Theory and Design Procedure of Helical Coiled Heat Exchangers: A Case Study

This chapter describes the theoretical framework and design methodology adopted for helical spiral heat exchangers intended for waste heat recovery. The analysis focuses primarily on thermohydraulic performance, fundamental design criteria, and the operational constraints that govern exchanger behavior under realistic working conditions. Helical coil arrangements are considered because their compact geometry provides a high surface-to-volume ratio and generally enhances heat transfer compared with conventional straight tubes. These advantages make them especially suitable for applications in which efficiency, limited installation space, and effective energy recovery are key requirements. A systematic design procedure is presented to determine the exchanger dimensions and operating parameters. The heat exchanger is divided into three functional regions: a preheating section, an evaporator, and a superheater. Each region is evaluated through iterative calculations involving local and overall heat transfer coefficients, fluid temperatures, pressure losses, heat duties, and total thermal efficiency. The methodology therefore accounts for variations in fluid properties and heat transfer mechanisms along the entire exchanger. Special attention is devoted to the effects of flow regime, tube curvature, and phase change. In particular, the Dean number is used to characterize secondary flows induced by curvature, which can significantly improve mixing and thermal performance while also affecting pressure drop. Although the selected application involves an Organic Rankine Cycle (ORC), the chapter concentrates mainly on exchanger design and optimization. The results demonstrate that geometry, working-fluid properties, operating conditions, and pinchpoint limitations strongly influence achievable performance and must be considered together to obtain a compact, efficient, and technically feasible design.