Abstract:
A feedwater heater for an HRSG is provided with a monitoring unit for detecting the presence of condensation in the feedwater heater. The monitoring unit includes a dielectric band around one of the tubes of the feedwater heater near the location where the feedwater is directed into the heater and a conductive band located around the dielectric band. The unit also includes a conductivity sensor installed between a ground on the feedwater heater and the conductive element. Hot gases containing moisture pass through the feedwater heater, and if the temperature of surfaces in the region of the tube around which the dielectric and conductive bands extend drops below the dew point of the gas, an electrically conductive condensate will appear those surfaces and on the tube and will flow over the dielectric band, completing an electric circuit between the tube and the conductive band. The conductivity sensor detects this and hence detects the presence of the condensation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application derives and claims priority from U.S. provisional application 60/557,626 filed Mar. 30, 2004. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   This invention relates in general to heat exchangers and, more particularly, to a process and apparatus for detecting condensation in a heat exchanger. 
   Natural gas represents a significant source of electrical energy in the United States. It burns with few emissions, and is available throughout much of the country. Moreover, the plants which convert it into electrical energy are efficient and, in comparison to hydroelectric projects and coal-fired plants, they are relatively easy and inexpensive to construct. In the typical plant, the natural gas burns in a gas turbine which powers an electrical generator. The exhaust gases—essentially carbon dioxide and steam—leave the gas turbine at about 1200° F. (649° C.) and themselves represent a significant source of energy. To harness this energy, the typical combined cycle, gas-fired, power plant also has a heat recovery steam generator (HRSG) through which the hot exhaust gases pass to produce steam which powers a steam turbine which, in turn, powers another electrical generator. The exhaust gases leave the HRSG at temperatures on the order of 150° F. (66° C.). 
   The HRSG basically comprises a series of heat exchanges housed in a duct. Water which is derived from condensing steam discharged from the steam turbine enters the HRSG at a feedwater heater where it undergoes a rise in temperature. The higher temperature water then flows into an evaporator where it is converted into steam, most if not all saturated steam. That steam flows into a superheater which converts it into superheated steam, and the superheated steam flows on to the steam turbine to power it. The hot gases derived from the combustion flow in the opposite direction, encountering the superheater, then the evaporator, and finally the feedwater heater. 
   Thus, the gases are at their coolest temperatures in the region of the feedwater heater and beyond. Natural gas contains traces of sulfur, and during the combustion the sulfur combines with oxygen to produce oxides of sulfur. Moreover, the combustion produces ample quantities of water in the form of steam. If the exhaust gases remain above the dew point for the gases, which is about 107° F. (42° C.), the oxides of sulfur pass out of the HRSG and into a flue. However, the low temperature feedwater has the capacity to bring the tubes at the downstream end of the feedwater heater below the dew point of the water in the exhaust gases, and when this occurs, water condenses on tubes. The oxides of sulfur in the flue gas unite with that water to form sulfuric acid which is highly corrosive. Other acids may likewise form. 
   In order to deter the formation of acids, operators of HRSGs control the temperature of the water entering the feedwater heater, so that it remains well above the dew point for the gases. This assures that no condensation occurs in the feedwater heater. And to be safe, the temperature of the entering water needs to be high, because the dew point temperature of the gases is difficult to predict in that it is a function of several parameters. If the temperature of the entering water could be lowered, the water would extract more energy from the gases, and they would pass beyond the feedwater heater at a lower temperature. 
   The problem of condensation in feedwater heaters or economizers is not confined solely to HRSGs installed downstream from gas turbines. Indeed, it can occur almost anywhere energy is extracted from hot gases flowing though a duct to heat the feedwater for a boiler. For example, many power plants convert the hot gases derived from the combustion of fossil fuels, such as coal or oil, directly into steam, and the boilers required for the conversion, to operate efficiently, should have feedwater heaters—heaters which should not produce condensation. Also, systems exist for producing steam from the hot gases derived from the incineration of waste, and they likewise have boilers including feedwater heaters that should not be subjected to condensation. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a schematic sectional view of an HRSG having a feedwater heater provided with a monitoring unit constructed in accordance with the present invention; 
       FIG. 2  is a fragmentary sectional view of the feedwater heater at the monitoring unit; and 
       FIG. 3  is an enlarged view of the activating terminal for the monitoring unit. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings a heat recovery steam generator (HRSG) A ( FIG. 1 ) contains a dew point monitoring unit B ( FIG. 2 ) which provides the HRSG A with a system that detects the presence of condensation in the HRSG A and produces and alarm or other signal. This enables the operator of the HRSG A to control the temperature of water entering the HRSG A so that surfaces within the HRSG A remain above the temperature at which condensate will form on them, yet not excessively above that temperature. 
   The HRSG A includes a duct  2  having an inlet end  4  and a discharge end  6  which leads into a stack or flue. Hot gases derived from the combustion of natural gas or some other fuel enter the duct  2  at the inlet end  4 , pass through it, and leave at the discharge end  6 . The gases contain carbon dioxide and steam and trace mounts of compounds which if united with liquid water can form corrosive substances such as acids. 
   In addition to the duct  2 , the HRSG includes several heat exchangers that are housed in succession within the duct  2  ( FIG. 1 ). Each has tubes made from low carbon steel and fins around the tubes. First, the gases flow through a superheater  10 , then through an evaporator  12 , and finally through a feedwater heater  14 , sometimes called an economizer. Water flows through these heat exchangers in the opposite direction. It enters the feedwater heater  14  as a liquid, where its temperature is elevated. The higher temperature water flows from the feedwater heater  14  into the evaporator  12  where it is converted into steam, mostly if not all saturated steam. The saturated steam enters the superheater  10  where it becomes superheated steam. The temperature of the gases drops as the gases pass through the superheater  10 , the evaporator  12  and the feedwater heater  14  and are at their coolest temperatures in the region of the feedwater heater  14  and beyond. To prevent the formation of corrosive acids, the temperature of surfaces within the feedwater heater  14  must remain above the dew point for the gases in the duct  2 . Typically, that temperature is about 107° F. (42° C.), but it does vary. Moreover, the dew point of the gases is difficult to predict, because it represents a function of several parameters. 
   The operator of the HRSG A maintains a measure of control over the temperature of the feedwater that enters the feedwater heater  14 . Preferably, that temperature should be low to extract maximum heat from the gases flowing through the duct  2 , yet it should remain above the dew point of the gases to avoid condensation from developing in the feedwater heater  14 . The monitoring unit B enables the operator of the HRSG to achieve these objectives. 
   The feedwater heater  14  includes ( FIG. 1 ) a header  20  and a collector  22 , as well as a succession of tubes  24  that extend vertically through the duct  2 , generally occupying the entire cross sectional area of the duct  2 , so that the hot gases must flow over them. All are formed from a metal, such as a low carbon steel and, of course, will conduct an electrical current. The header  20  extends across the duct  2  at the top of the duct  2 , and the collector  22  may do so as well, although in the alternative it may be in the bottom of the duct  2 . One end of each tube  24  is connected to the header  20  and the other end is connected to the collector  22 . The tubes  24  are fitted with fins  26  ( FIG. 2 ) which enhance the transfer of heat from the hot gases to the tubes  24  themselves and to the water within the tubes  24 . In addition, the feedwater heater  14  has an inlet  30 , which is connected to a source of feedwater and opens into the header  20 , and an outlet  32  which leads away from the collector  22  and is connected to the evaporator  12 . The relatively cool feedwater enters the header  20  through the inlet  30  and from the header  20  flows into the tubes  24  where it is heated by the hot gases and thus undergoes a rise in temperature. The heated feedwater flows from the tubes  24  into the collector  22  and thence into the outlet  32  which delivers it to the evaporator  12 . The surfaces of the inlet  30  and header  20  have the lowest temperatures of any surfaces in the feedwater heater  14 , and the same generally holds true for the tubes  24  where they are connected to the header  20 . One of the tubes  24 , preferably the one closest to the inlet  26 , immediately below its connection to the header  20  possess a bare surface  34  ( FIG. 2 ) that is devoid of fins  26 . Indeed, the bare surface  34  extends vertically between the header  20  and the first fins  26  on that tube  24 . 
   The monitoring unit B basically comprises ( FIG. 2 ) a ground terminal  40  somewhere on the metal feedwater heater  14 , preferably on the inlet  30 , and an actuating terminal  42  on the bare surface  34  of the one tube  24 . In addition, the monitoring unit B includes a conductivity meter  44  connected between the ground terminal  40  and the actuating terminal  42  with electrical leads  46  and  48 , respectively. The arrangement is such that the conductivity meter  44  will detect the completion of an electrical circuit between the ground terminals  40  and actuating terminal  42 . 
   The actuating terminal  42  includes ( FIG. 3 ) a dielectric band  50  which encircles the bare surface  34  of the one tube  24  slightly above the first fin  26  on that tube, it being spaced downwardly from the header  20 . Indeed, the spacing between the lower surface of the header  20  and the upper margin of the dielectric band  50  should be no greater than about 24 inches (62 cm). Moreover, the dielectric band  50  should be formed from a nonporous substance, so that it does not absorb condensate and of course it should withstand the temperatures to which the feedwater heater  14  is subjected. In addition to the dielectric band  50 , the activating terminal  42  includes an electrically conductive band  52  which surrounds the dielectric band  50 , tightly embracing the dielectric band  50  and retaining itself and the dielectric band  50  in a fixed position around the tube  24  without actually contacting the tube  24 . The conductive band  52  is formed from metal, preferably one, such as stainless steel, which resists corrosion but of course conducts electrical current. It may take the form of a pipe clamp. The electrical lead  46  is attached to the conductive band  52  and is thus electrically isolated from the tube  24  and the remainder of the feedwater heater  14 . Indeed, its end, with insulation stripped from it, may be simply inserted beneath the conductive band  52  and clamped against the dielectric band  50  by the conductive band  52 . 
   In the operation of the HRSG A, hot gases, the products of combustion of a fuel, such as natural gas, enter the duct  2  at its inlet end  4 . Here the gases exist at an extremely high temperature on the order of 1200° F. (649° C.). The gases pass through the superheater  10  where heat is extracted from them and then through the evaporator  12  where more heat is extracted. The temperature of the gases drops appreciably. When the gases encounter the feedwater heater  14  the temperature may have dropped to between 300° F. (149° C.) and 200° F. (93° C.). The dew point for the gases, although difficult to predict, is on the order of 107° F. (42° C.), so the surfaces of the feedwater heater  14  should remain above the dew point. Yet the feedwater  14  should maintain the surfaces of the feedwater heater  14  at a temperature only slightly above the dew point of the gases, perhaps 5° F. (2.8° C.) above the dew point. This enables the HRSG A to extract the maximum amount of heat from the gases without producing condensation and the corrosion that it causes. And the operator of the HRSG A does maintain a measure of control over the temperature of the water that enters the feedwater heater  14 . 
   Thus, to insure that the HRSG A operates most efficiently, the operator reduces the temperature of the feedwater while monitoring the conductivity meter  44 . As long as no condensation develops on the header  20  or the nearby regions of the tubes  24 , the conductivity meter  44  will not register an alarm or other signal. However, should the feedwater cool the header  20  and nearby regions of the tubes  24  to a temperature below the dew point of the gases, the moisture in the gases will condense on the header  20  and on the bare surface  34  of the one tube  24  and will flow downwardly over the upper margin of the dielectric band  50  and along the surface of the band  50  to the conductive band  52 . It completes an electrical circuit between the bare section  34  of the one tube  24  and the conductive band  52 . The conductivity meter  44  registers the completion of the circuit, thereby notifying the operator of the HRSG A that the temperature of the feedwater is too low. The operator can adjust the temperature of the feedwater upwardly in increments until the conductivity meter  44  no longer registers the presence of a circuit. This of course denotes the absence of a condensate. 
   Variations are possible. For example, the activating terminal  42  need not be on a tube  24 , but may be on some other surface, such as the side of the header  20 , where condensation will also occur. Irrespective of the location of the actuating terminal  42 , its dielectric and conductive elements need not extend completely around the surface on which it is mounted. Moreover, the HRSG A is depicted in its simplest form. It may include additional superheaters, evaporators and even feedwater heaters. The monitoring unit B may be used on heat exchanges other than feedwater heaters in HRSGs. Any instrument or sensor capable of detecting conductivity will suffice for the conductive meter  44 . Also, the monitoring unit B may be installed on an evaporator, such as the evaporator  12 . Should the unit B, when so installed, detect condensate, the operator can raise the evaporator boiling temperature. 
   PARTS LIST 
   Apparatus and Process for Detecting Condensation in a Heat Exchanger 
   
       
       A HRSG 
       B monitoring unit 
         2  duct 
         4  inlet end 
         6  outlet end 
         10  superheater 
         12  evaporator 
         14  feedwater heater 
         20  header 
         22  collector 
         24  tubes 
         26  fins 
         30  inlet 
         32  outlet 
         34  bare surface 
         40  ground terminal 
         42  activating terminal 
         44  conductivity meter 
         46  electrical lead 
         48  electrical lead 
         50  dielectric band 
         52  conductive band