Excel Spreadsheets for Estimation of an Overall Heat Transfer Coefficient
The Excel spreadsheet shown in the image at the left is set up to calculate the overall heat transfer coefficient, U, based on the indicated input values, which are: the wall thermal conductivity, k; the outside diameter of the tube, 2ro; the tube wall thickness, ro - ri; the outside convective heat transfer coefficient, ho; the inside convective heat transfer coefficient, hi; the outside fouling resistance, Rfo; and the inside fouling resistance, Rfi.
The Excel formulas are set up to calculate the overall heat transfer coefficient, U, using the equation discussed in the previous section:
1/U = 1/ho + Rfo + (ro - ri)/k + 1/hi + Rfi.
Values for the tube dimensions should be available from preliminary heat exchanger design calculations, which could be done using Excel spreadsheets like those available from the articles, "Excel Spreadsheet Templates for Preliminary Heat Exchanger Design," and "Excel Formulas for Pressure Drop in Shell and Tube Heat Exchanger Design." The table at the upper right gives BWG (Birmingham Wire Guage) wall thicknesses for heat exchanger tubes, which typically have diameters between 1/4" and 2".
The inside and outside convective heat transfer coefficients could be determined from the Excel spreadsheets that can be downloaded from the article, "Calculation of Forced Convection Heat Transfer Coefficients with Excel Spreadsheets."
Typical fouling resistance values for various combinations of fluids, temperatures and velocities are given in reference #1 below. Most of the fouling resistance values fall between 0.0005 and 0.01 ft2-oF-hr/Btu. For many common heat exchanger situations, the fouling resistance will be between 0.001 and 0.005 ft2-oF-hr/Btu.
The overall heat transfer coefficient calculated in the example Excel spreadsheet shown above turned out to be 116 Btu/hr-ft2-oF, which is amazingly close to the initial 'rough estimate' of 120 Btu/hr-ft2-oF, made in the article, "Preliminary Heat Exchanger Design Example several months ago!"
The example Excel spreadsheet uses U.S. units, however the spreadsheet template can be downloaded below with U.S. units or with S.I. units.
Click here to download this Excel spreadsheet in U.S. units.
Click here to download this Excel spreadsheet in S.I. units.