# Food Engineering Questions and Answers – Unit Operations – Heat Transfer by Convection-1

This set of Food Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Unit Operations – Heat Transfer by Convection-1”.

1. Which of the following is true about Natural Convection?
a) Low heat transfer
b) Due to gravity and natural buoyant forces
c) Dependent on viscosity, density and thermal conductivity
d) All of the mentioned

Explanation: Natural Convection has low heat transfer. It is due to gravity and natural buoyant forces and is dependent on viscosity, density and thermal conductivity.

2. __________ is when nucleate boiling transforms to film boiling.
a) Critical temperature
b) Leiden frost point
c) Critical temperature & Leiden frost point
d) None of the mentioned

Explanation: Leiden frost point is when nucleate boiling transforms to film boiling.

3. Statement 1: Nucleate Boiling is when a layer of film is formed in the beginning.
Statement 2: Boiling starts with the bottom layer which heats up, becomes light and goes up. That way everything becomes hot and there is not temperature difference and hence no heat transfer.
a) True, False
b) True, True
c) False, False
d) False, True

Explanation: Film Boiling is when a layer of film is formed in the beginning. Boiling starts with the bottom layer which heats up, becomes light and goes up. That way everything becomes hot and there is not temperature difference and hence no heat transfer.

4. Statement 1: When heat transfer takes between a solid surface and a fluid, an insulating film layer is formed. More is the agitation; more is the thickness of the film.
Statement 2: Within the film, the temperature varies from the wall to the temperature of the bulk.
a) True, False
b) True, True
c) False, False
d) False, True

Explanation: When heat transfer takes between a solid surface and a fluid, an insulating film layer is formed. More is the agitation; lesser is the thickness of the film. Within the film, the temperature varies from the wall to the temperature of the bulk.

5. Statement 1: NPr= (Cp μ)/k
Statement 2: If NPr is lesser than 1, Thermal Boundary layer is thinner than Hydrodynamic Boundary layer.
a) True, False
b) True, True
c) False, False
d) False, True

Explanation: NPr= (Cp μ)/k. Thermal Boundary layer is thicker than Hydrodynamic Boundary layer.
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6. Why is better to use a Jacketed vessel in the dairy industry than a stainless steel container directly?
a) To prevent milk from charring
b) To have a better temperature control
c) The temperature of food in the film = temperature of the wall which is very high. Hence, jacketed vessels are needed
d) All of the mentioned

Explanation: All the mentioned reasons are corrected.

7. Statement 1: hd/k is _____
Statement 2: _____ is the ratio of convective heat transfer to conductive heat transfer.
a) Prandtl number, Prandtl number
b) Nussel number, Nussel number
c) Pictet number, Pictet number
d) Fourier number, Fourier number

Explanation: hd/k is Nussel number. Nussel number is the ratio of convective heat transfer to conductive heat transfer.

8. The reciprocal of heat transfer co-efficient is _____
a) Conductance
b) Resistance
c) Density
d) Temperature difference

Explanation: The reciprocal of heat transfer co-efficient is Resistance.

9. Statement 1: Film-type condensation has a higher heat transfer co-efficient than drop-wise condensation.
Statement 2: Drop-wise condensation is easier to maintain than Film-type condensation
a) True, False
b) True, True
c) False, False
d) False, True

Explanation: Film-type condensation has a lower heat transfer co-efficient than drop-wise condensation. Film-type condensation is easier to maintain than Drop-wise condensation.

10. Statement 1: Drop-wise condensation takes place on a smooth surface.
Statement 2: All the resistance to heat transfer lies in the film formed during Film-type condensation.
a) True, False
b) True, True
c) False, False
d) False, True

Explanation: Drop-wise condensation does NOT take place on a smooth surface as the drops cannot adhere to the surface.

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