PLANT MANAGEMENT
T
raditional plate or shell and tube heat exchangers have long been used in processing industries. Today, however, with many new
applications involving high pressures, temperatures, and exposure to corrosive environments, more manufacturers are turning to compact printed circuit heat exchangers (PCHEs). A PCHE is a multi-layer heat exchanger consisting of thin, fl at metal plates into which fl uid fl ow microchannels are chemically etched in each layer to form a complex fl ow pattern. T e layers are then diff usion bonded together to create a dense heat exchanger with superior airfl ow and heat transfer properties. When designed this way, a heat
exchanger can be up to 85% smaller and lighter than traditional plate or shell and tube designs. In addition, PCHEs do not require excessive pipework, frames, or other associated structural elements, further reducing costs. “A high-quality diff usion bonded PCHE can withstand very high pressures of hundreds of bars and extreme temperatures beyond 800°C. As a result, PCHEs are well suited for a wide range of demanding applications, including oil and gas, hydrogen vehicle fueling stations, and aerospace,” says Dr Udo Broich, managing director of PVA Industrial Vacuum Systems.
DIFFUSION BONDING VERSUS BRAZING For many years, diff usion bonding has been used to join high strength and refractory metals that are either diffi cult or impossible by other means. T e process involves applying high temperature and pressure to the bonding part in a high- vacuum hot press such as those off ered by PVA TePla; this causes the atoms on solid metallic surfaces to intersperse and join. T e fi nal piece will have little or no interface lines or striations if the materials are similar; the interface of one material blends into the other, and vice versa. T e same result can also be achieved with dissimilar materials with the right equipment, material preparation, and process. T e key to the process is using diff usion bonding to join the layers over other alternatives, such as vacuum brazing. Although brazing is widely used for joining metals under normal conditions, it can be insuffi cient in high temperature, pressure,
The diff usion bonding process involves applying high temperature and pressure to the bonding part in a high-vacuum hot press
HEAT EXCHANGER ADVANCES
Compact printed circuit heat exchangers (PCHE), produced through a diff usion bonding process, surpass traditional alternatives in extreme temperature and pressure environments
or corrosion situations. Brazing is a joining process in which two or more metal items are joined together by melting and fl owing a fi ller metal into the joint. T e fi ller metal fl ows into the gap between the layers through capillary action. With a proper choice of fi ller material
and process parameters, brazing can also create high strengths and thermal resistant joints. However, as the fi ller metal always has a diff erent chemical composition from the bonding part materials, the properties of brazed components usually cannot reach that of a solid part. “In the case of brazing a PCHE, engineers must consider another issue: during brazing, molten fi ller metal can penetrate the microchannels and solidify, blocking the channels required for airfl ow. T is can render the PCHE quite ineff ective,” says Broich. “Since
diff usion bonding requires no fi ller metal and is a solid-state joining process, the microchannels remain intact. “When the layers of a PCHE are
diff usion bonded, the fi nal product retains the parent material’s mechanical, chemical, and thermal properties. Given the high strength and integrity of the material, PCHEs can withstand very severe operational conditions,” explains Broich. A signifi cant advantage of diff usion bonded PCHEs is that it signifi cantly reduces the size of the heat exchanger. “PCHEs have about 85% less mass and volume of traditional heat exchangers, while the microchannels provide a large surface area for heat exchange,” says Broich. “Achieving the same heat transfer rate with a standard [plate, or shell and tube] heat exchanger] design requires much more mass and volume.”
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