Abstract:
A method of cooling a ferrofluid seal that includes moving air from an atmospheric side of the ferrofluid seal over one or more heat dissipating surfaces within the ferrofluid seal through an air path formed in the ferrofluid seal by incorporating one or more vent openings in a housing case and one or more fan blades on a surface selected from the group consisting of an outside surface of a shaft and an inside surface of the housing case where the space surrounding the shaft communicates with the one or more vent openings and where the moving air is caused by the rotation of the surface having the one or more fan blades.

Description:
[0001]    This application is a Continuation Application of Ser. No. 11/161,910, filed on Aug. 22, 2005, which claims the benefit of US Provisional Patent Application No. 60/604,673, filed Aug. 26, 2004. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to the cooling of ferrofluid seals. Particularly, the present invention relates to self-cooling ferrofluid seals. 
         [0004]    2. Description of the Prior Art 
         [0005]    Ferrofluid seals are widely used in industry. Typically, a ferrofluid seal contains a magnetic circuit that is composed of stationary elements such as magnets and pole pieces, and a rotating element such as a shaft. Magnetic fluid is confined between the rotating and stationary elements by the magnetic field of the magnet and formed into a series of liquid O-rings, which provide sealing. 
         [0006]    Heat generation has been a common problem for ferrofluid seals that operate at high speed. The viscous heat generated by the magnetic fluid tends to heat up the seal to a level that does not allow for the proper operation of the seal. Typically, water cooling or another coolant is used to overcome this problem. When water is not available, externally introduced forced air convection methods (such as an external fan), or natural convection cooling methods are used. 
         [0007]    U.S. Pat. Nos. 4,674,109 and 5,421,892 are examples of liquid cooled ferrofluid seals. U.S. Pat. No. 4,674,109 (1987, Ono) discloses an x-ray tube device with an anode target capable of rotation and a cathode which generates electrons causing them to collide with the target set in a vacuum envelope, and with a shaft which supports and rotates the anode projecting outside the envelope. This x-ray tube device has a structure such that the target is cooled by coolant flowing through coolant channels in the shaft. A vacuum seal is maintained by seal means such as magnetic fluid seal between the envelope and the rotating shaft. The envelope and coolant channels are best maintained at ground potential, and thus have an intermediate potential, with high positive and negative voltages supplied to the anode target and cathode. 
         [0008]    U.S. Pat. No. 5,421,892 (1995, Miyagi) discloses a vertical heat treating apparatus that includes a cap body, which is movable up and down, for sealing a treatment vessel that holds objects to be treated. A rotary loading device is provided with a rotary shaft which extends into a through hole provided in the cap body, and a magnetic fluid seal member is provided around the rotary shaft. Heat-exchange media, such as water or ethylene glycol, is circulated within the rotary shaft, preferably to cool the rotary shaft. A temperature sensor may be provided in a housing for the rotary shaft, such that when the temperature exceeds a set temperature, the flow rate of the heat exchange medium is increased. Baffle plates may be provided about an upper surface of the cap body and opposed to the through hole in the cap body. In one embodiment of the invention, nitrogen gas is circulated through the through-hole in the cap body to prevent corrosive gas from contacting the shaft. Circumferential grooves are defined around the rotary shaft at locations where the heat exchange medium is admitted and discharged from the rotary shaft. Preferably, the heat exchange medium is circulated in the rotary shaft above and below the level of the magnetic seal. 
         [0009]    The following example uses the Peltier effect to cool a ferrofluid seal. U.S. Pat. No. 5,486,728 (1996, Hirama) discloses a micromotor. The micromotor includes a cylindrical rotor casing having a central through-hole, and a rotor having a cylindrical, magnetic rotor block fixed on a rotor shaft and inserted in the central through-hole of the rotor casing. First and second bearings support the rotor shaft for rotation and are fitted, respectively, in opposite ends of the central through-hole of the rotor casing and define a sealed rotor chamber therebetween. Stator coils are attached to the outer rotor circumference of the rotor casing, and a stator casing is joined to the rotor casing coaxially with the rotor so as to cover the stator coils. A magnetic fluid is filled in the sealed rotor chamber between the first and second bearings between which is disposed the magnetic rotor block. A series of Peltier elements are attached to the outer circumference of the stator casing and electrically connected to a power supply to adsorb heat generated by the operation of the components of the micromotor. 
         [0010]    Each of the listed methods has limitations. The natural convection cooling method is frequently unable to provide enough cooling effect, and the seal has a tendency to overheat at high speeds. The externally introduced forced air convection method requires additional space and parts to integrate the fan, which introduces design problems and higher costs. Further, the internal components are difficult to be cooled by this method. When water or other liquid coolant is used, there is always the concern that the liquid coolant may leak out of the cooling channels and cause equipment damage and process contamination. The use of Peltier devices adds additional cost, space and parts to integrate these Peltier devices and further requires power to be supplied to the Peltier devices to effect cooling. 
         [0011]    Therefore, what is needed is a cooling system for ferrofluid seals that eliminates coolant leaks. What is also needed is a cooling system that generates effective heat flow path and heat dissipation surface for both the stationary and rotating elements of the ferrofluid seal. What is further needed is a cooling system that generates effective airflow paths within the ferrofluid seal so that both its stationary and rotating elements can be cooled by forced convection. What is still further needed is a cooling system that generates airflow inside the seal to provide effective cooling to both the stationary and rotating elements of the seal. What is yet further needed is a cooling system that provides cooling simultaneously when the seal is operated and where the cooling effect increases proportionally with the operating speed of the seal. 
       SUMMARY OF THE INVENTION 
       [0012]    It is an object of the present invention to provide a cooling system that eliminates the potential for coolant leaks. It is another object of the present invention to provide a cooling system that has effective heat flow paths and heat dissipation surfaces for both the stationary and rotating elements of the ferrofluid seal. It is yet another object of the present invention to generate effective airflow paths within the ferrofluid seal so that both its stationary and rotating elements can be cooled by forced convection. It is a further object of the present invention to provide airflow inside the seal to provide effective cooling to both the stationary and rotating elements of the seal. It is yet a further object of the present invention to provide a cooling system that provides cooling simultaneously when the seal is operated and where the cooling effect increases proportionally with the operating speed of the seal. 
         [0013]    The present invention achieves these and other objectives by providing a self-cooling ferrofluid seal having at least a seal housing and a rotatable shaft. The seal housing includes a housing case and an external pole piece securely attached to the housing case. The housing case has a plurality of fins on the outside and may optionally include a plurality of vent channels and/or openings. The housing case is preferably made with heat dissipating materials. 
         [0014]    The housing case contains various ferrofluid seal members including one or more bearings, a second pole piece, a magnet, and a predetermined amount of magnetic fluid/ferrofluid. The combination of the external pole piece, the magnet and the second pole piece in combination with the rotatable shaft forms the magnetic fluid seal circuit. The magnetic fluid seal is located between the external pole piece and the rotatable shaft. The external pole piece may optionally have one or more grooves or ridges creating one or more stages. The magnetic flux of the magnet causes the magnetic fluid to be contained between the one or more stages and the rotatable shaft creating one or more liquid O-ring seals. Alternatively, the rotatable shaft may have one or more grooves or ridges creating the one or more stages, or both the rotatable shaft and the external pole piece may have the grooves or ridges. 
         [0015]    The pole pieces are made of a magnetic material and have an internal diameter sized to create an annular space between the pole pieces and the rotatable shaft. The portion of the rotatable shaft in the area of the one or more stages must also be made of a magnetic material. The rotatable shaft may be a solid rod or a hollow shaft. To facilitate heat transfer, a portion of the rotatable shaft may be made of a heat conducting material such as, for example, copper, while a magnetic portion on which the sealing stages are made is connected to the copper portion. Vent openings and channels may optionally be made into the shaft to provide effective air flow paths. The shaft may optionally include an inner shaft made of a heat dissipating material to help remove heat from the outer surface of the shaft. Heat dissipating surfaces such as, for example, fins may also be incorporated within a hollow shaft. 
         [0016]    A key feature of the present invention is the incorporation of one or more fan blades onto the outer surface of the shaft that are configured to move an air flow through the inside of the self-cooling ferrofluid seal and out through the vent openings, channels and heat dissipation surfaces of the housing or shaft or both. The one or more fan blades may be attached to the rotatable shaft individually or as a sleeve, or they may be an integral part of the shaft. The one or more fan blades may be outside and/or inside of the ferrofluid seal. 
         [0017]    As the shaft rotates, the one or more fan blades will generate the air flow to self-cool the ferrofluid seal. The air flow created by the one or more fan blades will force an air flow to pass through the air flow paths inside the shaft, the housing and the other ferrofluid seal components as well as across the heat dissipation surfaces of all of the components of the seal. The one or more fan blades may optionally be made of a heat conducting material to also act as a heat dissipating surface. The one or more fan blades may be placed in various locations inside the ferrofluid seal such as, for example, next to the pole piece, under the magnet, next to the bearings, etc., to provide cooling to these various components. 
         [0018]    The cooling effect of the present invention occurs simultaneously with the operation of the ferrofluid seal. As the rotation speed increases, so does the cooling effect. This effectively balances the viscous heat generation of the magnetic fluid. 
         [0019]    It should be noted that the housing of the ferrofluid seal may be made to rotate while the shaft is stationary. In this situation, the fan blades would preferably be attached to or incorporated into the housing instead of the shaft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0020]      FIG. 1  is a side plan view of the preferred embodiment of the present invention. 
           [0021]      FIG. 2  is a side plan view of the embodiment in  FIG. 1  showing the cooling fins of the housing. 
           [0022]      FIG. 3  is an end view of the embodiment in  FIG. 1  showing the rotating shaft with end fan. 
           [0023]      FIG. 4  is a cross-sectional view of the embodiment in  FIG. 1  showing the rotating shaft with fan blades and the ferrofluid seal. 
           [0024]      FIG. 5  is a cross-sectional view of the shaft of the present invention showing a rotating housing case with fan blades and the ferrofluid seal and heat dissipating fins within the hollow shaft. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0025]    The preferred embodiment(s) of the present invention is illustrated in  FIGS. 1-5 .  FIG. 1  illustrates an air-cooled, ferrofluid seal device  10 . Ferrofluid seal device  10  includes a seal housing  20 , a rotatable shaft  50  and an optional cover  100 . Turning now to  FIG. 2 , there is illustrated seal housing  20  and rotatable shaft  50 . Seal housing  20  includes a housing case  22  and a pole piece  24  securely attached to housing case  22 . Housing case  22  is made of a heat conducting material, preferably copper, forming a good heat flow path for pole piece  24 . Housing case  22  has a plurality of fins  26  and case vent channels  28 . The plurality of fins  26  provides an effective heat dissipation surface. 
         [0026]      FIG. 3  illustrates an end view of ferrofluid seal device  10  showing the rotatable shaft  50 , cover  100  and cover end opening  102 . Shaft end fan blade  40  is seen through cover end opening  102  securely attached to rotatable shaft  50 . Line  40 ′- 40 ″ indicates the cross-sectional view of ferrofluid seal device  10  illustrated in  FIG. 4 . 
         [0027]    Turning now to  FIG. 4 , there is illustrated a cross-sectional view of seal housing  20 , rotatable shaft  50 , and cover  100 . It should be understood that the illustrated components are not to scale but are exaggerated to facilitate an understanding of the present invention. Those of ordinary skill in the art are familiar with the manufacture of ferrofluid seals, the various magnetic circuit components and the size of the annular gap used to create the ferrofluid seal. 
         [0028]    As previously mentioned, seal housing  20  includes housing case  22  and pole piece  24 . Pole piece  24  is secured to housing case  22  preferably by securing hardware  66 . Pole piece  24  also contains a plurality of openings  62  in its peripheral flange for receiving securing hardware such as a bolt to secure ferrofluid seal device  10  to an apparatus that requires such a sealing device. Pole piece  24  may also incorporate a recessed portion  64  for receiving an O-ring or some other sealing material. 
         [0029]    The combination of pole piece  24  with a magnet  30  and second pole piece  32  constitutes a magnetic circuit  70 . It should be understood by those of ordinary skill in the art that pole piece  24 , magnet  30  and second pole piece  32  have inside diameters configured to create an annular space between their respective inside surfaces and the outside surface of rotatable shaft  50 . The magnetic flux gradient produced by magnetic circuit  70  causes the formation of a ferrofluid seal with shaft  50  when a magnetic fluid  60  is added to the annular space forming one or more seal stages between pole piece  24  and shaft  50 . Seal housing  20  also includes bearings  34  and a bearing retaining cap  36 . Bearing retaining cap  36  holds the bearings  34 , pole piece  32  and magnet  30  in a secure relationship within seal housing  20 . 
         [0030]    Housing case  22  further includes one or more case vent openings  44  that can be incorporated into housing case  22  to provide effective air flow paths for cooling the ferrofluid seal device  10 . Case vent openings  44  are either in continuous or intermittent communication with air spaces  49  within ferrofluid seal  10 . 
         [0031]    Rotatable shaft  50  may be solid or hollow and may be made of a good heat conducting material such as, for example, copper with a magnetic portion of shaft  50  on which the ferrofluid sealing stages are made being intimately attached to shaft  50 . Alternatively, rotatable shaft  50  may be made of a magnetic material with a good heat conducting portion. In yet another alternative embodiment and illustrated in  FIG. 4 , shaft  50  may optionally incorporate an inner shaft  48  that is made of a good heat conducting material to enhance heat dissipation. One or more shaft vent openings  46  may also optionally be incorporated in shaft  50  where a hollow rotatable shaft is used and may also be incorporated in inner shaft  48  when such a configuration is used. In addition, or instead of the optional vent openings  46 , inner shaft  48  may optionally include a plurality of heat dissipating fins  47  as illustrated in  FIG. 5 . 
         [0032]    The unique feature of the present invention is the incorporation of at lease one fan blade to create an air flow through the ferrofluid seal device  10  when the ferrofluid seal device  10  is operated. In the preferred embodiment, shaft  50  has a fan  40  with a plurality of blades attached to one end of shaft  50  adjacent cover end opening  102 . Fan  40  rotates when the shaft  50  is in operation causing an air flow to be drawn into the atmospheric side of the ferrofluid seal device  10  and out through case vent openings  44  and across fins  26 . It should be understood that each of the plurality of fan blades may be individually attached or attached in groups to shaft  50  or they may be made as an integral part of shaft  50 . Shaft  50  may optionally include one or more fan blades  42  in various locations along shaft  50  that coincide with seal device spaces  49  to further aid in the movement of air through seal device  10 . Fan blades  42  may be integrally formed into the surface of shaft  50  or may be securely attached to shaft  50 . It should be noted that in ferrofluid seals where the housing rotates around a stationary shaft, fan blades  42  could be incorporated at the inside surface of seal housing  20  or at the inside surface of any of the seal components in order to generate the air flow through the ferrofluid seal. 
         [0033]    During operation of ferrofluid seal device  10 , shaft  50  rotates and fan blades  42  generate a powerful air flow. The air flow generated by fan blades  42  passes through the air flow paths, i.e. spaces  49 , and openings  44  and  46  in the housing case  22  and the shaft  50 , respectively, and other parts of the ferrofluid seal device  10 , and across/against the heat dissipation surfaces of all parts of the seal device  10 , thus cooling the seal. Therefore, the cooling effect of the present invention occurs simultaneously with the operation of the ferrofluid seal device  10 . As the rotating speed increases, the cooling effect will also increase, effectively balancing the viscous heat generation of the magnetic fluid  62 . 
         [0034]    Although the preferred embodiment discloses a rotating shaft  50  and a stationary housing  20 , those of ordinary skill in the art will recognize that the housing  20  can be made to rotate while the shaft  50  is kept stationary. In this alternative configuration, the fan blades  42  would preferably be attached or integrated into the housing  20  instead of on the shaft  50  in order to create movement of air through the seal device  10 . 
         [0035]    Although the preferred embodiments of the present invention have been described herein, the above description is merely illustrative. Further modification of the invention herein disclosed will occur to those skilled in the respective arts and all such modifications are deemed to be within the scope of the invention as defined by the appended claims.