Heat Transfer:Quick recap(questions)

  • For the same inlet and outlet temperatures of hot and cold fluids, the Log mean Temperature Difference (LMTD) is
          (A) greater for parallel flow heat exchanger than for counter flow heat
          exchanger
          (B) greater for counter flow heat exchanger than for parallel flow heat
          exchanger
          (C) same for both parallel and counter flow heat exchangers
          (D) dependent on the properties of the fluids.

  • In a counter flow heat exchanger, for the hot fluid the heat capacity = 2 kJ/kgK, mass flow rate= 5 kg/s, inlet temperature = 150cC, outlet temperature = 100cC. For the cold fluid, heat capacity = 4 kJ/kgK, mass flow rate = 10 kg/s, inlet temperature = 20cC. Neglecting heat transfer to the surroundings, the outlet temperature of the cold fluid in cC is                            (A) 7.5
    (B) 32.5
    (C) 45.5
    (D) 70.0
  •  The logarithmic mean temperature difference (LMTD) of a counter flow heat exchanger is 20cC. The cold fluid enters at 20cC and the hot fluid enters at 100cC. Mass flow rate of the cold fluid is twice that of the hot fluid. Specific heat at constant pressure of the hot fluid is twice that of the cold fluid. The exit temperature of the cold fluid
    (A) is 40cC
    (B) is 60cC
    (C) is 80cC
    (D) cannot be determined




Answers:
1. (C)
2. (B)
3. (C)