CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is continuation of U.S. patent application Ser. No. 13/514,388, filed on Nov. 20, 2012, which is a national stage application under 35 U.S.C. §371(c) of prior-filed, co-pending PCT patent application serial number PCT/EP2010/068845, filed on Dec. 3, 2010, which claims priority to Italian Patent Application No. CO2009A000061, filed on Dec. 7, 2009, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    Exemplary embodiments relate generally to compressors and, more specifically, to the provision of thermal barriers for ensuring the smooth operation of a compressor over a wide temperature range. 
         [0004]    Description of the Prior Art 
         [0005]    A compressor is a machine which increases the pressure of a compressible fluid, e.g., a gas, through the use of mechanical energy. Compressors are used in a number of different applications and in a large number of industrial processes, including power generation, natural gas liquification and other processes. Among the various types of compressors used in such processes and process plants are the so-called centrifugal compressors, in which the mechanical energy operates on gas input to the compressor by way of centrifugal acceleration, for example, by rotating a centrifugal impeller. 
         [0006]    Centrifugal compressors can be fitted with a single impeller, i.e., a single stage configuration, or with a plurality of impellers in series, in which case they are frequently referred to as multistage compressors. Each of the stages of a centrifugal compressor typically includes an inlet conduit for gas to be compressed, an impeller which is capable of providing kinetic energy to the input gas and a diffuser which converts the kinetic energy of the gas leaving the impeller into pressure energy. 
         [0007]    A multistage compressor  100  is illustrated in  FIG. 1 . Compressor  100  includes a shaft  120  and a plurality of impellers  130 . The shaft  120  and impellers  130  are included in a rotor assembly that is supported through bearings  190  and  190 ′ and sealed to the outside through sealings  180  and  180 ′. 
         [0008]    The multistage centrifugal compressor  100  operates to take an input process gas from an inlet duct  160 , to increase the process gas pressure through operation of the rotor assembly, and to subsequently expel the process gas through an outlet duct  170  at an output pressure which is higher than its input pressure. The process gas may, for example, be any one of carbon dioxide, hydrogen sulfide, butane, methane, ethane, propane, liquefied natural gas, or a combination thereof. Between the impellers  130  and the bearings  190  and  190 ′, the sealings  180  and  180 ′ are provided to prevent the process gas from flowing through to the bearings  190 ,  190 ′. 
         [0009]    Each of the impellers  130  increases the pressure of the process gas. Each of the impellers  130  may be considered to be one stage of the multistage compressor  100 . Additional stages, therefore, result in an increase in the ratio of output pressure to input pressure. 
         [0010]    Compressors in oil and gas industries and power plants are operated with different gas temperatures. The temperature varies from cryogenic to very high temperature. The internal surfaces in boiled off gas application (BOG) compressors are subjected to cryogenic temperature while the outer surfaces of the compressor are exposed to atmospheric temperature. Due to the cryogenic temperature, thermal contraction occurs in the components. The contraction is not uniform due to variation in temperature on different parts. The non-uniform contraction reduces clearance and/or creates interference between the adjacent components and affects performance of the compressors. In BOG compressors, the differential thermal contraction between the sealings  180  and  180 ′ (in general, mechanical seals or dry gas seal type or DGS), the end head  140  and  140 ′ (which could also include heated seal carrier), the bearings  190  and  190 ′ and the shaft  120  creates interference between them and affects normal operation of the compressor. 
         [0011]    In order to remove or reduce thermal tension across the operating temperatures involved, dry gas seals  180  and  180 ′ are encapsulated in heated seal carriers  140  and  140 ′ that also act as thermal shields. 
         [0012]    It would be desirable to minimize thermal tension and stress on the dry gas seal and the end head by introducing a thermal barrier around the DGS to ensure smooth operation of the BOG compressor. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    Systems and methods according to these exemplary embodiments provide radial and axial thermal barriers to minimize thermal tension and stress on a mechanical seal and an end head by introducing a thermal barrier around the mechanical seal to ensure smooth operation of the BOG compressor. 
         [0014]    According to an exemplary embodiment, a compressor end head for providing a thermal barrier near a mechanical seal includes an inner end head and an outer end head. The outer end head includes an opening in the center for enclosing the inner end head, an outlet and grooves along side surfaces radially adjacent the opening. The inner head has an opening in the center, an inlet, grooves in the opening for enclosing an end portion of a compressor shaft and a flow path along an outer surface. 
         [0015]    According to another exemplary embodiment, a compressor end head for providing a thermal barrier near a mechanical seal includes an inner end head and an outer end head. The inner head includes an opening in the center, an inlet, grooves in the opening for enclosing an end portion of a compressor shaft. The outer end head includes an opening in a center for enclosing the inner end head, an outlet, grooves along side surfaces radially adjacent the opening, an inlet chamber connected to the inlet, an outlet chamber connected to the outlet and axial channels connecting the inlet chamber and the outlet chamber. 
         [0016]    According to a further embodiment, a compressor includes a shaft, a plurality of impellers, a plurality of seals, an inner end head and an outer end head adjacent the seals. The outer end head includes an opening in the center for enclosing the inner end head, an outlet and grooves along side surfaces radially adjacent the opening. The inner head has an opening in the center, an inlet, grooves in the opening for enclosing an end portion of a compressor shaft and a flow path along an outer surface. 
         [0017]    According to another exemplary embodiment, a compressor end head for providing a thermal barrier to a mechanical seal includes an inner end head having an inner end head opening, inner end head grooves in the inner end head opening, configured to place and seal an end portion of a compressor shaft; and an outer end head having an outer end head opening configured to enclose the inner end head. 
         [0018]    According to still another exemplary embodiment, a compressor includes a shaft, an inner end head having an inner end head opening, a plurality of inner end head grooves adjacent the inner end head opening configured to place and seal an end portion of the shaft; and an outer end head having an outer end head opening configured to enclose the inner end head. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The accompanying drawings illustrate exemplary embodiments, wherein: 
           [0020]      FIG. 1  illustrates a multistage compressor; 
           [0021]      FIG. 2  illustrates a dry gas seal end head according to exemplary embodiments; 
           [0022]      FIGS. 3 and 4  illustrate a cut view of a dry gas seal end head according to exemplary embodiments; 
           [0023]      FIGS. 5 and 6  illustrate inner and outer sides of an outer end head according to exemplary embodiments; 
           [0024]      FIG. 7  illustrates a cut view of an outer end head according to exemplary embodiments; 
           [0025]      FIGS. 8 and 9  illustrate internal and external cut views of an inner end head according to exemplary embodiments; 
           [0026]      FIG. 10  illustrates an oil flow path in an inner end head according to exemplary embodiments; 
           [0027]      FIG. 11  illustrates an oil flow path in an end head according to exemplary embodiments; 
           [0028]      FIGS. 12 and 13  illustrate a cut view of a dry gas seal end head according to exemplary embodiments; 
           [0029]      FIG. 14  illustrates a cut view of an outer end head according to exemplary embodiments; and 
           [0030]      FIGS. 15 and 16  illustrate a cut view of an inner end head according to exemplary embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
         [0032]    In exemplary embodiments, interference between a mechanical seal and an end head is prevented by providing axial thermal barriers around the mechanical seal to ensure smooth operation of a BOG compressor. 
         [0033]    For BOG applications, the mechanical seal (such as sealings  180  and  180 ′ of  FIG. 1 ) may include a dry gas seal encapsulated in a heated seal carrier as is known. The dry gas seal closes the compressor to seal the compressor from the outside. 
         [0034]    A dry gas seal may be in contact with an end head  140  and  140 ′ at the end of the compressor. Referring to  FIG. 2 , end head  200  may includes an inner end head  210  and an outer end head  220 . Each or both of the end heads  210  and  220  may be circular or may be some other shape but are illustrated as being circular in exemplary embodiments. End head  200  may be formed by, for example, welding the inner end head and outer end heads  210  and  220  in some embodiments. 
         [0035]    A circular or ring shaped outer end head  220  is illustrated in  FIGS. 5 and 6 . Outer end head  220  may include a circular opening  221  in the center within which inner end head  210  may be is circumferentially enclosed or fitted (as illustrated in  FIG. 2 ). 
         [0036]    Referring to  FIG. 2 , outer end head  220  includes a hot oil outlet  224  on an inner side surface  222 . Outer end head  220  also includes a circular groove  225  surrounding the circular opening  221  where the inner end head  210  may be welded with the outer end head  220  to form the circumferential enclosure. Outer end head  220  may include grooves  225  along both side surfaces (i.e. inner side surface  222  and outer side surfaces). Inner end head  210  includes a hot oil inlet  213 . 
         [0037]    Cut section views of the outer and inner surfaces of inner end head  210  are illustrated in  FIGS. 8 and 9 . Inner end head  210  includes a circular opening  211  in the center. As illustrated in  FIG. 8 , inner end head  210  includes a plurality of grooves  212  within the opening for facilitating the placement and sealing of the end portion of a compressor shaft. 
         [0038]    The diameter of inner end head  210  may be approximately equal to the diameter of circular opening  221  of outer end head  220  in order to facilitate the enclosure of inner end head  210  within outer end head  220 . 
         [0039]    As illustrated in  FIG. 9 , inner end head  210  may also includes an oil flow path  214  along an outer surface. Flow path  214  may be a helical flow path  214 . Flow path  214  along the outer surface may be formed between the grooves  212  which are on the inner surface of inner end head  210 . That is, the helical path  214  on the outer surface may correspond to the raised portion of the inner surface of the inner end head  210  between the grooves  212  (path  214  may be positioned on the outer surface corresponding to the raised portions between grooves  212  on the inner surface of the inner end head  210 ). In some embodiments, flow path  214  may correspond to the grooves  212 . When the inner end head  210  is welded to outer end head  220 , flow path  214  may provide a path for hot oil or gas to flow from inlet  213  to outlet  224 . 
         [0040]    End head  200  of  FIGS. 3 and 4  illustrates a helical flow path  214  and hot oil or gas outlet  224 . When viewed in conjunction with  FIG. 2 , hot oil or gas entering inlet  213  of inner end head  210  flows through helical flow path  214  to outlet  224  of outer end head  220 . 
         [0041]    The outer surface of inner end head  210  may include the helical flow path  214  as described above and illustrated in  FIG. 10 . The flow path  214  may be similar to a spiral path providing an axial thermal barrier as illustrated in  FIG. 11 . The flow path  214  as described herein provides a thermal barrier between the end head  210  and the DGS. 
         [0042]    In some embodiments, an additional thermal barrier may also be provided. Referring to  FIG. 7 , a hot oil/gas chamber  223  proximate the outer side surface of outer end head  220  (nearer to the DGS) reduces the thermal differential further. In this embodiment, oil in helical flow path  214  flows into chamber  223  and to outlet  224 . 
         [0043]    In order to prevent leakage from the helical flow path  214 , a light interference fit may be made between the inner end head  210  and the outer end head  220  in some embodiments. The inner end head  210  and the outer end head  220  can also be bolted to the compressor housing in some embodiments. 
         [0044]    In some embodiments, the helical flow path  214  may be substituted with straight holes in the outer end head  220  to provide heating to the inner end head  210  so that inner end head  210  and the dry gas seal can be maintained at required temperature to avoid interference between the dry gas seal and the inner end head  210  when the compressor handles or processes gas at cryogenic temperatures. 
         [0045]    Referring to  FIGS. 12 and 13 , an end head  300  includes inner end head  310  and outer end head  320  (corresponding to inner end head  210  and outer end head  220  of end head  200  as described above). Inner end head  310  includes a hot oil inlet  313 . Outer end head  320  includes hot oil outlet  324  and groove  325  for facilitating welding of inner end head  310  to outer end head  320 . Outer end head  320  also includes an inlet gas or oil chamber  326  and an outlet gas or oil chamber  327 . 
         [0046]    Chamber  326  is provided near the inner head hot oil inlet  313  for receiving the oil from inlet  313 . A plurality of passages  328  in the outer end head  320  (illustrated in  FIG. 14 ) facilitates oil flow from inlet chamber  326  to outlet chamber  327 . Outlet chamber  327  is connected to oil outlet  324 . In exemplary embodiments, there may be four passages (or channels or holes)  328 . 
         [0047]    Chambers  326  and  327  may be connected with each other through straight holes  328  in outer end head  320  in order to facilitate uniform hot oil flow along the axis of the inner end head  310 . 
         [0048]    Inner end head  310  may be in the form as illustrated in  FIGS. 15 and 16 . Inner end head  310  may also facilitate oil flow along its outer surface  315  from inlet  313  to outlet  324  of outer end head  320 . Inner end head  310  may provide a labyrinth seal. 
         [0049]    The term inner side surface of an outer end head (or the inner end head) as used herein may refer to the side of the end head that is facing an impeller (i.e. between an impeller and end of the shaft). The term outer side surface as used herein may refer to the side of the end head that is on a side not facing an impeller (i.e. side of the end head that faces toward the outside of the casing). 
         [0050]    The outer surface of the outer end head is adjacent the mechanical seal. The mechanical seal may be a dry gas seal (DGS). The inlet, the outlet, the chamber (of  FIG. 7 ) and the flow path may be for hot oil or gas. 
         [0051]    The inlet chamber  326  and the outlet chamber  327  provide a radial thermal barrier. 
         [0052]    Channels or passages  328  (of  FIG. 12 ) may be axial channels and provide an axial thermal barrier. 
         [0053]    Exemplary embodiments as described herein provide multiple advantages. A heating system according to exemplary embodiments provides a radial and axial thermal barrier. The thermal barrier reduces heat transfer between inlet and the zone surrounding the DGS leading to a smooth operation of the BOG compressor. The optimized flow path provides gradual change in temperature in radial and axial directions around the DGS and also reduces internal thermal stress. In addition, the heating system according to exemplary embodiments prevents interference between DGS and the end head. The system is simple and compact. The system also prevents interference and provides smooth operation of the BOG compressor at cryogenic temperatures. 
         [0054]    Exemplary embodiments as described provide an axial thermal barrier or an axial and a radial thermal barrier for handling temperature gradients in boiled off gas applications. The end head may be bolted to the compressor. The inner and outer heads may also be interference fitted to form the end head. 
         [0055]    The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items.
A compressor end head for providing a thermal barrier to a mechanical seal includes an inner end head having an inner end head opening, inner end head grooves in the inner end head opening, configured to place and seal an end portion of a compressor shaft; and an outer end head having an outer end head opening configured to enclose the inner end head.