After fermentation, a mixture of ethanol and water is sent to a small distillation column. At the top of the distillation column, a 95% ethanol solution at 65°C is produced at a flow rate of 3.5 kg/s. Your job is to design a double-pipe heat exchanger that will cool the 95% ethanol mixture to 40°C by using cooling water that is available at 10°C. Assume an outlet temperature of the cooling water of 55°C, and only use Schedule 40 pipe from the table below. Do not do viscosity corrections for heat transfer coefficients and the 95% ethanol solution will be pumped to the inside pipe. Assume that the heat capacity for the ethanol mixture is 2.55 kJ/kg.K and the heat capacity of the coolant is 4.18 kJ/kg.K. The density of the 95% ethanol solution is 0.804 g/mL Assume that the velocity of ethanol solution as 1 m/s as the initial guess. Check the accuracy of this calculation once you select the size of the pipe (the area should agree with the area required to achieve the desired heat transfer). The viscosity of the solution is given as 9.72 × 10–4 kg/(m · s) and the thermal conductivity is 0.175 W/(m · K).
1. The fluid specific heats do not vary with temperature.
2. The overall heat transfer coeff is constant throughout the heat exchanger
3. Heat exchange with ambient air is negligible
4. The flow is steady
5. The fouling factors are neglected.
You can also use the estimated dia of outer pipe = 5 inches.
The convective heat transfer coefficients for both the fluids can be calculated. After calculating the h's, the values of U, heat transfer area (A) and number of bends can be calculated.