Heat exchangers are described along with the basic definition, parts, types, applications, advantages, disadvantages, etc.
Let’s start heat exchangers!
We will start with the basics of heat exchangers!
If you are into a thermal system, you may have heard about heat exchangers.
Basically, as the name exchange suggests the heat exchangers are the device that transfers heat between two or more fluids.
So, the heat exchangers can be used in cooling as well as the heating processes.
The heat exchangers can be seen in various applications, such as,
The most familiar example would be the heat sink. It is used to cool the electronic devices to a fluid medium or liquid coolant. The heat sink is a kind of passive heat exchanger. We will see the types and classification in detail because there’s a vast classification of heat exchangers.
Let’s get in. As I have already said the classification of the heat exchangers is vast, firstly, let’s check out the components of the heat exchangers.
Heat exchanger has the following components,
The shell is the one in which the heat exchanger keeps everything inside of it.
In most of the heat exchangers the single pass shell is used. The shell side pass partition plate is provided to avoid the cross of temperatures between the hot fluid and cold fluid that causes the unfavourable temperature difference.
The baffle plates are used to perform mainly the two functions,
The baffle spacing is determined carefully because the spacing directly affects the heat transfer coefficient and pressure drop.
Usually, the standard baffle cut is 15% – 40%.
Baffles can be of different categories, like –
The tube used in the heat exchangers size ranges from 6.35mm to 63.5mm. The tubes carries either hot or cold fluid according to the process. Also, the number of tubes used depends upon the requirement.
The tube side passes gives the advantages of increasing the tube side fluid velocity hence improving the tube side heat transfer coefficient but at the expense of pressure drop.
The support needed for baffle plates is provided by the tie rods. The rods are generally made from solid metal bar.
Generally, four or more tie rods are used to support the baffle plates.
The spaces do the work of maintaining the space between the baffles. As we have already seen the space between baffles is an important parameter.
The tubes are the one end of tie rods is attached to the tube sheet also known as the tube plate. Due to the attachment the entire load of the tube bundle falls on the one or two tube sheets.
The sealing strip is responsible for reducing the amount of bypass streams of shell side fluid flowing from the clearance between shell side inside diameter and tube bundle diameter.
When the high temperature variation is given to the heat exchanger is expansion join become useful.
Now we will see the heat exchangers classification with their brief descriptions.
The classification of heat exchangers are based on the followings:
According to the transfer processes
a) Indirect-contact heat exchangers
b) Direct contact type heat exchangers
According to the number of fluids
a) Two fluid heat exchangers
b) Three fluid heat exchangers
According to the flow arrangement
a) Counter flow heat exchanger
b) Cross flow heat exchanger
c) Parallel flow heat exchanger
According to the construction
a) Shell and tube heat exchangers
b) Plate heat exchangers
c) Plate and shell heat exchanger
d) Plate fin heat exchanger
e) Spiral heat exchangers
Let’s learn all types of heat exchangers.
This classification is dependant upon the contact between the fluids. The flowing fluid is separated using the solid wall.
So as the name suggests the indirect heat exchanger as the fluid will flow separately separated by the solid wall between the two fluids.
The indirect contact ones are divided into three types.
There is no direct mixing of the fluids happening in this heat exchanger so don’t go by its name.
The storage type heat exchanger is also known as the regenerative heat exchanger. In this heat exchanger the fluid flow in an alternative way through the same flow passages and the heat transfer is intermittent.
In this heat exchanger the one side ow two fluid exchangers are immersed in a bed of finely divided solid materials.
This type of heat exchanger is commonly seen in drying, mixing, coal combustions and waste heat recovery systems.
In the direct contact type the two fluids comes directly in contact with each other. They exchange heat and then separated.
Let’s see some more types of direct contact type heat exchangers.
The two immiscible fluids are brought into direct contact in this heat exchangers. The immiscible fluids are the one which won’t mix to give the single phase.
The examples are water and oil.
In these exchangers one of the fluids is gas and other one is the liquid mostly low-pressure liquid.
Steam is partially condensed using the cooling water in this heat exchanger. Also, sometimes the water is heated with the waste heat steam through direct contact in the exchanger.
Examples are desuperheaters, open feedwater heaters.
The two fluid heat exchangers are the most commonly used. Because obviously there is need of two fluids for heat transfer for – ,
The three fluid heat exchangers are mostly used in the cryogenics and some chemical processes.
Like, air separation systems, helium-air separation units, etc.
This classification depends upon the fluid flow inside the heat exchangers. It has three types as follows.
The counter flow heat exchangers fluids flow in the opposite directions. They are generally used for liquid-liquid and condensing and gas cooling applications.
In this heat exchanger the one of fluid is in spiral flow whereas the other one is in the cross flow. The fluids flow in the normal direction to each other.
Though the effectiveness of this heat exchanger is lower than the counter flow heat exchanger.
These are also known as the co-current parallel stream. In this heat exchanger, the fluid stream enters at one end, then they flow parallel to each other in same direction and leave at another end.
The parallel flow heat exchanger has the lowest exchanger effectiveness compared to other arrangements.
Heat exchangers shall be as follows:
The shell and tube heat exchangers are popular and used in lots of applications. It has a tube box, shell, front and rear end headers, baffles, or fins.
As we have seen earlier, the baffles increase the turbulence of fluid and increases the efficiency.
There are few variations of shell and tube heat exchangers are available according to its flow rate and need of support etc.
It has series of tubes in which the fluid is present that needs either heating or cooling. One fluid runs inside the tubes other over the tubes.
The shell and tube heat exchangers are used in intercoolers, charging air for combustion engines. The fins are added to increase the heat transfer rates in these heat exchangers.
The plate heat exchangers consist of lots of thin plates. They are slightly separated and have large surface areas and small fluid flow for heat transfer.
Plate heat exchangers are now more practical with the advancements in gaskets and brazing technologies.
The plate and shell type heat exchanger combines the plate heat and shell tube heat exchanger.
The plate and shell exchangers offer high heat transfers, high pressure high operating temperatures, compact sizes, etc.
It has sandwiched passages consisting fins that increases the effectiveness of the unit.
In the spiral heat exchangers, the shell is replaced by the coiled tube. It allows the two fluids to flow parallelly to each other.
The spiral heat exchangers have the advantage of highly efficient use of space which can be used to gain some improvements in the performances.
Let’s take the working principle of the heat exchanger with shell & tube type.
We will try to understand it step by step.
The heat exchanger is worked based on the second law of thermodynamics. We have already learned that heat flows due to the temperature potential or temperature difference. It means always heat flows from a hotter body to a cooler body.
Heat can flow by three process,
Within these three processes, the conduction and convection processes are widely used in heat exchangers.
Take a shell and tube type, heat exchanger. It has two parts, shell and tubes. Many tubes are placed within the shell.
Two different types of fluids are used,
The cooling medium is passed through the tubes within the shell and the fluid which will be cooled is filled the shell around the tubes. In refrigerant shell and tube types chillers, the refrigerant is the medium to be cooled, basically, its a hot medium, and water is the cooling medium.
The process shall be as follows,
The temperature of the hot medium can be controlled to meet the process requirements by the followings,
To design a heat exchanger, the basic equation is considered as,
Q = U A ΔT,
Q: Heat transfer rate between the two fluids in watt,
U: Overall heat transfer coefficient in m2-K/w,
A: Heat transfer surface area in m2,
ΔT: Log mean temperature difference in oK,
Now, what is LMTD?
Let’s try to understand LMTD. Consider A side implies hot medium entry and cooling medium out and B side implies hot medium outlet and cooling medium entry.
LMTD or logarithmic mean temperature difference is expressed as,
ΔTA = Temperature difference of fluids at A
ΔTB = Temperature difference of fluids at B
The materials of heat exchangers are based on the applications and project requirements.
Few materials are listed for the basic idea,
We have already got a basic idea about heat exchangers, now, how to select a heat exchanger?
Let’s see the factors which need to be considered during heat exchanger design.
The applications of heat exchanges are as follows,
There are so many advantages, as follows:
There are few disadvantages, as follows,
There are a lot of manufacturers available across the globe, few of them are listed,
So, finally, we have learned the basics of heat exchanger along with the basic definition, parts, types, applications, advantages, disadvantages, etc.
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