References 231

coefficient based on the outer surface area of the tube and the tube length per pass, the outlet temperature of the cooling water, and the condenser effectiveness.

b. If, after prolonged usage, scaling (fouling) causes a resistance of 8 × 10−5 m2 K/W at the inner surface, determine the value of the overall heat transfer coefficient based on the outer surface area of the tube and the corresponding effectiveness of the condenser.

Properties of water: cp = 4178 J/kg.K, = 0.77 ×10−3 Pa.s, Pr = 5.2, and

k= 0.62 W/mK

7.6.A two-shell and four-tube pass shell and tube heat exchanger is designed to cool

2.2kg/s of oil (cp = 2.1 kJ/kg K) from 82 °C to 55 °C with water (cp = 4.2 kJ/ kg K) at an inlet temperature of 35 °C and flowing at 3 kg/s. The overall heat transfer coefficient is 820 W/m2 K. Determine the required heat transfer surface area. Compare the required area with that obtained from a single-shell and twotube pass shell and tube heat exchanger for the same flow rates and temperatures.

7.7.The following are the results from the test conducted on a finned-tube, crossflow heat exchanger used to recover the waste heat from the exhaust of a gas turbine. Gas turbine exhaust: flow rate 3 kg/s; specific heat 1100 J/kg K; inlet temperature 275 °C. Pressurized water (through the tubes): flow rate 0.75 kg/s; specific heat 4200 J/kg K; inlet temperature 40 °C and outlet temperature 130 °C. If the heat transfer surface area is 15 m2, determine the overall heat transfer coefficient assuming (a) both fluids unmixed and (b) one fluid mixed and the other unmixed.

7.8.An intercooler of the cross-flow type (single-pass) is used for cooling 3.2 kg/s

of air (cp = 1.1 kJ/kg K) at 115 °C with water (cp = 4.2 kJ/kg K) flowing through the tubes. The water flow rate is 2.9 kg/s and enters the intercooler at 40 °C. If the overall heat transfer coefficient based on the tube surface area of 35 m2 is 120 W/m2 K, determine the effectiveness of the intercooler and the outlet temperatures of air and water. Assume both fluids are unmixed.

References

Bejan, A., 1993. Heat Transfer, John Wiley & Sons, New York.

Bergman, T.L., Lavine, A.S., Incropera F.P., Dewitt, D.P., 2011. Fundamentals of Heat and Mass Transfer, seventh ed. John Wiley & Sons, NJ.

Bowman, R.A., Mueller, A.C., Nagle, W.M., 1940. Mean temperature difference in design. Trans. ASME 62 (4), 283–294.

Cengel,Y. A., 2003. Heat Transfer: A Practical Approach, second ed. McGraw-Hill, New York. Kays, W.M., London, A.L., 1984. Compact HeatExchangers third ed. McGraw-Hill, New

York.

Lienhard IV, J.H., Lienhard V, J.H., 2020. A Heat Transfer Textbook. Courier Dover Publications, Mineola, NY.