Three-extraction heating cold air preheating safety efficiency-improving system

Abstract

The utility model discloses a three-section extraction heating cold air preheating safety efficiency-improving system which comprises a high-pressure heater H1, a cold air heating unit and a steam-gas heater H2, three-section extraction steam of a steam turbine is connected with the high-pressure heater H1 through a pipeline of the steam-gas heater H2, a cold air inlet pipe FD2 and a heat exchange air outlet pipe FD4 are arranged on the steam-gas heater H2, and the cold air inlet pipe FD2 and the heat exchange air outlet pipe FD4 are connected with the high-pressure heater H1 through a pipeline of the steam-gas heater H2. The cold air heating unit is in heat exchange connection with a steam heater H2 through a cold air inlet pipe FD2 and a heat exchange air outlet pipe FD4, and furnace side cold air or hot air is used for cooling three-section extracted steam of a steam turbine; and safe operation of system equipment is ensured while the overall efficiency of the unit is improved.

Description

Technical Field

[0001] The utility model belongs to the technical field of thermal power plants, and particularly relates to a safety and efficiency improvement system for preheating cold air by triple extraction heating. Background Technique

[0002] After long-term operation of thermal power plants, serious tube blockage has occurred in some high-pressure heaters in thermal power plants, and even the overall energy consumption of the unit has been affected. It is necessary to replace the entire equipment to ensure the safe and efficient operation of the unit. After analysis, most of the high-pressure heaters with problems are the heaters corresponding to the first-stage extraction steam of the intermediate-pressure cylinder of the steam turbine. Since the superheated steam after once reheating re-enters the intermediate-pressure cylinder of the steam turbine to do work, the extraction steam temperature is still relatively high. However, the high-pressure feed water entering the high-pressure heater is just the feed water coming out of the feed water pump, and the temperature difference between the feed water temperature and the superheated steam of the steam turbine extraction steam is large, resulting in more damage to the heat exchange tubes.

[0003] In a conventional thermal power plant, the triple extraction steam of the steam turbine mostly directly enters the corresponding high-pressure heater to heat the high-pressure feed water, or after reducing a part of the superheat degree of the triple extraction steam by setting an external steam cooler with a partial high-pressure feed water flow, it continues to enter the corresponding high-pressure heater to heat the high-pressure feed water. Although this system solves the problem of the safe operation of the high-pressure heater, due to the throttling of the high-pressure feed water and the low overall efficiency of the steam turbine, the actual energy-saving effect is not significant. Content of the Utility Model

[0004] To solve the above technical problems, the utility model provides a safety and efficiency improvement system for preheating cold air by triple extraction heating.

[0005] The specific scheme is as follows:

[0006] A safety and efficiency improvement system for preheating cold air by triple extraction heating includes a high-pressure heater H1, a cold air heating unit, and a steam-gas heater H2. The triple extraction steam of the steam turbine is connected to the high-pressure heater H1 through the steam-gas heater H2 by pipelines. A cold air inlet pipe FD2 and a heat exchange outlet air pipe FD4 are arranged on the steam-gas heater H2. The cold air heating unit is heat exchange-connected to the steam-gas heater H2 through the cold air inlet pipe FD2 and the heat exchange outlet air pipe FD4.

[0007] The steam-gas heater H2 is an external steam-gas heater. An overheated steam inlet pipeline S1 and an overheated steam outlet pipeline S2 are arranged on the steam-gas heater H2. An electric shut-off valve GV1 and a steam extraction quick shut-off check valve RV1 are arranged on the overheated steam inlet pipeline S1. The triple extraction steam of the steam turbine is connected to the steam-gas heater H2 through the electric shut-off valve GV1 and the steam extraction quick shut-off check valve RV1 by pipelines. The steam-gas heater H2 is connected to the high-pressure heater H1 through the overheated steam outlet pipeline S2 by pipelines.

[0008] The high-pressure heater H1 is provided with a feed water pipe GG1, a water outlet pipe GG2, an emergency drain pipe SS1 and a normal drain pipe SS2. The inlet end of the feed water pipe GG1 is provided with a feed water pump, and the feed water pump is connected to the high-pressure heater H1 through the feed water pipe GG1. The outlet end of the water outlet pipe GG2 is provided with a next-stage high-pressure heater, and the high-pressure heater H1 is connected to the next-stage high-pressure heater through the water outlet pipe GG2. The emergency drain pipe SS1 is provided with an emergency drain electric shut-off valve GV2 and an emergency drain electric regulating valve RV2. The outlet end of the emergency drain pipe SS1 is provided with a drain expansion vessel, and the drain expansion vessel is connected to the high-pressure heater H1 through the emergency drain electric regulating valve RV2 and the emergency drain electric shut-off valve GV2. The normal drain pipe SS2 is provided with a normal drain electric shut-off valve GV3 and a normal drain electric regulating valve RV3. The outlet end of the normal drain pipe SS2 is provided with a deaerator, and the deaerator is connected to the high-pressure heater H1 through the normal drain electric regulating valve RV3 and the normal drain electric shut-off valve GV3.

[0009] A bypass cold air pipe FD3 is arranged between the cold air inlet pipe FD2 and the heat exchange outlet air pipe FD4. One end of the bypass cold air pipe FD3 is connected to the cold air inlet pipe FD2, and the other end of the bypass cold air pipe FD3 is connected to the heat exchange outlet air pipe FD4. The bypass cold air pipe FD3 is provided with a bypass shut-off valve V3 and a bypass regulating valve R2. The bypass shut-off valve V3 and the bypass regulating valve R2 are connected in series and then connected to the cold air heating unit.

[0010] The cold air heating unit includes a fan F, an air preheater H3 and a boiler B. The fan F is connected to the air preheater H3 through the cold air inlet pipe FD2 and the heat exchange outlet air pipe FD4. The fan F is also connected to the air preheater H3 through the bypass cold air pipe FD3. A hot air pipe FD5 is arranged between the air preheater H3 and the boiler B, and the air preheater H3 is connected to the boiler B through the hot air pipe FD5.

[0011] The fan F is provided with a fan inlet air duct FD1, a fan moving blade regulating mechanism R1 and a fan outlet electric shut-off valve V1. The fan inlet air duct FD1 is provided with an air filter IF and a silencer S. The air filter IF and the silencer S are arranged at the inlet end of the fan inlet air duct FD1. The fan F is connected to the cold air inlet pipe FD2 through the electric shut-off valve V1. The fan F is also connected to the bypass cold air pipe FD3 through the electric shut-off valve V1.

[0012] A secondary hot air pipe FD6 is also arranged at the outlet of the air preheater H3. The secondary hot air pipe FD6 is provided with a secondary hot shut-off valve V5, and the secondary hot air pipe FD6 is connected to the cold air inlet pipe FD2 through the secondary hot shut-off valve V5.

[0013] A cold air shut-off valve V2 is provided on the cold air inlet pipe FD2, and an outlet shut-off valve V4 is provided on the heat exchange outlet air pipe FD4. The cold air heating unit is connected to the cold air inlet pipe FD2 through the cold air shut-off valve V2, and the cold air heating unit is connected to the heat exchange outlet air pipe FD4 through the outlet shut-off valve V4. One end of the bypass cold air pipe FD3 is connected to the cold air inlet pipe FD2 through the cold air shut-off valve V2, and the other end of the bypass cold air pipe FD3 is connected to the heat exchange outlet air pipe FD4 through the outlet shut-off valve V4.

[0014] The utility model discloses a three-extraction heating cold air preheating safety and efficiency improvement system. By adding a new external steam-gas heater H2, the temperature of the third-stage extraction steam of the steam turbine is appropriately reduced, reducing the thermal stress deviation between the heated feed water and the superheated steam, thereby ensuring the safe and stable operation of the downstream high-pressure heater H1; moreover, by adding a new external steam-gas heater H2 to heat the cold air temperature on the boiler side, the cold air temperature entering the air preheater H3 can be increased, thereby effectively alleviating the ammonium bisulfate blockage of the air preheater and avoiding the low-temperature corrosion of the equipment; in addition, by adding a new external steam-gas heater H2 to heat the temperature of the hot secondary air on the boiler side, not only can the boiler combustion efficiency be improved, which is beneficial to the stable combustion of the unit at low load, but also the explosion-proof risk of the coal powder feeding system caused by heating the temperature of the primary hot air can be avoided; using the cold air or hot air on the furnace side to cool the third-stage extraction steam of the steam turbine; while improving the overall efficiency of the unit, ensuring the safe operation of the system equipment. Description of the Drawings

[0015] Figure 1 is a structural schematic diagram of the utility model.

[0016] Figure 2 is a structural schematic diagram of this structure with heating the secondary heat on the boiler side.

[0017] Among them, H1 is the high-pressure heater, H2 is the steam-gas heater, H3 is the air preheater, B is the boiler, F is the fan, S is the silencer, IF is the air filter, S1 is the superheated steam inlet pipe, S2 is the superheated steam outlet pipe, SS1 is the emergency drain pipe, SS2 is the normal drain pipe, GG1 is the water inlet pipe, GG2 is the water outlet pipe, FD1 is the fan inlet air duct, FD2 is the cold air inlet pipe, FD3 is the bypass cold air pipe, FD4 is the heat exchange outlet air pipe, FD5 is the hot air pipe, FD6 is the secondary heat air pipe, GV1 is the electric shut-off valve, RV1 is the extraction quick-closing check valve, GV2 is the emergency drain electric shut-off valve, RV2 is the emergency drain electric regulating valve, GV3 is the normal drain electric shut-off valve, RV3 is the normal drain electric regulating valve, R1 is the fan moving blade regulating mechanism, V1 is the fan outlet electric shut-off valve, V2 is the cold air shut-off valve, V3 is the bypass shut-off valve, R2 is the bypass regulating valve, V4 is the outlet shut-off valve, V5 is the secondary heat shut-off valve. Specific embodiments

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0019] A three-extraction heating cold-air preheating safety and efficiency improvement system specifically solves the problems of the safe operation and efficiency improvement of high-pressure heater equipment. In this embodiment, cold air or hot air on the furnace side is used to cool the three-stage extraction steam of the steam turbine; while improving the overall efficiency of the unit, the safe operation of the system equipment is ensured.

[0020] As Figure 1 shown, a three-extraction heating cold-air preheating safety and efficiency improvement system includes a high-pressure heater H1, a cold-air heating unit, and a steam-gas heater H2. The three-stage extraction steam of the steam turbine is connected to the high-pressure heater H1 through the steam-gas heater H2 by pipelines. A cold-air inlet pipe FD2 and a heat exchange outlet pipe FD4 are provided on the steam-gas heater H2. The cold-air heating unit is heat-exchange connected to the steam-gas heater H2 through the cold-air inlet pipe FD2 and the heat exchange outlet pipe FD4.

[0021] The steam-gas heater H2 is an external steam-gas heater. A superheated steam inlet pipe S1 and a superheated steam outlet pipe S2 are provided on the steam-gas heater H2. An electric shut-off valve GV1 and a stop check valve RV1 for rapid shut-off of extraction steam are provided on the superheated steam inlet pipe S1. The three-stage extraction steam of the steam turbine is connected to the steam-gas heater H2 through the electric shut-off valve GV1 and the stop check valve RV1 for rapid shut-off of extraction steam. The steam-gas heater H2 is connected to the high-pressure heater H1 through the superheated steam outlet pipe S2 by pipelines.

[0022] The high-pressure heater H1 is the No. 3 high-pressure heater. In this embodiment, an external steam-gas heater H2 is added between the three-stage extraction steam of the steam turbine and the original No. 3 high-pressure heater H1 of the steam turbine. The superheated steam outlet pipe S2 with reduced temperature after passing through the steam-gas heater H2 is sent to the No. 3 high-pressure heater H1. That is, after appropriately reducing the temperature of the three-extraction steam, the thermal stress deviation between the No. 3 high-pressure heater and the high-pressure feed water temperature on the water side is appropriately reduced to ensure the safe operation of the No. 3 high-pressure heater H1; the electric shut-off valve GV1 and the stop check valve RV1 for rapid shut-off of extraction steam of the three-stage extraction steam of the steam turbine ensure the safe operation of the steam turbine.

[0023] The high-pressure heater H1 is provided with a feed water inlet pipe GG1, a water outlet pipe GG2, an emergency drain pipe SS1 and a normal drain pipe SS2. A feed water pump is arranged at the water inlet end of the feed water inlet pipe GG1. The feed water pump is connected to the high-pressure heater H1 through the feed water inlet pipe GG1. The water outlet end of the water outlet pipe GG2 is provided with a next-stage high-pressure heater. The high-pressure heater H1 is connected to the next-stage high-pressure heater through the water outlet pipe GG2. An emergency drain electric shut-off valve GV2 and an emergency drain electric regulating valve RV2 are arranged on the emergency drain pipe SS1. A drain flash tank is arranged at the outlet end of the emergency drain pipe SS1. The drain flash tank is connected to the high-pressure heater H1 through the emergency drain electric regulating valve RV2 and the emergency drain electric shut-off valve GV2. A normal drain electric shut-off valve GV3 and a normal drain electric regulating valve RV3 are arranged on the normal drain pipe SS2. An deaerator is arranged at the outlet end of the normal drain pipe SS2. The deaerator is connected to the high-pressure heater H1 through the normal drain electric regulating valve RV3 and the normal drain electric shut-off valve GV3.

[0024] In this embodiment, the superheated steam cooled by the steam-gas heater H2 will enter the 3rd high-pressure heater H1 through the superheated steam outlet pipe S2 to heat the high-pressure feed water sent by the feed water pump through the feed water inlet pipe GG1. The heated high-pressure feed water is sent to the next-stage high-pressure heater through the water outlet pipe GG2.

[0025] The superheated steam after heat exchange with the high-pressure water will be cooled. The cooled superheated steam becomes condensate after being cooled by the 3rd high-pressure heater H1, and then enters the deaerator through the normal drain pipe SS2, the normal drain electric shut-off valve GV3 and the normal drain electric regulating valve RV3 to continue the thermodynamic cycle; in critical moments or at the initial stage of turbine startup, the condensate enters the drain flash tank through the emergency drain pipe SS1, the emergency drain electric shut-off valve GV2 and the emergency drain electric regulating valve RV2 and is recovered and then enters the condensate system.

[0026] A bypass cold air pipe FD3 is arranged between the cold air inlet pipe FD2 and the heat exchange outlet air pipe FD4. One end of the bypass cold air pipe FD3 is connected to the cold air inlet pipe FD2, and the other end of the bypass cold air pipe FD3 is connected to the heat exchange outlet air pipe FD4. A bypass shut-off valve V3 and a bypass regulating valve R2 are arranged on the bypass cold air pipe FD3. The bypass shut-off valve V3 and the bypass regulating valve R2 are connected in series to the cold air heating unit after being connected in series. The bypass shut-off valve V3 is used for equipment isolation, and the bypass regulating valve R2 is used for adjusting the cold air temperature or for use in case of equipment failure.

[0027] The cold air heating unit includes a fan F, an air preheater H3, and a boiler B. The fan F is connected to the air preheater H3 through a cold air inlet pipe FD2 and a heat exchange outlet pipe FD4. The fan F is also connected to the air preheater H3 through a bypass cold air pipe FD3. A hot air pipe FD5 is provided between the air preheater H3 and the boiler B, and the air preheater H3 is connected to the boiler B through the hot air pipe FD5. The cold air after heat exchange with the steam-gas heater H2 passes through the air preheater H3 and the hot air pipe FD5, and finally enters the boiler B for combustion support, reducing the combustion loss of the boiler.

[0028] The fan F is used to increase the cold air head and overcome the system resistance. An air inlet duct FD1, a moving vane regulating mechanism R1 of the fan, and an electric shut-off valve V1 at the fan outlet are provided on the fan F. An air filter IF and a silencer S are provided on the air inlet duct FD1 of the fan. The air filter IF and the silencer S are provided at the air inlet end of the air inlet duct FD1 of the fan. The air filter IF is used to purify the air, and the silencer S is used to reduce the operating noise. The fan F is connected to the cold air inlet pipe FD2 through the electric shut-off valve V1, and the fan F is also connected to the bypass cold air pipe FD3 through the electric shut-off valve V1.

[0029] The moving vane regulating mechanism R1 of the fan belongs to the prior art for those skilled in the art. It was written by Cheng Gang, and pages 405 to 406 of the book "Power Plant Centralized Control Operation" published by China Electric Power Press in January 2004 record the moving vane regulating mechanism of the fan.

[0030] A cold air shut-off valve V2 is provided on the cold air inlet pipe FD2, and an outlet shut-off valve V4 is provided on the heat exchange outlet pipe FD4. The cold air heating unit is connected to the cold air inlet pipe FD2 through the cold air shut-off valve V2, and the cold air heating unit is connected to the heat exchange outlet pipe FD4 through the outlet shut-off valve V4. One end of the bypass cold air pipe FD3 is connected to the cold air inlet pipe FD2 through the cold air shut-off valve V2, and the other end of the bypass cold air pipe FD3 is connected to the heat exchange outlet pipe FD4 through the outlet shut-off valve V4.

[0031] As Figure 2 shown, a secondary hot air pipe FD6 is also provided at the outlet of the air preheater H3. A secondary hot air shut-off valve V5 is provided on the secondary hot air pipe FD6. The secondary hot air pipe FD6 is connected to the cold air inlet pipe FD2 through the secondary hot air shut-off valve V5.

[0032] The specific working process of the three-extraction heating cold air preheating safety and efficiency improvement system is as follows:

[0033] The third-stage extraction steam of the steam turbine first enters the steam-gas heater H2 for heat exchange and temperature reduction, and then continues to enter the high-pressure heater H1 through the pipeline to heat the original high-pressure feed water;

[0034] The steam-gas heater H2 can also heat the cold air temperature on the boiler side. Since the primary cold air has a relatively small air volume during boiler combustion, it generally does not need to be adjusted and does not pass through the bypass regulating air; that is, the primary cold air enters the steam-gas heater H2 through the cold air inlet pipe FD2. After heat exchange with the steam, the temperature of the primary cold air increases and then enters the air preheater H3 through the heat exchange outlet air pipe FD4.

[0035] When heating the cold secondary air of the boiler, since the cold air volume of the boiler is large, the bypass regulating air needs to be opened. That is, a part of the secondary cold air enters the steam-gas heater H2 through the cold air inlet pipe FD2. After heat exchange with the steam, the temperature of the secondary cold air increases. Another part of the secondary cold air enters the air preheater H3 through the bypass cold air pipe FD3. The two parts of the secondary cold air are mixed and then enter the air preheater H3, ensuring the safe operation of the equipment on the steam side while utilizing the heat.

[0036] Although the increase in the cold air temperature helps to weaken the blockage of the air preheater H3 caused by ammonium bisulfate, at the same time, attention should be paid to the flue gas temperature after the air preheater H3 to avoid excessive increase in the flue gas temperature, resulting in an increase in the exhaust gas loss. Generally, a low-temperature economizer can be set in the flue after the furnace.

[0037] Since the superheat degree of the three-stage extraction steam is relatively large and the temperature is much higher than the hot secondary air temperature of the boiler, in this embodiment, the hot secondary air at the outlet of the air preheater can also be directly heated. That is, the hot secondary air enters the steam-gas heater H2 through the secondary hot air pipe FD6 and the cold air inlet pipe FD2 for heat exchange, as Figure 2 shown. It is preliminarily estimated that the hot air temperature can be increased by about 10°C, thereby more effectively utilizing the superheat degree of the extraction steam, obtaining a higher combustion efficiency, and avoiding the increase in the flue gas temperature at the outlet of the air preheater.

[0038] The technical means disclosed in the solution of the present utility model are not limited to the technical means disclosed in the above embodiments, but also include the technical solutions composed of any combination of the above technical features. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of the present utility model, several improvements and refinements can be made, and these improvements and refinements are also regarded as the protection scope of the present utility model.

Claims

1. A three-extraction heating cold air preheating safety and efficiency improvement system, characterized in that: It comprises a high-pressure heater (H1), a cold air heating unit and a steam-gas heater (H2); the three-stage steam extraction of the steam turbine is connected to the high-pressure heater (H1) through a pipeline of the steam-gas heater (H2); a cold air inlet pipe (FD2) and a heat exchange air outlet pipe (FD4) are provided on the steam-gas heater (H2); the cold air heating unit is connected to the steam-gas heater (H2) through the cold air inlet pipe (FD2) and the heat exchange air outlet pipe (FD4) for heat exchange.

2. The three-extraction heating cold air preheating safety and efficiency improvement system according to claim 1 is characterized by: The steam-gas heater (H2) is an external steam-gas heater. The steam-gas heater (H2) is provided with a superheated steam inlet pipeline (S1) and a superheated steam outlet pipeline (S2). The superheated steam inlet pipeline (S1) is provided with an electric shut-off valve (GV1) and a steam extraction fast-closing check valve (RV1). The three-stage steam extraction of the steam turbine is connected to the steam-gas heater (H2) pipeline via the electric shut-off valve (GV1) and the steam extraction fast-closing check valve (RV1). The steam-gas heater (H2) is connected to the high-pressure heater (H1) pipeline via the superheated steam outlet pipeline (S2).

3. The three-extraction heating cold air preheating safety and efficiency improvement system according to claim 1 is characterized by: The high-pressure heater (H1) is provided with an inlet pipe (GG1), an outlet pipe (GG2), an emergency drain pipe (SS1) and a normal drain pipe (SS2); a water supply pump is provided at the water inlet end of the inlet pipe (GG1), and the water supply pump is connected to the high-pressure heater (H1) through the inlet pipe (GG1); a high-pressure heater of the next stage is provided at the water outlet end of the outlet pipe (GG2), and the high-pressure heater (H1) is connected to the high-pressure heater of the next stage through the outlet pipe (GG2); an emergency drain electric shut-off valve (GV2) and an emergency drain electric shut-off valve (GV2) are provided on the emergency drain pipe (SS1); A regulating valve (RV2), a drain expansion tank is arranged at the outlet end of the emergency drain pipeline (SS1), and the drain expansion tank is connected to the high-pressure heater (H1) through the emergency drain electric regulating valve (RV2) and the emergency drain electric shut-off valve (GV2), a normal drain electric shut-off valve (GV3) and a normal drain electric regulating valve (RV3) are arranged on the normal drain pipeline (SS2), and a deaerator is arranged at the outlet end of the normal drain pipeline (SS2), and the deaerator is connected to the high-pressure heater (H1) through the normal drain electric regulating valve (RV3) and the normal drain electric shut-off valve (GV3).

4. The three-extraction heating cold air preheating safety and efficiency improvement system according to claim 1 is characterized by: A bypass cold air duct (FD3) is arranged between the cold air inlet pipe (FD2) and the heat exchange air outlet pipe (FD4); one end of the bypass cold air duct (FD3) is connected to the cold air inlet pipe (FD2), and the other end of the bypass cold air duct (FD3) is connected to the heat exchange air outlet pipe (FD4); a bypass shut-off valve (V3) and a bypass regulating valve (R2) are arranged on the bypass cold air duct (FD3); the bypass shut-off valve (V3) and the bypass regulating valve (R2) are connected in series and connected to the cold air heating unit.

5. The three-extraction heating cold air preheating safety and efficiency improvement system according to claim 4 is characterized by: The cold air heating unit comprises a fan (F), an air preheater (H3) and a boiler (B); the fan (F) is connected to the air preheater (H3) pipeline via a cold air inlet pipe (FD2) and a heat exchange air outlet pipe (FD4); the fan (F) is also connected to the air preheater (H3) pipeline via a bypass cold air pipe (FD3); a hot air pipe (FD5) is provided between the air preheater (H3) and the boiler (B); the air preheater (H3) is connected to the boiler (B) via the hot air pipe (FD5).

6. The three-extraction heating cold air preheating safety and efficiency improvement system according to claim 5 is characterized by: The fan (F) is provided with a fan air inlet duct (FD1), a fan blade adjustment mechanism (R1) and a fan outlet electric shut-off valve (V1); the fan air inlet duct (FD1) is provided with an air filter (IF) and a silencer (S); the air filter (IF) and the silencer (S) are arranged at the air inlet end of the fan air inlet duct (FD1); the fan (F) is connected to a cold air inlet pipe (FD2) through the fan outlet electric shut-off valve (V1); the fan (F) is also connected to a bypass cold air pipe (FD3) through the fan outlet electric shut-off valve (V1).

7. The three-extraction heating cold air preheating safety and efficiency improvement system according to claim 5 is characterized by: The outlet of the air preheater (H3) is also provided with a secondary hot air pipe (FD6), on which a secondary thermal shut-off valve (V5) is provided, and the secondary hot air pipe (FD6) is connected to the cold air inlet pipe (FD2) via the secondary thermal shut-off valve (V5).

8. The three-extraction heating cold air preheating safety and efficiency improvement system according to claim 4 is characterized by: The cold air inlet pipe (FD2) is provided with a cold air shut-off valve (V2), and the heat exchange outlet pipe (FD4) is provided with an outlet shut-off valve (V4); the cold air heating unit is connected to the cold air inlet pipe (FD2) via the cold air shut-off valve (V2), and the cold air heating unit is connected to the heat exchange outlet pipe (FD4) via the outlet shut-off valve (V4); one end of the bypass cold air duct (FD3) is connected to the cold air inlet pipe (FD2) via the cold air shut-off valve (V2), and the other end of the bypass cold air duct (FD3) is connected to the heat exchange outlet pipe (FD4) via the outlet shut-off valve (V4).

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