Abstract:
Steam power plants utilize high pressure and low pressure feedwater heaters to increase overall system efficiency. Both types of feedwater heaters generally include a subcooling zone that is separate from a condensing zone. The viability of the separation of the zones is critical to the efficiency of the overall system, specifically with regard to the inefficiencies inherent in the leakage of steam from the condensing zone into the subcooling zone. An improvement in the viability of that zone separation for such feedwater heaters is taught by the use of dual end plates for the subcooling zone to create a triple layer of separation between the condensing and subcooling zones to prevent unwanted and inefficient steam leakage.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/754,754 filed Jan. 21, 2013. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The instant invention relates to steam power plants and an improvement in the construction and use of feedwater heaters, both high pressure and low pressure feedwater heaters, having subcooling and condensing zones. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a steam power plant, feedwater heaters are used to gradually increase the temperature of the feedwater to the saturation temperature at boiler operating conditions. Preheating the feedwater improves the thermodynamic efficiency of the system, reduces the plant operating costs, and minimizes thermal shock to the boiler metal. A steam power plant may be equipped with a number of feedwater heaters. 
         [0004]    The energy used to heat the feedwater is usually derived from steam extracted from the steam turbine. Since the steam that would be used to perform expansion work in the turbine (and therefore generate power) is not utilized for that purpose, the extraction steam must be carefully optimized for maximum power plant thermal efficiency 
         [0005]    Feedwater heaters can be open or closed heat exchangers. In an open feedwater heater, the extraction steam is directly allowed to mix with the feedwater heater thereby heating it. 
         [0006]    A closed feedwater heater is typically a shell and tube heat exchanger wherein the feedwater passes through the tubes and is heated by turbine extraction steam flowing inside the shell on the outside of the tubes. Examples of such closed systems are shown in U.S. Pat. No. 2,412,573 to Fraser and U.S. Pat. No. 6,095,238 to Kawano. Additionally, steam power plant improvements have been numerous over the years, as shown by U.S. Pat. No. 2,729,430 to Sieder and U.S. Pat. No. 2,812,164 to Thompson. 
         [0007]    In a steam power plant, the feedwater heaters located upstream of the boiler feed pump are termed as high pressure feedwater heaters and those located downstream of the boiler feed pump are referred to as low pressure feedwater heaters. 
         [0008]    In high pressure feedwater heaters, the turbine extraction steam has a sizeable amount of superheat. Therefore, feedwater in high pressure feedwater is typically heated in three stages in three separate compartments: a desuperheating zone; a condensing zone; and a subcooling zone. Initial heating of the feedwater heater is carried out in the subcooling zone by subcooling the condensed turbine extraction steam. The secondary heating of the feedwater heater is carried out in the condensing zone from the condensing turbine extraction steam. The final heating of the heating of the feedwater is carried out in the desuperheating zone by the superheat in the turbine extraction steam. 
         [0009]    In low pressure heaters, the turbine exhaust steam has a lower amount of superheat. Therefore, feedwater in low pressure feedwater is typically heated in two stages in two separate compartments: a condensing zone; and a subcooling zone. Initial heating of the feedwater heater is carried out in the subcooling zone by the subcooling the condensed turbine extraction steam. The secondary and final heating of the feedwater heater occurs in the condensing zone from the condensing turbine extraction steam. 
       SUMMARY OF THE INVENTION 
       [0010]    The problem to be solved is as follows: In both the high pressure and low pressure feedwater heater, the extraction steam in the condensing zone has to be prevented from entering the subcooling zone. If the extraction steam enters the subcooling zone, then condensate in the subcooling will be heated instead of subcooled and, therefore, the entire function of the condensate subcooling will be nullified. In the prevailing feedwater heater designs the subcooling zone is isolated from the condensing zone by maintaining the water level above the entrance to the subcooling zone and employing an end plate at the end of the subcooling zone to separate the condensing zone from subcooling zone. The end plate is usually 2″-3″ thick and the tubeholes through the end plate are drilled to a tight tolerance. When the steam enters the tight spaces between the tube outer diameter and the end plate tube hole, it condenses and forms a water seal that prevents ingress of steam into the subcooling zone. 
         [0011]    Improper tube hole drilling tolerances, extended usage, normal wear and tear, or a combination thereof, can widening the gap between the outer diameter of the tube and the tube hole. In such scenarios, steam enters from the condensing zone, heating the condensate and compromising the performance of the subcooling zone, the entire heater, and the entire steam power plant. With each passing year the problem escalates until the decrease in efficiency is unsustainable. Eventually, the heater must be replaced. 
         [0012]    It is an object of the instant invention to prevent steam ingress into the subcooling zone through the end plate tubeholes since that is a major factor affecting the performance of feedwater heaters. 
         [0013]    According to the present invention, the ingress of steam into the subcooling zone can be avoided by using two end plates between the subcooling zone and condensing zone with a water seal in between for additional protection. According to the present invention, a feedwater heater is equipped with a subcooling zone that uses two end plates instead of one, and the tube holes in the end plates are drilled to tight tolerances. 
         [0014]    Additionally, a semi-circular plate is welded to two end plates. A flat plate is welded to the top of the end plates thereby creating an enclosure between the two end plates. Holes are drilled into the top plate connecting the two end plates to admit condensate into the enclosure, and holes are drilled at the bottom of the circular plate to drain the condensate. In this aspect of the invention a water dam is created with a minor flow of condensate through the enclosure. The water dam with a minor flow constitutes a water seal. 
         [0015]    The dual end plate with a water seal in between offers advantages of a triple layer of separation between the condensing and the subcooling zone. The present day technology has a single layer of separation. 
         [0016]    According to the present invention, the first layer of separation is provided by the condensate collected in the annular space between the tube outer diameter and the tube hole in the outer end plate. The annular space between the tube outer diameter and the tube hole in the outer end plates is filled with condensate at condensing zone steam saturation temperature. The absence of a heat sink creates less of an incentive for the condensing zone steam to enter the annular space between the tube hole and the outer diameter of the tube in the outer end plate. 
         [0017]    The second layer of separation, according to the present invention, is derived from the water dam between the outer and inner end plate. The enclosure between the inner and outer end plate is filled with condensate at or slightly below the condensing zone saturation temperature. Any steam from the condensing zone that might leak through the annular space between the tube outer diameter and the tube hole in the outer end plate is condensed by the water dam. 
         [0018]    The third layer of separation, according to the present invention, is created by the condensate occupying the annular space between the tube outer diameter and the tube hole in the inner end plate. Any steam from condensing zone that might leak through the annular space between the tube outer diameter and the tube hole in the outer end plate that due to some reason was not condensed by the condensate in the enclosure between the inner and outer end plate is condensed by condensate occupying the annular space between the tube outer diameter and the tube hole in the inner end plate. 
         [0019]    The triple barrier design, pursuant to this invention, consisting of the dual end plate with a annular condensate dam in between eliminates the ingress of steam into the subcooling zone. The performance of subcooling zone is preserved and the life of the feedwater heater is prolonged. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1 . is a schematic of a steam power plant system. 
           [0021]      FIG. 2  is a cutaway side view of a closed system high pressure feedwater heater. 
           [0022]      FIG. 2A  indicates the temperature changes of steam and feedwater inherent in high pressure feedwater heaters. 
           [0023]      FIG. 3  is a cutaway side view of a closed system low pressure feedwater heater. 
           [0024]      FIG. 3A  indicates the temperature changes of steam and feedwater inherent in low pressure feedwater heaters. 
           [0025]      FIG. 4  is a detailed view of the subcooling zone of a prior art feedwater heater. 
           [0026]      FIG. 5  is a cross sectional view of the subcooling zone of  FIG. 4  as viewed from the condensing zone. 
           [0027]      FIG. 6  shows a cutaway view of the prior art single end plate design for the subcooling zone of a standard low pressure feedwater heater. 
           [0028]      FIG. 7  is a detailed perspective view of the dual end plate subsystem of the present invention. 
           [0029]      FIG. 8  is a cross sectional view of the dual end plate subsystem of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    Referring to the drawings wherein like or similar references indicate like or similar elements throughout the several views,  FIG. 1  is a block diagram of a steam power plant system  1000  creating superheated steam to power steam turbine  11  driven electric generator  12  to produce electrical energy. The low pressure steam exiting the steam turbine is condensed in a steam surface condenser  13  using cold water from a cooling tower, lake, river or sea that enters on inlet  130 . Alternatively, the steam can be condensed in an air cooled condenser where the cold ambient air is used to condense the low pressure steam exiting the steam turbine  11 . The steam condensed in the water or air cooled condenser  13  is pumped by a condensate pump  14  into a low pressure feedwater heater  15 . The condensate entering the low pressure feedwater heater  15  through line  152  is termed feedwater. In the low pressure feedwater heater  15  the feedwater is heated by the steam extracted from the steam turbine  11  that enters at inlet  150 . The extraction steam condensed in the low pressure feedwater heater  15  is discharged through outlet  151  into the steam surface condenser  13 . The heated feedwater flows through line  155  into a deaerator  16  wherein the feedwater is deaerated using the steam extracted from the steam turbine  11  that enters the deaerator at inlet  160 . The heated feedwater from deaerator  16  is pumped by boiler feed pump  17  into a high pressure feedwater heater  18  at inlet  182  wherein the feedwater is further heated by the turbine extraction steam that enters the heater inlet  180 . The heated feedwater goes out through outlet  183  and enters the boiler  10 . The extraction steam condensed in the high pressure feedwater heater  18  is discharged through outlet  181  into a deaerator  16  through inlet  161 . Depending on the size, a power plant can have a multitude of low and high pressure feedwater heaters. 
         [0031]    The closed, high pressure feedwater heater  18  of  FIG. 1  is shown in greater detail in outline in  FIG. 2 . High pressure feedwater heater  18  has three zones: desuperheating zone  187 ; condensing zone  189 ; and subcooling zone  188 . In certain instances the high pressure feedwater heater  18  may be equipped with just a condensing zone and a subcooling zone. The high pressure feedwater heater  18  is equipped with a number of tubes that carry the feedwater. A representative tube  190  is shown in outline running through each of the zones. The feedwater enters tubeside of the heater through inlet  182 , travels through the tubes, and gets heated. The heated feedwater leaves the high pressure feedwater heater  18  through outlet  183  and flows into the boiler  10 , as shown in  FIG. 1 . Turbine extraction steam enters high pressure feedwater heater  18  at inlet  180  and condensate exits said closed high pressure feedwater heater  18  at outlet  181  and back to deaerator  16  also as shown in  FIG. 1 . In the subcooling zone  188 , the feedwater is heated by the subcooling condensate. In condensing zone  189 , the heat from the condensing extraction steam heats the feedwater. In the desuperheating zone  187 , the feedwater is further heated by the superheat in the extraction steam. Steam in the condensing zone is separated from the condensate exiting the subcooling zone by seal ring  191  and by end plate  185 . Inlet  184  allows only condensate into the subcooling zone  188 . 
         [0032]      FIG. 2A  illustrates the relative temperatures, plotted on the vertical axis of the graph, for steam and feedwater in subcooling, condensing and desuperheating zone in a high pressure feedwater heater. In  FIG. 2A , from left to right, the temperature of the extraction steam decreases in the desuperheating zone to a value close to the saturation temperature. In the condensing zone, the temperature of the extraction steam remains constant at the saturation temperature. In the condensing zone the extraction steam condenses on the tubes involving a phase change. In the subcooling zone the condensed extraction steam is subcooled to a temperature slightly higher than the feedwater inlet temperature. The flow of the feedwater inside the tubes is countercurrent to the of the flow of extraction steam and resulting condensate outside the tubes. The temperature of the feedwater, as shown in  FIG. 2A , increase as it flows through the tubes in the subcooling, condensing and desuperheating zone. 
         [0033]    Shown in  FIG. 3 , for comparison purposes to high pressure feedwater heater  18 , is low pressure feedwater heater  15 . A high or low pressure feedwater heater equipped with a subcooling zone will benefit from the present invention. Low pressure feedwater heater  15  has two zones: condensing zone  157  and subcooling zone  158 . A representative tube  159  is shown in outline running through each of the two zones. Feedwater enters the tube at inlet  155  travels through the subcooling and condensing zone and exits at outlet  156 . Turbine extraction steam enters low pressure feedwater heater  15  at inlet  150  and condenses in the condensing zone  157 . The condensed extraction steam enters the subcooling zone  158  at inlet  154  and gets subcooled as it loses its heat to the feedwater travelling inside the tubes. The subcooled condensate exits the low pressure feedwater heater at outlet  151 . 
         [0034]    Analogously to  FIG. 2A ,  FIG. 3A  shows relative temperatures, plotted on the vertical axis of the graph, for steam and feedwater temperatures in a low pressure feedwater heater. In  FIG. 3A , in the condensing zone, the temperature of the extraction steam remains constant at the saturation temperature as the extraction steam condenses. The temperature remains constant during phase change. In the subcooling zone, the temperature of the condensed turbine extraction steam decreases as the condensate loses its heat the feedwater travelling inside the tubes. The flow of feedwater is counter current to the flow of condensed extraction steam in the subcooling zone. The temperature of the feed water steadily increases as it travels through the subcooling and condensing zone. 
         [0035]    Prior art  FIG. 4  shows in detail the cross section detail of the subcooling zone  158  of a low pressure feedwater heater  15  in which the tubesheet  152  is shown with a representative tube  159  running there through. The end plate  200  forms the single, non-welded, barrier between subcooling zone  158  and condensing zone  157 . The longitudinal baffle  201 , semi-circular subcooling zone shroud  202  and the seal ring  153  together form the welded boundaries between said two zones. The welded boundaries constitute a permanent leak proof barrier between the condensing and subcooling zone. Condensate level  299  is shown by the sinusoidal broken line at the bottom of the condensing zone  157  and it is maintained well above the inlet  154  to the subcooling zone thereby ensuring that the steam from the condensing zone does not enter the subcooling zone. 
         [0036]    Prior art  FIG. 5  is a sectional view of end plate  200  as shown from the right (or condensing zone  157 ) side showing tube holes  203  through which tubes, such as representative tube  159 , run. The subcooling zone inlet  154  is shown in perspective in front of condensate outlet  151 . The welded boundaries between the condensing zone  157  (from which the view is taken) and said subcooling zone  158  are the longitudinal baffle  201  on the top, the semicircular longitudinal baffle  202  and the seal ring  153  (not shown). The end plate  200  containing the tube holes  203  and tubes running through the tube holes is the only non-welded boundary separating the subcooling zone  158  from the condensing zone (from which the view is taken). 
         [0037]    Prior art  FIG. 6  illustrates prior art for a subcooling zone  158 . The longitudinal baffle  201  and the longitudinal shroud  202  along with the seal ring  153  (not shown) form the welded barrier between the condensing zone and subcooling zone. A single end plate  200 , consisting of a multitude of tubes, such as representative tube  159 , running though a multitude of tube holes  203  forms the single non-welded barrier between the condensing zone  157  (not shown) and subcooling zone  158 . The tube holes  203  in the end plate  200  are drilled to a diameter which is slightly above that of the tubes so as to permit sliding the tubes through the tube hole  203  in end plate  200 . While it is anticipated that the water collected in the annular space between the tube outer diameter and the tube hole in the end plate  200  will form a permanent seal for the entire life of the heater, operational stresses using the prior art system prevent this from occurring. This is due, in part, to the fact that feedwater heaters are required to operate twenty-four hours a day for twenty-five to thirty years. 
         [0038]      FIGS. 7 and 8  show in detail the present invention over the prior art of  FIG. 6 , that is, the use of end plate system  2000  comprised of dual end plates  200  and  300  (with a water seal in between) together forming the triple barrier between subcooling zone  158  and condensing zone  157  (from which the view is taken) in a feedwater heater with a subcooling zone  158 . In  FIG. 7  for a feedwater heater with subcooling zone, the end plate system  2000  of the present invention is shown in part, with the inner end plate  200 , an outer end plate  300 . A semicircular plate  302  is welded to the inner end plate  200  and outer end plate  300 . A short horizontal plate  301  (not shown in its entirety) is welded to the top of the inner end plate  200  and the top of outer end plate  300  and the semicircular plate  302  is likewise welded to such end plates to form a water chamber between. The lips of the semicircular plate  302  are extended above the short horizontal plate  201  and bars are welded to the top of the inner end plate  200  and outer end plate  300  to form a water dam on top of the short horizontal plate  301 . Small holes (not shown) are drilled in the short horizontal plate  301  to allow condensate to enter the water chamber between the end plates  200  and  300 , having tube holes  203  and  303 , respectively. As shown in  FIG. 8 , drain holes (representative of which is shown as drain  305 ) is placed at the bottom of the semicircular plate  302  to drain the condensate. The intent is to keep the water chamber between the end plates flooded at all times and create a small water flow and avoid stagnation of condensate. 
         [0039]    According to the present invention, the ingress of steam into a subcooling zone, which has been one of the main reasons for degrading of performance of feedwater heaters worldwide, is eliminated by using a triple barrier design consisting of an inner end plate  200 , an outer end plate  300  and a water seal in between. 
         [0040]    The outer end plate  300 , with tightly drilled tube holes constitutes the first barrier. Steam condenses in the annular space between the outer diameter of the tubes and the tube hole. Condensate accumulated in the small annular gap prevents the entry of any additional steam. Due to normal wear and tear, extended usage or minor errors in end plate tube hole drilling, the annular gap between the tube outer diameter and the end plate tube hole could enlarge over time and steam from condensing zone could breach the first barrier. 
         [0041]    In such an event, the ingressing steam would come in contact with the second barrier, comprising condensate collected in the annular space between the inner and outer end plates, and condense. 
         [0042]    The inlet holes on the longitudinal baffle  201  on top and the drain  305  located at the bottom of the semi-circular cylinder  302  create a minor flow of condensate and prevent stagnation in the water chamber between inner and outer end plate. 
         [0043]    If, due to some unforeseen reason, steam from the condensing breaches the first and second barrier it is prevented from entering the subcooling zone by the third barrier comprising the inner end plate  200 . The condensate in the annular gap between the tube outer diameter and the inner end plate  200  tube holes  203  prevents the steam from the condensing zone from entering the subcooling zone. 
         [0044]    In this way, pursuant to this invention, the dual end plate with an annular condensate trough in between prevents the ingress of steam into the subcooling zone. The performance of subcooling zone is secured and the life of the feedwater is heater is prolonged. 
         [0045]    Although specific arrangements of components have been described herein, other suitable arrangements and components may be used as indicated with similar results in the viability of the seal between the subcooling and condensing zones of feedwater heaters, including, but not limited to, utilizing a plurality of such end plates to provide more than one water seal between said subcooling and condensing zones. 
         [0046]    Other modifications of the present invention will occur to those skilled in the art on reading the instant disclosure. Those modifications are intended to be covered within the scope of this invention such as, without limitation, the use of a plurality of plates and seals created thereby.