1. Classification of heat exchangers
1. Classification according to the use of heat exchangers
(1) Heater: The heater is used to heat the fluid to the required temperature, and the heated fluid does not undergo a phase change during the heating process.
(2) Preheater: The preheater is used to preheat the fluid to improve the efficiency of the entire process unit.
(3) Superheater: The superheater is used to heat saturated steam to make it superheated.
(4) Evaporator: The evaporator is used to heat the liquid to evaporate it.
(5) Reboiler: The reboiler is a special equipment in the distillation process, which is used to heat the condensed liquid and make it vaporized by heating.
(6) Cooler: The cooler is used to cool the fluid to the required temperature.
(7) Condenser: The condenser is used to condense saturated steam, so that it releases latent heat to condense and liquefy.
2. Classification according to the shape and structure of the heat transfer surface of the heat exchanger
(1) Tubular heat exchanger: Tubular heat exchanger transfers heat through the wall of the tube. According to the structure of the heat transfer tube, it can be divided into tubular heat exchange tube, casing heat exchanger, and coiled tube heat exchange. device, etc. Tubular heat exchangers are the most widely used.
(2) Plate heat exchanger: The plate heat exchanger conducts heat transfer through the plate surface. According to the structure of the heat transfer plate, it can be divided into a flat plate heat exchanger, a spiral plate heat exchanger, a plate fin heat exchanger and a heat exchanger. plate heat exchanger.
3. Classification by materials used in heat exchangers
(1) Metal material heat exchanger: Metal material heat exchanger is made of metal materials. Commonly used metal materials are carbon steel, alloy steel, copper and copper alloys, aluminum and aluminum alloys, titanium and titanium alloys. Due to the high thermal conductivity of metal materials, the heat transfer efficiency of this type of heat exchanger is high, and metal material heat exchangers are mainly used in production.
(2) Non-metallic material heat exchangers: Non-metallic material heat exchangers are made of non-metallic materials. Commonly used non-gold scrap materials include graphite, glass, plastic and ceramics. This type of heat exchanger is mainly used for corrosive materials due to the low thermal conductivity of non-metallic materials, so its heat transfer efficiency is low.
2. Heat exchanger structure and performance characteristics
(1) The structural form of the tubular heat exchanger
1. Shell and Tube Heat Exchanger
Shell and tube heat exchanger, also known as shell and tube heat exchanger, is a general standard heat exchange equipment. It has the advantages of simple structure, sturdy and durable, low cost, wide range of materials, convenient cleaning, strong adaptability, etc. It is the most widely used and occupies a dominant position in heat exchange equipment. Shell and tube heat exchangers are divided into the following types according to their structural characteristics.
(1) Fixed tube plate heat exchanger
It consists of shells, tube bundles, heads, tube sheets, baffles, nozzles and other components. Its structural feature is that two tube sheets are welded to the two ends of the shell respectively, and both ends of the tube bundle are fixed on the tube sheets. The whole heat exchanger is divided into two parts: the channel in the heat exchange tube and the place where it connects with both ends is called the tube side; the channel outside the heat exchange tube and the place where it connects is called the shell side. The cold and hot fluids flow continuously in the tube side and the shell side respectively. The fluid flowing through the tube side is called the tube (tube side) fluid, and the fluid flowing through the shell side is called the shell (shell side) fluid.
If the tube fluid passes through the tube pass at one time, it is called a single tube pass. When the heat transfer area of the heat exchanger is large and the number of tubes required is large, in order to improve the flow rate of the tube fluid, the heat exchange tubes are often divided into several groups on average, so that the fluid goes back and forth in the tube multiple times, which is called multi-tube pass. . The number of tube passes can be 2, 4, 6, or 8. If it is too large, it will increase the flow rate of the fluid in the tube, thereby increasing the convective heat transfer coefficient in the tube, but at the same time, it will increase the flow resistance. Therefore, the number of tube passes should not be too many, usually 2. The tube pass is the most common.
The shell fluid passes through the shell pass once, which is called a single shell pass. In order to increase the flow rate of the shell fluid, longitudinal baffles can also be placed in the direction parallel to the axis of the tube bundle to divide the shell side into multiple paths. The number of shell passes is the number of times the shell fluid travels back and forth along the axial direction of the shell within the shell pass.
The split range can increase the fluid velocity of the shell, increase the flow, and intensify the disturbance, which helps to strengthen the heat transfer. However, the splitting of the shell side not only increases the flow resistance, but also makes it difficult to manufacture and install, so it is less used in engineering. In order to improve the heat transfer on the shell side, baffles are usually used, and the purpose of strengthening heat transfer is achieved by setting baffles.
The advantage of the fixed tube sheet heat exchanger is its simple and compact structure. Within the same shell diameter, the number of tubes is the largest and the bypass is the least; each heat exchange tube can be replaced, and the inside of the tube is easy to clean. The disadvantage is that the shell side cannot be mechanically cleaned;
When the temperature difference between the heat exchange tube and the shell is large (greater than 50°C), temperature difference stress occurs, and an expansion joint needs to be installed on the shell, so the shell side pressure cannot be too high due to the strength of the expansion joint. The fixed tube plate heat exchanger is suitable for the case where the shell-side fluid is clean and not easy to scale, and the temperature difference between the two fluids is not large or the temperature difference is large but the shell-side pressure is not high.
(2) Floating head heat exchanger
The floating head type heat exchanger is characterized by its structure that one of the tube sheets at both ends is not fixedly connected to the shell, and can freely expand and contract in the axial direction in the shell. This end is called the floating head. The advantage of the floating head heat exchanger is that when there is a temperature difference between the heat exchange tube and the shell, when the shell or the heat exchange tube expands, they are not bound to each other, and no temperature difference stress will be generated; the tube bundle can be extracted from the shell, which is convenient for inside and between tubes. cleaning. The disadvantage is that the structure is more complicated, the amount of materials used is large, and the cost is high; if the sealing between the floating head cover and the floating tube sheet is not strict, internal leakage will occur, resulting in the mixing of the two media. The floating head heat exchanger is suitable for occasions where the temperature difference between the shell and the tube bundle wall is large or the medium on the shell side is easy to scale.
(3) U-tube heat exchanger
U-shaped tube heat exchanger is characterized in that there is only one tube sheet, the heat exchange tube is U-shaped, and both ends of the tube are fixed on the same tube sheet. The tube bundle can be freely stretched, and when there is a temperature difference between the shell and the U-shaped heat exchange tube, there will be no temperature difference stress. The advantages of U-shaped tube heat exchanger are simple structure, only one tube sheet, less sealing surface, reliable operation and low cost; the tube bundle can be drawn out, and the cleaning between the tubes is convenient.
The disadvantage is that it is difficult to clean the inside of the tube; because the tube needs to have a certain bending radius, the utilization rate of the tube sheet is low; the distance between the innermost tubes of the tube bundle is large, and the shell side is easy to short-circuit; the inner tube is broken and cannot be replaced, so the scrap rate is higher.
The U-tube heat exchanger is suitable for the occasions where the temperature difference between the tube and the shell wall is large or the medium on the shell side is easy to scale, but the medium on the tube side is clean and not easy to scale, and the high temperature, high pressure and strong corrosion are occasions. Generally, the medium with high temperature, high pressure and strong corrosiveness is routed in the pipe, which can reduce the high pressure space, make the seal easy to solve, save materials and reduce heat loss.
(4) stuffing box heat exchanger
The structural feature of the stuffing box heat exchanger is that only one end of the tube sheet is fixedly connected to the shell, and the other end is sealed with a stuffing box. The tube bundle can expand and contract freely without thermal stress caused by the temperature difference between the shell wall and the tube wall.
The advantage of the stuffing box heat exchanger is that the structure is simpler than the floating head heat exchanger, the manufacture is convenient, the consumables are few, and the cost is low; the tube bundle can be extracted from the shell, and the inside and between the tubes can be cleaned, which is convenient for maintenance. The disadvantage is that the pressure resistance of the stuffing box is not high, generally less than 4.0MPa; the shell-side medium may leak through the stuffing box, which is not suitable for flammable, explosive, toxic and valuable media.
The stuffing box heat exchanger is suitable for occasions where the temperature difference between the tube and the shell wall is large or the medium is easy to scale, which needs to be cleaned frequently and the pressure is not high.
(5) Kettle heat exchanger
The structural feature of the kettle heat exchanger is that an appropriate evaporation space is set on the upper part of the shell, and it also functions as a steam chamber. Tube bundles can be fixed tube sheet, floating head or U-tube. The kettle heat exchanger is easy to clean and maintain, can handle unclean and fouling media, and can withstand high temperature and high pressure. It is suitable for liquid-vapor heat exchange and can be used as a waste heat boiler with the simplest structure.
In addition to the above five types of shell and tube heat exchangers, there are other types such as intubation heat exchangers and sliding tube sheet heat exchangers.
2. Coiled tube heat exchanger
Coiled tube heat exchanger is a kind of heat exchange equipment with the simplest structure and the most convenient operation among the tube heat exchangers. Generally, according to the different heat exchange methods, the coil type heat exchanger is divided into two types: immersion type and spray type.
(1) Immersion coiled tube heat exchanger This type of heat exchanger is mostly made of bent metal tubes, made to suit the shape of the container, and immersed in the liquid in the container. The two fluids exchange heat inside and outside the tube, respectively. Several commonly used coil shapes.
The advantages of the immersed coil heat exchanger are simple structure, low price, easy anti-corrosion, and can withstand high pressure. The disadvantage is that the volume of the container is much larger than that of the coiled tube, and the heat transfer film coefficient of the fluid outside the tube is small, so it is often necessary to add a stirring device to improve its heat transfer efficiency.
(2) Spray coil heat exchanger, which is mostly used for cooling the hot fluid in the pipe. The coiled tubes fixed on the support are arranged on the same vertical plane, and the hot fluid enters from the lower tube and flows out from the upper tube.
The cooling water is evenly sprayed on the upper coil pipe by the spray device above the pipe, and drips down along the outer surface of the pipe, drops to the surface of the lower coil pipe, and finally collects in the bottom plate of the pipe. The device is usually placed in the outdoor air circulation place. When the cooling water is vaporized in the air, it can take away part of the heat to improve the cooling effect.
Compared with the immersion coil heat exchanger, the spray coil heat exchanger has the advantages of convenient maintenance and cleaning, and good heat transfer effect. The disadvantage is that it is bulky and covers a large area; the cooling water volume is large, and the spraying is not easy to be uniform. Coiled tube heat exchangers are often used in refrigeration devices and small refrigeration units because of their simple structure and easy operation.
3. Casing Heat Exchanger
The casing heat exchanger is a concentric casing formed by two straight pipes of different diameters. -43 shown. Each section of casing is called a pass, and the number of passes can be increased or decreased according to the requirements of the heat transfer area. During heat exchange, one fluid goes through the inner tube, the other fluid goes through the annular gap, and the wall of the inner tube is the heat transfer surface.
The advantages of the casing heat exchanger are that the structure is simple; it can withstand high pressure; the heat transfer area can be increased or decreased as required; the proper selection of the inner and outer diameters of the tube can increase the flow rate of the fluid, and the two fluids flow in countercurrent, which is beneficial to heat transfer. The disadvantage is that the metal consumption per unit of heat transfer area is large; there are many pipe joints, and maintenance and cleaning are inconvenient. This type of heat exchanger is suitable for heat exchange between high temperature, high pressure and small flow fluids.
(2) Structural form of plate heat exchanger
1. Flat heat exchanger
The plate heat exchanger is referred to as the plate heat exchanger, and its structure is shown in Figure 4-46. It is composed of a group of rectangular thin metal plates arranged in parallel, clamped and assembled on the bracket. The edges of the two adjacent plates are lined with gaskets, and a sealed fluid channel is formed between the plates after pressing, and the size of the channel can be adjusted by the thickness of the gasket.
On the four corners of each plate, a circular hole is opened, two of which are communicated with the flow channels on the plate surface, and the other two circular holes are not communicated. Their positions are staggered on adjacent plates to form channels for the two fluids, respectively. Cold and hot fluid flow alternately on both sides of the plate, and heat exchange is carried out through the metal plate.
Plate is the core component of plate heat exchanger. In order to make the fluid flow evenly across the plate surface, increase the heat transfer area, and promote the turbulence of the fluid, the plate surface is often punched into a concave-convex corrugated shape. There are dozens of corrugated shapes. The commonly used corrugated shapes include horizontal corrugations and herringbone corrugations. and arc-shaped ripples, etc.
The advantages of plate heat exchangers are compact structure and large heat exchange area provided by the unit volume of equipment; flexible assembly, the number of plates can be increased or decreased according to needs to adjust the heat transfer area; the corrugation of the plate surface makes the change of the cross section complicated, and the disturbance effect of the fluid is enhanced , with high heat transfer efficiency; easy disassembly and assembly, conducive to maintenance and cleaning.
The disadvantage is that the processing capacity is small; the operating pressure and temperature are limited by the performance of the sealing gasket material and should not be too high. The plate heat exchanger is suitable for occasions where cleaning is often required, the working environment is very compact, the working pressure is below 2.5 MPa, and the temperature is -35 °C ~ 200 °C.
2. Spiral Plate Heat Exchanger
Spiral plate heat exchanger is shown in picture 4-48, it is rolled by two parallel thin metal plates with a certain interval. Two thin metal plates form two concentric spiral channels, spacer columns are welded between the two plates to maintain the channel spacing, and cover plates are welded on both sides of the spiral plates. The cold and hot fluids pass through the two channels respectively, and the heat is exchanged through the thin plate.
The advantage of the spiral plate heat exchanger is that the fluid in the spiral channel reaches turbulent flow at a lower Reynolds number due to the action of inertial centrifugal force and the interference of the spacer column, and a higher flow rate is allowed, so the heat transfer coefficient is large;
Due to the high flow rate and the effect of inertial centrifugal force, the suspended matter in the fluid is not easy to deposit, so the spiral plate heat exchanger is not easy to scale and block; due to the long flow of the fluid and the complete reverse flow of the two fluids, it can be used in small It can make full use of the low temperature heat source; the structure is compact, and the heat transfer area per unit volume is about 3 times that of the shell and tube heat exchanger.
The disadvantage is: the operating temperature and pressure should not be too high, the current maximum operating pressure is 2MPa, and the temperature is below 400 ℃; because the entire heat exchanger is made of coils, once a leak is found, maintenance is very difficult.
3. Hot Plate Heat Exchanger
The hot plate heat exchanger is a new type of high-efficiency plate heat exchanger, and its basic heat transfer unit is a hot plate. The forming method is to spot-weld or roll-weld double-layer or multi-layer metal plates into various patterns according to the principle of equal resistance flow, and weld and seal the edges to form a whole.
The space between the plates is inflated under high pressure to achieve the flow channel structure of the best flow state. The thickness of each layer of metal plates can be the same or different, and the number of plates can be double or multiple layers, thus forming a variety of heat transfer surface forms of the heat plate, such as unequal thickness double layer heat plate, equal thickness double layer heat plate Plate, three-layer unequal-thickness hot plate, four-layer equal-thickness hot plate, etc., can be selected according to needs when designing.
The hot plate heat exchanger has the best flow state, low resistance and high heat transfer efficiency; it can be manufactured into various shapes according to engineering needs, and different plates can also be selected according to the performance of the medium. The hot plate heat exchanger can be used in various processes such as heating, heat preservation, drying, condensation, etc. As a new type of heat exchanger, it has broad application prospects.
(3) The structure of the heat pipe heat exchanger
The heat pipe heat exchanger with the heat pipe as the heat transfer unit is a new type of high-efficiency heat exchanger, which is composed of a shell, a heat pipe and a partition. As the main heat transfer element, the heat pipe is a heat transfer device with high thermal conductivity. It is a vacuum container whose basic components are shell, wick and working fluid. After the shell is evacuated and filled with an appropriate amount of working fluid, the closed shell forms a heat pipe. When the heat source supplies heat to one end of the shell, the working fluid absorbs heat from the heat source and evaporates and vaporizes. Under the action of the pressure difference, the steam carrying the latent heat is transported to the other end of the shell at a high speed, and releases the latent heat to the cold source to condense, and the condensate returns to the Hot end, boil again to vaporize. In this repeated cycle, heat is continuously transferred from the hot end to the cold end.
Schematic diagram of heat pipe
The heat pipe is divided into three types: liquid-absorbing core heat pipe, gravity heat pipe and centrifugal heat pipe according to the condensate circulation mode. The condensate of the liquid-absorbing core heat pipe returns to the hot end by the action of the capillary, and this heat pipe can work under the condition of weightlessness; the condensate of the gravity heat pipe flows back to the hot end by gravity, and its heat transfer is unidirectional, generally as vertical
