A hairpin heat exchanger (also known as a double-pipe heat exchanger) is a compact and efficient device used to transfer heat between two fluids. It consists of two concentric tubes bent into a U-shape (hairpin) — one fluid flows through the inner tube, while the other flows through the annular space between the inner and outer tubes. https://www.unitedheatexchangers.com/blog/what-is-a-hairpin-heat-exchanger/