What is thermal siphon effect? @Chemical Engineering Knowledge E014 HaiChuan【Video】

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Thermosiphon effect refers to the phenomenon in which a fluid loses density when heated and flows upward, while its density increases when it cools down, causing it to flow downward, thus creating a circular flow. This phenomenon is common in natural circulation systems, such as thermosiphon solar water heaters. Key point: Heating: When the fluid at the bottom is heated, its density decreases and it flows upward. Cooling: After the fluid cools at the top, its density increases and it flows downward. Cycle: This flow driven by the density difference creates a continuous cycle. Application: Solar water heater: Uses a thermosiphon cycle to heat water. Geothermal systems: Used for the utilization of geothermal energy. Industrial cooling: Some cooling systems also utilize this principle. Advantages: No external power required: relies on natural convection to save energy. Simple structure: low maintenance costs. Disadvantage: Limited efficiency: the circulation speed is slow, and its efficiency is lower than that of forced-circulation systems. Depends on temperature difference: A sufficient temperature difference is required for effective operation. In summary, thermosiphon phenomenon is a natural circulation process driven by temperature differences, and it is widely used in various fields

This post was last edited by tcdlb on 2025-3-3 16:39. The thermosiphon effect is a process in which heating causes part of the liquid to vaporize, resulting in a vapor-liquid mixture with lower density; the difference in density then serves as the driving force for this process. As the name implies, it is a siphon phenomenon driven by heat. In a siphon heat exchanger, when the liquid is heated, it expands in volume and becomes less dense and lighter, causing it to rise. Cold liquid from the surroundings then takes its place, thus creating a circulation. Circulation is driven by the density difference between the gas and liquid phases. Principle: The siphon effect is caused by the attractive forces between liquid molecules and differences in potential energy; in other words, it utilizes the pressure difference in the water column to push water upward before it flows back to lower areas. Since the water surface at the pipe openings is subjected to different atmospheric pressures, water flows from the side with higher pressure to the side with lower pressure, until the atmospheric pressures on both sides are equal and the water levels inside the container become the same; at that point, the flow of water stops. Using the siphon effect, the water in the container can be drained quickly. The siphon is an ancient invention of humanity; as early as the 1st century BC, people created a peculiar type of siphon. In fact, siphoning is not entirely caused by atmospheric pressure; siphoning can also occur in a vacuum. The force that makes a liquid rise is the cohesion between the molecules of the liquid. During siphoning, since more liquid flows out of the tube than flows in, the gravitational forces on both sides become unbalanced; consequently, the liquid continues to flow in one direction. As the liquid flows into the pipe, the pressure decreases the higher up it goes. If the tube through which the liquid rises is very tall, the pressure will drop to the point where bubbles form inside the tube (composed of air or other gaseous substances), and the operating height of the siphon is determined by the formation of these bubbles. Since bubbles cause the liquid to be interrupted, the forces between gas molecules at both ends of the bubble drop to zero, thereby disrupting the siphon effect; therefore, the tube must be completely filled with water. Under normal atmospheric pressure, a siphon functions better than in a vacuum, because the atmospheric pressure acting on both ends of the tube increases the overall pressure inside the siphon. The thermal cycling motion is known as the thermosiphon effect; the greater the temperature difference between the collector and the heat exchanger, the faster the circulation of water between them. In tube heat exchange, the thermosyphon heating system consists of aluminum heat dissipation tubes (outer tubes) and heating tubes (inner tubes). After the space between the inner and outer tube walls is evacuated and filled with a diffusing fluid, it becomes an innovative green heat transfer system for heating and cooling. When hot water at a high temperature is injected into the heating tube (inner tube), the diffusion liquid absorbs the heat around the heating tube, evaporates, and rapidly spreads toward the inner wall of the heat-releasing tube. Upon contacting the tube wall, it cools down and condenses, releasing heat in the process. The condensed diffusion liquid returns to its original location under the effect of gravity, where it is heated again, thus creating a cycle of heating → evaporation → condensation with heat release → re-heating, which leads to a rapid increase in the temperature of the surrounding environment and thereby achieves the purpose of heating. Similarly, when colder water is injected into the inner tube, the special diffusion fluid on the outer wall of the inner tube absorbs heat from the outer wall of the tube, rapidly lowering its temperature; the temperature around the tube wall drops instantly. This cycle repeats, causing the temperature of the surrounding environment to drop rapidly, thereby achieving the purpose of cooling. A high vacuum state always exists between the outer tube and the inner tube, and heat transfer is achieved through siphon action via a diffusion fluid; such heat transfer tubes are commonly referred to as “thermal siphon tubes”. Furthermore, as it is made of alloy aluminum with high heat exchange efficiency and high thermal conductivity, it is also known as an \"aluminum thermosyphon\". Reboiler: A thermosyphon reboiler relies on the hydrostatic head of the liquid in the bottom of the tower, as well as the density difference between the two phases within the reboiler, to generate a driving force that creates a thermosyphon effect. Thermosyphon reboilers utilize the density difference between the gas-liquid mixture in the reboiler and the liquid at the bottom of the tower as a driving force to increase the flow velocity of the fluid within the tubes, thereby reducing fouling and enhancing the heat transfer coefficient. These devices are compact and require little space; they can be divided into vertical thermosyphon reboilers and horizontal thermosyphon reboilers. Generally, in vertical thermosyphon systems, the process fluid flows through the tube side, while the heating steam flows through the shell side ； The evaporation side of the horizontal thermosyphon reboiler is not restricted; the flow path can be selected based on process requirements, such as the amount of evaporation and the susceptibility to scaling. The installation height of a horizontal thermosyphonic reboiler is lower than that of a vertical one; it has a greater circulation driving force and a larger circulation volume as well. Application: For the lubrication and cooling of the sealed end faces, double-end and tandem seals require a fluid circulation system for isolation. Although a pressure cycling system can meet this requirement, it is very uneconomical. In this case, a high-mounted siphon container is used; the heat generated by heat conversion creates a thermosiphonic temperature difference that drives the isolation fluid to circulate, thereby lubricating and cooling the sealed end faces. In the application of siphon effects in sealing systems, there should not be a large temperature difference between the sealing chamber and the siphon container, and the density difference is also small; therefore, throttling elements should be avoided in the connecting pipelines. The piping should have as few bends as possible, which facilitates the upward movement of low-density fluids. Due to the small temperature difference, the circulation volume must also be low; therefore, it is effective to install a pumping device inside the sealed chamber, which can also extend the service life of the siphon container.