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By Sheila Kennedy
Heat exchangers are getting increasingly compact and efficient. Enhanced, compact, ultra-compact, microscale and mesoscale variations are on the market or in development. Continuing innovations in nanotechnology and fuel cells promise further advances in heat exchanger miniaturization.
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The benefits of downsizing from shell-and-tube heat exchangers to compact heat exchangers include smaller size and weight, greater efficiency, increased mobility and reduced structural and support requirements. Significant capital and operating cost savings are possible with compact, ultra-compact and enhanced heat exchangers.
Compact heat exchangers are typically constructed with a large number of small channels to improve heat transfer coefficients. While compact plate-and-frame units generally follow the same construction methods, the performance differences are based on the flow channel patterns on the plates and the choice of materials.
The hybrid Plate & Shell Compact Heat Exchanger by Vahterus uses a system of internal flow directors that allows the four primary and secondary fluid connections to reside on one end of the unit to reduce the required space. Moreover, having all connections on one end makes it easier to install, insulate, remove and modify the connecting piping.
The majority of the heat exchangers used in the process industries are metallic, although other materials are available. Carbon internals stand up to sulfuric acid. Ceramic units can be used at high temperatures. Plate-and-frame heat exchangers in non-metallic materials, such as a graphite/fluoroplastic composite or a polymer, help prevent corrosion and fouling. Teflon and glass might be used where corrosion is a significant issue — Teflon is beneficial in abrasive environments. Polymer heat exchangers are available for heating, ventilating and air conditioning applications.
Some thin polymer film compact heat exchangers address deficiencies in thermal conductivity and deal with high pressure drops, as reported in the International Journal of Heat Exchangers. Corrugations on the internal channels promote better fluid mixing and increase the unit’s thermal performance. Polymer film heat exchanger plates coated with polyetheretherketone (PEEK), useful for its thermal, chemical and mechanical stability, have improved abrasion resistance, durability and adhesion.
Fuel cell technology has inspired new innovations in heat exchangers. For example, compact and efficient heat exchangers are crucial elements of portable fuel cell power supplies. This power source has a longer run time than batteries for a given weight, operates more quietly and efficiently than internal combustion engine generators, and supports distributed or mobile applications.
Mesoscopic Devices has developed portable solid oxide fuel cell (SOFC) generators the size of a child’s lunchbox. The heat exchangers are extremely compact, using mesoscale channels to achieve high power densities. The reduced size and thermal mass enable rapid start operation. High effectiveness and low pressure drops also are characteristic of these compact heat exchangers. Mesoscopic Devices supports additional specialized compact heat exchanger applications including extremely compact combustion-fired water heaters, condensing water from exhaust streams, boiling water with combustion systems, and rejecting heat.
Last year, the U.S. DOE awarded a Small Business Innovation Research grant to Acumentrics to investigate the feasibility of using a hybrid ceramic/metal heat exchanger with the company’s SOFC system. The fuel cell system, designed to provide clean power from a variety of fuels, will allow independence from the electrical grid or offer back-up power or grid augmentation.
Researchers in micro-scale engineering also are influencing thermal science and industrial manufacturing. For instance, microturbine-based cooling, heating and power systems that use compact, lightweight and affordable hot water heat exchangers are projected to help reduce the cost and consumption of fuels and reduce greenhouse gases.
Nanotechnology is likely to provide significant opportunities for transformational change in the scale and properties of heat exchangers. The use of nanocomposites is promising for its ability to improve efficiency, while developments in engineered nanomaterials and nanofluids might further change the face of heat exchanger design and fabrication.
To facilitate ongoing improvements, researchers and engineers need information about heat transfer conditions for each stream and the associated heat exchanger network. The Xchanger Suite by Heat Transfer Research, Inc. (HTRI) is a software program and database that supports the thermal design, analysis, rating and simulation of heat exchangers. As HTRI obtains and analyzes new data, it updates its methods and incorporates them into the next version of the software.
E-mail Contributing Editor Sheila Kennedy, managing director of Additive Communications, at Sheila@addcomm.com.
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