Abstract:
A method is provided for producing an exhaust gas aftertreatment or acoustic device ( 18 ) having a maximum operating temperature Tmax. The method includes the steps of providing a blanket ( 28 ) of silica fiber insulation material having a weight percentage of SiO 2  of greater than 65%; heating the blanket ( 28 ) so that all of silica fiber insulation material is raised to a temperature T greater than Tmax; and installing the blanket ( 28 ) in the device ( 18 ) after the heating step.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       MICROFICHE/COPYRIGHT REFERENCE 
       [0003]    Not Applicable. 
       FIELD OF THE INVENTION 
       [0004]    This invention relates to exhaust gas aftertreatment and/or acoustic systems and the devices used therein that utilize insulation blankets or batts. 
       BACKGROUND OF THE INVENTION 
       [0005]    Heat insulating batts and blankets are utilized in exhaust gas systems in order to provide heat insulation for acoustic and aftertreatment devices of the system to control the heat exchange to and from the devices. It is known to place such heat insulating blankets between adjacent wall surfaces of such device with the material of the heat insulation blanket being compressed to provide a desired installed density for the material to help maintain the heat insulating blanket in its mounted position via frictional forces between the blanket and the adjacent wall surfaces. Cost is typically a concern in any commercial system and one cost efficient heat insulation blanket material is made from silica fiber insulation material having a weight percentage of SiO 2  of greater than 65%. Unfortunately, when such a material was utilized in an exhaust gas aftertreatment device, the material failed after a period of time because the heat insulation blanket could not maintain adequate frictional engagement with the adjacent sidewalls in order to prevent destructive movement of the insulation blanket within the component. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with one feature of the invention, a method is provided for producing an exhaust gas aftertreatment or acoustic device having a maximum operating temperature Tmax. The method includes the steps of: providing a blanket of silica fiber insulation material having a weight percentage of SiO 2  of greater than 65%; heating the blanket so that all of silica fiber insulation material is raised to a temperature T greater than Tmax; and installing the blanket in the device after the heating step. 
         [0007]    As one feature, T is at least 1.05×Tmax. 
         [0008]    According to one feature, the installing step includes installing the blanket so that the blanket is compressed between two adjacent surfaces of the device to achieve an average installed density of 0.18 grams/cubic centimeter to 0.30 grams/cubic centimeter of the insulation material in the blanket. 
         [0009]    In one feature, during the heating step the blanket is an uncompressed state. 
         [0010]    As one feature, during the heating step the blanket is heated in a rolled state wherein the blanket has been formed into a roll having a central axis. In a further feature, during the heating step the blanket is rotated about the central axis. 
         [0011]    According to one feature, during the heating step the blanket is planar. 
         [0012]    In one feature, Tmax is within the range of 300° C. to 1100° C. 
         [0013]    As one feature, the installing step includes installing the blanket so that the blanket encircles a core of the device through which the exhaust gas passes. 
         [0014]    In one feature, the silica fiber insulation material has a weight percentage of SiO 2  of greater than 95%. 
         [0015]    In accordance with one feature of the invention, a method is provided for producing an exhaust gas aftertreatment or acoustic device having a maximum operating temperature Tmax. The method includes the steps of: providing a blanket of silica fiber insulation material having a weight percentage of SiO 2  of greater than 65%; heating the blanket so that all of silica fiber insulation material is raised to a temperature T greater than Tmax; and installing the blanket in the device after the heating step so that the blanket encircles a core of the device through which the exhaust gas passes and the blanket is compressed between two adjacent surfaces of the device to achieve an average installed density of 0.18 grams/cubic centimeter to 0.30 grams/cubic centimeter of the insulation material in the blanket. 
         [0016]    Other objects, features, and advantages of the invention will become apparent from a review of the entire specification, including the appended claims and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a diagrammatic representation of an exhaust gas system employing the invention; 
           [0018]      FIG. 2  is a section view of an exhaust system component employing the invention of  FIG. 1  taken from line  2 - 2  in  FIG. 1 ; 
           [0019]      FIG. 3  is a side elevational diagrammatic representation of a heat treatment process employed in the invention; 
           [0020]      FIG. 4  is a perspective view diagrammatic representation of an alternative heat treatment process employed in the invention; and 
           [0021]      FIG. 5  is a top plan view of yet another diagrammatic representation showing another alternate embodiment of a heat treatment process employed in the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    An exhaust gas system  10  is shown in  FIG. 1  in the form of a diesel exhaust gas aftertreatment system to treat the exhaust  12  from a diesel combustion process  14 , such as a diesel compression engine  16 . The exhaust  12  will typically contain oxides of nitrogen (NO x ) such as nitric oxide (NO) and nitrogen dioxide (NO 2 ) among others, particulate matter (PM), hydrocarbons, carbon monoxide (CO), and other combustion by-products. The system  10  includes one or more exhaust gas acoustic and/or aftertreatment devices or components  18 , with each device having a corresponding maximum operating temperature Tmax that can be achieved during operation of the system  10 . Examples of such devices  18  include catalytic converters, diesel oxidation catalysts, diesel particulate filters, gas particulate filters, lean NO x  traps, selective catalytic reduction monoliths, burners, manifolds, connecting pipes, mufflers, resonators, tail pipes, emission control system enclosure boxes, insulation rings, insulated end cones, insulated end caps, insulated inlet pipes, and insulated outlet pipes, all of any cross-sectional geometry, many of which are known. As those skilled in the art will appreciate, some of the foregoing devices  18  are strictly metallic components with a central core  19  through which the exhaust  12  flows, and other of the devices  18  can include a core  19  in the form of a ceramic monolithic structure and/or a woven metal structure through which the exhaust  12  flows. These devices  18  are conventionally used in motor vehicles (diesel or gasoline), construction equipment, locomotive engine applications (diesel or gasoline), marine engine applications (diesel or gasoline), small internal combustion engines (diesel or gasoline), and stationary power generation (diesel or gasoline). 
         [0023]      FIG. 2  shows one example of such a device  18  for use in the system  10  in the form of a catalytic unit  20  having a catalytic core  22 , a mount mat  24 , a cylindrical inner housing or can  26 , and heat insulating blanket or batt  28 , and a cylindrical outer housing or jacket  30 . The core  22  will typically be a ceramic substrate  32  having a monolithic structure with a catalyst coated thereon and will typically have an oval or circular cross section. The mounting mat  24  is sandwiched between the core  22  and the can  26  to help protect the core  22  from shock and vibrational forces that can be transmitted from the can  26  to the core  22 . Typically the mounting mat  24  is made of a heat resistant and shock absorbing-type material, such as a mat of glass fibers or rock wool and is compressed between the can and the carrier in order to generate a desired holding force. 
         [0024]    The heat insulating blanket  28  is made of a silica fiber insulation material having a weight percentage of SiO 2  of greater than 65%, and in preferred embodiments greater than 95%, and in highly preferred embodiments greater than 98%. Such material is known and commercially available, with one suitable example being supplied by BGF Industries, Inc. under the trade name SilcoSoft®, and another suitable example being supplied by ASGLAWO technofibre GmbH under the trade name Asglasil®. Such material is typically supplied in rolls, with the individual blankets  28  being die cut to the appropriate length and width for the corresponding device  18  after the material has been taken from the roll. Preferably, the blanket  28  is sandwiched or compressed in the annular gap  34  between the outer surface  36  of the can  26  and the inner surface  38  of the housing  30  to achieve an average installed density of 0.18 grams/cubic centimeter to 0.30 grams/cubic centimeter of the silica fiber insulation material of the blanket  28 . This provides sufficient frictional engagement between the blanket  28  and the surfaces  36  and  38  to suitably maintain the blanket in its desired location. It should be appreciated that while the blanket  28  is shown being compressed in the annular gap  34  between the cylindrical can  26  and housing  30 , the blanket  28  could be compressed between other adjacent surfaces of a device, including for example, a pair of planar adjacent surfaces, a pair of non-planar adjacent surfaces, a pair of conical adjacent surfaces, or any other pair of adjacent surfaces that can be found in acoustic or aftertreatment devices for exhaust systems. 
         [0025]    According to the invention, before the blanket  28  is installed into the device  18 , the blanket  28  is heat treated to achieve calcination of the silica fiber insulation material. In this regard, the blanket  28  is heated so that all of the silica fiber insulation material in the blanket  28  is raised to a temperature T greater than the maximum operating temperature Tmax of the device  18 . This heat treatment improves the resiliency and erosion resistance of the silica fiber insulation material and also eliminates the potential for a “thermoset” failure mode that can result if the silica fiber material were calcinated in-situ in the device  18  during operation of the system  10 . Preferably, this heat treatment takes place with the blanket  28  in an uncompressed or free state wherein there are no compressive forces being applied to the silica fiber insulation material of the blanket  28 . The temperature T preferably has some margin of safety above the maximum operating temperature Tmax of the device  18 , with one preferred margin of safety being 1.05×Tmax. 
         [0026]    As shown in  FIG. 3 , it is also preferred that the heat treatment take place using an in-line oven  40  wherein the silica fiber heat insulation material is unrolled from a supply roll  42  of the material and passed flat through the oven  40  on conveyor  43  so that the blanket  28  is planar during the heat treatment to reduce or prevent differential heating of the material of the blanket  28  and variation in thickness of the material in the blanket  28 . After heat treatment, the individual blankets  28  can be die cut to the desired length and width before installing in the device  18 . As an alternative, the complete supply roll  42  of the silica fiber heat insulation material can be heat treated, with or without rotation of the roll  42  about its center axis  44  in an oven  46 , as shown in  FIG. 4 . In this regard, it is believed that rotating the roll  42  about its axis  44  will serve to prevent a differential heating in the roll. Again, the individual blankets  28  can be die cut to the desired length and width after heat treatment and before installing in the device  18 . As yet an another alternative, the silica fiber insulation material can be die cut before heat treatment, with the blanket  28  being slightly oversized in length and width to account for shrinkage during heat treatment. The die cut blankets  28  can then be heat treated in an oven  40  or  44  while laying flat on a planar surface, as shown in  FIG. 5 . 
         [0027]    It has been found that by heat treating the silica fiber heat insulation material to the temperature T greater than Tmax before the blanket  28  is installed in the device  18 , the heat treated blanket  28  can be installed in a device  18  so that the blanket  28  is compressed between two adjacent surfaces of the device  18  and can maintain suitable frictional engagement with the surfaces over the desired life of the device  18  because the silica fiber insulation material of the blanket  28  maintains its resiliency and does not take on a “thermoset” from the max operation temperature Tmax of the device  18 . 
         [0028]    It should be appreciated that while the invention has been described herein in connection with a diesel combustion process in the form of a diesel compression engine  16 , the invention may find use in devices that are utilized in exhaust gas systems for other types of combustion processes, including other types of internal combustion engines, including, for example, internal combustion engines that use gasoline or other alternative fuels.