Helical Coil Heat Transfer in Mixing Vessels

Authors:

J.Y. Oldshue

A.T. Gretton 

This study investigates heat transfer in mixing vessels equipped with helical coils, using flat-blade turbine impellers under various conditions including different Reynolds numbers, impeller speeds, tube diameters, and baffle placements. Experimental results provide correlations for heating and cooling coefficients, demonstrating that factors such as tube spacing, D/T ratio, and power input significantly influence heat-transfer performance, while baffle placement affects power consumption more than heat-transfer efficiency. Comparisons with vertical tubes and jacketed tanks highlight the practical considerations for selecting heat-transfer surfaces in industrial mixing applications.

Key Learnings

• Helical coils are effective heat-transfer surfaces in mixing vessels and generally provide higher heat-transfer coefficients than jacketed tanks at equivalent power input.
• Flat-blade turbine impellers generate turbulence that significantly impacts heat transfer, with impeller size, speed, and power input directly affecting performance.
• Tube diameter and spacing influence heat-transfer efficiency, particularly for high-viscosity fluids, with wider spacing generally reducing the coefficient.
• Baffle placement has minimal effect on heat-transfer coefficient but can reduce power consumption when positioned slightly off the tank wall or inside the coil.
• Steady-state and unsteady-state operations produce consistent heat-transfer results, confirming the reliability of experimental correlations.
• Reynolds number, Prandtl number, and D/T ratio are key dimensionless parameters for predicting heat-transfer behavior in mixing systems.
• Comparisons with vertical tubes show that helical coils can provide more surface area and similar overall heat-transfer performance, with the choice often guided by economic and installation considerations.
• Accurate measurement of fluid and tube wall temperatures, using multiple thermocouples, is essential for reliable determination of heat-transfer coefficients.
• Power input per unit volume is a critical factor for correlating heat transfer and evaluating the efficiency of different mixer configurations.
• Exponential relationships and dimensionless correlations allow generalization of experimental data for various tank sizes, tube diameters, and fluid viscosities.