In almost all mechanical, chemical or electrical systems, heat is moved from one area to another or from one liquid to another. This heat transfer process is done through a heat exchanger. A heat exchanger is a mechanical device which uses transmits from one fluid to another. Heat exchangers are utilized all over the world for different applications to transfer heat. There are multiple types of heat exchangers, and a shell and tube heat exchanger (STHE) is one of them. The shell and tube heat exchanger has a bundle of tubes. The most popular type of STHE is the TEMAE type because of its robustness and simple design, as well as its wide operating pressure and temperature range. This article mainly describes different types, working, parts, and many other aspects of the shell and tube heat exchanger.
What is a shell and tube heat exchanger?
A shell and tube heat exchanger is a mechanical equipment that uses tubes installed in a cylindrical shell to exchange heat between two working fluids through thermal contact. Shell and tube heat exchanger is also called a tube and tube heat exchanger.
As the name suggests, this type of heat exchanger has an outer shell (a large pressure vessel) and a tube bundle. One liquid flow inside the tube while the other liquid flows over the tube. In this way, it transfers heat from one fluid to other.
The temperature of the fluid within the tube and shell is different. This temperature difference works as a driving force to exchange the temperature between the fluids.
The bundle of tubes may contain different types of tubes, including longitudinally finned and plain. These tubes may arrange in different configurations such as floating head configuration, fixed tube configuration, and U-shaped tube configuration.
These types of heat exchangers are best suitable for high-pressure applications. They are most commonly used in large chemical processes and oil refineries.
These exchangers have low maintenance costs, low purchasing costs, stable performance, and simple designing. The shell and tube heat exchanger has more heat transfer rate than other heat exchangers. However, these exchangers take more space for installation than plate heat exchangers having similar heat exchange capabilities.
Working Shell and Tube Heat Exchanger
The shell and tube heat exchanger has very simple and easy to work. In the working of this heat exchanger, one fluid moves in the tube while the other fluid moves inside the shell.
During the flow of these fluids, they exchange heat with each other. This means that a hot fluid transfers its heat to the cold fluid. Therefore, cold fluid enters through the inlet nozzle of the shell (or channel side or tube side) and discharges as a hot fluid through the outlet nozzle. Due to this heat transaction process, the temperature of the other fluid (i.e., hot fluid) will be lower at the outlet than at the inlet.
The heat transfer of the tube heat exchanger determines through the exposed surface, which is determined by the number of thermally conductive metal tubes. The fluid flows in this heat exchanger may have crossflow or parallel flow.
The above-given diagram represents the outlet and inlet nozzles on the channel side’s front header. This means that these heat exchanges have even numbers of tube passes. In this case, the outlet nozzle of the channel side is located on the read header. An increase in the number of tube passes increases the heat transfer coefficient and heat transfer rate.
The baffles and turbulator install within the shells and tubes to maximize the turbulence of the fluid in the shell and tube side.
For a better understanding, watch the following video:
Read Also: Working of Plate Heat Exchanger
Types of shell and tube heat exchangers
The shell and tube heat exchanger has the following major types:
- Floating head exchanger
- Fixed tubesheet exchanger
- U-tube heat exchanger
1) U-tube heat exchanger
In this type of shell and tube heat exchanger, the tube can be bent or straightened into a U-shape known as a U-shaped tube. Therefore, this heat exchanger is known as a U-tube heat exchanger. This heat exchanger uses an M-type rear head and any type of front header.
The design of the U-tube heat exchanger allows unlimited thermal expansion. In these exchangers, the tube bundle may also be detached for cleaning, and the small clearance between the shell and bundle may also be achievable. But it is difficult to mechanically clean the inner sides of the tubes. Therefore, U-shaped exchangers are best suitable for applications that have clean tube side fluid.
Advantages and Disadvantages of U-tube heat exchanger :-
|The tube bundle of this heat exchanger is removable for inspection and cleaning.||The inner surface of the tubes can only clean via chemical means.|
|It has a low initial cost.||The side passes of the tubes can’t be decreased or increased.|
|This heat exchanger has easy cleaning of the tube side headers and channels.||The velocity of the tube side may start corrosion on the inner sides of the bends.|
|It requires only one tubesheet.||You can’t mechanically clean the tubes of this exchanger.|
2) Fixed Tubesheet Exchanger
In the case of the fixed tubesheet exchangers, the tubesheet welds with the shell. This shell and tube heat exchanger has a simple and inexpensive structure that enables mechanical or chemical cleaning of the tube holes. Except for chemical cleaning, the tube’s outer surface of this exchanger is not accessible.
If there is a large temperature difference between the materials of the shell and tube, it may be compulsory to add expandable bellows to the shell in order to avoid unnecessary stresses due to expansion. Such types of bellows usually cause vulnerabilities and failures.
U-tubes or floating headers are typically employed in situations where the failure consequences are particularly severe.
This is a low-price design for all removable tube bundle but usually more costly than a low-pressure fixed tubesheet design.
Advantages and Disadvantages of Fixed Tubesheet Exchanger :-
|There is no internal leakage.||This heat exchanger has a relatively low service life.|
|It has small bypass seepage.||It has a large temperature difference between the tube and shell.|
|It has more heat transfer area than a floating head heat exchanger.||You can’t mechanically clean the shell side.|
|It has fewer gaskets.||It has a replaceable bundle|
3) Floating Head Exchanger
In the floating heat exchanger, the tubesheet at the end of the rear header doesn’t weld with the shell but can float or move. The diameter of the front header tubesheet is more than the shell, and the sealing method is the same as the fixed tubesheet exchanger.
The tube sheet at the shell’s rear header end has somewhat less diameter than the diameter of the shell so that a tube bundle can easily pass by the shell. By using a floating head exchanger, the thermal expansion can be taken into account, and the tube bundle can remove and clean easily.
These heat exchangers are best for the load’s associated applications with high pressures and high temperatures. However, these have more cost than equivalent fixed tubesheet exchangers.
Advantages and Disadvantages of Floating Head Exchanger :-
|You can clean it without eliminating the tube bundle.||It has more costs than the fixed exchanger.|
|The individual tube is replaceable.||The inner gasket of this exchanger has leakage issues.|
Read Also: Working of Cross-flow Heat Exchanger
Components of Shell and Tube Heat Exchanger
The shell and tube heat exchanger has the following major parts:
- Tube-Side Channel and Nozzles
- Shell and Shell-Side Nozzles
The tube is the fundamental part of the shell and tube heat exchanger. It provides a heat transfer surface between the fluid moving in the tube and another fluid moving out of the tube. These tubes can be welded or seamless.
They are generally made of copper alloy or steel. The aluminum, titanium, or nickel alloys may also be needed for certain applications. Seamless tubes manufacture by using the extrusion process; welded tubes are made by rolling strips on cylinders and welding. Therefore, welded pipes are usually inexpensive.
The tube may have a reinforced surface (finned) or exposed or an extended surface on the outside. Finned surface tubes use if one fluid has a significantly lower heat transfer coefficient than that of another fluid.
The tube is fixed in its place by inserting it into a tubesheet’s hole. After inserting, the tube extends in a groove cut in the hole or welded to the tubesheet where the tubes protrude from the surface.
This configuration stops the tube-side fluid from mixing with the shell-side fluid. This sheet is a grooved and perforated round metal plate that is properly drilled to accept tubes (triangular or square design), bolt rings, spacer rods, and gaskets that attach to the shell.
The distance between the tube hole’s centers is known as the tube pitch. Typically, this tube pitch is 1.25 times the outer diameter of the tube. Some other tube pitches also utilize to regulate the rate of shell-side fluid flowing through the tube bundle and to decrease shell-side pressure drop.
Read Also: Working of Crankshaft
3) Shell and Shell-Side Nozzles
The shell acts as a container for the fluid of the shell side. The nozzles work as the inlet port and outlet port. In maximum cases, the shell is constructed by rolling a metal plate of suitable size into a cylinder and welding the longitudinal joints. Such a type of shell has a circular cross-section.
The shells having a small diameter may also be constructed by cutting a tube of the required diameter up to a suitable length.
The shell’s roundness plays a most vital role in setting the maximum diameter of the insertable baffle. This also influences the effect of the leakage from the shell into the baffle.
4) Tube-Side Channel and Nozzles
These parts of the shell and tube heat exchanger only regulate the tube-side fluid flow into and out of the heat exchanger tube.
The fluid on the tube side has more corrosion than the shell-side fluid. Therefore, alloy material (well-suited with tube sheets and tubes) uses for the construction of these nozzles and channels.
The channel end is installed with the channel cover. These are round plates bolted with the channel flange. You may remove the channel end for the inspection of tubes without removing the tube side piping.
The below-given diagram represents the baffles:
- The baffles assist the tubes in maintaining their suitable position during installation and operation in order to stop the tube’s vibrations due to eddy currents generated by flow.
- The baffle guides the back-and-forth fluid flow of the shell-side across the tube field to improve heat transfer coefficient and speed.
- They play a big role in maintaining the tube spacing.
6) Pass Dividers
A pass divider requires in a single bonnet or channel for a heat exchanger, which has 2 tube-side passes. For an exchanger with more than one channel, both the bonnets and channels require pass drivers.
When the bonnet or channel is molded, the dividers are also molded in one piece, and then a surface treatment is applied to provide a smooth seating surface on the gasket for the connection between the tubesheet and dividers. When the channels are made of pipe or plate then the divider welds on its own place.
7) Tie Rods
The spacers and tie rods are utilized in heat exchangers due to the following reasons:
- They secure the baffle assembly together.
- They sustain the spacing of the selected baffle.
One end of the tie rod connects with the last baffle, and the other end connects with the tubesheet. These rods help to maintain the baffle assembly together.
The spacer installs over the tie rod between each baffle to hold the pitch for the selected baffle. The numbers of spacers and tie rods vary according to the shell diameter and the size of the spacers and tie rods.
Read Also: Working of Tie rod
Fluid Velocity inside Shell and Tube Exchanger
The high velocity of the fluid in a shell & tube heat exchanger decreases the fouling and increases heat transfer rate but generates erosion and rise pressure drop. Following are some fluids used in the shell and tube heat exchanger:
|Fluid Types||Fluid Velocity-Tube Side||Fluid Velocity-Shell Side|
|Gas /Vapor (High Pressure)||5 to 10 m/s||5 to 10 m/s|
|Gas /Vapor (Atmospheric Pressure)||10 to 30 m/s||10 to 30 m/s|
|Gas /Vapor (Vacuum Pressure)||50 to 70 m/s||50 to 70 m/s|
|Liquid||1 to 2 m/s||0.3 to 1 m/s|
Advantages and Disadvantages of Shell and Tube Heat Exchanger
Advantages of Shell and Tube Heat Exchanger
- The shell and tube heat exchanger has low cost than the plate heat exchanger.
- These types of heat exchangers have easy maintenance and simple construction.
- These are the best choice for high temperature and pressure applications instead of plate heat exchangers.
- The tube heat exchangers have a lower pressure drop (delta P/ΔP) than the plate heat exchangers.
- The shell and tube heat exchanger doesn’t foul so easily as a plate heat exchanger.
- These exchangers have steady and flexible designs.
- There are no dimensional limitations.
- These exchangers can be easily assembled and disassembled for cleaning, repairing, and maintenance.
- You can make the tube “double-walled” to decrease the possibility of tube-side fluid leaking into the shell-side fluid (or vice versa).
- These types of heat exchangers have more versatility than other types.
Disadvantages of Shell & Tube Heat Exchanger
- This heat exchanger has low efficiency than a plate exchanger.
- It requires more floor space for installation.
- The cooling capacity of these exchangers can’t be improved.
Application of Shell and Tube Heat Exchangers
The shell & tube heat exchanger is used in the following major applications and industries:
- They are used in marine applications.
- Power generation
- Chemical industries
- Metals and mining
- Power Plants
- Oil and Gas
- Fertilizer industries
- Cooling of engines, compressors, and turbines
- Refinery and Petrochemical
- These are also used in boilers.
Difference between Shell and Tube Heat Exchanger and Plate Heat Exchanger
|Shell and Tube Heat Exchanger||Plate Heat Exchanger|
|The shell and tube heat exchanger has lower heat transfer efficiency.||The plate heat exchange has more heat transfer efficiency than the tube heat exchanger.|
|It has difficult maintenance and cleaning.||It has easy cleaning and maintenance.|
|It requires more floor space than a plate heat exchanger.||This exchanger requires less floor space.|
|The shell and tube heat exchanger has a lower cost.||The plate heat exchanger has a high cost.|
|They have less pressure drop.||They have a high-pressure drop.|
|It is less efficient.||It is more efficient.|
What is the purpose of using baffles in shell and tube heat exchanger?
The baffles have the following two major functions:
- These parts of exchangers hold the tubes in place during process and assembly to stop tube vibrations generated due to flow-induced eddies.
- They move the shell-side backward and forward in the tube field and increase the speed and the heat transfer coefficient.
Why shell and tube heat exchanger is used?
Shell and tube heat exchangers are used in a large number of industrial applications because they have the capacity to remove process heat and preheat the feed water. They cool the engines, compressors, and turbines. These heat exchangers extract heat from one fluid and transfer heat to another desired fluid.
What principle does a shell and tube heat exchanger work on?
Shell and tube heat exchanger works according to the simple principle of the 2nd law of thermodynamics (i.e., the heat transfers from one body to another body because of their temperature difference).
What are the major parts of the shell-and-tube heat exchanger?
- Tube-Side Channel and Nozzles
- Shell and Shell-Side Nozzles