JP3227137B2 - Waste heat recovery boiler - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust heat recovery boiler, and more particularly to a structure of a heat transfer panel used for a superheater, a reheater, a economizer, and the like.

[0002]

2. Description of the Related Art Recently, as part of high-efficiency power generation, a combined cycle plant has attracted attention. This combined cycle plant first generates power using a gas turbine, recovers the retained heat in the exhaust gas discharged from the gas turbine using a waste heat recovery boiler, and further generates steam using the steam generated by the waste heat recovery boiler. By driving the turbines, power is generated by both turbines.

FIG. 5 is a schematic configuration diagram of a conventional combined cycle plant. In FIG.
The air A from the fuel and the fuel F from the fuel supply pipe 2 are mixed and burned in the combustor 3, and the gas turbine 4 is rotated by the combustion gas to generate power by the gas turbine 4.

The exhaust gas G discharged from the gas turbine 4 is introduced into an exhaust gas passage 6 of an exhaust heat recovery boiler. In the exhaust gas passage 6, a economizer 7, an evaporator 8, a drum 9, and a superheater 10 are arranged from the downstream side to the upstream side. In recent years, as the temperature of the exhaust gas from the gas turbine increases, the reheater 29 is disposed between the evaporator 8 and the superheater 10.
May be arranged.

On the other hand, feed water WF, which is a fluid to be heated, is supplied from a feed pump 11 to a economizer 7 through a feed pipe 12, preheated to a predetermined temperature, and then supplied to a drum 9. The feed water WF supplied to the drum 9 is naturally circulated or forcedly circulated in the order of the evaporator 8 and the drum 9 through the downcomer 13 of the drum 9, and is heated and separated into water and steam in the drum 9. . The water is again recycled to the downcomer 13, the evaporator 8 and the drum 9, and the steam is sent to the superheater 10, where the temperature is further increased, and then supplied to the steam turbine 15 from the main steam pipe 14. The power is generated by the steam turbine 15. When there is the reheater 29, the steam from the steam turbine 15 is sent to the reheater 29, where the steam is superheated, and the superheated steam is sent to the steam turbine 15 again to generate power. In the figure, 16 is a generator, and 17 is a condenser.

FIG. 6 is a perspective view of a heat transfer panel used for the economizer 7, the reheater 29, the superheater 10, and the like. As shown in the figure, water or steam introduced from the inlet lower header 19 flows upward from below while being heated by the heat transfer tubes 22 arranged in parallel, and the heat It is taken out and moves to the next heat transfer panel 18 via the connecting pipe 21.

As described above, in the conventional heat transfer panel 18, the fluid to be heated from the inlet lower header 19 flows in the heat transfer tube 22 in only one direction from the bottom to the top, and from the outlet upper header 20 through the connecting pipe 21 to the next. To the heat transfer panel 18.

FIG. 10 is a schematic structural view of a conventional exhaust heat recovery boiler. As shown in the figure, a superheater 41, an evaporator 42, and a economizer 43 are sequentially arranged from the upstream side in the flow direction of the exhaust gas G. Water sent from a water supply pump (not shown) is introduced into the inlet of the economizer 43, and heat is absorbed while repeating ascending and descending in the economizer 43.
4 to the drum 45.

The introduced water is mixed with boiler water in a drum 45, becomes saturated steam in an evaporator 42, is led to a superheater 41, and is superheated to a high temperature, after which it is heated to a steam turbine (not shown).
Supplied to

In this exhaust heat recovery boiler, when the calorific value of the exhaust gas decreases, the amount of evaporation decreases, so that the feed water flow rate is adjusted by a feed water flow control valve 46 disposed on the inlet side of the economizer 43. I have. At this time, due to the characteristics of the water supply pump, the internal pressure of the economizer 43 is maintained high even when the load decreases, thereby preventing steaming in the economizer 43. In particular, when performing the variable pressure operation, since the saturated steam temperature decreases in the superheater 41 and the evaporator 42, maintaining the internal pressure of the economizer 43 high by this method is effective for preventing steaming.

[0011]

The heat transfer panel 1 described above.
In FIG. 8, the heat transfer coefficient in the heat transfer tube 22 is substantially proportional to the 0.8 power of the flow rate in the pipe. Therefore, the higher the flow rate, the better the heat recovery rate, and the lower the flow rate in the pipe, the worse the heat recovery rate.

Meanwhile, recently, in order to shorten the installation period on site, there is a tendency that the waste heat recovery boiler 5 is replaced with a waste heat recovery boiler module 23 at a factory as shown in FIG. In this case, as shown in the figure, a heavy equipment for transportation 24 (for example, a dolly) is placed under the exhaust heat recovery boiler module 23,
A method is employed in which the exhaust heat recovery boiler module 23 is transported to a construction site without any change. Therefore, the width L1 of the exhaust heat recovery boiler 5 is wider than the width L2 of the heavy equipment 24 for transport in order to accommodate the heavy equipment 24 for transportation, and is considerably wider than the width L3 when the exhaust heat recovery boiler is not modularized.

Therefore, the number of heat transfer tubes of the heat transfer panel 18 is inevitably increased. Therefore, the flow velocity in each pipe decreases, and sufficient heat recovery is not performed. In order to compensate for this, it is necessary to increase the number of panels, which causes problems such as an increase in weight, an increase in size, and an uneconomical effect.

In order to solve these problems, a heat transfer panel 18 as shown in FIG. 8 has been proposed.
No. 125807). The heat transfer panel 18 includes a header 19 and a header 20 arranged at both ends of the heat transfer tube 22.
Is divided into a plurality of pieces to form divided panels, and each divided panel is connected by a panel connecting pipe 21.
By doing so, the flow velocity in each heat transfer tube 22 can be increased as compared with the conventional heat transfer panel, and the heat recovery efficiency can be increased.

However, this configuration is not without problems. In other words, the number of panel connecting pipes 21 increases according to the number of divisions of the heat transfer panel. However, the presence of the panel connecting pipes 21 not only increases the pressure loss but also may cause an unbalanced flow. In particular, when an unbalanced flow occurs in the case of a economizer, a heat transfer tube in which water does not flow partially occurs, so that steam is generated in the tube, causing a water hammer phenomenon, which has a disadvantage of damaging the heat transfer tube.

In the exhaust heat recovery boiler having the structure shown in FIG. 10, the fluid is flushed at the outlet of the flow control valve 44. In particular, in the variable pressure operation, the pressure difference between the inlet and the outlet of the flow control valve 44 is large. And the wear of the flow control valve 44 becomes remarkable.

In order to prevent this from occurring, it is necessary to design the fluid enthalpy at the outlet of the economizer 43 to be low. However, lowering the fluid enthalpy at the outlet of the economizer 43 results in a reduction in the amount of evaporation of the exhaust heat recovery boiler, which leads to a decrease in plant efficiency in a combined cycle plant or the like, which is preferable. Not that.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art, so that even if the number of heat transfer tubes in the heat transfer panel is increased, the flow rate in the tubes can be increased and the heat recovery can be maintained at a high heat recovery rate. It is to provide a recovery boiler.

[0019]

In order to achieve the above object, the present invention relates to a heat exchanger such as a superheater, a reheater, an evaporator, and a economizer disposed in an exhaust gas passage. At least one of the heat exchangers is intended for an exhaust heat recovery boiler including a plurality of heat transfer tubes arranged in a width direction of an exhaust gas passage, and heat transfer panels having headers connected to both ends of the heat transfer tubes. Is what you do.

At least a part of the heat transfer panel divides the heat transfer tube in the width direction of the exhaust gas passage, and is arranged on the same side as a header connected to one end of the heat transfer tube.
Have a temperature difference consisting another tube separated with a gap being location
And a header connected to the divided heat transfer tubes, and an intermediate header is connected to all the heat transfer tubes as a header connected to the other end .
The heat transfer panels are located in the downstream of the exhaust gas flow direction.
Heat transfer panel, second heat transfer panel toward upstream
And the fluid to be heated is introduced into the first heat transfer panel.
Heat transfer introduced from the mouth header and connected to the inlet header
Through pipe, through intermediate header, connected to outlet header
Through the heat transfer tubes and from the outlet header of the first heat transfer panel
Introduced into the inlet header of the second heat transfer panel and its inlet head
Through the heat transfer tube connected to the
The second heat transfer panel passes through the heat transfer tube connected to the header.
The heated fluid is discharged from the outlet header of the
While meandering in the exhaust gas passage
Exhaust gas while moving from the downstream side to the upstream side
And heat exchange is performed between them.

[0021]

DETAILED DESCRIPTION OF THE INVENTION The present invention is organized as described above arrangement, the tube flow rate of the heat transfer tube is faster, yet the heated stream
While the body repeats meandering in the exhaust gas passage,
While moving from the downstream in the flow direction to the upstream,
Heat is exchanged with the heat source, so that the heat recovery efficiency can be maintained high. Also, there is a temperature difference between the cold fluid passing through the inlet header and the hot fluid passing through the outlet header. Therefore, if the inlet header and the outlet header are constituted by one continuous header, the header and the heat transfer tube connected thereto are deformed or damaged by the temperature difference. That respect the present invention is temperature
Since the inlet header and the outlet header having a difference are separate pipes separated through a gap, deformation and damage of the header and the heat transfer tube due to the temperature difference are eliminated.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a front view of the heat transfer panel according to the first embodiment of the present invention.

In the heat transfer panel 18 shown in FIG. 1, a large number of heat transfer tubes 2 are provided in a gas passage extending vertically to the paper surface.
2, the heat transfer tube 22 is divided into approximately two right and left parts toward the paper surface, and a gas path prevention plate 27 is inserted in the middle part. The inlet header 19 is connected to one end of the heat transfer tube 22 (region a) on the right side of the drawing, while the outlet header 20 is connected to one end of the heat transfer tube 22 (region b) on the left side. The entrance header 19 and the exit header 20
Are completely separated from each other, and a gas path preventing plate 27 is arranged between the heat transfer tubes 22 corresponding to the gaps between them, as shown in the figure. Also, the other end of all the heat transfer tubes 22
They are connected to an intermediate header 25 provided at a position facing the entrance header 19 and the exit header 20, respectively.

In the heat transfer panel 18 having this structure, a fluid to be heated such as water is introduced from the inlet header 19, passes through the heat transfer tube 22 on the area a side, passes through the intermediate header 25, and passes through the heat transfer tube on the area b side. Exit header 20 after passing through 22
, And the fluid to be heated flows in two directions unlike the conventional one.

In the embodiment of the present invention, FIG.
Of the superheater 10, reheater 29, evaporator 8 and economizer 7 shown in Fig. 7 except for the evaporator 8, the superheater 10, reheater 29 and economizer 7 are constituted by the heat transfer panel 18. It is.

This is because, in the heat transfer tube 22 of the evaporator 8, the fluid to be heated is transferred while boiling, so that increasing the flow velocity in the tube hardly affects the improvement of the heat transfer efficiency. On the other hand, since the fluid to be heated in the heat transfer tube 22 in the superheater 10, the reheater 29, and the economizer 7 is only one phase of water or steam, increasing the flow rate increases the heat transfer efficiency accordingly. I do.

As in this embodiment, the entrance header 19
And the outlet header 20, the leftmost heat transfer tube 22a in the region a and the rightmost heat transfer tube 22b in the region b
Inevitably, the gap between them becomes larger. In this case, the exhaust gas G is short-passed through the gap to reduce the heat recovery efficiency. Therefore, the gas-pass prevention plate 27 is disposed to prevent the short-pass and increase the heat recovery efficiency.

In the first embodiment, the lengths of the entry header 19 and the exit header 20 are substantially equal, but in the case of the second embodiment shown in FIG.
Is changing the length. By alternately changing the lengths of the inlet header 19 and the outlet header 20 in this manner, the heat of the exhaust gas that has short-passed the divided portion of the panel A is recovered by the panel B located on the downstream side. In this embodiment, the entrance header 19 is short and the exit header 20
, But it is also possible to make the entrance header 19 longer and the exit header 20 shorter on the opposite side. In the figure
As shown, multiple heat transfer panels
Are arranged side by side from the downstream side toward the upstream side, and indicated by arrows.
So that the heated fluid is supplied to the inlet header 19 of the first heat transfer panel.
a introduced from a and connected to its inlet header 19a
Connects to the outlet header 20a through the pipe, through the intermediate header
Through the heat transfer tube, and the outlet head of the first heat transfer panel.
20a to the inlet header 19b of the second heat transfer panel.
Through the heat transfer tube connected to the inlet header 19b.
Connected to the outlet header 20b via the intermediate header.
Through the heat transfer tubes, the outlet header 20 of the second heat transfer panel
b into the third heat transfer panel entrance header 19c,
Through the heat transfer tube connected to the inlet header 19c,
The heat transfer tube connected to the outlet header 20c via the header
Of the second heat transfer panel from the outlet header 20c.
And finally the output of the fifth heat transfer panel.
It is discharged from the mouth header 20e. Heated in this way
While the fluid repeats meandering in the exhaust gas passage,
While moving from downstream to upstream in the flow direction.
It is configured to exchange heat with the gas.

The difference between the third embodiment shown in FIG. 3 and the first embodiment is that the heat transfer panel 18 shown in FIG.
Are arranged in a vertical position, whereas the heat transfer panel 18 in FIG. 3 is arranged in a horizontal position, and other configurations are the same as those in FIG.

FIG. 4 shows a fourth embodiment of the present invention. For combined cycle plants,
There is a disadvantage that steam is generated at the water supply outlet of the economizer 7 of the exhaust heat recovery boiler when the gas turbine is started or at a low load. At start-up, when the load is low, the exhaust gas temperature at the inlet of the exhaust gas passage 6 shown in FIG. 5 is lower than at the time of high load, so that the superheater 10 and the reheater arranged upstream of the economizer 7 29 and the amount of heat absorbed by the heat transfer panel 18 of the evaporator 8 is reduced.

Accordingly, the exhaust gas temperature at the inlet of the economizer 7 is higher because the amount of heat absorbed by the superheater 10, the reheater 29 and the evaporator 8 is smaller, and the temperature of the heat transfer panel 18 of the economizer 7 is higher. Conversely, the amount of heat absorption increases, and with the increase in the amount of heat absorption, among the heat transfer panels 18 of the economizer 7, the heat transfer panel 18 (close to the water supply outlet), that is, upstream of the flow of exhaust gas, Steaming occurs in the heat transfer panel 18) located upstream of the exhaust gas from the charcoal 7. When steaming occurs, a water hammer is generated at the same time, and the heat transfer tube 22 may be damaged, which is not preferable.

Therefore, when all of the heat transfer panels 18 shown in FIG. 1 or FIG. 2 are used for the economizer 7, the downwardly flowing heat transfer tubes flowing from the inlet header 19 of the heat transfer panel 18C at the water supply outlet to the intermediate header 25 are provided. When steam is generated in the pipe 22, there is a problem that the steam stagnates and a water hammer is generated.

Therefore, in the embodiment shown in FIG. 4, the heat transfer panel 18 at the water supply outlet where steam is expected to be generated is provided.
By setting C as the heat transfer panel 18 having only the upward flow, that is, the heat transfer panel 18 shown in FIG. 6, and the other heat transfer panels 18D to 18H as the heat transfer panels 18 shown in FIG. 1 or FIG. The heat recovery in the drum 7 can be enhanced, and water can be supplied to the drum 9 without stagnation even if steam is generated.

If the region where steam is generated is large, the heat transfer panel 18D is also made to be the heat transfer panel 18 having only the upward flow, and the diameter of the connecting pipe connecting the heat transfer panel 18C and the heat transfer panel 18D is reduced. Thus, it is possible to increase the flow velocity in the connecting pipe, eliminate stagnation of steam, and supply water from the heat transfer panel 18D to the heat transfer panel 18C.

FIG. 9 is a diagram for explaining a fifth embodiment of the present invention. As shown in the figure, the superheater 41 and the evaporator 4 extend from the upstream side to the downstream side in the flow direction of the exhaust gas G.
2. The high-temperature side economizer 43b and the low-temperature side economizer 43a are sequentially arranged.

The feedwater sent from a feedwater pump (not shown) is mixed with an appropriate amount of high-temperature water in the economizer 43a in order to prevent low-temperature corrosion, and is heated above the dew point of the exhaust gas G (about 45 ° C.). And is supplied to the economizer 43a. The water sent to the economizer 43a absorbs the heat from the exhaust gas G while repeating rising and falling, and then is supplied to the high-temperature economizer 43b composed of only the ascending pipe through the economizer connecting pipe 47. Is done. If the high-temperature side economizer 43b is composed of only the riser,
Even if steam is generated in it, it does not stay,
After all, there is no water hammer.

The fluid at the outlet of the high-temperature side economizer 43b (water or steam-containing fluid containing steam) is guided to the drum 45 through the drum connecting pipe 18, and is mixed with the can water in the drum 45. It flows into the evaporator 42. Saturated steam generated in the evaporator 42 is separated into steam and water in a drum 45 and is connected to the superheater connecting pipe 4.
The steam is guided to a superheater 41 by 9 and becomes superheated steam and is sent to a steam turbine (not shown).

The water supply amount control in accordance with the load change of the exhaust heat recovery boiler is performed by controlling the flow rate regulating valve provided in the middle of the economizer connecting pipe 47 connecting the economizer 43a and the economizer 43b. It is performed at 46. Therefore, low-temperature side economizer 43a
The inside can be maintained at a predetermined high pressure, and steaming in the low-temperature side economizer 43a can be prevented.

Further, since the high-temperature side economizer 43b is composed of only the rising pipe, there is no problem even if steaming occurs. Therefore, a design in which the fluid enthalpy at the inlet of the drum 45 is increased is possible. By increasing the amount of steam, plant efficiency of a combined cycle plant or the like can be designed to be higher.

Further, if the heat absorption amount in the high-temperature side economizer 43b is increased, the amount of evaporation can be increased accordingly, and the gas temperature at the inlet of the low-temperature economizer 43a decreases.
Further, there is an effect that steaming in the low-temperature side economizer 43a hardly occurs.

In the middle of the drum connecting pipe 48 communicating between the high-temperature side economizer 43b and the drum 45, a pressure loss applying section 50 made of an orifice or an orifice tube is provided.
And a switching valve 51. At a partial load, the fluid temperature at the outlet of the low-temperature side economizer 43a increases, and the flow control valve 46
When the flushing is likely to occur at the outlet, the switching valve 51 guides the fluid to the pressure loss applying section 50 having a large pressure loss, thereby preventing the flushing. Furthermore, from this, the fluid enthalpy at the inlet of the flow control valve 46 can be increased to near these limits, so that the evaporation amount can be further increased and the plant efficiency can be further improved.

[0042]

According to the first aspect of the present invention, the flow rate in the pipe can be increased , and the flow rate to be heated can be increased.
While the body repeats meandering in the exhaust gas passage,
While moving from the downstream in the flow direction to the upstream,
Heat is exchanged with the heat source, so that the heat recovery efficiency can be maintained high. Also, there is a temperature difference between the cold fluid passing through the inlet header and the hot fluid passing through the outlet header. Therefore, if the inlet header and the outlet header are constituted by one continuous header, the header and the heat transfer tube connected thereto are deformed or damaged by the temperature difference. That respect the present invention is temperature
Since the inlet header and the outlet header having a difference are separate pipes separated through a gap, deformation and damage of the header and the heat transfer tube due to the temperature difference are eliminated.

According to the second aspect of the present invention, the inlet header and the outlet header are separated through a gap, so that the space between the heat transfer tubes corresponding to the gap is opened. However, by disposing the gas path preventing member there, a short path of the exhaust gas can be prevented, and the heat recovery efficiency can be further improved.

According to a third aspect of the present invention, the gap between the inlet header and the outlet header of one heat transfer panel is set to be equal to the inlet header or the outlet header of the heat transfer panel disposed adjacent to the heat transfer panel. , The retained heat of the exhaust gas passing through the gap between the inlet header and the outlet header is recovered by the heat transfer panel on the downstream side, so that the heat recovery efficiency can be further increased.

[Brief description of the drawings]

FIG. 1 is a front view of a heat transfer panel according to a first embodiment of the present invention.

FIG. 2 is a plan view of a heat transfer panel according to a second embodiment of the present invention.

FIG. 3 is a front view of a heat transfer panel according to a third embodiment of the present invention.

FIG. 4 is a schematic configuration diagram around a economizer of a waste heat recovery boiler according to a fourth embodiment of the present invention.

FIG. 5 is a schematic configuration diagram of a conventional combined cycle plant.

FIG. 6 is a perspective view of a conventional heat transfer panel.

FIG. 7 is an explanatory diagram showing a state in which the modularized exhaust heat recovery boiler is transported.

FIG. 8 is a schematic configuration diagram of a conventionally proposed exhaust heat recovery boiler.

FIG. 9 is a schematic configuration diagram of an exhaust heat recovery boiler according to a fifth embodiment of the present invention.

FIG. 10 is a schematic configuration diagram of a conventional exhaust heat recovery boiler.

[Explanation of symbols]

 Reference Signs List 5 waste heat recovery boiler 6 exhaust gas passage 7 economizer 8 evaporator 9 drum 10 superheater 18 heat transfer panel 19 inlet lower header 19 20 outlet upper header 22 heat transfer tube 25 intermediate header 29 reheater 27 gas path prevention plate 41 superheater 42 Evaporator 43 Energy saving device 43a Low temperature side energy saving device 43b High temperature side energy saving device 45 Drum 46 Flow rate control valve 47 Energy saving device communication pipe 48 Drum communication pipe 50 Pressure loss giving section 51 Switching valve

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tetsuo Mimura 6-9 Takaracho, Kure City, Hiroshima Prefecture Inside the Kure Plant of Babkotsuk Hitachi Ltd. (72) Inventor Shosuke Miyake 2-6-1 Otemachi, Chiyoda-ku, Tokyo Inside Babkotsuk Hitachi Co., Ltd. (72) The inventor Hiroshi Saki ▼ 6-9 Takaracho, Kure City, Hiroshima Prefecture Inside the Kure Plant Babkotsuk Hitachi Co., Ltd. (56) References JP-A-55-105196 (JP, A) Sho-59-113667 (JP, U) Shokai Sho-59-139704 (JP, U) Shokai Sho-61-175701 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) F22B 1 / 18

Claims (4)

(57) [Claims] 1. Among heat exchangers such as a superheater, a reheater, an evaporator and a economizer disposed in an exhaust gas passage, at least one heat exchanger excluding the evaporator has a large number of heat transfer tubes. In the exhaust heat recovery boiler, which is arranged in the width direction of the exhaust gas passage, and includes a heat transfer panel to which a header is connected at both ends of the heat transfer tube, at least a part of the heat transfer panel includes a heat transfer tube that extends through the exhaust gas passage. As the header connected to one end of the heat transfer tube, the inlet header and the outlet header having a temperature difference composed of separate pipes arranged on the same side and separated via a gap are connected to the divided transfer header. Each of the heat transfer panels is connected to a corresponding one of the heat pipes, and an intermediate header is connected to all the heat transfer pipes as a header connected to the other end.
The first heat transfer panel and the second heat transfer panel toward the flow side
And the heated fluid is supplied to the inlet of the first heat transfer panel.
Heat transfer tubes introduced from the header and connected to the inlet header.
, Via the intermediate header, connected to the outlet header
Through the tube and from the outlet header of the first heat transfer panel to the second
To the heat transfer panel entrance header
Pass through the connected heat transfer tube, pass through the intermediate header, and exit
Through a heat transfer tube connected to the second heat transfer panel.
By being discharged from the outlet header, the fluid to be heated repeats meandering in the exhaust gas passage.
Do not shift from downstream to upstream in the exhaust gas flow direction.
An exhaust heat recovery boiler characterized in that heat is exchanged with the waste gas. 2. The exhaust heat recovery according to claim 1, wherein a gas path preventing member is arranged between the heat transfer tubes corresponding to a gap between an inlet header and an outlet header of the heat transfer tubes. boiler. 3. The heat transfer panel according to claim 1, wherein the heat transfer panels are arranged in a plurality of stages in the exhaust gas flow direction, and the lengths of the inlet header and the outlet header of adjacent heat transfer panels are different from each other. An exhaust heat recovery boiler, wherein a gap between the header and the outlet header is opposed to an inlet header or an outlet header of a heat transfer panel disposed next to the header. 4. The economizer according to claim 1, wherein the heat exchanger is a economizer, wherein the economizer is disposed on the high-temperature side economizer disposed on the upstream side in the exhaust gas flow direction and on the downstream side. The low-temperature side economizer, and the heat transfer panel of the low-temperature economizer is composed of a combination of an ascending pipe and a descending pipe provided with the inlet header, the outlet header, and the intermediate header, Wherein the heat transfer panel comprises only a riser provided with an inlet lower header and an outlet upper header.