Benzene flowing at 12.5 kg/s is to be cooled continuously from 80°C to 54°C by 10 kg/s of water available at 15.5°C. Using Table 10.6, estimate the surface area required for (a) cross-flow with six tube passes and one shell pass with neither of the fluids mixed and (b) a counterflow exchanger with one shell pass and eight tube passes with the colder fluid inside tubes.

Question

GIVEN

• Benzene coded by water in a heat exchanger

• Benzene flow rate m b= 12.5 kg/s

• Water flow rate m w= 10 mg/s

• Benzene temperatures

? (Tb,in) = 82°C

? (Tb,out) = 54°C

• Water inlet temperature (Tw,in) = 15.5°C

FIND

The surface area required for (a) Cross-flow, 6 tube passes, 1 shell pass, both unmixed (b) Counterflow, 8 tube passes, 1 shell pass, water in tubes

SKETCH



PROPERTIES AND CONSTANTS

the specific heat of water at 20°C (cpw) = 4182 J/(kg K)

the specific heat of Benzene at 68°C (cpb) = 1926 J/(kg K)

Answer 1

the specific heat of Benzene at 68°C (cpb) = 1926 J/(kg K)



The rate of heat transfer is



Solving for the effectiveness of the heat exchanger



(a) For unmixed cross-flow, the number of transfer units is NTU ? 0.7. By definition



The overall heat transfer coefficient (U), in the range of 280-850 W/(m2 K) between water and organic solvents. Therefore, use U = 565 + 50% W/(m2 K).



(b) NTU for counterflow is NTU ? 0.75