Methodology for determining the optimal fin dimensions in helically segmented finned tubes

Helical segmented fins of uniform profile are analyzed by means of the quasi one-dimensional fin theory coupled with the Logarithmic Mean Temperature Difference (LMTD) method from heat exchanger theory to determine the optimum fin dimensions. On one hand, the quasi one-dimensional fin theory is applied to establish the lower fin height limit, which is critical in this type of application. On the other hand, the LMTD method is used to define the optimal equilibrium point by way of the dimensionless overall heat transfer coefficient and the pressure drop. The methodology to be proposed for estimating the optimal dimensions of helical fins was applied to a bare tube with outside diameter of 50.8 mm (2 in) and a maximum transverse pitch based on a bare tube with outside diameter 2.25 larger. The computed results demonstrate that the optimal equilibrium point is affected by the lower fin height limits because it is unstable. Hence, two lower fin height limits are defined in order to determine a maximum deviation of the optimal equilibrium point, while the upper fin height limit is defined by physical–technical limiting factors. Thereby, the optimal equilibrium point predictions show a maximum deviation of 8% and a stable behavior under the influence of different thermal conditions and flow regimes associated with various helically segmented sizes, helical fin heights.