Hot water is used to heat air in a double pipe heat exchanger as shown in the following sketch. If the heat transfer coefficients on the water side and on the air side are 550 W/(m2K) and 55 W/(m2 K), respectively, calculate the overall heat transfer coefficient based on the outer diameter. The heat exchanger pipe is 5 cm, schedule 40 steel (k = 54 W/m K), with water inside.

Question

GIVEN

• A double pipe heat exchanger with water in inner tube and air in the annulus

• Heat transfer coefficients

? Water side ih = 550 W/(m2 K)

? Air side oh = 55 W/(m2 K)

• Inner pipe: 5 cm, schedule 40 steel

• Pipe thermal conductivity (k) = 54 W/(m K)

FIND

• The overall heat transfer coefficient based on the outer diameter (Uo). ASSUMPTIONS

• Uniform heat transfer coefficients

SKETCH



PROPERTIES AND CONSTANTS

for 5 cm, schedule 40 pipe

? Outside diameter (Do) = 2.375 in = 6.033 cm=0.0603 m

? Wall thickness (t) = 0.154 in=0.39 cm=0.0039 m

Answer 1

Inside diameter (Di) = Do – 2t = 6.033 cm – 2(0.39 cm) = 5.253 cm=0.0525 m

The overall heat transfer coefficient based on the outer tube diameter is