JPH1182913A - Method and device for controlling feed water temperature of exhaust heat recovery boiler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the low-temperature corrosion of a low-pressure economizer by regulating the flow rate of feed water at a low-pressure economizer outlet to be re-circulated to a low-pressure economizer so that the feed water temperature at the low-pressure economizer inlet is kept at and above the temperature sufficient for evaporating the condensate for the prescribed time or over when a boiler is started. SOLUTION: A temperature/flow rate controller 18 set the inlet feed water temperature of a low-pressure economizer 8 to two kinds of temperature, i.e., the temperature on the relatively high temperature side and the temperature on the relatively low temperature side depending on the magnitude of the load. The feed water temperature at an inlet of the low-pressure economizer 8 is once raised to 80 deg.C by increasing the re-circulation volume of feed water to the low-pressure economizer 8 through opening regulation of a flow rate regulation valve 16 since an exhaust heat recovery boiler is started, the temperature is kept for at least 0.15 hour to complete evaporation of the condensate at 80 deg.C, and then, the control is returned so that the feed water temperature at the inlet of the low-pressure economizer 8 is kept at 50 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method and an apparatus for controlling a feedwater temperature of an exhaust heat recovery boiler in a combined cycle power plant.

[0002]

2. Description of the Related Art As a plant most suitable for high-efficiency power generation and intermediate load operation, there is a combined power generation plant. This plant generates electricity by using a gas turbine, and has an exhaust heat recovery boiler that recovers the heat of the exhaust gas discharged from the gas turbine, and generates electricity by driving a steam turbine with the steam generated in the exhaust heat recovery boiler. It is. Such an exhaust heat recovery boiler will be described with reference to the drawings.

FIG. 7 is a system diagram of a conventional combined cycle power plant. The generator 2 is driven by the gas turbine 1, and the exhaust gas G of the gas turbine (GT) 1 is discharged from the exhaust heat recovery boiler (H
RSG) 3 and the heat is recovered. In the gas flow path of the exhaust heat recovery boiler 3, in order from a high temperature side to a low temperature side,
Superheater 4, high-pressure evaporator 5, high-pressure economizer 6, low-pressure evaporator 7
And a low-pressure economizer 8, and a low-pressure drum 9, a high-pressure drum 10 and the like are arranged outside the gas flow path. Then, water supplied to the exhaust heat recovery boiler 3 is finally discharged as steam from the superheater 4 and used for driving a steam turbine 12 connected to the generator 2.

[0004] The steam used in the steam turbine 12 is condensed in a condenser 13, and the water in the condenser 13 is supplied from a water supply passage 19 to a low-pressure economizer 8 by a condenser pump 14. The heated feed water at the outlet of the low-pressure economizer 8 is supplied to a high-pressure feed pump 15.
To the high pressure economizer 6 and the circulation water supply passage 20
And mixed with the feed water W to the low-pressure economizer 8. The feedwater temperature T _S1 at the inlet of the low-pressure economizer 8 is detected by a temperature detector 17, and the temperature / flow controller 18 controls the low-pressure economizer based on the feedwater flow at a feedwater flow detector 21 provided in a feedwater flow path 19. The opening degree of the flow control valve 16 provided in the circulation water supply flow path 20 to the low-pressure economizer 8 of the heated water supply at the outlet of the water heater 8 is adjusted.

[0005] The reason for mixing the feedwater at the outlet of the low-pressure economizer 8 of the combined cycle power plant with the feedwater at the inlet will be described. In recent combined cycle power plants, the installation of a deaerator is omitted for reasons such as reduction of equipment costs and simplification of the system, and a condenser deaeration method in which the condenser 13 has a deaeration function is adopted. Have been. In this case, boiler 3
Since the feedwater temperature T _S1 at the inlet of the low-pressure economizer 8 is as low as about 30 ° C., if water is supplied to the low-pressure economizer 8 at the same temperature, low-temperature corrosion occurs in the low-pressure economizer 8. As a countermeasure, as shown in the drawing, the heated feed water at the outlet of the low-pressure economizer 8 is mixed with the boiler feed water W (the feed water at the inlet of the low-pressure economizer 8) through the high-pressure feed pump 15 to cause low-temperature corrosion. Means for raising the temperature to a non-existent temperature are employed. And, the temperature is about 50 which is a temperature at which low temperature corrosion does not occur during rated operation.
° C, and this temperature is determined by the temperature detected by the temperature detector 17 and the circulating water supply flow path 20 at the upstream of the water supply flow path 1 at the junction.
The temperature / flow controller 18 controls the flow regulating valve 16 based on the flow rate of the water supplied by the flow rate detector 21 provided in the apparatus 9 to keep the flow rate constant.

[0006]

Conventionally, the low-temperature corrosion prevention temperature for raising the temperature of the boiler feed water W (the feed water at the inlet of the low-pressure economizer 8) is a predetermined load, for example, a minimum load of 20 to 30 from the start. The value is set to a constant value (about 50 ° C.) until the value reaches%.

However, when the boiler is started, the amount of heat recovery in the low-pressure economizer 8 is small because the temperature of the exhaust gas introduced into the exhaust heat recovery boiler is low, and the feedwater temperature T _{S2 at the} outlet of the low-pressure economizer 8 is low.
When the temperature is low and the temperature is about 50 ° C. or less, it is impossible to raise the feedwater temperature T _S1 at the inlet of the low-pressure economizer 8 to the set temperature (about 50 ° C.). It is unavoidable that the steam in the exhaust gas is dew-condensed in the steam generator 8 (the time required for dew condensation from the steam is about one hour from the start of the boiler), and this dew water has a supply water temperature T _S1 at the inlet of the low-pressure economizer 8. Even after the temperature rises to about 50 ° C, it takes a long time (approximately 3 hours or more) to complete the evaporation. Therefore, in a waste heat recovery boiler that is frequently started and stopped repeatedly, the heat transfer tube of the low-pressure economizer 8 is used. The disadvantage of low-temperature corrosion has occurred.

An object of the present invention is to eliminate low-temperature corrosion of a low-pressure economizer by evaporating dew condensation water in the low-pressure economizer in a short time when the boiler is started or when the boiler is under a low load. An object of the present invention is to provide a method and an apparatus for controlling a feedwater temperature of an exhaust heat recovery boiler that can be reduced.

[0009]

The present invention employs the following configuration to solve the above-mentioned problems. That is, in the exhaust heat recovery boiler that recirculates a part of the low-pressure economizer outlet feedwater heated by the low-pressure economizer to the low-pressure economizer inlet, when the boiler is started or the boiler is under a low load, the low-pressure economizer inlet is Low pressure economizer recirculates to the low pressure economizer inlet such that the feedwater temperature of the low pressure economizer is maintained at a temperature (e.g., 80 [deg.] C.) sufficient to evaporate the dew water of the low pressure economizer for at least 0.15 hours or more. This is a method for controlling the feedwater temperature of the exhaust heat recovery boiler that adjusts the flow rate of the outlet feedwater.

Further, instead of the feedwater temperature at the low-pressure economizer inlet, any one of a boiler load signal corresponding to the feedwater temperature, a physical quantity correlated with the load signal, or a feedwater quantity at the low-pressure economizer inlet is used. The low-pressure economizer is set so that the feedwater temperature at the low-pressure economizer inlet is maintained at a temperature (for example, 80 ° C.) or more sufficient to evaporate the dew condensation water of the low-pressure economizer for at least 0.15 hours based on the value. The flow rate of the low-pressure coal economizer outlet feedwater recirculated to the charcoal inlet may be adjusted.

The above-mentioned object of the present invention is also solved by the following constitution. That is, in a waste heat recovery boiler that recirculates a part of the low-pressure economizer outlet feedwater heated by the low-pressure economizer to the low-pressure economizer inlet, a boiler load signal and a boiler load Until the physical quantity correlated with the signal or the amount of water supplied to the low-pressure economizer inlet reaches a predetermined value, the temperature of the water supply at the low-pressure economizer is sufficient to evaporate the dew water of the low-pressure economizer (e.g., 80 ° C)
Adjust the flow rate of the low-pressure economizer outlet feedwater to be recirculated to the low-pressure economizer inlet so that the boiler load signal, a physical quantity correlated with the load signal, or the low-pressure economizer inlet waterfeed rate The exhaust heat recovery boiler for adjusting the flow rate of the low-pressure economizer outlet feedwater to be recirculated to the low-pressure economizer inlet so that the temperature of the low-pressure economizer becomes equal to or higher than the low-pressure economizer low-temperature corrosion prevention temperature (for example, 50 ° C.) This is a method for controlling the feedwater temperature.

The object of the present invention can also be solved by the following configuration. That is, a water supply system that feeds water to the low-pressure economizer inlet and a water supply system that recirculates feedwater that has passed through the low-pressure economizer to the low-pressure economizer inlet are provided. In an exhaust heat recovery boiler provided with recirculation flow rate adjusting means for adjusting a recirculation flow rate to an inlet, a feedwater temperature detecting means at a low-pressure economizer inlet, a feedwater flow detecting means at a low-pressure economizer inlet, and boiler start-up When the load is low or the load is low, the feedwater temperature at the low-pressure economizer inlet becomes higher than the temperature (for example, 80 ° C.) sufficient to evaporate the dew condensation water of the low-pressure economizer based on the predetermined detection values of the two detection means. And a recirculation flow control device for adjusting the recirculation flow rate of the feedwater passed through the low-pressure economizer to the low-pressure economizer inlet by the recirculation flow adjusting means to maintain the temperature for at least 0.15 hours or more. Feedwater temperature control equipment for waste heat recovery boiler , Or,

[0013] The feedwater temperature control device for the exhaust heat recovery boiler is provided with a means for detecting either a boiler load value, a physical quantity correlated with the boiler load value, or a feedwater quantity at the low-pressure economizer inlet, at the time of boiler startup or When the load is low, the recirculation flow control device determines that the feedwater temperature at the low-pressure economizer inlet is high enough to evaporate the dew condensation water of the low-pressure economizer until any of the detection means detects a predetermined detection value. (For example, 80 ° C.), the recirculation flow rate is adjusted by the recirculation flow rate adjusting means so that the recirculation flow rate of the feedwater passed through the low pressure economizer to the low pressure economizer inlet is further correlated with the boiler load value and the boiler load value. When either the relevant physical quantity or the water supply amount at the low-pressure economizer detects a predetermined detection value, the recirculation flow rate to the low-pressure economizer inlet is changed to the low-pressure economizer low-temperature corrosion prevention temperature (50 ° C).
(° C.) or more. The present invention can also be applied to an exhaust heat recovery boiler of a type in which installation of a deaerator is not omitted.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described based on the following embodiments. The system diagram of the combined cycle power plant to which the present invention is applied is the same as that shown in FIG. 7, but the configuration of the temperature / flow controller 18 is different from that of the prior art, and FIG. The configuration of the vessel 18 is shown together with that of the prior art.

The configuration of the temperature / flow rate controller 18 in the prior art is the same as that of the low pressure
Although the opening degree of the flow control valve 16 is controlled so that the inlet water supply temperature of the fuel cell becomes 50 ° C., in the present invention, the inlet water supply temperature of the low-pressure economizer 8 is set to a relatively high temperature side depending on the size of the load. The temperature and the temperature on the relatively low temperature side are set to two types.

In the examples shown in FIGS. 1 to 4, the supply water temperature on the relatively high temperature side is set to 80 ° C., and the temperature on the relatively low temperature side is set to 50 ° C. As shown in FIG. 3, the reason for setting the high-temperature side feedwater temperature to 80 ° C. is that if the temperature is 80 ° C. or more, the evaporation time of the dew condensation water of the moisture in the exhaust gas to the heat transfer tube of the low-pressure economizer 8 is reduced. Because the difference is small, and 90-100 ° C
, The recirculation flow rate from the circulating water supply channel 20 to the inlet of the low pressure economizer 8 increases, and the high pressure water pump 1
It is necessary to use a high-pressure water supply pump 15 having a capacity larger than 5 or a large capacity. In addition, by setting the supply water temperature at the low-pressure economizer 8 at 80 ° C., the heat of vaporization of water that has once condensed can be sufficiently supplied.

The inlet feed water temperature of the low-pressure economizer 8 is set to 50.
Because the dew point temperature of the moisture in the exhaust gas is about 45 ° C. even when the load on the gas turbine 1 is 100%, the setting of the economizer 8 is performed by setting the dew point temperature to 50 ° C. or higher. This is because low-temperature corrosion does not occur.

FIG. 2 (a) shows the time change characteristics of the outlet feedwater temperature and the inlet feedwater temperature of the low-pressure economizer 8 after the start of the boiler 3, that is, the start of the gas turbine (GT) 1.
And FIG. 2 (b). FIG. 2 (c) shows changes over time in the boiler load, the number of revolutions of the gas turbine, the amount of gas introduced into the boiler, and the gas temperature at the boiler outlet.

In the early stage of the boiler start-up, since the exhaust gas temperature is low, the amount of heat recovered in the economizer 8 is small, and the feedwater temperature at the outlet of the low-pressure economizer 8 is 50 ° C. or less. Meanwhile, the water supply temperature at the inlet of the low-pressure economizer 8 is also 50 ° C.
It is as follows. Therefore, in this case, since the temperature is equal to or lower than the dew point temperature shown in FIG. 2, moisture in the exhaust gas is condensed in the low-pressure economizer 8. Conventionally, while the recirculation flow rate to the low-pressure economizer 8 inlet is adjusted by the flow control valve 16 with the rise of the low-pressure economizer 8 outlet feedwater temperature, the low-pressure economizer 8 inlet feedwater temperature is set to 50 ° C. Had control.

In FIG. 2 (b), as the boiler load increases, the dew point temperature also increases. This is because the fuel consumption of the gas turbine increases as the boiler load increases. This is because when the amount increases, the amount of water in the exhaust gas also increases, and the dew point temperature also increases.

According to FIG. 2, according to the prior art, after a short time has passed since the water supply temperature at the outlet of the low-pressure economizer 8 became 150 ° C., the dew condensation was completed to evaporate for the first time. During this time, the dew condensation present in a local portion near the water supply inlet of the economizer 8 having a low temperature evaporates.

FIG. 3 shows the relationship between the feed water temperature and the evaporation completion time of the condensed water. If the supply water temperature is set at 50.degree. It can be seen that it takes three hours, but once the feedwater temperature at the inlet of the low-pressure economizer 8 is raised to 80 ° C. from the start of the boiler as in the present invention, the evaporation completion time at 80 ° C. for the condensed water is 0. It turns out that 15 hours is good.

In one embodiment of the present invention, as shown in FIG. 2, the recirculation flow rate of water supplied to the low-pressure economizer 8 is increased by adjusting the opening of the flow control valve 16 from the start of the boiler, thereby reducing the low pressure. The water supply temperature at the inlet of the economizer 8 is once raised to 80 ° C, and in order to complete the evaporation of dew condensation water at 80 ° C,
This temperature is maintained for 0.15 hours or more, and thereafter, the control is returned to the control for maintaining the feedwater temperature at the inlet of the low-pressure economizer 8 at a predetermined 50 ° C.

Here, the feed water temperature at the inlet of the low-pressure economizer 8 is 8
Since the value of the boiler load at 0 ° C. (according to the load signal of the gas turbine 1) corresponds to β, the recirculation of the water supply to the low-pressure economizer 8 by adjusting the opening of the flow control valve 16 from the start of the boiler. By increasing the flow rate and maintaining the recirculation flow rate of the feedwater for 0.15 hours or more after reaching the boiler load β, the evaporation of the condensed water can be completed. Further, instead of the boiler load β, it is also possible to use a high-pressure or low-pressure steam flow rate or a high-pressure main steam temperature correlated with the water supply amount W or the boiler load β to the low-pressure economizer 8.

A comparison between the present invention and the prior art in terms of the wetting time of the economizer 8 until the completion of the formation and evaporation of the condensed water (condensation time + the completion time of the evaporation), in the case of the present invention, the wetting time is 1 In the case of the prior art, the wetting time is 1 hour of dew condensation time and 3 hours of evaporation completion time, which is 1.15 hours, which is the sum of the time and the evaporation completion time of 0.15 hours.
Time.

Thus, the present invention reduces the wetting time by about 30% compared to the prior art, thereby about three times the life of the low pressure economizer 8 against cold corrosion. This has the effect that the thickness of the heat transfer tube of the economizer 8 can be reduced by using a steel material having a thickness of 1 mm in the present invention instead of using a steel material of 3 mm as a corrosion allowance.

Although the evaporation completion time 0.15 hours cannot be uniquely determined due to the difference in exhaust gas conditions and the combined cycle power plant, the exhaust gas temperature of the exhaust heat recovery boiler 3 is 100%.
If the gas temperature is 100 ° C. or more, the condensation is completed when the gas temperature is 100 ° C. or more, since the condensation water attached to the surface of the heat transfer tube of the low-pressure economizer 8 at the feed water temperature of 50 ° C. The time is shorter than 0.15 hours.

Here, after the recirculation flow rate to the low-pressure economizer 8 inlet side is increased to a predetermined temperature (80 ° C.), the timing for returning to the normal control of the predetermined temperature (50 ° C.) is as follows. As shown in FIG. 4, the flow rate controller 18 is controlled by the boiler load α or the water supply amount W at the inlet of the low-pressure economizer 8.
A method of changing the set value of the inlet feedwater temperature can be used.

As can be seen from FIGS. 2B and 2C, the boiler load in this case has a predetermined value α (α>
Until β), the evaporation of the condensed water at the inlet of the low-pressure economizer 8 has been completed.

As shown in FIG. 5, the flow control valve 16 is controlled by the boiler load α or the water supply amount W at the inlet of the low-pressure economizer 8.
There is a method of dividing the opening degree into two types and adjusting the feedwater recirculation flow rate to the low-pressure economizer 8 inlet. In the method shown in FIG. 5, when the boiler load is equal to or less than the predetermined value α from the start of the boiler, the water supply temperature at the inlet of the low-pressure economizer 8 is set to the predetermined temperature (8
0 ° C.), and then return to normal opening degree control of the flow control valve 16 so that the feedwater temperature at the inlet of the low-pressure economizer 8 becomes a predetermined value (50 ° C.). Things.

In place of the water supply amount W to the inlet of the boiler load α or the low-pressure economizer 8 shown in FIGS. 4 and 5, a high-pressure or low-pressure steam flow rate or a high-pressure main steam temperature correlated with the boiler load α is used. It is also possible.

In each of the above examples, the control from the start of the boiler has been described. However, the present invention is not limited to the start of the boiler, and the boiler load is reduced and the exhaust gas inlet portion of the low-pressure economizer 8 may be corroded at low temperature. Also applicable to Although FIG. 1 illustrates an example of a double-pressure exhaust heat recovery boiler having a high-pressure drum and a low-pressure drum, it is needless to say that the present invention is also applicable to a single pressure or triple pressure.

In addition, instead of adjusting the flow rate of recirculating water to the inlet of the low-pressure economizer 8 by the opening degree of the flow control valve 16, a variable-pressure pump 22 is used as shown in FIG. Control for adjusting the feedwater recirculation flow rate to the inlet may be performed.

[0034]

According to the present invention, the time during which the low-pressure economizer is exposed to the condensed water when the boiler is started up is reduced from about 3 hours (at 50 ° C.) to about 0.15 hours (80 ° C.).
) Is reduced to (1/20), so that low-temperature corrosion of the heat transfer tube of the low-pressure economizer can be significantly suppressed.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the operation of a temperature / flow rate controller at the inlet of a low-pressure economizer after the start-up of an exhaust heat recovery boiler to which the present invention and the prior art are applied.

FIG. 2 is a time change characteristic of a low-pressure economizer outlet water supply temperature after the start of the exhaust heat recovery boiler to which the present invention is applied (FIG. 2)
(A)) and the time change characteristics of the low-pressure economizer inlet feedwater temperature (FIG. 2 (b)). FIG. 2 (c) shows the boiler load, the gas turbine speed, the amount of gas introduced into the boiler, and the boiler outlet. FIG. 4 is a diagram showing a change in gas temperature over time.

FIG. 3 is a diagram showing the relationship between the temperature of feed water to the economizer of the waste heat recovery boiler to which the present invention is applied and the completion time of condensation water evaporation.

FIG. 4 is a diagram illustrating a method of changing a set value of a feedwater temperature at a low-pressure economizer according to a load or a feedwater quantity of an exhaust heat recovery boiler to which the present invention is applied.

FIG. 5 is a diagram showing a method of defining the opening of the flow control valve according to the load or the amount of supplied water of the exhaust heat recovery boiler to which the present invention is applied.

FIG. 6 is a system diagram of a combined cycle power plant to which the present invention is applied.

FIG. 7 is a system diagram of a combined cycle power plant to which the present invention is applied.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Gas turbine 2 Generator 3 Waste heat recovery boiler 4 Superheater 5 High pressure evaporator 6 High pressure economizer 7 Low pressure evaporator 8 Low pressure economizer 9 Low pressure drum 10 High pressure drum 12 Steam turbine 13 Condenser 14 Condenser pump 15 High pressure feed water pump 16 Flow rate control valve 17 Low pressure economizer inlet feed water temperature detector 18 Temperature / flow rate controller 19 Feed water flow path 20 Circulating feed water flow path 21 Feed water flow rate detector 22 Variable flow rate pump

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Hiroshi Yoshizaki, Inventor 6-9 Takaracho, Kure-shi, Hiroshima Pref. Inside the Kure Plant (72) Inventor Koichi Toyoshima 6-9 Takaracho, Kure-shi, Hiroshima Babcock-Hitachi, Ltd. Inside the Kure Factory (72) Inventor Atsushi Kawahara 6-9 Takaracho, Kure City, Hiroshima Pref. Babcock Hitachi Kure Factory

Claims (5)

[Claims] 1. A waste heat recovery boiler for recirculating a part of a low-pressure economizer outlet feedwater heated by a low-pressure economizer to an inlet of the low-pressure economizer, wherein the low-pressure economizer is activated when the boiler is started or when the boiler is under a low load. Low pressure economizer outlet feedwater recirculated to the low pressure economizer inlet so that the feedwater temperature at the coal appliance inlet is maintained above a temperature sufficient to evaporate the dew water of the low pressure economizer for at least 0.15 hours. A method for controlling the temperature of feed water of an exhaust heat recovery boiler, comprising adjusting a flow rate of water. 2. The low-pressure economizer inlet feedwater temperature is measured until the boiler load signal, the physical quantity correlated with the load signal or the low-pressure economizer inlet feedwater reaches a predetermined value. Adjust the flow rate of the low-pressure economizer outlet feedwater to be recirculated to the low-pressure economizer inlet so that the feedwater temperature is maintained above the temperature sufficient to evaporate the dew water from the low-pressure economizer for at least 0.15 hours The method for controlling the temperature of feed water of an exhaust heat recovery boiler according to claim 1, wherein: 3. A waste heat recovery boiler for recirculating a part of the low-pressure economizer outlet feed water heated by the low-pressure economizer to the low-pressure economizer inlet. ,
Until the physical quantity correlated with the boiler load signal or the water supply amount at the low-pressure economizer inlet reaches a predetermined value, the water supply temperature at the low-pressure economizer inlet is a temperature sufficient to evaporate the dew water of the low-pressure economizer. Adjust the flow rate of the low-pressure economizer outlet feedwater to be recirculated to the low-pressure economizer inlet so that the boiler load signal, a physical quantity correlated with the load signal, or the low-pressure economizer inlet waterfeed rate Adjusting the flow rate of the low-pressure economizer outlet feedwater to be recirculated to the low-pressure economizer inlet so that the temperature becomes equal to or higher than the low-pressure economizer low-temperature corrosion prevention temperature when the predetermined value is reached. Water supply temperature control method for recovery boiler. 4. A water supply system for supplying water to an inlet of the low-pressure economizer,
Recirculation flow adjusting means for providing a water supply system that recirculates feedwater that has passed through the low-pressure economizer to the low-pressure economizer inlet, and adjusts the recirculation flow rate of the feedwater that has passed through the low-pressure economizer to the low-pressure economizer inlet A wastewater heat recovery boiler comprising: a feedwater temperature detecting means at a low-pressure economizer inlet; a feedwater flow rate detecting means at a low-pressure economizer inlet; Based on the detected value, the feed water temperature at the low-pressure economizer inlet becomes higher than the temperature sufficient to evaporate the dew condensation water of the low-pressure economizer, and the recirculation flow rate adjusting means for maintaining the temperature for at least 0.15 hours or more. A recirculation flow rate control device for adjusting a recirculation flow rate of feedwater passing through the low-pressure economizer to the low-pressure economizer inlet. 5. A recirculation flow control device comprising: a boiler load value; a physical quantity correlated with the boiler load value; or a water supply amount at a low-pressure economizer inlet. The recirculation flow rate is adjusted so that the feedwater temperature at the low-pressure economizer inlet is higher than the temperature sufficient to evaporate the dew condensation water of the low-pressure economizer until any of the detection means detects a predetermined detection value. Means for controlling the recirculation flow rate of the feedwater passed through the low-pressure economizer to the low-pressure economizer inlet, and further controlling the boiler load value, a physical quantity correlated with the boiler load value, or the feedwater volume at the low-pressure economizer inlet. When any of the detection means detects the predetermined detection value, the control means controls the recirculation flow rate adjusting means so that the recirculation flow rate to the low-pressure economizer low temperature is equal to or higher than the low-pressure economizer low-temperature corrosion prevention temperature. 5. The method according to claim 4, wherein A feedwater temperature control device for an exhaust heat recovery boiler as described above.