Investigation on heat transfer in conical coiled heat exchangers

Abstract

The extensive use of heat exchangers in industries necessitates the high performance and compact sizes of the heat exchangers. With this necessity in industrial applications, various techniques are used to enhance the heat transfer. Some techniques are concerned with the modification in geometry or surface (passive); whereas, some others use an external power source (active). The selection of the proper heat transfer enhancement technique is important for efficient heat transfer. In passive techniques, the coiled tubes are most widely used. The coils accommodate large heat transfer area in a small volume, and have high heat transfer coefficients. The commonly used configurations in coiled type heat exchangers are helical coil configuration and conical coil configuration. The studies related to heat transfer and pressure drop for these configurations are very important for their selection for a given application. In the literature many studies are available on helical coils, and a few are available on spiral coils. Each one of these configurations has certain advantages and limitations. Conical coil is a coil that has a specified cone angle. These coils may offer the combination of advantages (high heat transfer coefficient, compact size, operations at high temperature, reduced induced stresses, elimination of expansion joints, high pressure capability, reduced fouling tendency, modular design) offered by helical and spiral coils. The conical coil with cone angle 0O is known as helical coil whereas the one with cone angle 180O is considered as spiral coil configuration. The present work aims to carry out heat transfer and pressure drop analysis for conical coil heat exchangers. The analysis is carried out using fifteen coils of five different cone angles (0O (helical), 45O, 90O, 135O and (spiral) 180O) and three different tube sizes (8×10, 10×12 and 12×15). The coils were fabricated in-house and used in a specially designed heat exchanger test setup for the experimentation. This work is the first attempt to study the heat transfer in conical coil heat exchangers. The empirical correlations developed provide a basis to evaluate heat transfer coefficient for heat exchanger designers. v Heat transfer coefficient, based on the overall temperature difference, is calculated using the „Wilson plot method‟. The effect of flow rates (Qh and Qc), effect of flow ratio (Qh/Qc), effect of cone angle (θ), and effect of tube diameter on heat transfer coefficient are studied. Heat exchanger‟s effectiveness as a function of flow ratio is predicted, which can be used to predict outlet temperature of the shell side and tube side fluid. The correlations for Nu as a function of flow parameter (Re/De), fluid parameter (Pr) and coil parameter (δ) are proposed. The friction factor is calculated by conventional Darcy-Weisbach equation. The effect of tube side flow (Re) on friction factor (f) for different cone angles (θ) and different tube diameters (di) is studied. The results show that Nu increases with increase in tube side flow (Qh) and flow ratio (Qh/Qc), whereas it reduces with increase in shell side flow (Qc), cone angle (θ) and tube diameter (d). Also, the friction factor (f) increases with cone angle (θ) and decreases with increase in tube side flow (Qh) and tube diameter (d).