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

1. What is the expression for heat transfer coefficient for a vertically placed condenser?

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

b) h_{VER}=0.743\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L)}]^{0.25}\)

c) h_{VER}=0.723\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L)} ]^{0.25}\)

d) h_{VER}=0.725\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L) }]^{0.25}\)

View Answer

Explanation: The heat transfer coefficients given by Nusselt’s theory for film-wise condensation on the surface of vertical tubes is as follows,

h

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

2. What is the expression for heat transfer coefficient for a horizontally placed condenser?

a) h_{HOR}=0.943\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L)}]^{0.25}\)

b) h_{HOR}=0.743\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L)}]^{0.25}\)

c) h_{HOR}=0.723\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L)}]^{0.25}\)

d) h_{HOR}=0.725\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L)}]^{0.25}\)

View Answer

Explanation: The heat transfer coefficients given by Nusselt’s theory for film-wise condensation on the surface of horizontal tubes is as follows,

h

_{HOR}=0.725\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L)}]^{0.25}\)

3. As thermal conductivity kF increases, the heat transfer coefficient ________

a) Increases

b) Decreases

c) Remains same

d) Jumps to a high value

View Answer

Explanation: As thermal conductivity kf increases, the heat transfer coefficient increases since thermal resistance of the condensate film decreases.

4. If there is a decrease in viscosity, the heat transfer coefficient ____________

a) Increase

b) Decrease

c) Remains same

d) Becomes zero

View Answer

Explanation: If the viscosity decreases or density increases, it will offer less frictional resistance and cause quick draining of the condensate, thereby causing an increase in heat transfer coefficient.

5. A high value of latent heat h_{fg} means that the heat transfer coefficient would __________

a) Increase

b) Decrease

c) Remains same

d) Becomes zero

View Answer

Explanation: A high value of latent heat means that for each kilo-Watt of heat transfer occurred, there will be thinner condensate thickness and hence the heat transfer coefficient augments.

6. An increase in diameter means a _________ heat transfer coefficient.

a) Larger

b) Smaller

c) Zero

d) Infinite

View Answer

Explanation: If the diameter increases then the thickness of the film at the bottom increases and hence a smaller heat transfer coefficient.

7. A large value of temperature difference will lead to more condensation and hence a __________ heat transfer coefficient

a) Larger

b) Smaller

c) Zero

d) Infinite

View Answer

Explanation: A large temperature difference will lead to more condensation on the wall and bigger condensate thickness which will lead to a small heat transfer coefficient.

8. An increase in number of tubes will lead to larger condensate thickness in the lower tubes leading to ____________ heat transfer coefficient

a) Larger

b) Smaller

c) Zero

d) Infinite

View Answer

Explanation: If the number of tubes increases then the condensate thickness too increases in the lower tubes leading to smaller heat transfer coefficient.

9. If the condensation occurs at the outside surface of the tubes, then the equation for the heat transfer coefficient does not change.

a) True

b) False

View Answer

Explanation: The constant of multiplication in the equation changes and the new equation is =

h

_{VER}=1.13\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L)}]^{0.25}\)

10. If the condensation occurs at the outside surface of the tubes, then the equation for the heat transfer coefficient cab be represented as which one of the following?

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

b) h_{VER}=1.13\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L)} ]^{0.25}\)

c) h_{VER}=0.723\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L)} ]^{0.25}\)

d) h_{VER}=0.725\([\frac{K^3 p^2 g h_{fg}}{μx(T_{sat} – T_L) }]^{0.25}\)

View Answer

Explanation: When the condensation occurs outside the tube surface the film formed is less thick and easily wiped which increases the heat transfer coefficient compared to the inner tube condensation, hence the constant is higher here.

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