Simplified optimum sizing and cost analysis for compact heat exchanger in VHTR

In this study, the optimum size of the compact heat exchanger has been developed based on its weight and pumping power for the reference design of 600 MWt very high temperature gas-cooled reactor (VHTR) system. Alloy 617 was selected as a construction material. The optimum size and a number of configurations for the reference design of the VHTR with an intermediate heat exchanger (IHX) were investigated and our initial calculations indicated that it has an unrealistically too large aspect ratio of the length and height due to its small-sized channels, which might cause manifolding problems and a large number of parallel modules with high thermal stress. The flow area and channel diameter were then adjusted to achieve a smaller aspect ratio in this paper. Achievement of this aspect ratio resulted in higher cost, but the cost increase was less than would have occurred by simply reducing the flow area by itself. The appropriate channel diameter is estimated to be less than 5.00 mm for the reference system. The effect of channel waviness enhanced the compactness and heat transfer performance, but unfavorably increased the aspect ratio. Therefore, the waviness should be carefully determined based on performance and economics. In this study, the waviness of the IHX is recommended to be selected between 1.0 and 2.5. Calculations show that reducing the duty dramatically decreases the aspect ratio, indicating that the compact heat exchanger is easy to be optimally designed for low duty, but many modules are required for high duty operation proportional to the operating power. Finally, the effect of working fluids was investigated, and it reveals that using carbon dioxide instead of helium in the secondary side reduces the size and cost by about 20% due to the lower pumping power in spite of its lower heat transfer capability by a factor of 4.