Heat Exchanger

A heat exchanger is a device that transfers heat from one medium to another—usually from a liquid to another liquid, a liquid to a gas, or a liquid to a solid surface, and vice versa. Heat exchangers may be used for heating or cooling purposes, though in industrial applications, most heat exchangers are used to keep equipment cool and functioning properly. If chemicals, liquids, gases, or machines are not kept within safe operating temperatures, accidents or injuries can occur. With an increased focus on carbon emissions, water usage, and climate impacts from heat exchangers, many industrial and mechanical engineers are now researching options to improve the efficiency of heat exchangers in various applications.

Background

Heat exchangers are common pieces of equipment found in a variety of engines and machines, both in the home and in industrial applications. The key facet of a typical heat exchanger is that the device transfers heat from one liquid (or gas) to another, but the two liquids or gases never come into contact with each other. For instance, most people are familiar with hot-water heating in a home, which may deliver heat through baseboard heaters or radiators. Hot-water heating is an example of heat exchange. The furnace burns natural gas or oil, which creates a flame. This flame burns over pipes that contain water. The water flowing through the pipes warms as the gas or oil burns. As the water circulates throughout the baseboard heaters or radiators in the home, it heats the air. Such heating systems are known as indirect contact heat exchangers. Less commonly used are direct contact heat exchangers, which actually do mix all or some of the fluids together. This process is often used when heat is being transferred across the same type of fluid.

Refrigerators and air conditioners contain heat exchangers, but they function in the opposite manner. Both refrigerators and air conditioners pull heat from an area and cool it in fluid. The radiator in a vehicle is a heat exchanger—water flows through the radiator to cool the engine, and fans in the car and the movement of the vehicle help to cool the radiator. A vehicle radiator provides examples of the additional tools that are often needed to ensure that a heat exchanger is functioning properly, such as fans, coolants, and condensers. Among the most common materials used for heat exchangers are steel, copper, bronze, stainless steel, aluminum, and cast iron.

Two common types of heat exchangers are shell and tube exchangers and plate/fin exchangers. In a shell and tube design, one fluid flows through one set of metal tubes, and the second fluid is housed in a shell that surrounds the tubes. Plate/fin heat exchangers have rows of thin metal plates; heat passes quickly between the surface areas of each plate. Other, less common designs include a coil-in-tank design, which includes one or two tubes inside a storage tank, or a tube-in-tube design, in which the tubes for heating and cooling directly contact each other. The directional flow of the fluid in a heat exchanger is also important. In a parallel-flow system, the hot and cold fluids flow parallel to each other. In a countercurrent flow, the fluids flow in opposite directions. In a cross-flow, the fluids flow in different directions and cross paths. All of these flows can affect the rate of heating and cooling.

Impact

When heat exchangers are used in industrial settings, energy efficiency—and preventing energy loss—is a common concern, as an inefficient heat exchanger can lead to significant losses. One primary concern is the fouling of a heat exchanger, which means that deposits have accumulated on the heat transfer surfaces. These deposits reduce the amount of heat that can be recovered and transferred and can slow down the flow of any fluid moving through the system, which means that additional energy is required to pump the same amount of fluid. That means increased use of oil or gas and increased emissions of these greenhouse gases. A system that has become inefficient in this way will also use increased water and electricity to compensate. These are all major concerns for a number of reasons, including environmental pollution and climate impacts from increased emissions; health problems related to air pollution; and issues with safely disposing of chemical wastes. From a business perspective, an inefficient or fouled heat exchange system will require increased maintenance costs and could present safety hazards in the workplace.

Conversely, an efficient heat exchange system can be used to reduce emissions and recapture energy that might otherwise have been lost. For instance, some heat exchangers capture steam or exhaust that is released during a manufacturing process and reuse this heat elsewhere. A solar water heating system is another type of efficient heat exchanger. It transfers solar energy directly to the water or air inside a home for heating. In 2012, professor of mechanical and industrial engineering Georgios Vatistas developed a new design for a heat exchanger that uses a vortex—a swirling flow—in the exchange process, which has been shown to be forty times more efficient than traditional heat exchangers. While the necessity of using heat exchangers in home and industrial settings is unquestioned, the drive to make them more efficient—reducing energy usage and costs, water usage, and emissions—will likely be a common goal in the continual development and improvement of these machines.

Bibliography

Casanueva-Robles, T., and T. R. Bott. "The Environmental Effect of Heat Exchanger Fouling: A Case Study." ECI Symposium Series, vol. RP2, 2005, pp. 278–82, dc.engconfintl.org/cgi/viewcontent.cgi?article=1035&context=heatexchanger2005. Accessed 21 Jan. 2017.

"Heat Exchangers for Solar Water Heating Systems." Energy.gov, energy.gov/energysaver/heat-exchangers-solar-water-heating-systems. Accessed 21 Jan. 2017.

"Hot Invention Cools Down Environment: Environmentally-Friendly Heat Exchanger Produced." PHYS.org, 15 Feb. 2012, phys.org/news/2012-02-hot-cools-environment-environmentally-friendly-exchanger.html. Accessed Jan. 21 2017.

Hussein, Hind Azeez Mohammed, Rozli Zulkifli, Wan Mohd Faizal Bin Wan Mahmood, and Raheem K. Ajeel. "Structure Parameters and Designs and Their Impact on Performance of Different Heat Exchangers: A Review." Renewable and Sustainable Energy Reviews, vol. 154, Feb. 2022, DOI: 10.1016/j.rser.2021.111842. Accessed 18 Jan. 2023.

"Industrial Heat Exchangers: What They Are, How They Work, and Why They Are Needed." STI Group,2014, setxind.com/downstream/industrial-heat-exchangers-what-they-are-how-they-work-and-why-they-are-needed/. Accessed Jan. 20 2017.

Müller-Steinhagen, H., et al. "Heat Exchanger Fouling: Environmental Impacts." Heat Transfer Engineering, vol. 30, no. 10–11, 2009, pp. 773–6, www.tandfonline.com/doi/abs/10.1080/01457630902744119#. Accessed 21 Jan. 2017.

Ronquillo, Romina. "Understanding Heat Exchanges." Thomas Publishing Company, 4 Oct. 2024, www.thomasnet.com/articles/process-equipment/understanding-heat-exchangers/. Accessed 20 Nov. 2024.

Shah, R.K., et al. "Opportunities for Heat Exchanger Applications in Environmental Systems." Applied Thermal Engineering, vol. 20, no. 7, 2000, pp. 631–50, www.sciencedirect.com/science/article/pii/S1359431199000459. Accessed 21 Jan. 2017.

Shah, Ramesh K., and Dušan P. Sekulić. Fundamentals of Heat Exchanger Design. John Wiley & Sons, 2003.

Woodford, Chris. "Heat Exchangers." ExplainThatStuff!, 12 Nov. 2016, www.explainthatstuff.com/how-heat-exchangers-work.html. Accessed Jan. 21 2017.