Lecture 9 : Heat exchanger I

Question 1

What are the 2 broad categories of heat exchangers?

Answer 1

1. Contact type heat exchanger
2. Non-contact type heat exchanger

Question 2

What is meant by contact type heat exchanger and list some examples [2]?

Answer 2

There is direct physical contact between product and heating/cooling streams

• Steam infusion
• Steam injection

Question 3

What is meant by non- contact type heat exchanger and list some examples [4]?

Answer 3

Product and heating/cooling mediums are physically separated, usually by a thin wall

• Plate exchanger
• Tubular heat exchanger
• Shell-and-tube (similar to tubular)
• scraped surface (not v impt)

Question 4

How does a plate heat exchanger work?

Answer 4

• Consists of plates stacked very close together
• Each plate alternates between product and heating medium
• Each plate has a gasket (rubber/polyester), to block 2 out of the 4 holes to direct fluid flow
• Fluids flow through the surface of the plate and flow through outlet

Question 5

In a plate heat exchanger, what are the 2 purposes of the gasket?

Answer 5

1. Prevent intermixing of fluid
2. Prevent leaks

Question 6

In a plate heat exchanger, why are surfaces corrugated (zigzag)?

Answer 6

Increase surface area of contact for more efficient heat transfer.

Question 7

What kind of fluids is the plate heat exchanger suitable for?

Answer 7

Low viscosity liquid foods (< 5 Pa s)

If solids are present, size of particulates should be < 0.3cm

• if high viscosity, fluid will get stuck and cant flow through the plates

Question 8

What is the phenomenon where solid particles get deposited the corrugated surfaces of plate heat exchanger called? What is the problem with it?

Answer 8

Fouling. Fouling is a problem as deposition of solids on the plates can decrease heat transfer rate

Question 9

What are some advantages of plate heat exchangers? [4]

Answer 9

1. High value for overall heat transfer coefficient
2. Compact design
3. Works well with small temperature differences between fluids
4. easy maintenance and cleaning
   - easy to stack / remove plates to alter heat transfer capacity, and can easily take plates apart

Question 10

What is the regeneration phenomenon in plate heat exchangers?

Answer 10

At the regeneration area, heat from heated product is transferred to incoming new, cold product.

Question 10

What are some disadvantages of plate heat exchangers? [3]

Answer 10

1. Narrow passageways leads to high pressure drop (increased friction), thus incurring higher costs to supply pressure for pumping
2. Potential for leakage
3. Does not work well with very high temperature fluids (gasket could melt) –> poses temperature limitations

Question 11

What is the equation for regeneration efficiency?

Answer 11

amount of energy supplied by regeneration / amount of energy required assuming no regeneration

Question 11

What are the benefits of regeneration?

Answer 11

1. Reduced operating costs – since you are using the heated product to heat up the new cold product and dont need additional cooling/heating medium
2. Improved efficiency: Less energy is required to achieve the desired level of heating or cooling

Question 11

How does a simple tubular heat exchanger work?

Answer 11

• One fluid is pumped through inner pipe, other fluid pumped through annular space
• Flow may be parallel/counter flow

Question 12

In a shell-and-tube heat exchanger, there are baffles in the annular space. What is the purpose of the baffles?

Answer 12

Usually, fluid takes the shortest path. However, adding baffles forces the fluid to travel longer distances.

Thus, to get the same volume of fluid to flow out but over a longer distance, it the fluid in the shell to move faster, increasing velocity and promoting turbulence.

Turbulent flow increases heat transfer coefficient and thus increases the rate of heat transfer.

Question 13

What is a way to increase fluid velocity and thus heat transfer efficiency in shell-and-tube heat exchangers?

Answer 13

Usage of multiple tubes within the shell –> multi pass tubes

• This makes the fluid in the shell pass through the tubes more times before exiting the heat exchanger, increasing the tube-side velocity and thus increase heat transfer efficiency

Question 14

How can you increase the number of shell and tube passes in a shell and tube heat exchanger?

Answer 14

Stack 2 shell and tube heat exchanger together with a vertical linkage

Question 15

How does steam infusion work?

Answer 15

[abit like spray dryer]

• Product in liquid state is pumped at the top of exchanger and allowed to flow in thin sheets in heating chamber
• steam is infused from a pipe at the side
• Heated products with condensed steam are released from chamber at the bottom

Question 16

In steam infusion, how does the temperature of products rise so quickly?

Answer 16

Temperature of products rise very quickly as heat is being released through condensation of steam (vapour + water) and the water has high heat capacity thus releasing a lot of heat.

Question 17

What kinds of applications is a steam infusion method useful for?

Answer 17

Useful for applications to dilute concentrated liquids (e.g. pasteurizing concentrated milk into more diluted milk)

• because steam carries water and thus steam infusion will dilute the product.

Note : If use hot air: the liquid product will be converted into solid as hot air will blow all the moisture content in product away (like spray dryer : liquid milk → milk powder)

Question 18

How does steam injection work?

Answer 18

Steam is injected directly into system with a nozzle.

Question 19

What is the difference between steam infusion and steam injection?

Answer 19

Heating speed
- Steam infusion can heat efficiently at higher steam flow rates, while steam injection can compromise between heat up times and efficiency.

Temperature exposure
Steam infusion prevents exposure to excessive temperatures while direct steam injection can cause fouling and discoloration.
- can damage the pipe but for steam infusion, it is a chamber inside

Question 20

Steam injection is suitable for what kinds of fluids?

Answer 20

Low viscosity fluids.

• Because steam is injected at a very high velocity, so ideally liquid should also move very fast to allow quick mixing and maximise heat transfer
• If liquid is viscous and move very slowly, while steam moves v fast. Steam cannot mix with the fluid as effectively so not very efficient

Question 21

What is the advantage of steam injection?

Answer 21

Fast heating (but slower than steam infusion)

Question 22

Whatis the disadvantage of steam injection?

Answer 22

Usually causes greater chemical degradation due to higher temperature difference between steam and incoming product

Question 23

What are the 2 types of flow (regarding direction of flow of hot and cold fluids)?

Answer 23

1. Parallel flow
2. Counter flow

Question 25

Overall, parallel flow is more efficient than counter flow because there is a large temperature gradient (difference) between the incoming hot and cold streams. True or False?

Answer 25

False.
- Although it may seem that parallel may be more efficient due to the larger temperature difference initially → larger gradient → faster heat transfer, the temperature gradient will become smaller v fast, thus heat transfer may not be as efficient

Question 26

Why is counter flow more efficient than parallel flow in general?

Answer 26

temperature gradient is always kept constant so greater heat transfer efficiency.

Question 27

Under what situations will you use parallel and counter flow?

Answer 27

• Parallel flow: if you just want to quickly cool / heat a product within a short period of time
• Counter flow : if you just want to cool / heat a product across a longer time and transport it over longer distances.

Question 28

In counter flow, what does “cross-over” of temperatures of both hot and cold streams mean?

Answer 28

The temperature of the cold fluid flowing out and temperature of hot fluid coming in could be equal.

This means if you are trying to cool/heat a product, the fact that the hot and cold fluids can reach same temp means its more efficient as compared to parallel flow.