Abstract:
A method of protecting a heat exchanger including a flue gas cooler with plastic heat recovery tubes arranged on a counterflow principle in heat exchange connection with the flue gases of a thermal power boiler. The heat recovery tubes are connected by an inlet chamber to an inlet tube and by an outlet chamber to an outlet tube, which form together with a heater a cycle in which a liquid heat exchange medium is recirculated by a pump. The method includes guiding vapor generating at the end part of the heat recovery tubes along a separate flow channel from the outlet chamber to the upper part of an expansion vessel, and guiding a liquid heat exchange medium from the lower part of the expansion vessel directly to the inlet chamber or to the inlet tube, in a vicinity of the inlet chamber, by a separate return duct, so as to enable natural circulation of the heat exchange medium in the heat recovery tubes without the use of external energy.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method of protecting a heat exchanger against stresses caused by boiling of a heat exchange medium, a protection circuit of a steam boiler and a steam boiler provided with an apparatus for protecting a heat exchanger. The invention especially relates to protecting a heat exchanger without external control or external energy. Preferably, the method and the protection circuit of a heat exchanger in accordance with the present invention are used in situations where heat is recovered from a flue gas flow of thermal power boilers in conditions where there is a risk of, on one hand, condensing of corrosive substances on heat exchange surfaces and, on the other hand, boiling of the water used as a heat exchange medium. 
       BRIEF SUMMARY OF THE INVENTION 
       [0002]    In modem thermal power plants, heat energy from flue gases is efficiently recovered by cooling the flue gases to a temperature as low as possible. A fluidized bed boiler used for the production of electricity is, in the following, provided as an example of such a process in accordance with the prior art. However, the method and the protection circuit of a heat exchanger in accordance with the present invention may be utilized in any kind of steam boiler plant. 
         [0003]    The chemical energy of a suitable fuel is converted in a fluidized bed boiler to heat energy by combusting it in a bed of inert material fluidized with air in a furnace of the boiler. Heat energy is recovered both directly with heat surfaces arranged to the furnace walls and with different heat exchangers arranged to the discharge channel of the flue gas. In the parts of the flue gas channel where the temperature of the flue gases and the temperature of the surfaces of the heat exchangers remain sufficiently high, it is possible to manufacture the heat exchangers of relatively inexpensive metal materials. 
         [0004]    When the flue gases cool down to a temperature low enough, for example, from 130° C. to 90° C., that the water vapor condenses in droplets on the surfaces of the heat exchanges, which are at temperatures lower than the acid and water dew point, compounds in the flue gases, for example, sulphur dioxide, may dissolve to water droplets and form compounds corroding the metal surfaces. Generally, the aim is to reduce corrosion by manufacturing the heat exchangers of a material that withstands corrosion as much as possible. Recently, especially, when the flue gases contain aggressive compounds, the manufacturers have started to manufacture heat exchangers of suitable plastic materials, too. 
         [0005]    In heat exchangers containing plastic pieces, the actual heat exchange tubes, which come into contact with flue gases, are usually U-formed plastic tubes, which are attached at the upper end to metal headers. The headers, on the other hand, are mounted to a recycling piping for a heat exchange medium, most usually, water. 
         [0006]    In the joints between the heat exchange piping and the headers, seals are used, which seals are manufactured of plastic or rubber material enduring well, in use, both corrosion and other stresses typical of the operating conditions, but their weakness is the mounting of the plastic tubes to the headers and, especially, the seals used in the joints. 
         [0007]    The seals of the joints have proved to poorly endure pressure strikes, which may be generated in situations, where the water in the liquid cycle of the heat exchanger is allowed, at least locally, to boil uncontrollably and to generate steam. When the steam in the water flowing in the plastic tubes and the headers condenses, local point-like pressure strokes are generated, which may directly hit the seals. The pressure strokes may also cause vibration in the whole heat exchanger, which gradually breaks the seals. 
         [0008]    The uncontrollable boiling of the heat exchange medium breaking seals typically results from a disturbance in the cooling water cycle. A disturbance in the cooling water cycle may result either from a power failure, which may stop the whole plant, including the liquid cycle of the heat exchanger, or from an operational disturbance in a circulation pump, or a breakdown of the whole pump or its drive motor. As far as an operational disturbance of the pump is concerned, it might be natural to try to solve the problem by stopping the whole combustion process of the boiler. The furnace, especially, a furnace of a fluidized bed boiler, provides, however, after-heat for some time, so that the transfer of heat to the cooling water does not stop immediately. Thereby, the liquid in the heat exchange tubes situated in the flue gas channel tends to continue to evaporate. 
         [0009]    Great Britain patent publication No. 629,298 discloses means for transmitting heat of the flue gases of a steam boiler to an air preheater comprising an expansion vessel in the main heat transfer circuit. French patent publication No. 2,564,746 discloses a heat exchanger with plastic U-shaped tubes in a plant for desulfurizing flue gases. 
         [0010]    The present invention solves, for example, the above-mentioned problem in such a way that an expansion vessel is mounted into the heat recovery cycle, in communication with a heat exchanger, so that the steam generated in the piping of a heat exchanger is allowed to be controllably discharged to the expansion vessel. 
         [0011]    Other characterizing features of a method of and an apparatus for protecting a heat exchanger, and a steam boiler, comprising means for protecting a heat exchanger, become evident in the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]    A method of and an apparatus for protecting a heat exchanger, and a steam boiler, comprising means for protecting a heat exchanger, are explained in more detail with reference to the accompanying drawings, in which 
           [0013]      FIG. 1  is a schematic view of a thermal power plant in accordance with the prior art; 
           [0014]      FIG. 2  is a schematic view of a protection circuit of a heat exchanger in accordance with a preferred embodiment of the invention; and 
           [0015]      FIG. 3  is a schematic view of a protection circuit of a heat exchanger in accordance with a second preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]      FIG. 1  schematically illustrates parts of a thermal power plant  10  in accordance with the prior art, as far as the parts are pertinent to the discussion of the present invention. Fuel  14  and combustion air  16  are introduced to a furnace  12  of the plant  10 , generating flue gases, the temperature of which is generally about 800° C. to about 950° C. Hot flue gases are introduced from the furnace along a flue gas duct  18  to a heat recovery section  20 , in which steam is generated by means of heat energy from the flue gases, and the temperature of the flue gases decreases, for example, to about 250° C. to about 450° C. The flue gases are supplied from the heat recovery section  20  to a regenerative preheater  22  for combustion air, in which preheater, the temperature of the flue gases further decreases, typically, to about 150° C. 
         [0017]    When the desire to utilize as great a share as possible of the heat energy of the flue gases, the flue gases may be guided from the regenerative preheater  22  for combustion air further through a flue gas blower  24  to a flue gas cooler  26 . In the cooler  26 , the heat energy of the flue gases is transferred to a medium, usually water, which is recycled by means of flow tubes  28   a  and  28   b  to a preheater  30  for combustion air. Thus, the combustion air, which is supplied by a blower  32 , is guided to the furnace  12  through a preheater  30  and a regenerative preheater  22 . 
         [0018]    Normally, the aim is to cool down the flue gases by the cooler  26  to a temperature as low as possible. When using metal heat exchange piping, the end temperature has to be above the acid dew point of the flue gas, at a minimum, about 100° C. When the heat exchange tubes coming into contact with the flue gas in the cooler  26  are made of plastic, flue gases may be cooled to a temperature below 100° C. 
         [0019]    The flue gases are guided form the cooler  26  to a stack  34 . The thermal power plant  10  also comprises many other parts, for example, flue gas cleaning equipment and ash treatment equipment. Since they are not important in view of the present invention, they are not illustrated in  FIG. 1 . 
         [0020]      FIG. 2  illustrates in more detail a heat exchanger  36 , comprising a flue gas cooler  26  and a combustion air preheater  30 , which heat exchanger also comprises a protection circuit  38  of a heat exchanger in connection with an atmospheric expansion vessel  52 , in accordance with a preferred embodiment of the present invention. 
         [0021]      FIG. 2  shows with arrows  40 ,  40 ′ a flue gas flow, which is cooled indirectly by a liquid heat exchange medium, i.e., in most cases, water, circulated in heat recovery tubes  42  of the heat exchanger  36 . The liquid cycle of the heat exchanger  36  comprises, in addition to heat recovery tubes  42 , recycling piping  28   a,    28   b,  in which liquid is recycled by a pump  44 . The recycling piping  28   a,    28   b  is connected with a combustion air preheater  30 , in which the medium is cooled again, when heating relatively cold combustion air supplied by a blower  32  by means of heat energy recovered from the flue gas. Alternatively, the heat exchanger  36  may comprise, instead of the combustion air preheater  30 , a heat exchanger of some other type, in which heat energy recovered from the flue gas heats a suitable medium. 
         [0022]    The heat recovery tubes  42  are U-formed tubes attached at their upper ends by means of seals  48  to the headers  46 ,  46 ′ in a disconnectable manner. One of the headers of the heat exchanger  36  is an inlet chamber  46 , to which an inlet tube  28   a  for a liquid cycle of the heat exchanger is connected. Correspondingly, one of the headers of the heat exchanger is an outlet chamber  46 ′, to which an outlet tube  28   b  of the liquid cycle is attached. The headers  46 ,  46 ′ are most usually of steel or of some other suitable metal or metal compound. However, they may, in some cases, also be of a plastic or suitable composite material. 
         [0023]    Heat recovery tubes  42  coming into contact with flue gas have been assembled to a vertical position in such a way that the gas possible in the tubes, especially steam, may easily rise upwards to the headers  46 ,  46 ′. Arrows  49  show the flow direction of water in the heat recovery tubes  42  and in the flow tubes  28   a  and  28   b.  Each U-tube  42  is usually connected as a so-called countercurrent heat exchanger. In other words, water flows in such a way that the incoming water flow, i.e., water flow flowing down from the inlet chamber  46  is on the cooler side, i.e., on the side of the outflowing flue gas  40 , and, correspondingly, the outflowing water flow, i.e., the water flow rising to the outlet chamber  46 ′ is on the hotter side, i.e., on the side of the coming flue gas flow  40 ′. 
         [0024]    By means of a countercurrent coupling, it is possible to minimize the end temperature of the flue gas. Moreover, if hot flue gas causes boiling of a medium in the tubes  42 , the boiling begins at the rising end portion of the tubes, which intensifies the liquid cycle. At the same time, possible steam bubbles accumulate to the outlet chamber  46 ′. 
         [0025]    It may be said that the heat recovery tubes  42  connected between the two headers  46 ,  46 ′ form a tube group  50 . The heat exchanger  36  may comprise two headers  46 ,  46 ′ and a tube group  50  therebetween, or as illustrated in  FIG. 3 , three headers  46 ,  46 ′,  46 ″ and two groups  50 ,  50 ′ connected in series, of which one is connected between the headers  46  and  46 ″ and the other between the headers  46 ″ and  46 ′. There may also be more than two tube groups connected in series and, in some cases, the heat exchanger may also comprise tube groups connected in parallel. 
         [0026]    When the heat recovery tubes  42  of the heat exchanger are made of plastic, the tubes must be attached to the headers  46 ,  46 ′,  46 ″ connecting them by using rubber or plastic seals  48 . These seals endure well the stresses caused by their normal operational conditions. It has, however, been shown that the seals do not endure intense pressure strokes, which they may receive, if the heat exchange medium is allowed to evaporate uncontrollably in the heat recovery tubes  42 . 
         [0027]    According to the present invention, there is a protection circuit  38  in connection with the heat exchanger  36 , which comprises an expansion vessel  52  and flow channels  54 ,  54 ′,  56 , which join at least some of the headers  46 ,  46 ′,  46 ″ to the expansion vessel  52 . In an arrangement in accordance with  FIG. 2 , an outlet chamber  46 ′ is connected with a tube  54 , which is connected at the upper end to the upper part of the expansion vessel  52 , above the liquid surface in the expansion vessel. On the other hand, a tube  56  is connected to the inlet chamber  46 , or in the vicinity thereof, the tube being connected at its upper end to the bottom part of the expansion vessel  52 . 
         [0028]    The flow channels  54 ,  54 ′ leading to the upper part of the expansion vessel  52  may each separately lead to the expansion vessel  52 , or they may, if so desired, be connected at their upper ends to one single flow channel leading to the expansion vessel. A return duct  56  leads from the expansion vessel  52  back to the inlet tube  28   a,  preferably, close to the junction point of the inlet tube  28   a  and the header  46 , or to the header  46 . In the embodiment illustrated in  FIG. 2 , a ventilation conduit  58  leads from the expansion vessel  52  to the atmosphere or to some other desired space. 
         [0029]    The expansion vessel  52  is situated at a level higher than the headers  46 ,  46 ′,  46 ″, whereby the liquid columns in the vessel  52  and in the flow channels  54 ,  54 ′ cause a desired overpressure in the medium of the heat exchanger. For example, when the expansion vessel  52  is situated five meters above the headers, the expansion vessel  52  may be kept atmospheric and still maintain about 0.5 bar over-pressure in the heat recovery tubes  42 . Preferably, the bottom of the expansion vessel is about three to about seven meters higher than the level of the headers. When the pump  44  is running, the flow resistance of the heat exchange tubes brings about that the surface of the liquid in the flow channel  54  connected with the outlet chamber  46 ′ is, by an amount caused by the pressure loss, lower than that in the expansion vessel  52 . 
         [0030]    The apparatus illustrated in  FIG. 2  operates in such a way that when the liquid circulation in the heat exchanger  36  is disturbed, for example, when the pump  44  stops, the liquid in the heat recovery tubes  42  begins locally to boil, and forms steam. The generated steam flows especially to the header  46 ′ and from there further along the flow channel  54  to the expansion vessel  52 . The steam accumulating in the headers  46 ′ and  46 ″ in the apparatus illustrated in  FIG. 3  is led to the upper part of the expansion vessel  52  along channels  54  and  54 ′. 
         [0031]    An advantage of the arrangement in accordance with the present invention is that it enables the liquid circulation in the heat recovery tubes  42  also when the pump  44  has stopped. This is based on the fact that when the pump  44  stops, it equalizes the liquid levels in different branches of a protection circuit  38 , but, especially, the hot flue gases impacting the rising part of the heat recovery tubes  42  heat the liquid in the rising part, whereby its density decreases. When the liquid boils in the rising part, a liquid/steam mixture begins to accumulate in the channel  54 , whereby the density of the medium column in the channel  54  considerably decreases and its upper surface rises substantially higher than the liquid surface in the expansion vessel  52 . Then, liquid begins to move from the channel  54  to the expansion vessel  52  and, further, from the bottom of the vessel  52  along the channel  56  to the inlet channel  46 . This so-called natural circulation thus ensures the circulation of liquid in the heat recovery tubes  42  completely without external energy. 
         [0032]    Further, two auxiliary water lines with valves are connected to the expansion vessel  52 , of which from one,  60 , fresh liquid may be supplied to the expansion vessel of a conventional water line of the plant and from the other  62 , for example, fire extinguishing water may be supplied. Line  62  is a backup system, which is used when the conventional water supply system has stopped, for example, due to a power failure. 
         [0033]    Preferably, flow channels  54 ,  54 ′,  56  are arranged from each header  46 ,  46 ′,  46 ″ to the heat expansion vessel  52  in such a way that each of the heat recovery tube groups  50 ,  50 ′ empties from steam. By doing so, it is possible to prevent the generation of a steam lock in the heat exchanger  36 . The flow channels  54 ,  54 ′ in connection with the end part of all heat recovery tube groups  50 ,  50 ′ are preferably led to the same height to the wall of the expansion vessel  52  and are connected there tangentially. Thereby, the steam flowing to the expansion vessel from one of the flow channels  54 ,  54 ′ disturbs as little as possible the steam flowing from the other one of the flow channels  54 ,  54 ′. Further, the flow channels  54 ,  54 ′are brought to the expansion vessel preferably in such a way that they open to the vessel  52  above the liquid surface thereof. 
         [0034]    In the above discussed embodiment, the expansion vessel  52  is illustrated as being in atmospheric pressure, which is the simplest embodiment of the invention, and requires only that the expansion vessel can be assembled high enough in relation to the heat exchanger  36 . If such high temperatures are used in the recycling water cycle that the pressurization with the liquid column is not sufficient to prevent the evaporation in a normal situation, it is possible to arrange the expansion vessel to be pressurized. A relief valve opening at a certain pressure is thereby connected to the ventilation conduit  58  of the expansion vessel, this relief valve releasing steam from the expansion vessel, if the pressure begins to rise too much. 
         [0035]      FIG. 2  illustrates further an additional preferred embodiment of the invention, i.e., an auxiliary cooler  64  connected to the recycling piping  28   a,  which cooler may be used to cool down the liquid recycling in the piping before it is boiling, and which may be used in connection with the above-described arrangement, but also, independently. The control of when to use this auxiliary cooler may be determined, for example, by the temperature of the liquid recycling in the piping, whereby the cooler may be taken into use automatically, guided by the control system. 
         [0036]    As is noted, from the above-described arrangement, a new method of solving problems related with the use of plastic heat exchangers, without the need for external auxiliary energy or control, is provided. It is to be understood from the above that the invention is discussed in view of the most preferred embodiments, and it is not intended to limit the scope of the invention from what is defined in the appended claims.