What is a shell and tube heat exchanger, and what are its primary components?
A shell and tube heat exchanger (STHE) is a type of heat exchanging device constructed using a large cylindrical enclosure, or shell, that has bundles of perfectly spaced tubing compacted in its interior. Heat exchanging is the transfer of heat from one substance or medium to a similar substance or medium. Shell and tube heat exchangers are the most common form of heat exchange design.
The components of a shell and tube heat exchanger can be broken down into the following parts:
- Shell
The shell is the heat exchanger’s outermost part which holds the tube bundle. It is commonly a cylindrical container constructed from steel or other appropriate substances
- Tubes or Tube Bundle
A collection of parallel tubes running along the length of the shell makes up the tube bundle. Depending on the specific use, the tubes can be composed of different materials, such as stainless steel, copper, or titanium. The diameter and thickness of the tubes are also important design parameters.
- Tube Sheets
Tube sheets are sturdy sheets that act as a barrier between the tube bundle and the shell. They are commonly constructed using steel and are fused to the shell to ensure a firm and leak-free closure. The tubes are inserted through holes in the tube sheets and are either expanded or welded in position.
- Baffles
Baffles are plates or rods that are placed inside the shell to regulate the movement of fluid around the tube bundle. These can be either longitudinal or transverse in orientation and are intended to enhance the effectiveness of heat transfer.
- Inlet and Outlet Nozzles
The inlet and outlet nozzles serve as the entry and exit points for fluids in the heat exchanger. These connections are usually placed at opposite ends of the shell and are attached to the tubes and the shell using flanges or other types of fittings.
- Expansion Joints
Expansion joints are flexible connectors that accommodate the tube bundle’s thermal expansion and contraction. Usually situated at the inlet and outlet of the heat exchanger, these joints are constructed using metal bellows or other flexible materials.
- Support Structures
Support structures hold heat exchangers in position, ensuring a stable foundation. Support structures can be either temporary or permanent and may be made of steel or other materials.
How does a shell and tube heat exchanger facilitate heat transfer between two fluids?
The process of a shell and tube heat exchanger provides a place for two fluids to exchange or transfer heat through conductive metals. In the shell and tube heat exchanger process, one fluid flows through the tubes while the other fluid flows through the shell.
What are the main advantages of using a shell and tube heat exchanger compared to other types of heat exchangers?
The advantages of a shell and tube heat exchanger include its simple design, robust characteristics, and relatively low purchase and maintenance costs. This type of heat exchanger is widely used in various cooling applications due to these benefits.
The simple design of a shell-and-tube heat exchanger makes it versatile and suitable for a range of cooling needs. It is commonly used to cool hydraulic fluid and oil in engines, transmissions, and hydraulic power packs. Additionally, with the right choice of materials, these heat exchangers can also be used to cool or heat other mediums such as swimming pool water or charge air.
One of the significant advantages of shell-and-tube heat exchangers is their ease of service, especially models with a floating tube bundle design where the tube plates are not welded to the outer shell. This makes maintenance and cleaning more accessible, contributing to efficient operation and extended lifespan. Furthermore, the cylindrical design of the housing provides excellent resistance to pressure, making shell-and-tube heat exchangers suitable for a wide range of pressure applications in various industries.
What are the different configurations of shell and tube heat exchangers, and what are their typical applications?
. Here are some common types of shell and tube heat exchangers, which are popular in different industries:
- Direct Contact Heat Exchangers– As the name suggests, in the direct contact heat exchangers, there is a direct contact between hot and cold fluids. There is no physical barrier present between them. This allows the fluids at different temperatures to mix directly. As a result of this, heat and mass transfer takes place simultaneously. A cooling tower comes under these types of heat exchangers.
- Co-current (Parallel) Flow Heat Exchangers– In these types of heat exchangers, both the hot, as well as the cold fluids flow in the same direction. The difference in the temperature of the hot and cold fluids continue to decrease consistently. The change in temperature occurs from one end to the other.
- Recuperators– Recuperators are among the most widely used types of heat exchangers. Evaporators used in automobile radiators and ice plants come under these types of heat exchangers.
- Regenerative Heat Exchangers– This is a popular type of heat exchanger. In a regenerative heat exchanger, the heat from a heat surface is stored and removed alternately. The surface is alternately moved in and out of the cold and hot fluid streams. At times, hot and cold currents are switched into or out of the heat surface.
How is the overall heat transfer coefficient determined in a shell and tube heat exchanger?
Heat exchanger setup typically involves two flowing fluids separated by a solid wall. Heat is first transferred from the hot fluid to the wall by convection, the heat is then transferred through the wall by conduction, and lastly the heat is transferred from the wall to the cold fluid again by convection. Any radiation effects are usually included in the convection heat transfer coefficients.
The main objective in the design of a heat exchanger is to determine the surface area required for the specified duty (rate of heat transfer) using the temperature differences available. The overall heat transfer coefficient is the reciprocal of the overall resistance to heat transfer, which is the sum of several individual resistances
What factors should be considered when designing a shell and tube heat exchanger for a specific process?
Tubes are the core of a shell & tube heat exchanger. They must be selected with consideration, taking into account factors such as thermal conductivity, corrosion resistance, and mechanical strength. The tube material, diameter, length, wall thickness (gauge), pitch, and layout (triangular, square, etc.) all influence the exchanger’s performance. Materials like carbon steel, duplex, and stainless steels are selected for their thermal conductivity, resistance to corrosion, and economics in shell and tube heat exchangers.
How do fouling and scaling impact the performance of a shell and tube heat exchanger, and what measures can be taken to minimize these issues?
These exchangers can suffer from fouling, which is the accumulation of unwanted material on the tube surfaces. Fouling reduces the heat transfer efficiency and increases the pressure drop, leading to higher energy consumption and operational costs.
Therefore, it is important to clean the shell and tube exchangers periodically to restore their performance and prevent corrosion or damage. However, cleaning also involves some costs and challenges, such as downtime, labour, equipment, safety, and environmental issues.
What materials are commonly used in the construction of shell and tube heat exchangers, and how is material selection made?
Traditional heat exchanger materials include:
Admiralty brass / RNB, Aluminium, Aluminium brass, Carbon steel, Copper, Cupronickel 70/30 and cupronickel 90/10,Duplex,Super Duplex, Monel, Stainless steel
However, a range of speciality materials is becoming increasingly popular
Hastelloy heat exchangers
Hastelloy is a nickel alloy best known for its corrosion resistance, combined with good temperature resistance. There are a variety of Hastelloy alloys each with slightly different properties, but the family overall has outstanding corrosion resistance, stress cracking resistance and are easy to weld and manipulate.
For example, Hastelloy heat exchangers are therefore well suited for use in chemical plants. Hastelloy can cope with corrosive fluids, including petrochemicals. It reduces the need for repairs, compared to less corrosion-resistant options, and therefore minimises any downtime.
Inconel heat exchangers
Inconel is part of a family of nickel-chrome-based superalloys. For example, Inconel 625 is approximately 65% nickel, 22% chromium, 9% molybdenum, and 4% niobium – but there are many other varieties of Inconel.
Similar to Hastelloy, Inconel has excellent corrosion resistant properties. These stand up against corrosive fluids, as well as oxidising agents. It’s also extremely strong and almost completely non-magnetic.
Inconel heat exchangers are therefore commonly used in corrosive environments such as chemical plants and environments with a high risk of oxidising metals which would otherwise lead to a lower lifespan of the heat exchanger.
Tantalum heat exchangers
Tantalum, previously known as tantalium, is a relatively rare material. It has a high melting point of 2,850°C and is almost entirely immune to chemical attack at temperatures below 150°C.
These properties make it extremely useful in chemical industries. For example, tantalum heat exchangers can be used in the process of vaporising (or boiling) strong acids.
If you’re not sure what material you need for your heat exchanger, see the lists below of appropriate materials for different applications.
Material for corrosion-resistant heat exchangers
Corrosion-resistant heat exchangers are common in many industries but perhaps most in chemical processing. Corrosion-resistant heat exchanger materials include:
Hastelloy, Inconel, Tantalum, Titanium, Zirconium
Materials for high-temperature heat exchangers
Some heat exchangers have to be able to function effectively and efficiently in high-temperature environments. These are known as HTHEs (high temperature heat exchangers). Appropriate materials include:
Nickel-based alloys such as Hastelloy and Inconel
Ferritic steels
Advanced carbon and silicon carbide composites (often for the highest temperature applications such as rocket nozzles)
How does the arrangement of tubes (e.g., single-pass, multi-pass) affect the efficiency and effectiveness of a shell and tube heat exchanger?
Single Pass: In a single-pass heat exchanger, the hot and cold fluids make just one pass through the unit. This design is simpler and more cost-effective but may provide less heat transfer efficiency as multipass configurations.
Multipass: In multipass heat exchangers, one or both fluids make multiple passes through the unit. This increases the contact time between the fluids, enhancing heat transfer efficiency. Multipass configurations are commonly used when a higher level of heat exchange is required.
What are the key maintenance practices required to ensure the long-term reliability and efficiency of a shell and tube heat exchanger?
Cleaning: Regular cleaning of tubes to prevent fouling and scaling. Cleaning methods can include chemical cleaning, mechanical cleaning, and high-pressure water jets.
Inspection: Routine checks for leaks, corrosion, and mechanical wear. Non-destructive testing methods like ultrasonic testing and radiographic inspection can help detect issues without dismantling the equipment.
Tube Replacement: Replacing damaged or corroded tubes to maintain efficiency. Keeping spare tubes and gaskets on hand can reduce downtime during maintenance.
How can thermal stress and vibration affect the operation of a shell and tube heat exchanger, and what design considerations can help mitigate these issues?
Solving Vibration Issues in Shell and Tube Heat Exchangers
Shell and tube heat exchangers are ubiquitous due to their reliability and adaptability. However, like all mechanical systems, they’re not without their challenges. One common issue experienced by designers and operators alike is vibration. Excessive vibration can lead to acoustic noise, mechanical wear, and even tube failure. Thankfully, there are several tried and true methods for addressing and mitigating these vibrations. Here are some of the best solutions for vibration issues in shell and tube heat exchangers.
Acoustic Vibration
The frequency of sound produced by the high-velocity gas or vapour flow can create acoustic (sound) vibration in a heat exchanger, which can range from a low-hum to a piercing scream like a jet engine!. By manipulating the flow patterns or modifying certain design elements (shell diameter), it’s possible to shift the frequency to a range that doesn’t resonate with the structural elements of the heat exchanger. For example, introducing `deresonating’ or `detuning’ baffles that run lengthwise along the shell, one can increase the acoustic frequency of the shell, and eliminate the potential for acoustic vibrations.
Lowering the Velocity on the Shell Side
One of the most direct ways to combat flow-induced vibrations is by reducing the velocity of the fluid on the shell side. When the fluid flows at a high speed over the tubes, it can induce vibrations. By checking that the design allows for optimal flow rates and not excessively high velocities, it’s possible to reduce the forces acting on the tubes, thereby decreasing the potential for vibration. This can be achieved with larger nozzles, increased baffle spans, and/or increased baffle cuts. Increasing the tubing pitch can help as well.
Try to Increase the Natural Frequency of the Tube
Every structure has a natural frequency at which it tends to vibrate if disturbed. If the flow-induced frequency matches this natural frequency, resonance occurs, amplifying the vibrations. By changing the material, geometry, or support conditions of the tubes, one can alter their natural frequency, so it doesn’t match with any frequencies the fluid flow might induce. This can be achieved by reducing the maximum unsupported tubing spans by reducing baffle spacing (more baffles), adding intermediate baffle supports, changing to no-tube-in-window (NTIW) baffle layouts, or adding specifically placed tube supports (aka FIVERS – Flow Induced Vibration Supports) in areas of high-velocity and/or larger tube-spans.
U-Bend Supports
Another area of concern for vibration in a shell & tube heat-exchanger is the U-bend area on U-tube (AEU, BEU, CEU, etc) type heat exchangers. Historically, the U-bend area was ignored for heat-transfer and vibration as shell-side process flow was minimal in this area due to nozzle locations. However, many process engineers and thermal designers are now making use of the U-bend area for “free” heat transfer, placing the shell inlet or outlet nozzle beyond the U-bend. On larger diameter exchangers, the unsupported span of the outer U-tubes can easily exceed the longest span in the rest of the bundle. Without careful consideration, these long U-bends can easily move and vibrate at low velocities.
U-bends can be supported via a full-support baffle (welded together from strips), or a grid mesh made of rods. Multiple supports may be required for longer outer U-tubes. The inner U-bends do not require support, and the exchanger designer should specify at which point support is required
What are some common troubleshooting techniques for diagnosing and addressing performance issues in shell and tube heat exchangers?
Fouling: Accumulation of deposits reduces heat transfer efficiency. Regular cleaning and proper filtration can mitigate fouling. Implementing a proper water treatment program can also help reduce fouling.
Leakage: Caused by corrosion or mechanical damage. Regular inspections and using corrosion-resistant materials can prevent leaks. In case of leakage, identifying the source and replacing or repairing the affected components is crucial.
Vibration: Resulting from flow-induced vibrations. Proper design and installation of baffles can minimize this issue. Ensuring adequate support and securing of tubes can also help reduce vibrations.