SEPARATORLESS BOILER

TECHNICAL FIELD

The present invention relates to a boiler with a small amount of retained water in a can body. The present application claims priority based on Japanese Patent Application No. 2012-060540 filed in Japan on Mar. 16, 2012 which is incorporated herein.

BACKGROUND ART

In a boiler (boiler with a small amount of retained water) having a can body structure in which an upper header and a lower header are connected by many water pipes heated by a burner and water levels in the water pipes are lower than upper ends of the water pipes when combustion of the burner is stopped as in Patent Document 1, if the water levels in the water pipes are excessively high, boiling in the water pipes becomes vigorous, many water droplets are discharged from the upper header together with steam, and it is impossible to obtain steam with high dryness. On the other hand, if the water levels in the water pipes are excessively low, dryness at an outlet of the can boy increases, though problems such as overheating and deformation of upper portions of the water pipes caused by heat of the combustion occur. Moreover, if a combustion load of the boiler changes, the water levels change as well. Therefore, it is very difficult to achieve water levels with which high dryness can be obtained and the overheating does not occur.

Furthermore, the dryness in the boiler with the small amount of retained water is also influenced by concentration of water (can water) in the water pipes. If the can water is concentrated due to evaporation, an amount of water droplets associated with the steam increases due to foaming of the can water and therefore the dryness further reduces, though the water levels are the same.

In order to cope with such problems, the prior-art boiler includes a separator. Steam with low dryness flows into the separator from the upper header and what is called steam-water separation for removing moisture in the steam with the low dryness is carried out in the separator. Then, steam with high dryness is supplied from the separator to a load side.

PRIOR ART DOCUMENT
Patent Document

Patent Document 1: Japanese Patent Application Laid-Open No. 2010-78204

SUMMARY OF THE INVENTION
Problems to be Solved by the Invention

Because the separator included in the structure disclosed in Patent Document 1 is difficult to manufacture and has many parts to be inspected after the manufacture, the structure including the separator involves high cost. On the other hand, if the structure does not include the separator, steam with desired dryness cannot be obtained.

The present invention has been made based on the above circumstances and its objects are to provide a separatorless boiler not including a separator and to provide a separatorless boiler with a small amount of retained water, with which desired dryness can be obtained, overheating of water pipes can be prevented, and a risk of corrosion of a can body can be reduced.

Means for Solving the Problems

The present invention has been made to solve the above problems and, in accordance with a first aspect of the invention, there is provided a separatorless boiler including: a can body which is formed by connecting an upper header and a lower header with many water pipes heated by a burner and in which water levels in the water pipes when combustion of the burner is stopped are lower than upper ends of the water pipes; a water supply means for supplying water for the boiler into the can body; and a control means which has an external water level detecting means, disposed outside the can body, communicating respectively with an internal space of the upper header and an internal space of the lower header through communication pipes, and having electrodes to detect an external water level in the can body, and which controls actuation of the water supply means according to the detected water level by the external water level detecting means,

wherein a downcomer pipe connecting a lower portion in the upper header and the lower header is provided,

the control means carries out a first control for controlling actuation of the water supply means so that the water level in the can body lowers when the detected water level by the external water level detecting means exceeds a first set water level and a second control for controlling actuation of the water supply means so that the water level in the can body rises when the detected water level becomes lower than or equal to a second set water level lower than the first set water level,

the first set water level is set to a dryness limit water level, a height of the upper header is set so that dryness of steam flowing out of the upper header becomes higher than or equal to set dryness as a result of the first control, and the second set water level is set to a water level which is higher than or equal to an overheating limit water level and which causes a circulation ratio of can water by the downcomer pipe to be higher than or equal to a set value. Even if “exceeds” is replaced with “becomes higher than or equal to” and “lower than or equal to” is replaced with “lower than”, these conditions are included in the invention.

With the invention according to the first aspect, because the first set water level is set to the dryness limit water level and the height of the upper header is set so that the dryness of the steam flowing out of the upper header becomes higher than or equal to the set dryness as a result of the first control, it is possible to maintain the predetermined dryness, though the boiler does not include a separator. Because the second set water level is set to the water level which is higher than or equal to the overheating limit water level and which causes the circulation ratio of the can water by the downcomer pipe to be higher than or equal to the set value, it is possible to maintain the predetermined circulation ratio of the can water by means of circulation of the can water through the downcomer pipe while preventing overheating of the water pipes.

In accordance with a second aspect of the invention, in the first aspect, the first set water level and/or the second set water level are/is adjusted according to one or a plurality of pressure in the can body, a temperature of supplied water, and a degree of concentration of the can water.

With the invention according to the second aspect, in addition to the effects of the invention according to the first aspect, an effect of being able to maintain the predetermined dryness even if one or the plurality of the pressure in the can body, the temperature of the supplied water, and the degree of concentration of the can water change(s) and/or an effect of being able to maintain the predetermined circulation ratio while preventing the overheating of the water pipes are/is exerted.

In accordance with a third aspect of the invention, in the first aspect, detection of the first set water level and the second set water level is carried out by a common electrode, determination of the first set water level by the control means is carried out based on detection of presence of water by the electrode or a lapse of a first set time since the detection of the presence of the water, and determination of the second set water level is carried out based on detection of absence of water by the electrode or a lapse of a second set time since the detection of the absence of the water.

With the invention according to the third aspect, in addition to the effects of the invention according to the first aspect, an effect of being able to maintain the predetermined dryness by adjusting the respective set times and/or an effect of being able to maintain the predetermined circulation ratio while preventing the overheating of the water pipes are/is exerted.

In accordance with the fourth aspect of the invention, in the first aspect, the external water level detecting means includes a first electrode for detecting the first set water level and a second electrode for detecting the second set water level, determination of the first set water level by the control means is carried out after detection of presence of water by the first electrode or a lapse of a third set time since the detection of the presence of the water, and the determination of the second set water level is carried out based on detection of absence of water by the second electrode or a lapse of a fourth set time since the detection of the absence of the water.

With the invention according to the fourth aspect, in addition to the effects of the invention according to the first aspect, an effect of being able to maintain the predetermined dryness by using the first electrode and the second electrode of the external water level detecting means and/or an effect of being able to maintain the predetermined circulation ratio while preventing the overheating of the water pipes are/is exerted.

Effects of the Invention

According to the present invention, it is possible to provide the separatorless boiler with the small amount of retained water, in which the desired dryness can be obtained, the overheating of the water pipes is prevented, and the risk of corrosion of the can body can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic block diagram of a vertical section showing a structure of a boiler according to embodiment 1 of the present invention.

FIG. 2 is a flowchart for explaining a control procedure in embodiment 1.

FIG. 3 is a flowchart for explaining another control procedure in embodiment 1.

FIG. 4 is a schematic block diagram of a vertical section showing a structure of a boiler according to embodiment 2 of the present invention.

FIG. 5 is a diagrammatic illustration of an external water level detecting device of a boiler according to embodiment 3 of the present invention.

FIG. 6 is a schematic block diagram of a vertical section showing a diagrammatic structure of a boiler according to embodiment 4 of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION
Embodiment 1

A boiler not including a separator (referred to as “separatorless boiler” and hereafter simply referred to as “boiler”) 10 according to embodiment 1 of the present invention will be described based on FIG. 1.

Structure of Embodiment 1

As shown in FIG. 1, the boiler 10 includes a can body 20, a burner 30 which can be switched between high combustion and low combustion, fireproof material 40, an external water level detecting device 50, a water supply pump 60, and a controller 70. Out of these members, the can body 20 includes a can body cover 21, a lower header 22, water pipes 23, and an upper header 24 as its main components.

Out of these main components, the can body cover 21 is formed by forming a plate material into a cylindrical shape and covers the water pipes 23 to isolate the water pipes 23 from outside. The lower header 22 is formed in a hollow ring shape, a water supply pipe 80 is connected to the lower header 22, and water for the boiler is supplied into the lower header 22 through the water supply pipe 80. A main drain pipe 81 is also connected to the lower header 22 and a main drain valve 82 is provided to the main drain pipe 81. By opening this main drain valve 82, can water existing in an internal space 22A of the lower header 22 can be discharged outside. Here, the can water refers to the water for the boiler and introduced into the can body 20 (the lower header 22, the water pipes 23, and the like).

Lower end sides of the water pipes 23 are connected to the lower header 22. The water pipes 23 are pipe-shaped members respectively communicating with the lower header 22 and the upper header 24 and storing the can water. By heating the water pipes 23 by combustion of the burner 30, the can water in the water pipes 23 boils. The respective water pipes 23 extend along a vertical direction in FIG. 1. In the present embodiment 1, the many water pipes 23 are disposed. An arrangement of the water pipes 23 includes an outer water pipe row 23A disposed along a first circumference at a distance of a first radius from a radial center of the can body 20 and an inner water pipe row 23B disposed along a second circumference at a distance of a second radius smaller than the first radius from the radial center of the can body 20. However, the water pipes 23 are not limited to those having the two rows, i.e., the outer water pipe row 23A and the inner water pipe row 23B and may include only one row or three or more rows.

Upper end sides of the respective water pipes 23 are connected to the upper header 24. Air bubbles produced by boiling phenomena caused in the water pipes 23 flow together with steam into the upper header 24. The upper header 24 is formed into a hollow ring shape similarly to the above-described lower header 22. Here, in the following description, a hollow space inside the upper header 24 is referred to as an internal space 24A. The internal space 24A communicates with opening portions 23s of the water pipes 23 while communicating with an opening portion 83s of a main steam pipe 83 through which the steam flows out. In the following description, the hollow space inside the lower header 22 is referred to as the internal space 22A.

As described above, the structure of the can body 20 in embodiment 1 is formed by vertically connecting the lower header 22 and the upper header 24 with the many water pipes 23.

A vertical dimension of the upper header 24 in embodiment 1 is set to be larger than those of the lower header 22 and the prior-art upper header 24 so that dryness of the steam becomes higher than or equal to set dryness in combination with a water level control (first control) for maintaining the dryness (described later). In other words, though the boiler 10 in embodiment 1 is not provided with a separator, a steam-water separating function to be performed by the separator is performed by the upper header 24. Therefore, the internal space 24A of the upper header 24 has the larger vertical dimension than the internal space 22A of the lower header 22 so as to perform the steam-water separating function.

The large vertical dimension of the internal space 24A of the upper header 24 in this manner means a long distance of a flow path from the opening portions 23s of the water pipes 23 positioned in a bottom face of the upper header 24 to the opening portion 83s of the main steam pipe 83 connected to a top face of the upper header 24. The increase in the distance of the flow path between the two opening portions (between the opening portions 23s and the opening portion 83s) results in performance of the steam-water separating function.

Here, in the prior-art upper header 24, the vertical dimension of the internal space 24A is about the same as a vertical dimension of an internal space 22A of the lower header 22. On the other hand, in the upper header 24 in embodiment 1, as shown in FIG. 1, the vertical dimension of the internal space 24A is much larger than that of the lower header 22. To put it concretely, the internal space 24A of the upper header 24 has such a height as to obtain a distance (time) necessary for a liquid droplet having a diameter of tens of micrometers and included in the steam to settle by gravitation against the rising steam and, as a result, to separate from the steam. If a rising speed of the steam is about 0.1 to 0.5 m/sec, for example, the height of the internal space 24A required to obtain the desired dryness (set dryness) is 200 mm at the minimum.

As is known, the slower the speed of the rising steam and the greater the height of the internal space 24A, the higher the performance of the steam-water separation becomes, in general. In embodiment 1, if the height of the internal space 24A is 200 mm or smaller, the time for the water droplets to separate from the steam is insufficient and the desired dryness of the steam cannot be obtained at an outlet (the opening portion 83s) of the can body 20. On the other hand, if the height of the internal space 24A is 700 mm or greater, the desired dryness can be obtained irrespective of a degree of concentration. However, depending on the degree of concentration of the can water, it is possible to obtain the desired dryness, even if the height of the internal space 24A is between 200 mm and 700 mm. Therefore, if the height of the internal space 24A is 700 mm or greater, the dimension is excessively large, weight increases, and the structure is not compact, which is disadvantageous in terms of cost.

Therefore, the above-described vertical dimension of the internal space 24A of the upper header 24 needs to be greater than or equal to a dryness height threshold or greater than the dryness height threshold. Here, the dryness height threshold refers to a threshold of the vertical dimension of the internal space 24A for obtaining the desired dryness of the steam (steam-water mixed fluid described later) discharged from the internal space 24A. The dryness height threshold is obtained by experiments or the like.

In the hollow ring-shaped internal space 24A of the upper header 24, a baffle plate 25 is provided. In embodiment 1, the baffle plate 25 is provided in the internal space 24A so that its plate face direction is parallel to a direction perpendicular to the vertical direction.

Here, if there is no member for intercepting the steam and the moisture included in the steam (the steam and the moisture included in the steam will be referred to as “steam-water mixed fluid” in the following description) between the opening portions 23s and the opening portion 83s, the moisture may flow straight into the opening portion 83s of the main steam pipe 83 together with the steam. However, if the baffle plate 25 is provided in the internal space 24A, the flow of the steam-water mixed fluid is intercepted by the baffle plate 25 between the opening portions 23s and the opening portion 83s. In this way, the moisture included in the steam-water mixed fluid does not flow straight and the moisture in the steam bonds when the steam collides with the baffle plate 25 and then the moisture drops. Moreover, as compared with a case without a baffle plate 25, it takes longer for the steam to reach the opening portion 83s. Therefore, major part of the moisture included in the steam is dropped by gravitation because it takes time for the moisture to reach the opening portion 83s, or adheres to the baffle plate 25 or an inner wall of the upper header 24, and therefore does not reach the opening portion 83s of the main steam pipe 83. In other words, the baffle plate 25 forms a flow path extending means for extending a flow path through which the steam-water mixed fluid flows. As described above, the baffle plate 25 has a function of further enhancing the performance of steam-water separation.

A shape of the baffle plate 25 will be described in detail. In a plan view of the baffle plate 25, the baffle plate 25 is provided to form a ring shape. In other words, the baffle plate 25 is provided almost throughout an annular area in the internal space 24A. Between an outer peripheral side of the baffle plate 25 and the inner wall 24c, a clearance A is provided. On the other hand, an inner peripheral side of the baffle plate 25 is attached at a plurality of circumferential positions to the inner wall 24c by welding or the like. Therefore, at portions of the inner peripheral side of the baffle plate 25 and the inner wall 24c and not attached to each other by welding or the like, clearances B exist. The clearances B are smaller than the clearance A.

In place of the structure shown in FIG. 1, a baffle plate 25 inclined downward from an inner peripheral side toward an outer peripheral side may be provided, for example. Also in this case, it is preferable to provide a clearance similar to the above-described clearance A between the outer peripheral side of the baffle plate 25 positioned at a lowest position and an inner wall 24c and clearances similar to the above-described clearances B may be provided between the inner peripheral side of the baffle plate 25 and the inner wall 24c. Moreover, a baffle plate 25 inclined downward from an outer peripheral side toward an inner peripheral side may be provided. Also in this case, it is preferable to provide a clearance similar to the above-described clearance A between the inner peripheral side of the baffle plate 25 positioned at a lowest position and an inner wall 24c and clearances similar to the above-described clearances B may be provided between the outer peripheral side of the baffle plate 25 and the inner wall 24c.

In place of the above-described structure, the baffle plate 25 may be formed into an arc shape, instead of being formed into the ring shape existing almost throughout the annular area of the internal space 24A. In this structure, the baffle plate 25 exists at a portion of the annular area of the internal space 24A. In this case, a notch portion may be formed on an outer peripheral side or an inner peripheral side of an end portion of the arc.

The baffle plate 25 is positioned at an upper position in the internal space 24A of the upper header 24. Therefore, the moisture in the steam-water mixed fluid when it reaches the baffle plate 25 is reduced more due to drop by gravitation or collision with the inner wall 24c when the baffle plate 25 is at the upper position than when the baffle plate 25 is at a lower position. If the steam-water mixed fluid with the reduced moisture collides with the baffle plate 25, the steam-water mixed fluid does not flow straight toward the opening portion 83s and therefore it takes time to reach the opening portion 83s. As described above, by providing the baffle plate 25 at the upper position in the internal space 24A, the steam-water separating performance of the baffle plate 25 is further enhanced.

Here, the attached position of the baffle plate 25 will be described. In the above-described structure, the baffle plate 25 is provided at the upper position in the internal space 24A. However, a structure in which the baffle plate 25 is provided at a position of the dryness height threshold described in the above first embodiment, i.e., at a point of the threshold of the vertical dimension of the internal space 24A is also preferable depending on a way of practicing.

On the other hand, an upper limit position where the baffle plate 25 is provided is set to such a height that the moisture staying on an upper face of the baffle plate 25 is not entangled (drawn) again in the steam flowing out of the opening portion 83s. In other words, the upper limit position of the attached position of the baffle plate 25 is set to a position at a predetermined distance from a ceiling face of the internal space 24A.

In FIG. 1, an upper end side of a downcomer pipe 84 communicates with a lower portion of the internal space 24A of the upper header 24. The downcomer pipe 84 is for returning the can water (the can water is often concentrated) existing in the internal space 24A into the lower header 22. Therefore, a lower end side of the downcomer pipe 84 communicates with the internal space 22A of the lower header 22. A concentrated drain pipe 85 is connected to the downcomer pipe 84 and a concentrated drain valve 86 is provided to the concentrated drain pipe 85. By opening the concentrated drain valve 86, the concentrated can water can be discharged to the outside. Here, whether the can water is concentrated to such a degree as to be necessary to be discharged is determined by an electric conductivity measurement sensor (not shown) for measuring electric conductivity by measuring electric conductivity of the can water existing in the downcomer pipe 84.

The burner 30 is provided to an upper portion of the can body 20. The burner 30 is positioned in a ring-shaped ring hole (not provided with a reference numeral) in the upper header 24 and forms a flame in an inner side (hereafter referred to as “combustion chamber 21A”) surrounded with the water pipes 23 inside the can body cover 21. For combustion in the combustion chamber 21A, fuel and air for combustion are supplied to the burner 30. The fireproof material 40 is provided to a lower portion of the can body 20. The fireproof material 40 closes the lower portion of the can body 20 (a portion (water pipe drawn portions) where the lower sides of the water pipes 23 are positioned and the inner side of the portion) to thereby form a radially center side of the inner water pipe row 23B into the combustion chamber 21A. The fireproof material 40 is also provided to a portion (water pipe drawn portions) where upper sides of the water pipes 23 are positioned on an upper side inside the can body cover 21.

The external water level detecting device 50 is for detecting a water level of the can water introduced into the can body 20 (the lower header 22, the water pipes 23, and the like). The external water level detecting device 50 includes a water level control cylinder 51 and a plurality of electrodes, i.e., a first electrode 52M and a second electrode 52S. The first electrode 52M is for controlling the water level during high combustion and the second electrode 52S is for controlling the water level during low combustion. The water level control cylinder 51 is made of conductive metal and formed into a substantially cylindrical shape with sealed opposite ends. A lower end of a communication pipe 87a is connected to an upper end portion of the water level control cylinder 51 and an upper end of the communication pipe 87a is connected to the internal space 24A of the upper header 24. An upper end of a communication pipe 87b is connected to the lower end portion of the water level control cylinder 51 and a lower end of the communication pipe 87b is connected to the internal space 22A of the lower header 22. In this way, the upper end portion and the lower end portion of the water level control cylinder 51 respectively communicate with the water pipes 23 through the upper header 24 and the lower header 22 and therefore the same water level as that of the can water introduced into the water pipes 23 is achieved in the water level control cylinder 51.

Between the first electrode 52M and the second electrode 52S, and the water level control cylinder 51, a voltage is applied. Then, from a change in the voltage caused when the can water comes in contact with the end of the first electrode 52M and the end of the second electrode 52S, it is possible to detect the water level in the water level control cylinder 51.

A water supply pump 60 is connected to the lower header 22 by the water supply pipe 80 and starts to supply water to the lower header 22 when actuated by control by the controller 70. A check valve 89 is provided to the water supply pipe 80 between the lower header 22 and the water supply pump 60 to prevent the supplied can water from flowing back from the lower header 22 side to the water supply pump 60 side.

Detection signals from various sensors such as the external water level detecting device 50 are input to the controller 70 and the controller 70 governs actuation of drive portions of the burner 30, the water supply pump 60, and the like in response to the detection signals and based on a water level control procedure stored in advance. The water level control procedure includes a high combustion water level control procedure and a low combustion water level control procedure.

(High Combustion Water Level Control Procedure)

The high combustion water level control procedure (an example of which is shown in FIG. 2) is formed to carry out a first control for turning off (stopping) the water supply pump 60 so that the water level in the can body 20 lowers when the detected water level by the first electrode 52M of the external water level detecting device 50 exceeds a high combustion first set water level H1 and a second control for turning on (driving) the water supply pump 60 so that the water level in the can body 20 rises when the detected water level becomes equal to or lower than a high combustion second set water level H2 which is lower than the high combustion first set water level H1.

The high combustion first set water level H1 is set to a water level (high combustion dryness limit water level) over which the set dryness cannot be maintained during the high combustion. A height of the upper header 24 is set so that dryness of the steam flowing out of the upper header 24 becomes equal to or higher than the set dryness as a result of the first control. The high combustion second set water level H2 is set to a water level which is higher than or equal to a water level (high combustion overheating degree limit water level) under which the water pipes overheat and which causes a circulation ratio of the can water by the downcomer pipe 84 to be higher than or equal to a set value (set circulation ratio). Although the set circulation ratio is set to the same value during the high combustion and the low combustion, it may be set to different values.

In embodiment 1, determination of the high combustion first set water level H1 by the controller 70 is carried out after a lapse of a high combustion first set time T1H since detection of presence of the water by the first electrode 52M and determination of the high combustion second set water level H2 is carried out after a lapse of a high combustion second set time T2H since detection of absence of the water by the first electrode 52M.

(Low Combustion Water Level Control Procedure)

Similarly to the high combustion water level control procedure, the low combustion water level control procedure (an example of which is shown in FIG. 3) is formed to carry out a first control for turning off the water supply pump 60 so that the water level in the can body 20 lowers when the detected water level by the second electrode 52S of the external water level detecting device 50 exceeds a low combustion first set water level L1 which is higher than the high combustion first set water level H1 and a second control for turning on the water supply pump 60 so that the water level in the can body 20 rises when the detected water level becomes equal to or lower than a low combustion second set water level L2 which is lower than the low combustion first set water level L1 and higher than the high combustion second set water level H2.

The low combustion first set water level L1 is set to a dryness limit water level during the low combustion. The height of the upper header 24 is set so that dryness of the steam flowing out of the upper header 24 becomes equal to or higher than the set dryness during the low combustion as a result of the first control. The low combustion second set water level L2 is set to a water level which is higher than or equal to the overheating degree limit water level and which causes the circulation ratio of the can water by the downcomer pipe 84 to be higher than or equal to the set value.

In embodiment 1, determination of the low combustion first set water level L1 by the controller 70 is carried out after a lapse of a low combustion first set time T1L since detection of presence of the water by the second electrode 52S and determination of the low combustion second set water level L2 is carried out after a lapse of a low combustion second set time T2L since detection of absence of the water by the first electrode 52S.

(Setting of Circulation Ratio)

Here, setting of the circulation ratio in the second control will be described. If the circulation ratio is low, pH of a portion of the lower header 22 to which the water of low pH is supplied becomes low, which increases a risk of corrosion. On the other hand, if the circulation ratio is increased, the supplied water of low pH and the can water of high pH and supplied into the lower header 22 through the downcomer pipe 84 mix with each other, which can maintain appropriate pH in the can body 20 and reduce the risk of corrosion. Furthermore, by mixing the low-temperature supplied water and the high-temperature can water, it is possible to obtain high temperature distribution throughout the can water, which can reduce the risk of corrosion due to dissolved oxygen.

The circulation ratio is defined as (evaporation amount+downcomer water amount)/(supplied water amount). The evaporation amount is an amount of steam flowing out of the upper header and the downcomer water amount is an amount of can water (circulating water) flowing down thorough the downcomer pipe. Here, if the evaporation amount is equal to the supplied water amount, the circulation ratio increases as the downcomer water amount increases.

The second control is for increasing the circulation ratio of the can water and timing of control of the water supply pump 60 so that the water level in the can body 20 rises, i.e., timing of determination that the water level in the can body 20 has lowered to the second set water level after the first control is when the water level has lowered to a target circulation ratio set water level. The target circulation ratio set water level is a water level higher than the overheating limit water level for preventing overheating of the water pipes 23.

The target circulation ratio set water level is a water level set in order to obtain the set circulation ratio as described above and the circulation ratio is 2 or lower in a through flow boiler. On the other hand, in the boiler 10 in embodiment 1, the target circulation ratio is properly set in the following range. A lower limit of the set circulation ratio is set based on a corrosion risk due to pH and dissolved oxygen concentration in the can body 20. An upper limit of the set circulation ratio is set based on a dryness limit, because it is difficult to control the dryness limit water level when the circulation ratio is excessively high. In the boiler 10 in embodiment 1, if the circulation ratio is 2 or lower for the highly corrosive water as in the through flow boiler, for example, the corrosion risk increases. Therefore, it is preferable that the circulation ratio is set to as high a value as possible, the value being over 2 and lower than or equal to a limit value.

Operation of Embodiment 1

First, a steam-water separating function of the boiler 10 having the above-described structure will be described and then the control of the dryness and the circulation ratio by the water level control will be described. The following explanation of the steam-water separating function is based on the premise that the water level control (first control), which will be described later, for maintaining the set dryness is being carried out.

(Steam-Water Separating Function)

If the water supply pump 60 is actuated to supply the can water into the water pipes 23 to a predetermined height and the burner 30 is caused to burn, the can water in the water pipes 23 boils and the steam including the water droplets (liquid droplets) flows from the opening portions 23s of the water pipes 23 into the internal space 24A of the upper header 24. The inflow changes according to the water level by the first control. Then, the steam flows toward an outlet (opening portion 83s) of the can body 20 while air bubbles produced by boiling and the water droplets rolled up due to bumping are entrained in the steam.

Here, in embodiment 1, the vertical dimension of the upper header 24 is set to such a height as to obtain the distance (time) necessary for the water droplet (liquid droplet) having a diameter of tens of micrometers and included in the steam to settle by gravitation against the rising steam and, as a result, to separate from the steam. The necessary vertical dimension needs to be changed depending on the rising speed of the steam. In the embodiment, if the rising speed of the steam is about 0.1 to 0.5 m/sec, for example, the vertical dimension of the internal space 24A is 200 mm to 700 mm. In this way, while the steam-water mixed fluid rises between the opening portions 23s of the water pipes 23 and the opening portion 83s of the main steam pipe 83, the water droplets (liquid droplets) included in the stem drop and the desired steam dryness can be obtained.

The upper header 24 has a larger vertical dimension of the internal space 24A than the upper header 24 in the prior art. In this way, it is possible to increase a length of the path (a length of the flow path) through which the steam-water mixed fluid flows from the opening portions 23s of the water pipes 23 to the opening portion 83s of the main steam pipe 83. As a result, a major part of the moisture included in the steam adheres to the inner wall of the upper header 24 or is dropped by the action of gravitation and therefore removed from the steam.

Then, a predetermined percentage of the steam-water mixed fluid from which the moisture in the steam is removed to some extent by increasing the vertical dimension of the internal space 24A of the upper header 24 collides with the baffle plate 25. As a result, the moisture in the steam adheres to the baffle plate 25, gathers, and is dropped by gravitation and, in this manner, the moisture is removed from the steam.

In addition, because the baffle plate 25 exists in the internal space 24A of the upper header 24, the steam-water mixed fluid does not flow straight toward the opening portion 83s in the internal space 24A of the upper header 24 and therefore it takes longer for the fluid to reach the opening portion 83s. Especially, the steam-water mixed fluid which tries to flow straight along the vertical direction from the opening portions 23s of the water pipes 23 toward the opening portion 83s of the main steam pipe 83 or the steam-water mixed fluid which tries to flow in a state close to the straight flowing state goes around to avoid the baffle plate 25 in flowing toward the opening portion 83s of the main steam pipe 83 due to the existence of the baffle plate 25. As a result, it takes time for the steam-water mixed fluid to reach the opening portion 83s and therefore the moisture included in the steam adheres to the inner wall 24c or is dropped by the action of gravitation. In this manner, by providing the baffle plate 25, the moisture in the steam is removed satisfactorily.

After the steam-water mixed fluid from which the major part of the moisture is removed as described above reaches an area above the baffle plate 25 or after the moisture in the steam is removed again to some extent above the baffle plate 25, the steam-water mixed fluid reaches the opening portion 83s of the main steam pipe 83. When the steam-water mixed fluid reaches the opening portion 83s, the desired dryness of the steam is achieved and the steam is sent through the main steam pipe 83 to a supply destination which requires supply of heat.

(Control of Dryness by Control of Water Level in High Combustion)

Next, the control of dryness in high combustion will be described based on FIG. 2. In the following description, the burner 30 is assumed to be continuing the high combustion. The controller 70 determines whether the high combustion first set time T1H has elapsed since detection of absence of water (the water level is not detected) by the first electrode 52M (a second control condition) in step S1 (hereafter “step SN” will be simply referred to as “SN”). If a result of the determination in S1 is YES, the water supply pump 60 is turned on in S2. Then, the water level in the can body 20 rises.

In S3, whether the high combustion second set time T2H has elapsed since detection of presence of water (the water level is detected) by the first electrode 52M (first control condition) is determined. If a result of the determination in S3 is YES as the water level in the can body 20 rises, the high combustion first set water level H1 is judged to have been reached and the procedure goes to S4 where the water supply pump 60 is turned off. As a result, the water level in the can body 20 is prevented from exceeding the high combustion first set water level H1 and therefore the predetermined dryness (0.98 or higher, for example) is maintained by the steam-water separating function of the upper header 24.

The high combustion second set time T2H by the time the water supply pump 60 is turned off is set with high concentration in the high combustion in mind. To put it more concretely, if a degree of concentration increases, the dryness decreases and therefore it is necessary to lower the water level. Therefore, it is preferable to adjust the second set time T2H so that the set time is shorter when the degree of concentration is higher depending on the degree of concentration of the can water. The degree of concentration can be detected by a concentration sensor (not shown) for detecting a combustion time or the degree of concentration of the can water.

(Control of Circulation Ratio by Water Level Control in High Combustion)

Next, the control of the circulation ratio in the high combustion will be described based on FIG. 2. If the water supply pump 60 is turned off in S4, the water level in the can body 20 lowers. If the high combustion first set time T1H has elapsed since the detection of absence of the water by the first electrode 52M for the high combustion in S1, the result of the determination is YES, i.e., the water level in the can body 20 is judged to have reached the high combustion second set water level H2, and the procedure proceeds to S2 where the water supply pump 60 is turned on. As a result, operation of the boiler 10 can be carried out at the predetermined circulation ratio.

(Control in Low Combustion Operation)

The water level in the low combustion operation of the boiler 10 is controlled based on a control procedure in FIG. 3. The control in the low combustion operation is different from that in the high combustion in that the dryness, the overheating degree of the water pipes, and the circulation ratio are controlled while the high water level is obtained overall by using the second electrode 52S for the low combustion, because boiling of the can water is gentle. A flow of the control is similar to the control procedure in FIG. 2 and will not be described, because S1, S2, S3, and S4 in FIG. 2 respectively correspond to S11, S12, S13, and S14 in FIG. 3.

(Effects of Embodiment 1)

According to the boiler 10 in embodiment 1 having the above-described structure, even if the structure does not include the separator, it is possible to send the steam of the desired dryness (set dryness) to the supply destination by means of the upper header 24 and the above-described water level control. As a result, the boiler 10 does not have the separator and attached members such as a pipe line for connecting the separator and the upper header 24 and therefore it is possible to cut cost related to materials of the separator and the attached members and cost required for manufacture of the separator and the attached members.

In the boiler 10 in embodiment 1, the vertical dimension of the internal space 24A of the upper header 24 is set to be larger than those of the lower header 22 and the prior-art upper header 24. In this way, it is possible to increase the length of the path (the length of the flow path) through which the steam-water mixed fluid flows from the opening portions 23s of the water pipes 23 to the opening portion 83s of the main steam pipe 83. As a result, the major part of the moisture included in the steam adheres to the inner wall of the upper header 24 or is dropped by the action of gravitation and therefore can be removed. In other words, by making the vertical dimension of the upper header 24 larger than those of the lower header 22 and the prior-art upper header 24, it is possible to allow the steam-water separating function to be exerted satisfactorily.

In the boiler 10 in embodiment 1, the baffle plate 25 is provided in the internal space 24A of the upper header 24. Therefore, the steam-water mixed fluid collides with the baffle plate 25 and the existence of the baffle plate 25 reduces the rising speed of the steam-water mixed fluid in the internal space 24A. As a result, the water droplets (liquid droplets) in the steam-water mixed fluid can be removed and the dryness of the steam can be increased. In other words, by disposing the baffle plate 25, it is possible to remove the moisture more satisfactorily from the steam-water mixed fluid.

Furthermore, in embodiment 1, the second set water levels H2 and L2 are set to the target circulation ratio set water levels which are higher than the heating limit water level for preventing the overheating and therefore the overheating of the water pipes can be prevented and the water level control can be carried out at the predetermined circulation ratio. As a result, the supplied water of low pH and the can water of high pH and supplied into the lower header 22 through the downcomer pipe 84 mix with each other in proper proportions, which can maintain appropriate pH in the can body 20 and reduce the risk of corrosion. Furthermore, by mixing the low-temperature supplied water and the high-temperature can water, it is possible to obtain high temperature distribution throughout the can water, which can reduce the risk of corrosion due to dissolved oxygen.

(Variation of Embodiment 1)

In embodiment 1, the detection of the first set water level and the second set water level is carried out by the common electrodes 52M and 52S, the determination of the first set water level by the controller 70 is carried out after the lapse of the set time since the detection of the presence by the electrodes 52M and 52S, and the determination of the second set water level is carried out after the lapse of the set time since the detection of the absence of the water by the electrodes 52M and 52S. However, the determination of the first set water level may be carried out immediately after the detection of the presence by the electrodes 52M and 52S or the determination of the second set water level may be carried out immediately after the detection of the absence of the water by the electrodes 52M and 52S.

Embodiment 2

Next, embodiment 2 of the invention will be described based on FIG. 4. In the invention, a target circulation ratio set water level may be adjusted according to any one or a plurality (two or three) of pressure in a can body, a temperature of supplied water, and a degree of concentration of can water (water in the can body). The higher pressure in the can body is advantageous to maintenance of the dryness but is disadvantageous to maintenance of a predetermined circulation ratio and prevention of overheating. In contrast, the higher temperature of the supplied water and the higher degree of concentration of the can water are disadvantageous to maintenance of the dryness but are advantageous to maintenance of the predetermined circulation ratio and prevention of the overheating. Therefore, the target circulation ratio set water level is raised when the pressure in the can body increases and is lowered when the temperature of the supplied water to the can body or the degree of concentration of the can water increases. In other words, the first set water level and the second set water level are raised as the pressure in the can body increases and as the temperature of the supplied water or the degree of concentration of the can water drops (reduces).

In embodiment 2, as shown in FIG. 4, the pressure is detected by a pressure sensor 90 for detecting the pressure (pressure of steam) in the can body 20 and the temperature is detected by a temperature sensor 100 for detecting the temperature of the supplied water in a water supply pipe 80. The first set time and the second set time are adjusted according to the detected pressure and the detected temperature. To put it concretely, timing of turning on of a water supply pump 60 is advanced (the first set time is shortened) and timing of turning off of the water supply pump 60 is delayed (the second set time is prolonged) as the pressure in the can body 20 increases and the temperature of the supplied water drops to thereby raise the water level.

To achieve this structure, a table in which the target circulation ratio set water level changing according to changes in the pressure in the can body and the temperature of the supplied water is stored in memory, for example. The target circulation ratio set water level corresponding to the detected pressure in the can body and the detected temperature of the supplied water is read from the table and a water level control by using the target circulation ratio set water level is carried out. The degree of concentration of the can water can be detected at a lower portion of a downcomer pipe, for example.

According to embodiment 2, it is possible to maintain the predetermined circulation ratio, even if the pressure in the can body and the temperature of the supplied water change.

Embodiment 3

Next, embodiment 3 of the invention will be described based on FIG. 5. Although the first set water level and the second set water level are detected by the common electrodes in embodiment 1, a first set water level and a second set water level are detected by separate electrodes in embodiment 3. In other words, as a high combustion first electrode 52M of an external water level detecting device 50, a first electrode 52M1 for detecting a high combustion first set water level H1 and a second electrode 52M2 for detecting a high combustion second set water level H2 are provided. Determination of the first set water level H1 by a controller 70 is carried out after detection of presence of water by the first electrode 52M1 or after a high combustion third set time since the detection of the presence of the water and determination of the second set water level H2 is carried out after detection of absence of water by the second electrode 52M2 or after a lapse of a high combustion fourth set time since the detection of the absence of the water. As a low combustion second electrode 52S of the external water level detecting device 50, a first electrode 52S1 for detecting a low combustion first set water level L1 and a second electrode 52S2 for detecting a low combustion second set water level L2 are provided. Determination of the low combustion first set water level L1 by the controller 70 is carried out after detection of presence of water by the first electrode 52S1 or after a low combustion third set time since the detection of the presence of the water and determination of the low combustion second set water level L2 is carried out after detection of absence of water by the second electrode 52S2 or after a lapse of a low combustion fourth set time since the detection of the absence of the water.

In embodiment 3, it is possible to adjust the respective set times according to any one or a plurality of pressure in a can body, a temperature of supplied water, and a degree of concentration of can water. With this structure, it is possible to maintain a predetermined circulation ratio, even if any one of or a plurality of the pressure in the can body, the temperature of the supplied water, and the degree of concentration of can water change(s).

Embodiment 4

Next, the boiler 10 in embodiment 4 of the invention will be described based on FIG. 6. Besides the boiler having the can body structure in which the plurality of water pipes 23 are disposed in annular arrangements as in embodiments 1 to 3 described above, the invention can be similarly applied to a boiler having a structure in which a plurality of water pipes 23 are disposed in a rectangular parallelepiped arrangement as in a boiler 10 shown in FIG. 6. In the following description of embodiment 4, structures which are different from those in embodiment 1 will be mainly described and components corresponding to those in embodiment 1 will be provided with the same reference numerals and will not be described.

The boiler 10 shown in FIG. 6 has a can body 20 including a rectangular parallelepiped lower header 22, a similarly rectangular parallelepiped upper header 24, and a group of water pipes having the plurality of water pipes 23 disposed to stand vertically between both the headers 22 and 24 from a viewpoint of space saving. In the group of water pipes forming the can body 20, adjacent ones of the water pipes 23 disposed on opposite outer sides in a longitudinal direction are respectively connected by connecting members (not shown) to form a pair of water pipe walls (not shown). Therefore, the can body 20 has a rectangular parallelepiped compartment formed by the lower header 22, the upper header 24, and the pair of water pipe walls. This compartment corresponds to the combustion chamber 21A in each of the above-described embodiments and is formed so that a flame from the burner 30 provided on one end side in the longitudinal direction flows toward an exhaust gas outlet 103 while carrying out a combustion reaction.

Although it is not shown in the drawing, a downcomer pipe 84 similar to that in each of the above embodiments is provided in the boiler 10 shown in FIG. 6. In other words, the upper header 24 in which the concentrated can water stays and the lower header 22 into which new water for the boiler is supplied are connected by the downcomer pipe 84 so that the can water in the upper header 24 naturally circulates. Here, a control of a water level of the boiler 10 shown in FIG. 6 is carried out in the same way as in embodiment 1 described above and will not be described specifically.

The invention is not limited to embodiments 1 to 4 described above. For example, the baffle plate 25 is used as a flow path extending means in the above-described embodiments. However, the flow path extending means is not limited to the baffle plate 25. For example, a labyrinth mechanism may be provided in the internal space 24A of the upper header 24 or the internal space 24A of the upper header 24 may be finely divided into small compartments with an inlet and an outlet positioned away from each other so as to extend the flow path through which the steam-water mixed fluid flows. Furthermore, a fine-mesh filter may be disposed for the steam-water separation.

Moreover, detection of the water level by the external water level detecting device 50 may be carried out not by using electrodes but by using a water level sensor (e.g., a differential pressure sensor) which can change a set water level. A target circulation ratio set water level itself may be changed and a water supply pump 60 may be turned on when the water level lowers to the changed target circulation ratio set water level.

EXPLANATION OF LETTERS OR NUMERALS

10 boiler

20 can body

22 lower header

23 water pipe

24 upper header

25 baffle plate

30 burner

50 external water level detecting device (external water level detecting means)

52M first electrode

52S second electrode

60 water supply pump (water supply means)

70 controller (control means)

80 water supply pipe

84 downcomer pipe

SEPARATORLESS BOILER

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