This article covers everything you need to know about shell and tube heat exchangers.
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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. They are classified according to their properties, tubing type, and other characteristics.
The use and popularity of shell and tube heat exchangers is due to the simplicity of their design and exceptionally efficient heat exchange rate. The process of a shell and tube heat exchanger involves the use of a liquid or steam that flows into the shell to heat the tubes. Four passes through the tubes is considered to be the most efficient and effective method of heat transfer.
Shell and tube heat exchangers are engineered using sophisticated and technical computer design parameters. The components of the device include the shell, shell cover, tubes, channel, channel cover, tube sheet, baffles, and nozzles. The specifications and standards for STHEs have been established by the Tubular Exchanger Manufacturers Association (TEMA).
Prior to producing a shell and tube heat exchanger, there are several pieces of data manufacturers require such as flow rates, inlet and outlet temperatures, pressure, pressure drop, resistance factors, physical properties of substances to be processed, line sizes, and the shell diameter. Added to these basic factors are more technical requirements that are used to determine the best process to produce the correct heat exchanger for the application.
The shell of a shell and tube heat exchanger is made of pipe or welded metal plates using materials that can withstand extreme temperatures and that are corrosion resistant. The inner shell must be round with a consistent diameter to minimize space between the baffled outer edge and the shell.
The type of channel or head depends on the use of the shell and tube heat exchanger, with bonnet type heads being the most common for use in applications where the head does not have to be removed frequently. Removable cover channels, necessary for maintenance, are flanged or welded. When frequent examination of the channel and tubes is necessary, a removable channel cover is necessary.
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.
A heat exchanger's purpose is to exchange heat from a hot substance on one side of the exchanger with a cold substance on the other side. Because of this, units are often subjected to wide ranges of temperatures.
Materials that are heated grow in length, while materials that are cooled contract. If there is no allowance for the expansion and contraction, materials in the exchanger could be stressed to the point of failing. This usually results in tubes buckling or being torn out of the tubesheets, but could also cause buckling in the shell or distortion of nozzle connections. All these side effects jeopardize the integrity of the exchanger, potentially making it unsafe to operate.
This is where an expansion joint comes in. This flexible element is built to withstand the changes in temperature and pressure.
Tube pitch is the distance from the center of a tube to the center of an adjacent tube. Tubes are laid out in a triangular or square pattern with the square pattern configuration being the easiest to clean and producing the least amount of turbulence. The square pitch arrangement allows vapors to rise between the tubes, an advantage over triangular and rotated square pitch tubing.
Baffles are used to direct the flow in the shell and tube sides such that the fluid velocity increases sufficiently to reach a high heat transfer coefficient and significantly reduce fouling, which is when unwanted materials form on the heat transfer surface. The accumulation of fouling increases heat transfer resistance and leads to poor heat exchanger performance. In horizontal shell and tube heat exchangers, the baffles support the tubes and prevent sagging or vibration damage.
Tie rods and spacers are supports and structural components designed to hold the baffles in place as well as maintain the space between the baffles. The number of rods and spacers is determined by the number of baffles and the diameter of the shell. Tie rods are screwed to the tube sheet and extend a bundle‘s length to the last baffle.
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.
In any situation where two pieces of material make contact, there will be an exchange or transfer of heat through a conductive surface. 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. In the diagram below, which is of a straight tube shell and tube heat exchanger, the shell inlet for the shell fluid to enter is at the top with the inlet for the tube fluid at the bottom right.
A shell and tube heat exchanger has two compartments or sections: the shell side and the tube side. When working with a shell and tube heat exchanger, it is important to decide on which side the hot fluid will enter and on which the cold fluid will enter; this decision is referred to as fluid allocation.
When there is a difference in pressure between the fluids, the lower pressure fluid enters through the shell inlet since the tubes are designed to handle high pressure.
When determining fluid flow for the shell side, it is important to know that the shell is more expensive to fabricate than the tubes, and it is more difficult to clean. The shell side has baffles that direct the flow of the fluid across the tube bundles.
Viscous fluids and fluids with a high flow rate are processed through the shell side where there is increased turbulence and an increased transfer coefficient that leads to improved heat transfer. Large temperature changes are normally performed on the shell side.
A necessity on the tube side is to create turbulent flow; this is achieved by installing turbulators inside the tubes through the holes in the tube sheet. Like the turbulence in the shell, the turbulence in the tubes increases the heat transfer capacity. A secondary function of the turbulators is to keep the inside of the tubes clean and unfouled. Tubes have lower turbulence and pressure drop and provide a streamlined flow.
Shell and tube heat exchangers can have one, two, four, six, or eight passes, which are written 1-1, 1-2, 1-4, and so on. The first number is the number of shells. The second number is the number of passes. The number of passes is the number of times the fluid passes through the fluid in the shell. In a single pass heat exchanger, the fluid goes through the shell once. As the number of passes increases, the heat transfer coefficient increases.
As can be seen in the graphic, cold fluid enters through the tube or shell inlet and is heated by conductive heating in the tubes; it then exits processed. The image below is a two pass shell and tube heat exchanger.
Turbulent flow in a shell and tube heat exchanger increases the heat transfer rate and avoids fouling in the tubes and shell walls. The constant turbulent flow has a self-cleaning effect that ensures continuous and optimum performance. Baffles that direct the flow in the shell create its turbulence while turbulators installed in the tubes create its turbulation.
The heat exchange process is facilitated by thermal contact between the two fluids contained in the shell and tubes. One of the fluids leaves cooler while the other leaves warmer or hot.
The stipulations for the manufacture, design, and construction of shell and tube heat exchangers have been outlined by the Tubular Exchangers Manufacturers Association (TEMA). The standards are divided into three categories: Class B, Class C, and Class R. The classification of a shell and tube heat exchanger is determined by the construction and structure of the shell and the type of service it provides.
TEMA Categories:
TEMA uses the front end or head, rear end, and the shell as a method of classifying and identifying shell and tube heat exchangers. The columns and rows of the chart below are used to classify and describe each type of shell and tube heat exchanger.
For easy identification of the different designs and assemblies, TEMA developed a three letter identification system—BEM, AEM, or NEN—for a straight tube and fixed tube sheet shell and tube heat exchanger.
The first letter is the descriptor for the front end stationary head type that identifies how the tube sheet is connected to the shell and channel—bolted or welded.
The second letter is the type of shell and defines the position of the inlets and outlets and the presence of longitudinal baffles and distribution plates.
The third letter identifies the rear end head type and includes the connection of the shell with the second tube sheet and the type of channel closure—bolted or welded.
Using the TEMA system, a BEM shell and tube heat exchanger has a bonnet header, one pass shell, and a fixed tube sheet.
Part of the classification process for shell and tube heat exchangers is dividing them into groups using their characteristics for easier understanding of their function and operation. One of the characteristics used to group them is their flow type.
The three flow types of shell and tube heat exchangers are parallel, counter, and cross. The design, operation, and applications necessitate the three flow types be used in combinations.
Parallel flow is when the shell and tube sides enter the heat exchanger at the same end and flow directly to the opposite end. The temperature change is the same for each fluid and increases or reduces by the same amount.
Counter flow is when the fluids are flowing in opposite directions, enter the heat exchanger at opposite ends, and discharge at opposite ends. The counter flow is the most popular and efficient type of heat exchanger.
In a cross flow shell and tube heat exchanger, the fluids flow perpendicular to each other at a 90o angle. One of the fluids in a cross heat exchanger changes state (just as in a steam system condenser where cooling water absorbs the steam), then is absorbed by the fluid that has remained in its liquid state.
A fixed tube sheet heat exchanger has straight tubes secured at both ends to the stationary tube sheets that are welded to the shell. A straight tube exchanger has a simple design and construction and is the least expensive type of heat exchanger. It is unable to accommodate large temperature variances between the fluids, which can be fixed by adding an expansion joint. An advantage of the fixed tube sheet heat exchanger is how easy it is to clean and maintain.
The name of the U tube shell and tube heat exchanger can be seen in the configuration of the tubes. The inlet and outlet valves for the tubes are located at one end of the heat exchanger. Fluids enter at the top of the tube sheet and exit through the lower half. The floating tubes at the U turn end of the exchanger can expand, which gives a U tube heat exchanger the ability to handle high temperature variances.
The inlet and outlet valves for a U tube shell and tube heat exchanger vary according to the design of the heat exchanger. In the diagram below, the shell fluid inlet is on the top left and its exit is on the bottom right.
The floating head design is similar to the U tube design without the U shape of the tubing. One end of the tubing is attached to the stationary tube sheet. The other end is unsecured, which allows it to expand and float. The floating head is capable of withstanding high temperature variances as the tubes expand. This type of heat exchanger is easy to clean and inspect since the tubes can be easily removed.
There are four types of floating head designs:
The tube bundle can be pulled out of the shell and has an abnormal clearance between the baffle outside diameter and the main shell inner diameter.
The removal of the tube bundle requires the dismantling of the tube bundle. It has normal clearance between the baffle diameter and shell inner diameter. .
The shell side is sealed with packing rings pressed into a stuffing box; this allows the tube sheet to slide back and forth. .
The fluids are sealed by O rings separated by a lantern ring.
There are applications that require heat transfer for viscous and sticky substances where the product accumulates on the internal surface of the heat exchanger. Scraped surface heat exchangers are designed to meet the needs of such applications and have scraping blades that remove built up materials from the internal surface of the heat exchanger. They are designed like other heat exchangers with the inclusion of rows of scraping blades inside the cylinder.
The blades of a scraped surface heat exchanger are spring loaded and spinning as they rub the surface and drain liquid from the exchanger. The typical number of blades is four but any number of blades can be used. The mitigating factor is as the number of blades increases so does the cost of the scraped surface heat exchanger. In essence, a greater number of blades is not necessary since the amount of time between scrapings is minimal.
Scraped surface heat exchangers can be mounted horizontally or vertically with the vertical design being preferred allowing liquids to flow downward due to gravity.
Shell and tube heat exchangers are used for a variety of applications and meet the needs of an assortment of industries. Since they are available in different configurations, they can be adapted to the requirements of any manufacturing or production operation.
Shell and tube heat exchangers are incorporated into the flow of processing equipment in refineries and factories to allow the smooth and efficient transfer or exchange of heat. Shell and tube heat exchangers account for 65% of heat exchangers on the market.
An important benefit of shell and tube heat exchangers is their cost. They are much less expensive than plate type coolers.
Heat exchangers have to be able to handle a wide range of temperatures, varying by application. Their ability to deal with extreme temperatures helps maintain production and keep operations moving. Shell and tube heat exchangers have a high temperature working capacity and can be adapted to fit any conditions.
The high pressure of a shell and tube heat exchanger requires the use of thick materials that makes the exchanger very heavy or too expensive if nickel alloys are used. High pressure creates major problems and leads to a loss of production. The shell and tubes of shell and tube heat exchangers are tested and designed to withstand the extremes caused by pressure variances and adhere to the Codes of the ASME and PED.
Pressure loss is a loss of energy and causes downstream pressure loss that slows the velocity of flow. Shell and tube heat exchangers are designed to deal with pressure loss and keep it to a minimum within the design criteria. There are several variables that are affected by pressure loss, one of them being a fouling of the shell and tubes. With the minimal pressure loss allowed by shell and tube heat exchangers, this problem is eliminated.
The design of shell and tube heat exchangers can be adjusted for adaptation to any production process. Changes in pipe diameter, number of pipes, length of pipes, pipe pitch, and pipe arrangement can be altered to specifically fit the needs of an application.
The multi-tube design of shell and tube heat exchangers allows for thermal expansion between the tubes and shell. This configuration gives the heat exchanger the ability to handle flammable and toxic fluids.
Food, beverage, dairy, and pharmaceutical industries rely on the use of shell and tube heat exchangers for the production of their consumer products and to ensure their products are safe, effective, and consistent. These four industries are closely monitored by the Food and Drug Administration (FDA) for the safety of consumers. The equipment they use must follow the guidelines and standards established by the FDA.
3-A Standards for the dairy industry are produced through the cooperation of equipment fabricators, professional sanitarians, and product processors. These three groups collaborate to develop the 3-A Sanitary Standards for dairy, food, and pharmaceutical industries. Their focus is on keeping equipment clean in place (CIP) and ensuring that it can be easily and manually cleaned.
3-ASSI maintains 70 sanitary standards for these categories:
API660 is a standard developed by the API regarding the design, materials, fabrication, inspection, testing, and shipping of shell and tube heat exchangers for use by the petroleum and petrochemical industries. The standards apply to heat exchangers, condensers, coolers, and reboilers.
The most widely used set of standards for shell and tube heat exchangers have been established by TEMA. The organization has developed identifications for every configuration of shell and tube heat exchanger as well as three categorizations for industry types. The stipulations between the categories are dependent on the working nature of the industry and whether it requires a heavier duty and more durable heat exchanger construction.
ASME Code VIII refers to the pressurized parts of a shell and tube heat exchanger, which are the tubes inside the shell. Section VIII is the section that is most strongly applied to shell and tube heat exchangers, with sections II and V occasionally being enforced. These sections stipulate materials and testing.
When products are manufactured in the United States but will be used elsewhere in the world, they must adhere to international standards. The PED is one of the international standards that applies to shell and tube heat exchangers. Included in the PED rules are:
The purpose of the adoption of these rules is the safety of products and workers.
The CRN is a number that is given by a Canadian province or territory regarding a boiler, pressure vessel, or fitting that has been approved for use in a province or territory. A shell and tube heat exchanger is defined as a pressure vessel in this context. The specifications for heat exchangers under the CRN approval system can be confusing since every province has their own unique requirements.
The CRN classification of heat exchangers is dependent on their size, fluids, and their pressure and temperature range with specifications for lethal substances and nonlethal substances that are specified by charts that guide manufacturer’s designs.
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