A. Heat transfer co-efficient over the entire heat exchanger is not constant
B. There exists an unsteady state
C. The heat capacity is not constant and there is a phase change
D. None of these
Related Mcqs:
- In a parallel flow heat exchanger, if the outlet temperature of hot and cold fluids are the same, then the log mean temperature difference (LMTD) is_________________?
A. Minimum
B. Maximum
C. Zero
D. Infinity - In a liquid-liquid heat exchanger, for the same process temperature, the ratio of the LMTD in parallel flow to the LMTD in counter flow is always__________________?
A. < 1
B. > 1
C. 1
D. ∞ - Hot water (0.01 m3 /min) enters the tube side of a counter current shell and tube heat exchanger at 80°C and leaves at 50°C. Cold oil (0.05 m3/min) of density 800 kg/m3 and specific heat of 2 kJ/kg.K enters at 20°C. The log mean temperature difference in °C is approximately ?
A. 32
B. 37
C. 45
D. 50 - Out of the following four assumptions used in the derivation of the equation for LMTD [LMTD = (Δt1 – Δt2)/ln(Δt1/Δt2)], which one is subject to the largest deviation in practice ?
A. Constant overall heat transfer co-efficient.
B. Constant rate of fluid flow
C. Constant specific heat
D. No partial phase change in the system - In a 1-1 concurrent heat exchanger, if the tube side fluid outlet temperature is equal to the shell side fluid outlet temperature, then the LMTD is_________________?
A. ∞
B. 0
C. Equal to the difference between hot and cold fluids inlet temperature
D. Equal to the difference between hot fluid inlet temperature and cold fluid outlet temperature - A wall has two layers of materials A and B; each made of a different material. Both the layers have the same thickness. The thermal conductivity of material A is twice that of B. Under the equilibrium, the temperature difference across the wall is 36°C. The temperature difference across the layer A is ______________ °C?
A. 6
B. 12
C. 18
D. 24 - A long iron rod initially at a temperature of 20°C has one end dipped in boiling water (100°C) at time, t = 0. The curved surface of the rod is insulated so that heat conduction is one dimensional in the axial direction. The temperature at a distance 100 mm from the dipped end becomes 40°C at time, t = 200 s. The same temperature is achieved at a distance of 200 mm from the dipped end at time ___________________?
A. t = 283 s
B. t = 356 s
C. t = 400 s
D. t = 800 s - Harmonic mean temperature difference is given by_________________?
A. √(ΔT1 . ΔT2)
B. 2 (ΔT1 . ΔT2)/(ΔT1 + ΔT2)
C. 2 (ΔT1 . ΔT2)/(ΔT1 – ΔT2)
D. (ΔT1 – ΔT2)/(ΔT1 . ΔT2) - LMTD for counter-flow and parallel flow heat exchanger will be the same, when the_________________?
A. Cold fluid is heated to a certain temperature by condensing steam (isothermal fluid)
B. Outlet temperature of both the hot and cold fluid are same
C. Outlet temperature of hot fluid is less than the outlet temperature of the cold fluid
D. None of these - It is not recommended to use a 1-2 shell and tube heat exchanger for a particular heat duty, whenever the LMTD correction factor is___________________?
A. > 0.75
B. < 0.75
C. < 0.50
D. < 0.25
