Natural vacuum type seawater desalination system based on mixed condensation and gravity pressurization exhaust

Abstract

The invention discloses a natural vacuum type seawater desalination system based on mixed condensation and gravity pressurization exhaust, a flash evaporator and a mixed condenser are arranged at positions at a first height and a second height from a to-be-desalinated seawater plane, the first height is not lower than the second height, the arrangement position of a surface condenser is lower than that of the flash evaporator, and the mixed condenser is connected with the mixed condenser. A gas outlet of the flash evaporator is connected to a first inlet of the mixed condenser through the vacuum compressor, an outlet of the mixed condenser is connected to an inlet of a to-be-cooled medium channel of the surface type condenser, and an outlet of the to-be-cooled medium channel is connected to a second inlet of the mixed condenser through first water. The surface type condenser is provided with a cooling medium channel, a cooling medium flows into the cooling medium channel to cool a gas-liquid mixture entering the to-be-cooled medium channel, the to-be-cooled medium channel is provided with an exhaust port, and the gas-liquid mixture entering the surface type condenser is pressurized to one atmospheric pressure through the height difference between the mixed type condenser and the surface type condenser. According to the invention, efficient and continuous emission of non-condensable gas is realized.

Description

Technical Field

[0001] This invention relates to a seawater desalination system, and more particularly to a natural vacuum seawater desalination system based on mixed condensation and gravity pressurization exhaust. Background Technology

[0002] Natural vacuum seawater desalination systems utilize the principle that one atmosphere of pressure can support a 10-meter-high water column, creating a vacuum environment inside the evaporator. Through a siphon effect, seawater is drawn into the evaporator for low-pressure flash evaporation. Therefore, natural vacuum seawater desalination systems can significantly reduce the power consumption of seawater pumps, have high system efficiency, and have broad application prospects. However, because seawater contains dissolved non-condensable gases such as carbon dioxide, nitrogen, and oxygen, and the solubility of these gases generally increases with decreasing pressure, these non-condensable gases are more easily mixed into the natural vacuum seawater desalination system. If these non-condensable gases are not removed from the system in a timely manner, it will seriously affect the vacuum level inside the flash evaporator, leading to a decrease in system operating efficiency.

[0003] Existing seawater desalination systems typically use vacuum pumps to remove non-condensable gases. These pumps need to pressurize the low-pressure non-condensable gases to atmospheric pressure, resulting in high power consumption due to the relatively large operating pressure ratio. The seawater desalination system proposed in Chinese patents CN109809516A and CN111943299A does not use a vacuum pump for non-condensable gas venting; instead, it uses water filling for venting. This is a phased venting method, which causes fluctuations in the operating pressure within the flash evaporator within a certain range. Furthermore, the system also requires phased switching of operating components or equipment, placing high demands on system operation control. Summary of the Invention

[0004] To address the aforementioned deficiencies in the prior art, the present invention aims to provide a natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust, thereby solving the problem of efficient and continuous emission of non-condensable gases in natural vacuum seawater desalination systems.

[0005] The technical solution of this invention is as follows: A natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust includes a flash evaporator, a vacuum compressor, a hybrid condenser, a surface condenser, and a first water pump. The flash evaporator is positioned at a first height above the seawater to be desalinated, and the hybrid condenser is positioned at a second height above the seawater to be desalinated. The first height is not lower than the second height. The surface condenser is positioned lower than the flash evaporator. The inlet of the flash evaporator is connected to the seawater to be desalinated via a pipeline. The gas outlet at the top of the flash evaporator is connected to the inlet of the vacuum compressor, and the outlet of the vacuum compressor is connected to the hybrid condenser. The first inlet of the mixing condenser is connected to the outlet of the mixing condenser, which is connected to the inlet of the cooling medium channel of the surface condenser. The outlet of the cooling medium channel of the surface condenser is connected to the inlet of the first water pump, and the outlet of the first water pump is connected to the second inlet of the mixing condenser. The surface condenser is provided with a cooling medium channel in which cooling medium flows to cool the gas-liquid mixture entering the cooling medium channel. The cooling medium channel is provided with an exhaust port. The height difference between the mixing condenser and the surface condenser causes the gas-liquid mixture entering the surface condenser to be pressurized to one atmosphere under the action of gravity.

[0006] Further, the system includes a liquid storage tank and a second water pump. The outlet of the mixing condenser is connected to the inlet of a three-way valve. The first outlet of the three-way valve is connected to the inlet of the liquid storage tank. The second outlet of the three-way valve is connected to the inlet of the channel for the medium to be cooled. The outlet of the liquid storage tank is connected to the inlet of the second water pump. The outlet of the second water pump is connected to the third inlet of the mixing condenser. The position of the liquid storage tank is lower than the mixing condenser but higher than the surface condenser.

[0007] Furthermore, a third water pump is included, which drives the liquid discharged from the outlet at the bottom of the flash evaporator into the cooling medium channel.

[0008] Furthermore, the outlet at the bottom of the flash evaporator is connected to the inlet of the third water pump, and the outlet of the third water pump is connected to the inlet of the cooling medium channel.

[0009] Furthermore, the first height and the second height are not less than 9.5 meters.

[0010] Furthermore, the surface condenser is positioned at sea level.

[0011] Furthermore, the flash evaporator operates at a temperature range of 10–30°C, and its operating pressure range is the saturated seawater pressure corresponding to seawater at 10–30°C. The mixing condenser operates at a temperature range of 16–36°C, and its operating pressure is higher than the saturated water pressure at 16–36°C.

[0012] Furthermore, the working pressure range of the liquid storage tank is 10–90 kPa.

[0013] Furthermore, the vacuum compressor is one of the following: a rotary vane vacuum compressor, a screw vacuum compressor, a reciprocating piston vacuum compressor, a scroll vacuum compressor, or a centrifugal vacuum compressor.

[0014] Furthermore, the surface condenser is one of the following: shell-and-tube heat exchanger, plate heat exchanger, plate-fin heat exchanger, coaxial heat exchanger, and spiral plate heat exchanger.

[0015] This invention places a mixing condenser at a height no lower than that of a flash evaporator. The condensed low-pressure fresh water and non-condensable gas are fully mixed in the mixing condenser. According to the principles of fluid mechanics, the gas-liquid mixture can be continuously pressurized by gravity. When the pressure of the gas-liquid mixture increases to atmospheric pressure, it is stored in a surface condenser with an exhaust port. The non-condensable gas can then be directly discharged into the external environment, thereby achieving the effect of continuous exhaust.

[0016] Compared with the prior art, the advantages of the technical solution of the present invention are as follows: 1. Vacuum compressors have a low operating pressure ratio. A vacuum compressor only needs to pressurize the low-pressure water vapor and non-condensable gases after flash evaporation to a pressure higher than the saturated water pressure corresponding to the operating temperature of the mixing condenser. For example, if the flash temperature of the flash evaporator is 25℃ and the operating temperature of the mixing condenser is 36℃, the corresponding flash pressure is approximately 3170 Pa and the saturated water pressure is approximately 5948 Pa. If the outlet pressure of the vacuum compressor is 6300 Pa, then the operating pressure ratio of the vacuum compressor is only 6300 / 3170 = 1.99, far lower than 101325 / 3170 = 31.96. This lower operating pressure ratio not only facilitates efficient operation and lower power consumption of the vacuum compressor but also effectively reduces the selection requirements for vacuum compressors.

[0017] 2. High heat exchange efficiency of hybrid condensation. Hybrid condensation is a direct contact condensation method with no solid wall thermal resistance, resulting in fast heat and mass transfer rates and high heat transfer efficiency. Because it eliminates the need for numerous metal heat transfer tubes, hybrid condensers have a compact structure and lower manufacturing and maintenance costs. Furthermore, since cooling water and steam are directly mixed, there is no need to worry about scaling inside the heat transfer tubes affecting heat exchange, and the requirements for cooling water quality are lower.

[0018] 3. Gravity booster pumps are highly efficient and reliable. They are equivalent to pumps with 100% adiabatic efficiency and require no external power consumption, making them energy-saving and highly efficient. Furthermore, gravity booster pumps only require a water tank, pipes, and valves; they lack complex control units or moving parts, resulting in high reliability and low maintenance costs.

[0019] 4. In the scheme of adding a storage tank and a second water pump, the volume fraction of non-condensable gases at the outlet of the mixing condenser can be effectively controlled. The height of the storage tank is between that of the mixing condenser and the surface condenser. The working pressure of the internal circulation formed by the mixing condenser, storage tank, and second water pump is lower than the ambient atmospheric pressure. Therefore, the solubility of non-condensable gases in the internal circulating fresh water is low, and the non-condensable gas load in the mixing condenser will not increase significantly due to pressure changes in the circulating water (the working pressure of the storage tank drops to the working pressure of the mixing condenser). By adjusting the flow rate of the internal circulating fresh water, the volume fraction of non-condensable gases at the outlet of the mixing condenser can be controlled below 50%, which is beneficial for the efficient carrying of non-condensable gases by gravity pressurization of the condensed fresh water.

[0020] 5. In the scheme of adding a third water pump, the flash evaporation heat energy is utilized efficiently. The low-temperature seawater after flash evaporation is used to cool the gas-liquid mixture composed of gravity-pressurized freshwater and non-condensable gases, realizing the reuse of flash evaporation heat energy. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust according to Embodiment 1 of the present invention.

[0022] Figure 2 This is a schematic diagram of a natural vacuum seawater desalination system based on hybrid condensation and gravity pressurization exhaust according to Embodiment 2 of the present invention. Detailed Implementation

[0023] The present invention will be further described below with reference to embodiments, but these are not intended to limit the scope of the invention.

[0024] Example 1, referring to Figure 1 , Figure 1 This is a schematic diagram of a natural vacuum seawater desalination system based on hybrid condensation and gravity pressurization exhaust provided in this embodiment. The system includes a flash evaporator 1, a vacuum compressor 2, a hybrid condenser 3, a surface condenser 5, a first water pump 8, and a third water pump 6.

[0025] The flash evaporator 1, the third water pump 6, and the surface condenser 5 form a seawater circulation loop. Specifically, the inlet of the flash evaporator 1 is connected to the environment of the seawater to be desalinated via a pipeline. The outlet at the bottom of the flash evaporator 1 is connected to the inlet of the third water pump 6. The outlet of the third water pump 6 is then connected to the inlet of the cooling medium channel 5a of the surface condenser 5. The outlet of the cooling medium channel 5a of the surface condenser 5 is then connected to the external seawater environment. Generally, both the inlet of the flash evaporator 1 and the outlet of the cooling medium channel 5a of the surface condenser 5 can be connected to the natural marine environment. The operating temperature range of the flash evaporator 1 is 10–30°C, and the operating pressure range of the flash evaporator 1 is the saturated seawater pressure corresponding to seawater at 10–30°C.

[0026] The mixing condenser 3, the surface condenser 5, and the first water pump 8 constitute a freshwater circulation loop. Specifically, the outlet of the mixing condenser is connected to the inlet of the cooling medium channel 5b of the surface condenser 5, the outlet of the cooling medium channel 5b is connected to the inlet of the first water pump 8, and the outlet of the first water pump 8 is connected to the second inlet of the mixing condenser 3. Additionally, the cooling medium channel 5b is equipped with an exhaust port to discharge non-condensable gases, and an exhaust valve 7 is installed at the exhaust port. The cooling medium channel 5b is also equipped with a freshwater outlet (not shown) to discharge the freshwater formed after seawater desalination. The surface condenser 5 can be a shell-and-tube heat exchanger, a plate heat exchanger, a plate-fin heat exchanger, a coaxial heat exchanger, or a spiral plate heat exchanger. The operating temperature range of the mixing condenser 3 is 16–36°C, and the operating pressure of the mixing condenser 3 is higher than the saturated water pressure at 16–36°C. The temperature range of the freshwater condensed in the surface condenser 5 is 13–33°C.

[0027] Vacuum compressor 2 is located between flash evaporator 1 and mixing condenser 3. Specifically, the gas outlet at the top of flash evaporator 1 is connected to the inlet of vacuum compressor 2, and the outlet of vacuum compressor 2 is connected to the first inlet of mixing condenser 3. Vacuum compressor 2 can be a rotary vane vacuum compressor 2, screw vacuum compressor 2, reciprocating piston vacuum compressor 2, scroll vacuum compressor 2, or centrifugal vacuum compressor 2.

[0028] To achieve natural vacuum and gravitational pressurization, a flash evaporator 1 is installed at a first height above the surface of the seawater to be desalinated, and a mixing condenser 3 is installed at a second height above the surface of the seawater to be desalinated. The first height is not lower than the second height, and generally, both the first and second heights are not less than 9.5 meters. In this embodiment, the flash evaporator 1 is set at a height of 9.7 meters, and the surface condenser 5 is set at the same height as the surface of the seawater to be desalinated. The height difference between the mixing condenser 3 and the surface condenser 5 is 9.5 meters. This height difference causes the gas-liquid mixture entering the surface condenser 5 to be pressurized to one atmosphere under the influence of gravity.

[0029] The working process of the natural vacuum seawater desalination system based on mixed condensation and gravity pressurization exhaust in this embodiment is as follows: The vacuum compressor 2 operates continuously, causing the internal pressure of the flash evaporator 1 to gradually decrease. The seawater to be desalinated flows into the low-pressure flash evaporator 1 due to the siphon effect. Due to the sudden pressure change, some of the seawater flowing into the flash evaporator 1 flashes instantly. The low-temperature, low-pressure water vapor and non-condensable gases generated by flash evaporation are compressed by the vacuum compressor 2. The compressed high-temperature, medium-pressure mixed gas enters the mixed condenser 3. At the same time, the fresh water in the cooling medium channel 5b of the surface condenser 5, after being slightly pressurized by the first water pump 8, enters the mixed condenser 3, cooling the high-temperature, medium-pressure flash evaporation water vapor to medium-temperature, medium-pressure fresh water. The gas-liquid mixture composed of medium-temperature, medium-pressure fresh water and non-condensable gases is discharged from the mixed condenser 3 by gravity and then naturally pressurized into the cooling medium channel 5b of the surface cooler to be cooled, thereby forming usable fresh water. The non-condensable gases, pressurized to one atmosphere, can be discharged into the external atmosphere through the exhaust valve 7. In this embodiment, the low-temperature, low-pressure seawater after flash evaporation in flash evaporator 1, after being pressurized by gravity, still has a pressure lower than the ambient atmospheric pressure. The low-temperature, low-pressure seawater is then pressurized by the third water pump 6 to a pressure slightly higher than the ambient atmospheric pressure before entering the cooling medium channel 5a of the surface condenser 5. It exchanges heat with the gas-liquid mixture composed of medium-temperature, medium-pressure fresh water and non-condensable gases in the medium channel 5b to be cooled, utilizing the cooling capacity of the flash-evaporated low-temperature, low-pressure seawater. Subsequently, the ambient temperature, low-pressure seawater is discharged into the external seawater environment. The cooled low-temperature, ambient pressure fresh water, relying on the atmospheric pressure siphon effect and slightly pressurized by the first water pump 8, re-enters the mixing condenser 3.

[0030] As can be seen from the above process, this system can achieve continuous discharge of non-condensable gases. The main power-consuming equipment in the system is the vacuum compressor 2. The first water pump 8 and the third water pump 6 are only used to overcome fluid flow pressure loss and have low power consumption.

[0031] Example 2, refer to Figure 2 , Figure 2 This is a schematic diagram of a natural vacuum seawater desalination system based on hybrid condensation and gravity pressurization exhaust provided in this embodiment. The system includes a flash evaporator 1, a vacuum compressor 2, a hybrid condenser 3, a surface condenser 5, a three-way valve 4, a liquid storage tank 9, a first water pump 8, a second water pump 10, and a third water pump 6.

[0032] The flash evaporator 1, the third water pump 6, and the surface condenser 5 form a seawater circulation loop. Specifically, the inlet of the flash evaporator 1 is connected to the environment of the seawater to be desalinated via a pipeline. The outlet at the bottom of the flash evaporator 1 is connected to the inlet of the third water pump 6. The outlet of the third water pump 6 is then connected to the inlet of the cooling medium channel 5a of the surface condenser 5. The outlet of the cooling medium channel 5a of the surface condenser 5 is then connected to the external seawater environment. Generally, both the inlet of the flash evaporator 1 and the outlet of the cooling medium channel 5a of the surface condenser 5 can be connected to the natural marine environment. The operating temperature range of the flash evaporator 1 is 10–30°C, and the operating pressure range of the flash evaporator 1 is the saturated seawater pressure corresponding to seawater at 10–30°C.

[0033] The mixing condenser 3, three-way valve 4, surface condenser 5, and first water pump 8 constitute a freshwater circulation loop. Specifically, the outlet of the mixing condenser 3 is connected to the inlet of the three-way valve 4, the second outlet of the three-way valve 4 is connected to the inlet of the cooling medium channel 5b of the surface condenser 5, the outlet of the cooling medium channel 5b is connected to the inlet of the first water pump 8, and the outlet of the first water pump 8 is connected to the second inlet of the mixing condenser 3. Additionally, the cooling medium channel 5b is equipped with an exhaust port to discharge non-condensable gases, and an exhaust valve 7 is installed at the exhaust port. The cooling medium channel 5b is also equipped with a freshwater outlet (not shown) to discharge the freshwater formed after seawater desalination. The surface condenser 5 can be a shell-and-tube heat exchanger, a plate heat exchanger, a plate-fin heat exchanger, a coaxial heat exchanger, or a spiral plate heat exchanger. The operating temperature range of the mixing condenser 3 is 16–36°C, and the operating pressure of the mixing condenser 3 is higher than the saturated water pressure at 16–36°C. The temperature range of the freshwater condensed in the surface condenser 5 is 13–33°C.

[0034] The mixing condenser 3, three-way valve 4, liquid receiver 9, and second water pump 10 form an internal circulation loop. Specifically, the outlet of the mixing condenser 3 is connected to the inlet of the three-way valve 4, the first outlet of the three-way valve 4 is connected to the inlet of the liquid receiver 9, the outlet of the liquid receiver 9 is connected to the inlet of the second water pump 10, and the outlet of the second water pump 10 is connected to the third inlet of the mixing condenser 3. The operating pressure range of the liquid receiver 9 is 10–90 kPa.

[0035] Vacuum compressor 2 is located between flash evaporator 1 and mixing condenser 3. Specifically, the gas outlet at the top of flash evaporator 1 is connected to the inlet of vacuum compressor 2, and the outlet of vacuum compressor 2 is connected to the first inlet of mixing condenser 3. Vacuum compressor 2 can be a rotary vane vacuum compressor 2, screw vacuum compressor 2, reciprocating piston vacuum compressor 2, scroll vacuum compressor 2, or centrifugal vacuum compressor 2.

[0036] The flash evaporator 1, the mixing condenser 3, and the surface condenser 5 are positioned in the same locations as in Example 1. The liquid storage tank 9 is positioned at a height difference of 1.4 meters from the mixing condenser 3, and is lower than the mixing condenser 3 but higher than the surface condenser 5.

[0037] The working process of the natural vacuum seawater desalination system based on hybrid condensation and gravity pressurization exhaust in this embodiment is as follows: the vacuum compressor 2 runs continuously, causing the internal pressure of the flash evaporator 1 to gradually decrease. The seawater to be desalinated flows into the low-pressure flash evaporator 1 due to the siphon effect. Due to the sudden pressure change, some of the seawater flowing into the flash evaporator 1 flashes instantly. The low-temperature, low-pressure water vapor and non-condensable gas generated by the flash evaporation are compressed by the vacuum compressor 2. The compressed high-temperature, medium-pressure mixed gas enters the hybrid condenser 3. Meanwhile, the fresh water in the cooling medium channel 5b of the surface condenser 5, after being slightly pressurized by the first water pump 8, enters the mixing condenser 3 as low-temperature, medium-pressure fresh water. The fresh water in the storage tank 9, after being slightly pressurized by the second water pump 10, also enters the mixing condenser 3, cooling the high-temperature, medium-pressure flash steam to medium-temperature, medium-pressure fresh water. The gas-liquid mixture composed of medium-temperature, medium-pressure fresh water and non-condensable gas is discharged from the mixing condenser 3 by gravity and, after natural pressurization, part of it enters the storage tank 9 from the first outlet of the three-way valve 4, and the other part enters the cooling medium channel 5b of the surface cooler from the second outlet of the three-way valve 4 to be cooled, thus forming usable fresh water. The non-condensable gas, pressurized to one atmosphere, can be discharged into the outside atmosphere through the exhaust valve 7. In this embodiment, the low-temperature, low-pressure seawater after flash evaporation in flash evaporator 1, after being pressurized by gravity, still has a pressure lower than the ambient atmospheric pressure. The low-temperature, low-pressure seawater is then pressurized by the third water pump 6 to a pressure slightly higher than the ambient atmospheric pressure before entering the cooling medium channel 5a of the surface condenser 5. It exchanges heat with the gas-liquid mixture composed of medium-temperature, medium-pressure fresh water and non-condensable gases in the medium channel 5b to be cooled, so as to utilize the cooling capacity of the flash-evaporated low-temperature, low-pressure seawater. Subsequently, the ambient temperature, low-pressure seawater is discharged into the external seawater environment. The cooled low-temperature, ambient pressure fresh water, relying on the atmospheric pressure siphon effect and slightly pressurized by the first water pump 8, re-enters the mixing condenser 3. The medium-temperature, medium-pressure fresh water in the storage tank 9, relying on the atmospheric pressure siphon effect and slightly pressurized by the second water pump 10, re-enters the mixing condenser 3.

[0038] This system enables continuous discharge of non-condensable gases, and the main power-consuming equipment in the system is the vacuum compressor 2. The first water pump 8, the second water pump 10, and the third water pump 6 are only used to overcome the pressure loss of fluid flow and have low power consumption.

[0039] Under a specific design condition: (1) The sea surface temperature is 30℃; (2) The operating parameters of flash evaporator 1 are 25℃ / 3170Pa; (3) The outlet parameters of vacuum compressor 2 are 93.2℃ / 6300Pa; (4) The operating parameters of the hybrid condenser 3 are 36℃ / 6300Pa; (5) The inlet parameters of the surface condenser 5 on the fresh water side are 36℃ / 101325Pa and the outlet parameters are 33℃ / 101325Pa. (6) The seawater inlet parameters of the surface condenser 5 are 25℃ / 101325Pa and the outlet parameters are 30.3℃ / 101325Pa; (7) The operating parameters of the liquid storage tank 9 are 36℃ / 20000Pa; (8) The isentropic efficiency of vacuum compressor 2 is 0.8; (9) The minimum operating temperature difference of the surface condenser 5 is 3℃. Calculations show that flash evaporation of 1 kg of fresh water in flash evaporator 1 requires 116.3 kg of seawater, and the power consumption of the vacuum compressor is 0.036 kWh / kg. Correspondingly, the amount of fresh water required for cooling in hybrid condenser 3 is 201.5 kg, and the amount of seawater required for cooling in surface condenser 5 is 115.3 kg.

Claims

1. A natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust, characterized in that, The system includes a flash evaporator, a vacuum compressor, a mixing condenser, a surface condenser, and a first water pump. The flash evaporator is positioned at a first height above the seawater to be desalinated, and the mixing condenser is positioned at a second height above the seawater to be desalinated. The first height is not lower than the second height. The surface condenser is positioned lower than the flash evaporator. The inlet of the flash evaporator is connected to the seawater to be desalinated via a pipeline. The gas outlet at the top of the flash evaporator is connected to the inlet of the vacuum compressor. The outlet of the vacuum compressor is connected to the first inlet of the mixing condenser. The outlet of the mixing condenser is connected to the inlet of the cooling medium channel of the surface condenser. The outlet of the cooling medium channel of the surface condenser is connected to the inlet of the first water pump. The outlet of the first water pump is connected to the second inlet of the mixing condenser. The surface condenser has a cooling medium channel through which cooling medium flows to cool the gas-liquid mixture entering the cooling medium channel. The cooling medium channel has an exhaust port. The height difference between the mixing condenser and the surface condenser causes the gas-liquid mixture entering the surface condenser to be pressurized to one atmosphere under gravity.

2. The natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust as described in claim 1, characterized in that, The system includes a liquid storage tank and a second water pump. The outlet of the mixing condenser is connected to the inlet of a three-way valve. The first outlet of the three-way valve is connected to the inlet of the liquid storage tank. The second outlet of the three-way valve is connected to the inlet of the channel for the medium to be cooled. The outlet of the liquid storage tank is connected to the inlet of the second water pump. The outlet of the second water pump is connected to the third inlet of the mixing condenser. The liquid storage tank is positioned below the mixing condenser but above the surface condenser.

3. The natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust as described in claim 1 or 2, characterized in that, It includes a third water pump, which drives the liquid discharged from the bottom outlet of the flash evaporator into the cooling medium channel.

4. The natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust as described in claim 3, characterized in that, The outlet at the bottom of the flash evaporator is connected to the inlet of the third water pump, and the outlet of the third water pump is connected to the inlet of the cooling medium channel.

5. The natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust as described in claim 1, characterized in that, The first height and the second height are not less than 9.5 meters.

6. The natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust as described in claim 5, characterized in that, The surface condenser is positioned at sea level.

7. The natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust as described in claim 1, characterized in that, The flash evaporator operates at a temperature range of 10–30°C and at a pressure range equal to the saturated seawater pressure corresponding to seawater at 10–30°C. The mixing condenser operates at a temperature range of 16–36°C and at a pressure higher than the saturated water pressure at 16–36°C.

8. The natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust as described in claim 2, characterized in that, The flash evaporator operates at a temperature range of 10–30°C and at a pressure range equal to the saturated seawater pressure corresponding to seawater at 10–30°C. The mixing condenser operates at a temperature range of 16–36°C and at a pressure higher than the saturated water pressure at 16–36°C. The storage tank operates at a pressure range of 10–90 kPa.

9. The natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust as described in claim 1, characterized in that, The vacuum compressor is one of the following: rotary vane vacuum compressor, screw vacuum compressor, reciprocating piston vacuum compressor, scroll vacuum compressor, or centrifugal vacuum compressor.

10. The natural vacuum seawater desalination system based on hybrid condensation and gravity-pressurized exhaust as described in claim 1, characterized in that, The surface condenser is one of the following: shell and tube heat exchanger, plate heat exchanger, plate-fin heat exchanger, coaxial heat exchanger, and spiral plate heat exchanger.

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