JPH0758121B2 - Recycling controller for economizer - Google Patents

Description

Description: [Industrial field of use] The present invention relates to a economizer recirculation control device for controlling the circulation amount of a recirculation line provided in parallel with a economizer.

[Prior Art] Conventionally, some heat recovery steam generators and the like are provided with a recirculation line for a economizer.

The construction of this type of boiler is shown in FIG. As a basic configuration, the waste gas passage 1 from the combustion chamber to the chimney is provided with an evaporator 2 connected to the drum 3 in order to generate steam by utilizing the waste heat. Furthermore, in order to effectively utilize the waste heat,
A coal economizer 4 is provided on the downstream side of the waste gas from the evaporation unit 2, and cold water from a water supply pump 6 and a water supply control valve 7 is passed through the coal economizer 4 to warm it, and then put into the drum 3. In reality, in many cases, the superheater 5 is provided on the upstream side of the evaporator 2 and the steam generated from the drum 3 is taken out as superheated steam.

Next, the recirculation line system is constructed as follows.

That is, the water supply line 8 that reaches the drum 3 through the economizer 4.
In parallel with the economizer 4, a recirculation line 9 from the economizer outlet to the economizer inlet is provided, and a pump 10 and a control valve 11 are provided in the middle of the recirculation line 9.

The water from the water supply pump 6 enters the drum 3 through the economizer 4, but part of the water is circulated to the economizer 4 again from the recirculation line 9 by the pump 10. The water that has entered the drum 3 is circulated and evaporated in the evaporating unit 2, and the water is superheated by the superheater 5 and finally used. The water supply control valve 7 controls the flow rate according to the amount used, and keeps the level of the drum 3 constant.

The recirculation line 9 has two purposes.

The first is the inlet temperature T1 of the economizer 4 when the feed water temperature is low.
To prevent low temperature corrosion by keeping the temperature above the condensation temperature. That is, if the water supplied to the economizer 4 is too cold, it will corrode by condensing dew on the outside. Therefore, the once warmed water is circulated through the recirculation line 9 so that the inlet temperature T1 of the economizer 4 is maintained above the condensation temperature.

The second is to prevent steaming at low load. When the load is low, that is, when the amount of evaporation is small, the temperature of the water inside the economizer 4 rises and steam is generated. In other words, it causes steaming. If steaming occurs, the level will fluctuate drastically, and piping will cause hammering and other inconveniences. Therefore, in order to prevent steam from being generated in the economizer 4, a recirculation line 9 is provided. That is, if the circulation amount is increased, the heat exchange amount is reduced, and the outlet temperature of the economizer 4 is reduced.
T2 will be lowered.

[Problems to be Solved by the Invention] However, a conventional control device for a recirculation line system is performed as follows.

Normally, set the subtractor set temperature S1 slightly higher than the condensation temperature and compare the inlet temperature T1 of the economizer 4 with this,
When <S1, the control valve 11 is opened to raise the temperature T1, and when S1 <T1, the control valve 11 is closed to lower the temperature T1. To prevent. For example, the set temperature S1 is set to 40 ℃, the inlet temperature of the economizer 4
The control valve 11 controls T1 to be 40 ° C.

On the other hand, when the load is low, the subtractor set temperature S1 is set to a high value so that steaming is not likely to occur in the economizer, and the economizer 4 is set to this high set value. Steaming is prevented by comparing the inlet temperatures T1 of the two and adjusting the opening of the control valve 11. For example, set temperature S1 to 15
After resetting to 0 ℃, when T1> S1, the control valve 11 is opened to increase the amount of return from the outlet of the economizer 4 which is in a high temperature state to the inlet, and the exit temperature of the economizer 4 is increased. Lower T2.

However, in the above control method, since only the inlet temperature T1 of the economizer 4 is controlled, the exit temperature T2 of the economizer 4 is
Will change greatly. That is, if S1 is set so as to prevent steaming under any load, T2 will decrease as the load increases.

As described above, lowering the temperature of T2 more than necessary depending on the load is very uneconomical from the viewpoint of energy.

An object of the present invention is to solve the above problems and to provide a control device that can switch between normal operation and low load more economically and smoothly.

[Means for Solving the Problem] The present invention keeps the inlet temperature of the economizer at a temperature equal to or higher than the dew condensation temperature and controls it by controlling the circulation amount of a recirculation line provided in parallel with the economizer of the boiler. In a economizer recirculation control device for preventing steaming under load, a first control system that detects the inlet temperature of the economizer and maintains the inlet temperature at a predetermined value equal to or higher than the dew condensation temperature; The second control system that detects the outlet temperature of the reactor and keeps the outlet temperature slightly lower than the saturation temperature of the drum, and the first control system is selected during normal operation based on the magnitude of the outputs of the first control system and the second control system. When the load is low, the second control system is selected, and a high signal selector for controlling the circulation amount of the recirculation line by the output of the selected control system is provided.

[Operation] The circulation amount of the recirculation line is reduced by the second control system when the load is low, that is, when the heat and evaporation power of the waste gas are small and the water supply amount from the economizer to the drum is small. It is maintained to keep the outlet temperature of the vessel slightly below the saturation temperature of the steam drum. Therefore, even when the load is low, the supply from the outlet of the economizer to the drum is always kept in the water state, and steaming at the low load is prevented.

Next, when the load increases, the high signal selector automatically switches from the second control system to the first control system, and during normal operation, the first control system causes the inlet temperature of the economizer to change the condensation temperature. It is controlled so as to maintain the above predetermined value. Moreover, before and after this switching, the temperature difference between the outlet temperature of the economizer and the saturation temperature is small, and the energy loss is extremely small.
Therefore, it is possible to switch economically and smoothly between normal operation and low load.

[Embodiment] An embodiment of the present invention will be described below with reference to the accompanying drawings.

In FIG. 3, the recirculation lines of the exhaust heat recovery boiler and the economizer are the same as those already described. That is, the waste gas passage 1 from the combustion chamber to the chimney is provided with the evaporation unit 2 connected to the steam drum 3, the economizer 4, and the superheater 5, and the water supply pump 6, the water supply control valve 7, the economizer. A recirculation line 9 is provided in parallel with the economizer 4 in the water supply line 8 that reaches the drum 3 through
A pump 10 and a control valve 11 are provided in the middle of the recirculation line 9.

However, unlike the prior art, in addition to the first sensor 21 that detects the inlet temperature T1 of the economizer 4, a second sensor 22 that detects the outlet temperature T2 of the economizer 4 and the saturation temperature of the drum 3 are detected. Third
A sensor 23 is provided.

FIG. 1 is a circuit diagram showing a control device, which has two control systems (controllers). The first control system is the first sensor
It comprises a subtractor 24 that compares the detected temperature T1 of 21 with the set temperature S1 and outputs a deviation, and a proportional-plus-integral regulator 25 connected to this. The second control system compares the detected temperature T2 of the second sensor 22 with the detected temperature T3 of the third sensor 23 and outputs a difference (T3-T2), and compares the output with the set temperature S2. It is composed of a subtractor 27 that outputs a deviation and a proportional-plus-integral regulator 28 connected to the subtractor 27. Further, the control device outputs the output of the first control system and the second control system.
The output of the control system is compared, the control system with the larger output is selected, and the signal selector 29 for controlling the output of the proportional-plus-integral regulator 25 or 28 of the control system to be given to the control valve 11 is selected. Have.

Next, the control method will be described with reference to FIG.

When the load on the left side of Fig. 2 is low, the economizer 4
If the inlet temperature T1 is controlled to be kept at the set temperature S1 (a temperature slightly higher than the condensation temperature), the outlet temperature T2 of the economizer 4 coincides with the saturation temperature T3 and steaming occurs.

Therefore, in order to prevent steaming, initially, the opening of the control valve 11 is increased to increase the circulation amount by the automatic operation of the second control system, and the temperature difference ΔT = T3−T2 is a constant value ( The temperature is controlled to reach the set temperature S2) (see line A in FIG. 2). That is, the outlet temperature T2 of the economizer 4 is set to the saturation temperature.
Control to keep it slightly lower than T3.

The load is gradually changed from low load to high load with respect to time t. Then, the temperature of the inlet temperature T1 of the economizer 4 gradually decreases. Then, if the temperature T1 drops to the set temperature S1, this time the automatic control of the first control system is started, and the temperature T1
Is maintained at the set temperature S1 (see line B in FIG. 2). If the temperature T1 is kept constant, the outlet temperature T2 of the economizer 4 will gradually decrease (see line C in FIG. 2).

Next, a method of switching between the second control system and the first control system will be described.

In this embodiment, the signal selector 29 is used to smoothly switch between the two control systems, and the first control system controls when the second control system controls the output deviation.
Switch to control system. The advantage of this switching method is that the switching timing can be determined only by the magnitude relation between the outputs of the first and second control systems without determining the load state.

In detail, in FIG. 2, when the load is low, the circulation amount is not so much needed to control T1 constant. Then, the temperature of T2 will rise far. In order to suppress this temperature rise, the circulation amount is increased at the beginning. As a result, the temperature of T1 becomes relatively high, but the set value S1 is not changed unlike the conventional case.

(1) When the load is low, the temperature of T1 is much higher than the set value S1 (for example, 40 ° C.), so the control output of 25 in the first control system is conversely small with respect to the set value S1. In contrast,
In the second control system, since the temperature difference T3-T2 of the temperature T2 is small, the deviation with respect to the set temperature S2 (for example + 5 ° C) becomes large. Therefore, the output signal of the second control system is
It becomes higher than the output signal of the control system, and the signal selector 29 selects the second control system.

After all, when the load is low, the temperature difference is
It is automatically operated so that T3-T2 becomes constant value (+ 5 ℃).

(2) When the load increases, the hot air increases and the evaporation power increases. Therefore, the temperature difference T3-T2 can be maintained at the set value S2 even if the amount of water supply increases and the circulation amount decreases, and the output of the proportional-plus-integral controller 28 decreases. On the other hand, since the temperature T1 decreases, the output of the proportional integral regulator 25 increases, and finally reaches the set value S1. Therefore, in the signal selector 29, the second
It is judged that the control output from the first control system is larger than that of the control system, and the control output is automatically switched to the first control system.

Therefore, during normal operation, the opening degree of the control valve 11 is controlled by the first control system so that the temperature T1 matches the set temperature S1, and automatic operation is performed.

[Effects of the Invention] In summary, according to the present invention, since the temperature difference between T3 and T2 is controlled to be constant at low load, the circulation amount of the recirculation line is controlled by the second control system at low load. The outlet temperature of the economizer is slightly lower than the saturation temperature of the drum, that is, it is kept in a water state and steaming is prevented.

Further, when the normal operation is started, the selector automatically switches the second control system to the first control system, and even before and after this switching, the outlet temperature of the economizer has a temperature difference from the saturation temperature. Few. Therefore, it is possible to switch between the normal operation and the low load extremely economically and smoothly as compared with the case where the set point regarding one kind of temperature (the inlet temperature of the economizer) is switched midway as in the conventional control. .

[Brief description of drawings]

FIG. 1 is a block diagram of a coal economizer recirculation control device showing an embodiment of the present invention, FIG. 2 is a diagram illustrating its control state, and FIG. 3 is a configuration of a waste heat recovery boiler to which the present invention is applied. FIG. In the figure, 1 is a waste gas passage, 2 is an evaporator, 3 is a drum, 4 is a economizer, 5 is a superheater, 6 is a water supply pump, 7 is a water supply control valve, 8 is a water supply line, and 9 is a recirculation line. , 10 is a pump,
11 is a control valve, 21, 22 and 23 are sensors, 24 is a subtractor of the first control system, 25 is a proportional integrator of the first control system, 26 and 27 are subtractors of the second control system, and 28 is A proportional integrator of the second control system, and 29 is a signal selector.

Claims (1)

[Claims] 1. By controlling the circulation amount of a recirculation line provided in parallel with a economizer of a boiler, the inlet temperature of the economizer is kept above the condensation temperature and steaming at a low load is prevented. In the economizer recirculation control device configured as described above, the first control system that detects the inlet temperature of the economizer and keeps the inlet temperature at a predetermined value equal to or higher than the condensation temperature, and the exit temperature of the economizer is detected. Second, to keep the outlet temperature slightly below the saturation temperature of the drum
Depending on the output of the control system and the outputs of the first control system and the second control system, the first control system is selected during normal operation, the second control system is selected during low load, and the output of the selected control system is used again. A economizer recirculation control device comprising a selector for controlling the circulation amount of the circulation line.