This article covers everything you need to know about shell and tube heat exchangers.
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The shell and tube heat exchanger (STHE) is a highly utilized device engineered for thermal energy exchange. It comprises a sizable cylindrical shell housing numerous parallel tubes. These tubes are strategically organized within the shell to enable heat transfer between two different fluids. Appreciated for their efficiency and versatility, shell and tube heat exchangers come in multiple designs tailored to specific applications, tube configurations, and various attributes.
These heat exchangers are favored for their straightforward and efficient design in transferring heat. They operate by circulating a fluid or steam through the shell, which subsequently warms the tubes within. Achieving optimal performance often involves passing the fluid through the tubes four times, generally considered an effective tactic for heat transfer efficiency.
Shell and tube heat exchangers are designed using advanced computational techniques and precise engineering parameters. Key components of these devices include the shell, shell cover, tubes, channel, channel cover, tube sheet, baffles, and nozzles. The design and manufacturing standards for shell and tube heat exchangers are regulated by the Tubular Exchanger Manufacturers Association (TEMA).
Before manufacturing a shell and tube heat exchanger, several critical data points are required from the manufacturers. These include flow rates, inlet and outlet temperatures, pressure levels, pressure drops, resistance factors, the physical properties of the substances being processed, pipe dimensions, and the shell diameter. In addition to these fundamental aspects, detailed technical specifications are necessary to ensure the production of a heat exchanger that meets the specific needs of the application.
The shell of a shell and tube heat exchanger is constructed from pipes or welded metal plates, selected for their ability to endure high temperatures and resist corrosion. The interior of the shell is designed to be cylindrical with a uniform diameter to reduce the gap between the baffled perimeter and the shell itself.
The choice of channel or head type for a shell and tube heat exchanger varies based on its application. Bonnet-type heads are typically used when the head does not need frequent removal. For maintenance purposes, channels with removable covers are either flanged or welded. When regular access to the channel and tubes is required, a removable cover for the channel is essential.
Tubes of a shell and tube heat exchanger are welded or extruded and made from carbon steel, stainless steel, titanium, Inconel, or copper. Tube diameters of 0.625 inch (16mm), 0.75 inch (19mm), or one inch (25mm) are used in compact heat exchangers. The thickness of the tubes is chosen for pressure, temperature, thermal stress, and resistance to corrosion at lengths of 6 to 24 feet or 2 meters to 7 meters. Tubes that are longer reduce shell diameter and result in high shell pressure drop.
The tube sheet is a plate or sheet that is perforated with holes for the insertion of pipes or tubes and designed to support the tubes on either end of the cylindrical shell. The shell extends beyond the tube sheets and is sealed on both ends to form the enclosed chamber that is covered by the heads.
The main function of a heat exchanger is to transfer heat between a hot fluid on one side and a cold fluid on the other. As a result, these units frequently experience a wide range of temperatures.
Materials expand when heated and contract when cooled. Without proper allowances for these changes, the materials in the heat exchanger could become overstressed and fail. This can lead to issues such as tube buckling, tubes being dislodged from the tube sheets, shell deformation, or distortion of nozzle connections. Such problems compromise the heat exchanger's structural integrity and could render it unsafe for operation.
To address this, an expansion joint is used. This flexible component is designed to absorb the stresses caused by temperature and pressure variations.
Tube pitch refers to the measurement from the center of one tube to the center of the neighboring tube. Tubes can be arranged in either a triangular or square configuration. The square pattern is generally preferred for its ease of cleaning and minimal turbulence. This arrangement also facilitates the ascent of vapors between the tubes, offering a benefit compared to the triangular and rotated square pitch layouts.
Baffles are installed to control the fluid flow within the shell side of a heat exchanger. Their purpose is to enhance the velocity of the fluid, which improves the heat transfer efficiency and helps reduce fouling. Fouling refers to the buildup of unwanted materials on the heat transfer surfaces, which impedes heat transfer and degrades the performance of the heat exchanger. In horizontal shell and tube heat exchangers, baffles also play a crucial role in supporting the tubes, preventing them from sagging or experiencing damage due to vibrations.
Tie rods and spacers serve as structural supports for the baffles within a heat exchanger, ensuring they stay securely in position and maintaining the proper spacing between them. The quantity of rods and spacers required is based on the number of baffles and the diameter of the shell. Tie rods are attached to the tube sheet and span the length of the tube bundle, extending to the final baffle to provide necessary support.
The concept and operation of a shell and tube heat exchanger are rather simple and are based on the flow and thermal contact of two liquids. The name of a shell and tube heat exchanger serves as an explanation of the process, which is the exchanging of temperature between two fluids. In a heat exchanger, a heated or hot fluid will flow around a cold fluid and transfer heat in the direction of the flow of the cold fluid.
Whenever two materials come into contact, heat will be transferred through the conductive surfaces between them. Shell and tube heat exchangers are designed to facilitate this heat transfer, allowing two different fluids to exchange heat through metal surfaces.
In a shell and tube heat exchanger, one fluid moves through the tubes while the other flows around the tubes within the shell. For instance, in a straight tube shell and tube heat exchanger, the shell fluid enters through the top inlet, while the tube fluid enters from the bottom right inlet.
These heat exchangers consist of two main sections: the shell side and the tube side. Proper fluid allocation is essential, determining which side will handle the hot fluid and which will handle the cold fluid.
In cases where there is a pressure differential between the fluids, the lower pressure fluid is directed through the shell inlet, as the tubes are built to withstand higher pressures.
When configuring fluid flow on the shell side, it's crucial to remember that the shell is costlier to manufacture compared to the tubes and is also more challenging to clean. Baffles inside the shell help manage the fluid flow, directing it across the tube bundles.
The shell side is typically used for handling viscous fluids and those with high flow rates, as it provides enhanced turbulence and a higher heat transfer coefficient. This setup is particularly effective for managing large temperature differences.
To ensure turbulent flow on the tube side, turbulators are installed inside the tubes through the openings in the tube sheet. This turbulence enhances the heat transfer efficiency, similar to the effect in the shell side. Additionally, turbulators help maintain the cleanliness of the tubes by preventing fouling. While the tubes generally have lower turbulence and pressure drop, they facilitate a smoother flow of the fluid.
Shell and tube heat exchangers are categorized by the number of passes, which can range from one to eight or more. This is denoted as 1-1, 1-2, 1-4, etc., where the first number represents the number of shells and the second indicates the number of passes. Each pass refers to the number of times the fluid circulates through the shell side. For instance, a single-pass heat exchanger allows the fluid to traverse the shell only once. Increasing the number of passes typically enhances the heat transfer coefficient.
The illustration demonstrates that cold fluid enters through the inlet of the tube or shell and undergoes heating via conductive methods within the tubes, then exits after being processed. The diagram below depicts a heat exchanger with a two-pass shell and tube design.
In a shell and tube heat exchanger, turbulent flow enhances the efficiency of heat transfer and reduces the risk of fouling on the tubes and shell surfaces. The consistent turbulence has a self-cleaning effect that helps maintain optimal performance. Flow baffles in the shell generate turbulence, while internal turbulators in the tubes contribute to this effect.
Heat exchange occurs through thermal contact between the fluids circulating within the shell and tubes. This process results in one fluid exiting at a lower temperature, while the other exits at a higher temperature.
The Tubular Exchangers Manufacturers Association (TEMA) has established guidelines for the design, manufacture, and construction of shell and tube heat exchangers. These guidelines are categorized into three classes: Class B, Class C, and Class R. The classification of a shell and tube heat exchanger is based on its construction specifics and the type of service it is intended to deliver.
TEMA Classifications:
TEMA classifies shell and tube heat exchangers based on their front end or head, rear end, and shell configurations. The chart below uses columns and rows to categorize and detail each type of shell and tube heat exchanger.
To simplify the identification of various designs and configurations, TEMA has created a three-letter identification system—BEM, AEM, or NEN—for straight tube and fixed tube sheet shell and tube heat exchangers.
The first letter indicates the type of front end stationary head, specifying how the tube sheet is attached to the shell and channel—whether by bolting or welding.
The second letter represents the shell type, detailing the arrangement of inlets and outlets as well as the presence of longitudinal baffles and distribution plates.
The third letter denotes the rear end head type, covering the connection between the shell and the second tube sheet and the channel closure method—bolted or welded.
For example, a BEM shell and tube heat exchanger features a bonnet header, a single pass shell, and a fixed tube sheet.
To better understand the function and operation of shell and tube heat exchangers, they are categorized based on their characteristics. One key characteristic for classification is the type of flow they utilize.
Shell and tube heat exchangers can be grouped into three main flow types: parallel, counter, and cross. Each type has distinct design, operational, and application requirements, and often these flow types are used in various combinations.
In parallel flow, both the shell and tube sides enter the heat exchanger at the same end and move directly to the opposite end. This means that the temperature change in each fluid is uniform, with each fluid heating or cooling by the same degree.
Counter flow involves fluids moving in opposite directions. They enter the heat exchanger from opposite ends and exit from the other ends. This flow arrangement is the most efficient and commonly used type of heat exchanger due to its effective heat transfer capabilities.
Cross flow heat exchangers feature fluids flowing perpendicular to each other at a 90-degree angle. In this design, one fluid changes state (as seen in steam condensers where cooling water absorbs steam), and the other fluid, which remains in its liquid form, absorbs the heat.
A fixed tube sheet heat exchanger consists of straight tubes that are attached at both ends to stationary tube sheets, which are welded directly to the shell. This type of heat exchanger is known for its straightforward design and construction, making it one of the most economical options available. However, it has a limitation in handling significant temperature differences between the fluids, though this issue can be mitigated by incorporating an expansion joint. One of the main advantages of a fixed tube sheet heat exchanger is its simplicity in cleaning and maintenance.
The U-tube shell and tube heat exchanger derives its name from the U-shaped configuration of the tubes. The inlet and outlet valves are positioned at one end of the heat exchanger, with fluids entering through the upper portion of the tube sheet and exiting from the lower portion. The tubes are free to expand at the U-bend, enabling the U-tube heat exchanger to accommodate significant temperature differences.
The placement of inlet and outlet valves in a U-tube shell and tube heat exchanger can vary depending on the design. In the diagram below, the shell fluid enters from the top left and exits from the bottom right.
The floating head design resembles the U-tube design but without the U-shaped tubes. In this design, one end of the tubing is fixed to a stationary tube sheet, while the other end is left unattached, allowing it to expand and float freely. This flexibility enables the floating head design to handle significant temperature differences as the tubes expand. Additionally, this type of heat exchanger is easy to clean and inspect because the tubes can be conveniently removed.
Floating head designs are categorized into four distinct types:
The tube bundle can be removed from the shell, featuring an unusual clearance between the baffle's outer diameter and the inner diameter of the main shell.
Removing the tube bundle necessitates its disassembly. This design maintains a standard clearance between the baffle diameter and the inner diameter of the shell.
The shell side is sealed using packing rings that are compressed into a stuffing box, enabling the tube sheet to slide back and forth smoothly.
The fluids are sealed by O-rings separated by a lantern ring.
In certain applications, where heat transfer involves viscous or sticky substances, materials can build up on the internal surface of the heat exchanger. Scraped surface heat exchangers are specifically designed for these situations, featuring scraping blades that continuously remove accumulated material from the interior surface. Structurally, they resemble other heat exchangers but are equipped with rows of scraping blades inside the cylinder.
The blades in a scraped surface heat exchanger are spring-loaded and rotate, effectively scraping the surface while draining liquid from the exchanger. Typically, four blades are used, though the number can vary. However, increasing the number of blades also raises the cost of the heat exchanger. In practice, a higher blade count is often unnecessary since the intervals between scrapings are already brief.
Scraped surface heat exchangers can be installed either horizontally or vertically, with the vertical configuration being preferred as it allows liquids to flow downward naturally due to gravity.
Shell and tube heat exchangers are versatile and serve various applications across multiple industries. Their diverse configurations allow them to be customized to meet the specific needs of different manufacturing and production processes.
In refineries and factories, shell and tube heat exchangers are integrated into processing equipment to facilitate efficient heat transfer. They represent about 65% of the heat exchangers available in the market.
A significant advantage of shell and tube heat exchangers is their cost-effectiveness. They are considerably more affordable compared to plate-type coolers.
Heat exchangers must accommodate a broad range of temperatures, depending on the application. Their capacity to handle extreme temperatures is crucial for maintaining production and operational efficiency. Shell and tube heat exchangers offer high-temperature resilience and can be customized to meet various conditions.
Due to the high pressure they endure, shell and tube heat exchangers require robust materials, which can make them heavy or costly, especially if nickel alloys are used. High pressure can lead to significant issues and production delays. The shell and tubes are engineered and tested to handle pressure fluctuations while complying with ASME and PED standards.
Pressure loss represents a loss of energy and results in reduced downstream pressure, slowing the flow velocity. Shell and tube heat exchangers are designed to minimize pressure loss within acceptable limits. They are also equipped to manage issues like fouling of the shell and tubes, thereby preventing related problems.
The design of shell and tube heat exchangers is highly adaptable to various production processes. Adjustments can be made to pipe diameter, quantity, length, pitch, and arrangement to suit specific application requirements.
The multi-tube structure of shell and tube heat exchangers accommodates thermal expansion between the tubes and shell, making them suitable for handling flammable and toxic fluids.
Shell and tube heat exchangers are essential in the food, beverage, dairy, and pharmaceutical industries for producing consumer products while ensuring safety, efficacy, and consistency. These sectors are regulated by the Food and Drug Administration (FDA), and the equipment used must comply with FDA guidelines and standards.
3-A Standards for the dairy industry are created through the collaboration of equipment manufacturers, professional sanitarians, and product processors. These stakeholders work together to establish the 3-A Sanitary Standards for the dairy, food, and pharmaceutical sectors, emphasizing the importance of clean-in-place (CIP) systems and ensuring equipment can be easily cleaned manually.
3-ASSI upholds 70 sanitary standards for these categories:
API 660 is a standard established by the API that covers the design, materials, fabrication, inspection, testing, and shipping of shell and tube heat exchangers used in the petroleum and petrochemical industries. This standard applies to various equipment, including heat exchangers, condensers, coolers, and reboilers.
TEMA has established the most commonly used standards for shell and tube heat exchangers. The organization provides specific identifications for each configuration of these heat exchangers and categorizes them into three industry types. The criteria for these categories depend on the operational demands of the industry and whether a more robust and durable heat exchanger construction is needed.
ASME Code VIII addresses the pressurized components of a shell and tube heat exchanger, specifically the tubes within the shell. Section VIII is most relevant to these heat exchangers, though Sections II and V may also be applicable, covering material specifications and testing requirements.
Products manufactured in the United States for use internationally must comply with global standards. One such standard is the PED, which applies to shell and tube heat exchangers. The PED guidelines include:
These rules are adopted to ensure the safety of both products and workers.
The CRN (Canadian Registration Number) is issued by a Canadian province or territory to approve a boiler, pressure vessel, or fitting for use within that jurisdiction. In this context, a shell and tube heat exchanger is classified as a pressure vessel. The CRN approval process can be complex, as each province or territory has its own set of requirements.
The CRN classification of heat exchangers is based on factors such as size, fluids, and their pressure and temperature ranges. Specifications vary for lethal and non-lethal substances, with detailed charts guiding manufacturers' designs.
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