E5.8 Overall Heat Transfer Coefficient

Example 5.8: It is desired to heat cold water from 80 °F to 120 °F using hot water, which is cooled from 160 °F to 100 °F. Find the overall heat transfer coefficient if the following data apply:

Heat transfer, Q = 3.992´10⁵ Btu/hr,

Inside film transfer coefficient, h_i = 948.9 Btu/(hr·ft²·°F);

Outside film transfer coefficient, h_o = 1217.7 Btu/(hr·ft²·°F);

Inside fouling resistance, R_i = 0.001 (hr·ft²·°F)/Btu,

Outside fouling resistance, R_o = 0.001 (hr·ft²·°F)/Btu,

Inside diameter, d_i = 0.1112 ft,

Outside diameter, d_o = 0.125 ft.

Logarithmic temperature difference, ΔT_lm = 28.85 °F.

Solution:

Inside heat transfer coefficient based on outside diameter, h_io:

Heat transfer resistance in a clean heat exchanger, 1/U_C:

Overall heat transfer coefficient, U_C :

Fouling resistance of inside pipe based on outside diameter, R_dio:

Total fouling resistance, R_d:

Dirty (design) heat transfer resistance, 1/U_D:

Dirty (design) heat transfer coefficient, U_D:

Depending upon heat load and the type of arrangement, heat transfer area (A) can be calculated.