Ultra-high temperature exhaust heat recovery system and operation control method

Abstract

This invention discloses an ultra-high temperature rise waste heat recovery system and its operation control method, including an electric heat pump, a flash tank, and a multi-stage steam compressor unit. A low-temperature heat source is connected to the cold end of the electric heat pump, and the hot end of the electric heat pump is connected to the inlet of the flash tank. The multi-stage steam compressor unit consists of several steam compressors connected in series. The steam outlet of the flash tank is connected to the inlet of the first-stage steam compressor, and the exhaust end of the last-stage steam compressor is connected to an output pipeline. An anti-surge backflow pipeline is installed between the exhaust end of the last-stage steam compressor and the inlet end of the first-stage steam compressor, and an anti-surge backflow valve is installed on the anti-surge backflow pipeline. An outlet vent valve is installed on the output pipeline. This invention provides a waste heat recovery system consisting of an electric heat pump, a flash tank, and a centrifugal steam compressor connected in series, which can significantly increase the pressure and temperature of the produced high-pressure steam compared to the original low-temperature waste heat, achieving a large temperature rise and upgraded recovery of the original low-temperature waste heat.

Description

Technical Field

[0001] This invention relates to the field of waste heat recovery technology, and in particular to an ultra-high temperature rise waste heat recovery system and its operation control method. Background Technology

[0002] In industrial production processes such as chemical, pharmaceutical, food, textile, and new energy industries, a large amount of low-grade waste heat resources with a temperature of 60-70℃ are generated, including low-temperature exhaust steam, low-temperature hot water, and low-temperature materials. Efficiently converting this low-grade heat energy into saturated steam with a pressure of 300-400 kPa·A and a temperature of 130-140℃, which can be directly used in production processes, is one of the core technological requirements for achieving energy conservation, emission reduction, and lower production costs in the industrial sector.

[0003] Currently, the mainstream technologies for producing process steam using low-temperature waste heat include heat pump technology, negative pressure flash evaporation technology, and mechanical vapor recompression (MVR) technology. Among these, heat pumps can improve the heat energy grade through working fluid circulation, negative pressure flash evaporation can convert hot water into low-pressure steam, and MVR uses a compressor to pressurize and heat the steam. However, existing technologies still have several shortcomings in practical applications:

[0004] First, the temperature rise capability of a single technology route has a clear upper limit. Due to the thermodynamic characteristics of the heat pump working fluid and the pressure ratio limitations of single-stage or two-stage compressors, the total saturated temperature rise of existing single recovery systems from the low-temperature heat source inlet to the high-pressure steam outlet is usually no more than 40-50°C, which is difficult to meet the process requirements with a large temperature rise range. For example, the upper limit of the pressure ratio of conventional single-stage or two-stage centrifugal compressors is only 4-5, which cannot directly compress the negative pressure flash steam of 45-60 kPa.A to the process steam pressure of 300-400 kPa.A; the energy efficiency ratio, saturated temperature rise, and maximum output steam pressure of the heat pump system also have upper limits, and the operating efficiency and economy decrease significantly under large temperature rise conditions.

[0005] Secondly, simple series-connected multi-device combinations present challenges in startup control and equipment redundancy. If the heat pump, flash evaporator, and compressor are directly connected in series, two types of faults are highly likely to occur during system startup: first, insufficient steam production before a stable negative pressure is established in the flash tank leads to insufficient compressor intake and triggers surge; second, a mismatch between steam production and exhaust rates causes uncontrolled overpressure in the flash tank. Therefore, existing series systems typically require an additional independent vacuum device for the flash tank to establish a negative pressure environment during startup, increasing equipment investment, system complexity, and maintenance costs.

[0006] Third, the reliability and energy efficiency are poor under high-pressure-ratio compression conditions. For multi-stage compression systems, if the pressure ratio distribution of each stage is unreasonable and there is a lack of effective interstage temperature control, it will lead to excessive single-stage pressure ratio and uncontrolled interstage overheating. This will not only significantly reduce the variable efficiency of the compressor and increase compression power consumption, but also increase the risk of surge, aggravate equipment heat loss, shorten equipment service life, and make it difficult to adapt to high-temperature-rise waste heat recovery conditions in the long term.

[0007] Fourth, the adaptability to operating conditions and compatibility with the working fluid are insufficient. The operation and control logic of the existing waste heat recovery system is imperfect. When faced with fluctuations in waste heat load during the production process, it is difficult to quickly stabilize the pressure and temperature parameters at various points in the system, which can easily lead to problems such as operating parameters exceeding limits or equipment mismatch. In addition, some low-temperature waste heat working fluids contain corrosive impurities. If they directly enter the compression unit, they will cause corrosion to the equipment. The existing system requires additional anti-corrosion treatment, which further increases the system cost and complexity.

[0008] In summary, existing low-temperature waste heat recovery technologies have shortcomings such as insufficient total temperature rise capacity, low operating efficiency under high temperature rise conditions, complex start-up control requiring additional vacuum equipment, and poor adaptability to operating conditions. They cannot simultaneously meet the requirements of high temperature rise, high energy efficiency, and high reliability in waste heat recovery. Summary of the Invention

[0009] The purpose of this invention is to address the shortcomings of the prior art by providing an ultra-high temperature rise waste heat recovery system and its operation control method.

[0010] The objective of this invention is achieved through the following technical solution: an ultra-high temperature waste heat recovery system, comprising an electric heat pump, a flash tank, and a multi-stage steam compressor unit. A low-temperature heat source is connected to the cold end of the electric heat pump, and the hot end of the electric heat pump is connected to the water inlet of the flash tank. The multi-stage steam compressor unit consists of several stages of steam compressors connected in series. Adjacent stages of steam compressors are connected by interstage pipelines, and interstage desuperheating devices are installed on the interstage pipelines. The steam outlet of the flash tank is connected to the steam inlet of the first-stage steam compressor, and the exhaust end of the last-stage steam compressor is connected to an output pipeline. An anti-surge backflow pipeline is installed between the exhaust end of the last-stage steam compressor and the steam inlet of the first-stage steam compressor, and an anti-surge backflow valve is installed on the anti-surge backflow pipeline. An outlet vent valve and a steam supply valve are installed on the output pipeline.

[0011] Preferably, each stage of the steam compressor is equipped with an inlet temperature detection unit at the inlet end, and each stage of the steam compressor is equipped with an exhaust temperature detection unit and an exhaust pressure detection unit at the exhaust end. The first stage of the steam compressor is also equipped with an inlet pressure detection unit at the inlet end. An output desuperheating device and an output pipeline temperature detection unit are installed on the output pipeline, with the output pipeline temperature detection unit located downstream of the output desuperheating device.

[0012] Preferably, the anti-surge return pipeline is equipped with a return cooling device.

[0013] Preferably, the multi-stage steam compressor unit includes a three-stage centrifugal steam compressor. The pressure ratio of each stage of the centrifugal steam compressor is distributed according to the principle of thermodynamic ladder matching, and the total pressure ratio of the multi-stage steam compressor unit is 5-8. The interstage desuperheating device between each stage of the steam compressor controls the steam superheat at the inlet of the subsequent stage steam compressor to 5-10℃.

[0014] An operation control method for an ultra-high temperature rise waste heat recovery system includes the following specific steps: S1: Start the electric heat pump and pass a low-temperature heat source into it. After absorbing the heat from the low-temperature heat source, the electric heat pump produces hot water with a preset temperature range at the hot end. The hot water is then fed into the flash tank. S2: The multi-stage steam compressor unit runs to the preset low speed state, the anti-surge backflow valve maintains the preset opening state, the opening of the outlet vent valve is adjusted, and the pressure ratio generated by the low speed operation of the compressor is used to create a negative pressure in the flash tank, and the hot water in the flash tank flashes to generate low-pressure saturated steam. S3: Control the steam compressor to continuously increase its speed. During the speed increase, the pressure at the steam inlet of the first-stage steam compressor is used as the control target. Control the electric heat pump to reach the preset operating load state. Based on the operating load state of the electric heat pump and the pressure before and after the outlet vent valve, calculate the target opening of the outlet vent valve to keep the flash steam production and the steam venting amount of the outlet vent valve in balance, thereby maintaining the pressure in the flash tank stable within the target flash pressure range. S4: When the speed of the steam compressor increases to the preset switching speed, the opening of the outlet vent valve is adjusted with the pressure at the exhaust end of the last stage steam compressor as the control target, so that the pressure at the exhaust end of the last stage steam compressor is stabilized within the preset exhaust pressure range. S5: After the steam compressor speed reaches the rated operating speed range, first increase the operating load of the electric heat pump and increase the steam production of the flash tank, and then gradually reduce the opening of the anti-surge backflow valve to maintain the stability of the steam compressor inlet pressure and steam volume. S6: After the anti-surge return valve is completely closed, reduce the opening of the outlet vent valve to increase the compressor outlet pressure. When the steam pressure at the exhaust end of the final stage steam compressor reaches the target steam delivery pressure, open the steam delivery valve and deliver steam to the outside.

[0015] Preferably, in step S3, the target opening degree of the outlet vent valve is determined in the following way: Based on the preset operating load state, the theoretical steam production of the current electric heat pump is obtained, and the measured values ​​of the pressure before and after the outlet vent valve are obtained to obtain the pressure difference across the outlet vent valve. Based on the pressure difference across the outlet vent valve and the theoretical steam production, the target opening of the outlet vent valve is obtained through the opening-flow-pressure difference characteristic curve.

[0016] As a preferred approach, the target opening obtained based on the opening-flow-pressure difference characteristic curve is used as the feedforward control reference, while the measured pressure value at the inlet of the first-stage steam compressor is used as the feedback signal, and the opening of the outlet vent valve is corrected by the PID algorithm.

[0017] As a preferred option, during operation, the exhaust temperature and interstage pressure of each stage of the steam compressor are monitored in real time. The interstage desuperheating device adopts water spray desuperheating. The interstage desuperheating device calculates the corresponding steam saturation temperature based on the interstage pressure, and uses steam superheat as the controlled variable. The water spray volume is adjusted in real time through a PID algorithm to stabilize the steam superheat at the inlet of the subsequent steam compressor at 5-10℃.

[0018] Preferably, in step S2, the preset low-speed state is when the steam compressor speed is less than 50% of the rated speed; in step S3, the target flash pressure range is 45-60 kPa.A.

[0019] Preferably, during the speed-up process of the steam compressor, the opening degree of the anti-surge backflow valve is controlled in segments according to the speed range: When the steam compressor speed is 0-20Hz, the anti-surge backflow valve remains fully open; When the steam compressor speed is 20-30Hz, the opening of the anti-surge backflow valve decreases linearly from 100% to 80% as the speed increases; When the steam compressor speed is 30-45Hz, the opening of the anti-surge backflow valve decreases linearly from 80% to 60% as the speed increases; When the speed of the steam compressor is 45-50Hz, the ratio of the compressor outlet pressure to the inlet pressure is used as the real-time pressure ratio, and the ratio of the steam flow rate to the pressure at the inlet of the first-stage steam compressor is used as the real-time converted flow rate parameter. Based on the real-time pressure ratio and the preset pressure ratio-converted flow rate anti-surge safety curve, the theoretical converted flow rate parameter of the anti-surge margin is calculated. The difference between the real-time converted flow rate parameter and the theoretical converted flow rate parameter is taken as the anti-surge margin. Based on the PID algorithm, the opening of the anti-surge return valve is adjusted in a closed loop using the anti-surge margin as the controlled variable.

[0020] The beneficial effects of this invention are: 1. This invention provides a waste heat recovery system consisting of an electric heat pump, a flash tank, and a centrifugal steam compressor connected in series. This system significantly increases the pressure and temperature of the produced high-pressure steam compared to the original low-temperature waste heat. The produced steam can be reused as a high-grade heat source for various production processes, achieving a significant temperature rise and upgrading of the original low-temperature waste heat recovery. This effectively solves the problems of direct emission and waste of various low-temperature waste heat. Compared to traditional waste heat recovery methods, this system can upgrade a 60-70°C low-temperature heat source to 130-140°C saturated steam, with a total temperature rise exceeding 70°C, achieving an ultra-high temperature rise, far superior to the existing MVR system which can only achieve a maximum temperature rise of 40-50°C.

[0021] 2. The design pressure ratio of the multi-stage steam compressor unit can reach 5-8. When the compressor outlet vent valve is opened, the outlet back pressure is about 100 kPa.A. When the steam compressor pressure ratio reaches about 2, the inlet can reach the pressure required for flash evaporation (45-60 kPa.A). Therefore, when the compressor starts, it can run at low speed and, with the outlet anti-surge valve at a large opening, achieve the effect of vacuuming the flash tank and establishing negative pressure flash evaporation pressure without the need for a special vacuuming device.

[0022] 3. During system startup, targeted calculations ensure the electric heat pump operates at a low load, maintaining a balance between flash steam production and venting. The venting valve is adjusted to stabilize pressure throughout the system. Once the steam compressor reaches its operating speed, the electric heat pump is gradually loaded, the anti-surge backflow valve and outlet venting valve are closed, ultimately enabling external steam delivery. This control sequence fundamentally avoids the defects of traditional startup methods, such as "surge due to insufficient compressor intake" or "safety valve tripping due to flash tank overpressure." It also reduces the need for a separate flash vacuum system, significantly improving system startup success rate and automation level.

[0023] 4. The multi-stage centrifugal steam compressor allocates the pressure ratio of each stage according to the principle of cascade and sets up interstage cooling devices, so that each stage operates in the high-efficiency range, effectively avoiding the low efficiency and surge risk under single-stage pressure ratio exceeding the limit and double-stage high pressure ratio, and improving the safety and reliability of operation under high temperature rise conditions. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the pipeline connection of the ultra-high temperature rise waste heat recovery system of the present invention.

[0025] In the diagram: 1. Electric heat pump, 1a. Cold end, 1b. Hot end, 2. Flash tank, 2a. Water level monitoring device, 3. Flash tank output pressure detection unit, 4. Gas-liquid separator, 5. Interstage piping, 6. Anti-surge reflux piping, 7. Multistage steam compressor unit, 7a. Steam compressor, 8. Interstage desuperheating device, 9. Exhaust end pressure detection unit, 10. Exhaust end temperature detection unit, 11. Inlet end temperature detection unit, 12. Buffer tank, 13. Reflux desuperheating device, 14. Anti-surge reflux valve, 15. Inlet end pressure detection unit, 16. Output piping, 17. Output desuperheating device, 18. Outlet vent valve, 19. Output piping temperature detection unit, 20. Steam supply valve. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0027] Those skilled in the art should understand that, in the disclosure of this invention, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the above terms should not be construed as limiting this invention.

[0028] It is understood that the term "a" should be understood as "at least one" or "one or more", that is, in one embodiment, the number of an element can be one, while in another embodiment, the number of the element can be multiple, and the term "a" should not be understood as a limitation on the number.

[0029] like Figure 1As shown, an ultra-high temperature waste heat recovery system includes an electric heat pump 1, a flash tank 2, and a multi-stage steam compressor unit 7. A low-temperature heat source is connected to the cold end 1a of the electric heat pump 1, and the hot end 1b of the electric heat pump 1 is connected to the water inlet of the flash tank 2. The multi-stage steam compressor unit 7 consists of several stages of steam compressors 7a connected in series. Adjacent stages of steam compressors 7a are connected by an interstage pipeline 5, and an interstage desuperheating device is installed on the interstage pipeline 5. The steam outlet of the flash tank 2 is connected to the steam inlet of the first-stage steam compressor 7a, and the exhaust end of the last-stage steam compressor 7a is connected to an output pipeline 16. An anti-surge backflow pipeline 6 is installed between the exhaust end of the last-stage steam compressor 7a and the steam inlet end of the first-stage steam compressor 7a. An anti-surge backflow valve 14 is installed on the anti-surge backflow pipeline 6, and an outlet vent valve 18 and a steam supply valve 20 are installed on the output pipeline 16.

[0030] The overall operating principle of the ultra-high temperature waste heat recovery system is as follows: The low-temperature heat source first enters the cold end 1a of the electric heat pump 1. After the electric heat pump 1 absorbs its heat, high-temperature hot water of about 90°C is produced at the hot end 1b and enters the flash tank 2. Under the working state, the flash tank 2 is under a negative pressure of 45-60 kPa.A. Under the negative pressure condition, the hot water flashes to produce low-pressure saturated steam of 80-85°C. The hot water with a lower temperature after flashing returns to the electric heat pump 1 to absorb heat again, realizing the cycle. The generated low-temperature saturated steam is pressurized to a higher pressure and temperature by the multi-stage steam compressor unit 7 to form finished steam, which is output to the target user through the output pipeline 16 to realize the recovery and utilization of heat.

[0031] Among them, the electric heat pump 1 is essentially a heat lifting device driven by electric energy and operating in a reverse Carnot cycle. It is the core heat lifting component of this ultra-high temperature rise waste heat recovery system. It is located at the forefront of the entire heat recovery chain and undertakes the key functions of "absorbing low-grade waste heat and raising the temperature to produce high-temperature hot water". It is the first-level power source for the entire ultra-high temperature rise waste heat recovery system to achieve "ultra-high temperature rise" (total saturated temperature rise exceeding 70°C).

[0032] The flash tank 2 is equipped with a water level monitoring device 2a. By monitoring the liquid level and replenishing water in a timely manner, the liquid level in the flash tank 2 is controlled within a reasonable range, thus maintaining the stability of the hot water circulation in the tank.

[0033] A gas-liquid separator 4 is installed between the flash tank 2 and the steam inlet of the primary steam compressor 7a. The gas-liquid separator 4 separates the low-pressure saturated steam produced by the negative pressure flash evaporation into gas and liquid, removes liquid water droplets entrained in the steam, and ensures the safe and stable operation of the multi-stage steam compressor 7a.

[0034] Each stage of steam compressor 7a is equipped with an inlet temperature detection unit 11 at the steam inlet end, and each stage of steam compressor 7a is equipped with an exhaust temperature detection unit 10 and an exhaust pressure detection unit 9 at the exhaust end. The first stage steam compressor 7a is also equipped with an inlet pressure detection unit 15 at the steam inlet end. The output pipeline 16 is equipped with an output desuperheating device 17 and an output pipeline temperature detection unit 19, with the output pipeline temperature detection unit 19 located downstream of the output desuperheating device 17.

[0035] By setting up an inlet pressure detection unit 15 and an outlet pressure detection unit, the actual pressure ratio of each stage of the steam compressor 7a can be calculated in real time. At the same time, by monitoring the interstage pressure, abnormal pressure fluctuations can serve as early warning signals for surge and pipeline blockage faults, thereby improving the fault response speed.

[0036] The temperature detection unit can serve as a feedforward control input for the interstage desuperheating device. By combining the saturation temperature corresponding to the exhaust pressure, it can accurately calculate the required desuperheating range, adjust the interstage desuperheating water volume in advance, and improve the response speed of temperature control. On the other hand, it can monitor the risk of compressor exhaust overheating in real time. Once the temperature exceeds the limit, it can trigger desuperheating enhancement or load reduction protection to avoid problems such as impeller material performance degradation and excessive thermal stress caused by high temperature, thus extending the service life of the equipment.

[0037] A reflux desuperheating device 13 is installed on the anti-surge reflux pipeline 6. Since the final-stage steam compressor 7a discharges high-pressure, high-temperature superheated steam, while the input steam of the first-stage steam compressor 7a is low-pressure saturated slightly superheated steam at 80-85℃, if the high-temperature reflux steam directly mixes into the inlet, it will cause the superheat of the first-stage inlet steam to exceed the standard significantly, deviating from the optimal inlet state designed for the compressor. This results in a decrease in compressor efficiency, an increase in compression power consumption, and a gradual increase in the outlet temperature of subsequent stages, further burdening the interstage desuperheating device. The reflux desuperheating device 13 can cool the reflux steam to a range suitable for the first-stage inlet steam temperature, ensuring that the superheat of the mixed inlet steam is stably maintained within the design range. This guarantees that each stage of the steam compressor 7a always operates within its high-efficiency operating range, while simultaneously preventing overload of the interstage and outlet desuperheating systems.

[0038] The flash tank 2 is equipped with a flash tank output pressure detection unit 3 at the steam outlet, which is used to detect the steam pressure output from the flash tank 2.

[0039] The multi-stage steam compressor unit 7 includes a three-stage centrifugal steam compressor 7a. The pressure ratio of each stage of the centrifugal steam compressor 7a is distributed according to the thermodynamic ladder matching principle. The total pressure ratio of the multi-stage steam compressor unit 7 is 5-8. The interstage desuperheating device between each stage of the steam compressor 7a controls the steam superheat at the inlet of the subsequent steam compressor 7a to 5-10℃.

[0040] Each level of pipeline 5 is equipped with a buffer tank 12.

[0041] In this application, the steam compressor 7a is a centrifugal steam compressor 7a.

[0042] An operation control method for an ultra-high temperature rise waste heat recovery system includes the following specific steps: S1: Start the electric heat pump 1 and introduce a low-temperature heat source into the electric heat pump 1. After absorbing the heat from the low-temperature heat source, the electric heat pump 1 produces hot water with a preset temperature range at the hot end 1b. The hot water is then fed into the flash tank 2.

[0043] In this step, the temperature of the hot water is about 90℃. The hot water that has cooled down after flash evaporation in the flash tank 2 is returned to the electric heat pump 1 to absorb heat again.

[0044] S2: The multi-stage steam compressor unit 7 runs to the preset low speed state, the anti-surge backflow valve 14 maintains the preset opening state, and the opening of the outlet vent valve 18 is adjusted. The pressure ratio generated by the low-speed operation of the compressor is used to create a negative pressure in the flash tank 2, and the hot water in the flash tank 2 flashes to generate low-pressure saturated steam.

[0045] In this step, the preset low-speed state is when the speed of the steam compressor 7a is less than 50% of the rated speed.

[0046] By utilizing the inherent pressure ratio of the multi-stage steam compressor 7a at low speed, and in conjunction with the flow regulation of the outlet vent valve 18, a negative pressure environment is directly created by suction from the flash tank 2. This eliminates the need to configure a separate vacuum pump set for the flash tank 2, reducing equipment investment, floor space, and maintenance costs, and simplifying the system composition.

[0047] During the startup phase, the anti-surge return valve 14 maintains a preset opening. Under this preset opening, the opening of the anti-surge return valve 14 is greater than 80%. Under this large opening, a large amount of steam circulates fully inside the compression unit, significantly increasing the actual flow rate of the compressor. This ensures that the compressor operating point is far from the surge boundary under low-speed and low-intake-volume conditions, avoiding compressor surge failure caused by insufficient flash steam production in the initial startup phase.

[0048] At the same time, the establishment of negative pressure and the start of flash steam generation are synchronized, and the transition of operating conditions is smooth, avoiding the pressure shock of the traditional mode of first drawing a vacuum and then turning on water to generate steam, so that flash tank 2 can smoothly transition from normal pressure to the target negative pressure range.

[0049] S3: Control the steam compressor 7a to continuously increase its speed. During the speed increase, the pressure at the steam inlet of the first-stage steam compressor 7a is used as the control target. Control the electric heat pump 1 to reach the preset operating load state. Based on the operating load state of the electric heat pump 1 and the pressure before and after the outlet vent valve 18, calculate the target opening of the outlet vent valve 18 to keep the flash steam production and the steam venting amount of the outlet vent valve 18 in balance, thereby maintaining the pressure in the flash tank 2 stable within the target flash pressure range.

[0050] In this step, the preset operating load state of the electric heat pump 1 is set at 25% load, and the target flash pressure range is 45-60 kPa.A.

[0051] The target opening degree of the outlet vent valve 18 is determined in the following way: Based on the preset operating load state, the theoretical steam production of the current electric heat pump 1 is obtained, along with the measured values ​​of the pressure before and after the outlet vent valve 18, to obtain the pressure difference across the outlet vent valve 18. Based on the pressure difference across the outlet vent valve 18 and the theoretical steam production, the target opening degree of the outlet vent valve 18 is obtained through the opening-flow-pressure difference characteristic curve. The theoretical steam production can be determined through the performance parameter table corresponding to the electric heat pump 1, and the steam production corresponding to 25% load condition can be found by referring to the performance parameter table.

[0052] The valve flow characteristic curve, also known as the valve opening-flow-differential pressure characteristic curve, can be provided by the valve manufacturer. This curve is the core engineering characteristic curve of the valve, used to quantitatively characterize the relationship between the valve opening (relative lift of the valve disc), the pressure difference between the fluids before and after the valve, and the flow rate of the medium passing through the valve.

[0053] Furthermore, the target opening obtained based on the opening-flow-pressure difference characteristic curve is used as the feedforward control reference, and the measured pressure value at the inlet of the first-stage steam compressor 7a is used as the feedback signal to correct the opening of the outlet vent valve 18 through a PID algorithm.

[0054] This step uses the inlet pressure of the primary compressor (i.e., the pressure of flash tank 2) as the control target to adjust the outlet vent valve 18, directly anchoring the core parameters of the flash operation. This counteracts the enhanced suction capacity effect caused by the compressor speed increase, preventing a continuous decrease in flash pressure and stabilizing the flash pressure within the target range, ensuring uniform and stable flash steam production and steam parameters. The electric heat pump 1 maintains a preset low-load operation, keeping the flash steam production at a stable and controllable level. Combined with precise adjustment of the vent valve opening, this achieves a dynamic balance between steam production and venting, preventing overpressure in flash tank 2 and safety valve activation caused by steam production exceeding venting, and also preventing excessive negative pressure and liquid carryover in the steam caused by venting exceeding steam production.

[0055] The opening adjustment of the outlet vent valve 18 adopts a control strategy of "feedforward reference + PID feedback correction". Based on the load of the electric heat pump 1, the pressure difference across the valve and the valve flow characteristic curve, the target opening of the vent valve is pre-calculated, which makes the valve adjustment feedforward, greatly reduces the lag and oscillation risk of pure PID regulation, makes the pressure regulation smoother during the acceleration process, and improves the stability and control accuracy of the acceleration process.

[0056] S4: When the speed of steam compressor 7a increases to the preset switching speed, the opening of the outlet vent valve 18 is adjusted with the pressure at the exhaust end of the last stage steam compressor 7a as the control target, so that the pressure at the exhaust end of the last stage steam compressor 7a is stabilized within the preset exhaust pressure range.

[0057] S5: After the speed of the steam compressor 7a reaches the rated operating speed range, first increase the operating load of the electric heat pump 1 and increase the steam production of the flash tank 2, and then gradually reduce the opening of the anti-surge backflow valve 14 to maintain the stability of the inlet pressure and steam volume of the steam compressor 7a. S6: After the anti-surge return valve 14 is completely closed, reduce the opening of the outlet vent valve 18 to increase the compressor outlet pressure. When the steam pressure at the exhaust end of the final stage steam compressor 7a reaches the target steam delivery pressure, open the steam delivery valve 20 and deliver steam to the outside.

[0058] During operation, the exhaust temperature and interstage pressure of each stage of steam compressor 7a are monitored in real time. The interstage desuperheating device adopts water spray desuperheating. The interstage desuperheating device calculates the corresponding steam saturation temperature based on the interstage pressure. With steam superheat as the controlled variable, the water spray volume is adjusted in real time through PID algorithm to stabilize the steam superheat at the inlet of the subsequent steam compressor 7a at 5-10℃.

[0059] Furthermore, during system operation, during the speed-up process of steam compressor 7a, the opening degree of anti-surge backflow valve 14 is controlled in segments according to the speed range: When the speed of the steam compressor 7a is 0-20Hz, the anti-surge backflow valve 14 remains fully open; When the speed of the steam compressor 7a is 20-30Hz, the opening of the anti-surge backflow valve 14 decreases linearly from 100% to 80% as the speed increases; When the speed of the steam compressor 7a is 30-45Hz, the opening of the anti-surge backflow valve 14 decreases linearly from 80% to 60% as the speed increases; When the speed of steam compressor 7a is 45-50Hz, the ratio of compressor outlet pressure to inlet pressure is used as the real-time pressure ratio, and the ratio of steam flow rate to pressure at the inlet of the first-stage steam compressor 7a is used as the real-time converted flow rate parameter. Based on the real-time pressure ratio and the preset pressure ratio-converted flow rate anti-surge safety curve, the theoretical converted flow rate parameter of the anti-surge margin is calculated. The difference between the real-time converted flow rate parameter and the theoretical converted flow rate parameter is taken as the anti-surge margin. Based on the PID algorithm, the opening of the anti-surge return valve 14 is adjusted in a closed loop using the anti-surge margin as the controlled variable.

[0060] In this application, within the 0-45Hz speed range, the steam compressor 7a has a low pressure ratio and small flow rate, and the surge risk mode is simple. During this stage, a preset linear opening rule is adopted, and the control logic is simple and reliable, which can ensure safety in the low-speed stage and avoid adjustment oscillation.

[0061] When the speed is in the mid-to-high speed transition range of 45~50Hz, the compressor pressure ratio increases rapidly, and the operating point gradually approaches the rated operating range, significantly increasing the risk of surge. At the same time, the operating parameters tend to stabilize, providing conditions for closed-loop regulation. At this point, switching to margin PID control can not only accurately control the safety margin, but also gradually close the anti-surge valve and reduce backflow losses, providing a smooth transition for subsequent full-speed loading and closing of the anti-surge valve.

[0062] Assume the inlet flow rate of the steam compressor is F; the inlet pressure of the first-stage steam compressor is P1; and the outlet pressure of the last-stage steam compressor is P2. Then the pressure ratio y = P2 / P1, and the equivalent flow rate parameter x = F / P1; Taking a design inlet pressure of 57.8 kPa·A, a design outlet pressure of 400 kPa·A, and a design steam flow rate of 9.25 t / h as an example, several points are selected from the characteristic curve of the steam compressor, as follows: When the inlet flow rate F = 8.5 t / h, the pressure ratio is 6.228374; When the inlet flow rate F = 9.25 t / h, the pressure ratio is 6.92042; When the inlet flow rate F = 9.6 t / h, the pressure ratio is 7.61256; Based on the above data points, substituting them into a linear equation, the relationship between the reduced flow rate parameter x and the pressure ratio y can be calculated, resulting in the following piecewise function: ; The above function serves as a preset pressure ratio-equivalent flow rate anti-surge safety curve. Based on the pressure ratio y, the theoretical equivalent flow rate parameter can be calculated. The real-time equivalent flow rate parameter is calculated using the measured values ​​of the inlet pressure of the primary steam compressor and the measured value of the inlet flow rate of the steam compressor, obtained from the pressure detection unit. The difference between the real-time equivalent flow rate parameter and the theoretical equivalent flow rate parameter is then taken. x is used as the asthma prevention margin. x is the controlled variable of the PID controller (with a preset target safety margin), and the valve opening command is output through deviation calculation.

[0063] When the anti-surge margin Δx decreases (the operating point is close to the surge line), it means that the actual margin is less than the target margin. At this time, the PID output increases, the anti-surge return valve is opened wider, more outlet steam flows back to the steam inlet, the steam inlet flow increases, and Δx rises back to the target safe range. When Δx is too large (sufficient margin, wasteful backflow), it means that the actual margin is greater than the target margin. The PID output decreases, the anti-surge backflow valve is closed, useless backflow is reduced, and the system operating efficiency is improved.

[0064] This invention is not limited to the preferred embodiments described above. Anyone can derive other products in various forms under the guidance of this invention. However, regardless of any changes in shape or structure, any technical solution that is the same as or similar to this application falls within the protection scope of this invention.

Claims

1. A waste heat recovery system for ultra-high temperature rise, characterized in that, The system includes an electric heat pump, a flash tank, and a multi-stage steam compressor unit. A low-temperature heat source is connected to the cold end of the electric heat pump, and the hot end of the electric heat pump is connected to the water inlet of the flash tank. The multi-stage steam compressor unit consists of several steam compressors connected in series. Adjacent steam compressors are connected by interstage pipelines, and interstage desuperheating devices are installed on the interstage pipelines. The steam outlet of the flash tank is connected to the steam inlet of the first-stage steam compressor, and the exhaust end of the last-stage steam compressor is connected to an output pipeline. An anti-surge backflow pipeline is installed between the exhaust end of the last-stage steam compressor and the steam inlet of the first-stage steam compressor. An anti-surge backflow valve is installed on the anti-surge backflow pipeline, and an outlet vent valve and a steam supply valve are installed on the output pipeline. The operation control method of the ultra-high temperature rise waste heat recovery system includes the following specific steps: S1: Start the electric heat pump and pass a low-temperature heat source into it. After absorbing the heat from the low-temperature heat source, the electric heat pump produces hot water with a preset temperature range at the hot end. The hot water is then fed into the flash tank. S2: The multi-stage steam compressor unit runs to the preset low speed state, the anti-surge backflow valve maintains the preset opening state, the opening of the outlet vent valve is adjusted, and the pressure ratio generated by the low speed operation of the compressor is used to create a negative pressure in the flash tank, and the hot water in the flash tank flashes to generate low-pressure saturated steam. S3: Control the steam compressor to continuously increase its speed. During the speed increase, the pressure at the steam inlet of the first-stage steam compressor is used as the control target. Control the electric heat pump to reach the preset operating load state. Based on the operating load state of the electric heat pump and the pressure before and after the outlet vent valve, calculate the target opening of the outlet vent valve to keep the flash steam production and the steam venting amount of the outlet vent valve in balance, thereby maintaining the pressure in the flash tank stable within the target flash pressure range. S4: When the speed of the steam compressor increases to the preset switching speed, the opening of the outlet vent valve is adjusted with the pressure at the exhaust end of the last stage steam compressor as the control target, so that the pressure at the exhaust end of the last stage steam compressor is stabilized within the preset exhaust pressure range. S5: After the steam compressor speed reaches the rated operating speed range, first increase the operating load of the electric heat pump and increase the steam production of the flash tank, and then gradually reduce the opening of the anti-surge backflow valve to maintain the stability of the steam compressor inlet pressure and steam volume. S6: After the anti-surge return valve is completely closed, reduce the opening of the outlet vent valve to increase the compressor outlet pressure. When the steam pressure at the exhaust end of the final stage steam compressor reaches the target steam delivery pressure, open the steam delivery valve and deliver steam to the outside.

2. The ultra-high temperature rise waste heat recovery system according to claim 1, characterized in that, Each stage of the steam compressor is equipped with an inlet temperature detection unit at the steam inlet end, and each stage of the steam compressor is equipped with an exhaust temperature detection unit and an exhaust pressure detection unit at the exhaust end. The first stage of the steam compressor is also equipped with an inlet pressure detection unit at the steam inlet end. An output desuperheating device and an output pipeline temperature detection unit are installed on the output pipeline, with the output pipeline temperature detection unit located downstream of the output desuperheating device.

3. The ultra-high temperature rise waste heat recovery system according to claim 1, characterized in that, The anti-surge return pipeline is equipped with a return cooling device.

4. The ultra-high temperature rise waste heat recovery system according to claim 1, characterized in that, The multi-stage steam compressor unit includes a three-stage centrifugal steam compressor. The pressure ratio of each stage of the centrifugal steam compressor is distributed according to the thermodynamic ladder matching principle. The total pressure ratio of the multi-stage steam compressor unit is 5-8. The interstage desuperheating device between each stage of the steam compressor controls the steam superheat at the inlet of the subsequent stage steam compressor to 5-10℃.

5. The ultra-high temperature rise waste heat recovery system according to claim 1, characterized in that, In step S3, the target opening degree of the outlet vent valve is determined in the following way: Based on the preset operating load state, the theoretical steam production of the current electric heat pump is obtained, and the measured values ​​of the pressure before and after the outlet vent valve are obtained to obtain the pressure difference across the outlet vent valve. Based on the pressure difference across the outlet vent valve and the theoretical steam production, the target opening of the outlet vent valve is obtained through the opening-flow-pressure difference characteristic curve.

6. The ultra-high temperature rise waste heat recovery system according to claim 5, characterized in that, The target opening obtained based on the opening-flow-pressure difference characteristic curve is used as the feedforward control reference. At the same time, the measured pressure value at the steam inlet of the first-stage steam compressor is used as the feedback signal, and the opening of the outlet vent valve is corrected by the PID algorithm.

7. The ultra-high temperature rise waste heat recovery system according to claim 1, characterized in that, During operation, the exhaust temperature and interstage pressure of each stage of the steam compressor are monitored in real time. The interstage desuperheating device adopts water spray desuperheating. The interstage desuperheating device calculates the corresponding steam saturation temperature based on the interstage pressure. With steam superheat as the controlled variable, the water spray volume is adjusted in real time through PID algorithm to stabilize the steam superheat at the inlet of the subsequent steam compressor at 5-10℃.

8. The ultra-high temperature rise waste heat recovery system according to claim 1, characterized in that, In step S2, the preset low-speed state is when the steam compressor speed is less than 50% of the rated speed; in step S3, the target flash pressure range is 45-60 kPa.A.

9. The ultra-high temperature rise waste heat recovery system according to claim 1, characterized in that, During the speed-up process of the steam compressor, the opening degree of the anti-surge backflow valve is controlled in segments according to the speed range: When the steam compressor speed is 0-20Hz, the anti-surge backflow valve remains fully open; When the steam compressor speed is 20-30Hz, the opening of the anti-surge backflow valve decreases linearly from 100% to 80% as the speed increases; When the steam compressor speed is 30-45Hz, the opening of the anti-surge backflow valve decreases linearly from 80% to 60% as the speed increases; When the speed of the steam compressor is 45-50Hz, the ratio of the compressor outlet pressure to the inlet pressure is used as the real-time pressure ratio, and the ratio of the steam flow rate to the pressure at the inlet of the first-stage steam compressor is used as the real-time converted flow rate parameter. Based on the real-time pressure ratio and the preset pressure ratio-converted flow rate anti-surge safety curve, the theoretical converted flow rate parameter of the anti-surge margin is calculated. The difference between the real-time converted flow rate parameter and the theoretical converted flow rate parameter is taken as the anti-surge margin. Based on the PID algorithm, the opening of the anti-surge return valve is adjusted in a closed loop using the anti-surge margin as the controlled variable.

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