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## Homework Statement

(c) A heat exchanger is to be used to heat a process liquid within the tubes using saturated steam at 100oC. The tubes have an inside diameter of 20 mm and outside diameter of 22 mm. It is estimated that the inner surface heat transfer coefficient will be 4.2 kW m-2 K-1 and the outer surface heat transfer coefficient will be 15.4 kW m-2 K-1 when the exchanger is clean. In order to allow for possible fouling during use you should assume a fouling factor of 1.12 × 10-4 m2 K W-1 will be applicable. Estimate:

## The Attempt at a Solution

The overall heat transfer coefficient in use

R_f= 1.12∙10^(-4) m^2 K W^(-1)

R_A= R_INNER 〖+R〗_OUTER 〖+R〗_f

R_A= 1/4.2+1/15.4+1.12=1.423 m^2 K 〖kW〗^(-1)

U= 1/R_A = 1/1.423= 0.703 kW m^2 K^(-1)

Ive been provided some feedback with respect to the above answer, "But remember the U you can get from here is the U, based on average area [Uave], between inner and outer dia of the heat exchanger surface." would someone be able to explain this for me ? From the notes I have then answer should be correct.[/B]