Abstract:
An electrical resistance heater element has a metal tube containing an electrical resistance coil that is electrically insulated from the metal tube by magnesium oxide powder, the coil being bonded to a conductor pin that protrudes from an open end of the tube. The powder is sealed against moisture by placing at least the open end of the tube within a chamber and immersing the open end within liquid silicone. A gas is pumped into the chamber to pressurize the silicone sufficiently to cause some of it to encroach into the powder through the open end of the tube. The heater element is installed within a heat exchanger with the silicone remaining uncured.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority to provisional application Ser. No. 60/653,763, filed Feb. 17, 2005. 

   FIELD OF THE INVENTION 
   This invention relates in general to heater elements for process heat exchangers and in particular to a method of conditioning heater elements to resist moisture. 
   BACKGROUND OF THE INVENTION 
   Heat exchangers with electrical resistance heater elements are commonly used for heating fluids in processing plants, such as chemical plants. Typically, a number of electrical resistance heater elements are located within a tank of the heat exchanger. Each heater element  21  comprises a metal tube containing a coiled resistance wire for generating heat as electrical current passes through it. The coiled wire is insulated from the metal tube by an insulation powder, which is typically magnesium oxide packed tightly within the tube surrounding the coiled wire. While magnesium oxide provides excellent electrical insulation, it is a desiccant, thus it attracts moisture from the surrounding atmosphere. The penetration of moisture reduces the ability of the insulation powder to insulate. 
   In the past, heater elements of this nature have been kept in low humidity rooms and/or baked in an oven with their ends open to drive off any moisture. Then, when ready for use, the heater element is mounted to a header plate and seals are placed over the open ends. For example, a liquid sealant may be poured over the open ends and cured. While these methods work, improving the resistance of the insulation is desirable. 
   SUMMARY 
   In this invention, a dielectric liquid is applied under pressure to the open ends of the tube to cause some of the liquid to encroach into the insulation powder. Preferably the dielectric liquid is substantially at room temperature while it is forced into the open end of the tube. In the preferred embodiment, the dielectric liquid is uncured liquid silicone, which remains uncured at the completion of the treatment. Also, the heater element is preferably baked in an oven prior to the step of applying the liquid silicone to the open ends. 
   The liquid silicone may be applied under pressure to the open ends in various manners. In one method, the open end of the heater tube, or alternately the entire heater tube, is placed within a chamber and immersed in the dielectric liquid. Gas is pumped into the chamber to apply the pressure to the liquid silicone. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic sectional view illustrating one type of a process circulation heater constructed in accordance with this invention, and showing only one of the heater elements. 
       FIG. 2  is an enlarged sectional view of the connector pin portion of one of the heater elements of the circulation heater of  FIG. 1 . 
       FIG. 3  is a sectional view of the header portion of the circulation heater of  FIG. 1 , with the heater tank removed, additional heater elements installed, and illustrating one step in the conditioning process. 
       FIG. 4  is a view similar to  FIG. 3 , but illustrating the step of  FIG. 3  in connection with a heater with a temporary installation housing. 
       FIG. 5  is a schematic sectional view illustrating another method of conditioning the heater elements of the circulation heater of  FIG. 1 . 
       FIG. 6  is a schematic sectional view illustrating another method of conditioning the heater elements of the circulation heater of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , circulation heater  11  is of a conventional type used for heating fluids in processing plants, such as chemical plants, or for other heat exchanging uses. Heater  11  has a tank  13  that is generally cylindrical. Tank  13  has ports  15  and  16  on its sidewall near opposite ends for circulating a fluid through tank  13 . One end of tank  13  is closed, and the other has an opening encircled by a flange  17 . Thermal insulation  19  is typically located on the exterior of tank  13 . Circulation heater  11  is shown as an example only, and it could be other types, such as a flanged heater or screw plug heater. 
   A number of electrical resistance heater elements  21  are located within tank  13 . In  FIG. 1 , only one of the heater elements  21  is shown, but normally a number of heater elements  21  would be utilized, as illustrated in  FIGS. 3 and 4 . Each heater element  21  comprises a metal tube or sheath  23 , which in this example, is bent to form a U-shaped bend  25  and two open ends  26 . An electrical connector pin  27  protrudes from each open end  26  of each heater element  21 . Open ends  26  extend through mating holes provided in a header plate  33 . Typically, each sheath  23  is brazed or welded to header plate  33 . Header plate  33  has holes  34  circumferentially spaced around its outer edge for bolting header plate  33  to flange  17  of tank  13 . 
   As shown in  FIG. 2 , each connector pin  27  is an electrical conductor that is joined, as by brazing, to a coiled wire  29 . Wire  29  has a high electrical resistance for generating heat as electrical current passes through it. Wire  29  extends continuously from one connector pin  27  through U-shaped bend  25  and to the other connector pin  27 . A typical material for wire  29  is a nickel chromium alloy. 
   Coiled wire  29  is insulated from metal sheath  23  by an insulation powder  31 . Insulation powder  31  is preferably magnesium oxide, and it is packed tightly within sheath  23  surrounding coiled wire  29 . Prior to the complete assembly of heater element  21  to header plate  33 , insulation powder  31  is exposed to atmosphere at open end  26 . While magnesium oxide provides excellent resistance, it is a desiccant, thus it attracts moisture from the surrounding atmosphere. The penetration of moisture reduces the ability of insulation powder  31  to insulate. Utilizing a process to be described subsequently reduces the tendency of insulation  31  to attract moisture. 
   Referring again to  FIG. 1 , a cylindrical housing  35  is joined to header plate  33  and encloses connector pins  27 . In this embodiment, housing  35  is welded to the outer side of header plate  33  surrounding connector pins  27 , but it could be attached in other manners. In this example, housing  35  has an end cap  37  that is releasable. Some circulation heaters  11  have housings  35  that are sealed to atmosphere and capable of containing internal pressure up to a desired amount. Other housings  35  are open to atmosphere. Housing  35  is employed to protect the wires and connectors (not shown) that join connector pins  27 . Various types of insulating boots may be secured over each open end  26  surrounding each connector pin  27 . 
   In the example of  FIG. 1 , heater elements  21  have been processed or conditioned to retard moisture entry. Further, an optional layer of sealant  39 , such as silicone, has been cured in place on header plate  33 . Sealant  39  was poured on header plate  33  over the open ends  26  of heater elements  21 , leaving only connector pins  27  exposed. Sealant  39  preferably cures at room temperature when exposed to air, and is allowed to cure after it is poured onto header plate  33 . 
   The first step in conditioning or moisture-proofing heater elements  21  is to heat them for a sufficient amount time at a sufficient temperature to remove as much moisture as practical in insulation  31 . This step is normally performed in an oven, and it may be done prior to or after assembly of heater elements  21  with header plate  33 . Alternately, the removal of moisture step may be done both before and after assembly of heater elements  21  with header plate  33 . Insulation  31  will be exposed to the atmosphere in the oven at open ends  26 . Then, heater elements  21  may be attached to header plate  33 , preferably by welding, brazing or staking open ends  26 . Also, compression fittings may be used to attach heater elements  21  to header plate  33 . In the example of  FIG. 3 , housing  35  is then welded to header plate  33 . 
   Referring still to  FIG. 3 , housing  35  in this example is a pressure chamber containing type. A dielectric liquid  41  is poured on top of header plate  33  before applying any sealant  39  ( FIGS. 1 and 2 ). A preferred dielectric is high dielectric uncured liquid silicone of a type that does not cure when exposed to air. The viscosity may vary widely, such as from 25 centipoise to 25000 centipoise. The level of silicone  41  could entirely immerse connector pins  27 , but this is not necessary as long as it covers open ends  26  ( FIG. 2 ) and comes into contact with insulation powder  31  of heater elements  21 . 
   Housing  35  typically has a port  43 , and this port is connected to an air pressure source  45 , which applies air pressure to the interior of housing  35 . No heat is required, and dielectric liquid  41  remains uncured. The amount and duration of the air pressure may vary, and typically is about 100 psi for five to ten minutes. The application of pressure to dielectric liquid  41  causes some of the liquid to enter insulation powder  31  ( FIG. 2 ), filling and sealing the spaces between the individual grains of insulation powder  31 . The amount of dielectric liquid  41  that actually enters open ends  26  is small, typically only migrating about 0.5 to 2.0 inches inward into insulation powder  31 . Often, the extent of migration is about 0.75 inch. After dielectric liquid  41  has migrated the typical distance, it tends to plug up insulation powder  31  and not migrate any further, regardless of the amount of time air pressure is applied. Dielectric liquid  41  may reach its full penetration depth in less than five minutes. Dielectric liquid  41  does not cure after entering insulation  31 , rather remains a liquid. 
   After a sufficient time under pressure is reached, air pressure source  45  is disconnected and the excess dielectric liquid  41  removed from header plate  33 . It is not necessary to thoroughly clean dielectric liquid  41  from header plate  33  and open ends  26 . If desired, sealant layer  39  ( FIG. 1 ) may then be poured and cured around connector pins  26 . 
     FIG. 4  shows a method that is applicable for housings  35  ( FIG. 3 ) that will not contain pressure. In this method, after heater elements  21  are welded into header plate  33  and before welding housing  35  to header plate  33 , an installation fixture or housing  47  is temporarily connected to header plate  33 . Installation chamber or housing  47  has an annular seal  49  that seals its face to header plate  33  at a point surrounding heater element open ends  26  and radially inward from bolt holes  34 . Installation housing  47  has a cap  51  and a port  53 . Installation housing  47  may be secured to header plate  33  in different manners. In this example, installation housing  47  has a flange with holes  57  that align with holes  34  in header  33  for receiving bolts. Once secured, the operator introduces dielectric liquid  41 , closes cap  51  and applies pressure through air pressure source  45  ( FIG. 3 ) and port  53 . After pressure has been applied for the desired amount of time, the operator removes dielectric liquid  41  and unbolts installation housing  47  from header plate  33 . Permanent housing  35  will then be attached in a conventional manner, such as by welding. 
   Some heater units do not employ heater elements welded to a header plate as described above.  FIG. 5  shows one method for utilizing this process without a header plate  33  ( FIG. 1 ). A chamber or housing  59  is utilized that has holes  61  for receiving one or more heater elements  21  (only one shown in  FIG. 5 ). Chamber  59  has a base  60  with holes  61 , each receiving one of the open ends  26  of a heater element  21 . A seal  63  for each hole  61  seals around sheath  23 . Chamber  59  has a cap  65  for access and a port  67  for application of air pressure. Dielectric liquid  41  is placed on chamber base  60  in the same manner as in connection with the other embodiments. Air pressure is applied through port  67  to dielectric liquid  41  for a selected time. Subsequently, heater element  21  is removed from chamber  47  and mounted to a heater assembly. 
     FIG. 6  discloses still another method of conditioning individual heater elements  21  prior to installation into a heater assembly. In  FIG. 6 , a pressure chamber  69  is constructed for holding one or more heater elements  21 . For safety, pressure chamber  69  may be located within a vertical hole with a substantial portion below ground level  70 . In one example, pressure chamber  69  is a cylindrical tank about 15 feet long. Pressure chamber  69  has a removable top  71  and an air inlet port  73  though its sidewall. Heater elements  21  are suspended from top  71  by hangers  77  that engage hooks  75 . The lower ends of hangers  77  engage the U-shaped bends  25 . The open ends  26  of heater elements  21  locate near or touch the bottom of chamber  69 . Often, some heater elements  21  will be longer than others, and hangers  77  of different lengths may be employed to hang them at positions so that all of the open ends  26  are located near the bottom of chamber  69 . 
   Dielectric liquid  41  is introduced to a level above open ends  26 . One dispensing and recovery method employs an external reservoir  79  that has a vent  81 . A tube  83  leads from the bottom of reservoir  79  through the sidewall of pressure chamber  69  and to a point near the bottom of pressure chamber  69 . A valve  85  in tube  83  is opened to allow dielectric liquid to flow from reservoir  79  into pressure chamber  69 . Valve  85  is closed once the selected amount of dielectric fluid  41  is dispensed. Air pressure is then supplied through port  73  for a selected time interval. After removing the air pressure, heater elements  21  are removed from pressure chamber  69  and utilized with other heater assemblies. 
   Periodically, dielectric liquid  41  needs to be removed from pressure chamber  69  to avoid contamination. Some operators may wish to remove dielectric liquid  41  after each treatment. To do so, prior to bleeding off the air pressure in pressure chamber  69 , the operator opens valve  85 . The air pressure will push dielectric liquid  41  from pressure chamber  69  up through tube  83  into reservoir  79 . After recovering substantially all of dielectric liquid  41 , the operator closes valve  85  and bleeds off air pressure from pressure chamber  69 . 
   Although not shown, some heater elements are straight, rather than U-shaped, and have open ends on opposite ends. These heater elements could be treated by completely immersing them within a pressure chamber such as pressure chamber  69 . Pressure chamber  69  could be oriented horizontally, rather than vertical, if desired. Heater elements, other than tubular ones, could also be treated in accordance with  FIG. 6 . Any type of encased or enclosed heater assembly containing compacted magnesium oxide could be treated in the manner described, including strip and cartridge heater elements. 
   The invention has significant advantages. The method seals the insulation powder to resist the entry of moisture. The electrical resistance of the powder thus does not deteriorate with time. 
   While the invention has been shown in only a few of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.