A counter-flow double pipe heat exchanger is used to cool a lubricating oil. The flow rate of cooling water through the inner pipe (Di = 25 mm) is 0.21 kg/s and the oil flows through the annulus (Do = 45 mm) at a rate of 0.1 kg/s. Oil and water enter at temperatures of 100 and 30 C respectively. How long must the tube be made if the outlet oil temperature is 60 C? The Nusselt number for oil side is 5.56. F. water side, the Nusselt number can be estimated using he following equation:
Pr = Cp μ/k Nu = hD/k
Please clearly mention your assumptions.
Nu = 0.023Re0.8 Pr0.4
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Average T
Deg. C
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Cp
J/kg.0
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Viscosity
N.s/m2
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Conductivity
W/m.K
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Properties
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Oil
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80
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2131
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3.25E-02
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0.138
0.625
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Water
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35
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4178
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7.25E-04
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