Repeat Problem 8.6, but assume that a fouling factor of 0.001 (hr °F ft2)/Btu based on the tube outside diameter has developed over time.
From Problem 8.6: Hot water is used to heat air in a double pipe heat exchanger. If the heat transfer coefficients on the water side and on the air side are 100 Btu/(h ft2 °F) and 10 Btu/(h ft2), respectively, calculate the overall heat transfer coefficient per unit length based on the outer diameter. The heat exchanger pipe is 2 inch, schedule 40, made of steel (k = 54 W/(m K)), with water inside. Express your answer in Btu/(h °F) and W/(m °C).
GIVEN
A double pipe heat exchanger with water in inner tube and air in the annulus
Heat transfer coefficients
Water side (hi ) = 1000 Btu/(h ft2 °F)
Air side (ho ) = 10 Btu/(h ft2 °F)
Inner pipe: 2 inch, schedule 40, made of steel
Pipe thermal conductivity (k) = 54 W/(m K)
Fouling factor (Rd) = 0.001 (h °F ft2)/Btu, based on the tube outside area
ASSUMPTIONS
Uniform heat transfer coefficients
The given fouling factor is based on the outer tube diameter (Do)