Clean Coefficient, U_C and Fouling Resistance, R_d : Once film coefficients are known, clean coefficient for heat exchangers can be found. It is important to allow for fouling of exchangers. The relationships involved are listed in Table 5.7.

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.