Impact of Mechanical Design Issues on Printed Circuit Heat Exchangers

by Renaud Le Pierres, David Southall, & Stephen Osborne
Heatric Division of Meggitt (UK) Limited, United Kingdom

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Power conversion cycle efficiency is a key factor in the design of new generations of power plants irrespective of the heat source. In response to increasingly demanding emission regulations throughout the world there is now a drive to improve on the existing cycles, a drive which is supported by public opinion. This has resulted in the promotion of several new concepts for power conversion cycles. Of these new cycles the Supercritical Carbon Dioxide (SCO2) Brayton cycle is one of the most promising. Over the last few years many SCO2 brayton cycles have been designed to operate at pressures ranging between 20 MPa and 30 Mpa and with turbine inlet temperature between 500 °C and 600°C. In order to obtain higher efficiencies wide use is made of heat exchangers and recuperators. To achieve efficiences of 45% and above intermediate heat exchangers and recuperators must be able to provide very close temperature approaches while withstanding demanding operating pressure and temperature combinations that require very high mechanical integrity.

Printed Circuit Heat Exchangers (PCHEs) have been used on the Barber-Nicholls/Sandia National Laboratory test loop for the cooler, low temperature recuperator and a high temperature recuperator applications. The compact diffusion bonded PCHE offers many advantages which contribute to the performance, safety and viability of the SCO2 brayton cycle. The use of diffusion bonded construction delivers high mechanical integrity. Their high surface area per unit volume allows a closer temperature approach in a more compact space envelope compared to an equivalent Shell and Tube heat exchanger. The PCHE can be manufactured with a wide range of materials including high grade alloys which are useful for higher temperature or pressure applications where the use of SS316L is restricted. The PCHE has great flexibility to including variable angles and flow paths to increase heat exchange while balancing pressure drop as well as the potential for multi-stream heat exchange.

Many papers have been published on the PCHE for use in power generation by various entities other than Heatric, predominently for nuclear applications associated with Gen IV cycles. A lot of these papers have been built on assumptions resulting from interpretation or reverse engineering of public domain information provided by Heatric who have been designing and manufacturing PCHEs for more than 25 years. This has resulted in many papers with various, often contradictory, claims about PCHE performance which ignore the impact of mechanical design issues on the design of PCHEs.

The aim of this paper is to review some of these assumptions in order to clarify and correct them, as well as to introduce some of the mechanical design considerations and challenges which significantly affect the final geometry of the PCHE being engineered condition.