Abstract:
Disclosed is a Once-Through Steam Generator (OTSG) coil ( 52 ) and method, comprising a plurality of vertically arranged serpentine conduits ( 90 ) in a horizontal heat recovery steam generator (HRSG) that replaces a traditional natural circulation HP evaporator for producing super-critical steam. The OTSG comprises a lower equalization header system ( 130 ) that promotes system stability in multiple operating conditions. The equalization header allows a partial flow of fluid from the lower serpentine curved flow path ( 120 ) through an equalization conduit ( 125 ) into the equalization header ( 130 ) Disclosed also are: a flow restriction device in serpentine conduits; drainage structure from serpentine conduits through the equalization header, a drainage expansion section to accommodate stresses, and drainage bypass connections; and flow through serpentine conduits in upstream and downstream directions, mixed flow directions and longitudinally staggered directions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Patent Application Ser. No. 62/062,055, filed Oct. 9, 2014, the contents of which are expressly incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Natural gas and fuel oil serve as the energy source for much of the currently generated electricity. To this end, the gas or fuel oil undergoes combustion in a turbine which powers an electrical generator. The products of combustion leave the turbine as an exhaust gas quite high in temperature so that the exhaust gas represents an energy source in itself. This energy is captured in a heat recovery steam generator (“HRSG”) that produces superheated steam that powers another electrical generator. 
         [0003]    Generally, an HRSG comprises a casing having an inlet and an outlet and a succession of heat exchangers—that can include a superheater, an evaporator, and an economizer arranged in that order within the casing between the inlet and outlet. 
         [0004]    Such heat exchangers for an HRSG can have multiple banks of coils, the last of which in the direction of the gas flow can be a feedwater heater. The feedwater heater receives condensate that is derived from low-pressure steam discharged by the steam turbine, and elevates the temperature of the water. Then the warmer water from the feedwater heater flows for example into one or more economizers, boiler feed pumps or evaporators, which convert it into saturated steam. That saturated steam flows on to a superheater which converts it into superheated steam. From such a superheater, the superheated steam can flow to the steam turbine. 
         [0005]    Generally, in the above-discussed process, most HRSGs produce superheated steam at three pressure levels—low pressure (LP), intermediate pressure (IP) and high pressure (HP). An HRSG can thus have one or more superheaters and also can have what are termed an LP Evaporator, an HP Economizer, and an IP Economizer. 
         [0006]    An overall illustration of a system which features an HRSG using a natural circulation system appears in U.S. Pat. No. 6,508,206 B1 (hereafter “&#39;206 Patent”), which &#39;206 Patent is incorporated by reference. FIG. 4 of the &#39;206 Patent illustrates an arrangement with a superheater 18 located at the farthest position upstream. Downstream from the superheater 16 in the internal HRSG flow path is at least one evaporator 18 which has in fluid flow connection therewith a steam drum shown located atop of the evaporator. That steam drum is located outside of the HRSG internal exhaust gas flow path. The HRSG in the &#39;206 Patent also has a feedwater heater 20. 
         [0007]    The superheated steam produced by an HRSG has typically been below the critical point pressure of steam. Industry trends to build power plants of larger scale and of greater efficiency have evolved into a need for such plants to operate above, or just below, the critical pressure of water. 
         [0008]    In a natural circulation HRSG, water is first evaporated into saturated steam. This takes place in the high pressure (HP) evaporator coil and drum combination, which is simply referred to herein as “HP evaporator section” (HPEVAP). In such an HP evaporator coil and drum combination, the evaporator coil is located within the internal exhaust flow path of the HRSG, while the drum is located exterior to the internal exhaust flow path of the HRSG, with the HP evaporator coil and drum being in fluid flow connection with one another. In the HPEVAP, the density difference of steam and water at saturation conditions is the driving force to cause water and/or steam to circulate from a steam drum through downcomer pipes to the HPEVAP coil tubes, and through risers back to the steam drum. This circulation of saturated water in the HPEVAP is what distinguishes a natural circulation HRSG from other types of HRSGs. 
         [0009]    Another type of HRSG is a system that uses a once-through steam generator, commonly referred to in the art as an “OTSG”. In an OTSG, the working fluid does not recirculate through the heating surface as with a natural circulation HRSG system. Rather, with an OTSG the working fluid makes one pass through each individual parallel HPEVAP conduit and then exits the OTSG. U.S. Pat. No. 6,019,070 to Duffy (“Duffy ‘070’ Patent”) discloses an HRSG having an OTSG with what are designated therein as circuit assemblies. Those circuit assemblies in the Duffy &#39;070 Patent each comprises a serpentine shaped heat exchange tube with U-bend shaped portions and vertically oriented linear portions, positioned within the HRSG internal gas flow path. 
         [0010]    U.S. Pat. No. 6,189,491 to Wittchow, et al. (“Wittchow &#39;491 Patent”) also discloses an HRSG having an OTSG with vertically disposed steam-generator tubes within the HRSG gas flow path. U.S. Pat. No. 8,959,917 to Berndt, et al. (“Berndt &#39;917 Patent”) discloses an HRSG that uses an OTSG, while U.S. Patent Application Zhang having Pub. No. US 2013/0180228 A1, discloses an HRSG with a supercritical evaporator arrangement (“Zhang &#39;228 Applic.”) The said Duffy &#39;070 Patent, Wittchow &#39;491 Patent, Berndt &#39;917 Patent and Zhang &#39;228 Applic. are incorporated herein as if fully set forth herein. 
         [0011]      FIG. 1  of the present application shows an overall layout of a system that illustrates use of an HRSG similar to that shown in FIG. 3 of the &#39;206 Patent.  FIG. 1  of the present application discloses a gas turbine G that discharges hot exhaust gases into an “HRSG”, which extracts heat from the gases to produce steam to power a steam turbine S. The gas turbine G and steam turbine S power the generators E that are capable of producing electrical energy. The steam turbine S discharges steam at a low temperature and pressure into a condenser CN, where it is condensed into liquid water. The condenser CN is in flow connection with a condensate pump CP that directs the water back to the HRSG as feedwater. 
         [0012]    Generally, the heat exchangers comprise coils that have a multitude of tubes that usually are oriented vertically and arranged one after the other transversely across the interior of the casing. The coils are also arranged in rows located one after the other in the direction of the hot gas flow depicted by the arrows in  FIGS. 2-7  of the present application. The tubes contain water in whatever phase its coils are designed to accommodate. The length of the tubes can be as great as about 90′ tall. 
       SUMMARY OF DISCLOSURES 
       [0013]    As discussed above, in order to maximize cycle efficiency, an HRSG generally contains multiple pressure levels of superheated steam generation and steam reheat. The current invention will allow the operating pressure range to be increased to include steam production at supercritical pressures. Since only one pressure system (for a given working fluid), and nominally the high pressure (HP) system, can run at supercritical pressure, it is desirable to maintain natural circulation for the other pressure levels, typically the intermediate pressure (IP) and low pressure (LP) systems. There can be other pressure systems and nomenclature. This summary does not limit the type of HRSG that can be used. 
         [0014]    At pressures approaching the critical point of water, the density difference of water and steam at saturation conditions is much less than it is at lower pressures. Under such conditions, the hydrodynamics that drive the flow in a natural circulation evaporator are diminished to the point where another method is required to ultimately generate flow for the plant generation needs. In this case, it is practical to design and operate the HPEVAP as a once through steam generator (OTSG) in which, as noted, the working fluid does not recirculate through the heating surface but rather makes one pass through each individual parallel HPEVAP tube conduit and then exits the HPEVAP section. The OTSG as shown in  FIG. 2  replaces the typical HPEVAP in a natural circulation HRSG. Though OTSG&#39;s are known in the HRSG art, there is a need for the production of sub-critical and supercritical steam by means employed in using an OTSG to handle the operating conditions in a stable and mechanically acceptable design. The stratification of two-phase flows, critical heat fluxes, and instability are major concerns for designers of OTSGs. In supercritical conditions the working fluid exists as a single phase fluid and is sensibly heated as it passes through the parallel circuits of the HPEVAP. 
         [0015]    In the present disclosures, the OTSG is configured to comprise a group of individual serpentine tubes having vertical tube sections, and bends toward the top and bottom that are in flow connection with the vertical tube sections. Water can be introduced into the inlet of the OTSG group of tubes from the exit of the HP Economizer via pressure from the HP Feedwater Pump. The water can then be heated as it moves through the serpentine tubes in the OTSG coil, absorbing heat from the exhaust gas. At pressures slightly lower than the critical point the fluid exits as two phase or slightly superheated steam. At pressures at or above the critical point, the fluid exits the OTSG having properties consistent with the temperature. The OTSG operates under a high mass flux. As with other HRSGs, the supercritical water/steam fluid exiting the evaporator coil can be heated further in coil sections upstream in the gas path, absorbing heat from even higher temperature gas and increasing the temperature further to maximize the steam cycle efficiency. 
         [0016]    Accommodation of pressure differences that may exist between the individual conduits that contain serpentine tubes, is provided by an inter-association among those conduits to assist in balancing the pressure. Such equalization promotes pressure stability among the tube circuits. The configuration and location of the inter-association conduits utilize natural forces to aid in separating liquid from steam in the process, to promote two phase separation below the lower U-bends of the individual serpentine conduit sections. This assists in directing water into the equalization conduits, which promotes flow stability. More particularly, the disclosure preferably provides headers interconnected among the individual conduits. The headers are preferably positioned beneath the bottom of the lower U-bends of the serpentine tubes transverse to the internal exhaust gas flow. Further, the disclosure preferably includes a flow restriction device in flow connection with individual serpentine tubes positioned to improve flow distribution and flow stability, and preferably located toward the inlet of the serpentine tube. Moreover, the disclosure preferably provides drainage from the serpentine tube. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a general schematic of a combined cycle power system having an HRSG, that can use the present invention; 
           [0018]      FIG. 2  is a sectional view of an embodiment with an HRSG, and illustrating a once-through vertical tubed supercritical evaporator coil; 
           [0019]      FIG. 3  is a sectional view of a first embodiment of the invention showing part of the floor and roof of an HRSG, shown in a representative scale of height to width; 
           [0020]      FIG. 4  is the preferred embodiment of the invention, showing an inlet at the bottom and an odd number of tube rows; 
           [0021]      FIG. 5  is a plan view of a preferred first embodiment of the invention in a staggered tube pitch arrangement, with an oblong bubble ( 123 ) toward the left end indicating the ends of one row of vertical sections of a group of individual conduits tube sections; The diagram beneath the drawing indicates that a darkened circular area represents upward flow through a vertical tube section  108  of an individual conduit, while an “x” illustrates downward flow through an adjacent vertical tube section  108 , in alternating fashion; 
           [0022]      FIG. 6  is a plan view of an alternate embodiment of the invention showing an in-line tube pitch arrangement for the individual conduits, with an oblong bubble ( 123 ) toward the left end indicating the ends of one row of vertical sections of a group of individual conduits tube sections; as with  FIG. 5 ; the diagram beneath the drawing indicates that a darkened circular area represents upward flow through a vertical tube section  108 ′ of an individual conduit  90 ′, while an “x” illustrates downward flow through an adjacent vertical tube section  108 ′, in alternating fashion; 
           [0023]      FIG. 7  is an embodiment of the invention wherein the individual intermediate conduits incorporate an expansion loop to address differential tube expansion; 
           [0024]      FIG. 8  is an embodiment of the invention showing an alternate inlet location at the top and an even number of tube rows; 
           [0025]      FIG. 9  is an alternate embodiment of the invention showing a mixture of counter current flow and co-current flow, with an inlet header towards the bottom; and 
           [0026]      FIG. 10  is an alternate embodiment of the invention showing a mixture of counter current flow and co-current flow, with an inlet header towards the top. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0027]    The following detailed description illustrates the claimed invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the disclosure, describes several embodiments, adaptations, variations, alternatives, and uses of the disclosure, including what is presently believed to be the best mode of carrying out the claimed invention. Additionally, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The disclosure is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
         [0028]    For the following description, we will refer to the supercritical water/steam mixture and the sub-critical water/steam mixture as “fluid”. This should not infer that the behavior of sub-critical water and steam are the same as supercritical water/steam. 
         [0029]    Referring to  FIG. 2 , an HRSG  20  has a casing  23  within which are heat exchangers. Hot exhaust gases, such as discharged from a gas turbine (e.g., turbine G of  FIG. 1 ), enter the casing  23  and pass through a duct having an inlet  25  and an outlet  27 , such as indicated by arrows in  FIGS. 1 and 2 . During such process, that gas passes through heat exchangers. 
         [0030]    The HRSG casing  23  has a floor  30 , a roof  32 , and sidewalls that extend upwardly from the floor  30  to the roof  32 . The heat exchangers are positioned within the casing  23 . The floor  30  and roof  32  extend between the sidewalls so that the floor  30 , sidewalls and roof  32  help to form the internal duct of the HRSG casing  23 , through which the exhaust gas passes. 
         [0031]      FIG. 2  shows an HRSG casing with an exemplary sequential arrangement of heat exchangers. In  FIG. 2 , in a longitudinal direction from left to right, in the direction of the arrow showing exhaust gas flow, are a first reheater  36 , followed by a first high pressure (HP) superheater  39 , then downstream therefrom a second HP superheater  42  followed by a second reheater  44 . 
         [0032]    In the interest of minimizing the disruption to a “typical” horizontal gas flow, the disclosed vertical tube once-through HP evaporator (OTSG)  47  is shown in  FIG. 2  in a preferred position. As such, it replaces a natural circulation HPEVAP in an HRSG. For maintaining the balance of the HRSG as normally supplied, a horizontal gas path is preferable. 
         [0033]    The OTSG  47  comprises a large coil  52 , shown in  FIG. 3 . Coil  52  comprises individual serpentine tubes assembled into a module of convenient size for transportation, and will be further described. Downstream from the OTSG  47  can be a high pressure (HP) economizer system  56 , followed downstream by an intermediate pressure (IP) system  59 , which can be then followed by a low pressure (LP) system  61 . Downstream therefrom can be a feedwater heater system  63  (e.g., such as discussed and disclosed in the &#39;206 Patent). 
         [0034]    The coil  52  is supported from its roof structure  42  hanging in a steel frame, shown partially in  FIG. 3  as the roof beams  65  and floor beams  67 . The exhaust gas is contained inside the steel frame by an insulated casing and liner system typically found in HRSG&#39;s and partially shown in  FIG. 2  as the roof  32  and floor  30 . 
         [0035]    Turning now from the  FIG. 2  description to a more detailed discussion of the OTSG  47  and its coil  52 , the coil  52  comprises a plurality of individual heat exchange conduits illustrated as tubes  90 .  FIG. 3  shows a sub-group  70  of tubes  90 , with the number of individual tube conduits reduced for purposes of illustration.  FIG. 4  shows an even more detailed elevation view of a subgroup  70 . 
         [0036]    Referring to  FIG. 4 , in general, the OTSG  47  has an inlet header  75 , which can be a pipe, that can receive fluid from an inlet conduit  78  that is connected to the outlet of the HP Economizer  56  (depicted in  FIG. 2 ). The OTSG  47  also has an outlet header  82  that is in fluid flow connection with an outlet conduit  86 . Conduit  86  can lead to fluid flow connection with the inlet  87  of an external separator  88 , the outlet  89  of which can lead to flow connection with the inlet of the HP Superheater  44  (depicted in  FIG. 2 ). 
         [0037]    Located between the inlet header  75  and outlet header  82  are a group of individual heat exchange conduits  90 . The elevation view of  FIG. 4  shows one such conduit  90 . The top plan view of  FIG. 5  shows that the conduit sub-group  70  comprises a plurality of such individual conduits  90  that are shown in elevation in  FIG. 4 . 
         [0038]    Each individual conduit  90  can be a tube that has an inlet end  94  and an outlet end  98 . The inlet header  75  and outlet header  82  are preferably cylindrical bodies arranged normal to the exhaust gas flow, with openings along their lengths to which the inlet ends  94  and outlet ends  98  of tubes  90  are respectively secured, such as by welding. 
         [0039]    As shown in  FIG. 4 , from the inlet conduit end  94 , the conduit  90  can preferably comprise a flow restriction device  100  through which fluid flows. The pressure drop associated with the flow restriction device  100  improves flow distribution and flow stability. From flow restriction device  100 , the conduit  90  generally extends into a serpentine tube section  104  ( FIGS. 4 and 5 ). Serpentine tube section  104  generally comprises a series of vertical tube sections  108 , which comprise a middle portion  109 . As known in the art, those vertical sections  108  can comprise a portion  111  having heat exchange fins (which portions  111  are shown enlarged in  FIG. 4 ), and portion  113  which have no fins. The finned portion  111  is illustrated as overlapping the middle portion  109 . 
         [0040]    The conduits  90  also have a series of non-linear sections which are curved or bent, such as illustrated as a plurality of upper U-bend sections  115  and lower U-bend sections  120 . The first of the vertical sections  108  of conduit  90  is designated  121  in  FIG. 4 . The flow restriction device  100  is incorporated into the flow path of the first sections  121 , preferably before flow passes into the middle portion  109  of section  121 . 
         [0041]    Thus, in the preferred embodiment, flow within an individual conduit  90  comprises upward flow through a vertical tube section  108  to an upper U-bend section  115 , and then subsequent downward flow through an adjacent vertical tube section  108  to a lower U-bend section  120 . At the last of the series of vertical tube sections  108 , fluid flows upwardly through conduit outlet end  98  into outlet header  82 . Thus the flow through a conduit  90  is a continuing circuit of alternating upward and downward paths until flow through outlet end  98  reaches the outlet header  82 . 
         [0042]    As seen in the plan view of  FIG. 5 , in the subgroup  70  a number of individual conduits  90  are arranged in parallel in general alignment with the internal longitudinal exhaust gas flow path. Vertical tube sections  108  are aligned in a transverse plane that is generally perpendicular to the longitudinal exhaust flow path, to make up “rows”  123  of tube sections  108 . Rows  123  are thus arranged normal to the path of the hot exhaust gas.  FIG. 5  illustrates the direction of upward fluid flow and downward flow through an exemplary conduit  90  located at the bottom of  FIG. 5 . As noted in the description of  FIG. 5 , a darkened circular area designates upward flow through a vertical tube section  108 , while an “x” illustrates downward fluid flow through a tube section  108 . 
         [0043]    In the  FIG. 5  preferred embodiment, the vertical tube sections  108  illustrated in  FIG. 4  are arranged in a staggered fashion, where each tube section  108  in the tube row  123  is positioned at the midpoint of the transverse spacing of the upstream and/or downstream tube row. The tube sections  108  are thus staggered in a longitudinal direction in the exhaust flow path in an alternate offset pattern. In this arrangement a vertical section longitudinally downstream from an adjacent vertical section is longitudinally offset therefrom in an alternating pattern so that the vertical sections are not in longitudinal alignment In the particular embodiment of  FIG. 5 , a first group of the vertical sections  108  are in longitudinal alignment with one another, and a second group of the vertical sections  108 , are in longitudinal alignment with one another, so that the first and second groups are themselves offset relative to each other longitudinally. Such offset and staggered arrangement is known in the art as “staggered pitch.” 
         [0044]      FIG. 6  shows an alternate embodiment of the serpentine arrangement, wherein the vertical tube sections  108 ′ are arranged in an in-line pitch, so that the tube sections  108 ′, upper U-bends  115 ′ and lower U-bends  120 ′ in each individual conduit  90 ′ are longitudinally aligned from front to rear of each conduit  90 ′. Such alignment is known in the art as “in-line pitch.” 
         [0045]    Referring to  FIG. 4 , the flow restriction device  100  can be in the nature of an orifice or constricted tube. The orifice is sized based upon the required pressure drop and flow rates. A device  100  is preferably placed in the first tube section  121  section of the first tube row  123  downstream of the inlet header  75  as shown in  FIG. 4 . The location of the flow restriction device  100  is preferably between the inlet header  75  and the finned portion  111  of the first tube section  121 . The pressure drop associated with the flow restriction device improves flow distribution and flow stability. 
         [0046]    Now attention is directed toward an arrangement for equalization among the individual conduits  90 . Toward the bottom of each lower U-bend section  120  is an intermediate equalization conduit  125 . Intermediate conduit  125  can be a relatively short piece of pipe or tube, which has its upper inlet end connected toward the bottom of U-bend section  120 , preferably in the middle thereof. Intermediate conduit  125  allows fluid flow from the bottom center of each lower U-bend  120  to flow into an equalization conduit in the form of a header  130 . Each equalization header  130  is preferably a cylindrical pipe oriented normal to the exhaust gas flow of the HRSG, and spans the width of one tube row  123  within one coil  52 . The outlet ends of intermediate conduits  125  are connected to the header equalization conduits  130  preferably toward the top thereof. Preferably the connection of the outlet end of intermediate conduit  125  to the header conduit  130  is generally directly beneath the connection of the inlet end of intermediate conduit  125  to its respective lower U-bend  120 . 
         [0047]    As shown In  FIG. 4 , each equalization header  130  is connected at its underside to a drain  133 , such as a pipe, to be in fluid flow connection therewith. The drain pipes  133  extend through the casing floor  30 . A bellows expansion joint  136  is connected with drain  133  to accommodate tube expansion during operation, while sealing the exhaust gas inside the floor  30 . The drain pipes  133  can be open and shut as by valves  134 , such as by the illustrated gate valves  134 , ball valves, or other valves known in the art. The valves  134  can be operated so that the drain pipes  133  can carry fluid to a disposal point during times when the OTSG coil  52  may need to be emptied of fluid. 
         [0048]    Drain bypass conduits  140 , which can be pipes or tubes, connect desired adjacent drains  133 . Bypasses  140  allow a relatively small amount of flow to circulate between the pair of drain pipes  133  with which the bypasses  140  are connected. The movement of fluid through the bypasses  140  is stimulated by fluid movement within the drain pipes  133  to thus reduce stagnation of fluid within the separate drain pipes  133 , and create a cooling effect on the drain pipes  133 . Such cooling can be beneficial for situations in which process conditions and the metallurgy of the drain pipes  133  require that they be cooled during operation. 
         [0049]    System hydrodynamics and differing heat absorption of different individual conduits  90  can create destabilization and pressure difference between individual conduits  90 . Such a pressure difference causes flow through the equalization intermediate conduits  125  and equalization headers  130  to occur to balance those pressure differences. Such pressure balancing has a stabilizing effect on the flow through the conduits  90 . 
         [0050]    As the fluid flows downwardly through vertical tube sections  108  into each lower U-bend section  120 , the fluid is subject to the forces of gravity and the centrifugal force of the fluid as it turns in the lower U-bend section  120 . Water, being of higher density than steam, will be forced to the interior surface of the extrados of the U-bend section  120  by the centrifugal and gravitational forces. Particularly in the case of two-phase flow, it is desirable to redistribute only water flow through the equalization headers  130 . The high mass flux of the fluid through each tube row  123  plus the forces on the higher density fluid in the lower U-bend sections  120  ensures that only water is present in the equalization intermediate conduit  125  and equalization header  130  during subcritical operation. 
         [0051]    The interior diameter of pipe forming the individual conduits  90  are a function of the specific design details and can for example be about 0.5″ in. to about 2″. The shape of the arc of the bend in U-bends  120  is preferably of a generally semi-circular shape. The bend centerline radius of a U-bend  120  can be, for example, about 1.5 to about 3.0 centerline conduit diameters. The thickness of the wall of the individual conduits  90  can be based upon material type, diameter, operating temperatures and pressures. 
         [0052]    The equalization intermediate conduits  125  are preferably pipe having nominal diameter in the range of about 0.25″ to about 1.0″. The inside diameter of the equalization conduit conduits  125  is preferably smaller than the inside diameter of the individual conduits  90 . The smaller inside diameter of the equalization intermediate conduits  125  relative to the inside diameter of its respective individual tube facilitates only a relatively small amount of flow through the intermediate equalization conduit as compared to the amount of flow through the lower U-bend sections  120 , to pressurize the equalization headers  130 . In subcritical operation the flow through intermediate conduits  125  would include liquid water, which promotes stability of the system. In a preferred embodiment the inside diameter of an equalization conduit  125  is noticeably smaller than the inside diameter of its respective individual conduit  90 . In a preferred embodiment the ratio of the inside diameter of an equalization conduit  125  to the inside diameter of its respective individual conduit  90  is about ⅓ to about ½. 
         [0053]    The drains  133  are preferably pipe having a nominal diameter of about 1.5″ to about 2″. A bellows expansion joint  140  is used with drain pipe  133  to take up expansion during operation while sealing the exhaust gas inside the floor  30 . The drain pipes  133  carry fluid to a disposal point during times when the tubes  90  may need to be emptied of fluid. Drain bypasses  144  connect adjacent drains  133  and allow a small amount of flow to circulate through the drain pipes  133  for situations where process conditions and the metallurgy of the drain pipes  133  require that they be cooled during operation. 
         [0054]      FIG. 7  depicts an embodiment where each equalization intermediate conduit  125 ″ is formed into an expansion loop shaped, or bowed, section  127 ″. It is expected for coils of this type to have temperature variations between adjacent vertical tube sections  108  in the same tube row  123  due to external variations in heat input. Large variations in temperature can cause stress in the connections between individual conduits  90 , intermediate conduits  125  and headers  130 . The equalization headers  130  effectively anchor the lower U-bend sections  120  and restrict differential growth in adjacent individual conduits  90 . The looped or bowed configuration of section  127 ″ allows it to flex during expansion and contraction so that each pair of vertical tube sections  108 ″ connected directly to intermediate conduit  125 ″ can move independently of other adjacent vertical tube sections  108 ″ in their tube rows  123 ″. In  FIG. 7  the expansion loop section  127 ″ can comprise many configurations including for example “C”, “V”, “U” or “L” shape, and can have vertical and horizontal sections not in the same plane. Such configuration can allow independent growth or contraction of each vertical tube section  108 - 108 ′″. The amount of temperature variation in adjacent conduits  90  will determine if the embodiment of  FIG. 7  is employed or not, depending on thermal-mechanical analysis of the fluid flow, geometry and materials used. The expansion loop shaped configuration  127 ″ flexes and bends during operation to thus provide flexibility in adjusting to temperature variations that can exist between different individual conduits  90 ″, to thus reduce and avoid damage or failures from creep stresses and fatigue stresses. 
         [0055]      FIG. 8  depicts an alternate embodiment where the inlet header  75 ′″ is positioned toward the top of the coil  52 ′″. Fluid moves through the serpentine portions of the coil  52 ′″ in the same manner as heretofore described, but with the first row  123 ′″ starting with fluid flow in the downward direction. In the  FIG. 8  embodiment the flow restriction device  100 ′″ is located below the inlet header  100 ′″, and above the middle portion  109 ′″ of the first vertical section  121 ′″. 
         [0056]    Preferably, the overall serpentine fluid flow path flows counter current to the exhaust gas. Alternatively, the configuration could be a serpentine flow path flowing co-current to the exhaust gas. In the case of such reversed flow, the locations of the inlet headers  75 ,  75 ′,  75 ″ and  75 ′″ and outlet headers  82 ,  82 ′,  82 ″ and  82 ′″ are switched with each other. The location of the first vertical tube section  121 ,  121 ′,  121 ″ or  121 ′″, rather than being to the far right or farther away from the HRSG inlet, would be to be the farthest upstream of the vertical tube sections closer to the HRSG inlet  25 . The repositioning of the inlet headers  75 ,  75 ′,  75 ″ or  75 ′″ could be above or below such corresponding repositioned first vertical section  121 ,  121 ′,  121 ″ or  121 ′″. There can also be mixed flow embodiments with both co-current and counter-current sections. 
         [0057]    Alternatively, there could be a mixture of counter current flow and co-current flow in the same serpentine flow path.  FIGS. 9 and 10  show two alternative embodiments featuring such mixed flow. Such mixture can occur regardless of the position of the inlet header being at the bottom or top of the flow path.  FIG. 9  shows the inlet header  75 ″″ at the bottom of the flow path, while  FIG. 10  shows the inlet header  75 ′″″ at the top of the flow path. As a general exemplary description, the first three or four tube rows  113 ″″ ( FIG. 9 ) or  113 ′″″ ( FIG. 10 ) can flow co-current to the internal exhaust gas flow before changing direction through a loop back section  150 ″″ ( FIG. 9 ) or  150 ′″″ ( FIG. 10 ), respectively, to flow counter current to that flow path. The flow through the individual conduits  90 ″″ or  90 ′″″, respectively, terminates into the outlet header  82 ″″ or  82 ′″″, respectively. Benefits of this mixed flow path include more efficient phase change in subcritical conditions in the co-current flow path, and then changing to counter current flow to complete the heating of fluid as required. 
         [0058]    The number of tube rows  123 ′″ and the relative position of the inlet  78 ′″ and outlet  86 ′″ is a function of the exhaust gas conditions and the amount of heating surface needed to heat the fluid. The invention is not limited by the number of tube rows  123  depicted in the figures, or the relative positions of the inlet  78  and outlet  86 . The invention is not limited by the number of individual conduits or serpentine sections in the transverse direction, nor in the number of coils  52  that these serpentine tubes can form and that are placed in the HRSG exhaust path. 
         [0059]    In operation, for startup and low load operation the system can be operated at subcritical conditions. During all modes of operation flow entering the inlet header  75  is subcooled so that the water inlet temperature is below the saturation temperature. The system is designed to maintain this requirement by employing economizer-inlet approach temperature control. In order to avoid gravity controlled flow regimes a minimum tube mass flux is desired. That flow preferably is at least about 400 kg/ms2. Lower mass flux may be acceptable in certain specific designs and/or operating modes. Flow stability during startup and low load conditions are particularly important, and preferably should be kept above about 400 kg/ms2. As noted, the inclusion of a flow restriction device and pressure equalization headers serve to stabilize flow and reduce localized temperature and pressure differences in the coil. 
         [0060]    For startup and low loads in subcritical operation, an HRSG with an OTSG, such as the OTSG  47  of  FIG. 2 , can be placed in a flow control mode. The outlet steam/water mixture can be separated in an external separator such as external separator  88  of  FIG. 2 , where the water can be recycled to the plant condenser, for example, or to another point in the system, e.g., atmospheric blowoff tank, an economizer connection, a dedicated flash tank, or other place such as known in the art. Once sufficient heat is available to the OTSG  47  to produce superheated steam, the flow control is preferably changed to be based upon steam outlet temperature and other parameters. Thereafter, pressure can be increased to supercritical operation. 
         [0061]    Changes can be made in the above constructions without departing from the scope of the disclosure. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.