This set of Heat Transfer Operations Multiple Choice Questions & Answers (MCQs) focuses on “Condensers Overview”.

1. Which one of the following best describes the function of a condenser?

a) Solid feed to a liquid phase product

b) Vapour feed to a liquid phase product

c) Liquid feed to a solid phase product

d) Liquid feed to a vapour phase product

View Answer

Explanation: Condensers use the property of condensation which is a convective heat transfer process that is associated with a change in the phase of a fluid from the gas to a liquid phase.

2. The convective heat transfer coefficients for condensation in Condensers usually lie in the range ____________

a) 0-200 W/m^{2}K

b) 60-300 W/m^{2}K

c) 300-1000 W/m^{2}K

d) 2500-6000 W/m^{2}K

View Answer

Explanation: Condensation as well as evaporation have high convective heat transfer coefficients because they are related to the latent heat of vaporization of water which is equal to 2260 W/m

^{2}K.

3. Which one of the following condensation phenomenon is usually avoided in Condensers?

a) Drop-wise condensation

b) Film condensation

c) Boundary layer condensation

d) Horizontal surface Drop-wise condensation

View Answer

Explanation: Film wise condensation has lower heat transfer coefficient due to covering the wall surface and hence it reduces the efficiency of the condenser.

4. Which one of the following is not correct about the condensation regimes in a condenser?

a) If a condensing liquid does not wet a surface, then drop wise condensation will not take place on it

b) Drop-wise condensation gives a higher heat transfer rate than film wise condensation

c) Reynolds number of condensing liquid is based on its mass flow rate of the outgoing condensed fluid

d) Suitable coating or vapour additive is used to promote film wise condensation

View Answer

Explanation: Suitable coating or vapour additive is not used to promote film wise condensation it is rather used to promote Drop-wise condensation.

5. Which one of the following is the correct classification of condensation process?

a) Batch and Semi-batch condensation

b) Drop-wise and Film-wise condensation

c) Boundary layer and Thermal layer condensation

d) Nucleate and non-nucleate condensation

View Answer

Explanation: The two classification of condensation process are Drop-wise and Film-wise condensation where in drop wise by the name, the water condenses as droplets and as a film in the latter.

6. A plate condenser is usually designed to be kept vertical. How would the condensation coefficient change if due to site constraints it now has to be kept at an angle of 30o to the horizontal?

a) 1.53% reduction in condensation coefficient

b) 50% reduction in condensation coefficient

c) 16% reduction in condensation coefficient

d) 84% reduction in condensation coefficient

View Answer

Explanation: The condensation coefficient of a condenser placed at an angle α is given by –

h

_{INC}=\(0.943[\frac{K^3 p^2 g h_{fg}}{\mu L(T_{sat} – T_L)}sin\alpha]^{0.25}\)

Hence \(\frac{h_{INC}}{h_{VER}}\) = sinα

^{0.25}= sin30

^{0.25}=\((\frac{1}{2})^{0.25}\)=0.5

^{0.25}=0.84.

7. If we place a condenser at an angle of α to the vertical, then the convective heat transfer coefficient can be represented as ____________

a) h_{INC} = 0.943 \(0.943[\frac{K^3 p^2 g h_{fg}}{\mu L(T_{sat} – T_L)}sin\alpha]^{0.25}\)

b) h_{INC} = 0.943 \(0.943[\frac{K^3 p^2 g h_{fg}}{\mu L(T_{sat} – T_L)}tan\alpha]^{0.25}\)

c) h_{INC} = 0.943 \(0.943[\frac{K^3 p^2 g h_{fg}}{\mu L(T_{sat} – T_L)}cos\alpha]^{0.25}\)

d) h_{INC} = 0.943 \(0.943[\frac{K^3 p^2 g h_{fg}}{\mu L(T_{sat} – T_L)}cosec\alpha]^{0.25}\)

View Answer

Explanation: The condensation coefficient of a condenser placed at an angle α is given by –

h

_{INC}=\(0.943[\frac{K^3 p^2 g h_{fg}}{\mu L(T_{sat} – T_L)}sin\alpha]^{0.25}\)

Hence \(\frac{h_{INC}}{h_{VER}}\)=sinα

^{0.25}, where the relation is as a sin function.

8. What is the ratio of convective heat transfer coefficients of a inclined condenser to a vertical condenser?

a) \(\frac{h_{INC}}{h_{VER}}\) = cosecα^{0.25}

b) \(\frac{h_{INC}}{h_{VER}}\) = cosα

c) \(\frac{h_{INC}}{h_{VER}}\) = sinα^{0.25}

d) \(\frac{h_{INC}}{h_{VER}}\) = tanα^{0.25}

View Answer

Explanation:

h

_{INC}=\(0.943[\frac{K^3 p^2 g h_{fg}}{\mu L(T_{sat} – T_L)}sin\alpha]^{0.25}\)

And

h

_{VER}=\(0.943[\frac{K^3 p^2 g h_{fg}}{\mu L(T_{sat} – T_L)}]^{0.25}\)

Hence,\(\frac{h_{INC}}{h_{VER}}\) = sinα

^{0.25}.

9. In a vertical setup of a Condenser, the value of convective heat transfer coefficient depends on the thermal conductivity as ___________

a) k^{0.25}

b) k^{0.33}

c) k^{0.75}

d) k^{-0.5}

View Answer

Explanation: As we know the formula for convective heat transfer coefficient is

h

_{VER}=\(0.943[\frac{K^3 p^2 g h_{fg}}{\mu L(T_{sat} – T_L)}]^{0.25}\)

Hence, h

_{VER}is proportional to K

^{0.75}

10. Frictional losses in condensers is usually ____________

a) Very high

b) Very low

c) Low

d) Negligible or 0

View Answer

Explanation: The frictional losses are very low but not necessarily equal to zero. The frictional losses can be tube turns, rough walls, pressure drops, etc.

11. For pure vapour as well as mixed liquid, the condensation is isothermal.

a) True

b) False

View Answer

Explanation: In the case of pure vapour the condensation is isothermal but in the case of mixed vapour, every component has different boiling points and hence the system is not isothermal.

12. As the liquid condenses and flows down the condenser wall, the film thickness _________ and the heat transfer coefficient _________

a) Increases, decreases

b) Increases, Increases

c) Decreases, Increases

d) Decreases, decreases

View Answer

Explanation: As the wall thickness increases, the heat transfer coefficient decrease leading to more heat transfer resistance as the film is bad conduction of heat.

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