Page 413 - Handbook of Energy Engineering Calculations

2. Determine the actual U for the heat exchanger

Enter Fig. 5 at the bottom with the clean heat-transfer coefficient of U = 100
2
2
Btu/(h · ft · °F) [567.8 W/(m · °C)] and project vertically upward to the
0.002 fouling-factor curve. From the intersection with this curve, project
horizontally to the left to read the design or actual heat-transfer coefficient as
2
2
U = 78 Btu/(h · ft · °F) [442.9 W/(m · °C)]. Thus, the fouling of the tubes
a
2
causes a reduction of the U value of 100 – 78 = 22 Btu/(h · ft · °F) [124.9
2
W/(m · °C)]. This means that the required heat-transfer area must be
increased by nearly 25 percent to compensate for the reduction in heat

transfer caused by fouling.