Abstract:
A protected carbon steel pipe for fire tube heat exchange devices, particularly boilers, comprising internally, at least along a portion of its length, at least one bonded layer of corrosion-resistant material.

Description:
[0001]    The present invention relates to a protected carbon steel pipe for fire tube heat exchange devices, particularly boilers. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is known that fire tube heat exchange devices exist which provide pipes designed to convey flue gases generated by combustion in appropriate furnaces, such pipes being provided within a vessel which contains the fluid to be heated; among such devices, boilers for generating hot water or another heat transfer fluid are particularly important. 
         [0003]    The pipes comprised within said devices are made of carbon steel in order to ensure optimum quality of the welded joints between the pipes and the structures of the devices, which are also made of carbon steel; however, in the case of devices, such as for example condensing boilers, in which the water vapor contained in the flue gases condenses inside the flue gas conveyance pipes, condensation forms which attacks strongly by corrosion the wall of the pipes. 
         [0004]    Pipes of the described type are not typical only of boilers, but can be present also in other devices of industrial thermal cycles, such as for example condensers, economizers and heat exchangers. 
       SUMMARY OF THE INVENTION 
       [0005]    The aim of the present invention is to provide a carbon steel pipe adapted to convey flue gases which is entirely protected against the danger of corrosion caused by condensation and further ensures high efficiency in the transmission of heat from the flue gases to the fluid, and in particular to the water, to be heated. 
         [0006]    The proposed aim is achieved by a protected carbon steel pipe for fire tube heat exchange devices, particularly boilers, according to the invention, characterized in that it comprises the features disclosed in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Further characteristics and advantages will become better apparent from the description of some preferred but not exclusive embodiments of the protected carbon steel pipe for fire tube heat exchange devices, particularly boilers according to the invention, illustrated by way of non-limiting example in the accompanying drawings, wherein: 
           [0008]      FIG. 1  is a longitudinal sectional view of a fire tube boiler with a pipe according to the present invention; 
           [0009]      FIG. 2  is a partial sectional view, taken along the line II-II of  FIG. 1 ; 
           [0010]      FIGS. 3 to 25  show the same sectional view according to variations. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0011]    With reference to the  FIGS. 1 and 2 , the reference numeral  1  generally designates a fire tube boiler with a burner  2 , a furnace  3 , a flue gas reversal chamber  4   a,  a vessel  5  which contains the water to be heated with couplings  5   a,    5   b  respectively for inflow and outflow, pipes  6  designed to convey the gases generated by combustion in the furnace which arrive from the reversal chamber  4   a  and are sent to the output chamber  4   b  according to the arrows shown in  FIG. 1 . 
         [0012]    The boiler  1  is of the type known as condensing boiler, and therefore the water vapor contained in the flue gases condenses therein as they flow within the pipes such as  6 ; the structure of said boiler is made of carbon steel. 
         [0013]    An important characteristic of the invention consists in that the pipe  6  is made of carbon steel and comprises internally, bonded thereto, a layer  7  made of a corrosion-resistant material, such as aluminum or stainless steel. 
         [0014]    In this manner, the dual need to be able to weld the pipe to the carbon steel structure of the boiler and to protect said pipe against the aggression of the condensation generated in the flue gases is achieved in an optimum manner. 
         [0015]    As can be seen from  FIG. 1 , the layer  7  is present along the entire length of the pipe  6 , but it should be clearly noted that such layer might be provided only in the part of the pipe toward the outlet of the flue gases. 
         [0016]    An embodiment of the pipe according to the invention is shown in  FIG. 3 : the carbon steel pipe  8  comprises, bonded thereto, a layer  9  made of corrosion-resistant material and accommodates internally a coaxial sleeve  10 , which is closed by at least one plug  10   a , likewise made of corrosion-resistant material. 
         [0017]    An interspace  11  for conveying the flue gases in a reduced cross-section is thus provided: the consequent increase in speed effectively helps to increase heat exchange between the flue gases and the water to be heated. 
         [0018]    A further increased efficiency of said exchange occurs in the variation of  FIG. 4 , in which a carbon steel pipe  12  has, bonded thereto, a layer  13  of corrosion-resistant material, and a sleeve  14 , closed by a plug  14   a,  has ribs  14   b  which extend monolithically from it and which, by entering an interspace  15  through which the flue gases flow, make contact with the layer  13 , transmitting thereto, and ultimately to the water to be heated, heat by conduction. 
         [0019]    An identical situation occurs in the variations of  FIGS. 5 ,  6 ,  7 , and  8 , changing only the shape of the cross-section of the ribs: while the ribs of the solution of  FIG. 4  are shaped so as to have a cross-section with a rounded cusp, the ribs of the variations of said figures respectively have a rectangular cross-section  16 , a triangular cross-section  17  in which the thickness decreases gradually toward the central region  18 , and a rectangular cross-section with an end face  19 . 
         [0020]    The variation of  FIG. 9  provides, bonded to a carbon steel pipe  20 , a first layer  21  made of corrosion-resistant material, and a second layer  22 , also made of corrosion-resistant material, which provides ribs  22   a  adapted to make contact, by entering an interspace  23  through which the flue gases flow, with a sleeve  24  closed by a plug  24   a,  thus providing a situation which is similar to the one described earlier. 
         [0021]    Variations of the cross-sections of the ribs identical to the ones shown in  FIGS. 5 ,  6 ,  7 ,  8  are visible in  FIGS. 10 ,  11 ,  12 ,  13 : therefore, there is no need to deal with these variations. 
         [0022]    The variations of  FIGS. 14 to 18  replicate the constructive embodiments shown in  FIGS. 9 to 13 , with the only difference related to the fact that there is just one layer made of corrosion-resistant material bonded to the carbon steel pipe: thus, for example, the variation of  FIG. 14  provides, bonded to a carbon steel pipe  25 , only a layer  26  made of corrosion-resistant material, which is provided with a ribs  26   a  which make contact with a sleeve  27 . 
         [0023]      FIG. 19  illustrates an embodiment in which a first layer  29 , made of corrosion-resistant material, and a second layer  30 , also made of corrosion-resistant material, are bonded to a carbon steel pipe  28 ; ribs  30   a  protrude from said second layer and are alternated with ribs  31  a which protrude from a sleeve  31 , leaving spaces  32  between said ribs for the flow of the flue gases: ribs  31   a  extend until they make contact with the layer  30  in the presence of references  31   b  which ensure correct positioning. 
         [0024]    A variation of the embodiment of  FIG. 19  is shown in  FIG. 20 : the only difference is the absence of the layer  29  bonded to a carbon steel pipe  33 , and therefore only a layer  34  made of corrosion-resistant material and provided with the ribs as described above, is present. 
         [0025]    The variation shown in  FIG. 21  is now described: it comprises, bonded to a carbon steel pipe  35 , a layer  36  made of corrosion-resistant material, which is provided with variously shaped ribs  36   a  arranged alternately with respect to variously shaped ribs  37   a  which protrude from a sleeve  37  and are adapted to make contact in the presence of references  37   b  with the wall of the layer  36 . 
         [0026]    Another variation is described with reference to  FIG. 22 , in which the reference numeral  38  designates a carbon steel pipe, which comprises internally two flue gas conveyance modules, designated generally by the reference numerals  39  and  40  respectively, which are delimited by a closed wall made of corrosion-resistant material. 
         [0027]    The wall of the module  39  comprises a portion  41 , which is bonded to the wall of the pipe  38  substantially along half of the circumferential extension thereof, and a straight portion  42 , which extends transversely, and likewise the wall of the module  40  comprises a portion  43  bonded to the wall of the pipe  38  and a straight portion  44 ; the straight portions  42  and  44  are in mutual contact. 
         [0028]    The described configuration allows to obtain the dual result of protecting the wall of the pipe  38  against contact with the flue gases, and this is done by the portions  41  and  43  of the walls of the modules, and of providing an intense transmission of heat from the flue gases to the water contained in the boiler which strikes the outer surface of the pipe  38 , determined by the presence of the portions  42  and  44  of said walls which make contact with the flue gases at the region where said flue gases have a particularly high temperature. 
         [0029]      FIG. 23  illustrates another variation of the invention, which provides, inside the pipe  38 , six flue gas conveyance modules which are substantially shaped like wedges and are designated respectively by the reference numerals  45 ,  46 ,  47 ,  48 ,  49 ,  50 . 
         [0030]    The walls of the module, which are made of corrosion-resistant material, are identical and comprise an arc-like portion,  45   a  for the module  45 , bonded to the wall of the pipe  38 , and two straight portions  45   b,    45   c  for said module, which protrude from the ends of said arc-like portion toward the axis of said pipe; the straight portions of the individual modules are in mutual contact. 
         [0031]    Of course, this embodiment, too, ensures the functional characteristics stated with reference to the embodiment shown in  FIG. 22 . 
         [0032]    As regards the variation of  FIG. 24 , it differs from the embodiment of  FIG. 23  only in that inside the pipe  38  there are twelve flue gas conveyance modules  51  instead of the six modules provided in the embodiment of  FIG. 23 . 
         [0033]    The variation shown in  FIG. 25  provides for the presence, inside the pipe  38 , of a continuous layer  52 , provided with protrusions such as  53  which protrude toward the axis of the pipe and thus provide, as in the previously described variations, optimum conditions both as regards the protection of the pipe  38  against corrosion and for high efficiency in heat transfer from the flue gases contained in the pipe  38  to the water to be heated. 
         [0034]    The described invention is susceptible of numerous other modifications and variations, all of which are within the scope of the appended claims: thus, for example, it is important to stress the fact that the various means for protecting the carbon steel pipe made of corrosion-resistant material may cover different lengths within the described pipes. 
         [0035]    The disclosures in Italian Patent Applications No. MN2005A000023 and No. MN2006A000012 from which this application claims priority are incorporated herein by reference.