Heat Transfer Multiple Choice Questions & Answers (MCQs) on “Condensation”.
1. Condensation refers to a change from the
a) Solid to a liquid phase
b) Vapor to a liquid phase
c) Liquid to a solid phase
d) Liquid to a vapor phase
Answer: b
Clarification: Condensation is a convective heat transfer process that is associated with a change in the phase of a fluid.
2. Condensation process is very common in
(i) Boilers
(ii) Condensers
(iii) Evaporators
Identify the correct statements
a) i and ii
b) ii and iii
c) i, ii and iii
d) i and iii
Answer: c
Clarification: This process is very common in power plants and refrigeration systems.
3. The convective coefficients for condensation usually lie in the range
a) 30-300 W/m2 K
b) 60-3000 W/m2 K
c) 300-10000 W/m2 K
d) 2500-10000 W/m2 K
Answer: d
Clarification: The convective coefficient for condensation should be high because condensation refers to a change from the vapor to a liquid phase.
4. Drop wise condensation usually occurs on
a) Oily surface
b) Glazed surface
c) Smooth surface
d) Coated surface
Answer: a
Clarification: It generally occurs on oily surface. It is the convective heat transfer process that is associated with a change in the phase of a fluid.
5. Consider the following statements
(i) If a condensing liquid does not wet a surface, then drop wise condensation will not take place on it
(ii) Drop wise condensation gives a higher transfer rate than film wise condensation
(iii) Reynolds number of condensing liquid is based on its mass flow rate
(iv) Suitable coating or vapor additive is used to promote film wise condensation
Identify the correct statement
a) i and ii
b) ii, iii and iv
c) iv only
d) i, ii and iii
Answer: d
Clarification: All are correct except the last one because suitable coating or vapor additive is not used to promote film wise condensation.
6. Depending upon the behavior of condensate up on the cooled surface, the condensation process are classified into how many distinct modes?
a) 1
b) 2
c) 3
d) 4
Answer: b
Clarification: Film condensation and drop wise condensation.
7. A plate condenser was designed to be kept vertical. How would the condensation coefficient be effected if due to site constraints, it has to be kept at 60 degree to the horizontal?
a) 1.53% reduction in condensation coefficient
b) 2.53% reduction in condensation coefficient
c) 3.53% reduction in condensation coefficient
d) 4.53% reduction in condensation coefficient
Answer: c
Clarification: h VER = 0.943 [k 3 p 2 g h f g/δ l (t sat – t s)] 0.25, h INC = 0.943 [k 3 p 2 g sin α h f g/δ l (t sat – t s)] 0.25.
8. Saturated steam is allowed to condense over a vertical flat surface and the condensate film flows down the surface. The local coefficient of heat transfer for condensation
a) Remains constant at all heights of the surface
b) Decreases with increasing distance from the top of the surface
c) Increases with increasing thickness of film
d) Increases with increasing temperature differential between the surface and vapour
Answer: b
Clarification: It decreases with increasing thickness of condensate film.
9. In condensation over a vertical surface, the value of convection coefficient varies as
a) k 0.25
b) k 0.33
c) k 0.75
d) k -0.5
Answer: a
Clarification: h = 0.943 [k 3 p 2 g h f g/δ l (t sat – t s)] 0.25.
10. For film wise condensation on a vertical plane, the film thickness δ and heat transfer coefficient h vary with distance x from the leading edge as
a) δ decreases, h increases
b) Both δ and h increases
c) δ increases, h decreases
d) Both δ and h decreases
Answer: c
Clarification: Thickness increases and heat transfer coefficient decreases.