Original Title: [Dry Goods] Structures and Working Principles of 22 Kinds of Evaporation and Crystallization Equipment, Illustrated Central-circulation tubular evaporator The heating chamber of the central circulation tube evaporator is composed of a vertical heating tube bundle (boiling tube bundle). In the center of the tube bundle, there is a tube with a larger diameter, called the central circulation tube, whose cross-sectional area is generally 40-100% of the total cross-sectional area of the heating tube bundle. When the heating medium is introduced between the tubes for heating, the heated area of the liquid per unit volume in the heating tube is larger than that in the central circulating tube, so that the relative density of the liquid in the heating tube is small, thereby causing a density difference between the liquid in the central circulating tube and that of the heating tube, and the density difference causes the solution to flow in a natural circulation manner of descending from the central circulation tube and then ascending from the heating pipe. The circulation speed of the solution depends on the density difference produced by the solution and the length of the tube. The greater the density difference, the longer the tube, and the greater the circulation speed of the solution. However, due to the limitation of the total height, the length of the heating tube of this type of evaporator is generally 1 ~ 2 m, the diameter is 25 ~ 75 mm, and the length-diameter ratio is 20 ~ 40. Performance characteristics: The central circulation tube evaporator has the advantages of compact structure, convenient manufacture and reliable operation, so it is widely used in industry and is called "standard evaporator". But in fact, due to the limitation of structure, its circulation speed is relatively low (generally below 0.5m/s); and because the solution circulates continuously in the heating tube, its concentration is always close to the concentration of the finished solution, so the boiling point of the solution is high and the effective temperature difference is reduced. In addition, the cleaning and maintenance of the equipment is not convenient enough. Externally heated evaporator The structural feature of the externally heated evaporator is that the heating chamber is separated from the separation chamber, which not only facilitates cleaning and replacement, but also reduces the total height of the evaporator. Because the heating pipe is long (the ratio of the pipe length to the pipe diameter is 50-100), and the solution in the circulating pipe is not heated, the circulating speed of the solution is high, which can reach 1.5m/s. Rising film evaporator The heating chamber of the rising film evaporator is composed of one or several vertical long tubes. The diameter of the heating tube is usually 25 ~ 50mm, and the ratio of the tube length to the tube diameter is 100 ~ 150. After being preheated, the raw material liquid enters from the bottom of the evaporator, and the heated steam is condensed outside the tube. When the solution is heated and boiled, it vaporizes rapidly, and the generated secondary steam rises at a high speed in the tube, driving the liquid to flow upward along the inner wall of the tube in a film shape, and the rising liquid film continues to evaporate due to heating. Therefore, the solution is gradually concentrated in the process of rising from the bottom to the top of the evaporator, and the concentrated solution enters the separation chamber and is discharged from the bottom of the separator after being separated from the secondary steam. The secondary steam velocity at the outlet of the heating tube under normal pressure shall not be less than 10 m/s, generally 20 ~ 50 m/s, and sometimes 100 ~ 160 m/s or higher during decompression operation. Performance characteristics: Expand the full text The climbing film evaporator is suitable for the solution with large evaporation capacity (i.e. dilute solution), heat sensitivity and easy foaming, but not suitable for the solution with high viscosity,rotovap distillation
, crystal precipitation or easy scaling. Horizontal evaporator It is basically similar to the structure of the horizontal shell and tube condenser. According to the way of liquid supply, it can be divided into shell and tube evaporator and dry evaporator. Horizontal shell and tube evaporators are widely used in closed saline circulation systems. Performance characteristics: Compact structure, good contact between liquid and heat transfer surface, high heat transfer coefficient. However, it needs to be filled with a large amount of refrigerant, and the liquid column will have a certain impact on the evaporation temperature. And when the concentration of the saline is reduced or the brine pump is shut down for some reason, the saline may be frozen in the pipe. If the refrigerant is Freon, it is difficult for the lubricating oil dissolved in the Freon to return to the compressor. In addition, it is necessary to stop working during cleaning. Vertical tube type cold water tank evaporator Spiral tube evaporator The common point of vertical tube and spiral tube evaporators is that the refrigerant evaporates in the tubes, and the whole evaporator tube group is immersed in the box (or pool, tank) filled with secondary refrigerant. In order to ensure that the secondary refrigerant circulates in the box at a certain speed, a longitudinal partition is welded in the box and a spiral stirrer is installed. The coolant flow rate is generally 0.3 ~ 0.7 m/s to enhance heat transfer.
Performance characteristics: Vertical tube and spiral tube evaporators can only be used in open cycle systems, so the secondary refrigerant must be non-volatile substances, such as saline and water. If saline is used,50l rotovap
, the evaporator tubes are easily oxidized, and the saline is easy to absorb moisture and reduce the concentration. These two evaporators can directly observe the flow of secondary refrigerant and are widely used in saline refrigeration system with ammonia as refrigerant. Cooling tube bank A cooling calandria is an evaporator used to cool the air. It is widely used in low temperature cold storage. The refrigerant flows and evaporates in the cooling tube, and the cooled air outside the tube as the heat transfer medium makes natural convection. Performance characteristics: The biggest advantage of the cooling calandria is that it is simple in structure, easy to make, and causes less dry loss to the non-packaged food stored in the warehouse. However, the heat transfer coefficient of the calandria is low, and the defrosting operation is difficult, which is not conducive to the realization of automation. For the ammonia direct cooling system, the seamless steel tube is welded, and the smooth tube or wound finned tube is used; for the Freon system, the wound or set finned copper tube is mostly used. Coil type calandria Coil type jacking pipe gravity liquid supply or ammonia pump liquid supply can be used; single-row and double-row coil type wall calandria can be used for an ammonia pump liquid supply system and a gravity liquid supply system of a bottom-in and top-out type, and a single coil type calandria can also be used for an ammonia pump top in and bottom out liquid supply system and a thermal expansion valve liquid supply system. Performance characteristics: The coiled calandria has the advantages of simple structure, easy manufacture, small liquid storage and strong applicability. Its main disadvantage is that the steam produced in the lower section of the calandria can not be drawn out in time and can only be discharged after passing through the full length of the calandria, so the heat transfer coefficient is small and the vapor-liquid two-phase flow resistance is large. Air cooler (air cooler) Air cooler is a complete set of equipment composed of axial flow fan and cooling calandria. It relies on the fan to force the air in the warehouse to flow through the cooling pipes in the box for heat exchange, so as to cool the air and achieve the purpose of reducing the temperature of the warehouse. The air cooler can be divided into dry type, wet type and dry-wet mixed type according to the way of cooling air. Among them, the refrigerant or secondary refrigerant flows in the calandria and passes through the tube wall to cool the air outside the tube, which is called dry air cooler; the sprayed secondary refrigerant liquid directly exchanges heat with the air, which is called wet air cooler; the mixed air cooler has a secondary refrigerant spraying device in addition to cooling the calandria. Dry air coolers commonly used in cold storage can be divided into ceiling type and floor type according to their installation positions. They are all composed of air cooling pipes, ventilators and defrosting devices, and the cooling pipes in the air cooler are all sheet-type. Large dry air coolers are often floor-mounted. External circulation type evaporation equipment The evaporator is characterized in that the heating pipe is lengthened and the heating chamber is arranged outside the evaporator, so that the total length of the evaporator can be reduced, and meanwhile, the circulating pipe is not heated by steam, so that the natural circulation speed of the solution is higher. Basket evaporator It is an improvement of the central circulation tube evaporator. The heating chamber is like a hanging basket, which is hung at the lower part of the evaporator shell and can be taken out from the top for easy cleaning and replacement. The heat medium enters that heat chamber through the central steam pipe, cbd crystallization equipment
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, and there is an annular channel between the outer wall of the heating cham and the inner wall of the evaporator shell, which acts like a central circulating pipe. During operation, the solution descends along the annulus and ascends along the heating tube, forming a natural circulation. Generally, the cross-sectional area of the annular gap is about 100-150% of the total area of the heating tube, so the solution circulation rate is relatively high (about 1-1.5m/s). Since the boiling liquid is in contact with the evaporator shell at a lower temperature, there is less heat loss. Performance characteristics: The basket-type evaporator is suitable for evaporating the solution which is easy to scale or has crystal precipitation. Its disadvantage is that the structure is complex and the amount of equipment materials required for unit heat transfer surface is large. Levin evaporator The structure of the Levin evaporator is characterized in that a boiling chamber is additionally arranged at the upper part of the heating chamber. In this way, the solution in the heating chamber can be vaporized only when it rises to the boiling chamber due to the action of this additional liquid column. A longitudinal partition is installed above the boiling chamber to prevent bubbles from growing. In addition, since the circulation pipe is not heated, the driving force for the circulation of the solution is large. The height of the circulating pipe is generally 7 ~ 8 m, and its cross-sectional area is about 200 ~ 350% of the total cross-sectional area of the heating pipe. Therefore, the flow resistance in the circulating pipe is small, and the circulating speed can be as high as 2 to 3 m/s.
Performance characteristics: The advantages of Levin evaporator are high circulation speed and good heat transfer effect. Because the solution does not boil in the heating tube, it can avoid the precipitation of crystals in the heating tube, so it is suitable for the treatment of solutions with crystal precipitation or easy scaling. Its disadvantage is that the equipment is huge and the required plant is high. In addition, since the static pressure of the liquid layer is large, the pressure of the heating steam is required to be high. Forced circulation evaporator The above evaporators are all natural circulation evaporators, that is, the circulation of the solution is caused by the density difference between the heating tube and the circulating tube. The circulation speed is generally low, so it is not suitable to deal with the solution with high viscosity, easy scaling and a large number of precipitated crystals. For the evaporation of such solutions, a forced circulation type evaporator may be used. This kind of evaporator uses external power (circulating pump) to make the solution circulate at a high speed in a certain direction. The circulation speed can be controlled by adjusting the flow of the pump. Generally, the circulation speed is above 2.5m/s. Performance characteristics: This kind of evaporator has the advantages of large heat transfer coefficient and good adaptability to materials with high viscosity or easy crystallization and scaling, but its power consumption is large. Falling film evaporator The difference between the falling-film evaporator and the rising-film evaporator is that the feed liquid is added from the top of the heating tube. Under the action of its own gravity, the solution flows downward along the inner wall of the tube in a film shape and is evaporated and concentrated. The vapor-liquid mixture enters the separation chamber from the bottom of the heating tube. After gas-liquid separation, the finished liquid is discharged from the bottom of the separator. In order to make the solution form a uniform film on the wall, the top of each heating tube needs to be equipped with a liquid film distributor. There are many types of film distributor, three of which are commonly used. A cylinder with a spiral groove is used as the draft tube, and the liquid flows down along the groove and is distributed on the inner wall of the whole tube. The lower part of the draft tube is a cone, and the bottom of the cone is concave downward to prevent the liquid flowing down along the cone slope from gathering in the center. The liquid descends along the inner wall of the heating tube in a film shape through the tooth gap. Performance characteristics: The falling film evaporator can evaporate the solution with higher concentration, and is also suitable for the material with higher viscosity. However, it is not suitable for the solution which is easy to crystallize or scale. In addition, because the liquid film is not easy to distribute evenly in the tube, its heat transfer coefficient is smaller than that of the climbing film evaporator. Scraper film evaporator The scraper film evaporator uses the scraper action of the rotating scraper to distribute the liquid on the wall of the heating tube. Its outstanding advantage is that it has strong adaptability to materials, such as materials with high viscosity, heat sensitivity, easy crystallization and scaling. A heating steam jacket is arranged outside the shell of the scraper film evaporator, and a rotatable stirring scraper is arranged inside the shell of the scraper film evaporator, wherein the rotating scraper can be fixed or movable. The gap between the former and the inner wall of the shell is 0.75 ~ 1.5 mm, and the gap between the latter and the wall of the shell varies with the number of revolutions of the stirring shaft. After the feed liquid is added from the upper part of the evaporator along the tangential direction, the solution forms a downward spiral film on the inner wall of the shell under the drive of gravity and the rotary scraper, and is continuously evaporated and concentrated in the descending process to obtain the finished liquid at the bottom. In some cases, the solution may be evaporated to dryness to obtain a solid product directly from the bottom. Performance characteristics: The disadvantages of this kind of evaporator are complex structure, large power consumption, small heat transfer area, generally 3 ~ 4 m2, the maximum is not more than 20 m2, so its processing capacity is small. MVR falling film evaporator The material stock solution is added from the upper tube box of the heat exchanger, the material is distributed into each heat exchange tube through the liquid distributor, a uniform liquid film is formed along the inner wall of the heat exchange tube, the liquid film in the tube is heated by the heating steam of the shell side in the downward flow process, and the liquid film is boiled and evaporated while flowing downward. And that material at the bottom end of the heat exchange tube become concentrated solution and secondary steam. The concentrated solution falls into the lower channel box, and the secondary steam enters the gas-liquid separator. In the gas-liquid separator, liquid droplets entrained in the vapor are removed, and the pure vapor is transferred from the separator to the compressor. The compressor compresses the secondary steam and delivers it as heating steam to the shell side of the heat exchanger for the heat source of the evaporator. And that continuous evaporation proces is realized. Performance characteristics: 1. High heat exchange efficiency 2. Small floor area And 3, the retention time of the materials is short, and the materials are not easy to deteriorate. 4. It is suitable for materials with higher viscosity.
Scope of application: The falling-film evaporator is suitable for the pre-concentration process of MVR evaporation and crystallization, and can evaporate materials with high viscosity, especially heat-sensitive materials, but it is not suitable for the treatment of materials with crystallization. MVR forced circulation evaporator The forced circulation evaporator consists of an evaporation separator, a heat exchanger and a forced circulation pump. The material in the heat exchange tube of the heat exchanger is heated by the steam outside the heat exchange tube to raise the temperature. Under the action of the circulating pump, the material rises into the evaporation separator, and the material is evaporated in the evaporation separator due to the decrease of the static pressure of the material. And secondary steam generate by evaporation overflows from that material, the material is concentrate to generate supersaturation so as to enable crystals to grow, the supersaturated material enters the for circulating pump and enters the heat exchanger under the action of the circulating pump, and the material is circulated in this way to be continuously evaporated and concentrated or concentrated and crystallized. The crystal slurry is output from the circulating pipeline by a discharge pump. The secondary steam in the evaporation separator is delivered to the compressor after being purified by the separation and defoaming device at the upper part of the evaporation separator, and the compressor compresses the secondary steam and delivers the secondary steam to the shell side of the heat exchanger to be used as the heating steam of the evaporator, so as to realize the continuous evaporation of heat energy circulation. Performance characteristics: 1. Low heat transfer coefficient ； 2. The heat exchange surface is not easy to form scale or crystallization 。 Scope of application: It is suitable for the evaporation concentration or evaporation crystallization process of high-viscosity materials which are easy to scale and produce crystallization. MVR Evaporation OSLO Crystallizer OSLO evaporative crystallizer consists of OSLO evaporator, heat exchanger and forced circulation pump. The material in the heat exchange tube of the heat exchanger is heated by the steam outside the heat exchange tube to raise the temperature. Under the action of the circulating pump, the material rises to the OSLO evaporative crystallizer, and the material is evaporated in the OSLO evaporative crystallizer due to the decrease of the static pressure of the material. The secondary steam generated by evaporation overflows from the material, and the material is concentrated to generate supersaturation. The supersaturated solution descends in the central tube of the OSLO evaporation crystallizer and fully contacts with the small crystals in the solution to further grow the crystals. The larger crystals are elutriated by the elutriation column, and the large crystals are precipitated below the elutriation column and transported to the thickener by the crystal slurry pump. Smaller crystals continue to grow in the OSLO crystallizer. The clarified liquid is delivered to the heat exchanger by the forced circulation pump for further heating, and the material is continuously evaporated and concentrated or concentrated and crystallized in this way. The secondary steam in the OSLO evaporation crystallizer is delivered to the compressor after being purified by the separation and defoaming device at the upper part of the separator. The compressor compresses the secondary steam and delivers it to the shell side of the heat exchanger to be used as the heating steam of the evaporator. And that heat energy circulation continuous evaporation is realize. Main features: 1, that crystal granularity is large and uniform; 2. Large equipment volume and high cost Scope of application: It is suitable for the production of materials requiring large crystal size. MVR evaporation DTB crystallizer DTB type crystallizer is a typical crystallizer with internal circulation of crystal slurry. Because the crystallizer is provided with the inner draft tube, the circulating channel is formed, so that the crystal slurry has a good mixing condition, the supersaturation degree can be quickly eliminated in the evaporative crystallization, and the supersaturation degree of the solution can be kept at a lower level. It is especially suitable for products with steep solubility curve. The DTB type crystallizer has good performance and high production strength, can produce larger crystal grains, and is not easy to scab in the crystallizer. It has become one of the main forms of continuous crystallizer. Performance characteristics: High production strength, large crystal particles and stable performance. Scope of application: It is suitable for the production of materials with large crystal size and high production intensity. Vertical crystallization box The vertical crystallization box is usually used for the products with small output and short crystallization period Horizontal crystallization box Horizontal crystallization box is often used for those with large output and long cycle. (1) the volume is large, and the power consumed by the crystal suspension stirring is small; (2) materials with higher crystallization speed can be operated in series for continuous crystallization. The best control for continuous operation is to make the solution start to generate crystal nuclei at the inlet, and generate enough crystal nuclei soon after entering the equipment. These crystal nuclei are suspended in the solution and grow into crystals as the solution moves slowly in the tank. And finally discharging from the other end of the crystallization tank.
Vacuum crystal boiling pot It is suitable for products with fast crystallization speed,hemp extraction centrifuge
, easy natural crystallization and large crystal requirement, and can control the evaporation speed and feeding speed of the solution with simple structure. Source: Water, Gas and Solid Environmental Protection Technology Collection Return to Sohu to see more Responsible Editor:. toptiontech.com