Two very challenging problems facing the U.S. and the world are energy security and global climate change, largely due to dependence on fossil fuels. Cost-effective technologies have been developed that are capable of substantial energy savings through improved energy efficiency.

The prototype of the air preheater.

Microchannel heat exchanger technology is capable of improving the efficiency of natural gas appliances. The technology is also useful for other applications involving combustion processes, such as fuel cell systems that utilize steam reforming to generate hydrogen.

In many applications where fuel is combusted, high-temperature heat is generated and then transferred to another media, such as air or water. Energy losses in this process include the unused heat content of the exhaust gas. Exhaust gas losses are reduced by improving heat exchange to extract more energy from the combustion gas. To squeeze the most amount of energy from the exhaust, a heat exchanger is used to preheat the incoming combustion air (or fuel) with the exhaust. Eventually, water condenses from the exhaust gas as it cools, which is usually corrosive, so components that come into contact with the water must be corrosion-resistant. These condensing systems (i.e. condensing furnaces, condensing boilers, etc.) are a primary application for this technology.

Microchannel heat exchangers, such as the Combustion Gas Heat Exchanger, are smaller in size and have proven more effective than conventional clamshell and tubular heat exchangers currently used in condensing furnaces. The prototype Combustion Gas Heat Exchanger (see figure) is a stainless steel air recuperator designed and tested in a fuel cell demonstration system. It has a design duty of 3.5 kW, exhibits very low pressure drop, and operates with more than 90 percent heat exchange effectiveness. The device weighs 14 times less than conventional offerings, and is almost 30 times smaller than a commercial heat exchanger having similar duty, yet lower effectiveness.

The small size and effective heat transfer in microchannel heat exchangers and microreactors have similarly shown advantages in fuel reformers for fuel cells. A system containing a steam reforming micro-reactor and a vaporizing microchannel heat exchanger, both heated from a combustion gas stream and designed for very low pressure drop, enables rapid start of a fuel reforming system. A demonstration system was heated from ambient temperature to an operating temperature over 700 °C in 5 seconds, and reached full operating capacity in less than 30 seconds.

Multiple devices of similar design have been fabricated and tested on various fuel cell and thermal systems. All working prototypes have been fabricated using photochemical machining (PCM) and high-temperature diffusion brazing. Early process development has been done with alternative fabrication methods, including shim stamping, brazing, and laser welding.

For more information, contact Gordon Graff at This email address is being protected from spambots. You need JavaScript enabled to view it. or 509-375-6786.


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This article first appeared in the August, 2019 issue of Tech Briefs Magazine.

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