Material selection and optimisation of a high-temper[at]ure compact heat exchanger

Abstract

The Pebble Bed Micro Model (PBMM) plant currently employs a large conventional shell-and-tube heat exchanger as recuperator at the inlet to the heat source in order to preheat the nitrogen. The thermal inertia of the current PBMM recuperator influences the dynamic behaviour of the plant and the heat exchanger configuration is also different from the actual Pebble Bed Modular Reactor (PBMR) plant, which will employ a compact heat exchanger. A need therefore exists to design a high-temperature compact heat exchanger for the PBMM plant. This study includes the material selection and optimization of the proposed heat exchanger design. The material chosen for this recuperator is a tungsten-copper alloy with 68 wt% W and 32 wt% Cu. A mini-channel heat exchanger with a counter flow configuration is the chosen type of heat exchanger. The heat exchanger is made up of two types of plates that are stacked alternatively. These plates are then either diffusion bonded or brazed in order to form the complete heat exchanger. The final geometry of the recuperator is as follows: - Total length -- 0.75 m - Total width -- 0.518 m - Total height -- 0.8 m - Number of channels wide -- 370 - Number of hot channels high -- 200 - Number of cold channels high -- 200 - Plate thickness - 1 mm - Fin thickness -- 0.4 mm - Channel height -- 1 mm - Channel width - 1 mm The proposed recuperator only occupies 10% of the footprint area and 13% of the volume of the shell-and-tube recuperator. Its affectivity is 95%, which results in an increase of 2% in the cycle affectivity. The response time for the compact recuperator is about 25% that of the shell-and-tube recuperator.