BLOG
Specializing in the research, development, production, sales, equipment consulting, maintenance, and accessory services of heat exchange technology, equipment, and systems
Category >
25
2018
-
06
Knowledge of heat exchangers
Author:
Summary:
Heat exchangers can be classified by material into metal heat exchangers and ceramic heat exchangers. Metal heat exchangers can be further divided by structure into wall-type heat exchangers and mixed-type heat exchangers. Ceramic heat exchangers can be classified by structure into single-loop heat exchangers and multi-loop heat exchangers. Classification of metal heat exchangers: Types of wall-type heat exchangers include jacketed heat exchangers, which are made by installing a jacket on the outer wall of a container, featuring a simple structure; however, the heating surface is limited by the container wall, resulting in a low heat transfer coefficient. To improve the heat transfer coefficient and ensure uniform heating of the liquid inside the vessel, a stirrer can be installed inside the vessel. When cooling water or a non-phase-changing heating agent is introduced into the jacket, it can also be set in the jacket.
Heat exchangers can be divided into metal heat exchangers and ceramic heat exchangers based on material.
Metal heat exchangers can be classified into wall-type heat exchangers and mixed-type heat exchangers based on structure.
Ceramic heat exchangers can be classified into single-loop heat exchangers and multi-loop heat exchangers based on structure.
Classification of metal heat exchangers.
Types of wall-type heat exchangers.
Jacketed heat exchangers are made by installing a jacket on the outer wall of the container, with a simple structure; however, the heating surface is limited by the container wall, and the heat transfer coefficient is not high. To improve the heat transfer coefficient and ensure uniform heating of the liquid inside the kettle, a stirrer can be installed inside the kettle. When cooling water or a non-phase-changing heating agent is introduced into the jacket, spiral baffles or other measures to increase turbulence can also be set in the jacket to enhance the heat transfer coefficient on one side of the jacket. To supplement the insufficient heat transfer area, snake tubes can also be installed inside the kettle. Jacketed heat exchangers are widely used for heating and cooling in reaction processes.
Immersion snake tube heat exchangers are made by bending metal tubes into various shapes that adapt to the container and immersing them in the liquid inside the container. The advantages of snake tube heat exchangers are their simple structure, ability to withstand high pressure, and the possibility of being made from corrosion-resistant materials; their disadvantages include low turbulence of the liquid inside the container and a small heat transfer coefficient outside the tube. To improve the heat transfer coefficient, a stirrer can be installed inside the container.
Spray heat exchangers are fixed in rows on a steel frame, with hot fluid flowing inside the tubes and cooling water sprayed evenly from above, hence also called spray coolers. The outer layer of the spray heat exchanger has a high turbulence liquid film, significantly increasing the heat transfer coefficient compared to immersion types. Additionally, these heat exchangers are mostly placed in areas with air circulation, where the evaporation of cooling water also removes some heat, helping to lower the cooling water temperature and increase the driving force for heat transfer. Therefore, compared to immersion types, the heat transfer effect of spray heat exchangers is greatly improved.
The annular heat exchanger is made of concentric tubes of different diameters connected by U-shaped elbows. In this type of heat exchanger, one fluid flows inside the tube while another flows in the annular space, both achieving relatively high flow rates, thus resulting in a large heat transfer coefficient. Additionally, in annular heat exchangers, the two fluids can flow in pure countercurrent, leading to a larger logarithmic mean driving force. The structure of annular heat exchangers is simple, can withstand high pressure, and is convenient to use (the number of tube sections can be increased or decreased as needed). Especially because annular heat exchangers have the advantages of a large heat transfer coefficient, large driving force for heat transfer, and the ability to withstand high pressure, almost all heat exchangers used in ultra-high pressure production processes (for example, in high-pressure polyethylene production processes with an operating pressure of 3000 atmospheres) are annular types.
Shell-and-tube heat exchangers (also known as tube bundle heat exchangers) are the most typical wall-type heat exchangers, with a long history of industrial application and still dominating among all heat exchangers today.
Shell-and-tube heat exchangers mainly consist of a shell, tube bundle, tube sheets, and end caps, with the shell often being circular and containing parallel tube bundles fixed at both ends to the tube sheets. In shell-and-tube heat exchangers, one fluid flows inside the tubes, referred to as the tube side; the other fluid flows outside the tubes, referred to as the shell side. The wall surface of the tube bundle serves as the heat transfer surface. To improve the heat transfer coefficient of the fluid outside the tubes, a certain number of transverse baffles are usually installed inside the shell. Baffles not only prevent fluid short-circuiting and increase fluid velocity but also force the fluid to flow through the tube bundle multiple times along a specified path, greatly increasing turbulence. Commonly used baffle shapes include segmental and disc shapes, with the former being more widely used. Each time the fluid passes through the tube bundle is called one tube pass, and each time it passes through the shell is called one shell pass. To increase the fluid velocity inside the tubes, appropriate partitions can be set in the end caps to evenly divide all tubes into several groups. This way, the fluid can pass through only part of the tubes each time and return through the tube bundle multiple times, referred to as multi-tube passes. Similarly, to increase the fluid velocity outside the tubes, longitudinal baffles can be installed in the shell to allow the fluid to pass through the shell space multiple times, referred to as multi-shell passes. In shell-and-tube heat exchangers, due to the different temperatures of the fluids inside and outside the tubes, the temperatures of the shell and tube bundle also differ. If the temperature difference is significant, large thermal stresses may occur inside the heat exchanger, potentially causing the tubes to bend, break, or detach from the tube sheets. Therefore, when the temperature difference between the tube bundle and the shell exceeds 50°C, appropriate temperature difference compensation measures should be taken to eliminate or reduce thermal stress.
Mixed-type heat exchangers.
Mixed heat exchangers rely on the direct contact of cold and hot fluids for heat transfer, avoiding the thermal resistance of the heat transfer wall and the dirt on both sides. As long as the contact between the fluids is good, there will be a large heat transfer rate. Therefore, in any situation where fluid mixing is allowed, mixed heat exchangers can be used, such as in gas scrubbing and cooling, cooling of circulating water, mixed heating between steam and water, and condensation of steam, etc. Its applications are widespread in chemical and metallurgical enterprises, power engineering, air conditioning engineering, and many other production sectors.
According to different uses, mixed heat exchangers can be divided into the following types:
(1) Cooling towers (also known as cooling water towers).
In this type of equipment, water that has been heated during production is cooled and then recycled using natural or mechanical ventilation methods to improve the economic efficiency of the system. For example, cooling water from thermal power plants or nuclear power stations is cooled in water cooling towers before being recycled. This method has been widely used in practical engineering.
(2) Gas scrubbers (also known as scrubbers).
In industry, this type of equipment is used for various purposes to wash gases, such as absorbing certain components from gas mixtures with liquids, removing dust from gases, and humidifying or drying gases. However, its most widespread use is for cooling gases, with water being the most common liquid used for cooling. The spray chamber, widely used in air conditioning engineering, can be considered a special form of this equipment. The spray chamber can cool air like a gas washing tower, and it can also heat the air. However, it has disadvantages such as high water quality requirements, large footprint, and high energy consumption by water pumps. Therefore, it is not commonly used in general buildings or is only used as a humidification device. However, it is still widely used in textile factories, cigarette factories, and other places where humidity regulation is the main purpose!
(3) Jet-type heat exchanger
In this type of equipment, a high-pressure fluid is ejected from a nozzle, forming a very high speed, and a low-pressure fluid is introduced into a mixing chamber to come into direct contact with the jet for heat transfer, and both enter the diffusion tube, where they reach the same pressure and temperature at the outlet of the diffusion tube before being delivered to the user.
(4) Mixed-type condenser
This type of equipment generally uses direct contact between water and steam to condense the steam.
Thermal storage heat exchanger
The thermal storage heat exchanger is used for thermal storage heat exchange equipment. It is filled with solid materials to store heat. It is generally constructed with refractory bricks to form a fire grid (sometimes using metal corrugated strips, etc.). Heat exchange occurs in two stages. In the first stage, hot gas passes through the fire grid, transferring heat to the fire grid for storage. In the second stage, cold gas passes through the fire grid, receiving the stored heat from the fire grid and being heated. These two stages alternate. Typically, two thermal storage units are used alternately, meaning when hot gas enters one unit, cold gas enters the other. It is commonly used in the metallurgical industry, such as in the thermal storage chamber of a steelmaking furnace. It is also used in the chemical industry, such as in air preheaters or combustion chambers in gas furnaces, and in thermal storage cracking furnaces in synthetic oil plants.
Thermal storage heat exchangers are generally used in situations where the requirements for medium mixing are relatively low.
Ceramic heat exchanger
The ceramic heat exchanger is a new type of tubular high-temperature heat recovery device, mainly composed of silicon carbide, which can be widely used in metallurgy, machinery, building materials, chemical industry, and other sectors to directly recover waste heat from high-temperature flue gas emissions of 850-1400℃ from various industrial kilns, to obtain high-temperature combustion air or process gas.
The heat exchange element material of this developed device is a new type of silicon carbide engineering ceramic, which has excellent properties of high-temperature resistance and thermal shock resistance, showing no cracks after being cooled from 1000℃ to room temperature more than 50 times; its thermal conductivity is equivalent to that of stainless steel; it has good corrosion resistance in oxidative and acidic media. Structurally, it successfully solves the thermal compensation and effectively addresses the gas sealing issues. This device has high heat transfer efficiency and significant energy-saving effects, used for preheating combustion air or heating certain process gases, which can save primary energy, with fuel savings of up to 30%-55%, and can enhance the process, significantly improving production capacity.
The production process of the ceramic heat exchanger is basically the same as that of kiln tools, with thermal conductivity and oxidation resistance being the main application properties of the material. Its principle is to place the ceramic heat exchanger close to the flue outlet where the temperature is higher, without the need for cold air mixing and high-temperature protection. When the kiln temperature is 1250-1450℃, the flue outlet temperature should be 1000-1300℃, and the ceramic heat exchanger can recover waste heat up to 450-750℃, sending the recovered hot air into the kiln to mix with gas for combustion, thus directly reducing production costs and increasing economic benefits.
The ceramic heat exchanger has developed well under the limitations of metal heat exchangers because it better addresses issues of corrosion resistance and high-temperature resistance. Its main advantages are: good thermal conductivity, high-temperature strength, good oxidation and thermal shock resistance. It has a long lifespan, low maintenance, reliable and stable performance, and is easy to operate.
Design requirements
With the development of the economy, various types and kinds of heat exchangers are developing rapidly, and new structures and new materials of heat exchangers are constantly emerging. To meet the needs of development, China has established standards for certain types of heat exchangers, forming a series. A well-designed heat exchanger should meet the following basic requirements during design or selection:
(1) Reasonably achieve the specified process conditions;
(2) Structure is safe and reliable;
(3) Easy to manufacture, install, operate, and maintain;
(4) Economically reasonable.
In a floating head heat exchanger, one end of the tube sheet is fixed to the shell, while the other end can float freely within the shell. The shell and tube bundle are free to expand, so when there is a large temperature difference between the two media, no thermal stress occurs between the tube bundle and the shell. The floating head end is designed to be detachable, allowing the tube bundle to be easily inserted or withdrawn from the shell. (It can also be designed to be non-detachable). This provides convenience for maintenance and cleaning. However, this heat exchanger has a more complex structure, and the small cover at the floating end makes it impossible to know the leakage situation during operation. Therefore, special attention must be paid to its sealing during installation.
The structure of the floating head part of the floating head heat exchanger can be designed in various forms according to different requirements. In addition to ensuring that the tube bundle can move freely within the equipment, the convenience of maintenance, installation, and cleaning of the floating head part must also be considered.
When designing, the outer diameter Do of the floating head tube sheet must be considered. This outer diameter should be less than the inner diameter Di of the shell, and it is generally recommended that the gap b1 between the floating head tube sheet and the inner wall of the shell be 3-5mm. This way, when the hook ring at the floating head is removed, the tube bundle can be withdrawn from the shell for maintenance and cleaning. The floating head cover can only be assembled after the tube bundle is installed, so the design should consider ensuring the necessary space for the assembly of the floating head cover.
The hook ring plays an important role in ensuring the sealing of the floating head end and preventing leakage between media. With the development of design and manufacturing technology for floating head heat exchangers, as well as the accumulation of long-term usage experience, the structural form of the hook ring has also been continuously improved and perfected.
The hook ring is generally of an open structure, requiring reliable sealing, simple and compact structure, and ease of manufacturing and disassembly.
Floating head heat exchangers have accumulated rich experience in long-term use due to their high reliability and wide adaptability. Although they have faced challenges from emerging new types of heat exchangers in recent years, this has also continuously promoted their own development. Therefore, they still dominate among various heat exchangers to this day.
The tubes form the heat transfer surface of the heat exchanger, and the size and shape of the tubes have a significant impact on heat transfer. When using small diameter tubes, the heat transfer area per unit volume of the heat exchanger is larger, making the equipment more compact, with less metal consumption per unit heat transfer area and a higher heat transfer coefficient. However, manufacturing is troublesome, the tubes are prone to scaling, and they are not easy to clean. Large diameter tubes are used for viscous or dirty fluids, while small diameter tubes are used for cleaner fluids.
The selection of tube materials should be determined based on the pressure, temperature, and corrosiveness of the medium.
The arrangement of tubes on the tube sheet not only considers the compactness of the equipment but also takes into account the properties of the fluid, structural design, and manufacturing aspects. There are four standard arrangements for tubes on the tube sheet: equilateral triangle and corner equilateral triangle arrangements, suitable for clean shell-side media that do not require mechanical cleaning. Square and corner square arrangements allow small bridges between tubes to form a straight channel, facilitating mechanical cleaning, generally used in cases where the tube bundle can be cleaned between the tubes.
Additionally, for multi-tube pass heat exchangers, a combined arrangement method is often used, where each pass generally adopts a triangular arrangement, while square arrangements are often used between passes, making it easier to arrange the baffle positions.
When the diameter of the heat exchanger is large and there are many tubes, the heat transfer tubes must be arranged as much as possible within the curved space around the tube bundle. This not only effectively increases the heat transfer area but also prevents the shell-side fluid from short-circuiting in the curved area, which could adversely affect heat transfer.
The selection of the center distance of heat transfer tubes on the tube sheet must consider the compactness of the structure, heat transfer effectiveness, as well as the strength of the tube sheet and the space required for cleaning the outer surface of the tubes. In addition, the method of fixing the tubes on the tube sheet must also be considered. If the spacing is too small, when using welded connections, the welds of two adjacent tubes may be too close, making it difficult to ensure the quality of the welds due to heat influence; if using expansion joints, the squeezing force may cause excessive deformation of the tube sheet, losing the bonding force between the tubes and the tube sheet. Generally, the center distance of the heat transfer tubes is not less than 1.25 times the outer diameter of the tubes.
When the required heat transfer area of the heat exchanger is large, and the tubes cannot be made too long, the diameter of the shell must be increased to accommodate more tubes. At this time, to increase the flow rate in the tube pass and enhance heat transfer effectiveness, the tube bundle must be divided into passes, allowing the fluid to flow sequentially through each pass of the tube bundle.
To make the heat exchanger multi-tube pass, a certain number of baffles can be installed in the tube boxes at one or both ends.
Advantages and disadvantages of floating head heat exchangers
Advantages:
(1) The tube bundle can be extracted for easy cleaning of the tubes and shell side;
(2) The temperature difference between the media is not limited;
(3) Can operate at high temperatures and pressures, generally with temperatures less than or equal to 450 degrees and pressures less than or equal to 6.4 MPa;
(4) Can be used in situations with severe scaling;
(5) Can be used in situations where the tube pass is prone to corrosion.
Disadvantages:
(1) Small floating heads are prone to internal leakage;
(2) High metal material consumption, with costs increased by 20%;
(3) Complex structure
Manufacturing process
Select the manufacturing materials and grades for the heat exchange equipment, conduct chemical composition testing of the materials, and after mechanical performance is qualified, perform straightening on the steel plates, methods include manual straightening, mechanical straightening, and flame straightening.
Material preparation - marking - cutting - edge processing (flaw detection) - forming - assembly - welding - welding quality inspection - assembly welding - pressure testing
Quality inspection
Chemical equipment not only inspects raw materials before manufacturing but also requires ongoing checks during the manufacturing process.
Quality inspection content
Inspection during the equipment manufacturing process includes inspection of raw materials, inspection between processes, and pressure testing, with specific content as follows:
(1) Inspection of the dimensions and geometric shapes of raw materials and equipment parts;
(2) Chemical composition analysis, mechanical performance analysis tests, and metallographic structure inspection of raw materials and welds, collectively referred to as destructive testing;
(3) Inspection of internal defects in raw materials and welds, with inspection methods being non-destructive testing, which includes: radiographic testing, ultrasonic testing, magnetic particle testing, penetrant testing, etc.;
(4) Equipment pressure testing, including: hydraulic testing, medium testing, and airtight testing.
Pressure testing and airtightness testing:
The completed heat exchanger should undergo pressure testing or additional airtightness testing on the connections of the heat exchanger tube sheet, tube pass, and shell pass. Pressure testing includes hydraulic testing and gas pressure testing. Generally, the heat exchanger undergoes hydraulic testing, but if due to structural or support reasons, it cannot be filled with liquid or if operating conditions do not allow residual test liquid, gas pressure testing can be used.
If the medium is highly toxic, poses a significant hazard, or if there is no allowance for trace leakage between the tube and shell passes, additional airtightness testing must be conducted.
Quality inspection methods
The sequence of pressure testing for heat exchangers is as follows:
For fixed tube sheet heat exchangers, first conduct shell-side pressure testing while checking the connections between the heat transfer tubes and the tube sheet, then conduct tube-side pressure testing;
For U-tube heat exchangers, kettle-type reboilers (U-tube bundles), and packed column heat exchangers, first use a test pressure ring for shell-side pressure testing while checking the joints, then conduct tube-side pressure testing.
Floating head heat exchangers and kettle-type reboilers (floating head tube bundles) should first use test pressure rings and special tools for floating heads to conduct pressure tests on the tube heads. For kettle-type reboilers, a special shell for pressure testing the tube heads should also be equipped, followed by pressure testing of the tube side and finally the shell side.
Pressure testing of overlapping heat exchanger joints can be done individually. When the heat exchangers are interconnected, pressure testing of the tube side and shell side should be conducted after the overlapping assembly.
Installation Method
The foundation for installing the heat exchanger must be adequate to prevent the heat exchanger from sinking or to prevent excessive deformation from being transmitted to the heat exchanger's connection pipes. There are generally two types of foundations: one is a brick saddle foundation, where the heat exchanger is placed directly on the saddle foundation without saddle supports, allowing it to move freely with thermal expansion. The other is a concrete foundation, where the heat exchanger is securely connected to the foundation through saddle supports and anchor bolts.
Before installing the heat exchanger, a strict inspection and acceptance of the foundation quality should be conducted, focusing on the following main items: the condition of the foundation surface; foundation elevation, plan position, shape, and main dimensions, as well as whether the reserved holes meet actual requirements; the position of the anchor bolts should be correct, the threading should be good, and the nuts and washers should be complete; the surface of the foundation where the shims are placed should be flat, etc.
After the foundation is accepted, shims should be placed on the foundation before installing the heat exchanger. The surface of the foundation where the shims are placed must be leveled to ensure good contact between the two. The thickness of the shims can be adjusted to ensure the heat exchanger reaches the designed horizontal height. After placing the shims, the stability of the heat exchanger on the foundation can be increased, and its weight can be evenly transmitted to the foundation through the shims. Shims can be classified into flat shims, inclined shims, and open shims. Among them, inclined shims must be used in pairs. There should be shims on both sides of the anchor bolts, and the installation of shims should not hinder the thermal expansion of the heat exchanger.
After positioning the heat exchanger, a level should be used to level the heat exchanger, allowing all connection pipes to be connected without stress. After leveling, the inclined shims can be welded to the base, but should not be welded to the flat shims or sliding plates below. When installing two or more overlapping heat exchangers, the lower heat exchanger should be aligned and the anchor bolts fully secured before installing the upper heat exchanger. Before installing the tube bundle heat exchanger, the core should be extracted for inspection and cleaning, and care should be taken to protect the sealing surfaces and baffles during the extraction of the tube bundle. When moving and lifting the tube bundle, it should be placed on a dedicated support structure to avoid damaging the heat exchange tubes.
Depending on the type of heat exchanger, sufficient space should be left at both ends of the heat exchanger to meet the requirements for cleaning and maintenance. The fixed head cover end of the floating head heat exchanger should have enough space to allow the tube bundle to be extracted from the shell, and the outer head cover end must also leave more than one meter of space for the installation and removal of the outer head cover and floating head cover.
Sufficient space should be left at both ends of the fixed tube sheet heat exchanger to allow for the extraction and replacement of tubes. Additionally, when cleaning the inside of the tubes mechanically, both ends can be used for brushing operations. The fixed head cover of the U-tube heat exchanger should leave enough space to extract the tube bundle, and sufficient space should also be left at the opposite end for disassembling the shell.
The heat exchanger must not operate under conditions exceeding those specified on the nameplate. The temperature and pressure drop of the media in the tube and shell sides should be monitored regularly, and the leakage and fouling conditions of the heat exchange tubes should be analyzed. The shell-and-tube heat exchanger utilizes tubes to facilitate heat exchange, cooling, condensation, heating, and evaporation processes between the materials inside and outside. Compared to other equipment, the surface area in contact with corrosive media is significantly larger, leading to a high risk of corrosion, perforation, and loosening leaks at the joints. Therefore, the methods for corrosion prevention and leak prevention for heat exchangers require more consideration than for other equipment. When steam is used to heat or water is used to cool the heat exchanger, the solubility of dissolved substances in the water generally increases after heating, while substances like calcium sulfate show little change. Cooling water is often reused, and due to evaporation, salts concentrate, leading to deposits or fouling. Additionally, the presence of corrosive dissolved gases and chloride ions in the water causes equipment corrosion, with corrosion and fouling alternating, exacerbating the corrosion of steel. Therefore, cleaning is necessary to improve the performance of the heat exchanger. As the difficulty of cleaning increases rapidly with the thickness of the fouling layer or deposits, the cleaning interval should not be too long and should be regularly checked, repaired, and cleaned based on the characteristics of the production unit, the nature of the heat exchange medium, the corrosion rate, and the operating cycle.
Heat exchangers are widely used in daily life, such as heating radiators, condensers in turbine installations, and oil coolers on spacecraft. They are also widely used in industries such as chemical, petroleum, power, and atomic energy. Their main function is to ensure that the process requirements for specific temperatures of the media are met, and they are also one of the main devices for improving energy utilization.
A heat exchanger can be a standalone device, such as a heater, cooler, or condenser; or it can be a component of a process device, such as a heat exchanger in an ammonia synthesis tower.
Due to limitations in manufacturing processes and scientific levels, early heat exchangers could only adopt simple structures, with small heat transfer areas, large volumes, and heavy weights, such as snake tube heat exchangers. With the development of manufacturing processes, a type of shell-and-tube heat exchanger gradually emerged, which not only has a larger heat transfer area per unit volume but also has better heat transfer efficiency, becoming a typical heat exchanger in industrial production over the years.
Development History
In the 1920s, plate heat exchangers appeared and were applied in the food industry. Heat exchangers made of plates instead of tubes have a compact structure and good heat transfer efficiency, leading to the development of various forms. In the early 1930s, Sweden first produced spiral plate heat exchangers. Subsequently, the UK manufactured a plate-fin heat exchanger made of copper and its alloys using brazing methods for cooling aircraft engines. By the end of the 1930s, Sweden produced the first plate-and-shell heat exchanger for use in pulp mills. During this period, to address the heat exchange issues of highly corrosive media, attention began to be paid to heat exchangers made from new materials.
Around the 1960s, due to the rapid development of space technology and cutting-edge science, there was an urgent need for various high-efficiency compact heat exchangers. Coupled with advancements in technologies such as stamping, brazing, and sealing, the manufacturing process of heat exchangers was further improved, thus promoting the vigorous development and widespread application of compact plate heat exchangers. Additionally, starting from the 1960s, typical shell-and-tube heat exchangers were further developed to meet the needs of heat exchange and energy saving under high temperature and high pressure conditions. In the mid-1970s, to enhance heat transfer, heat pipe heat exchangers were created based on the research and development of heat pipes.
Heat exchangers can be classified into three categories based on the method of heat transfer: mixed type, thermal storage type, and wall type.
Mixed heat exchangers are those that exchange heat through the direct contact and mixing of cold and hot fluids, also known as contact heat exchangers. Since the two fluids must be separated promptly after mixing and exchanging heat, this type of heat exchanger is suitable for heat exchange between gas and liquid fluids. For example, in cooling towers used in chemical plants and power plants, hot water is sprayed from the top down, while cold air is drawn in from the bottom up. Heat exchange occurs on the surface of the water film, droplets, or mist, where hot water and cold air come into contact, cooling the hot water and heating the cold air, which then relies on the density difference of the two fluids for timely separation.
Thermal storage heat exchangers utilize the alternating flow of cold and hot fluids over the surface of thermal storage media (fill material) in a thermal storage chamber to exchange heat, such as the thermal storage chamber for preheating air below a coking furnace. This type of heat exchanger is mainly used to recover and utilize the heat from high-temperature waste gases. Equipment designed for the purpose of recovering cold is called a cold storage device, which is mostly used in air separation units.
In wall-type heat exchangers, the cold and hot fluids are separated by solid walls, and heat exchange occurs through these walls, hence they are also known as surface heat exchangers. This type of heat exchanger is the most widely used.
Wall-type heat exchangers can be classified into tubular, plate, and other types based on the structure of the heat transfer surface. Tubular heat exchangers use the surface of tubes as the heat transfer surface, including serpentine heat exchangers, double-pipe heat exchangers, and shell-and-tube heat exchangers; plate heat exchangers use plates as the heat transfer surface, including plate heat exchangers, spiral plate heat exchangers, plate-fin heat exchangers, plate-shell heat exchangers, and umbrella plate heat exchangers; other types of heat exchangers are designed to meet specific requirements, such as scraped surface heat exchangers, rotary heat exchangers, and air coolers.
The relative flow direction of fluids in heat exchangers generally has two types: co-current and counter-current. In co-current flow, the temperature difference between the two fluids at the inlet is the largest and gradually decreases along the heat transfer surface, reaching the minimum temperature difference at the outlet. In counter-current flow, the temperature difference between the two fluids along the heat transfer surface is more uniform. Under the condition of constant inlet and outlet temperatures of the cold and hot fluids, when both fluids do not undergo phase change, the average temperature difference in counter-current flow is maximized while it is minimized in co-current flow.
Under the condition of achieving the same amount of heat transfer, using counter-current flow can increase the average temperature difference and reduce the heat transfer area of the heat exchanger; if the heat transfer area remains unchanged, using counter-current flow can reduce the consumption of heating or cooling fluids. The former can save equipment costs, while the latter can save operating costs, so counter-current heat exchange should be adopted as much as possible in design or production.
When either or both of the cold and hot fluids undergo a phase change (boiling or condensation), the temperature of the fluid itself does not change during the phase change, as only latent heat of vaporization is released or absorbed. Therefore, the inlet and outlet temperatures of the fluids are equal, and at this point, the temperature difference between the two fluids is independent of the flow direction chosen. In addition to co-current and counter-current flow, there are also cross-flow and diverted flow.
In the heat transfer process, reducing the thermal resistance in wall-type heat exchangers to improve the heat transfer coefficient is an important issue. The thermal resistance mainly comes from the thin layer of fluid that adheres to the heat transfer surface on both sides of the wall (known as the boundary layer) and the fouling layer that forms on the walls during the use of the heat exchanger, while the thermal resistance of the metal wall is relatively small.
Increasing the flow rate and turbulence of the fluid can reduce the thickness of the boundary layer, lower thermal resistance, and improve the heat transfer coefficient. However, increasing the flow rate will increase energy consumption, so a reasonable balance should be made between reducing thermal resistance and lowering energy consumption during design. To reduce the thermal resistance caused by fouling, efforts can be made to delay the formation of fouling and regularly clean the heat transfer surface.
Generally, heat exchangers are made of metal materials, among which carbon steel and low-alloy steel are mostly used for manufacturing medium and low-pressure heat exchangers; stainless steel is mainly used for different corrosion resistance conditions, and austenitic stainless steel can also be used as materials resistant to high and low temperatures; copper, aluminum, and their alloys are mostly used for manufacturing low-temperature heat exchangers; nickel alloys are used under high-temperature conditions; non-metallic materials, in addition to making gasket parts, have begun to be used for making non-metallic corrosion-resistant heat exchangers, such as graphite heat exchangers, fluoroplastic heat exchangers, and glass heat exchangers.
Operation of heat exchangers
Unit structure
Heat exchange units serve as a direct bridge between the primary heat network and users, obtaining heat from the primary heat network and automatically and continuously converting it into the hot water and heating water needed by users, suitable for air conditioning (heating and cooling), heating, domestic hot water (bathing), or other heat exchange circuits (such as floor heating, process water cooling, etc.). The heat exchange unit consists of a plate heat exchanger, circulating water pump, make-up water pump, filter, valves, unit base, heat metering device, distribution box, electronic instruments, and automatic control system. The steam or high-temperature water from the heat source enters the plate heat exchanger through the water supply inlet on the primary side of the unit, while the low-temperature return water on the secondary side is filtered to remove impurities and also enters the plate heat exchanger through the circulating pump. The two different temperature waters undergo heat exchange, and the secondary side delivers heat to the heat users.
Edit this section
Common problems and solutions
During the production process, due to the erosion of the heat exchanger tube sheet by water, cavitation, and the corrosion of trace chemical media, leaks often occur at the welds of the tube sheet, leading to the mixing of water and chemical materials. This makes it difficult to control the production process temperature, resulting in the generation of other products, severely affecting product quality and lowering product grades. After leaks occur at the condenser tube sheet welds, companies usually use traditional welding methods for repairs. Internal stress can easily develop inside the tube sheet and is difficult to eliminate, causing leaks in other heat exchangers. Companies conduct pressure tests to check the repair status of the equipment, repeatedly welding and experimenting, requiring 2 to 4 people several days to complete the repairs. After a few months of use, the tube sheet welds corrode again, leading to waste of human, material, and financial resources, and increased production costs. By utilizing the corrosion resistance and erosion resistance of Fushilan polymer composite materials, and by protecting new heat exchangers in advance, not only are the existing weld and sand hole issues effectively managed, but it also prevents chemical substances from corroding the metal surface and weld points of the heat exchanger after use. During future regular maintenance, Fushilan polymer composite materials can also be applied to protect exposed metals; even if leaks occur after use, they can be repaired in a timely manner using Fushilan technology, avoiding prolonged welding repairs that affect production. It is precisely due to this meticulous management that the probability of heat exchanger leaks has been greatly reduced, not only lowering the procurement costs of heat exchanger equipment but also ensuring product quality and production time, thereby enhancing product competitiveness.
Edit this section
New type of heat exchanger
Pneumatic spray finned tube heat exchanger
Russia proposed an advanced method, namely the pneumatic spraying method, to improve the performance of finned surfaces. The essence is to use a high-speed cold or slightly heated fluid containing particles to spray powder particles onto the fin surface. This method can spray not only metals but also alloys and ceramics (metal-ceramic mixtures), resulting in various surfaces with different properties. In practice, the contact resistance of the fin base is one of the factors limiting the installation of fins on tubes. To evaluate the finned tube heat exchanger components, experimental studies were conducted. The experiment involved spraying ac-aluminum on the fin surface and adding 24a white electric furnace alumina. By organizing the data obtained from the experiment, the contact resistance of the fin base can be evaluated. Comparing the efficiency of the studied fins with calculated data, the conclusion is that the contact resistance of the pneumatic sprayed fin base has no substantial impact on efficiency. To confirm this, a metallographic structure analysis was conducted on the transition zone between the base (tube) and the surface (fin).
Analysis of the transition zone samples indicates that there are no micro-cracks with poor tightness along the entire length of the connecting boundary. Therefore, the pneumatic spraying method promotes the formation of branch boundaries that enhance the interaction between the surface and the base, facilitating the penetration of powder particles into the matrix, which indicates high adhesion strength, physical contact, and the formation of metal chains. Thus, the pneumatic spraying method can not only be used for shaping but also to fix fins manufactured by conventional methods onto the surface of heat exchanger tubes, and it can also be used to reinforce the base of ordinary fins. It is expected that the pneumatic spraying method will be widely applied in the production of compact and efficient heat exchangers.
Spiral baffle heat exchanger
In shell-and-tube heat exchangers, the shell side is usually a weak link. Typically, ordinary arc baffles can create a convoluted flow path system (zigzag flow path), which can lead to significant dead zones and relatively high back-mixing. These dead zones can exacerbate fouling on the shell side, adversely affecting heat transfer efficiency. Back-mixing can also distort and reduce the average temperature difference. The consequence is that, compared to piston flow, arc baffles reduce net heat transfer. Superior arc baffle shell-and-tube heat exchangers are difficult to meet high thermal efficiency requirements, and are often replaced by other types of heat exchangers (such as compact plate heat exchangers). Improving the geometry of ordinary baffles is the first step in developing the shell side. Although sealing strips and additional measures such as deflection baffles have been introduced to improve the performance of heat exchangers, the main drawbacks of ordinary baffle designs still exist.
To address this, the United States proposed a new scheme, recommending the use of spiral baffles. The advancement of this design has been confirmed by fluid dynamics research and heat transfer test results, and this design has been patented. This structure overcomes the main drawbacks of ordinary baffles. The design principle of the spiral baffle is simple: specially designed plates with a circular cross-section are installed in a 'pseudo-spiral baffle system', with each baffle occupying one-quarter of the cross-section of the heat exchanger shell, angled towards the axis of the heat exchanger, maintaining an inclination to the axis. The periphery of adjacent baffles connects and forms a continuous spiral at the outer circumference. The axial overlap of the baffles can also achieve a double spiral design to reduce the span of the supporting tubes. The spiral baffle structure can meet relatively wide process conditions. This design offers great flexibility, allowing for the selection of the optimal spiral angle for different operating conditions; overlapping baffles or double spiral baffle structures can be chosen based on specific situations.
New type of twisted tube heat exchanger
The Swedish company Alares developed a flat tube heat exchanger, commonly referred to as a twisted tube heat exchanger. Brown Company in Houston, USA made improvements. The manufacturing process of the spiral flat tube includes two processes: 'flattening' and 'thermal twisting'. The improved twisted tube heat exchanger is as simple as traditional shell-and-tube heat exchangers, but has many exciting advancements, achieving the following technical and economic benefits: improved heat transfer, reduced fouling, true counterflow, lower costs, no vibration, space-saving, and no baffle elements.
Due to the unique structure of the tubes, both the tube side and shell side are in a spiral motion simultaneously, promoting turbulence. This heat exchanger has a total heat transfer coefficient that is 40% higher than conventional heat exchangers, while the pressure drop is almost equal. When assembling the heat exchanger, a mixed method of spiral flat tubes and smooth tubes can also be used. This heat exchanger is manufactured strictly according to ASME standards. It can replace any place where shell-and-tube heat exchangers and traditional devices are used. It can achieve the best values obtained by ordinary shell-and-tube heat exchangers and plate-frame heat transfer equipment. It is estimated to have broad application prospects in the chemical and petrochemical industries.
Non-brazed wire-wound spiral tube heat exchanger
The spiral tube heat exchanger (TA) with metal wire wound around the tube as ribs (fins) generally uses welding methods to fix the metal wire onto the tube. However, this method has a series of impacts on the overall quality of the equipment, as the brazing method will 'deduct' a significant portion of the surface of the tube and metal wire from heat transfer. More importantly, the rapid aging and breakage of the solder can cause blockage in machines and equipment, leading to premature damage.
Previous:
Previous:
Heat exchange equipment solutions
Chongqing Southwest Saiwei Heat Exchange Equipment Co., Ltd. is committed to providing customers with excellent heat exchange equipment solutions. With the rapid development of the industrial field, the demand and application of heat exchange equipment are becoming more and more extensive. The company deeply knows that each customer's needs are unique, so customized solutions have become our core competitiveness.
2025-08-13
Heat exchanger material and corrosion resistance
Heat exchanger material and corrosion resistance
2025-07-08
Chongqing Southwest Saiwei Heat Exchange Equipment Co., Ltd. boosts the take-off of China's heat exchange technology.
2025-05-30
Contact Information
Telephone:
Address:
Baishiyi, Jiulongpo District, Chongqing
E-mail:
Copyright©2024 Chongqing Southwest Saiwell Heat Exchange Equipment Co., Ltd. Powered by:www.300.cn [SEO]
Quick Connect
Chongqing Southwest Saiwell Heat Exchange Equipment Co., Ltd.
Specializing in the research, development, production and sales, equipment consulting, maintenance, and accessory services of heat exchange technology, equipment, and systems, and undertaking the design and installation of thermal system engineering, energy conservation and emission reduction, and energy conservation and emission reduction system engineering.
- Phone 86-023-63521288
- Whats App
- Messages
- Back