A heat exchanger is a device built for efficient heat transfer from one medium to another
Heat transfer also known as heat flow as transfer of thermal energy is the movement of heat from one place to another
When an object is at adifferent temperature from its surroundings heat transfer occurs so that the body and the surroundings reach the same temperature at thermal equilibrium
Such heat transfer always occurs from a region of high temperature to another region of lower temperature
So, a heat exchanger is a component that allows the transfer of heat from one fluid ( liquid or gas) to another fluid
Heat transfer is usually required
To heat a cooler fluid by means by a hotter fluid
To reduce the temperature of a hot fluid by means of a cooler fluid
To boil a liquid by means of a hotter fluid
To condense a gaseous fluid by means of a cooler fluid
To boil a liquid while condensing a hotter gaseous fluid
In aheat exchanger the media may be separated by asolid wall , as that they never mix or they may be in direct contact.
In heat exchangers wher the media is separated by a solid wall , the heat is transferred from the hot fluid to the metal isolating the two fluids and then to the cooler fluid
The basic design of a heat exchanger normally has two fluids of different temperatures separated by some conducting medium
Types of heat exchangers
Based on the direction of flow , heat exchangers are of 3 types
1) parallel flow: in parallel flow heat exchangers the two fluids enter the heat exchanger from the same end with a large temperature difference
As the fluids transfer heat , hotter to cooler , the temperature of the two fluids approach each other
Thus they have a large temperature difference at the inlet and a small temperature difference at the inlet and a small temperature difference at the outlet .
2) counter flow : this exists when the two fluids flow in opposite directions . each of the fluids enter the heat exchangers at the opposite ends because the cooler fluid exits the counter flow heat exchanger , the cooler fluid will approach the inlet temperature of the hot fluid .
These type of heat exchangers have temperature difference along the heat transfer length . these are used for liquid – liquid condensing and gas cooling applications
Units are usually mounted vertically when condensing vapour and mounted horizontally when handling high concentrations of solids
3) Cross flow :
This exists when one fluid flows perpendicular to the second fluid
That is one fluid flows through tubes and the second fluid passes around the tubes at 90 degrees.it is generally used as a condenser. These are applicable when one of the fluid changes state .
Eg: steam condenser , the steam existing the turbine enters the condenser shell side , and the cool water flowing in the tube absorbs the heat from the steam , condesing it into water
Large volumes of vapor may be condensed using this type of heat exchangers flow
Among the 3 types of heat exchangers counter current flow is most efficient when compared with other because the average temperature difference between the two fluids over the length of the heat exchanger is maximized.
At the same operating conditions , operating the 3 heat exchangers the counter flow heat exchanger will result in a greater haeat transfer rate than operating in parallel flow
Based on the flow heat exchangers are classified into two types as such
1) single pass heat exchangers - when a heat exchanger fluid passes only once
2) multipass heat exchangers - when a heat exchanger fluid passes more than once
Based on the function heat exchangers are classified into two types
1) regenerative : these heat exchangers use the same fluid for heating and cooling
The same fluid is both cooling fluid and cooled fluid . that is the hot fluid leaving a system gives up its heat to regenerate or heat up the fluid returning to the system.
These are found in high temperature systems where a portion of systems fluid is removed form the main process and then returned.
Because the fluid removed from the main process contain energy , the heat from the fluid leaving the main system is sued to reheat the returning fluid instead of being rejected to an external cooling medium to improve efficiency.
2) non regenerative: the hot fluid is cooled by fluid from a separate system and the enrgy removed is not returned to the system.
The most commonly used heat exchangers are
1) plate heat exchangers
2) shell tube heat exchangers
Plate heat exchangers :
Thes consists of thin plates joined together with a small amount of space between each plate typically mainatained by a small rubber gasket .
This consists of plates inside tubes to separate hot and cold fluids.
These set of tubes is called the tube bundles and can be made up of several types of tubes .
Shell and tube exchangers are typically used for high pressure applications
Several thermal design features should be t5aken into account when designing the tubes in this type of heat exchangers .
Tube diameter , tube thickenss, tube length , tube pitch , tube layout , baffle design play a major role.
Shell and tube heat exchangers :
These comprises of multiple tubes through which liquid flows
Set of tubes in a container called shell
The fluid flowing inside the tube is called tube side fluid and the fluid flowing on the outside of the tube is shell side fluid .
The tunes are divided into two sets
First set contains the liquid to be heated or cooled
Second set contain the liquid responsible for triggering the heat exchange
Selection of heat exchangers:
This depends on the heat transfer rate
Size and weight
Material of construction
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