Patent Publication Number: US-6668762-B1

Title: Indirect fired process heater

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to an indirect fired process heater for heating process fluids such as natural gas or oil or any liquids or any gas. In particular, the present invention is directed to an indirect fired process heater wherein heat transfer fluid is heated in order to heat the process fluid. 
     2. Prior Art 
     Indirect fired process heaters are known to heat process fluids such as a liquid or a gas which might be employed in chemical, petroleum, or other industrial applications. For example, natural gas in a pipe that passes through a pipeline transmission/distribution system may be periodically heated for transmission purposes. Keeping the natural gas above a certain temperature will prevent water from condensing and/or freezing in or on a natural gas pipeline. Another industrial application would be as a preheater for further processing, such as natural gas processing. A further application of indirect fired process heaters is in fuel gas conditioning units. 
     In a standard indirect fired process heater, a quantity of heat transfer fluids is initially heated in a vessel with the fluids remaining static in the vessel. Heat retained by the heat transfer fluid is transferred to the process fluid. Thus, the process fluid is indirectly heated rather than directly heated. An indirect fired process heater provides more uniform temperature control than a direct fired heater and also reduces the likelihood of fire or explosion when heating combustible process fluids such as natural gas. The heat transfer fluid may be of different types, one type being a mixture of glycol and water. Ethylene glycol, propylene glycol or other types of glycol might be utilized. 
     Sams (U.S. Pat. No. 5,921,206) discloses an example of a conventional indirect process fluid heater with a novel baffle system. As in indirect fired process heaters to date, the entire vessel would be filled with heat transfer fluid medium. 
     It would be desirable to provide an indirect fired process heater which is more efficient than existing indirect fired process heaters. 
     It would be desirable to provide an indirect fired process heater that requires less heat process fluid to be heated than conventionally required for an equivalent output. 
     It would also be desirable to provide an indirect fired process heater that can start up from cold shutdown condition to full flow operation in a substantially shorter time period than a conventional indirect fired heater. 
     It would be desirable to provide an indirect fired process heater wherein the length of the heater could be decreased and the weight of the heater could be decreased from a conventional indirect fired heater. 
     It would be desirable to provide an indirect fired heater that can operate with low-nox burners which will reduce nox. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved indirect fired process heater apparatus and method. The apparatus includes a toroidal shell having an outer cylinder and a smaller diameter inner cylinder. The outer cylinder and inner cylinder together form a fluid tight enclosure for containing heat transfer fluid. 
     A plurality of helical heat transfer fluid coils are positioned within the toroidal shell and are coaxial therewith. The helical heat transfer coils have a radius less than the inner cylinder. The heat transfer fluid coils contain a heat transfer fluid which passes therein and therethrough. The heat transfer fluid is directed from the heat transfer fluid coils through a line into the toroidal shell where the heat transfer fluid circulates and thereafter is returned by a pump via a line back to the heat transfer fluid coils. A closed loop, circulating system is thereby formed. 
     A burner at one end of the vessel supplies heat to an axial passageway formed by the helical heat transfer fluid coils. Heat from the burner is directed into and through the axial passageway by a fan, fan/blower or natural draft type burners. The heat directed by the fan/blower or natural gas burner passes generally axially through the axial passageway. 
     A plurality of helical process fluid heating coils are positioned within the apparatus and are coaxial with but independent from the heat transfer fluid coils. The process fluid heating coils pass through the toroidal shell so that the process coils are in heat exchange relationship with the heat transfer fluid. The helical process fluid coils each have an axial diameter which is intermediate between the outer cylinder and the inner cylinder. The process fluid, such as natural gas, enters through an intake, passes through the helical process fluid heating coils, and thereafter exits through an outlet. 
     Hot combustion products (hereinafter referred to as “flue gases”) generated by the burner passes into and through the axial passageway and thereafter reverses direction and passes through an annulus formed by the exterior of the heat transfer fluid coils and the inner cylinder of the toroidal shell. Thereafter, these cooled flue gases are permitted to move out of an exhaust stack extending radially from the apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side, sectional view of an indirect fired process heater apparatus constructed in accordance with the present invention; 
     FIG. 2 is an end view of the indirect fired process heater apparatus shown in FIG. 1; and 
     FIG. 3 is a simplified schematic diagram of the indirect fired process heater shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments discussed herein are merely illustrative of specific manners in which to make and use the invention and are not to be interpreted as limiting the scope of the instant invention. 
     While the invention has been described with a certain degree of particularity, it is to be noted that many modifications may be made in the details of the invention&#39;s construction and the arrangement of its components without departing from the spirit and scope of this disclosure. It is understood that the invention is not limited to the embodiments set forth herein for purposes of exemplification. 
     Referring to the drawings in detail, FIG. 1 illustrates a cross-sectional view of an indirect fired process heater apparatus  10  constructed in accordance with the present invention while FIG. 2 illustrates an end view. The apparatus includes a toroidal shell  12  having an outer cylinder  14  and a smaller diameter inner cylinder  16 . The outer cylinder  14  and the inner cylinder  16  together form a fluid tight enclosure for containing heat transfer fluid as will be described in detail herein. The outer cylinder  14  along with end walls  18  and  20  form the exterior of an enclosed containment vessel. In the present embodiment, the containment vessel is in the form of a cylinder having an axis  22  illustrated by dashed lines. 
     A plurality of helical heat transfer coils  24  are positioned within the containment vessel and are coaxial therewith, having the same axis shown by dashed line  22 . The heat transfer coils  24  contain a heat transfer fluid which passes therein and therethrough as shown by the cut away portion. The heat process coils  24  may be of various dimensions and, in a preferred embodiment, are 4 inches or less in diameter. 
     The heat transfer fluid may be any number of fluids and, in one application, is a mixture of water and glycol. Various types of glycol may be employed. It will be understood that various other fluids may be employed which are suitable for the selected design pressure and temperature conditions. 
     The heat transfer fluid enters through an intake  26  to the helical coils, passes through the helical coils  24  and thereafter exits through an out take  28 . Thereafter, the heat transfer fluid is directed through a line  30  into the toroidal shell  12  formed by the outer cylinder  14  and inner cylinder  16 . The heat transfer fluid circulates through the toroidal shell and thereafter returns with force from pump  38  via line  32  back to the intake  26 . In the present embodiment, the heat transfer fluid enters one end of the vessel and exits the same end although other arrangements are possible. 
     An added advantage of the present invention is that since the toroidal shell forms the exterior of the apparatus, the normal required insulation or refractory lining of the inner shell  16  is eliminated. 
     A fluid expansion tank  34  in communication with the toroidal shell is provided to accommodate expansion of the heat transfer fluid when heated. It will be appreciated from the foregoing that the heat transfer fluid is in a circulating, closed loop system. 
     A burner  40  at one end of the vessel, in this case end wall  20 , supplies heat to an axial passageway formed by the helical heat transfer fluid coils  24 . Hot flue gas from the burner  40  is directed into and through the axial passageway by a fan  42  or blower, or natural draft burners visible in FIG.  2 . 
     The flue gases directed by the fan  42  or blower or natural draft burner passes generally axially through the axial passageway toward the opposite end wall  18 . 
     A plurality of helical process fluid heating coils  44  are positioned within the apparatus  10  and are coaxial with but independent from the heat transfer fluid coils  24 . Stated in other words, the fluid system of the heat transfer fluid coils is independent from the fluid system of the process fluid coils. 
     The process fluid heating coils  44  pass through the toroidal shell  12  so that the coils  44  are in heat exchange relationship with the heat transfer fluid of glycol and water. As shown in the present embodiment, the process fluid, such as natural gas, enters through an intake  45 , passes and circulates through the helical process heating coils  44  and thereafter exits through an out take  48 . In the embodiment shown, the process fluid enters one end of the vessel and exits the same end but other arrangements are possible. 
     The helical process fluid heating coils  44  have an axial diameter or diameters which are intermediate between the outer cylinder  14  and the inner cylinder  16 . By way of example but not by way of limitation, the helical process fluid heating coils may be 4″ or less in diameter. Accordingly, heat from the heat transfer fluid is passed to the process fluid, such as natural gas. 
     Hot flue gas generated by the burner  40  passes into and through the axial passageway formed by the helical heat transfer fluid coils  24 . The flue gas generated by the burner  40  and moved by the fan  42 , blower or natural draft burners thereafter reverses direction as shown by arrows  50 . The hot flue gases make a 180° turn and pass through an annulus formed by the exterior of the heat process coils  24  and the inner cylinder  16  of the toroidal shell  12 . Heat from the flue gas is also transferred to the heat transfer fluid while in the toroidal shell. Thereafter, flue gases are permitted to move in the direction shown by arrow  52  through and out of an exhaust stack  54  extending radially from the apparatus. 
     In the preferred embodiment disclosed herein, the toroidal shell  12 , the heat process coils  24  and the process heating coils  44  are all coaxial with each other. 
     The operation of the apparatus  10  in the present invention will be accomplished by initially heating the heat transfer fluid in the helical heat process coils  24  with hot flue gas generated from the burner  40  and directed by the fan  42  or blower or natural draft burner through an axial passageway formed by the heat transfer coils  24 . The heat transfer fluid is circulated via a pump  38  through the helical heat transfer coils and thereafter directed to the toroidal shell  12  having an outer cylinder and inner cylinder to form a fluid tight enclosure. Heat from the heat transfer fluid is transferred to the process fluid. The relatively cooler heat transfer fluid is thereafter circulated back to the heat transfer coils by a pump  38  so that a closed loop fluid system is formed. In one embodiment, the circulating heat transfer fluid is heated up to approximately 250° F., although other temperatures are possible. 
     The process fluid to be processed, such as natural gas, is directed into the apparatus  10  and through a plurality of the helical process heating coils  44  wherein the process heating coils pass through the toroidal shell in heat transfer relationship with the heat transfer fluid. 
     The flue gas generated by the burner  40  and directed by fan  42  or blower is directed through the axial passageway and thereafter through an annulus formed by a space between the heat transfer coils  24  and the inner cylinder  16  of the toroidal shell  12 . 
     FIG. 3 illustrates a simplified schematic diagram of the operation of the indirect fired process heater  10  of the present invention. Box  80  diagrammatically depicts the toroidal shell  12  which forms a containment vessel for the heater apparatus  10 . The helical heat transfer fluid coils  82  pass through the cylindrical toroidal shell  80  having an outer cylinder and an inner cylinder. Heat transfer fluid in the coils  82  passes into the toroidal shell and circulates from the apparatus as shown by arrow  84  and past a thermometer  86 . The heat transfer fluid is moved by a pump  88  and thereafter circulated back through the heat transfer coils as illustrated by arrow  90 . 
     Burner  40  illustrated by box  92  includes a valve  94  for regulating air moved by a fan or blower  96  driven by a motor  98 . The burner also includes a valve  100  for regulating a fuel gas line  102  so that fuel to the burner is delivered as shown by arrow  104 . A line  106  with a valve  108  may be provided for a pilot light mechanism. 
     A thermometer  110  monitors temperature of the heat transfer fluid in the toroidal shell. An exhaust stack  112  draws off the products of combustion from the burner  92  which have passed through the vessel. An expansion tank  114  provides room for expansion of the heat transfer fluid when heated. 
     Finally, process fuel line  116  shows an inlet which passes a thermometer  118  and thereafter through the helical process fluid coils  120  which pass through the toroidal shell. The process fluid is thereby heated. Thereafter, the process fluid is directed to an outflow  122  and passes a temperature sensor  124 . The temperature sensor  124  operates a control mechanism  126  which controls the air valve  94  and fuel valve  100  to increase or decrease heat to the apparatus in order to maintain a desired outflow temperature of the process fluid. 
     EXAMPLE 
     In one example of an application of the present invention, an indirect fired process heater  10  constructed in accordance with the teachings of the present invention may be compared to the typical, prior art indirect fired process heater wherein a vessel is filled with heat transfer fluid. The heat transfer fluid in the typical prior art heater remains static in the vessel and is not circulated. 
     The following are equivalent heater units in that each transfer three million (3,000,000) BTU/hr to a process fluid, such as natural gas, during similar flow conditions: 
     
       
         
           
               
               
               
             
               
                   
               
               
                   
                   
                 APPLICANT&#39;S 
               
               
                   
                 STANDARD HEATER 
                 HEATER 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Shell Diameter (inches) 
                 60 
                 56 
               
               
                 Shell Length (feet) 
                 24.6 
                 15 
               
               
                 Weight in Pounds 
                 33,700 
                 18,400 
               
               
                 (including heat transfer fluid) 
               
               
                 Heat Transfer Fluid in Gallons 
                 2,588 
                 248 
               
               
                   
               
            
           
         
       
     
     As can be seen by the foregoing, an indirect fired process heater constructed in accordance with the present invention would be approximately half the weight of a standard indirect process heater. An indirect fired process heater of the present invention would require a much smaller heat transfer volume charge, requiring only {fraction (1/10)} of the heat transfer fluid. The overall size of the vessel would also be reduced from a standard indirect process heater. 
     Finally, because of the size and fluid reductions, the present invention may be started up from cold condition to full flow use condition in a substantially shorter time. 
     Whereas, the present invention has been described in relation to the drawings attached hereto, it should be understood that other and further modifications, apart from those shown or suggested herein, may be made within the spirit and scope of this invention.