Patent Publication Number: US-2017363288-A1

Title: Fired heater with heat pipe preheater

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates and claims priority to U.S. Provisional Application No. 62/352,099, filed on Jun. 20, 2016, the disclosure of which is incorporated herein specifically by reference in its entirety. 
    
    
     FIELD 
     This invention concerns a fired heater with a heat pipe preheater. 
     BACKGROUND 
     Air preheaters are used to improve the thermal efficiency of fired heaters. In a common application, a heat exchanger is used to transfer heat from the flue gas exiting a fired heater to its incoming combustion air. Such installations typically require significant ducting to route flue gas to the location of the heat exchanger and then to a flue gas stack and additional ducting to route incoming combustion air from the heat exchanger to the burners of the fired heater. In addition, forced draft fans are generally needed to drive the incoming combustion air through the restrictive heat exchanger and induced draft fans are generally needed to pull flue gas through the heat exchanger and out the flue gas stack. 
     It would therefore be desirable to provide new air preheating systems for fired heaters that avoid some of the drawbacks of existing systems. 
     SUMMARY 
     We have now developed an improved fired heater with air preheating provided by one or more heat pipes. The fired heater may include at least one burner for combusting a fuel stream and an air stream and producing heated exhaust gases; a hot gas flow path and at least one conduit containing a process fluid to be heated by heat transfer from the heated exhaust gases; and an air preheater comprising at least one heat pipe having a first section exposed to the heated exhaust gases and a second section exposed to the air stream, wherein the heat pipe is positioned and arranged to transfer heat from the heated exhaust gases to the air stream, wherein the at least one heat pipe contains a working fluid sealed within the heat pipe, wherein said working fluid transfers heat from the heated exhaust gas to the air stream to be preheated. 
     A method is also provided for operating a fired heater. The method includes combusting a fuel stream and an air stream to produce heated exhaust gases; exposing the heated exhaust gases to a conduit transporting a process fluid to be heated by heat transfer from the heated exhaust gases; and exposing the heated exhaust gases to at least one heat pipe downstream of the conduit; transferring heat from the heated exhaust gases to a working fluid in the at least one heat pipe; and transferring heat from the working fluid to the air stream to preheat the air stream. 
    
    
     
       DRAWINGS 
         FIG. 1  is a schematic illustrating a fired heater including a heat pipe preheater according to one or more embodiments of the present invention. 
         FIG. 2  is a detail view illustrating the heat pipe preheater of the fired heater of  FIG. 1 . 
         FIG. 3  is a detail view illustrating a heat pipe preheater for a fired heater according to one or more embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Improved fired heaters are provided which incorporate heat pipe air preheaters for preheating combustion air. As used herein, the term “fired heater” refers to a direct-fired heat exchangers that use heat of combustion to raise the temperature of a material flowing through one or more coils throughout the heater. The material flowing through the one or more coils can be any type of material to be heated, such as a process fluid. For example, the material can be a feed material for another process, such as feed for a cracking unit. In some embodiments, the fired heater may be a furnace for a delayed coker unit and the process fluid may be a residual oil from a vacuum distillation unit. 
     In some embodiments, the improved fired heaters can avoid one or more of the disadvantages associated with traditional air preheater heat exchangers. For example, in some embodiments the air preheater may be contiguous with the convection or radiant section of the fired heater, thereby avoiding or limiting the ducting required to direct the heated exhaust gases to the heat exchanger. In addition, the improved fire heater can utilize one or more heat pipes that are positioned and arranged to limit the restriction of flow of the heated exhaust gases or combustion air through the air preheater. By limiting the pressure drop across the air preheater, it is possible to provide air preheating without the assistance of an induced draft fan (e.g., on the heated exhaust gas side of the preheater). Accordingly, in some embodiments, exhaust gases can flow unaided by draft fans and entirely by natural draft using the thermo siphon effect created by combustion of fuel gas at the burners of the fired heater. 
     The improved fired heater can include at least one burner for combusting a fuel stream and an air stream and producing heated exhaust gases; a hot gas flow path; at least one conduit containing a process fluid to be heated by heat transfer from the heated exhaust gases; and an air preheater comprising at least one heat pipe having a first section exposed to the heated exhaust gases and a second section exposed to the air stream. The heat pipe is positioned and arranged to transfer heat from the heated exhaust gases to the air stream. The heat pipe contains a working fluid sealed within the heat pipe, and the working fluid transfers heat from the heated exhaust gas to the air stream to be preheated. 
     In any embodiment, the air preheater can include a plurality of heat pipes, each having an evaporator section extending into the hot gas flow path, which passes through a passageway in the air preheater. The passageway may be contiguous with a convection section of the fired heater (or the radiant section if the fired heater does not include a convection section). Each heat pipe also may include a condensing section extending into a passageway for receiving a flow of combustion air for the fired heater. The condensing section of the heat pipe may be elevated relative to the evaporator section of the heat pipe. For example, the heat pipe may have a center axis extending through the evaporator section and condenser section and the center axis may form an angle of at least 10 degrees with respect to the horizon. 
     Preferably, the number, size, and arrangement of the heat pipes in the passageway may be configured to provide a pressure drop of less than 0.8 inch w.c. (inch water column), more preferably less than 0.7 inch w.c., more preferably less than 0.6 inch w.c., more preferably less than 0.5 inch w.c., more preferably less than 0.4 inch w.c., more preferably less than 0.3 inch w.c., more preferably less than 0.2 inch w.c., more preferably less than 0.1 inch w.c., more preferably less than 0.05 inch w.c. for a flow velocity of 11 ft/sec at the flue gas side inlet of the air preheater. 
     Because such little resistance to flow is provided by the air preheater, heated flue gas may move through the convection section and air preheater solely by natural draft. Similarly, the air stream for combustion may be fed through the air preheater solely by natural draft. In other embodiments, induced draft fans might also be employed, though it is expected that their demand would be substantially reduced in comparison to the case where a conventional heat exchanger is employed for the air preheater. For example, in any embodiment an induced draft fan, providing a motive pressure of less than 0.8 inch w.c., such as less than 0.5 inch w.c., or less than 0.1 inch w.c., can be employed between the air preheater and flue gas stack. Further, in some embodiments, the heated exhaust gas stream may be cooled in a tubular air preheater before it contacts the heat pipes of the air preheater. 
     In any embodiment, operating velocity for the flue gas through the inlet of flue gas side of the preheater may be 0.1 to 25 ft/sec, but is preferably in the range of 0.1 to 15 ft/sec, and even more preferably between 0.1 to 11 ft/sec, or 1 to 11 ft/sec, or 2 to 11 ft/sec, or 3 to 11 ft/sec. or 4 to 11 ft/sec, or 5 to 11 ft/sec. 
     The improved fired heater may further include an exhaust stack positioned downstream of the convection section. In some embodiments, the exhaust stack may be in substantial vertical alignment with the convection and/or radiant sections of the fired heater as well as the passageway of the air preheater containing the evaporator sections of the heat pipes. The evaporator sections of the heat pipe may be positioned at a same level or above the conduit containing the process fluid of the convection section and below or inside the exhaust stack. 
     An exemplary embodiment is illustrated in  FIG. 1 . Fired heater  10  includes air preheater  12 , having a plurality of heat pipes  28 , which preheats ambient combustion air before it passes through ducting  14  to burners  16  where the combustion air and a fuel are ignited. Heat produced from the combustion of the air and fuel heats radiant coil  20 , and the process fluid passing therethrough, in radiant section  18 . The heated exhaust gases pass through convection section  22  where the gases heat convection coil  24  and the fluid passing therethrough. The fluid heated in convection coil  24  may be the same or different fluid than the process fluid heated in radiant coil  20 . 
     The heated exhaust gases then pass through passageway  26  of the air preheater  12  where sections of the heat pipes  28  extending into passageway  26  are heated. This heat is then transferred to other sections of the heat pipes  28  which heat the combustion air passing through air preheater  12 . The heated exhaust gases then pass through stack  30  before they are released into the atmosphere. 
     As illustrated in  FIG. 1 , air preheater  12  may be contiguous to the convection section  22  of the fired heater (or it may be contiguous with the radiant section  18  in cases where the fired heater does not include a separate convection section). This avoids the need for additional ducting of the exhaust gas to a heat exchanger placed in a different location on the ground plot. The heat pipe air preheater  12  may be sufficiently small and lightweight to be secured directly to the fired heater  10  between the fired heater  10  and the stack  30  (i.e., the heat pipes may be disposed in a heated exhaust gas passageway  26  that is in substantial vertical alignment with the convection section  22  or radiant section  18  and also in substantial vertical alignment with stack  30 ). 
     The air preheater  12  of  FIG. 1  is shown in greater detail in  FIG. 2 . The air preheater  12  generally includes a plurality of heat pipes  28  that have first sections that extend into heated exhaust gas passageway  26  and second sections that extend into combustion air passageway  32 . Each heat pipe  28  is partly filled with a working fluid, such as water or a hydrocarbon, and is sealed. The heated exhaust gas passing through passageway  26  transfer heat to the evaporator sections of the heat pipes  28  to evaporate the working fluid and the heated vapor flows to the other, condenser end, where it gives up heat to the incoming combustion air flowing over the condenser sections of the heat pipes  28 . The condenser ends are elevated related to the evaporator ends so that condensed working fluid flows back under gravity to the evaporator ends. The condenser ends are preferably elevated by 10 degrees or more than the condenser ends (i.e., the heat pipe has a center axis extending through the first section and second section and the center axis forms an angle of at least 10 degrees with respect to the horizon). 
     Each heat pipe  28  generally includes an outer container and a working fluid contained therein. The outer container isolates the working fluid from the heated exhaust gases and the combustion air. The container preferably may be made of carbon steel. Passageway  26  and passageway  32  are also separated by a divider (tube sheet) or other structure to maintain physical separation between the flue gases and the combustion air. 
     The working fluid within the heat pipe  28  is selected to have a vapor temperature range appropriate to the intended operations. The vapor pressure over the operating temperature range should be sufficiently great to avoid high vapor velocities, which can cause flow instabilities. The fluid should exhibit good thermal stability, a vapor pressure not too high or low over the temperature range, a high latent heat, high thermal conductivity, low liquid and vapor viscosities, and acceptable freezing or pour point. The selection should also be based on thermodynamic considerations which are concerned with the various limitations to heat flow occurring within the heat pipe such as viscous, sonic, capillary, entrainment and nucleate boiling levels. While a wicking material is not required with pipe positions that are disclosed herein, it is not excluded. 
     Exemplary working fluids include acetone and other ethers, alcohols such as ethanol, methanol, propanol, and butanol, hydrocarbons, such as toluene, perhalocarbons, naphthalene, Dowtherm™ heat transfer fluids and water. Mercury may be suitable, but may be unpreferred for environmental reasons. Liquid metals such as sodium, lithium and sodium/potassium alloy may be useful in high temperature applications but are not usually required in the present applications. 
     In any embodiment the air preheater may include additional features to improve performance of the preheater. For example, the air preheater may include a tubular heat exchanger upstream of the heat tubes, which may be helpful in some cases to further cool the heated exhaust gases before they are exposed to the heat tubes. As illustrated in  FIG. 3 , combustion air may be fed through the interior of tubular heat exchanger  42  via manifold  40 . Heat from the heated exhaust gases is transferred to the air through the heat exchanger  42  so that the heated exhaust gases passing through passageway  48  is cooled prior to transferring heat to heat pipes  46 . The combustion air enters into passageway  44  where it is then further heated by heat pipes  46 . Such a configuration may be particularly useful in cases where the temperature of the heated exhaust gases would cause the heat pipes  46  to exceed the critical temperature of the working fluid or would otherwise cause the heat pipes  46  to operate outside of their ideal range. The heat pipes can include various features to improve heat transfer to or from the heat pipes, such as fins or other heat-transferring elements. 
     The following embodiments are also provided: 
     Embodiment 1 
     A fired heater comprising: at least one burner for combusting a fuel stream and an air stream and producing heated exhaust gases; a hot gas flow path and at least one conduit containing a process fluid to be heated by heat transfer from the heated exhaust gases; and an air preheater comprising at least one heat pipe having a first section exposed to the heated exhaust gases and a second section exposed to the air stream, wherein the heat pipe is positioned and arranged to transfer heat from the heated exhaust gases to the air stream, wherein the at least one heat pipe contains a working fluid sealed within the heat pipe, wherein said working fluid transfers heat from the heated exhaust gas to the air stream to be preheated. 
     Embodiment 2 
     The fired heater or method of any other Embodiment, wherein the air preheater comprises a plurality of heat pipes. 
     Embodiment 3 
     The fired heater or method of any other Embodiment, wherein the first section of the heat pipe extends into the hot gas flow path. 
     Embodiment 4 
     The fired heater or method of any other Embodiment, wherein the air preheater includes a passageway for passage of the heated exhaust gases therethrough. 
     Embodiment 5 
     The fired heater or method of any other Embodiment, wherein the passageway is contiguous with a convection section of the fired heater, the convection section including the at least one conduit containing the process fluid, and wherein the passageway is fluidly connected to the hot gas flow path. 
     Embodiment 6 
     The fired heater or method of any other Embodiment, wherein the air preheater provides a pressure drop of less than 0.8 inch w.c. for a flow velocity of 11 ft/sec through an inlet of the passageway. 
     Embodiment 7 
     The fired heater or method of any other Embodiment, further comprising an exhaust stack positioned downstream of the convection section, wherein the first section of the heat pipe is positioned at a same level or above the conduit of the convection section and below or inside the exhaust stack. 
     Embodiment 8 
     The fired heater or method of any other Embodiment, wherein the second section of the heat pipe is elevated relative to the first section of the heat pipe. 
     Embodiment 9 
     The fired heater or method of any other Embodiment, wherein the heat pipe has a center axis extending through the first section and second section and the center axis forms an angle of at least 10 degrees with respect to the horizon. 
     Embodiment 10 
     The fired heater or method of any other Embodiment, wherein the heated flue gas moves through the convection section and air preheater solely by natural draft. 
     Embodiment 11 
     The fired heater or method of any other Embodiment, wherein the air stream is fed through the air preheater and to the at least one burner solely by a forced draft fan. 
     Embodiment 12 
     The fired heater or method of any other Embodiment, wherein the air stream is fed through the air preheater and to the at least one burner solely by a forced draft fan. 
     Embodiment 13 
     The fired heater or method of any other Embodiment, wherein the fired heater is a delayed coker. 
     Embodiment 14 
     The fired heater or method of any other Embodiment, wherein the heated exhaust gas stream is cooled in a tubular air preheater before it passes by the first section of the heat pipe. 
     Embodiment 15 
     A method for operating a fired heater comprising: (a) combusting a fuel stream and an air stream to produce heated exhaust gases; (b) exposing the heated exhaust gases to a conduit transporting a process fluid to be heated by heat transfer from the heated exhaust gases; and (c) exposing the heated exhaust gases to at least one heat pipe downstream of the conduit; (d) transferring heat from the heated exhaust gases to a working fluid in the at least one heat pipe; and (e) transferring heat from the working fluid to the air stream to preheat the air stream. 
     Embodiment 16 
     The method of Embodiment 15, wherein steps (a)-(e) are performed without the assistance of an induced draft fan.