310204e Heat Transfer And Evaporation

Question 1

Define energy

Answer 1

Energy is the ability to do work

Question 2

Define heat transfer

Answer 2

Heat transfer is the exchange of heat from a hot fluid to a cold fluid to either raise the temperature of the cold fluid or lower the temperature of the hot fluid.

Question 3

Heat transfer occurs in 3 different ways, what are they?

Answer 3

1. Conduction
2. Convection
3. Radiation

Question 4

What is thermal conduction?

Answer 4

Thermal conduction is the movement of heat from a region of high temperature to a region of low temperature

Question 5

What is the equation for expressing the amount of heat that flows through substance per unit of time?

Answer 5

Question 6

Define convection

Answer 6

Convection is the physical transfer of the stored thermal energy (heat) of a substance from one location to another
Air and water are 2 fluids that store heat and then transfer the heat by convection.

Question 7

Define radiation

Answer 7

Radiation is heat that transfers through space from a hot object to a cold object in the form of electromagnetic waves
Heat from the sun that reaches the earth warms an object to a temperature higher than the temperature of the ambient air through radiation

Question 8

True or false
Air requires more than 4 times the mass and 800 times the volume to do the same amount of cooling as water.

Answer 8

True

Question 9

What is a Condenser?

Answer 9

Condensers are heat exchangers that condense vapor into liquid by removing heat
During condensation latent heat is removed and the temperature and pressure of the condensing fluid should remain the same

Question 10

What are the 2 types of condensers?

Answer 10

1. Surface condensers, where the coolant does not contact the vapours or condensate
2. Direct contact condensers, where the coolant, vapour and condensate are in direct contact.

Question 11

If there is a choice which of the 2 fluids in heat exchanger should be run inside of the tubes and why?

Answer 11

The fluid that has the greatest tendency to foul the tube should be run through the inside of the tube because the inside of the tube is easier to clean.

Question 12

True or false
Materials that are good electrical insulators are good heat conductors

Answer 12

False, materials that are good electrical insulators are also poor heat conductors

Question 13

Define Co-current flow

Answer 13

When the fluids in a heat exchanger flow in the same direction

Question 14

Define counter current flow

Answer 14

When the fluids in a heat exchanger flow in opposite directions

Question 15

What is the process for tube cleaning in a U-tube shell and tube heat exchanger?

Answer 15

The tubes must be chemically cleaned. If this does not work, the tube bundle must be replaced.

Question 16

Explain why the floating head in a shell and tube heat exchanger is necessary

Answer 16

The floating hat is necessary to allow the head to move
As the exchanger heats and cools, the tubes will expand and change length
If both ends of the tubes are rigidly fastened, thermal expansion will damage the tubes.

Question 17

List 4 factors that vary the airflow rate through an air-cooled condenser

Answer 17

1. The number of fans running
2. The speed of the fans
3. The pitch of the fan blades
4. The position of the inlet and outlet louvers of the cooler.

Question 18

What features make the plate heat exchanger a very effective heat transfer device?

Answer 18

The plate heat exchanger uses metal plates to transfer heat.
This makes it effective because the fluids are exposed to a larger surface area

Question 19

What are 2 common methods for cooling water using an evaporative process?

Answer 19

1. Spray ponds
2. Cooling towers

Question 20

Mechanical draft cooling towers can either be an induced or forced draft design. Explain the difference between the 2 designs.

Answer 20

In a forced draft cooling tower, the fan is in the ambient airstream entering the cooling tower and the air is blown through the unit
In the induced draft cooling tower, the fan is located at the exit of the airflow and draws air through the tower.

Question 21

What does air recycling refer to with cooling towers?

Answer 21

Air recycling is when the wet air expelled at the top of the cooling tower is blown back into the air intake by the wind.

Question 22

Condensers are classified based on the interaction between the condensing vapor and the cooling fluid
State the 2 classifications

Answer 22

1. Surface condensers, where the coolant does not contact the vapours or condensate
2. Direct contact, where the coolant, vapour and condensate are in direct contact

Question 23

What is the objective of evaporation in a process evaporator

Answer 23

The objective of evaporation is to concentrate a solution or slurry feed
Ex. Recovering salt from seawater

Question 24

What are the 2 main components of a process evaporator

Answer 24

1. The heat exchanger
2. The vapour body

Question 25

True or false
Lowering the pressure in the vapor body below atmospheric pressure (vacuum) in the evaporator lowers the boiling point of the water

Answer 25

True

Question 26

What prevents the liquid from leaving a rising film evaporator along with the vapour?

Answer 26

The deflector plate above the tubes

Question 27

What evaporator design is used to avoid boiling the product on the heating surfaces?

Answer 27

A forced circulation evaporator.

Question 28

What serves as the heat source in the second effect in a multiple effect evaporator

Answer 28

The hot vapours produced in the first effect evaporator provide the heat source in the second effect
The water boils in a sequence of vessels where each vessel is held at a lower pressure than the last
Because the boiling temperature of water decreases as pressure decreases, the vapour boiled off in one vessel can be used to heat the next.
Only the first vessel, the one at the highest pressure, requires an external source of heat

Question 29

What causes lower vapour pressures in evaporators following the first effect

Answer 29

The pressures become progressively lower due to the condensing of the vapours as they move through the heat exchangers

Question 30

What is the process of crystallization?

Answer 30

Crystallization separates solids and liquids through a process that saturates a solution by lowering its temperature until the solid material precipitates out in the form of crystals.

Question 31

When are vacuum crystallizers used over standard crystallizers?

Answer 31

Vacuum crystallizers are used when the solution is heat sensitive

Question 32

What maintains the vacuum in a vacuum crystallizer?

Answer 32

The vacuum is maintained with a steam injector and a barometric condenser.