Abstract:
To suppress corrosion of heat transfer tubes of a boiler under an influence of water, there is provided a boiler apparatus equipped with a unit for gathering boiler water ( 3 ) in a boiler  2  and a unit for adding the gathered boiler water ( 4 ) to supply water that has not been supplied to the boiler ( 2 ) yet. With this construction, boiler water with concentrated silica, which is an alkaline and corrosion preventive ingredient, is gathered from the boiler ( 2 ) in combustion by the boiler water gathering unit ( 3 ). The gathered boiler water is added to supply water stored in a supply water tank ( 15 ) by the boiler water adding unit ( 4 ), and this supply water is adjusted to have a water quality allowing suppression of corrosion of a connecting portion between a lower header ( 6 ) and water tubes ( 7 ) of the boiler ( 2 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a boiler apparatus capable of suppressing corrosion generated in heat transfer tubes of a boiler or an economizer under an influence of water. 
         [0003]    2. Description of the Related Art 
         [0004]    Boilers, such as water tube boilers and cylindrical boilers, are used as heat sources in various industries, and supply steam to apparatuses using steam, such as heating apparatus, dryers, and production apparatuses. Boilers, which generate steam by heating supply water, are equipped with heat transfer tubes, such as water tubes and smoke tubes. The heat transfer tubes are formed of a non-passive metal, such as carbon steel, so the portions of the heat transfer tubes coming into contact with the boiler water suffer damage due to corrosion attributable to the influence of the boiler water, which may fatally affect the service life of the steam boiler. Thus, to operate the boiler in a stable manner for a long period of time, it is necessary to effectively suppress corrosion of the heat transfer tubes. 
         [0005]    In the boiler, an economizer (a supply water pre-heater) is installed in the gas duct (see page 28 of “Companion to Small Once-Through Boilers” issued by the Japan Society for Small Once-Through Boilers). In a boiler of this construction, the latent heat of the exhaust gas is recovered by the economizer and is used to pre-heat supply water, thereby making it possible to reduce the amount of fuel used, which is advantageous from the economic point of view. The economizer is provided with heat transfer tubes for effecting heat exchange between supply water and exhaust gas. Since the heat-transfer tubes are formed of a non-passive metal, such as carbon steel, the portions of the heat transfer tubes coming into contact with supply water suffer damage due to corrosion caused under the influence of the supply water, which may fatally affect the service life of the economizer. Thus, to use the economizer in a stable manner for a long period of time, it is necessary to effectively suppress corrosion of the heat transfer tubes. 
         [0006]    Corrosion of the heat transfer tubes is likely to occur in portions continuously kept in contact with supply water or boiler water. For example, in a one-through boiler, which is a type of water tube boiler, the connecting portions between the heat transfer tubes and a lower header tend to undergo corrosion. In the economizer, supply water constantly exists in the heat transfer tubes during operation of the boiler, so corrosion is likely to occur. Generally speaking, when the water contains a lot of corrosion promoting ingredients, such as sulfate ions, chloride ions, or dissolved oxygen, and contains little corrosion suppressing ingredient, such as silica, the possibility of corrosion of the heat transfer tubes is high. In addition to thinning corrosion, the heat transfer tubes are subject to local corrosion, which may cause minute perforation, thereby being damaged. Here, the local corrosion is a boring-like corrosion occurring from the side where the heat transfer tubes and the water come into contact with each other toward the opposite side in a thickness direction, that is, a pitting occurring in a wall-thickness direction of the heat transfer tubes, which leads to damage of the heat transfer tubes in a short period of time. 
         [0007]    To suppress corrosion of the heat transfer tubes formed of a non-passive metal, the boiler is operated so as to maintain a pH of the boiler in a range of 11 to 12. Supply water including city water, underground water, or the like as a water source usually contains alkali ingredients (bicarbonate and carbonate), and the alkali ingredients undergo thermal decomposition inside the boiler to produce a hydroxide. This hydroxide is concentrated as steam is generated, and the pH of the boiler water increases. The concentration rate is controlled such that the pH of the boiler water is within the above-mentioned range through blow operation, for example, blow from the bottom of the lower header (boiler body bottom blow) or blow from a down corner of a water separator connected to the lower header (separator blow). Thus, in the case of a water quality in which a sufficient amount of alkali ingredients is contained, it is possible to suppress corrosion of the heat transfer tubes without having to add a water treatment agent, such as alkali chemicals, to the supply water. Further, in the steam boiler, the concentration of the dissolved oxygen is reduced as the boiler water boils, which contributes to suppression of corrosion of the heat transfer tubes. In the heat transfer tubes, however, the water temperature at a portion where the supply water flows in, for example, at the connecting portion between the heat transfer tubes and the lower header, is relatively low, so concentration or boiling does not easily occur. Thus, even when the concentration rate of the boiler water is controlled to be within a predetermined range, an increase in pH or deoxidation does not easily occur, which means corrosion may be generated. 
         [0008]    On the other hand, in the economizer mentioned above, temperature rise of the supply water due to heat exchange with the exhaust gas is approximately 120° C. at the most, and the alkali ingredients in the supply water do not easily undergo thermal decomposition, nor does concentration take place. That is, in the heat transfer tubes, the pH of the supply water does not increase, so, as compared with the case of the boiler, corrosion is more likely to be promoted. Further, the interior of the heat transfer tubes is in a pressurized state, so boiling of the supply water does not take place, and the concentration of the dissolved oxygen is not reduced, so corrosion is likely to be promoted. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the present invention to suppress generation of corrosion in the heat transfer tubes of a boiler or an economizer under an influence of water. 
         [0010]    The present invention has been made to achieve the object as described above. According to a first aspect of the present invention, a boiler apparatus is characterized by including: a boiler; means for gathering boiler water in the boiler; and means for adding the gathered boiler water to supply water that has not been supplied to the boiler yet. 
         [0011]    According to the first aspect of the invention, some boiler water with concentrated silica, which is an alkali, corrosion suppressing ingredient, is gathered from a boiler in a combustion state by the boiler water gathering means. Then, the gathered boiler water is added to the supply water prior to its supply to the boiler by the boiler water adding means, and the supply water is adjusted to have a water quality allowing suppression of corrosion generated in the heat transfer tubes of the boiler. 
         [0012]    According to a second aspect of the present invention, a boiler apparatus is characterized by including: a boiler; an economizer for pre-heating supply water for the boiler by exhaust gas from the boiler; means for gathering boiler water in the boiler; and means for adding the gathered boiler water to supply water that has not been supplied to the economizer yet. 
         [0013]    According to the second aspect of the invention, some boiler water with concentrated silica, which is an alkali, corrosion suppressing ingredient, is gathered from the boiler in a combustion state by the boiler water gathering means. Then, the gathered boiler water is added to the supply water prior to its supply to the economizer by the boiler water adding means, and the supply water is adjusted to have a water quality allowing suppression of both corrosion generated in the heat transfer tubes of the boiler and corrosion generated in the heat transfer tubes of the economizer. 
         [0014]    Further, according to the third aspect of the invention, in a boiler apparatus of the first and second aspects of the invention, it is characterized in that the gathered boiler water is added to a supply water tank for storing supply water to be supplied to the boiler. 
         [0015]    According to the third aspect of the invention, the heated boiler water is mixed with the supply water stored in the supply water tank. As the temperature of the supply water increases, the concentration of dissolved oxygen is reduced, and the water is adjusted to have a water quality allowing suppression of corrosion generated in the heat transfer tubes of the boiler or the economizer. 
         [0016]    According to the present invention, it is possible to suppress corrosion generated in heat transfer tubes of a boiler or an economizer under an influence of water. As a result, damage of the boiler and the economizer is prevented, thereby making it possible to supply steam in a stable manner for a long period of time. 
         [0017]    Next, embodiment modes of the present invention will be described. The present invention is suitably applicable to a boiler apparatus in which supply water is supplied to a boiler and heated therein to thereby generate steam. 
       FIRST EMBODIMENT MODE 
       [0018]    First, a boiler apparatus according to a first embodiment mode of the present invention will be described. A boiler apparatus according to the first embodiment mode of the present invention is mainly equipped with a boiler, boiler water gathering means, and boiler water adding means. 
         [0019]    The boiler generates steam by heating supply water, and has various types of construction: a multi-tubular water tube boiler, a mono-tubular water tube boiler, a cylindrical boiler, etc. Of those boilers, a multi-tubular water tube boiler, in particular, is suitably employed since it is small and requires little installation space, is superior in boiler efficiency, and is easy of control and treatment. 
         [0020]    The lower portion of the so-called boiler body is connected to a supply tank through a water supply channel. Connected to the upstream side of the supply water tank is water treatment equipment for performing a predetermined water treatment according to the quality of raw water, such as city water, industrial water, or underground water, for example, a water softener, an ion exchange equipment, a reverse osmosis membrane equipment, nano-filtration membrane equipment, deoxidation equipment, or chemical dosing equipment for a water treatment agent. That is, supply water processed by the water treatment equipment is stored in the supply water tank, and this supply water is stored in the boiler body through the water supply channel. On the other hand, the upper portion of the boiler body is connected to a water separator through a steam supply channel, and the lower portion of the water separator is connected to the lower portion of the boiler body through a down corner to recover the separated water. 
         [0021]    In the first embodiment mode, the boiler water gathering means is equipped with a boiler water gathering channel and a boiler water gathering valve. To gather concentrated boiler water from within the boiler body, the boiler water gathering channel is connected, for example, to the down corner or the lower portion of the boiler body, and this boiler water gathering channel is provided with the boiler water gathering valve. The boiler water gathering valve is usually opening/closing-controlled continuously or intermittently during combustion in the boiler. 
         [0022]    Here, in order to prevent the boiler water from being excessively concentrated to cause a problem such as carry-over, a predetermined proportion of boiler water is discharged continuously or intermittently from within the boiler body to maintain a proper boiler water concentration. That is, in the above-mentioned boiler, to discharge concentrated boiler water as blow waste water, a boiler water delivery channel is connected to the down corner or the lower portion of the boiler body, and a blow valve is provided in the boiler water delivery channel. Thus, in the above-mentioned boiler constructed as described above, even if no such boiler water gathering means as mentioned above is provided, it is possible to utilize the boiler water delivery channel as it is as the boiler water gathering channel, and to utilize the blow valve as it is as the boiler water gathering valve in the first embodiment mode, the boiler water adding means is equipped with a boiler water supply channel. One end of the boiler water supply channel is connected to the boiler water gathering channel from the down corner or the lower portion of the boiler body. On the other hand, the other end of the boiler water supply channel is connected, for example, to the supply water tank and the water supply channel. That is, the boiler water gathered from within the boiler body is supplied to the upstream side of the boiler body through the boiler water gathering channel and the boiler water supply channel. 
         [0023]    When supplying the gathered boiler water into the supply water tank, it is desirable to mix the supply water stored in the supply water tank uniformly with the boiler water. For example, an inflow portion for the supply water and an inflow portion for the boiler water are arranged in close proximity to each other, and the supply water and the boiler water are uniformly mixed with each other by the flow of the supply water into the supply water tank. Further, for example, a circulation channel is connected to the supply water tank, and the supply water in the supply water tank is circulated by a circulation pump, thereby mixing the supply water and the boiler water uniformly with each other. Further, for example, an agitator is provided in the supply water tank, and the supply water in the supply water tank is agitated, thereby mixing the supply water and the boiler water uniformly with each other. 
         [0024]    Further, it is possible to provide a water quality meter, for example, a pH meter, in the supply water tank or the water supply channel. Based on the detection results obtained by this water quality meter, opening/closing control is effected on the boiler water gathering valve, thereby making it possible to adjust the amount of boiler water added to the supply water more accurately. 
         [0025]    In the following, an operation of the boiler apparatus of the first embodiment of the present invention mode will be described. First, the supply water stored in the supply water tank is supplied to the boiler body through the water supply channel, and is stored therein as boiler water. This boiler water usually contains an alkali ingredient, such as bicarbonate or carbonate, and silica, which is a corrosion suppressing ingredient. 
         [0026]    When combustion is performed in the boiler, the boiler water is boiled through heating, and produces steam. This steam is supplied to an apparatus using steam provided in a facility, such as a plant. Here, the alkali ingredient contained in the boiler water undergoes thermal decomposition through heating to produce a hydroxide, increasing the pH of the boiler water. The silica contained in the boiler water is concentrated as the steam is produced, and functions to form a corrosion prevention film inside the boiler body. 
         [0027]    Here, the boiler apparatus is operated so as to maintain the pH of the boiler water at a value allowing suppression of corrosion of the non-passive metal forming the boiler body, more specifically, in the pH range of 11 to 12, and as to maintain an electrical conductivity involving no carry-over (e.g., 4000 μS/cm or less). That is, during combustion in the boiler, opening/closing control is effected on the boiler water gathering valve, and a predetermined proportion (e.g., an amount corresponding to 10 to 20% of the supply water amount) of alkaline boiler water is delivered from the boiler body. The boiler water delivered from the boiler body, which is alkaline and contains concentrated silica, is supplied into the supply water for the boiler through the boiler water gathering channel and the boiler water supply channel, and is mixed therewith, thereby increasing the pH and the silica concentration of the supply water. 
         [0028]    Here, the pH of the supply water mixed with the boiler water is adjusted to a value allowing suppression of corrosion of the non-passive metal forming the lower portion of the boiler body (in particular, the connecting portion between the heat transfer tubes and the lower header in the case of a once-through boiler), and more specifically, to a pH range of 9 to 11.5. The silica concentration of the supply water mixed with the boiler water is adjusted to be not less than a predetermined value allowing formation of a film on the non-passive metal forming the lower portion of the boiler body (in particular, the connecting portion between the heat transfer tubes and the lower header in the case of a once-through boiler). The supply water the pH and the silica concentration of which have been adjusted is supplied to the boiler body through the water supply channel. Thus, in the first embodiment mode of the present invention, the alkali ingredient and silica in the supply water are recovered while being concentrated in the boiler body, and are added to the supply water for recycling. 
         [0029]    As described above, according to the first embodiment mode of the present invention, it is possible to suppress corrosion of the heat transfer tubes of the boiler under the influence of water. As a result, damage of the boiler is prevented, thereby making it possible to supply steam in a stable manner for a long period of time. 
       SECOND EMBODIMENT MODE 
       [0030]    Next, a steam boiler apparatus according to a second embodiment mode of the present invention will be described. A steam boiler apparatus according to the second embodiment mode is mainly equipped with a boiler, an economizer, boiler water gathering means, and boiler water adding means. Here, a detailed description of the components that are the same as those of the first embodiment mode of the present invention will be omitted, and the description will be centered only on the difference. 
         [0031]    The economizer is provided in the gas duct of the boiler, and effects heat exchange between the exhaust gas from the boiler and the supply water for the boiler to pre-heat the supply water. The supply water inlet of the economizer is connected to the supply water tank through the water supply channel. On the other hand, the supply water outlet of the economizer is connected to the lower portion of the boiler body. 
         [0032]    As in the first embodiment mode of the present invention, the boiler water gathering means is equipped with the boiler water gathering channel and the boiler water gathering valve. To gather concentrated boiler water from within the boiler body, the boiler water gathering channel is connected, for example, to the down corner or the lower portion of the boiler body, and this boiler water gathering channel is provided with the boiler water gathering valve. Usually, opening/closing control is effected on the boiler water gathering valve continuously or intermittently during combustion in the boiler. 
         [0033]    In the second embodiment mode of the present invention, the boiler water adding means is equipped with a storage vessel, a boiler water supply channel, and an addition pump. The storage vessel stores boiler water gathered from the boiler body through the boiler water gathering channel, and one end of the boiler water supply channel is connected to this storage vessel. The other end of the boiler water supply channel is connected to the upstream side of the economizer, for example, to the supply water tank or the water supply channel. The addition pump is provided in the boiler water supply channel. That is, the boiler water, which is delivered continuously or intermittently from the boiler body, is stored in the storage vessel, and then supplied to the upstream side of the economizer through the boiler water supply channel. Further, the addition pump is controlled so as to operate continuously or intermittently during combustion in the boiler, that is, while supply water is being supplied to the boiler body. 
         [0034]    The boiler water from the boiler body is heated when delivered. Thus, to recover the heat of the boiler water, it is possible to provide a heat exchanger in the supply water tank or the water supply channel. In this case, the boiler water supply channel is connected such that boiler water is supplied from the storage vessel to the heat exchanger, and that the cooled boiler water is supplied to the supply water prior to the supply to the economizer. Here, in the case in which the heat exchanger is provided in the supply water tank, the supply water is heated by the heat of the boiler water, and deoxidation of the supply water is promoted. Thus, this construction makes it possible to effectively suppress corrosion of the heat transfer tubes of the economizer due to dissolved oxygen. 
         [0035]    When no heat exchanger is provided, it is desirable to supply the boiler water in the storage vessel directly into the supply water stored in the supply water tank. When the boiler water in the heated state is directly supplied to the supply water, the supply water is heated by the heat of the boiler, and deoxidation of the supply water is promoted. Thus, this construction makes it possible to effectively suppress corrosion of the heat transfer tubes of the economizer due to dissolved oxygen. 
         [0036]    Further, as in the first embodiment mode of the present invention, a water quality meter, e.g., a pH member, may be provided in the water supply channel. By operating the addition pump based on the detection results obtained by this water quality meter, it is possible to more accurately adjust the amount of boiler water added to the supply water. 
         [0037]    In the following, the operation of the boiler apparatus of the second embodiment mode of the present invention will be described. First, as in the first embodiment mode, the steam boiler apparatus is operated while continuously or intermittently delivering a predetermined proportion (e.g., an amount corresponding to 10 to 20% of the supply water amount) of alkaline boiler water from the boiler body so as to maintain the pH of the boiler water within the range of 11 to 12. That is, during combustion in the boiler, opening/closing control is performed on the boiler water gathering valve to deliver alkaline boiler water from the boiler body, and this boiler water is supplied to the storage vessel through the boiler water gathering channel. Then, the boiler water in the storage vessel, which is alkaline and contains concentrated silica, is mixed with the supply water flowing on the upstream side of the economizer, increasing the pH and the silica concentration of the supply water. 
         [0038]    Here, the pH of the supply water mixed with the boiler water is adjusted to a value allowing suppression of corrosion of the non-passive metal forming the economizer, and more specifically, to a pH range of 9 to 11.5. The silica concentration of the supply water mixed with the boiler water is adjusted to be not less than a predetermined value allowing formation of a film on the non-passive metal forming the economizer. The supply water the pH and the silica concentration of which have been adjusted is supplied to the economizer, and then is supplied to the boiler body through the water supply channel. Thus, in the second embodiment mode of the present invention, the alkali ingredient and silica in the supply water are recovered while being concentrated in the boiler body, and are added to the supply water for recycling. 
         [0039]    As described above, according to the second embodiment mode, it is possible to suppress corrosion of the heat transfer tubes of the boiler and the economizer under the influence of water. As a result, damage of the boiler and the economizer is prevented, thereby making it possible to supply steam in a stable manner for a long period of time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    In the accompanying drawings: 
           [0041]      FIG. 1  is a schematic diagram showing a construction of a boiler apparatus according to a first embodiment of the present invention; 
           [0042]      FIG. 2  is a schematic diagram showing a construction of a boiler apparatus according to a second embodiment of the present invention; 
           [0043]      FIG. 3  is a schematic diagram showing a construction of a boiler apparatus according to a third embodiment of the present invention; and 
           [0044]      FIG. 4  is a schematic diagram showing a construction of a boiler apparatus according to a fourth embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0045]    In the following, a first embodiment of the present invention will be described in detail with reference to the corresponding figure.  FIG. 1  is a schematic diagram showing a construction of a boiler apparatus according to the first embodiment of the present invention. The boiler apparatus  1  is mainly equipped with a boiler  2 , boiler water gathering means  3 , and boiler water adding means  4 . 
         [0046]    The boiler  2  is a so-called multi-tubular water tube boiler, and is equipped with a boiler body  8  in which a large number of water tubes (heat transfer tubes)  7  are arranged upright between an upper header  5  and a lower header  6 . The upper header  5  is connected to a water separator  9  through a steam supply channel  10 , and the upper portion of the water separator  9  is connected to a load apparatus (not shown) through steam piping  11 . To recover separated water, the lower portion of the water separator  9  is connected to the lower header  6  through a down corner  12 . Further, connected to the gas duct  13  of the boiler  2  is an exhaust pipe  14  for discharging the exhaust gas produced through combustion. 
         [0047]    The lower header  6  is connected to a supply water tank  15  through a water supply channel  16 , which is provided with a supply water pump  17 . Connected to the upstream side of the supply water tank  15  is a makeup water supply channel  18 , which is provided with, from the upstream side a water softener  19  and a deoxidizer  20  in the stated order. 
         [0048]    The water softener  19  is a water treatment apparatus for removing hardness ingredients (calcium ions, magnesium ions, etc.) from raw water, such as city water, industrial water, underground water, etc. to prevent scale generation in the water tubes  7 . The oxidizer  20  is a water treatment apparatus for removing dissolved oxygen from raw water to prevent corrosion of the interior of the lower header  6 , the water tubes  7 , the water supply channel  16 , etc. 
         [0049]    To circulate stored supply water, a circulation channel  21  is connected to the side surface of the supply water tank  15 , and a circulation pump  22  is provided in the circulation channel  21 . Here, a supply water inlet  23  of the circulation channel  21  is connected to the lower portion of the supply water tank  15 , and a supply water outlet  24  of the circulation channel  21  is connected to the upper portion of the supply water tank  15 . That is, the supply water in the supply water tank  15  is extracted from the lower portion of the supply water tank  15 , and then fed back to the upper portion of the supply water tank  15 . 
         [0050]    The boiler water gathering means  3  is equipped with a boiler water gathering channel  25  and a boiler water gathering valve  26 . One end of the boiler water gathering channel  25  is connected to the down corner  12 , and the boiler water gathering valve  26  is provided in the boiler water gathering channel  25 . The other end of the boiler water gathering channel  25  is connected to a filter portion  27  for cleaning the gathered boiler water. Here, to maintain the proper boiler water concentration during combustion in the boiler  2 , a controller (not shown) performs opening/closing control on the boiler water gathering valve  26  so as to continuously or intermittently deliver a predetermined proportion of boiler water from within the boiler body  8 . 
         [0051]    The boiler water adding means  4  is equipped with a boiler water supply channel  28 . One end of the boiler water supply channel  28  is connected to the filter portion  27 . The other end of the boiler water supply channel  28  is connected to the lower portion of the supply water tank  15  at a position in close proximity to the supply water inlet  23 . 
         [0052]    In the following, operation of the boiler apparatus  1  according to the first embodiment of the present invention will be described. First, raw water flowing through the makeup water supply channel  18  undergoes water softening through ion exchange at the water softener  19 , and then deoxidation at the deoxidizer  20 . The deoxidized soft water is supplied to the supply water tank  15  as supply water and is stored therein. Then, by operating the supply water pump  17 , the supply water in the supply water tank  15  is supplied through the water supply channel  16 , and is stored in the boiler body  8  as boiler water. 
         [0053]    During combustion in the boiler  2 , the boiler water is boiled through heating of the boiler body  8  to thereby produce steam. This steam is supplied to the water separator  9  through the steam supply channel  10 , and the water in the steam is separated to enhance the dryness of the steam. Then, the steam is supplied to the load apparatus through the steam piping  11 . On the other hand, the water separated at the water separator  9 , that is, the separated water, is fed back into the boiler body  8  through the down corner  12 . 
         [0054]    The alkali ingredient (i.e., bicarbonate or carbonate) contained in the supply water undergoes thermal decomposition in the boiler body  8  through heating and produces a hydroxide and is concentrated to increase the pH of the boiler water. At the same time, the silica, which is a corrosion suppressing ingredient contained in the supply water, is also concentrated. During combustion in the boiler  2 , to maintain the pH of the boiler water in the range of 11 to 12, opening/closing control is performed on the boiler water gathering valve  26  to thereby periodically discharge from within the boiler body  8  alkaline boiler water in an amount corresponding to 10 to 20% of the supply water amount. That is, a portion of the alkaline boiler water is supplied to the filter portion  27  through the boiler water gathering channel  25 . 
         [0055]    At the filter portion  27 , sludge or the like contained in the boiler water is removed by filtration to thereby clean the boiler water. The filtered boiler water is supplied to the supply water tank  15  through the boiler water supply channel  28  by the steam pressure within the boiler body  8 . Inside the supply water tank  15 , the supply water and the boiler water are circulated through the circulation channel  21  to be uniformly mixed with each other by operating the circulation pump  22 . As a result, the pH and the silica concentration of the supply water to which the boiler water is added increase. Here, control of the opening time and the opening interval of the boiler water gathering valve  26  is effected to adjust the amount of boiler water added to the supply water such that the pH of the supply water is adjusted to a range of 9 to 11.5 which allows suppression of corrosion at the connecting portion between the lower header  6  and the water tubes  7 . 
       Second Embodiment 
       [0056]    Next, the second embodiment of the present invention will be described in detail with reference to the corresponding figure. The second embodiment is a modification of the first embodiment of the present invention.  FIG. 2  is a schematic diagram showing a construction of a boiler apparatus according to the second embodiment of the present invention. In  FIG. 2 , components that are the same as those of the first embodiment of the present invention are indicated by the same reference numerals, and a detailed description of such components will be omitted. 
         [0057]    In the second embodiment of the present invention, the water supply channel  18  extends toward the bottom of the supply water tank  15 . That is, the supply water that has undergone treatment at the water softener  19  and the deoxidizer  20  flows to the bottom of the supply water tank  15 . Further, the other end of the boiler water supply channel  28  is connected to the lower portion of the supply water tank  15  at a position in close proximity to the end of the water supply channel  18 . That is, the flow of supply water heading for the bottom of the supply water tank  15  collide with the flow of boiler water coming in through the boiler water supply channel  28 . 
         [0058]    In the following, operation of the boiler apparatus  1  according to the second embodiment of the present invention will be described. First, raw water flowing through the makeup water supply channel  18  undergoes water softening through ion exchange at the water softener  19 , and then deoxidation at the deoxidizer  20 . The deoxidized soft water is supplied to the supply water tank  15  as supply water and is stored therein. Then, by operating the supply water pump  17 , the supply water in the supply water tank  15  is supplied through the water supply channel  16 , and is stored in the boiler body  8  as boiler water. 
         [0059]    During combustion in the boiler  2 , the boiler water is boiled through heating of the boiler body  8 , to thereby produce steam. This steam is supplied to the water separator  9  through the steam supply channel  10 , and the water in the steam is separated to enhance the dryness of the steam. Then, the steam is supplied to the load apparatus through the steam piping  11 . On the other hand, the water separated at the water separator  9 , that is, the separated water, is fed back into the boiler body  8  through the down corner  12 . 
         [0060]    The alkali ingredient (i.e., bicarbonate or carbonate) contained in the supply water undergoes thermal decomposition in the boiler body  8  through heating and produces a hydroxide and is concentrated to increase the pH of the boiler water. At the same time, the silica, which is a corrosion suppressing ingredient contained in the supply water, is also concentrated. During combustion in the boiler  2 , to maintain the pH of the boiler water in the range of 11 to 12, opening/closing control is performed on the boiler water gathering valve  26  to thereby periodically discharge from within the boiler body  8  alkaline boiler water in an amount corresponding to 10 to 20% of the supply water amount. That is, a portion of the alkaline boiler water is supplied, to the filter portion  27  through the boiler water gathering channel  25 . 
         [0061]    At the filter portion  27 , sludge or the like contained in the boiler water is removed by filtration to thereby clean the boiler water. The filtered boiler water is supplied to the supply water tank  15  through the boiler water supply channel  28  by the steam pressure within the boiler body  8 . Inside the supply water tank  15 , the supply water and the boiler water collide to be uniformly mixed with each other. As a result, the pH and the silica concentration of the supply water to which the boiler water is added increase. Here, control of the opening time and the opening interval of the boiler water gathering valve  26  is effected to adjust the amount of boiler water added to the supply water such that the pH of the supply water is adjusted to a range of 9 to 11.5 which allows suppression of corrosion at the connecting portion between the lower header  6  and the water tubes  7 . 
       Third Embodiment 
       [0062]    Next, a third embodiment of the present invention will be described in detail with reference to the corresponding figure. 
         [0063]      FIG. 3  is a schematic diagram showing a construction of a boiler apparatus according to the third embodiment of the present invention. In  FIG. 3 , the components that are the same as those of the first embodiment and the second embodiment are indicated by the same reference numerals, and a detailed description of such components will be omitted. A boiler apparatus  29  according to the third embodiment is mainly equipped with an economizer  30  in addition to the boiler  2 , the boiler water gathering means  3 , and the boiler water adding means  4 . 
         [0064]    In the boiler  2 , the gas duct  13  is connected to an exhaust gas inlet  31  of the economizer  30 . Further, the exhaust pipe  14  is connected to an exhaust gas outlet  32  of the economizer  30 . In an exhaust gas circulation space  33  of the economizer  30 , there is provided a heat transfer tube  34  causing supply water to circulate for heat exchange with the exhaust gas, and the outlet of the heat transfer tube  34  is connected to the lower header  6 . The inlet side of the heat transfer tube  34  is connected to the water supply channel  16 . 
         [0065]    The water softener  19  is provided in the makeup water supply channel  18 . Further, inside the supply water tank  15 , there is provided a heat exchanger  35  for effecting heat exchange between the boiler water discharged from the boiler body  8  and the supply water, and the other end of the boiler water gathering channel  25  is connected to a boiler water inlet  36  of the heat exchanger  35 . 
         [0066]    In the third embodiment of the present invention the boiler water adding means  4  is equipped with the boiler water supply channel  28 , a storage vessel  37 , and an addition pump  38 . The storage vessel  37  is connected to a boiler water outlet  39  of the heat exchanger  35  through a boiler water recovery channel  40  in order to store the boiler water discharged from within the boiler body  8  and cooled by the heat exchanger  35 . The storage vessel  37  is connected to the lower portion of the supply water tank  15  at a position in close proximity to the supply water inlet  23  through the boiler water supply channel  28 , and the boiler water supply channel  28  is provided with the addition pump  38 . The addition pump  38  is controlled by a control portion (not shown) during combustion in the boiler  2 , more specifically, so as to operate in conjunction with the supply water pump  24 . 
         [0067]    In the following, operation of the boiler apparatus  29  according to the third embodiment of the present invention will be described. First, raw water flowing through the makeup water supply channel  18  is softened by the water softener  19  through ion exchange. This soft water is supplied to the supply water tank  15 , and is stored therein. Then, the supply water in the supply water tank  15  is supplied to the economizer  30  through the water supply channel  16  by operating the supply water pump  17 . This supply water undergoes heat exchange with the exhaust gas at the heat transfer tube  34 , and is pre-heated to a predetermined temperature. This pre-heated supply water is stored in the boiler body  8  as boiler water. 
         [0068]    During combustion in the boiler  2 , the boiler water is boiled through heating of the boiler body  8  to thereby produce steam. This steam is supplied to the water separator  9  through the steam supply channel  10 , and the water in the steam is separated to enhance the dryness of the steam. Then, the steam is supplied to the load apparatus through the steam piping  11 . On the other hand, the water separated at the water separator  9 , that is, the separated water, is fed back into the boiler body  8  through the down corner  12 . 
         [0069]    The alkali ingredient (i.e., bicarbonate or carbonate) contained in the supply water undergoes thermal decomposition in the boiler body  8  through heating and produces a hydroxide and is concentrated to increase the pH of the boiler water. At the same time, the silica, which is a corrosion suppressing ingredient contained in the supply water, is also concentrated. During combustion in the boiler  2 , to maintain the pH of the boiler water in the range of 11 to 12, opening/closing control is performed on the boiler water gathering valve  26  to thereby periodically discharge from within the boiler body  8  alkaline boiler water in an amount corresponding to 10 to 20% of the supply water amount. That is, a portion of the alkaline boiler water is supplied to the heat exchanger  35  through the boiler water gathering channel  25 . At the heat exchanger  35 , heat exchange is effected between the boiler water and the supply water in the supply water tank  15  to thereby heat the supply water, and the dissolved oxygen in the supply water is reduced. The boiler water, cooled at the heat exchanger  35  is supplied to the storage vessel  37  through the boiler water recovery channel  40 , and is stored therein. 
         [0070]    Next, during operation of the supply water pump  17  (that is, during supply of supply water), the addition pump  38  is operated. The boiler water in the storage vessel  37  is supplied to the supply water tank  15  through the boiler water supply channel  28  by a discharge pressure of the addition pump  38 . Inside the supply water tank  15 , the supply water and the boiler water are circulated through the circulation channel  21  and are uniformly mixed with each other by operating the circulation pump  22 . As a result, the pH and the silica concentration of the supply water to which the boiler water is added increase. Here, the operation time and the operation interval of the addition pump  38  are adjusted to control the amount of boiler water added to the supply water such that the pH of the supply water is kept in a range of 9 to 11.5 which allows suppression of corrosion of both the heat transfer tube  34  and connecting portion between the lower header  6  and the water tubes  7 . 
       Fourth Embodiment 
       [0071]    Next, the fourth embodiment of the present invention will be described in detail with reference to the corresponding figure. The fourth embodiment of the present invention is a modification according to the third embodiment of the present invention.  FIG. 4  is a schematic diagram showing a construction of a boiler apparatus according to the fourth embodiment of the present invention. In  FIG. 4 , the components that are the same as those of the first embodiment, the second embodiment, and the third embodiment of the present invention are indicated by the same reference numerals, and a detailed description of such components will be omitted. 
         [0072]    In the fourth embodiment, a boiler water delivery channel  41  is connected to the down corner  12 , and the boiler water delivery channel  41  is provided with a blow valve  42 . One end of the boiler water gathering channel  25  is connected to the bottom of the lower header  6 , and the boiler water gathering channel  25  is provided with the boiler water gathering valve  26 . Here, to maintain the proper boiler water concentration during combustion in the boiler  2 , a controller (not shown) performs opening/closing control on the blow valve  42  and the boiler water gathering valve  26  so as to discharge a predetermined proportion of boiler water continuously or intermittently from within the boiler body  8 . 
         [0073]    In the fourth embodiment of the present invention, the water supply channel  18  extends toward the bottom of the supply water tank  15 . That is, the supply water that has undergone treatment at the water softener  19  and the deoxidizer  20  flows to the bottom of the supply water tank  15 . Further, the other end of the boiler water supply channel  28  is connected to the lower portion of the supply water tank  15  at apposition in close proximity to the end of the water supply channel  18 . That is, the flow of supply water heading for the bottom of the supply water tank  15  collide with the flow of boiler water coming in through the boiler water supply channel  28 . 
         [0074]    In the following, operation of the boiler apparatus  29  according to the fourth embodiment of the present invention will be described. First, raw water flowing through the makeup water supply channel  18  is softened by the water softener  19  through ion exchange. This soft water is supplied to the supply water tank  15 , and is stored therein. Then, the supply water in the supply water tank  15  is supplied to the economizer  30  through the water supply channel  16  by operating the supply water pump  17 . This supply water undergoes heat exchange with the exhaust gas at the heat transfer tube  34 , and is pre-heated to a predetermined temperature. This pre-heated supply water is stored in the boiler body  8  as boiler water. 
         [0075]    During combustion in the boiler  2 , the boiler water is boiled through heating of the boiler body  8  to thereby produce steam. This steam is supplied to the water separator  9  through the steam supply channel  10 , and the water in the steam is separated to enhance the dryness of the steam. Then, the steam is supplied to the load apparatus through the steam piping  11 . On the other hand, the water separated at the water separator  9 , that is, the separated water, is fed back into the boiler body  8  through the down corner  12 . 
         [0076]    The alkali ingredient (i.e., bicarbonate or carbonate) contained in the supply water undergoes thermal decomposition in the boiler body  8  through heating and produces a hydroxide and is concentrated to increase the pH of the boiler water. At the same time, the silica, which is a corrosion suppressing ingredient contained in the supply water, is also concentrated. During combustion in the boiler  2 , to maintain the pH of the boiler water in the range of 11 to 12, opening/closing control is performed on the blow valve  42  and the boiler water gathering valve  26  to thereby periodically discharge from within the boiler body  8  alkaline boiler water in an amount corresponding to 10 to 20% of the supply water amount. That is, a portion of the alkaline boiler water is discharge to the exterior of the system through the boiler water delivery channel  41 . Further, a portion of the alkaline boiler water is supplied to the heat exchanger  35  through the boiler water gathering channel  25 . At the heat exchanger  35 , heat exchange is effected between the boiler water and the supply water in the supply water tank  15  to thereby heat the supply water, and the dissolved oxygen in the supply water is reduced. The boiler water cooled at the heat exchanger  35  is supplied to the storage vessel  37  through the boiler water recovery channel  40 , and is stored therein. 
         [0077]    Next, during operation of the supply water pump  17  (that is, during supply of supply water), the addition pump  38  is operated. The boiler water in the storage vessel  37  is supplied to the supply water tank  15  through the boiler water supply channel  28  by the discharge pressure of the addition pump  38 . Inside the supply water tank  15 , the supply water and the boiler water collide to be uniformly mixed with each other. As a result, the pH and the silica concentration of the supply water to which the boiler water is added increase. Here, the operation time and the operation interval of the addition pump  38  are adjusted to control the amount of boiler water added to the supply water such that the pH of the supply water is kept in a range of 9 to 11.5 which allows suppression of corrosion of both the heat transfer tube  34  and connecting portion between the lower header  6  and the water tubes  7 .