heat exchanger is a device that transfers heat between two or more fluids. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. Heat exchangers are widely used in a variety of applications, including:
Heating and cooling systems
Power plants
Chemical processing
Food processing
Refrigeration
Air conditioning
2. 1.1 Introduction
Heat exchangers are devices that provide the transfer of thermal
energy between two or more fluids at different temperatures. Heat
exchangers are used in a wide variety of applications, such as power
production, process, chemical and food industries, electronics,
environmental engineering, waste heat recovery, manufacturing
industry, air-conditioning, refrigeration, space applications, etc.
3. 1.2CONSTRUCTION OF HEAT EXCHANGERS
A heat exchanger consists of heat-exchanging elements such
as a core or matrix containing the heat transfer surface, and
fluid distribution elements such as headers or tanks, inlet and
outlet nozzles or pipes, etc. Usually, there are no moving parts
in the heat exchanger; however, there are exceptions, such as
a rotary regenerator in which the matrix is driven to rotate at
some design speed and a scraped surface heat exchanger in
which a rotary element with scraper blades continuously
rotates inside the heat transfer tube. The heat transfer surface
is in direct contact with fluids through which heat is
transferred by conduction. The portion of the surface that
separates the fluids is referred to as the primary or direct
contact surface. To increase heat transfer area, secondary
surfaces known as fins may be attached to the primary surface
4. 1.3 Classification of Heat Exchangers
1.3.1 Types of Heat Exchangers
Double-pipe exchanger
Shell and tube exchangers
Plate and frame exchangers
Plate-fin exchangers.
Spiral heat exchangers.
Air cooled heat exchangers
Agitated vessels.
Fired heaters.
5. 1.3.2 Based on transfer process
Indirect Contact – Shell & Tube Heat Exchangers
Direct Contact – Cooling Towers
1.3.3 Based on phase of fluids
Gas-Liquid exchangers
Liquid-Liquid exchangers
Gas-Gas heat exchangers
1.3 Classification of Heat Exchangers
6. 1.3.4 Based on construction
Tubular
a. Double pipe heat exchanger
b. Shell and tube heat exchangers
c. Spiral heat exchangers
Plate-type
a. Plate and frame heat exchangers
b. Spiral plate heat exchangers
Extended Surface
a. Plate-fin exchanger
b. Tube-fin exchanger
1.3 Classification of Heat Exchangers
7. 1.3 Classification of Heat Exchangers
1.3.5 Based on flow arrangements
Parallel flow / Co-current flow
Counter flow
Cross flow
8. The most uncomplicated heat transfer
device is Double-pipe heat exchanger.
Which has the characteristics of two
pipes overlapping as shown in the
picture. Divided into Parallel Flow
and Counter Flow
9. 2.1 THE OVERALL HEAT TRANSFER COEFFICIENT
A heat exchanger typically involves two flowing fluids
separated by a solid wall. Heat is first transferred from the hot
fluid to the wall by convection , through the wall by
conduction, and from the wall to the cold fluid again by
convection. Any radiation effects are usually included in the
convection heat transfer coefficients. The thermal resistance
network associated with this heat transfer process involves
two convection and one conduction resistances, as shown in
Figure. Here the subscripts i and o represent the inner and
outer surfaces of the inner tube. For a double-pipe heat
exchanger, we have 𝑨𝒊 = 𝝅𝑫𝒊𝑳 and 𝑨𝒐 = 𝝅𝑫𝒐𝑳, and the
thermal resistance of the tube wall in this case is
𝑅𝑤𝑎𝑙𝑙 =
ln(
𝐷𝑜
𝐷𝑖
)
2𝜋𝑘𝐿
10. where k is the thermal conductivity of the wall material and L is the length of
the tube. Then the total thermal resistance becomes:
In the analysis of heat exchangers, it is convenient to combine all the thermal resistances in the
path of heat flow from the hot fluid to the cold one into a single resistance R, and to express the
rate of heat transfer between the two fluids as:
Where :
U = Overall heat transfer coefficient
h = convection heat transfer coefficient
R = thermal resistance
Total heat transfer coefficient (Overall heat transfer coefficient):
11. For the case where the pipe wall is
very thick and has a very high k value,
the conduction resistance is
approximately zero (Rwall ≈ 0 )
Therefore, the Overall heat transfer
coefficient (U) value can be written as
follows:
𝟏
𝑼
≈
𝟏
𝒉𝒊
≈
𝟏
𝒉𝒐
Representative values of the overall heat transfer coefficients in heat
exchangers
12. 2.2 Analysis of Heat Exchangers
In considering the analysis of heat transfer of heat exchanger equipment Must
assume the following assumptions:
The heat exchanger must be a Steady flow device.
Fluid properties And the velocity of the fluid is constant.
No changes in Kinetic and potential energy.
Has a specific heat capacity.
No heat conduction in the direction of the length of the pipe.
The outer walls of heat exchanger are insulated.
No heat loss to the environment.
Heat transfer occurs between only 2 types of fluids.
13. The rate of heat transfer from the hot side fluid = The rate of heat transfer
into the cold side fluid.
Where:
the subscripts c and h stand for cold and hot fluids, respectively, and
𝑚𝑐, 𝑚ℎ = mass flow rates
Cpc, Cph = specific heats
Tc, out, Th, out = outlet temperatures
Tc, in, Th, in = inlet temperatures
14. 2.3 THE LOG MEAN TEMPERATURE DIFFERENCE METHOD
Consider Newton’s Law of Cooling
𝑄 = 𝑈𝐴𝑠∆𝑇𝑚
where ∆𝑇𝑚 is the difference between average temperatures
Both types of fluids
To find the value ∆𝑇𝑚 is determined by the flow of
Fluid that flows in the same direction in
double pipe heat exchanger. will get the equation for ∆𝑇𝑚 as:
∆𝑇𝑙𝑚 =
∆𝑇1 − ∆𝑇2
ln(
∆𝑇1
∆𝑇2
)
15. To calculate ∆𝑇𝑙𝑚, dividing the flow of fluid into 2 types which are:
Both types of fluids flow in the same direction (Parallel flow)
Two fluids flowing in opposite directions (Counter flow)
For (Parallel flow) (Counter flow)
∆𝑇𝑙𝑚 =
∆𝑇1 − ∆𝑇2
ln(
∆𝑇1
∆𝑇2
)
16. 2.4 Multipass and Cross-Flow Heat Exchangers: Use of a Correction
Factor
The Calculation for ∆𝑇𝑚 using the formula mentioned above Will be used for double pipe
heat exchangers that are parallel flow or counter flow only.
For Cross flow or Multipass shell and tube heat exchanger, Correction factor: F is required to
calculate ∆𝑇𝑚.
where F will depend on The shape of the heat exchanger The temperature of the fluid at the
entrance And the fluid temperature at the exit
∆𝑇𝑙𝑚 = 𝐹∆𝑇𝑙𝑚,𝐶𝐹
17. Bellow the correction factor F charts for common shell-and-tube and cross-flow heat exchangers
(from Bowman, Mueller, and Nagle, Ref. 2):
18. 2.5 THE EFFECTIVENESS–NTU METHOD
Heat exchanger method analysis method 2 is the NTU Effectiveness method. Which is commonly used to
find the rate of heat transfer And the fluid temperature at the device exit.
By knowing the various values such as the type and size of the device. Fluid flow rate And the temperature
of the fluid at the entrance.
NTU method is calculated using the relationship of Number of Transfer Unit (NTU) and Heat Transfer
Effectiveness (𝜀)
𝜀 =
𝑄
𝑄𝑚𝑎𝑥
=
𝐴𝑐𝑡𝑢𝑎𝑙 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑟𝑎𝑡𝑒
𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑝𝑜𝑠𝑠𝑖𝑏𝑙𝑒 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑟𝑎𝑡𝑒
Where:
𝑄 = 𝐶𝑐 𝑇𝑐,𝑜𝑢𝑡 − 𝑇𝑐,𝑖𝑛 = 𝐶ℎ 𝑇ℎ,𝑜𝑢𝑡 − 𝑇ℎ,𝑖𝑛
𝑄𝑚𝑎𝑥 = 𝐶𝑚𝑖𝑛∆𝑇𝑚𝑎𝑥= 𝐶𝑚𝑖𝑛 𝑇ℎ,𝑖𝑛 − 𝑇𝑐,𝑖𝑛
Number of Transfer Unit (NTU) 𝑁𝑇𝑈 =
𝑈𝐴𝑠
𝐶𝑚𝑖𝑛
=
𝑈𝐴𝑠
(𝑚𝑐𝑝)𝑚𝑖𝑛
Capacity Ratio (c) 𝑐 =
𝐶𝑚𝑖𝑛
𝐶𝑚𝑎𝑥
19. 2.6 SELECTION OF HEAT EXCHANGERS
Heat Transfer Rate
Cost
Pumping Power
Size and Weight
Type
Materials
Other Considerations
20. References:
Heat exchangers, Selection, Rating, and Thermal Design, Third Edition,
Sadik Kakaç, Hongtan Liu, Anchasa Pramuanjaroenkij.
Heat Transfer, A Practical Approach, Second Edition, Yunus A. Cengel.
Fundamentals of heat exchanger design, Ramesh K. Shah, Dusˇan P.Sekulic.
Heat Exchanger Design Handbook, Second edition, Kuppan Thulukkanam