Question 1

For an ideal black body

Accepted Answer

An ideal black body absorbs all incident radiation, hence its absorptivity is 1. Reflectivity, emissivity, and transmissivity values for an ideal black body do not match the options given.

Question 2

In a liquid-liquid heat exchanger, for the same process temperature, the ratio of the LMTD in parallel flow to the LMTD in counter flow is always

Accepted Answer

In a liquid-liquid heat exchanger, the Log Mean Temperature Difference (LMTD) in parallel flow is always less than the LMTD in counter flow for the same process temperatures. This is because the temperature gradient between the two fluids decreases more rapidly in parallel flow, leading to a lower average temperature difference across the heat exchanger.

Question 3

For concentrating an aqueous solution of a material like anhydrous Na2SO4, whose solubility decreases with rise in temperature, the most suitable evaporator is a __________ evaporator.

Accepted Answer

A vacuum evaporator is most suitable for concentrating solutions like anhydrous Na2SO4, whose solubility decreases with a rise in temperature, because it operates at reduced pressure, allowing the solvent (water) to evaporate at lower temperatures, which is beneficial for such solutes.

Question 4

Small scale evaporation is done in a

Accepted Answer

Small scale evaporation is typically done in a steam jacketed kettle, which allows for the controlled heating necessary for the evaporation process.

Question 5

Leidenfrost point is a term concerned with the

Accepted Answer

The Leidenfrost point refers to the temperature at which a liquid, upon coming into contact with a surface significantly hotter than its boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. This phenomenon is observed when a liquid is in a state of near-boiling on a surface that is much hotter than the liquid's boiling point, leading to the formation of a vapor layer that insulates the liquid from the surface, allowing it to levitate and move around rather than evaporate quickly.

Question 6

In a single effect evaporator, the economy is

Accepted Answer

The answer of In a single effect evaporator, the economy...

Question 7

At steady state the temperature variation in a plane wall, made of two different solids I & II is shown below: The thermal conductivity of material „I?

Accepted Answer

In steady state, the temperature gradient is inversely proportional to the thermal conductivity. If the temperature gradient in material I is steeper than in material II, then the thermal conductivity of material I is smaller than that of material II.

Question 8

Mode of heat transfer involved in the cooling of air cooled internal combustion engine is

Accepted Answer

The answer of Mode of heat transfer involved in the...

Question 9

Which of the following is not used as a medium for high temperature heating ?

Accepted Answer

Mercury is not used as a medium for high temperature heating due to its relatively low boiling point (356.7°C) compared to other options listed, which limits its application in high temperature processes. Additionally, mercury's toxicity poses significant health and environmental risks.

Question 10

In counter flow compared to parallel flow,

Accepted Answer

In counter flow heat exchangers, the log mean temperature difference (LMTD) is greater, allowing for a more efficient heat transfer. This efficiency means that for a given heat transfer rate, less surface area is required compared to parallel flow heat exchangers.

Question 11

The inside heat transfer co-efficient in case of turbulent flow of liquid in the tube side in a 1-2 shell and tube heat exchanger is increased by __________ times, when the number of tube passes is increased to 8.

Accepted Answer

The answer of The inside heat transfer co-efficient in case...

Question 12

Heat exchangers operating, when the asymptotic range is reached,

Accepted Answer

When the asymptotic range is reached in heat exchangers, the temperature difference between the two fluids becomes very small in some parts of the heat exchanger. This effectively makes part of the heating surface inactive because the driving force (temperature difference) for heat transfer is minimal, leading to very low heat transfer rates in those areas.

Question 13

In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred

Accepted Answer

In a regenerative air preheater, heat is transferred by heating an intermediate material (such as chequor bricks) with hot flue gas. This hot material then transfers the heat to the air, effectively preheating it before it enters the combustion process or space to be heated, like coke ovens. This method does not involve direct contact between the hot flue gas and the air, nor does it rely solely on a metallic wall for heat transfer.

Question 14

In case of evaporators, liquid entrainment results primarily due to

Accepted Answer

Foaming of the solution can cause liquid entrainment in evaporators as bubbles can carry liquid droplets with the vapor.

Question 15

A multiple effect evaporator has a capacity to process 4000 kg of solid caustic soda per day, when it is concentrating from 10% to 25% solids. The water evaporated in kg per day is

Accepted Answer

To find the water evaporated, calculate the initial and final mass of the solution. Initial mass = 4000 kg / 0.10 = 40000 kg. Final mass = 4000 kg / 0.25 = 16000 kg. Water evaporated = Initial mass - Final mass = 40000 kg - 16000 kg = 24000 kg.

Question 16

Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube

Accepted Answer

In laminar flow through a long tube, asymptotic conditions are reached when the exit fluid temperature equals the wall temperature, indicating thermal equilibrium.

Question 17

The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is

Accepted Answer

The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe with constant surface temperature is 3.66. This value is derived from the analytical solution of the thermal energy equation under the specified conditions.

Question 18

Economy of a multiple effect evaporator is not influenced much by the

Accepted Answer

The answer of Economy of a multiple effect evaporator is...

Question 19

For evaporation of viscous solution in a multiple effect evaporator, the preferred feeding scheme is

Accepted Answer

Backward feeding in a multiple effect evaporator is preferred for the evaporation of viscous solutions because it allows the solution with the highest viscosity to be heated with the lowest temperature steam, reducing the viscosity and making it easier to handle.

Question 20

What is the absorptivity of a black body?

Accepted Answer

A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. Therefore, its absorptivity is 1.

Quiz 8: Heat Transfer and Crystallisation