Applications of turbulators in heat exchangers

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Enhancing heat transfer: The so-called enhancement of heat transfer in heat exchangers refers to the adoption of certain technical measures, through the analysis and calculation of various factors that affect heat transfer, in order to increase the heat transfer capacity of heat exchange equipment, or to reduce its volume while maintaining the existing heat transfer capacity. The methods commonly used to enhance heat transfer in heat exchangers fall into three categories: increasing the heat transfer area (F) ； Increase the heat transfer temperature difference ； Increase the heat transfer coefficient (K). Methods to enhance heat transfer in heat exchangers 1. Increasing the heat transfer area F: Increasing the heat transfer area is the most common and simplest method used to improve heat transfer efficiency. In the process of increasing the heat transfer area of exchangers, simply enlarging the size of the equipment to boost the heat transfer area or adding more units to increase the heat transfer capacity not only requires higher investment in equipment, but also results in larger floor space requirements; moreover, the improvement in heat transfer efficiency is not significant. This approach has now been abandoned. Currently, the most common approach is to enhance heat transfer efficiency by appropriately increasing the heat transfer area per unit volume of the equipment. For example, in heat exchangers, materials such as finned tubes, corrugated tubes, and plate-fin heat transfer surfaces, which offer a large heat transfer area per unit volume, are widely used. By employing these materials, the heat transfer area per unit volume of each individual piece of equipment can be significantly increased, thereby achieving efficient and compact heat exchange systems. 2. Increase the heat transfer temperature difference Δt: Increasing the heat transfer temperature difference Δt in a heat exchanger is one of the common methods used to enhance its heat exchange efficiency. During the operation of the heat exchanger, increasing the pressure of the steam inside the radiant heating tubes, raising the temperature of the hot water used for heating, replacing tap water with cooler deep well water for the condenser cooling water, and lowering the temperature of the cooling water in the air cooler – all these measures can directly increase the heat transfer temperature difference Δt of the heat exchanger. However, there is a limit to increasing the heat transfer temperature difference Δt of the heat exchanger; we cannot rely on it as the primary method to enhance heat transfer efficiency. During use, we should consider whether the actual process or equipment conditions permit it. For example, when increasing the steam temperature of the radiant heating panels, we must not exceed the radiation intensity allowed by radiant heating; the increase in the steam temperature of these panels is actually a limited one, and improving the heat transfer efficiency of the heat exchanger can only be achieved to a certain extent by increasing the temperature difference Δt for heat transfer ； At the same time, we should recognize that an increase in the heat transfer temperature difference will increase the irreversibility of the entire thermal system, thereby reducing its availability. Therefore, one should not solely pursue an increase in the heat transfer temperature difference, but also take into account the rational use of energy throughout the entire thermal system. 3. Increase the heat transfer coefficient (K) – The most effective measure to improve the heat transfer performance of a heat exchanger is to strive to increase its heat transfer coefficient (K). The value of the heat transfer coefficient (K) of a heat exchanger is actually determined by the total thermal resistance in the heat transfer process; the greater the total thermal resistance, the lower the value of the heat transfer coefficient (K) ； The lower the heat transfer coefficient (K) of a heat exchanger, the worse its heat transfer performance. During operation, the total thermal resistance of a heat exchanger is the sum of its individual thermal resistances; therefore, to change the heat transfer coefficient, it is necessary to analyze each of these individual thermal resistances in the heat transfer process. How to control each individual heat resistance in the heat transfer process of a heat exchanger is key to determining its heat transfer coefficient. The methods mentioned above for enhancing heat transfer efficiency are more or less employed in heat exchangers. However, expanding the heat transfer area and increasing the temperature difference for heat transfer are often limited by factors such as space, equipment, funding, and practical constraints, making it impossible to enhance heat transfer indefinitely. Therefore, the main focus of current research on improving heat transfer in heat exchangers is on finding ways to increase this efficiency by controlling the heat transfer coefficient (K) of the heat exchanger. The most commonly used technique at present to increase the heat transfer coefficient (K) of heat exchangers is the addition of flow disruptors inside the heat exchange tubes. Through the action of these flow disruptors, the thermal resistance in the heat transfer process is reduced, thereby increasing the heat transfer coefficient (K) of the heat exchanger. The use of perturbators on heat exchangers to enhance heat transfer: In the process of heat transfer, the main factors that affect the heat transfer coefficient (K) of a heat exchanger include the flow conditions of the liquids inside and outside the heat exchanger, as well as the shape and size of the heat transfer surfaces. To improve the heat transfer coefficient of heat exchangers and enhance their heat transfer efficiency, various enhancement elements and measures have been developed both domestically and internationally. These mainly include the use of threaded tubes, corrugated tubes, tapered tubes, high-pitch multi-groove tubes, fully double-sided spiral finned tubes in heat exchangers, as well as the addition of flow disruptors within the heat transfer tubes to enhance heat transfer inside them. Among these, the use of flow perturbers in heat exchange tubes to enhance heat transfer has been employed in industry for many years. It enables a significant increase in the overall heat transfer coefficient of heat exchangers, allowing for **reduction in the required heat transfer area, lessening the weight of the equipment, and saving large amounts of metal material. Its numerous advantages are attracting increasing attention. 1. Types and common characteristics of flow perturber reinforcement elements. There are various forms of flow perturber reinforcement elements; the most commonly used ones include wire-based elements, metal coils, disc-shaped components, twisted iron strips, and wing-shaped elements. These turbulence-enhancing elements share one common feature: in the heat exchange tubes of the heat exchanger, these additives can effectively reduce the overall thermal resistance during the heat transfer process, **thereby increasing the heat transfer coefficient (K) of the heat exchanger and significantly enhancing its heat transfer performance. 2. Principle of heat transfer enhancement by turbulators: Studies have shown that turbulator additives introduced into the heat exchange tubes of a heat exchanger can cause the liquid flowing within these tubes to move in a distinct spiral pattern. In other words, adding turbulator additives to the heat exchange tubes of a heat exchanger is equivalent to introducing porous materials with a porosity of ε≥95% into those tubes. When the liquid flowing through these tubes passes by these turbulator additives, a significant diffusion flow effect occurs within the flow channel; at low Reynolds numbers (Re≥300), this diffusion flow promotes the transition of the liquid in the tubes to a turbulent state. The total thermal resistance of fluid in a turbulent state is the lowest among all flow states of fluids. Since the thermal resistance of fluid flowing in a turbulent state within the heat exchange tubes is very low, the heat transfer coefficient (K) of the heat exchanger will **increase**. At high heat transfer coefficient (K) values, the effect of flow disruptors in enhancing heat transfer within the heat exchanger becomes very significant. Of course, the effect of the perturbators in the heat exchanger on enhancing heat transfer for different media flowing through the heat exchange tubes varies. Furthermore, the shape of the turbulence elements inside the heat exchange tubes and the method of installing them on the heat transfer surface affect both heat transfer and flow resistance; their optimal design can generally be determined through experiments. For example, experiments show that filling the entire length of the pipe with spiral wires results in a lower flow resistance compared to installing spiral coils intermittently, while maintaining the same heat transfer performance. There are many other views regarding the principle of using perturbators to enhance heat transfer; some experts believe that this enhancement is due to an increase in the heat transfer area and roughness, which is undoubtedly correct. However, experiments have shown that even when not installed in close contact with the wall surface, axial fixers positioned at the center of the flow channel can increase the α value. Some explain this by suggesting that the filler generates continuous vortices, creating a central rotating flow in the direction of flow; under the influence of centrifugal force, this facilitates thorough mixing between the fluid at the center of the tube and the fluid in the wall boundary layer. This thins the boundary layer and enhances heat transfer. Overall, the theory regarding the use of perturbators to enhance heat transfer is still incomplete and inconsistent; some data come solely from experiments, and more researchers are needed to develop and utilize this area of research. 3. Characteristics of heat transfer enhancement by flow disruptors: Adding flow disruptor additives to the heat exchange tubes in heat exchangers results in the most notable characteristic being an **increase in the heat transfer coefficient on the inner side of the heat exchange tubes**. Tests have shown that by adding turbulator additives to the heat exchange tubes of a heat exchanger, the heat transfer coefficient on the inner side of these tubes can be increased by more than 3.5 times compared to tubes without such additives. In addition to reducing metal consumption, flow disruptors enhance heat transfer, which in turn improves the efficiency of heat utilization in factories and lowers energy consumption. Currently, some designs aim for high heat intensity; whereas in shell-and-tube exchangers, due to their low heat transfer efficiency, the main approach adopted in design is to increase the logarithmic mean temperature difference, which leads to a significant increase in energy consumption. Taking the atmospheric and vacuum distillation units in refineries as an example, the heat transfer temperature difference is 60°C; assuming the temperatures of the hot and cold fluids to be 260–200°C, heat transfer accounts for 21.5% of the total heat flow. If this temperature difference is reduced to 33°C, the heat transfer losses can be lowered to 10%. Heat exchangers that use turbulators to enhance heat transfer can significantly reduce the heat transfer temperature difference while maintaining the same heat transfer efficiency, thereby reducing heat losses and enabling better energy matching to achieve energy savings and reduced consumption. Another advantage of using perturbators to enhance heat transfer is that it can effectively suppress the formation of fouling. Scaling is a very tricky problem for heat exchangers. Dirt reduces heat transfer efficiency; it has poor thermal conductivity, at only 1/30 to 1/50 that of steel. For carbon steel tube oil coolers, when the scale thickness reaches 2 mm, their operating efficiency decreases by 30% compared to when they are new and free of scale. The American Heat Transfer Research Company has conducted years of research on the fouling problem in heat exchangers and found that the formation and growth of fouling are primarily related to the temperature and flow rate of the fluid. The higher the fluid temperature, the greater the temperature difference between the fluid and the wall surface; meanwhile, a lower flow rate increases the likelihood of fouling occurring. To remove fouling on the tube side, some manufacturers abroad increase the flow velocity inside the tubes (V=2–3 m/s), but this results in a high pressure drop and significant energy consumption. In heat exchangers that use perturbators to enhance heat transfer, fouling on the equipment’s tube side is significantly reduced. Firstly, due to the diffusive flow of the fluid, the temperature gradient in the medium is low, which suppresses the formation and growth of fouling ； Secondly, due to the high turbulence degree of the diffusive flow (the perturbators act like static agitators), impurities in the fluid do not tend to deposit as scale. Cleaning heat exchangers enhanced by turbulators is very convenient. For short-term cleaning, it is not necessary to remove the reinforcement elements; the tube side can be washed with clean water at a flow rate of V > 0.8 m/s. Experiments show that. When the water velocity reaches 0.8 m/s, the flow will generate strong diffusive vortices, exerting a significant erosive effect on the pipe walls. Therefore, the flow disruptors and scale on the tube wall can be removed quite cleanly. If cleaning is performed after long-term operation, the reinforcement elements can be removed to clean the turbulence generators and the tube walls separately, which is also easy to do. Heat transfer elements enhanced with turbulators are very easy to install and remove, require simple daily maintenance, and are particularly useful for upgrading and optimizing existing equipment.