Exploring the fundamental workings and diverse applications of heat exchangers, this article delves into their pivotal role in efficiently transferring thermal energy between fluids. From their basic principles to advanced simulations using ANSYS Fluent, discover how heat exchangers drive innovation across industries, optimizing processes and enhancing energy utilization.
What is a heat exchanger?
In a heat exchanger, the hot fluid transfers its heat to the cold fluid without the two fluids mixing together. The phase of any fluid can be gas or liquid.
In an ideal heat exchanger, the temperatures of the two fluids at the entrance are different, but at the exit, the temperatures of both fluids are the same. However, this situation is uncommon. In some cases, the outlet temperatures of the fluids are deliberately kept different because a fluid with a specific temperature is required at the outlet.
How does a heat exchanger work?
In each heat exchanger, there is at least one inlet and one outlet for each fluid. In some cases, the number of fluids can be more than two, or there may be multiple ducts for a single fluid inside the exchanger. The channels inside the exchanger can have different geometries, which are selected based on the designer’s preference and the application’s requirements.
In all types of exchangers, two fluids are separated by a wall through which they exchange heat without mixing. In many cases, fins are used to increase the contact surface area and enhance heat transfer.
Types of heat exchanger
Heat exchangers are available in various types, each tailored to meet specific applications and operating conditions. Some of the common types include:
Shell and tube heat exchanger
In this heat exchanger model, one fluid flows through a large tank, which is usually cylindrical. Inside this tank, there are tubes through which another fluid moves; these tubes can be U-shaped. This heat exchanger model is very popular and widely used in various industries, such as oil refineries and SAGD facilities. It can operate at high pressures and temperatures.
Shell and tube heat exchanger with one shell pass and one tube pass, from Incropera et al., Fundamentals of Heat and Mass Transfer, 7th edition, Wiley.
Plate heat exchanger
In this type of heat exchanger, a series of plates are placed together and sealed with gaskets, creating a closed environment between the plates for fluid movement. In these closed environments, hot and cold fluids flow alternately. The plates typically have a textured surface with a complex design to enhance heat transfer. This heat exchanger model is commonly used in industries such as food processing and oil refining. For example, this type of heat exchanger can be used to pasteurize milk. However, these exchangers are not suitable for high-pressure applications.
Sketch of plate heat exchanger, from Robinson et al., mass and heat transfer: analysis of mass contactors and heat exchangers
Hrale Stainless Steel plate Heat Exchanger
Double-pipe heat exchanger
This type of heat exchanger is made of two concentric cylinders. Fluids can enter both cylinders from one direction and exit from the other side, or they can enter from opposite directions. A double-pipe heat exchanger has a simple structure, making it cost-effective to construct and install. Additionally, it can be manufactured in various dimensions to suit different applications. Despite its simplicity, it is versatile and effective for many heat transfer processes. Double-pipe heat exchangers are commonly used in applications such as oil cooling, water heating, chemical processing, and HVAC systems.
Double–pipe heat exchanger (cocurrent flow), from Robinson et al., mass and heat transfer: analysis of mass contactors and heat exchangers
Finned-Tube Heat Exchanger
These models of heat exchangers are similar to shell and tube heat exchangers, with the addition of numerous fins to the tubes. These fins significantly increase the heat transfer contact surface, thereby enhancing heat transfer efficiency. Such exchangers find extensive use in HVAC systems, particularly when the fluid passing through the shell section is gas. Car radios also commonly incorporate this type of heat exchanger.
Schematic representation of core region of a fin–tube heat exchanger, from heat transfer enhancement in cross-flow heat exchangers using oval tubes and multiple delta winglets by S. Tiwari et al.
Heat exchangers applications
Heat exchangers find diverse applications across numerous industries due to their ability to efficiently transfer thermal energy between fluids or between a fluid and a solid surface. Some common applications include:
- HVAC Systems: Heat exchangers play a vital role in both heating, ventilation, and air conditioning (HVAC) systems within buildings. These integral components facilitate heat transfer between indoor air and a refrigerant or water loop, effectively regulating temperature and humidity levels. In HVAC systems, heat exchangers absorb heat from indoor air and transfer it to refrigerants, which are then released outdoors, contributing to cooling. Conversely, heat pumps utilize heat exchangers to transfer heat from the outdoor environment to warm the building’s interior, enhancing heating efficiency. Whether it’s in AC units or heat pumps, the functionality of heat exchangers is pivotal in maintaining comfortable indoor environments and efficient energy utilization.
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- Power plants: Heat exchangers serve a critical function in power generation, playing essential roles in various types of power plants. In fossil fuel power plants, they convert thermal energy from burning fuel into steam, which subsequently drives turbines to produce electricity. Additionally, condensers, another type of heat exchanger, are employed to convert exhaust steam back into water, creating closed-loop systems. These components are pivotal not only in fossil fuel power plants but also in nuclear power plants and renewable energy systems like solar thermal power plants, where they facilitate the transfer of heat to water or steam, enabling the generation of electricity through turbine-driven processes.
- Food processing: Heat exchangers are extensively utilized in diverse food processing applications to ensure product safety and quality. They play crucial roles in pasteurization, sterilization, cooking, drying, and cooling processes within the food and beverage production industry. By efficiently transferring heat, heat exchangers aid in heating liquids to kill bacteria during pasteurization, sterilizing food products, and facilitating the cooling process to maintain freshness and preserve product integrity.
- Oil and Gas Industry: Heat exchangers are pivotal in oil refineries and natural gas plants, facilitating heating, cooling, and condensing of hydrocarbons and process fluids. They aid in crude oil distillation, gas purification, and product condensation, ensuring efficient processing and product quality in these industries.
How does a heat exchanger work in a boiler
In a boiler, high-temperature gases are produced by the combustion process. Pipes pass inside the boiler, carrying the desired fluid, usually water. The gases transfer their heat to this fluid, raising its temperature and possibly changing its phase to steam. Increasing the number of tubes, optimizing their arrangement, and adding fins can enhance heat transfer. Boilers are used in many industries, including fossil fuel power plants and some HVAC systems.
Heat exchanger diagram
In each heat exchanger diagram, channels for the entry and exit of hot fluid and cold fluid can be seen. The path of heat transfer between these two fluids inside the heat exchanger can vary depending on the application. Fluids can enter in the same direction (parallel flow) or in opposite directions (counterflow). The diagram helps visualize how thermal energy is transferred from the hot fluid to the cold fluid, illustrating the temperature gradients and flow paths that drive this process. This content can be observed in all figures of this article.
Simulation of heat exchangers by ANSYS Fluent
ANSYS Fluent is known for its ability to simulate a wide range of heat exchanger models across diverse industrial applications. The software allows users to create detailed models of complex geometries and provides various meshing methods for accurate simulations. With ANSYS Fluent, users can effectively simulate turbulent flow regimes, a crucial aspect of many heat exchanger applications.
In addition, the software offers advanced post-processing capabilities, enabling users to extract comprehensive performance data from the simulations. As a result, ANSYS Fluent serves as a valuable tool for analyzing and optimizing heat exchanger performance. However, it is essential for users to ensure the accuracy of their simulations by validating the results against experimental data or established benchmarks.
At CFDLAND, we have completed numerous Heat exchanger projects using ANSYS Fluent, all of which can be viewed in our CFD shop. Additionally, you can submit your CFD projects through the Order Project section. Trust our team of CFD experts to handle even the most complex simulations with proficiency and precision.
Temperature contour of a heat exchanger based on ANSYS Fluent simulation, from “Performance Improvement of Shell & Tube Heat Exchanger with Baffle CFD Simulation”
Conclusion
In conclusion, heat exchangers stand as indispensable components across industries, enabling efficient heat transfer for myriad applications. From HVAC systems to power generation and food processing, their versatility and effectiveness drive innovation and sustainability. With advanced simulation tools like ANSYS Fluent, engineers can continue to refine heat exchanger designs, optimizing performance and contributing to a more energy-efficient future.
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