203 Final

Question 1

The goal of heat exchangers is to exchange as much heat as possible in the smallest area. What are some reasons this would be a goal?

Answer 1

The larger the piece of equipment is, the more expensive it is because the materials cost money.
Space inside a plant is valuable so we want the equipment to be compact.
Thicker walls between fluids increases the size and results in less heat transferred, so we make the walls as thin as possible, but still strong enough to hold up to the flow .

Question 2

What is the term for hot and cold fluids flowing in opposite directions?

Answer 2

Countercurrent

Question 3

What is the term for hot and cold fluids flowing in the same direction?

Answer 3

Cocurrent

Question 4

What is the term for hot and cold fluids flowing at right angles to each other?

Answer 4

Crossflow

Question 5

What does a small Biot number indicate about the temperature distribution in a cooling particle?

Answer 5

The temperature is the same throughout the entire particle

Question 6

What does an intermediate Biot number indicate during particle cooling?

Answer 6

The temperature of the fluid in contact with the particle differs from the bulk fluid, and the particles temperature varies across it.

Question 7

What does a large Biot number suggest about fluid and particle temperature?

Answer 7

The fluid in contact with the particle is the same temperature as the bulk fluid.

Question 8

The Biot number is a ratio of what to what?

Answer 8

Interior resistance to surface resistance

Question 9

α is a function of what variables ?

Answer 9

Cs

ρs

ks

Question 10

True or false.
Cocurrent heat exchangers are the most efficient at transferring heat?

Answer 10

This is false.

Countercurrent ( opposite flow direction ) heat exchangers are typically more efficient, as they maintain a more uniform temperature gradient between the two fluids throughout the exchanger length

Question 11

If the circumference of a cylinder were to increase, how would that affect the Biot number?

Answer 11

the Biot number would increase

The circumference increasing causes the radius and the characteristic length both to increase leading to a higher Biot number.

Question 12

If the density of a substance were to increase, how does that affect the time it takes to cool an object?

Answer 12

The time it takes to cool an object will increase

The cooling time of an object is influenced by its thermal inertia, which depends on density, specific heat capacity, and thermal conductivity. When density increases, assuming k and Cp stay the same, then thermal diffusivity (α) decreases leading to a lower temperature change and longer cooling time.
Greater density = more thermal mass = slower cooling

Question 13

For a 1-2 shell and tube heat exchanger, what do the one and two represent?

Answer 13

The one indicates that there is one pass for the fluid inside the shell. Essentially, the shell side fluid flows through the shell in a single pass.

The two indicates that the tube side fluid makes two passes through the tubes

Question 14

What are baffles?

Answer 14

Baffles are internal plates or guides that are placed within the shell of the heat exchanger to support the tubes, prevent tube vibration and sagging, and direct the flow to have higher heat transfer coefficient.

Question 15

What is the distance between two baffles called?

Answer 15

Baffle spacing

Question 16

For a heat exchanger problem :

Oil is being cooled while the water absorbs heat in the heat exchanger. What sign would the Q of oil and the Q of water have?

Answer 16

The oil is losing heat, so the Q of the oil would be negative.
The water is gaining heat, so the heat transfer for the water would be positive .

Question 17

When choosing a U value and there is a range which you should you choose?

Answer 17

As a conservative estimate, we should choose the U that gives us the largest area which means we choose a low U.

Question 18

What happens to the heat transfer area (A) when the overall heat transfer coefficient (U) is small?

Answer 18

When U is small, the heat transfer area (A) must be large larger to achieve the same rate of heat transfer (Q). This is because a smaller U means a less efficient heat transfer, so a larger surface area is needed to compensate for the lower efficiency.

Question 19

What are the benefits of countercurrent heat exchangers compared to cocurrent?

Answer 19

Countercurrent heat exchange is more attractive than cocurrent because it requires a smaller heat transfer area and cost less to accomplish the same heat transfer.

Question 20

What is a heat exchanger pinch point?

Answer 20

The pinch point in a heat exchanger refers to the location where the temperature difference between the hot and cold fluids is the smallest. If the temperature difference at the pinch point is too small, large surface areas or very efficient heat exchange is needed to transfer sufficient heat. It it determines the limiting factor for heat transfer.

Question 21

When can a system be considered transient or at unsteady state?

Answer 21

A system can be considered transient or at unsteady state if the temperatures are changing and not yet at equilibrium.

An example would be putting an ice cube in a warm drink, this will cause the ice to initially melt and the drink to cool down, but this will not occur instantaneously .

Question 22

What is Fo? What does it mean when Fo =1, and what does it mean when
Fo > 1?

Answer 22

This is the fourier number which accounts for cooling time and object size; Fo = 1 means enough time is passed for temperature change across the system.
Fo > 1 means the system is approaching steady state.

Question 23

What is the Bi number?

Answer 23

The Bi number is a dimensionless number for transient heat transfer called the Biot number. It describes the relative importance of interior resistance and surface resistance to thermal conduction.

Bi = interior resistance / surface resistance

Question 24

What L is used in the Fo number?

Answer 24

The characteristic length, which is the volume over the surface area.

Question 25

What is the thermal diffusivity? α

Answer 25

The thermal diffusivity is a property of the material that relates its ability to conduct thermal energy to its ability to store it. Higher means it is a good thermal conductor.

Question 26

Nu and Bi have very similar equations. What’s the difference?

Answer 26

Bi uses thermal conductivity of the solid, Nu uses the thermal conductivity of the fluid.

Question 27

If talking about a copper wire, what can we assume about the Bi number?

Answer 27

Copper is highly conductive so it would be fair to assume a small Biot number.

Question 28

If we have forced convection, will the Biot number be very small,intermediate , or very large?

Answer 28

Very large

Question 29

If a hot object is plunged into cold water, it cools, but not instantaneously. What two factors govern the cooling rate of an object ?

Answer 29

The film resistance at the surface of the object, characterized by the h value for the situation. The rate of heat flow out of the interior of the object.

Question 30

What does a small Biot number indicate regarding heat resistance?

Answer 30

A small Biot number indicates that the main resistance to heat transfer is the film.

Question 31

What does a large Biot number indicate regarding heat resistance?

Answer 31

A large Biot number indicates that the main resistance to heat transfer is the conduction of heat out of the body.

Question 32

What are the three broad types of exchangers?

Answer 32

The recuperator or through-the-wall nonstroring exchanger The direct-contact nonstoring exchanger The regenerator, accumulator, or heat-storing exchanger

Question 33

What is a recuperator heat exchanger?

Answer 33

A recuperator is a type of heat exchanger where the two flowing streams are separated by a wall, and heat must pass through this wall.

Question 34

Why are recuperator heat exchangers less effective than direct contact exchangers?

Answer 34

Recuperator’s are less effective because the presence of the wall hinders the flow of heat between the fluids

Question 35

In what situations are recuperators used?

Answer 35

Recuperator are used when the fluids are not allowed to contact each other, such as in gas-gas systems, miscible liquids, dissolving solids, or reactive chemicals Examples: Shell and tube exchanger Cold finger and bayonet exchanger Plate exchanger Spiral plate exchanger Automobile Radiator Combat exchanger

Question 36

How do direct contact non-storing exchangers work?

Answer 36

In direct contact non-storing exchangers, the streams contact each other intimately with the hotter stream transferring heat directly to the colder stream

Question 37

When are direct contact non-storing exchangers typically used?

Answer 37

They are used when the two contacting phases are mutually insoluble and do not react with each other, meaning they cannot be used with gas-gas systems.

Question 38

What are the three broad types of direct contact exchangers?

Answer 38

Gas-solid exchangers Fluid-fluid exchangers (where the fluids are mutually immiscible) Exchangers where one fluid dissolves in the other (such as air-water systems)

Question 39

Give an example of a direct contact exchanger where one fluid dissolves in the other.

Answer 39

An example is the water cooling tower, where water (the liquid phase) dissolves or evaporates in air ( the gas phase). This is most widely used type of exchanger in industry.

Question 40

Why is the direct-contact exchanger important in air-water systems?

Answer 40

The direct-contact exchanger is important because water dissolves or evaporates in air, which is critical for applications like cooling towers

Question 41

How do regenerators work?

Answer 41

In regenerators, a hot steam of gas, transfers its heat to an intermediary usually a solid, which then later releases the stored heat to a second stream of cold gas

Question 42

What challenge arises when using fluidized beds for counter current heat exchange of solids?

Answer 42

The main challenge when using fluidized beds for counter current heat exchange of solids is vigorous back mixing of the solid particles. The back mixing disrupts the desired plug flow pattern and causes severe deviations from ideal countercurrent flow, reducing the effectiveness and predictability of heat transfer. It makes it difficult to maintain a well defined temperature gradient, and efficient heat exchanged between the two solid streams.

Question 43

What is another simpler method for countercurrent heat exchange of solids?

Answer 43

A simpler method is to use a liquid stream as a go between.

Question 44

What are the two key pieces of information required for designing a recuperator?

Answer 44

The two key pieces of information are: 1. the overall heat transfer coefficient U, which accounts for the total resistance to heat transfer, (including film and wall resistance) 2. The contacting pattern of the two phases

Question 45

What does the overall heat transfer coefficient U account for in a recuperator design?

Answer 45

The overall heat transfer coefficient U accounts for the overall resistance to heat transfer caused by the wall, including individual film resistances and wall resistance.

Question 46

In shell and tube exchangers what do the variables M,C, and T refer to?

Answer 46

M refers to the Shell side flow rate C refers to the specific heat on the shell side T refers to the temperature on the shell side

Question 47

In shell and tube exchangers what do the variables m, c, and t refer to?

Answer 47

m refers to the tube side flow rate c refers to the specific heat on the tube side t refers to temperature on the tube side

Question 48

Why are shell and tube exchangers still widely used in the industry, despite being less efficient than countercurrent plug flow?

Answer 48

They are widely used because they are often more convenient, compact and less expensive to construct for a given duty

Question 49

How are less efficient shell and tube exchangers treated in performance analysis?

Answer 49

They are treated as counter current plug flow units with a “fudge factor” f,which accounts for the lowered contacting efficiency. This factor is between zero and one.