A waste heat cascade utilization system for oil-free screw air compressors based on high-temperature heat pumps

Abstract

This invention discloses a waste heat cascade utilization system for an oil-free screw air compressor based on a high-temperature heat pump. The system includes an air compressor, a high-temperature heat pump unit, a high-temperature heat exchanger, a low-temperature heat exchanger, a buffer water tank, and a circulating water pump. High- and low-temperature heat exchangers are connected in parallel between the air compressor outlet and the existing cooler. A cascade utilization mode is adopted, where the high-temperature section undergoes direct heat exchange and the low-temperature section is coupled with a high-temperature heat pump for temperature enhancement, integrating the two heat sources to output high-temperature hot water at approximately 90°C. This invention, through the cascade utilization of waste heat via direct heat exchange in the high-temperature section and coupling the low-temperature section with a high-temperature heat pump, effectively improves the utilization value of waste heat from the oil-free screw air compressor, achieving full and efficient waste heat recovery, meeting the heat requirements of high-temperature processes, and reducing the capacity and cost of the heat pump configuration. The system retains the original cooling backup structure and is equipped with a buffer water tank to ensure the safe operation of the air compressor and the stable operation of the heat pump, avoiding air compressor overheating shutdowns and frequent heat pump start-ups and shutdowns.

Description

Technical Field

[0001] This invention relates to the field of waste heat recovery technology for air compressors, specifically a waste heat cascade utilization system for oil-free screw air compressors based on a high-temperature heat pump. Background Technology

[0002] Oil-free screw air compressors are widely used in industrial production. During the compression process, they generate a large amount of high-temperature waste heat, with the compressed air temperature at the compressor outlet reaching 170-190℃. Traditional air compressor waste heat recovery often uses a single-stage heat exchange method, directly exchanging heat between high-temperature compressed air and low-temperature water. This results in the degradation of high-grade heat energy, low energy utilization efficiency, and significant energy loss. Some technologies only recover waste heat from the high-temperature section of the air compressor, while the waste heat from the low-temperature section below 85℃ remains unutilized and still requires cooling tower cooling, leading to energy waste.

[0003] Existing heat pump waste heat recovery technologies mostly utilize air compressor exhaust as the heat source, but excessively high heat source temperatures can lead to decreased heat pump system efficiency and even affect equipment lifespan. Simultaneously, during low-temperature waste heat recovery, the heat pump load fluctuates significantly, easily causing frequent start-ups and shutdowns, impacting equipment stability. Furthermore, oil-free screw air compressors have high requirements for exhaust temperature stability, and existing waste heat recovery systems lack reliable backup cooling solutions. When the heat exchange branch fails, the air compressor is prone to overheating and shutdown. Summary of the Invention

[0004] The purpose of this invention is to overcome or at least partially solve the above problems by proposing a waste heat cascade utilization system for an oil-free screw air compressor based on a high-temperature heat pump.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a waste heat cascade utilization system for an oil-free screw air compressor based on a high-temperature heat pump, comprising an air compressor, a high-temperature heat pump unit, a high-temperature heat exchanger, a low-temperature heat exchanger, a buffer water tank, and a circulating water pump;

[0006] The air compressor includes a compressor, a cooler, and an air consumption point. The compressor outlet is provided with a branch line, which is respectively connected to the primary side inlet of the high-temperature heat exchanger, the primary side inlet of the low-temperature heat exchanger, and the inlet of the cooler. The outlet of the cooler, the primary side outlet of the high-temperature heat exchanger, and the primary side outlet of the low-temperature heat exchanger converge and are connected to the air consumption point.

[0007] The secondary side of the high-temperature heat exchanger is connected to a circulation pipeline consisting of hot water return and hot water supply, which is used to directly heat the hot water return.

[0008] The secondary outlet of the low-temperature heat exchanger is sequentially connected to the buffer water tank, the circulating water pump, and the heat pump evaporator of the high-temperature heat pump unit. The outlet of the heat pump evaporator is connected back to the secondary inlet of the low-temperature heat exchanger, forming an independent low-temperature heat exchange loop.

[0009] The high-temperature heat pump unit is composed of a heat pump evaporator, a heat pump compressor, a heat pump condenser, and a heat pump throttling valve connected in sequence. The heat pump condenser is connected to the circulation pipeline of the hot water return and hot water supply.

[0010] The inlet and outlet of the cooler are respectively connected to the cooling water inlet and the cooling water outlet.

[0011] Preferably, each branch pipe at the compressor outlet is equipped with an electric regulating valve, and the cooler is the original cooler of the air compressor and its matching cooling tower, serving as a backup cooling branch for the system.

[0012] Preferably, the primary inlet of the high-temperature heat exchanger is supplied with high-temperature compressed air at 170-190°C. After heat exchange, the air outlet temperature drops to 85°C, and the secondary side heats the 80°C hot water return to 90-95°C.

[0013] Preferably, the primary inlet of the low-temperature heat exchanger is connected to compressed air at 85°C, and the air outlet temperature drops to below 40°C after heat exchange; the secondary side heats the circulating water from 35°C to 45°C to provide a low-temperature heat source for the high-temperature heat pump unit.

[0014] Preferably, the buffer water tank is used to stabilize the water flow and thermal inertia of the low-temperature heat exchange circulation loop, thereby preventing the high-temperature heat pump unit from frequently starting and stopping.

[0015] Preferably, the heat pump condenser and the secondary side of the high-temperature heat exchanger are connected in parallel to the hot water circulation pipeline, and the hot water heated by the two is combined and output from the hot water supply end.

[0016] This invention also proposes a method for the cascade utilization of waste heat from an oil-free screw air compressor based on a high-temperature heat pump, comprising the following steps:

[0017] S1. The high-temperature compressed air discharged from the air compressor at 170-190℃ enters the primary side of the high-temperature heat exchanger through the branch pipeline, where it exchanges heat with the hot water return water on the secondary side, heating the 80℃ hot water return water to 90-95℃, and the compressed air flows out after cooling down to 85℃.

[0018] S2. Compressed air cooled to 85°C is diverted into the primary side of the low-temperature heat exchanger to exchange heat with the circulating water on the secondary side, heating the circulating water from 35°C to 45°C. The compressed air is cooled to below 40°C and then delivered to the point of use. The heated circulating water is then delivered to the heat pump evaporator of the high-temperature heat pump unit through a buffer water tank and a circulating water pump to release heat and provide a low-temperature heat source for the heat pump.

[0019] S3. In the high-temperature heat pump unit, the heat pump compressor compresses the working fluid to generate high-temperature and high-pressure heat, which heats the hot water return water through the heat pump condenser. The heated hot water merges with the hot water output from the high-temperature heat exchanger and is output from the hot water supply end.

[0020] S4. The air flow rate entering the heat exchanger is adjusted by the electric regulating valves on each branch pipeline. At the same time, the cooler branch serves as a backup cooling system to ensure that the air compressor exhaust temperature is stable below 40℃.

[0021] Preferably, in step S2, the circulating water drops back to 35°C after releasing heat in the heat pump evaporator, and then flows back to the secondary side inlet of the low-temperature heat exchanger to complete the circulation.

[0022] Preferably, in step S4, when the heat exchanger branch fails or the load is insufficient, the compressed air is switched to the cooler branch and cooled by cooling water to ensure the stable operation of the air compressor.

[0023] Compared with existing technologies, this invention provides a waste heat cascade utilization system for oil-free screw air compressors based on high-temperature heat pumps. Through direct heat exchange in the high-temperature section and coupling of the high-temperature section with a high-temperature heat pump, the waste heat utilization method effectively improves the utilization value of waste heat from oil-free screw air compressors, achieving full recovery and efficient utilization of waste heat. This can meet the heating demands of processes at higher temperatures while reducing the configuration capacity and investment cost of the heat pump unit. The system retains the original cooling backup structure and is equipped with a buffer water tank, ensuring safe operation of the air compressor and stable operation of the heat pump unit, avoiding air compressor overheating shutdowns and frequent heat pump start-stops. The overall system adopts a parallel connection method, making the modification of the original air compressor system simple and convenient. It can effectively replace traditional heating equipment, reduce energy consumption, and has good practicality and economic benefits. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the overall structure of the waste heat cascade utilization system of an oil-free screw air compressor based on a high-temperature heat pump according to the present invention.

[0025] In the diagram: 1. Air compressor; 1a. Compressor; 1b. Cooler; 1c. Gas consumption point; 2. High-temperature heat pump unit; 2a. Heat pump evaporator; 2b. Heat pump compressor; 2c. Heat pump cooler; 2d. Heat pump throttle valve; 3. High-temperature heat exchanger; 4. Low-temperature heat exchanger; 5. Buffer water tank; 6. Circulating water pump. Detailed Implementation

[0026] The present invention will be further described in detail below with reference to the accompanying drawings.

[0027] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this description, those skilled in the art can make creative modifications to this embodiment as needed, but as long as they are within the scope of the claims of the present invention, they are protected by patent law.

[0028] This invention discloses a waste heat cascade utilization system for oil-free screw air compressors based on a high-temperature heat pump, which solves the technical problems in the prior art. The overall concept is as follows:

[0029] Example 1

[0030] Please see Figure 1 A waste heat cascade utilization system for an oil-free screw air compressor based on a high-temperature heat pump includes an air compressor 1, a high-temperature heat pump unit 2, a high-temperature heat exchanger 3, a low-temperature heat exchanger 4, a buffer water tank 5, and a circulating water pump 6. The air compressor 1 includes a compressor 1a, a cooler 1b, and an air consumption point 1c. The outlet of the compressor 1a is connected to three parallel branches via a pipeline 1, namely a high-temperature heat exchanger branch, a low-temperature heat exchanger branch, and a standby cooling branch. Each branch inlet is equipped with an electric regulating valve, which can adjust the compressed air flow rate of each branch according to the air compressor exhaust temperature and heat demand.

[0031] The primary side of the high-temperature heat exchanger 3 is connected to the high-temperature compressed air from the compressor outlet, and the secondary side is connected to the hot water circulation pipeline, and is connected in parallel with the heat pump condenser 2c; the primary side of the low-temperature heat exchanger 4 is connected to the compressed air cooled by the high-temperature heat exchanger, and the secondary side is connected to the buffer water tank 5, the circulating water pump 6, and the heat pump evaporator 2a to form an independent low-temperature circulation loop; the high-temperature heat pump unit 2 is composed of the heat pump evaporator 2a, the heat pump compressor 2b, the heat pump condenser 2c, and the heat pump throttle valve 2d connected in sequence, and the heat pump condenser 2c is connected to the hot water circulation pipeline; the inlet of the cooler 1b is connected to the compressed air from the compressor outlet, and the outlet is connected to the gas consumption point 1c. The water side of the cooler 1b is connected to the cooling water inlet and the cooling water outlet, serving as the system's backup cooling system.

[0032] This embodiment uses a 132kW single-stage oil-free screw air compressor for field testing and verification. The system working process and steady-state operating parameters are as follows:

[0033] Compressed air flow path: Normal temperature and pressure air enters compressor 1a and is compressed into high temperature compressed air of about 182°C. It is then divided into three paths through the main pipeline: The first path enters the primary side of high temperature heat exchanger 3, where it exchanges heat with the hot water return water on the secondary side and flows out after the temperature drops to about 84.5°C; the second path enters the primary side of low temperature heat exchanger 4, where it exchanges heat with the circulating water on the secondary side and flows out after the temperature drops to about 39.7°C; the third path is a backup branch. When the heat exchange branch fails or the load is insufficient, the compressed air enters cooler 1b and is cooled to below 40°C by cooling water before flowing out; the three compressed air paths are combined and delivered to the air consumption point 1c to ensure stable air consumption.

[0034] Hot water circulation path: The 80.2℃ hot water return is divided into two paths: the first path enters the secondary side of the high-temperature heat exchanger 3, absorbs the heat from the compressed air, is heated to 89.6℃, and then flows out to the hot water supply end; the second path enters the heat pump condenser 2c of the high-temperature heat pump unit 2, absorbs the heat released by the heat pump working fluid, is heated to 89.8℃, and then merges with the hot water flowing out of the high-temperature heat exchanger 3, and together they are output from the hot water supply end to meet the industrial heat demand.

[0035] Low-temperature circulating water flow path: The circulating water on the secondary side of the low-temperature heat exchanger 4 absorbs heat from the compressed air, is heated from 34.5℃ to 44.6℃, and then flows out into the buffer water tank 5. After the buffer water tank 5 stabilizes the flow rate and temperature, it is transported to the heat pump evaporator 2a by the circulating water pump 6. The heat pump working fluid absorbs heat from the circulating water in the evaporator. After the circulating water cools down to 34.5℃, it flows back to the secondary side inlet of the low-temperature heat exchanger 4 to complete the circulation.

[0036] The heat pump unit operates as follows: The heat pump working fluid absorbs heat from the low-temperature circulating water in the heat pump evaporator 2a and evaporates into low-pressure steam. It is then compressed into high-temperature and high-pressure steam by the heat pump compressor 2b and enters the heat pump condenser 2c, where it releases heat to the hot water return water. It then condenses into a high-pressure liquid and returns to the heat pump evaporator 2a after being throttled and depressurized by the heat pump throttling valve 2d, thus completing the heat pump cycle.

[0037] Once the system is running stably, the air compressor's built-in cooling system does not need to be started; the waste heat recovery system alone is sufficient to meet the cooling requirements. The actual operating parameters are as follows:

[0038] Air compressor input power: 131.2kW

[0039] High-temperature heat exchanger primary side inlet temperature: 182℃

[0040] Primary side outlet temperature of the high-temperature heat exchanger: 84.5℃

[0041] Secondary side inlet water temperature of high-temperature heat exchanger: 80.2℃

[0042] Secondary side outlet water temperature of high-temperature heat exchanger: 89.6℃

[0043] Primary side inlet temperature of the low-temperature heat exchanger: 84.5℃

[0044] Primary side outlet temperature of the low-temperature heat exchanger: 39.7℃

[0045] Secondary side inlet water temperature of the low-temperature heat exchanger: 34.5℃

[0046] Secondary side outlet water temperature of the low-temperature heat exchanger: 44.6℃

[0047] Buffer tank water temperature: 44.6℃

[0048] Heat pump evaporator inlet water temperature: 44.6℃

[0049] Heat pump evaporator outlet water temperature: 34.5℃

[0050] Heat pump condenser inlet water temperature: 80.2℃

[0051] Heat pump condenser outlet water temperature: 89.8℃

[0052] Compressor discharge temperature: 39.7℃

[0053] Compressed air flow rate: 20.1 Nm³ / min

[0054] Secondary flow rate of high-temperature heat exchanger: 5.59 m³ / h

[0055] Secondary flow rate of the low-temperature heat exchanger: 2.39 m³ / h

[0056] High-temperature heat pump unit condenser flow rate: 3.65 m³ / h

[0057] System energy efficiency and economic benefits:

[0058] Heat recovered by the high-temperature heat exchanger: 61.13 kW

[0059] Heat recovered by the low-temperature heat exchanger and the high-temperature heat pump together: 40.74 kW

[0060] High-temperature heat pump input power: 12.65kW

[0061] High-temperature heat pump COP: 3.22

[0062] Total heat recovered by the coupling system: 101.87 kW

[0063] Compared to using a natural gas boiler to produce hot water: the cost saving rate is about 81%, saving about 290,100 yuan / year in energy costs (calculated based on: average electricity price of 0.75 yuan / kWh, natural gas price of 4.5 yuan / m³, comprehensive efficiency of gas boiler of 90%, and annual operating time of 7000h).

[0064] In this embodiment, the system achieves full-stage cascade recovery and efficient utilization of waste heat from the oil-free screw air compressor, with a total recovered heat of 101.87kW and a high-temperature heat pump COP of 3.22, resulting in significant energy-saving benefits. The air compressor exhaust temperature remains stable below 40℃, and there are no over-temperature shutdowns. The buffer water tank effectively stabilizes heat source fluctuations, and the heat pump unit does not experience frequent start-stop issues, ensuring safe and stable equipment operation.

[0065] The above description of the embodiments is provided to facilitate understanding and use of the present invention by those skilled in the art. It is obvious to those skilled in the art that various modifications can be made to the embodiments, and the general principles described herein can be applied to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A waste heat cascade utilization system for an oil-free screw air compressor based on a high-temperature heat pump, characterized in that, It includes an air compressor (1), a high-temperature heat pump unit (2), a high-temperature heat exchanger (3), a low-temperature heat exchanger (4), a buffer water tank (5), and a circulating water pump (6). The air compressor (1) includes a compressor (1a), a cooler (1b), and an air consumption point (1c). The outlet of the compressor (1a) is provided with a branch pipe, which is connected to the primary side inlet of the high-temperature heat exchanger (3), the primary side inlet of the low-temperature heat exchanger (4), and the inlet of the cooler (1b). The outlet of the cooler (1b), the primary side outlet of the high-temperature heat exchanger (3), and the primary side outlet of the low-temperature heat exchanger (4) converge and are connected to the air consumption point (1c). The secondary side of the high-temperature heat exchanger (3) is connected to a circulation pipeline consisting of hot water return and hot water supply, which is used to directly heat the hot water return. The secondary outlet of the low-temperature heat exchanger (4) is connected in sequence to the buffer water tank (5), the circulating water pump (6) and the heat pump evaporator (2a) of the high-temperature heat pump unit (2). The outlet of the heat pump evaporator (2a) is connected back to the secondary inlet of the low-temperature heat exchanger (4) to form an independent low-temperature heat exchange loop. The high-temperature heat pump unit (2) is composed of a heat pump evaporator (2a), a heat pump compressor (2b), a heat pump condenser (2c), and a heat pump throttling valve (2d) connected in sequence. The heat pump condenser (2c) is connected to the circulation pipeline of hot water return and hot water supply. The inlet and outlet of the cooler (1b) are respectively connected to the cooling water inlet and the cooling water outlet.

2. The waste heat cascade utilization system of an oil-free screw air compressor based on a high-temperature heat pump according to claim 1, characterized in that: Electric regulating valves are installed on each branch pipeline at the outlet of the compressor (1a). The cooler (1b) is the original cooler of the air compressor and its matching cooling tower, serving as a backup cooling branch of the system.

3. The waste heat cascade utilization system of an oil-free screw air compressor based on a high-temperature heat pump according to claim 1, characterized in that: The high-temperature heat exchanger (3) is supplied with high-temperature compressed air at 170-190°C on the primary side. After heat exchange, the air outlet temperature drops to 85°C. The secondary side heats the hot water return water at 80°C to 90-95°C.

4. The waste heat cascade utilization system of an oil-free screw air compressor based on a high-temperature heat pump according to claim 1, characterized in that: The primary side inlet of the low-temperature heat exchanger (4) is connected to compressed air at 85°C. After heat exchange, the air outlet temperature drops to below 40°C. The secondary side heats the circulating water from 35°C to 45°C to provide a low-temperature heat source for the high-temperature heat pump unit (2).

5. A waste heat cascade utilization system for an oil-free screw air compressor based on a high-temperature heat pump according to claim 1, characterized in that: The buffer water tank (5) is used to stabilize the water flow and thermal inertia of the low-temperature heat exchange circulation loop, and to prevent the high-temperature heat pump unit (2) from starting and stopping frequently.

6. A waste heat cascade utilization system for an oil-free screw air compressor based on a high-temperature heat pump according to claim 1, characterized in that: The heat pump condenser (2c) and the secondary side of the high-temperature heat exchanger (3) are connected in parallel to the hot water circulation pipeline. The hot water heated by the two is combined and output from the hot water supply end.

7. A method for cascade utilization of waste heat from an oil-free screw air compressor based on a high-temperature heat pump, characterized in that, Includes the following steps: S1. The high-temperature compressed air of 170-190℃ discharged by the compressor (1a) of the air compressor (1) enters the primary side of the high-temperature heat exchanger (3) through the branch pipeline, and exchanges heat with the hot water return water on the secondary side, heating the 80℃ hot water return water to 90-95℃, and the compressed air flows out after cooling down to 85℃. S2. The compressed air cooled to 85°C is diverted into the primary side of the low-temperature heat exchanger (4) to exchange heat with the circulating water on the secondary side, heating the circulating water from 35°C to 45°C. The compressed air is cooled to below 40°C and then delivered to the gas consumption point (1c). The heated circulating water is delivered to the heat pump evaporator (2a) of the high-temperature heat pump unit (2) through the buffer water tank (5) and the circulating water pump (6) to release heat and provide a low-temperature heat source for the heat pump. S3. In the high-temperature heat pump unit (2), the heat pump compressor (2b) compresses the working fluid to generate high-temperature and high-pressure heat, which heats the hot water return water through the heat pump condenser (2c). The heated hot water merges with the hot water output from the high-temperature heat exchanger (3) and is output from the hot water supply end. S4. The air flow rate entering the heat exchanger is adjusted by the electric regulating valves on each branch pipeline. At the same time, the cooler (1b) branch serves as a backup cooling system to ensure that the air compressor exhaust temperature is stable below 40°C.

8. A method for cascade utilization of waste heat from an oil-free screw air compressor based on a high-temperature heat pump, as described in claim 7, characterized in that: In step S2, the circulating water releases heat in the heat pump evaporator (2a) and then drops back to 35°C, before flowing back to the secondary inlet of the low-temperature heat exchanger (4) to complete the circulation.

9. A method for cascade utilization of waste heat from an oil-free screw air compressor based on a high-temperature heat pump, as described in claim 7, characterized in that: In step S4, when the heat exchanger branch fails or the load is insufficient, the compressed air is switched to the cooler (1b) branch and cooled by cooling water to ensure the stable operation of the air compressor.

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