The heat exchanger is an indispensable equipment to realize the heat exchange and transfer in the chemical production process. In the heat exchange, there are often some corrosive and highly oxidizing materials. Therefore, the materials used to manufacture the heat exchanger are required to have strong corrosion resistance. The classification of heat exchangers is broad: reactor pressure vessel condenser reaction pot helical plate heat exchanger corrugated tube heat exchanger tubular heat exchanger plate heat exchanger helical plate heat exchanger tube-shell heat exchanger volume heat exchanger Floating head heat exchanger Tube heat exchanger Heat pipe heat exchanger Steam-water heat exchanger Heat exchange unit Graphite heat exchanger, Air heat exchanger Titanium heat exchanger Heat exchange equipment, the material required to manufacture the heat exchanger has strong corrosion resistance . It can be made of graphite, ceramics, glass and other non-metallic materials, as well as stainless steel, titanium, tantalum, zirconium and other metal materials. However, those made of graphite, ceramics, glass and other materials have shortcomings such as fragility, large volume, and poor thermal conductivity. Heat exchangers made of rare metals such as titanium, tantalum and zirconium are too expensive, while stainless steel is difficult to withstand many corrosive media , And produce intergranular corrosion.  

In the industrial production of petroleum, chemical industry, light industry, pharmaceuticals, energy and other industries, heat exchangers often need to heat low-temperature fluids or cool high-temperature fluids, vaporize liquids into steam or condense steam into liquids. These processes are closely related to heat transfer, so they can all be completed by heat exchangers.  

With the development of economy, various types and types of heat exchangers are developing rapidly, and heat exchangers with new structures and new materials are constantly emerging. In order to meet the needs of development, my country has established standards for certain types of heat exchangers and formed a series. The complete heat exchanger should meet the following basic requirements during design or selection:   

(1) Reasonably realize the specified process conditions;   

(2) The structure is safe and reliable;   

(3) Easy to manufacture, install, operate and maintain;   

(4) Economically reasonable.  

The tube sheet at one end of the floating head heat exchanger is fixed to the shell, while the tube sheet at the other end can float freely in the shell. The shell and tube bundle are free to expand. Therefore, when the temperature difference between the two media is large, the tube bundle and There is no thermal stress between the shells. The floating head end is designed as a detachable structure, so that the tube bundle can be easily inserted or drawn out of the shell. (It can also be designed to be non-detachable). This provides convenience for maintenance and cleaning. However, the structure of the heat exchanger is more complicated, and the floating end small cover cannot know the leakage situation during operation. Therefore, pay special attention to its seal 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 the free movement of the tube bundle in the equipment, the convenience of maintenance, installation and cleaning of the floating head part must also be considered.  

The outer diameter Do of the floating head tube plate must be considered in the design. The outer diameter should be smaller than the inner diameter Di of the shell. Generally, it is recommended that the gap between the floating head tube plate and the inner wall of the shell b1=3~5mm. In this way, when the hook loop from the floating head is removed, the tube bundle can be withdrawn from the housing. To facilitate maintenance and cleaning. The floating head cover can only be assembled after the tube bundle is installed, so the necessary space for the floating head cover during assembly should be considered in the design.  The hook ring plays an important role in ensuring the sealing of the floating head and preventing the leakage of the medium. With the development of the design and manufacturing technology of the crosshead heat exchanger, and the accumulation of long-term use experience, the structure of the hook and loop has also been improved and perfected.  The hook ring is generally a split structure, which requires reliable sealing, simple and compact structure, easy to manufacture and easy to disassemble.  

Floating head heat exchanger with its high reliability and wide adaptability, has accumulated rich experience in the process of long-term use. Although it has been challenged by the emerging new heat exchangers in recent years, it has in turn promoted its own development. So far, it has been dominant in various heat exchangers.  

The tube constitutes the heat transfer surface of the heat exchanger, and the size and shape of the tube have a great influence on heat transfer. When a small diameter tube is used, the heat exchange area per unit volume of the heat exchanger is larger, the equipment is more compact, the metal consumption per unit heat transfer area is small, and the heat transfer coefficient is also higher. However, it is troublesome to manufacture, and the tube is easy to scale and difficult to clean. Large-diameter tubes are used for fluids with high viscosity or dirty, and small-diameter tubes are used for cleaner fluids.  

The choice of pipe material should be determined according to the pressure, temperature and corrosivity of the medium.  

The arrangement of the heat exchanger tubes on the tube plate not only considers the compactness of the equipment, but also considers the nature of the fluid, the structural design, and the processing and manufacturing conditions. There are four standard arrangements of pipes on the tube plate: equilateral triangle and corner equilateral triangle arrangement, which is suitable for cleaning with shell side media and does not require mechanical cleaning. The square and corner square arrangement can make the small bridge between the tubes form a straight channel, which is convenient for mechanical cleaning. It is generally used for the occasions where the tube bundle can be drawn out of the tube for cleaning.  

In addition, for multi-tube pass heat exchangers, a combined arrangement method is often used. In each pass, a triangular arrangement is generally used, and a square arrangement is often used between passes, which facilitates the arrangement of the partitions.  When the diameter of the heat exchanger is large and there are many tubes, the heat exchange tubes must be arranged in the arcuate space around the tube bundle as much as possible. This not only can effectively increase the heat transfer area, but also prevents the shell side fluid from being short-circuited in the arcuate area and adversely affecting heat transfer. The choice of the center distance of the heat exchange tubes on the tube plate not only considers the compactness of the structure and the heat transfer effect, but also the strength of the tube plate and the space required for cleaning the outer surface of the tube. In addition, the method of fixing the tube on the tube plate should also be considered. If the distance is too small, when the welding connection is adopted, the welding seams of two adjacent pipes are too close, and the quality of the welding seam is not easily guaranteed due to heat; if the expansion connection is adopted, the extrusion force may cause excessive deformation of the tube sheet and loss The bonding force between the tube and the tube sheet. The center distance of the generally used heat exchange tube is not less than 1.25 times the outer diameter of the tube.  When the heat exchange area required by the heat exchanger is large, and the tube cannot be made too long, the diameter of the shell must be increased to arrange more tubes. At this time, in order to increase the flow rate of the tube side and increase the heat transfer effect, the tube bundle must be divided so that the fluid flows through the tube bundle in turn.  In order to make the heat exchanger into a multi-tube pass, a certain number of baffles can be placed in the tube boxes at one or both ends.

The advantages and disadvantages of floating head heat exchanger: Pros:   

(1) The tube bundle can be drawn out to facilitate cleaning of the tube and shell side;   

(2) The temperature difference between the media is not limited;   

(3) It can work under high temperature and high pressure, generally the temperature is less than or equal to 450 degrees, and the pressure is less than or equal to 6.4 MPa;   

(4) can be used in the occasions with more serious fouling;   

(5) can be used in the occasions where the pipe is easily corroded.  

Cons:   

(1) Small floating heads are prone to internal leakage;   

(2) The consumption of metal materials is large, and the cost is 20% higher;   

(3) Complex structure   

Manufacturing process   

Select the materials and grades of the heat exchange equipment to check the chemical composition of the materials. After the mechanical properties are qualified, the steel plate is corrected. The methods include manual correction, mechanical correction and flame correction.

Material preparation-marking-cutting-edge processing (flaw detection)-forming-assembly-welding-welding quality inspection-assembly welding-pressure test   

Quality inspection   

Chemical equipment not only inspects raw materials before manufacturing, but also inspects at any time during the manufacturing process.  

Quality inspection content and methods:   

Inspection during the manufacturing process of equipment, including raw material inspection, inter-process inspection and pressure test. The specific contents are as follows:   

(1) Inspection of the size and geometry of raw materials and equipment parts;  

(2) The chemical composition analysis, mechanical property analysis test, and metallographic inspection of raw materials and welds are collectively referred to as destruction tests;   

(3) The inspection of raw materials and welds internal defects, the inspection method is non-destructive testing, which includes: Testing, ultrasonic testing, magnetic particle testing, penetration testing, etc.;

(4) Equipment pressure test, including: water pressure test, medium test, air tightness test, etc.  

Pressure test and air tightness test:   

The finished heat exchanger shall be subjected to pressure test or additional air-tightness test on the connection joints, tube side and shell side of the heat exchanger tube sheet. The pressure test includes water pressure test and air pressure test. Heat exchangers are generally subjected to hydraulic test, but due to structural or support reasons, liquid cannot be filled or operating conditions do not allow residual test liquid, air pressure test can be used.  

If the medium is extremely toxic, highly hazardous, or small leakage is not allowed between the tube and the shell side, the air tightness test must be added.

The pressure test sequence of the heat exchanger is as follows:   

The fixed tube-sheet heat exchanger first performs a shell-side pressure test, and at the same time checks the connection joints of the heat exchange tube and the tube sheet, and then performs a tube-side pressure test;   U-shaped tube heat exchanger, kettle reboiler (U-shaped tube bundle) and For stuffing box heat exchangers, use test pressure rings for shell-side pressure test, while checking joints, and then pipe-side pressure test;

Floating head heat exchangers and kettle reboilers (floating head tube bundles) first use test pressure rings and floating head special tools to test the tube head pressure. For the kettle reboiler, it should be equipped with a special shell for tube head pressure test. Tube side pressure test, and finally shell side pressure test;   

The pressure test of overlapping heat exchanger joints can be carried out by a single unit. When each heat exchanger is connected, the tube side and shell side pressure test should be carried out after overlapping assembly.  

Installation:   

The foundation for installing the heat exchanger must be satisfied so that the heat exchanger does not sink, or the pipe transfers excessive deformation to the heat exchanger tube. The foundation is generally divided into two types: one is a brick saddle foundation. The heat exchanger is directly placed on the saddle foundation without a saddle support. The heat exchanger and the foundation are not fixed, and can expand with the needs of thermal expansion. Move freely. The other is a concrete foundation. The heat exchanger is firmly connected to the foundation by anchor bolts through saddle supports.  

The basic quality inspection and acceptance work should be strictly carried out before installing the heat exchanger. The main items are as follows: basic surface overview; foundation elevation, plane position, shape and main dimensions and whether the reserved holes meet actual requirements; anchor bolt location Whether it is correct, whether the thread is in good condition, whether the nut and washer are complete; whether the foundation surface for placing the shim is flat, etc.  

After the foundation acceptance is completed, put the shim on the foundation before installing the heat exchanger. The surface of the foundation where the shim is placed must be leveled so that the two can be in good contact. The thickness of the shim can be adjusted so that the heat exchanger can reach the designed level. After the shim is placed, the stability of the heat exchanger on the foundation can be increased, and its weight can be evenly transferred to the foundation through the shim. The pad iron can be divided into flat pad iron, inclined pad iron and open pad iron. Among them, the inclined shim must be used in pairs. There should be pad irons on both sides of the anchor bolts, and the installation of the pad irons shall not hinder the thermal expansion of the heat exchanger.  

After the heat exchanger is in place, the heat exchanger needs to be leveled with a spirit level, so that all pipes can be connected to the pipes without force. After leveling, the oblique shim can be welded firmly to the Shiba base, but it must not be welded to the flat shim or sliding plate below. When two or more overlapping heat exchangers are installed, the lower heat exchanger should be aligned and the anchor bolts should be fully fixed before installing the upper heat exchanger. Before installation of the drawable tube bundle heat exchanger, the core should be checked, cleaned, and the sealing surface and baffle should be protected when drawing the tube bundle. When moving and lifting the tube bundle, the tube bundle should be placed on a special supporting structure to avoid damage to the heat exchange tubes.  

According to the form of the heat exchanger, there should be enough space at both ends of the heat exchanger to meet the conditions (operation) cleaning and maintenance needs. The fixed head cover end of the floating head heat exchanger should have enough space to be able to withdraw the tube bundle from the shell, and the outer head cover end must also leave a position of more than one meter to install and remove the outer head cover and the floating head cover.  

Sufficient space should be left at both ends of the fixed tube-sheet heat exchanger so that the tubes can be extracted and replaced. Also, when cleaning the inside of the pipe mechanically. Both ends can be used to scrub the tube. The fixed head cover of the U-shaped tube heat exchanger should leave enough space to draw out the tube bundle, or leave enough space at its opposite end to allow the shell to be removed.  

The heat exchanger must not be operated under conditions exceeding the conditions specified on the nameplate. The temperature and pressure drop of the tube and shell side media should be monitored frequently, and the leakage and fouling of the heat exchange tube should be analyzed. Shell-and-tube heat exchangers use pipes to exchange, cool, condense, heat, and evaporate the materials inside and outside of the pipe. Compared with other equipment, the surface area of other corrosive media contacts appears to be very large, causing corrosion to perforate the junction. The risk of slack leakage is very high. Therefore, the anti-corrosion and anti-leakage methods of the heat exchanger must be considered more than other equipment. When the heat exchanger is heated by steam or cooled by water, the dissolved substances in the water will be heated after heating. Most of the solubility will be improved, while the calcium sulfate type of substance is almost unchanged. Cooling water is often recycled. Due to the evaporation of water, the salt is concentrated and deposits or dirt are generated. In addition, the corrosive dissolved gases and chloride ions in the water cause equipment corrosion, which alternates between corrosion and scaling, which intensifies the corrosion of steel. Therefore, it must be cleaned to improve the performance of the heat exchanger. Since the difficulty of cleaning increases rapidly with the increase of the thickness or deposition of the scale layer, the cleaning interval should not be too long. It should be based on the characteristics of the production device, the nature of the heat exchange medium, the corrosion rate and the operating cycle. Perform inspection, repair and cleaning.  

Heat exchangers are widely used. Heating radiators used in daily life, condensers in steam turbine installations, and oil coolers on space rockets are all heat exchangers. It is also widely used in the chemical, petroleum, power and atomic energy industries. Its main function is to ensure the specific temperature required by the medium in the process, and it is also one of the main equipment to improve energy utilization.  

The heat exchanger can be a separate device, such as a heater, cooler, and condenser; it can also be a component of a process equipment, such as a heat exchanger in an ammonia synthesis tower.  

Due to the limitation of manufacturing technology and scientific level, the early heat exchangers can only adopt simple structures, and the heat transfer area is small, bulky and heavy, such as coiled heat exchangers. With the development of manufacturing technology, a shell-and-tube heat exchanger is gradually formed. It not only has a larger heat transfer area per unit volume, but also has a better heat transfer effect. It has long been a typical heat exchanger in industrial production. Heater.  Plate heat exchangers appeared in the 1920s and were used in the food industry. The heat exchanger made of plate instead of tube has compact structure and good heat transfer effect, so it has gradually developed into various forms. In the early 1930s, Sweden made the spiral plate heat exchanger for the first time. Then the British used brazing to produce a plate-fin heat exchanger made of copper and its alloy materials for the heat dissipation of aircraft engines. At the end of the 1930s, Sweden produced the first plate and shell heat exchanger for pulp mills. During this period, in order to solve the heat exchange problem of strong corrosive media, people began to pay attention to heat exchangers made of new materials.  

Around the 1960s, due to the rapid development of space technology and cutting-edge science, a variety of high-performance and compact heat exchangers were urgently needed. Coupled with the development of stamping, brazing and sealing technologies, the heat exchanger manufacturing process was further improved. This has promoted the vigorous development and wide application of compact plate heat exchangers. In addition, since the 1960s, in order to meet the needs of heat exchange and energy saving under high temperature and high pressure conditions, typical shell and tube heat exchangers have also been further developed. In the mid-1970s, in order to enhance heat transfer, heat pipe heat exchangers were created on the basis of research and development of heat pipes.  

Heat exchangers can be divided into three types: hybrid type, heat storage type and partition type according to different heat transfer methods.  

Hybrid heat exchanger is a heat exchanger that exchanges heat through direct contact and mixing of cold and hot fluids, also known as contact heat exchangers. Since the two fluids must be separated in time after they are mixed and exchanged, this type of heat exchanger is suitable for heat exchange between gas and liquid. For example, in the cooling water towers used in chemical plants and power plants, hot water is sprayed from top to bottom, while cold air is sucked in from bottom to top, on the surface of the water film of the filling or on the surface of droplets and water droplets, hot water and cold air Contact with each other for heat exchange, the hot water is cooled, the cold air is heated, and then separated in time by the density difference between the two fluids.  

Regenerative heat exchanger is a heat exchanger that uses cold and hot fluids to alternately flow through the surface of the regenerator (filler) in the regenerator to exchange heat, such as the regenerator for preheating air under the coke oven. This type of heat exchanger is mainly used to recover and utilize the heat of high-temperature exhaust gas. The similar equipment for the purpose of recovering cold is called cold storage, which is mostly used in air separation plants.  

The cold and hot fluids of the dividing wall heat exchanger are separated by solid dividing walls and heat exchange through the dividing walls. Therefore, it is also called surface heat exchanger. This type of heat exchanger is the most widely used.  

Dividing wall heat exchangers can be divided into tube type, plate type and other types according to the structure of the heat transfer surface. Tubular heat exchangers use the surface of the tube as the heat transfer surface, including coiled tube heat exchangers, double-pipe heat exchangers and shell-and-tube heat exchangers, etc.; plate surface heat exchangers use the plate surface 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, etc.; other types of heat exchangers are heat exchangers designed to meet certain special requirements , Such as scraped surface heat exchangers, turntable heat exchangers and air coolers.  The relative flow of the fluid in the heat exchanger generally has two types: forward flow and reverse flow. When flowing downstream, the temperature difference between the two fluids at the inlet is the largest, and gradually decreases along the heat transfer surface until the temperature difference at the outlet is the smallest. In countercurrent, the temperature difference between the two fluids along the heat transfer surface is more evenly distributed. Under the condition that the inlet and outlet temperatures of the cold and hot fluids are constant, when the two fluids have no phase change, the average temperature difference between the countercurrent and the downstream is the largest.  Under the condition of completing the same heat transfer, the use of counterflow can increase the average temperature difference and reduce the heat transfer area of the heat exchanger; if the heat transfer area remains the same, the use of counterflow can reduce the consumption of heating or cooling fluid. The former can save equipment costs, and the latter can save operating costs, so countercurrent heat exchange should be used as much as possible in design or production.  

When there is a phase change (boiling or condensing) in both or one of the cold and hot fluids, since only the latent heat of vaporization is released or absorbed during the phase change, the temperature of the fluid itself does not change, so the inlet and outlet temperatures of the fluid are equal. At this time, the temperature difference between the two fluids has nothing to do with the choice of fluid flow direction. In addition to the downstream and countercurrent flows, there are also flow directions such as cross-flow and baffle flow.  

In the heat transfer process, it is an important issue to reduce the thermal resistance in the partition heat exchanger to improve the heat transfer coefficient. The thermal resistance mainly comes from the thin layer of fluid sticking to the heat transfer surface on both sides of the partition wall (called boundary layer) and the dirt layer formed on both sides of the wall during the use of the heat exchanger. The thermal resistance of the metal wall is relatively small.

Increase the flow velocity and turbulence of the fluid, which can thin the boundary layer, reduce the thermal resistance and increase the heat transfer coefficient. However, increasing the fluid flow rate will increase energy consumption, so a reasonable coordination should be made between reducing thermal resistance and reducing energy consumption during design. In order to reduce the thermal resistance of dirt, try to delay the formation of dirt and clean the heat transfer surface regularly.  

Generally, heat exchangers are made of metal materials. Among them, carbon steel and low-alloy steel are mostly used to manufacture medium and low-pressure heat exchangers. In addition to being mainly used for different corrosion resistance conditions, austenitic stainless steel can also be used as a High and low temperature materials; copper, aluminum and their alloys are mostly used in the manufacture of low temperature heat exchangers; nickel alloys are used in high temperature conditions; non-metallic materials are used in the production of non-metallic materials except for gasket parts. Corrosion heat exchangers, such as graphite heat exchangers, fluoroplastic heat exchangers and glass heat exchangers. Operation of the heat exchanger   When the heat exchanger starts to run, if the heat exchanger is found to be uneven in cold and heat, you should check whether the air is not cleaned, whether the heat exchange plate is added incorrectly, or whether it is usually blocked, etc., and take corresponding effective measures. When it is found that the heat exchanger has two kinds of media in collusion, especially the flammable and explosive media, stop immediately, find out and replace its perforated or cracked plates. The heat exchanger strictly controls the temperature and pressure not to exceed the allowable value, otherwise it will accelerate the aging of the sealing gasket. Because the equipment is full of medium during the operation of the heat exchanger, it is not allowed to firmly clamp the bolts under pressure. When tightening the clamping bolts and nuts of the heat exchange fins, the plate bundle distance between the two heads should be strictly controlled, otherwise the heat exchange plates or the sealing gaskets will be easily damaged. The sliding rollers on the movable head of the heat exchanger should be regularly refueled to prevent rust, so as to ensure flexible and easy disassembly. Under normal circumstances, the heat exchanger does not need to be stopped. When the resistance drops above the allowable value, the backwash has no obvious effect, the production capacity suddenly drops, the medium is interlocked or the medium leaks in large quantities and cannot be controlled. Find the reason. Clean up or replace damaged heat exchanger parts.