11 - Heat Exchangers

Question 1

Limitations of parallel flow exchangers

Answer 1

Not efficient heat transfer towards the end

Limit on how hot the cold fluid can be at the exit

Question 2

Compact heat exchanger

Answer 2

Has a large heat transfer surface area per unit volume
Area density (beta) > 700m^2/m^3

Question 3

What is cross-flow?

Answer 3

In compact heat exchangers, the two fluids usually move perpendicular to each other (can be mixed or unmixed flow)

Question 4

Shell-and-tube heat exchanger

Answer 4

Contains large number of tubes packed in a shell with their axes parallel to that of the shell
Heat transfer takes place as one fluid flows inside the tubes while the other flows outside the tubes through the shell
Most common type of heat exchanger in industrial applications

Question 5

Regenerative heat exchanger

Answer 5

Involves the alternate passage of the hot and cold fluid streams through the same flow area

Question 6

Dynamic-type regenerator

Answer 6

Involves a rotating drum and continuous flow of the hot and cold fluid through different portions of the drum so that any portion of the drum passes periodically through the hot stream, storing heat, and then through the cold stream rejecting this stored heat

Question 7

Condenser

Answer 7

One of the fluids is cooled and condenses as it flows through the heat exchanger

Question 8

Boiler

Answer 8

One of the fluids absorbs heat and vaporises

Question 9

What is the overall heat transfer coefficient dominated by?

Answer 9

The smaller convection coefficient

Question 10

Fouling factor

Answer 10

Performance of heat exchangers usually deteriorates with time as a result of accumulation of deposits on heat transfer surfaces, causing additional resistance to heat transfer, represented by fouling factor Rf

Question 11

How can the fouling factor increase/decrease?

Answer 11

Increases with operating temperature and length of service

Decreases with fluid velocity

Question 12

What to consider when selecting a heat exchanger

Answer 12

Achieves specified temperature change in a fluid stream of known mass flow rate - log mean temperature difference (LMTD) method
Predict outlet temperatures of hot and cold fluid streams in a specified heat exchanger - effectiveness-NTU method

Question 13

LMTD

Answer 13

Exact representation of the average temperature difference between hot and cold fluids
Always less than arithmetic mean temperature difference

Question 14

Points about counter-flow heat exchangers

Answer 14

Cold fluid will be heated to inlet temperature of hot fluid, but outlet of cold will never exceed hot inlet
LMTD is always greater than that of a parallel-flow heat exchanger
Therefore can have smaller surface area and smaller heat exchanger to achieve specified heat transfer rate

Question 15

Correction factor (F)

Answer 15

Depends on geometry of heat exchanger and inlet and outlet temperatures of the two streams
F = 1 for a condenser or boiler

Question 16

Steps to select a heat exchanger

Answer 16

Select type of heat exchanger suitable for application
Determine any unknown inlet or outlet temperature and the heat transfer rate using an energy balance
Calculate LMTD and correction factor (if necessary)
Obtain the value of the overall heat transfer coefficient U
Calculate the heat transfer surface area A
Complete by selecting a heat exchanger that has a heat transfer surface equal to or larger than A

Question 17

What type of fluid is more suitable for the shell side and tube side?

Answer 17

More viscous fluid for shell side as larger passage area and thus lower pressure drop
Higher pressure fluid for tube side