Diffusion Bonded PCB Heat Exchangers for Harsh Environments
Traditional plate or shell and tube heat exchangers have long been used in the process industries. Today, however, with many new applications involving pressures, high temperatures and exposure to corrosive environments, more and more manufacturers are turning to compact printed circuit heat exchangers (PCHE).
A PCHE is a multi-layer heat exchanger made of thin flat metal plates in which microfluid flow channels are chemically etched into each layer to form a complex flow pattern. The layers are then diffusion bonded to create a dense heat exchanger with superior airflow 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. Additionally, PCHEs do not require excessive associated piping, frames, or other structural elements, further reducing costs.
“A high-quality diffusion-bonded PCHE can withstand very high pressures of several hundred bars and extreme temperatures in excess of 800°C. As a result, PCHEs are well suited for a wide range of demanding applications, including oil and gas, hydrogen vehicle service stations and aerospace,” said Dr. Udo Broich, Managing Director of PVA. Industrial Vacuum Systems GmbH, Wettenberg, Germany.
Dr. Broich, who wrote his doctoral thesis on joining technology, focused on vacuum soldering and diffusion bonding for 25 years. PVA TePla AG is a global manufacturer of industrial furnaces and PulsPlasma nitriding systems.
Diffusion bonding versus soldering
For many years diffusion bonding has been used to join high strength and refractory metals that are difficult or impossible by other means. The process consists of applying high temperature and pressure to the part to be bonded in a high-vacuum heat press such as those offered by PVA TePla; this causes atoms on solid metal surfaces to intersperse and join together.
The final part will have few or no interface lines or ridges if the materials are similar; the interface of one material blends into the other, and vice versa. The same result can also be achieved with different materials with the proper equipment, material preparation and process.
The key to the process is using diffusion bonding to join the layers over other alternatives, such as vacuum soldering. Although brazing is widely used to join metals under normal conditions, it may be insufficient in situations of high temperature, pressure or corrosion.
Brazing is a joining process in which two or more metallic elements are joined together by melting and flowing a filler metal into the joint. The filler metal flows into the space between the layers by capillary action.
With proper choice of filler material and process parameters, brazing can also create high strength, heat resistant joints. However, because the filler metal always has a different chemical composition than the bonding part materials, the properties of the brazed components usually cannot match those of a solid part, according to Dr. Broich.
“When brazing a PCHE, engineers have to consider another issue: during brazing, molten filler metal can enter the microchannels and solidify, blocking the channels needed for airflow. . This can make PCHE quite ineffective,” Dr. Broich said. “Because diffusion bonding requires no filler metal and is a solid-state assembly process, the microchannels remain intact.”
“When the layers of a PCHE are bonded by diffusion, the final product retains the mechanical, chemical and thermal properties of the base material. Given the high strength and integrity of the material, PCHEs can withstand very severe operating conditions,” explained Dr. Broich.
A significant advantage of diffusion bonded PCHEs is that they significantly reduce the size of the heat exchanger.
“PCHEs have about 85% less mass and volume than traditional heat exchangers, while microchannels provide a large surface area for heat exchange,” Dr. Broich said. “Achieving the same heat transfer rate with a standard [plate, or shell and tube heat exchanger] the design requires much more mass and volume.
Growth of markets for PCHE linked by diffusion
Due to the inherent advantages of diffusion bonded PCHEs, many industries are adopting this evolving technology to improve heat transfer in various applications.
oil and gas
The compact and efficient heat transfer capabilities of PCHEs are ideal for many oil and gas applications, including preheaters, superheaters, gas compression coolers, high temperature recuperators and liquefied natural gas (LNG) exchangers ).
For example, in offshore LNG production, natural gas is converted to liquid for safe transportation or storage when pipelines are unavailable. LNG takes up only a fraction of the space in its liquid form, but natural gas must be cooled to around -260°F as part of the process, generating heat.
A PCHE is a practical and compact solution for areas with little space such as on board ships.
Hydrogen refueling stations
PCHEs are a requirement at fueling stations for hydrogen vehicles. The stored hydrogen must first be cooled to approximately -40°C before being transferred to the vehicle tank. This cooling process avoids potential damage due to excessive temperature which can damage the tanks during filling.
“To fill a tank, pressurized hydrogen is distributed at approximately 1,000 bars; this process generates excessive heat,” Dr. Broich said. “Because it is essential that the temperature does not exceed the critical limits of the tank, the hydrogen is pre-cooled to around -40°C. The PCHE in the service station cooling loop must withstand pressure of 1,000 bar or more and temperatures as low as -50°C. The compact design of the PCHEs also facilitates integration with the hydrogen dispenser housing.
Aluminum Heat Sink Applications: Electric Vehicles and Aerospace
Aluminum heatsinks are popular for thermal management of many critical devices where weight is a factor, including automotive and aerospace. Aluminum heat sinks are also commonly used with electric batteries.
The most common aluminum alloys used in heat sinks and heat exchangers are the 6000 series alloys. However, magnesium and silicon are the main alloying elements and are difficult to join with methods such as brazing. .
“In many heatsink applications, solder flux is often prohibited. Vacuum brazing therefore remains an assembly technology using filler metals based on an aluminum-silicon eutectic alloy. However, these alloys have a melting point of 580°C, very close to the melting point of the base material, thus providing only a tiny process window to achieve high quality bonds,” said Dr Broich.
He notes that PVA TePla has developed a diffusion bonding process to successfully join high-alloy aluminum materials like 6061 over the years.
Oven design optimization for diffusion bonding
There are different approaches to oven design and process implementation for diffusion bonding of PCHE. Advances in heat presses in high vacuum furnaces allow for superior pressure control and rapid cooling systems that improve bonding, increase yields and dramatically reduce cycle time.
Manufacturers such as PVA TePla offer multi-cylinder systems with large pressing plates that can accommodate various parts. The largest, the company’s MOV 843 HP, can handle substrates as wide as 950mm (37.4”) x 1300mm (51.18”), which is quite a large area for diffusion bonding.
The pressing force is 8000 kN. Research is underway to increase this geometric limitation as well.
The integrated press provides remarkably consistent pressure across the entire surface by controlling and synchronizing each cylinder. The MOV also comes with pressure transducers built into the bottom of the pressing plate.
Individual hydraulic cylinders can be adjusted in software to achieve uniformity even over large areas based on sensor feedback.
Dr. Broich emphasizes that high bond strength is critical for applications involving PCHEs in extreme environments.
“It requires a high-strength design [when a high vacuum hot press is needed] of the pressing unit[pressingramsandplatens)totransfer800or1000tonsat1000°Corevenhighertemperatures”saidDrBroich[pressingramsandplatens)totransfer800or1000tonsat1000°Corevenhighertemperatures”saidDrBroich[cylindresdepressageetplateaux)pourtransférer800ou1000tonnesà1000°Coumêmeàdestempératuresplusélevées»adéclaréleDrBroich[pressingramsandplatens)totransfer800or1000tonsat1000°Corevenhighertemperatures”saidDrBroich
Additionally, the pressing unit should have a design that minimizes mass in the hot zone while providing even force distribution throughout the workspace and pressing platen, according to Dr. Broich.
“The design of a diffusion bonding machine must allow for the shortest possible cycle times while saving energy by reducing its thermal mass; this is very important for productivity and profitability,” he stressed.
To produce high quality diffusion bonded PCHE, working with an expert partner like PVA TePla can be essential for manufacturers who need process and technology support.
“Most of our customers are not familiar with this technology. We support them with basic technical process knowledge so that they can properly use the machine to produce high-quality diffusion-bound PCHE,” said Dr. Broich.
Due to the growing need for this precision technology in the United States, PVA TePla is opening a Diffusion Bonding Technology Center in Corona, California. This summer, a full-scale industrial furnace corresponding to the capacities offered in Germany will be available.
PVA TePla will be able to demonstrate the machine’s bonding capabilities, run R&D samples and provide processing services.
“In the US and Germany, we can demonstrate bond quality before a customer decides to invest in a machine. We handle most of the process development elements to prove the technology and help scale up production volumes. Alternatively, we can provide full-scale contract processing to customers if they outsource production,” Dr Broich said.