Where: f
α is convective heat transfer coefficient from
fluid to tube wall, W/m2·K; w α is convective heat transfer
coefficient from water film to air, W/m2·K; f
λ is conductive
heat transfer coefficient from fluid to tube wall, W/m2·K;
o d is diameter of exterior tube, mm; i d is diameter of
interior tube, mm; of R is heat resistance of oil inside tubes,
m2·K/ W.