# Food Processing Unit Operations Questions and Answers – Heat Transfer in Evaporators

This set of Food Processing Unit Operations Questions & Answers for Exams focuses on “Heat Transfer in Evaporators”.

1. What are modes of heat transfer in evaporators?
a) Conduction and convention

Explanation: Heat transfer in evaporators is governed by conduction and convention. Calculations on evaporators can be supported by combining mass and energy balances with the principles of heat transfer.

2. What controls the overall heat transfer coefficient in falling film evaporators?
a) Film thickness
b) Velocity
c) Liquid properties
d) Film thickness, velocity, liquid properties

Explanation: Falling film evaporators controls the overall heat transfer coefficient by difference in temperature, thickness of the film, properties of the liquid, velocity across the film layer. These are important design modules which help in high heat transfer rates.

3. What is the cause of convection heat transfer in evaporators?
a) Convection is heat transfer by neutrons
b) Convection is heat transfer by mass motion of a heat source
c) Convection is mass transfer by mass motion of a fluid
d) Convection is heat transfer by mass motion of a fluid

Explanation: Convection is heat transfer by mass motion of a fluid like water, air. Due to this movement the heated fluid moves away from the source of heat and thus carrying energy with it.

4. What is conduction?
a) Conduction is heat transfer by means of molecular agitation
b) Conduction is mass transfer by means of molecular agitation
c) Conduction is heat transfer by means of electron agitation
d) Conduction is heat transfer by means of atom agitation

Explanation: It is heat transfer by molecular agitation within a material without any motion of the material as a whole. If one end of a metal rod is heated, the energy transfers toward the colder end as the higher speed particles collides with the slower ones with a net transfer of energy to the slower ones.

5. What is evaporation?
a) Process of substance in a liquid state changing to a gaseous state
b) Process of substance in a solid state changing to a gaseous state
c) Process of substance in a liquid state changing to a solid state
d) Process of substance in a gaseous state changing to a liquid state

Explanation: Evaporation is the process of a substance in a liquid state changing to a gaseous state due to an increase in temperature and pressure.When the molecules of the liquid collide, they transfer energy to each other.
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6. Which of the following statements about rate of evaporationare correct?
Statement 1: Quantity of heat required to evaporate each kg of water.
Statement 2: Maximum allowable temperature of the liquid,
a) True, False
b) True, True
c) False, True
d) False, False

Explanation: The basic factors that affect the rate of evaporation are i) Rate of heat transfer to the liquid, ii) Quantity of heat required to evaporate each kg of water, iii) Maximum allowable temperature of the liquid, iv) Pressure at which the evaporation takes place, v) Changes that occur in the product during the evaporation process.

7. What are the principle functions of evaporator?
a) Heat exchanger and separator
b) Heat exchanger
c) Heat exchanger
d) Evaporate

Explanation: Heat exchanger and separator are the two main principle functions of evaporator. Evaporation is used to pre-concentrate food, to increase the solid content of food and to reduce the water content of a liquid product.

8. What is meant by heat transfer coefficient?
a) The heat transfer coefficient is the heat transferred per unit volume per kelvin
b) The heat transfer coefficient is the heat transferred per unit area per kelvin
c) The heat transfer coefficient is the heat transferred per unit volume per Celsius
d) The heat transfer coefficient is the heat transferred per unit area per Celsius

Explanation: It is the heat transferred per unit area per kelvin. The area included in the equation represents the area over which the heat transfer takes place. The areas for each flow are different as they signify the contact area for each fluid side.

9. How do you increase heat transfer coefficient?
a) By increasing Reynolds number
b) By decreasing Reynolds number
c) By increasing velocity
d) By increasing Reynolds number and velocity

Explanation: To increase the heat transfer coefficient by increasing Reynolds number and velocity of fluid when it flows through a pipe. This is due to higher velocity increases turbulence, which in turn provides a more efficient radial transport of heat.

10. What factors affect heat transfer coefficient?
a) Viscosity, density, specific heat
b) Viscosity
c) Density
d) Specific heat

Explanation: The factors affecting overall heat transfer coefficient are physiochemical properties of fluids like viscosity, density, specific heat, thermal conductivity, geometry of the evaporator and velocity of flowing fluids.

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