Heat
exchangers are mechanical components which are used to transfer heat from fluid
with high temperature to the fluid with low temperature. This process of heat
exchange between the fluids happens due to the phenomena’s called thermal
conduction and thermal convection (Bari 2015). Heat exchanger transfer heat by
first absorbing the heat from fluid with high temperature through the process
of convection, then transferring it to the other side having fluid with low
temperature through the process called conduction and then finally transferring
it to the fluid with low temperature through the process called convection
(Vendat 2000).
Figure 1 working of spiral heat exchanger
(Vendat 2000)
Heat
exchangers are used in all those applications where heat is needed to be
absorbed from the environment or rejected to the environment. Some examples of
heat exchangers are domestic water heater which absorb heat from its high
temperature surrounding (high temperature of surrounding is due to burning of
fossil fuel) and exchange it with the low temperature water (Vendat 2000).
Other example of heat exchanger is heat rejected into atmosphere by the vehicle
engine where heat exchanger is a fin type heat exchanger which absorb heat of
combustion from the engine and deliver it to the surrounding air (Bari 2015).
Figure 2 domestic water heater
(Bari 2015)
One
of the applications of heat exchanger is the recovery of heat from any
industrial or domestic heat source where a lot of heat is being rejected into
the environment (Bari 2015). This setup of utilizing the wasted heat is called
waste heat recovery system and it is only possible by the use of heat
exchanger. Heat exchanger makes use of high temperature fluid coming out of any
process as a waste fluid and absorbs its heat (Vendat 2000). Then that
particular heat is utilized for any process required for example heating the
incoming raw material. As shown in the example below the exhaust coming out of
the gas turbine is being utilized by the heat exchanger to heat water convert
it into steam. Similar to this the heat exchanger can be utilized in any waster
heat recovery system.
Figure 3 waste heat recovery system
(Bari 2015)
Heat
exchangers are devices used to transfer heat between two mediums of different
temperature. There are different types of heat exchangers and their performance
depends upon different parameters. Some of the parameters are (Vendat 2000)
Type
of flow in spiral heat exchanger
Type of flow include laminar and
turbulent.in laminar flow particles moves in a straight line and do not change
their position because of which less heat transfer take place. Turbulent flow
occurs when Reynolds number exceed above 2000.in this type of flow every
particle of medium flow randomly due to which high heat transfer take place.
Parallel
and counter flow spiral heat exchanger
When both fluids flow parallel to each
other initially temperature difference is high but as the fluid flow
temperature difference decreases due to which lees heat is tranfered.on
contrary in counter flow there is greater temperature difference between two
mediums because of which more heat is transferred
Material of spiral heat exchanger
Different materials have different
thermal conductivity. Higher the thermal conductivity higher will be the heat
transfer and vice versa. Spiral
heat exchanger consists of two parallel plates folded in spiral shape. Hot
fluid enters from the centre and exit at the periphery while cold fluid enters
from the periphery and exit from the centre. From one of the study it is
concluded that heat transfer rate (HTR) of spiral heat exchanger is more than
other type of heat exchanger. More heat transfer take place due to greater
contact area (Jamshid Khorshidi 2016).
Material Selection for
Spiral Heat Exchanger
In
the case of heat exchanger from all the factors involve in heat exchanger
design two factors, one the fluid which is being used as a medium in heat
exchanger and second the material from which the heat exchanger is manufactured
are the most important of all. Both of
these factors decide the working, performance, efficiency and amount of the
heat that a heat exchanger can transfer. So the first phase in design of a
spiral heat exchanger is the selection of material for the structure of the
heat exchanger (Jamshid Khorshidi 2016). Material used
in the manufacturing of spiral heat exchanger will define the strength of
spiral heat exchanger, thermal conductivity of spiral heat exchanger, eight of
spiral heat exchanger and cost of manufacturing of spiral heat exchanger.
Selection of material is very critical for the effective working of the heat
exchanger so that’s why the proper selection process has to be established for
this process. The selection process of the spiral heat exchanger material will
consist of number of properties or the requirements which a material should
fulfil in order to design an effective spiral heat exchanger.
Following
is the list of mechanical properties which material should have in order to get
selected (Jamshid Khorshidi 2016)
High
conductivity
Material
selected for the job should have high conductivity in terms of heat and energy
so that maximum heat can be transfer between fluids. The role of material in
transfer of heat in spiral heat exchanger is very important. Material absorbed
heat from hot fluid using convective heat transfer coefficient between material
and fluid. Heat absorbed from fluid is then transfer between two sides of
material using conduction and the again between material and fluid using
convection. All this mechanism of heat transfer in spiral heat exchanger is
depending on the material conductivity. Higher the conductivity of material
higher will be the performance and higher will be the heat transfer in spiral
heat exchanger for a given temperature difference (Kothandaraman, 2006).
Strength
Temperature
produce stresses in material which can cause leakage in spiral heat exchanger
so material should have good strength. The spiral heat exchanger will be under
stress during its operation. This stress can be due to the pressure of the
fluid which is moving inside the spiral of the spiral heat exchanger. The
stress can also be due to the high temperature of hot fluid, the thermal stresses
in material. During heat exchanger the cold fluid will gain heat due to which
it can expand inside spiral heat exchanger. This thermal expansion of cold
fluid cause stresses in walls of spiral heat exchanger. So the material
selected for the manufacturing of spiral heat exchanger should have high
strength to resist the stress produce in spiral heat exchanger (Kothandaraman,
2006).
Density
Density
of the selected material should be low to keep the weight of the spiral heat
exchanger low. Density of material is defined as the mass per unit volume of
the material. Greater the density of material from which spiral heat exchanger
is manufactured greater will be the mass per unit volume and thus greater will
be the weight of the spiral heat exchanger. Material with greater density will
be difficult to handle and maintain, so material with the lowest density
possible should be selected for the spiral heat exchanger (Kothandaraman, 2006).
Thermal
Expansion
Material
natural response to heat is the expansion of material in all directions. This
expansion of material during heating is called thermal expansion. In spiral
heat exchanger the thermal expansion of material can cause distortion in
complicated design and can make the entire setup useless. Material with low thermal
expansion is recommended for manufacturing of spiral heat exchanger as it will
have lower thermal stress which makes it secure and safe to use.
Environmental
effect
Material
for the manufacturing of spiral heat exchanger should be selected after considering
the environmental effect of all shortlisted materials available for the
manufacturing of spiral heat exchanger. Material selected for heat exchanger
should not affect environment in any aspect which include working,
manufacturing and disposal. Material should neither react chemically with the
working fluids of spiral heat exchanger nor should it react with the
surrounding environment elements of spiral heat exchanger (Kothandaraman,
2006).
Manufacturing
Material
selected for heat exchanger should be easy to manufacture and can be
manufactures using traditional machines. Material should have excellent
machinability (material should be easy to machine into required shape of spiral
heat exchanger and weldability (material should allow the welding of any required
component of spiral heat exchanger). Other than this the material of the spiral
heat exchanger should have excelled surface texture and good support for paint
jobs.
Cost
Material
selected for heat exchanger should be a cost effective option in terms of every
aspect like material cost, availability, manufacturing cost and disposal cost.
Lower material cost means the raw material required for the manufacturing of
spiral heat exchanger will be cheap which means less cost of final product.
Less manufacturing cost of spiral heat exchanger means less initial investment
is required and less final product cost (Kothandaraman, 2006).
Material
Mechanical and Thermal Properties
Based
on the above discussion on the material mechanical and thermal properties required
for the spiral heat exchanger, below is list of mechanical and thermal
properties which the selected material for further process of spiral heat
exchanger should have (Kothandaraman, 2006).
1. High
Thermal Conductivity
2. Lower
Thermal expansion
3. High
Strength (Yield Strength)
4. Low
Material Density
5. Machinability
6. Weldability
7. Environmental
Aspects
8. Cost
Based
on the information obtain from the literature review of different spiral heat
exchanger following are the materials
recommended for the manufacturing of the spiral heat exchanger as they all are
excelled in the mechanical and thermal properties required for the spiral heat
exchanger.
1. Copper
2. Aluminum
3. Cast
Iron
4. Stainless
Steel
|
Properties |
Copper |
Aluminum
6061 T6 |
Cast
Iron |
Stainless
Steel |
|
Conductivity W/mK |
385 |
205 |
80 |
50 |
|
Expansion per C * 10^-6 |
17 |
24 |
12 |
13 |
|
Strength MPa |
80 |
218 |
276 |
300 |
|
Density Kg/m^3 |
8960 |
2800 |
7874 |
8000 |
|
Machinability |
Excellent |
Good |
Satisfactory |
Good |
|
Weldability |
Satisfactory |
Good |
Poor |
Excellent |
|
Cost |
High |
High |
Low |
Moderate |
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