[Chapter 1] What is a Shell and Tube Heat Exchanger?
A shell and tube heat exchanger (STHE) is a heat exchanger that consists of a huge cylindrical casing, or shell, with bundles of properly spaced tubing compacted inside. The transmission of heat from one material or medium to another comparable substance or medium is known as heat exchanging. The most common type of heat exchanger is a shell and tube, heat exchanger. Their qualities, tubing type, and other criteria are used to classify them.
Shell and tube heat exchangers are widely used and popular because of their simple design and high heat transfer efficiency. A liquid or steam goes into the shell to heat the tubes in a shell and tube heat exchanger. Heat transmission is thought to be most efficient and effective when four passes through the tubes are made.
[Chapter Two] Design of Shell and Tube Heat Exchangers
Heat exchangers, both shell and tube, are designed utilising sophisticated and technical computer design parameters. The shell, shell cover, tubes, channel, channel cover, tube sheet, baffles, and nozzles are all parts of the device. The Tubular Exchanger Manufacturers Association has set specifications and standards for STHEs (TEMA).
Several pieces of information are required before a shell and tube heat exchanger can be manufactured, including flow rates, intake and output temperatures, pressure, pressure drop, resistance factors, physical qualities of the substances to be treated, line diameters, and shell diameter. More technical criteria are added to these fundamental parameters to establish the optimal technique for producing the proper heat exchanger for the application.
Design of a Shell and Tube Heat Exchanger
Shell
The shell of a shell and tube heat exchanger is built of pipe or welded metal plates made of corrosion-resistant materials that can sustain high temperatures. To reduce the gap between the baffled outer edge and the shell, the inner shell must be circular with a uniform diameter. At lengths of 6 to 24 feet (2 metres to 7 metres), the thickness of the tubes is chosen for pressure, temperature, thermal stress, and corrosion resistance. Longer tubes result in a smaller shell diameter and a higher shell pressure drop.
Heads or Channels
The type of channel or head used in a shell and tube heat exchanger is determined by the application, with bonnet style heads being the most popular for applications where the head is not removed regularly. Flanged or welded removable cover channels are required for maintenance. A removable channel cover is required when regular examination of the channel and tubes is required.
Tubes
Tubes in a shell and tube heat exchanger are constructed of carbon steel, stainless steel, titanium, Inconel, or copper and are welded or extruded. Compact heat exchangers have tube diameters of 0.625 inches (16mm), 0.75 inches (19mm), or one inch (25mm). At lengths of 6 to 24 feet (2 metres to 7 metres), the thickness of the tubes is chosen for pressure, temperature, thermal stress, and corrosion resistance. Longer tubes result in a smaller shell diameter and a higher shell pressure drop.
Sheet Tube
The tube sheet is a perforated plate or sheet that is meant to hold the tubes on each end of the cylindrical shell and is perforated with holes for the insertion of pipes or tubes. The shell extends beyond the tube sheets and is sealed on both ends to create the enclosed chamber that the heads cover.
Expansion Joint
The goal of a heat exchanger is to transfer heat from a hot material on one side to a cold substance on the other. As a result, units are frequently exposed to a broad variety of temperatures.
Heat causes materials to lengthen, whereas cooling causes them to compress. Materials in the exchanger might be strained to the point of failure if there is no room for expansion and contraction. Tubes commonly buckle or tear out of the tube sheets as a result of this, but it can also induce buckling in the shell or nozzle connection distortion. All of these negative effects endanger the exchanger's integrity, making it potentially dangerous to use.
An expansion joint is used in this situation. This flexible part is designed to endure temperature and pressure variations.
Pitch of the Tube
The distance between the centre of one tube and the centre of another tube is known as tube pitch. The tubes are arranged in a triangle or square arrangement, with the square shape being the easiest to clean and causing the least turbulence. Unlike triangular and rotational square pitch tubing, the square pitch design enables vapours to rise between the tubes.
Baffles
Baffles are used to control the flow in the shell and tube sides so that the fluid velocity rises to a high enough level to achieve a high heat transfer coefficient and considerably prevent fouling, which is the formation of undesired materials on the heat transfer surface. Fouling builds up in heat exchangers, increasing heat transfer resistance and resulting in poor heat exchanger performance. Baffles support the tubes in horizontal shells and tube heat exchangers and prevent sagging or vibration damage.
Spacers and Tie Rods
Tie rods and spacers are structural and support components that keep the baffles in place and maintain the distance between them. The number of baffles and the diameter of the shell dictates the number of rods and spacers. Tie rods are fastened to the tube sheet and stretch the length of the bundle all the way to the final baffle.
[Chapter Three] How Shell and Tube Heat Exchangers Work
A shell and tube heat exchanger has a basic idea and operation that is based on the flow and thermal contact of two liquids. The method of exchanging temperature between two fluids is described by the name of a shell and tube heat exchanger. A heated or hot fluid flows around a cold fluid in a heat exchanger, transferring heat in the direction of the cold fluid's movement.
There will be a heat exchange or transfer across a conductive surface in every case when two pieces of material make touch. A shell and tube heat exchanger is a device that allows two fluids to exchange or transfer heat through the use of conductive metals.
What Are Shell and Tube Heat Exchangers and How Do They Work?
One fluid runs through the tubes while the other flows through the shell in a shell and tube heat exchanger. The intake for the shell fluid is at the top of the schematic of a straight tube shell and tube heat exchanger, while the inlet for the tube fluid is at the bottom right.
The shell side and the tube side of a shell and tube heat exchanger are two compartments or portions. Fluid allocation is vital when dealing with a shell and tube heat exchanger since it determines which side the hot fluid will enter and which side the cool fluid will enter.
Because the tubes are intended to bear high pressure, the lower pressure fluid enters via the shell intake when there is a pressure differential between the fluids.
Side of the Shell
When estimating fluid flow on the shell side, keep in mind that the shell is more expensive to build than the tubes, and cleaning it is more difficult. The fluid flow is directed across the tube bundles by baffles on the shell side.
Fluids that are viscous or have a high flow rate are handled on the shell side, where there is more turbulence and a higher transfer coefficient, resulting in better heat transfer. On the shell side, large temperature variations are usually made.
Side of the Tube
On the tube side, turbulent flow is required, which is accomplished by placing turbulators within the tubes through perforations in the tube sheet. Turbulence in the tubes, like turbulence in the shell, improves heat transmission capacity. The turbulators also serve to keep the insides of the tubes clean and free of debris. Tubes create a streamlined flow with less turbulence and pressure drop.
Passes
One, two, four, six, or eight passes are marked 1-1, 1-2, 1-4, and so on in shell and tube heat exchangers. The first number indicates how many shells there are. The number of passes is the second number. The number of passes refers to the number of times the fluid flows through the shell fluid. The fluid only passes through the shell once in a single pass heat exchanger. The heat transfer coefficient increases as the number of passes increases.
A Shell and Tube Heat Exchanger in Use
Cold fluid comes by tube or shell input and is heated by conductive heating in the tubes before being treated, as shown in the diagram. A two-pass shell and tube heat exchanger is shown below.
Turbulent flow in a shell and tube heat exchanger improves heat transfer rates while also preventing fouling in the tubes and shell walls. The self-cleaning action of the continuously turbulent flow assures ongoing and optimal performance. Turbulence is created by baffles that guide the flow in the shell, while turbulators put in the tubes generate turbulation.
Thermal contact between the two fluids enclosed in the shell and tubes aids the heat exchange process. One of the fluids evaporates at a lower temperature, whereas the other evaporates at a higher temperature.
[Chapter Four] Types of Shell and Tube Heat Exchangers
The Tubular Exchangers Manufacturers Association has established guidelines for the production, design, and construction of shell and tube heat exchangers (TEMA). Class B, Class C, and Class R are the three categories of standards. The architecture and structure of the shell, as well as the sort of service it offers, define the categorization of a shell and tube heat exchanger.
TEMA Classifications:
· Chemical Processing (Class B)
· General Commercial Applications (Class C)
· Petroleum and Large-Scale Applications (Class R)
TEMA classifies and identifies shell and tube heat exchangers based on the front end or head, the back end, and the shell. Each form of shell and tube heat exchanger is classified and described using the columns and rows of the chart below.
TEMA devised a three-letter identification system—BEM, AEM, or NEN—for a straight tube and fixed tube sheet shell and tube heat exchanger to make different designs and assemblies easier to identify.
The first letter is a description for the front end stationary head type, and it indicates whether the tube sheet is bolted or welded to the shell and channel.
The position of the inlets and outlets, as well as the existence of longitudinal baffles and distribution plates, are defined by the second letter, which denotes the kind of shell.
The third letter denotes the kind of rear-end head, which comprises the connection between the shell and the second tube sheet, as well as the channel closing method (bolted or welded).
A BEM shell and tube heat exchanger using the TEMA system comprises a bonnet header, one pass shell, and a fixed tube sheet.
Types of Flow
The division of shell and tube heat exchangers into groups based on their features is a part of the categorization process for a clearer understanding of their function and operation. The flow type is one of the criteria used to categorise them. Shell and tube heat exchangers have three flow types: parallel, counter, and cross. The three flow types must be utilised in conjunction for the design, operation, and applications.
When the shell and tube sides enter the heat exchanger at the same end and flow directly to the opposite end, this is known as parallel flow. Each fluid's temperature changes by the same amount and increases or decreases by the same amount.
When fluids flow in opposing directions, they enter the heat exchanger at opposite ends and exit at opposite ends, which is known as counter flow. The most common and effective form of heat exchanger is the counter flow. The fluids flow perpendicular to each other at a 90o angle in a cross flow shell and tube heat exchanger. One of the fluids in a cross heat exchanger changes state (similar to how cooling water absorbs steam in a steam system condenser), and is subsequently absorbed by the fluid that has stayed in its liquid form.
Types of Shell and Tube Heat Exchangers
TEMA Type M Fixed Tube Sheet
Straight tubes are fastened at both ends to stationary tube sheets that are welded to the shell in a fixed tube sheet heat exchanger. A straight tube heat exchanger is the least expensive form of heat exchanger due to its basic design and construction. It can't handle high temperature differences between the fluids, but that can be addressed by adding an expansion joint. The fixed tube sheet heat exchanger has the benefit of being simple to clean and maintain.
Heat Exchanger with U Tubes
The tubes' arrangement reveals the name of the U tube shell and tube heat exchanger. At one end of the heat exchanger, the tube inlet and exit valves are positioned. Fluids enter at the top and escape at the bottom of the tube sheet. A U tube heat exchanger can manage high-temperature variations because the floating tubes at the U turn end of the exchanger may expand. The input and output valves for a U tube shell and tube heat exchanger differ depending on the heat exchanger's design. The shell fluid input is on the top left and the outlet is on the bottom right in the diagram below.
TEMA Type S Floating Head Heat Exchanger
The floating head is similar to the U tube design except that the tubing is not in the U form. The tubing is joined to the stationary tube sheet on one end. The opposite end is open, allowing it to expand and float. As the tubes expand, the floating head can resist large temperature variations. Because the tubes can be quickly removed, this form of heat exchanger is simple to clean and examine.
There are four different sorts of floating head designs to choose from:
1. TEMA P Outside Packed Stuffing Box
2. TEMA W Outside Packed Lantern Ring
3. TEMA S Floating Head with Backing Device
4. TEMA T Pull Through
Type T of TEMA or Type AKT of TEMA
The tube bundle may be dragged out of the shell, and the baffle outer diameter and the main shell inner diameter have an irregular clearance.
Type S of TEMA or Type AES of TEMA
The tube bundle must first be dismantled before it can be removed. The distance between the baffle diameter and the inner diameter of the shell is typical.
Type P or Type AEP of the TEMA
Packing rings pressed into a stuffing box seal the shell side, allowing the tube sheet to glide back and forth.
Type W TEMA or Type AJW TEMA
O rings seal the fluids, which are separated by a lantern ring.
[Chapter Five] Benefits of Shell and Tube Heat Exchangers
Shell and tube heat exchangers are utilised in a number of applications and industries to satisfy their demands. They may be customised to meet the needs of any manufacturing or production business since they come in a variety of configurations.
Shell and tube heat exchangers are used in the flow of processing equipment in refineries and industries to provide a smooth and effective heat transfer. 65 per cent of heat exchangers on the market are shell and tube, heat exchangers. Cost Advantages of Shell and Tube Heat Exchangers
The cost of shell and tube heat exchangers is a significant advantage. They are considerably less costly than plate coolers.
Capacity of Heat
Heat exchangers must be able to withstand a wide range of temperatures, which varies depending on the application. Their capacity to withstand severe temperatures aids in maintaining production and operations. Heat exchangers made of shell and tube have a high temperature working capability and may be customised to match any situation.
Pressure
The high pressure causes serious issues and results in a loss of output. Shell and tube heat exchangers' shells and tubes have been tested and engineered to tolerate high-pressure variations. Loss of Pressure
Pressure loss is an energy loss that produces downstream pressure loss, slowing the flow velocity. Shell and tube heat exchangers are made to handle pressure loss and maintain it to a bare minimum.
Pressure Loss
Pressure loss has an impact on a number of factors, one of which is fouling of the shell and tubes. This difficulty is solved by using shell and tube heat exchangers, which have a low-pressure loss.
Anodes
A protective layer on the surface of a shell and tube heat exchanger is created by sacrificial anodes in the end covers. The anodes' aim is to apply cathodic protection to heat exchanger sections in order to extend their life. Anodes are used to protect the framework from erosion and corrosion. They are mounted on tube sheets, tube ends, and channel sections.
Adjustments
Heat exchangers made of shell and tube may be customised to fit any industrial process. Pipe diameter, number of pipes, length of pipes, pipe pitch, and pipe arrangement may all be changed to meet the demands of a given application.
Expansion of Heat
Shell and tube heat exchangers have a multi-tube design that allows for thermal expansion between the tubes and the shell. The heat exchanger can now handle flammable and poisonous fluids thanks to this arrangement.
[Chapter Six] Standards and Regulations for Shell and Tube Heat Exchangers
Shell and tube heat exchangers are used extensively in the food, beverage, dairy, and pharmaceutical sectors to guarantee that their consumer goods are safe, effective, and consistent. The Food and Drug Administration (FDA) keeps a tight eye on these four businesses to ensure customer safety. The FDA's requirements and regulations must be followed by the equipment they utilise.
Standards and Regulations for Shell and Tube Heat Exchangers
3-A Sanitary Requirements (3-ASSI)
3-A Dairy industry standards are developed via collaboration between equipment fabricators, professional sanitarians, and product processors. The 3-A Sanitary Standards for the dairy, food and pharmaceutical sectors are developed in collaboration by these three organisations. Their main focus is on keeping equipment clean in place (CIP) and making it easy to clean manually.
3-The ASSI has 70 hygienic criteria for the following categories.
- Vessels' Fillers
- Fittings & Valves
- Mixers & Pumps
- Heat Exchangers
- Conveyors & Feeders
- Instruments
- Concentrating Devices
- Equipment for Farm/Raw Milk
- Equipment for Cheese & Butter
Standard 660 of the American Petroleum Institute (API660)
The API 660 standard covers the design, materials, manufacture, inspection, testing, and shipment of shell and tube heat exchangers for use in the petroleum and petrochemical industries. Heat exchangers, condensers, coolers, and reboilers are all covered by the regulations.
Manufacturers of Tubular Exchangers Association (TEMA)
TEMA produced the most extensively used set of standards for shell and tube heat exchangers. The organisation has created identifications for every form of shell and tube heat exchanger, as well as three industrial classifications. The distinctions between the categories are determined by the industry's working nature and if it needs a heavier-duty and more robust heat exchanger design.
American Society of Mechanical Engineers (ASME)
The pressurised sections of a shell and tube heat exchanger, which are the tubes inside the shell, are referred to as ASME Code VIII by the American Society of Mechanical Engineers (ASME). Provision VIII is the section that applies the most to shell and tube heat exchangers, with sections II and V being enforced on occasion. Materials and tests are specified in these sections.
Pressure Equipment Directive (PED)
Products made in the United States but intended for use elsewhere in the globe must comply with international standards. One of the worldwide standards for shell and tube heat exchangers is the PED. The PED regulations cover the following topics:
- Materials
- Standards that are consistent
- Requirements essential
- Market monitoring
- Assessment of conformity
The establishment of these guidelines is aimed at ensuring the safety of both products and workers.
Conclusion
- A shell and tube heat exchanger (STHE) is a type of heat exchanger that is made up of a large cylindrical enclosure, or shell, with tubing bundles compacted inside.
- Shell and tube heat exchangers are widely used and popular owing to their simple design and high heat transfer efficiency.
- The shell, shell cover, tubes, channel, channel cover, tube sheet, baffles, and nozzles are all parts of a shell and tube heat exchanger.
- One fluid runs through the tubes while the other flows through the shell in a shell and tube heat exchanger.
- The Tubular Exchangers Manufacturers Association establishes guidelines for the production, design, and construction of shell and tube heat exchangers (TEMA).
