fluid chamber is disposed about the shaft, so that the fluid is in communication with the seal face. As these mechanical seals are frequently used in a double or tandem configuration, the chamber may be disposed along the shaft, between and including the two mechanical seals. Often, a cooling fluid reservoir is added, with an auxiliary pump to circulate the fluid between the reservoir and the chamber. However, the addition of an auxiliary pump adds cost, requires additional space, and adds another component that is subject to failure, thereby reducing reliability. 
     There are many applications where a mechanical seal is subjected to fluid at the ID of the face. One of the most common is that of an unpressurized tandem seal where the barrier fluid is in contact with the ID of the primary seal and at the OD of the secondary seal. There is circulation of the barrier fluid into and out of the seal chamber by means of some type of pumping device that is usually part of the secondary seal rotating element. This circulation is adequate for cooling the secondary seal but is less than satisfactory for cooling the primary seal. This lack of cooling performance for the primary seal is due to the inability of the fluid to circulate to the ID of the seal. 
     Another application where cooling is needed at the seal face is in a vertical pump gear box seal oriented with the gear box oil at the ID of the seal. Gravity ensures that oil is at the ID of the face. However, during dynamic operation this fluid can not circulate with the bulk fluid in the gear box. This leads to increased seal temperature and possibly coking of the oil at one or both of the seal faces. Coking leads to increased leakage and damage to seal faces. 
     Rather than use an auxiliary pump, other configurations have built a “pumping rotor” into the system. See, e.g., U.S. Pat. No. 4,466,619 to Adams U.S. Pat. No. 4,560,173 to Adams et al. A slotted sleeve is fitted concentrically about the shaft, whereby the rotational movement of the shaft aids in circulating fluid along a fairly linear path, drawing the fluid from the reservoir into the chamber through an inlet, moving it radially around the shaft, and pushing it out of the chamber through an outlet and back into the reservoir. Another type of seal uses screw-type threads on the shaft to move the fluid between an inlet and an outlet. However, the fluid may only be moved in one direction in the chamber, between the inlet and the outlet, and away from the mechanical seal. 
     Because of the seal mechanisms themselves, it is generally not possible to position an inlet or outlet directly adjacent a seal face. Thus there is a space in the chamber between the inlet and outlet, which define the path of circulation, and the seal face, where the heat is generated and where the fluid will be heated the most. This causes a “dead end” space in the chamber between the seal faces and the respective inlet and outlet, where the cooling fluid is substantially stagnant, and does not circulate with the rest of the fluid. In the “pumping rotor” configuration discussed above, the radial circulating action occurs in a “band” that is aligned with the inlet and outlet; fluid outside this band remains substantially uncirculated. In the screw type circulator discussed above, these dead spots occur on either side of the inlet and outlet, as the fluid is substantially circulated only between the inlet and outlet. In the double or tandem configuration, there is generally a band or path of circulation between the seals, but there is inadequate circulation directly at the seal faces, where circulation is most necessary. 
     Thus there exists the need for a circulation device for mechanical seals which provides circulation to the seal faces, preferably without the use of auxiliary pumps, which can circulate fluid about the seal faces beyond the respective inlet and outlet locations, and which operates under rotation of the shaft in either direction. 
     SUMMARY OF THE INVENTION 
     The present invention is embodied in a cooling assembly for a shaft-mounted mechanical seal, which provides a chamber about the shaft and seal. A closed loop fluid path about the outer diameter of the shaft preferably provides both axial and radial pumping action upon rotation of the shaft, and effectively circulates cooling fluid in the chamber to cool the faces of the seal. The design of the cooling assembly circulates fluid in “dead end” spaces at the seal faces, beyond the location of the inlet or outlet, and more thoroughly circulates the fluid within the chamber. This results in reduced face temperature, elimination of coking and increased seal life. 
     An important feature of the present invention is the ability to circulate the fluid within the “dead end” space at the opposed faces of the seal. Although fluid cannot be pumped beyond this point, the present invention provides both radial and axial circulation, lifting the heated fluid away from the opposed seal faces and replacing it with cooler fluid. 
     Another important feature of the present invention is that it operates effectively upon rotation of the shaft in either direction. Thus the mechanical seal remains cooled regardless of the direction of shaft rotation. 
     The increased circulation the present invention provides also reduces the operating temperature at the seal faces, extending the life of the seal and reducing the chance of failure. 
     Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The details and features of the present invention may be more fully understood by referencing the detailed description and drawings, in which: 
     FIG. 1 is an elevation view of a first embodiment of the present invention. 
     FIG. 2 s a plan view of the sleeve of FIG. 1 with the sleeve shown sectioned and laid flat. 
     FIG. 3 is an elevation view of a second embodiment of the present invention. 
     FIG. 4 is a plan view of an alternative embodiment of the sleeve with the sleeve shown sectioned and laid flat. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a vertical pump gear box seal  10  with oil (or cooling fluid) A at the ID. A housing  20  encloses a mechanical seal  30  disposed about a vertically oriented shaft  40 . The seal has two faces—a stationary face  32 , coupled to the housing  20 , and a rotating face  34 , coupled to the shaft  40 . The housing  20  also encloses a fluid chamber  50 , which contains the cooling fluid A supplied through inlet  52  from a reservoir (not shown). The fluid chamber  50  surrounds the shaft, and provides fluid to the faces of the seal. Preferably, a sleeve  42  is coupled concentrically about the shaft, and the outer surface of the sleeve  42  defines a closed loop fluid path  44 . Preferably, the closed loop path is a groove having a base  43  and two spaced apart walls  45  (See FIG.  2 ). 
     With reference to FIG. 2, the sleeve  42  has been sectioned and laid flat to see the preferred groove that is manufactured (or milled) into the sleeve. The closed loop fluid path  44  is preferably a continuous groove about the sleeve, most preferably sinusoidal in circumferential profile and spanning substantially the axial dimension of the chamber  50 . In other words, the high point of the closed loop  44  is near the high point of the chamber  50 , while the low point of the closed loop  44  is near the low point of the chamber  50 . It will be appreciated by those skilled in the art that the circumferential profile of the closed loop can be a variety of shapes, that preferably substantially spans the axial dimension of the chamber  50 . The sinusoidal design, in particular, however, offers excellent cooling performance due to the minimization of turbulence in the oil. Turbulence is reduced because the entrance point (upper right of sleeve in FIG. 1) and the return point (lower left of sleeve in FIG. 1) are tangential to the direction of motion which results in a smooth transition for change in flow direction. Reducing turbulence increases cooling. Alternatively, when no sleeve is used the outer surface of the shaft  40  can define the closed loop fluid path  44 . 
     As the shaft  40  and the sleeve  42  rotate, the closed loop fluid path  44  rotates also. Because the closed loop fluid path substantially spans the axial dimension of the chamber, it appears to move up and down. Viewing the sleeve from a fixed viewpoint, as the shaft rotates the closed loop fluid path appears to oscillate up and down in a sinusoidal fashion. This axial movement takes the cooling liquid from the top of the chamber and “pumps” it down to the face of the seal at the bottom of the chamber, then lifts the heated fluid away from the seal at the bottom of the chamber and brings it to the top. In other words, when oil engages the entrance point in the sleeve groove, it is pumped to the left, because of shaft rotation, as shown by the oil circulation arrow. When it reaches the lower left part of the groove the flow direction is changed to pump the oil back up and out to the bulk of the oil. This results in a constant supply of cooler oil being delivered to the entire ID of the stationary seal face. One advantage to this design is its bi-directionality, i.e., circulation will occur regardless of shaft rotation direction. In addition to the axial motion, the rotational motion of the sleeve and the closed loop fluid path create a radial circulation about the shaft as well. Thus effective circulation of the fluid is achieved, increasing the heat transfer and reducing the temperature at the seal faces. 
     FIG. 3 illustrates another embodiment of the invention, where the shaft  40  is horizontally oriented in a tandem seal or double seal arrangement. A housing  20  encloses two mechanical seals—a primary seal  30  and a secondary seal  31 —disposed about a horizontally oriented shaft  40 . Each seal has two faces—a stationary face  32 , 33  coupled to the housing  20 , and a rotating face  34 , 35  coupled to the shaft  40 . The housing  20  also encloses a fluid chamber  50 , which contains the cooling fluid. The fluid chamber  50  surrounds the shaft, and provides fluid to the faces of the seals. The chamber encloses an area about the shaft from the inner diameter of the primary seal  30 , extending along the shaft to the outer diameter of the secondary seal  31 . 
     It will be appreciated by those skilled in the art that the orientation of the seals may be varied without departing from the present invention. In another embodiment, the chamber extends from the outer diameter of the primary seal to the inner diameter of the secondary seal. In yet another embodiment, the chamber extends along the shaft from the inner diameter of the primary seal to the inner diameter of the secondary seal. 
     A sleeve  42  is coupled concentrically about the shaft, and the outer surface of the sleeve  42  defines a closed loop fluid path  44 . The closed loop fluid path  44  is a continuous groove about the sleeve, preferably sinusoidal in circumferential profile. The closed loop fluid path  44  is located partially within the chamber portion containing the primary seal. Distal from this position, and located axially along the shaft at substantially the location of the secondary seal, are the inlet  52  and outlet  54  which communicate with the cooling fluid reservoir (not shown). The pumping action of the closed loop fluid path  44  draws cooling fluid from the inlet, circulates the fluid through the chamber, and expels the fluid to the fluid reservoir through the outlet  54 . Preferably, the inlet and outlet are offset radially about the shaft from one another, but are located at substantially the same axial position. However, it will be appreciated by those skilled in the art that the inlet  52  and outlet  54  may be located at other locations. 
     As demonstrated above, the present invention may be used equally as well in a wide variety of seal configurations. The cooling assembly may be used with a single mechanical seal, as well as in a tandem or double-seal configuration, or configurations utilizing more than two mechanical seals. 
     As noted above, there are many possible variations of the groove design. For example, FIG. 4 illustrates a linear groove design  80  having an angular shape. The sinusoidal groove design of FIG.  2  and the linear groove design of FIG. 4, as well as a plain sleeve, were tested in the laboratory. Face temperature was measured and a comparison is shown in Table 1 below. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Comparison of Sleeve Designs 
               
             
          
           
               
                   
                 Sleeve Design 
                 Face Temperature (° F.) 
               
               
                   
                   
               
               
                   
                 Plain Sleeve 
                 290 
               
               
                   
                 Linear Groove (FIG. 4) 
                 260 
               
               
                   
                 Sinusoidal Groove (FIG. 2) 
                 208 
               
               
                   
                   
               
             
          
         
       
     
     Whereas the linear groove sleeve shows an improvement over a plain sleeve, the sinusoidal groove offers significant cooling over both. 
     Although the invention has been described in detail with reference only to the preferred embodiments, those having ordinary skill in the art will appreciate that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, the invention is defined with reference to the following claims.