Heat Transfer Modeling in a Double Wall Helical Coil Heat Exchanger

Double wall helical coil heat exchangers are widely used in a variety of applications. These heat exchangers are used anywhere that requires an added layer of protection from cross-fluid contamination such as in indirect water heaters. Here, the coil is heating a potable water tank with coil water which is often infused with a glycol mixture. Since contamination of the potable water would be detrimental to human health, the added protection of a double wall coil is necessary. Unfortunately, the double wall coil is inherently difficult to design due to unknown intertubular contact. This intertubular contact is hidden in nature and difficult to evaluate by nondestructive means. This paper uses STAR-CCM+ to create a computational fluid dynamic model of the double wall helical coil in respect to GAMA indirect water heater continuous draw test conditions. Using CFD, the model is validated with single wall helical coil Nusselt number correlations determined from experimentation. Then, a parametric study is performed to determine what parameters of the coil can be changed to effectively overcome the intertubular thermal loss. From this study, it is found that tube length has a linear positive relationship with coil output density and can be used to calculate necessary lengths for a desired output. Also, it was found that the experimental Nusselt number correlations identify with the results calculated by the double wall CFD model when intertubular contact percentage is used as a correction factor.