In a heat exchanger, as shown in accompanying figure, air flows over brass tubes of 1.8- cm-ID and 2.1-cm-OD that contain steam. The convection heat-transfer coefficients on the air and steam sides of the tubes are 70 W/(m2 K) and 210 W/(m2 K), respectively. Calculate the overall heat transfer coefficient for the heat exchanger (a) based on the inner tube area, (b) based on the outer tube area but assume that a fouling factor of 0.00018 (m2 K)/W has developed on the inside of the tube during operation.

Question

GIVEN

• Air flow over brass tube containing steam

• Tube diameters Inside (Di) = 1.8 cm = 0.018 m

Outside (Do) = 2.1 cm = 0.021 m

• Convective heat transfer coefficients Air side h o= 70 W/(m2 K)

Steam side h i= 210 W/(m2 K)

• Fouling factor on the inside of the tube (Rd) = 0.0018 (m2 K)/W

FIND

• The overall heat transfer coefficient for the heat exchanger based on

(a) the inner tube area (Ui) and (b) the outer tube area (Uo)

ASSUMPTIONS

• The heat transfer coefficients are uniform over the transfer surfaces

SKETCH

Answer 1

From the solution to



without fouling. The overall heat transfer coefficient with fouling (Ud) can be calculated by rearranging



(a) Based on the inner tube area



(b) To base the overall heat transfer coefficient on the outer tube area, the fouling factor must also be based on the outer tube area