Consider a steam power plant.
The feed water must be converted to steam before it can turn the turbine and generate
power.
This is usually done in a boiler where the heat from the hot burning gases is transferred
to the water.
After the steam has expanded through the turbine, it must be condensed to liquid water before
it can be pumped back into the boiler.
This is accomplished by a condenser where cooling water cools and condenses the exhaust
steam from the turbine.
One thing that both - boiler and condenser have in common is that there is a transfer
of heat between two fluids without them mixing.
These devices are referred to as Heat exchangers which are very commonly used in the industry
and are the focus of this lesson.
So let’s get started.
Heat Exchangers are the devices that facilitate the exchange of heat between two fluids that
are at different temperatures while keeping them from mixing with each other.
They are used in a wide range of applications –
from heating and air-conditioning systems in a household to process industries and power
plants.
Heat exchangers are often given specific names to reflect the applications for which they
are used.
For example, a boiler is one in which the fluid absorbs heat and vaporizes.
A condenser is a heat exchanger in which one fluid is cooled and condensed as it flows
through the heat exchanger.
Or a space heater where the heat is transferred from a hot fluid to the air surrounding the
device.
Heat exchangers can also be classified based on flow arrangement and type of construction.
The simplest heat exchanger is a double pipe heat exchanger that consists of concentric
tubes in which the hot and the cold fluid move in the same or opposite direction.
If the fluids are moving in the same direction, then we would get a temperature distribution
as shown here.
This configuration is known as parallel flow heat exchanger.
On the other hand, if the fluids move in the opposite direction then we would get this
temperature distribution and the setup is commonly referred to as counter flow heat
exchanger.
The two fluids can also move perpendicular to each other.
This flow configuration is known as crossflow.
Crossflow heat exchangers are further classified as finned and unfinned heat exchangers.
Fins are added to increase the total surface area available for heat transfer.
The plate fins force the fluid to flow through a particular interfin spacing and prevent
it from moving parallel to the tubes and as a result the flow is considered to be unmixed.
On the other hand, in the absence of these fins the fluid is free to move in the transverse
direction and is said be to mixed.
As one might expect, the nature of mixing condition significantly influences the heat
exchanger performance.
Some common types of heat exchangers in industrial applications are shell and tube heat exchangers,
plate heat exchangers and finned tube heat exchangers.
Shell and tube heat exchangers contain a large number of parallel tubes packed in a shell.
Heat is exchanged as one fluid flows through the tubes while a second fluid flows outside
the tubes through the shell.
Baffles are placed in the shell to force the shell-side fluid to flow across the shell
to enhance the heat transfer and to support and maintain uniform spacing between the tubes.
Shell and tube heat exchangers can be further classified according to the number of shell
and tube passes involved.
For e.g. this configuration represents one-shell pass and two tube passes, and this one represents
two shell passes and four tube passes.
Plate and frame heat exchangers or simply plate heat exchangers consist of a series
of plates with corrugated flat flow passages mounted in a frame using a screw press.
Hot and cold fluids flow in alternate passages and thus each cold fluid is surrounded by
two hot fluid streams resulting in a very effective heat transfer.
It is well suited for liquid to liquid heat transfer if the two are at similar pressures.
Moreover, this configuration can easily grow with increasing demand for heat transfer by
simply adding more plates.Finned tube heat exchangers have tubes with extended outer
surface area or fins to enhance the heat transfer rate from the additional area of fins.
They are normally used in applications involving heat transfer between air and a liquid where
the heat transfer coefficient is much higher on one side of the heat exchanger than on
the other.Another commonly used heat exchanger in the process industry is a regenerative
heat exchanger or regenerator.
It involves the alternate passage of hot and cold fluid streams through the same flow area.
Heat from the hot fluid is intermittently stored in a thermal storage medium before
it is transferred to the cold fluid.
To accomplish this the hot fluid is brought into contact with the heat storage medium,
the fluid is displaced with the cold fluid, which absorbs the heat.
Now that we have some understanding of different types of heat exchangers, let’s try to understand
how they work and how they can be analyzed.
Any heat exchanger typically involves two flowing fluids separated by a solid wall.
Heat that is being convected by the hot fluid is conducted through the wall and then convected
again by the cold fluid.
Flow on either side of the heat exchanger is usually complex and local values vary from
one point to the other.
Therefore, In the analysis of heat exchangers, it is convenient to work with an overall heat
transfer coefficient that accounts for the contribution of all these effects on the heat
transfer.
You can think of this as a lumped parameter describing the overall efficiency of the heat
exchanger.
It is one of the main specification of a heat exchanger that is provided by a manufacturer
or a customer when they place an order for a custom heat exchanger.
For a double pipe heat exchanger, the thermal resistance of the tube wall is given by this
expression.
We can then get the total thermal resistance between the two fluids as shown here.
Now we can express the overall heat transfer between the two fluids using this simple expression
where U is the overall heat transfer coefficient given by this expression.
Another thing to keep in mind while considering the long-term performance of heat exchangers
is the accumulation of deposits on heat transfer surfaces over time.
The layers of deposits represent additional resistance to heat transfer and causes the
heat transfer to decrease.
The net effect of these accumulations on heat transfer can be accounted by using the Fouling
Factor, Rf, representing thermal resistance introduced by fouling.
Fouling can occur due to scaling or precipitation of solid deposits, corrosion or even due to
the formation of algae on the surface over time.
Therefore, fouling should be considered in the design and selection of heat exchangers,
especially for applications where it is likely to occur.
Perhaps you are wondering why we have two overall heat transfer coefficients Uc and
Uh for a heat exchanger.
This is because every heat exchanger has two heat transfer surface areas Ac and Ah, which
in general are not equal.
For example, consider the case where one side of the tube wall is finned and the other is
not.
The surface area of the finned side will be several times that of the unfinned side.
We can now write the overall heat transfer coefficient including the fouling and the
fin effects like this.
Here eta-zero represents the overall surface efficiency of a finned surface and is given
by this expression.
Using the heat transfer analysis for extended surfaces it can be written in this form.
This brings us to the end of this lesson.