Tough Heat Transfer Operations Questions on “Packed Beds – Bed Temperature Profiles”.
1. The particles are usually _______ packed near the wall of the tube.
a) Tightly
b) Loosely
c) Very tightly
d) Negligibly
Answer: b
Clarification: Particles at the centre form hexagonal or octagonal close packed structure which sort of ends near the walls, hence they are loosely packed near the walls.
2. The presence of particles makes the inside heat transfer coefficient higher than empty tube because the superficial velocity is _______ the actual velocity.
a) Little higher than
b) Very higher than
c) Very less than
d) Little less than
Answer: b
Clarification: The Superficial velocity given by the relation Vs = ∅Va, where ∅ is the porosity, hence as ∅<<1, Vs>>Va.
3. The heat transfer coefficient ________ with increasing flow rate.
a) Increases
b) Decreases
c) Remains the same
d) Drops drastically
Answer: b
Clarification: As the heat transfer coefficient is hwall Dp/Kg=1.94 Re0.5 Pr0.33 and the Reynolds number is directly proportional to Vs, hence a power of 0.5 decreases h with increasing Re.
4. In the expression given below, what is the expression Di? This given ratio defines voidage which is controlled to obtain maximum heat transfer coefficient.
∈ = Dp/Di
a) Internal diameter of Pellets
b) Internal diameter of Tube
c) Internal diameter of Bed
d) Internal diameter of Pellets
Answer: b
Clarification: This ratio is used to signify the porosity of the bed without explicitly calculating the porosity. This ratio helps us to know the temperature profiles and guess the efficiency of the packed bed.
5. The value of dp/di in the range _______ gives maximum heat transfer coefficient.
a) 0.15-0.2
b) 2-3
c) 0.6-0.9
d) 0.001-0.002
Answer: a
Clarification: The value of dp/di in 0.15-0.2 or di/dp = 5-10 is best suited for heat transfer operations in the packed bed as this ratio perfectly fits the turbulence and mixing required.
11. For very low dp/di, the temperature pattern is similar to that of complete mixing and turbulent flow.
a) True
b) False
Answer: b
Clarification: When we have very low Dp/Di, which means the pellet size is very small, the flow becomes completely laminar inside the bed as the void spaces come very close to one another.
6. For very high dp/di, the mixing is very large and the heat transfer coefficient is ______
a) Low
b) Very low
c) High
d) Very high
Answer: c
Clarification: The heat transfer coefficient increases because there is now rapid mixing of the Fluid passing through the voids.
7. Which one is the correct temperature profile for high dp/di Packed bed setup?
a) Laminar temperature profile
b) Turbulent Temperature profile
c) Laminar near the walls and turbulent at the centre
d) Turbulent near the walls and laminar at the centre
Answer: c
Clarification: When we have a high Dp/Di, which means the pellet size is large, the flow becomes completely laminar near the walls and turbulent at the bed as the void spaces are more near the walls.
8. Which one is the correct temperature profile for very low dp/di packed bed setup?
a) Laminar temperature profile
b) Turbulent Temperature profile
c) Laminar near the walls and turbulent at the centre
d) Turbulent near the walls and laminar at the centre
Answer: a
Clarification: When we have very low Dp/Di, which means the pellet size is very small, the flow becomes completely laminar inside the bed as the void spaces come very close to one another.
9. The renowned equation used to calculate the pressure drop in a non-ideal packed bed is known as______________
a) Sieder Tate Equation
b) Kozeny Carman Equation
c) Ergun Equation
d) Arrhenius Equation
Answer: c
Clarification: The Ergun equation expresses the friction factor as the Reynolds number and is commonly used to calculate the pressure drop in packed beds having non laminar flow regime.