This set of Heat Transfer Operations Multiple Choice Questions & Answers (MCQs) focuses on “Dryers – Kelvin Equation”.
1. A closed vacuum or high-pressure vessel cannot be applied for which one of the following drying operation?
a) Steam drying
b) Flash drying
c) Furnace drying
d) Drum dryers
View Answer
Explanation: High pressure cannot be used for steam drying as it would increase the boiling point and the steam would condense.
2. What happens in airless drying?
a) High pressure drying in the chamber
b) Low pressure drying in the chamber
c) High temperature drying
d) Steam is produced by compressing the evaporated water
View Answer
Explanation: Airless drying requires an airtight and well-insulated hood around the drying section of the machine where the substance is dried by steam heated cylinders in which the steam is produced by compressing the evaporated water.
3. 70 – 90% reduction in thermal energy requirements can be achieved by which one of the following drying methods?
a) Drum drying
b) Airless drying
c) Steam drying
d) Salt drying
View Answer
Explanation: Airless drying requires an airtight and well-insulated hood around the drying section of the machine where the substance is dried by steam heated cylinders in which the steam is produced by compressing the evaporated water. This condensed water vapour can provide 70 – 90% of the total heat energy required for drying.
4. The necessary condition of drying is that it only takes place if the wet material contains ________ moisture than the equilibrium value for its environment.
a) Less
b) More
c) Same amount of
d) Very less amount of
View Answer
Explanation: Evaporation can only occur when the surrounding is dry and if the surrounding is more wet, then there would rather be condensation on the surface rather than drying.
5. A capillary-porous material may be _______________
a) Hygroscopic
b) Non-hygroscopic
c) Hydrophobic
d) Hydrophilic
View Answer
Explanation: A capillary material which is porous may be non-hygroscopic (having little or no tendency to absorb moisture) which means that the moisture content of the given system now exerts complete vapour pressure.
6. What does the Kelvin equation tell us about?
a) The liquid vapour pressure is less than normal
b) The liquid vapour pressure is very less than normal
c) The liquid vapour pressure is more than normal
d) The liquid vapour pressure is equal to normal
View Answer
Explanation: The Kelvin equation given as –
PK=PO exp \([-\frac{2\sigma v}{d_p RT}] \) Signifies that the liquid-vapour pressure is less than the normal.
7. Which one of the following equation correctly represents the lowering in vapour pressure due to capillaries?
a) PK=PO exp\([\frac{2\sigma v}{d_p RT}] \)
b) PK=PO exp\([-\frac{\sigma v}{d_p RT}] \)
c) PK=PO exp\([-\frac{2\sigma v}{d_p RT}] \)
d) PK=PO exp\([-\frac{2\sigma v}{d_p R}] \)
View Answer
Explanation: The Kelvin equation for the lowering in vapour pressure due to capillaries is given as-
PK=PO exp\([-\frac{2\sigma v}{d_p RT}] \)
8. What is the term PO in the Kelvin equation given below?
a) PO is the superheated vapour pressure
b) PO is the saturation vapour pressure
c) PO is the sub-cooled vapour pressure
d) PO is the elevated vapour pressure
View Answer
Explanation: The Kelvin equation for the lowering in vapour pressure due to capillaries is given as-
PK=PO exp\([-\frac{2\sigma v}{d_p RT}] \)
Where, PK is the capillary-vapour pressure, PO is the saturation vapour pressure, σ is the surface tension, ν is the molar liquid volume and dp is the capillary diameter.
9. If given the surface tension of water as 73×10-3 N/m and diameter of capillary as 1mm, calculate the ratio of capillary-vapour pressure of water in this solid to the saturation vapour pressure at 25℃.
a) 99.8%
b) 90%
c) 98%
d) 97%
View Answer
Explanation: The Kelvin equation for the lowering in vapour pressure due to capillaries is given as-
PK=POexp\([-\frac{2\sigma v}{d_p RT}] \) Where, PK is the capillary-vapour pressure, PO is the saturation vapour pressure, σ is the surface tension, ν is the molar liquid volume and dp is the capillary diameter.
\(\frac{P_K}{P_O}=exp[-\frac{2\sigma v}{d_p RT}] = exp[-\frac{2x73x10^{-3}x22.4×10^{-3}}{8.314x298x10^{-3}}]\)=0.998.
10. What is the term V in the Kelvin equation?
a) Molar liquid volume
b) Molal liquid volume
c) Mole fraction of liquid
d) Normal liquid volume
View Answer
Explanation: The Kelvin equation for the lowering in vapour pressure due to capillaries is given as-
PK=PO exp\([-\frac{2\sigma v}{d_p RT}] \)
Where, PK is the capillary-vapour pressure, PO is the saturation vapour pressure, \(\sigma\) is the surface tension, ν is the molar liquid volume and dp is the capillary diameter.
11. The vapour pressure of a liquid is always same at a given temperature and pressure and hence does not change for highly porous solids.
a) True
b) False
View Answer
Explanation: The vapour pressure changes for porous particles and the changed vapour pressure can be represented as a function of its diameter by the Kelvin equation.
12. The ratio of the moisture-vapour pressure to the saturation value at the same temperature is called __________________
a) Relative humidity
b) Absolute humidity
c) Humidity
d) Percentage humidity
View Answer
Explanation: The definition of relative humidity is the ratio of the moisture-vapour pressure to the saturation value at the same temperature.
Sanfoundry Global Education & Learning Series – Heat Transfer Operations.
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