Thermal Unit Operation

(ChEg3113)

Instructor: Mr. Tedla Yeshitila (M.Sc.)

Lecture 4- Example on types of heat flow and types

of heat exchanger

Today…

• Review

• Example on parallel and counter current heat

flow

• Heat exchanger types

Review LMTD (Log Mean Temperature Difference) is defined as:

𝐿𝑀𝑇𝐷 =𝜟𝑇1−𝜟𝑇2

ln(𝜟𝑇1/𝜟𝑇2)=𝜟𝑇𝑙𝑚

For constant overall heat transfer coefficient:

𝑄 = 𝑈0𝐴𝑜(𝐿𝑀𝑇𝐷), and 𝑄 = 𝑈𝑖𝐴𝑖(𝐿𝑀𝑇𝐷)

Simply, 𝑄 = 𝑈𝐴(𝐿𝑀𝑇𝐷)

Where 𝜟𝑇𝑙𝑚: Log mean temperature difference (K)

𝑄 : Heat transfer rate (J/hr)

Uo: Overall heat transfer coefficient (J/hr-m2-K)

Ao: Cross sectional heat transfer area (m2)

Chapter 2

Classification of Heat Exchanger Equipment's

Assumptions:

1. Constant overall heat transfer coefficients (U) over the

entire length of pipe. In reality, it is subjected to deviation.

2. Constant flowrate, obeying steady state conditions

3. Constant heat transfer (specific heat (Cp) is constant)

4. No partial phase changes into the system. i.e. no partial

condensation or vaporization. i.e. the derivation is

applicable for sensible heat changes and when

vaporization/condensation is isothermal over the whole

length of path

5. No heat loss. i.e. negligible

Chapter 2

Classification of Heat Exchanger Equipment's

Each of the three types of heat exchangers (Parallel, Cross and

Counter Flow) has their own advantages and disadvantages.

But of the three, the counter flow heat exchanger design is the

most efficient when comparing the heat transfer rate per unit

surface area.

The efficiency of a counter flow heat exchanger is due to the

fact that the average T (difference in temperature) between the

two fluids over the length of the heat exchanger is maximized.

Therefore, the log mean temperature for a counter flow heat

exchanger is larger than the log mean temperature for a similar

parallel or cross flow heat exchanger.

Chapter 2

Classification of Heat Exchanger Equipment's

Example 1:

Consider the following example of a heat exchanger operated

under identical conditions as a counter flow and then a parallel

flow heat exchanger.

T1: represents the hot fluid

temperature

T1in: 200K

T1out: 145K

Uo: 70 J/hr-m2-K

Ao : 75m2

T2: represents the cold fluid

temperature

T2in: 80K

T2out: 120K

Determine 𝜟𝑇𝑙𝑚 and 𝑄 for both counter flow and then a parallel

flow heat exchanger.

Chapter 2

Classification of Heat Exchanger Equipment's Solution:

For counter flow:

𝜟𝑇𝑙𝑚 =(200−120)−(145−80)

ln200−120

145−80

=72K, and

𝑄 = 70J/hr−m2−K ∗ 75m2 ∗72K=3.8*105J/hr

For parallel flow:

𝜟𝑇𝑙𝑚 =(200−80)−(145−120)

ln200−80

145−120

=61K, and

𝑄 = 70J/hr−m2−K ∗ 75m2 ∗61K=3.2*105J/hr

The results demonstrate that for a given heat exchanger

operating at the same conditions, counter flow will result in a

greater heat transfer rate than parallel flow.

Chapter 2

Classification of Heat Exchanger Equipment's Example 2:

Given the concentric parallel-flow heat exchanger, calculate the

tube length necessary to perform the heat exchange requirements

given below:

Oil:

moil= 0.15kg/s

Tin= 100oC

Tout= 60oC

Cp=2,131J/kg. oC

h=38.8 W/m2.oC

Water:

Tin= 25oC

Tout= 50oC

h=2,250W/m2.oC

Di=30mm

Do=50mm

Chapter 2

Classification of Heat Exchanger Equipment's Solution:

Assumptions:

1. Negligible heat loss to surrounding. i.e. all heat is transferred

between the two fluid

2. Thin walled between the two fluid. i.e. there is no conductive

heat resistance

Steps to solve the problem:

1. Calculate q

𝑞 = 𝑚 𝐶𝑝𝛥𝑇 = 0.15 ∗ 2131 ∗ 100 − 60 = 𝟏𝟐, 𝟕𝟖𝟔𝑾

2. Calculate U

𝑈 = (1

ℎ𝑜𝑖𝑙+

1

ℎ𝑤𝑎𝑡𝑒𝑟)−1=38.1 W/m2.oC

Chapter 2

Classification of Heat Exchanger Equipment's 3. Calculate 𝜟𝑇𝑙𝑚 For parallel flow:

𝜟𝑇1 = 75oC, 𝑎𝑛𝑑𝜟𝑇2 = 10oC, then

𝜟𝑇𝑙𝑚 =(75)−(10)

ln75

10

=32.2oC, and

4. Calculate L

𝑄 = 𝑈0𝐴𝑜𝜟𝑇𝑙𝑚 = 𝑈𝐴𝜟𝑇𝑙𝑚 but A=𝛱 DL, then

𝐿 =𝑄

𝑈𝛱D𝜟𝑇𝑙𝑚

L parallel = 110.6m

Chapter 2

Classification of Heat Exchanger Equipment's Example 3:

Given the concentric counter-flow heat exchanger, calculate the

tube length necessary to perform the heat exchange requirements

given below:

Oil:

moil= 0.15kg/s

Tin= 100oC

Tout= 60oC

Cp=2,131J/kg. oC

h=38.8 W/m2. oC

Water:

Tin= 25oC

Tout= 50oC

h=2,250 W/m2. oC

Di=30mm

Do=50mm

Chapter 2

Classification of Heat Exchanger Equipment's Solution:

From previous example:

𝑄 = 𝟏𝟐, 𝟕𝟖𝟔𝑾and 𝑈= 8.1 W/m2.oC

For counter flow:

𝜟𝑇1 = 50oC, 𝑎𝑛𝑑𝜟𝑇2 = 35oC,then

𝜟𝑇𝑙𝑚 =(50) − (35)

ln5035

=42oC

𝐿 =𝑄

𝑈𝛱D𝜟𝑇𝑙𝑚

L counter= 84m

The temperature difference greater in the tube, you need

less surface area for same amount of heat transfer, so we

need shorter length of pipe for counter flow HX.

Chapter 2

Classification of Heat Exchanger Equipment's

In actuality, most large heat exchangers are not purely parallel

flow, counter flow, or cross flow; they are usually a

combination of the two or all three types of heat exchangers.

This is due to the fact that actual heat exchangers are more

complex than the simple components shown in the idealized

figures used to depict each type of heat exchanger.

The reason for the combination of the various types is to

maximize the efficiency of the heat exchanger within the

restrictions placed on the design. That is, size, cost, weight,

required efficiency, type of fluids, operating pressures, and

temperatures, all help determine the complexity of a specific

heat exchanger.

Chapter 2

Classification of Heat Exchanger Equipment's One method that combines the characteristics of two or more heat

exchangers and improves the performance of a heat exchanger is to have

the two fluids pass each other several times within a single heat exchanger.

If the fluids pass each other only once, the heat exchanger is called a

single-pass heat exchanger.

When a heat exchanger's fluids pass each other more than once, a heat

exchanger is called a multi-pass heat exchanger.

Chapter 2

Classification of Heat Exchanger Equipment's

Commonly, the multi-pass heat exchanger reverses the flow in

the tubes by use of one or more sets of "U" bends in the tubes.

The "U" bends allow the fluid to flow back and forth across

the length of the heat exchanger.

A second method to achieve multiple passes is to insert baffles

on the shell side of the heat exchanger. These direct the shell

side fluid back and forth across the tubes to achieve the multi-

pass effect.

Chapter 2

Classification of Heat Exchanger Equipment's The principal types of heat exchanger used in the chemical

process and industries are listed below:

1. Double-pipe exchanger: the simplest type, used for cooling

and heating.

2. Shell and tube exchangers: used for all applications

3. Plate and frame exchangers (plate heat exchangers): used for

heating and cooling.

4. Plate-fin exchangers

5. Spiral heat exchangers

6. Air cooled: coolers and condensers

7. Direct contact: cooling and quenching

8. Agitated vessels

9. Fired heaters

Chapter 2

Classification of Heat Exchanger Equipment's A tubular heat exchanger (double pipe) is essentially a jacket

around a pipe. The working fluid (often steam) enters the jacket on one side of the heat exchanger and leaves on the other side. Inside the pipe is the mixture which you want to heat or cool.

It consists of two concentric tubes in which one is inside the other, so there are inner tube and outer tube.

Heat is exchanged through the walls of the device in accordance to the second law of thermodynamics, which requires that heat flow from higher to lower temperatures.

Therefore, if it is desired to cool off the fluid in the pipe, the working fluid must be cooler than the fluid in the pipe.

In the beginning there was big temperature difference, then the difference decreases in x direction.

The length would be very long, so you can coil or find a way to compact it.

Chapter 2

Classification of Heat Exchanger Equipment's

Shell and tube heat exchanger: Assume tube side flow from

left to right and shell side from left to right, so it is counter

current flow.

The side where the flow comes into the tubes called front-end

header and where the flow goes out of the tube is called rear-

end side. And it is one shell pass and one tube pass.

These are the most commonly used in industries.

They are too heavy to use in transport and aerospace. But in

terms of cost and value for money they are best HX.

The application range can be from 100kW to MW.

By putting baffle, the flow in the shell side goes down then

comes back upward again, so the outlet becomes in the top.

And it is two shell pass and one tube pass, while the flow type

become cross flow.

Chapter 2

Classification of Heat Exchanger Equipment's

The other function of baffle is it support the tube better

because the tubes are long.

Simultaneously, we are changing the heat flow from counter

current flow to cross flow. And the effectiveness of these types

of HXs are better.

The baffles can be used in different angle.

The other modification can be by connecting each tube, so the

flow goes from one to another

Chapter 2

Classification of Heat Exchanger Equipment's Plate and frame (plate) HX: in such HX fluid 1 passes between

two plates, and fluid 2 passes between the next two plates.

We can redirect also into the other plates similarly.

These types of HX are compact, and can be flexible because when you need heat transfer rate because the production rate is increased, so you just add some more plates.

The disadvantage is they cannot take large pressure differences. E.g. liquids, milk industries

But they not used as condenser or evaporator, heating and ventilation industries because refrigerant side the pressure is in MPa and in liquid side in kPa, so it result large pressure difference and the plate becomes bend then you will have problem with seal.

• They are very well suited for liquid-liquid type HX.

• You can use corrugated plate instead of flat which gives more resistance to bending and also more turbulence.

Chapter 2

Classification of Heat Exchanger Equipment's

Regenerative type: there is porous material which absorb a lot

of heat, and first fluid one flow to increase the heat till it is

heated, then fluid two flow which is the one needed to be

heated.

Usually, both stream never flow at the same time.

It can be static or dynamic (using slowly rotating wheel)

E.g. In concentrating solar panel, the problem is you can`t get

solar energy at the night so you can`t generate electricity. But

if you store the heat you can use it later, so rock used for this

purpose.

Chapter 2

Classification of Heat Exchanger Equipment's

Condenser: are cooler whose primary purpose is removal of

latent heat instead of sensible heat. There is change from gas

phase to liquid phase.

Evaporator: are employed for the concentration of solution by

evaporation of water.

If any other fluid is vaporized beside water, it is called

vaporizer.

Radiator: e.g. In your car: lot of tubes with fins and fan at the

back to cool the water inside at high pressure

At the end of this class:

• You will be able to use LMTD equation and solve problems

depending on the heat flow type

• You will be able to understand different types of heat

exchangers

End of lecture - 4