The heat transfer characteristics of various insulated long polygonal ducts are analyzed using one-dimensional Plane Wedge Thermal Resistance (PWTR) and Plate Thermal Resistance (PTR) models. It is found that errors generated by the PWTR model are all positive and those generated by the PTR model are all negative. Thus, the Combined Plate Wedge Thermal Resistance (CPWTR) model generated by implementing PWTR and PTR models and equating the proportion factors can neutralize the positive and negative errors and generate reasonably accurate results when compared with the two-dimensional numerical analysis. Most errors generated by the one-dimensional CPWTR model are within ±1% for practical sizes and insulated thickness for air-conditioning and refrigeration applications. Meanwhile, the PTR model can be used to estimate accurately the maximum or minimum surface temperatures, respectively, with hot or cold fluids inside an insulated triangular, rectangular or pentagonal duct.
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