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Evaporator concept
发布者:admin  日期:2011-2-14 9:56:29  点击:2040

evaporator & vaporizer
Evaporator and the film is divided into two major categories of recycling.
Mainly by the heating chamber and the evaporation chamber composed of two parts. Evaporation of the liquid heating chamber to provide the heat needed to promote vaporization of the liquid boiling; evaporation chamber so that complete separation of the two phase. Steam generated in the heating chamber with a large number of liquid foam, to a larger space after the evaporation chamber, the condensed liquid by itself or in addition to the role of foam, etc. and vapor separation. Usually demister in the top of evaporation chamber.
Evaporator operating pressure divided by pressure, compression and decompression 3. Solution in the evaporator by the movement of points are: ① recycling. Boiling the solution several times in the heating chamber by heating the surface, such as the central circular tube, hanging baskets, external thermal, Levin type and forced circulation type and so on. ② One-way type. Boiling a solution in the heating chamber by heating the surface, no circulation, that is, concentrate discharge line, such as the rise film, falling film, mixing film and centrifugal thin film and so on. ③ direct contact type. Heating medium in direct contact with the solution of heat transfer, such as the submerged combustion evaporator. Evaporation device in operation, to spend a lot of steam heating, in order to save the heating steam, can be multi-effect evaporators and steam recompression evaporator. Evaporators are widely used in chemical, light industry and other sectors.
Evaporator vaporizer medicine
Volatile anesthetics showed a liquid at room temperature. Evaporator to be effective in evaporation of volatile liquid anesthetic gases, and can accurately adjust the concentration of anesthetic vapor output. Anesthetics evaporation needs heat, the temperature around the evaporator is to determine the rate of evaporation of volatile anesthetics main factors. Modern anesthesia machines are widely used type of temperature compensation of a flow evaporator, the fresh gas flow in the temperature or change, through the automatic compensation mechanism to maintain a constant evaporation rate of volatile anesthetics, thus leaving the evaporator to ensure that volatile anesthetics output levels stable. Since the boiling point of different volatile inhaled anesthetics and saturation vapor pressure and other physical characteristics, so that, exclusive of the evaporator with drugs, such as the evaporator enflurane, isoflurane, evaporator, each can not be universal. Modern anesthesia machines multi-evaporator loop placed in the anesthesia breathing, the oxygen flow associated with a single connection, evaporation out of steam inhalation anesthetics mix it with the main flow for patients with inhalation.

Evaporation operation in Industry
Evaporation is to use the heating method, the solution containing non-volatile solutes heated to boiling conditions, so that part of the solvent vapor was removed, thereby enhancing the concentration of solute in the solvent unit operations. Application of industrial production operation of evaporation are the following occasions:
1, concentrated products made directly from dilute solution or concentrated solution of re-treatment (such as cooling crystallization) Preparation of solid products, such as the concentration of electrolytic caustic soda solution, concentrated solution of sugar and various fruit juice concentrate, etc.;
2, while the concentrated solution and recovery of solvents, such as organic phosphorus pesticide solution concentration of benzene from benzene, alcohol, medicine production, leachate evaporation, etc.;
3, in order to obtain the pure solvent, such as desalination.
In short, in the chemical industry, food industry, pharmaceutical industry, the evaporation operation is widely used.
L film evaporator

Evaporation characteristics of the operation
Project, the evaporation process is separated from the solution part of the solvent, the solute remained in solution, therefore, is the operation of evaporation of the solution in a volatile solvent and non-volatile solute separation. As the solvent evaporation rate depends on the heat transfer rate, so the evaporation is a heat transfer operation, evaporation equipment for heat transfer equipment, however, the process of evaporation heat transfer compared with the general operation, has the following characteristics:
1, elevation of boiling point solution
Because the solution does not contain volatile solutes, therefore, at the same temperature, the solution than the pure solvent vapor pressure is small, that is, at the same pressure, the boiling point higher than that of pure solvent, the higher the concentration, which more significant impact.
2, material and process characteristics
The material in the enrichment process, often in the heat of solute or impurity deposited on the surface, the formation of crystalline precipitates scale layer of heat transfer; some solutes are heat-sensitive, and long residence time at high temperature perishable; some materials have greater resistance or high viscosity and so on.
3, the energy recovery
Solvent evaporation process is the process of evaporation, latent heat of vaporization as a great solvent, it is a big energy evaporation unit operation. Therefore, the operation of evaporation energy is an important issue to be considered.

Classification of operation of evaporation
1, by evaporation points:
Natural evaporation: the solution temperature below the boiling point of evaporation, such as sea salt, this case, the only solution because the surface of the solvent vaporization, solvent evaporation rate is low.
Evaporation: the solution is heated to boiling point, so that evaporation in the boiling state. Industry are all basically the evaporation of such operations.
2, by heating points:
It is the direct heat source heating fuel and air mixture, so that high temperature combustion and flue gas flame spray nozzle directly into the solution evaporated to heat the solution, evaporation of the solvent evaporation process.
Indirect heat source heating the container partitions to pass the solution evaporated. That the partitions in the heat exchanger the heat transfer process.
3, according to operating pressure points:
Can be divided into pressure, pressure and vacuum (vacuum) evaporation operation. Obviously, the heat-sensitive materials, such as antibiotics solution, juice, etc. should be carried out under vacuum. The high viscosity material to be heated by compression heat source (such as conducting oil, salt, etc.) evaporation
4, according to the number of valid points:
Can be divided into single-effect and multi-effect evaporation. If the evaporation of the secondary steam condensate is no longer used directly, known as single-effect evaporation. If the second effect steam heated steam as the next, and serial number of the evaporator, the evaporation process is the multi-effect evaporation.
The characteristics of the evaporator
Partitions currently used evaporator heat transfer, according to the solution to stay in the case of the evaporator can be divided into one-way loop type and type two categories.
First, the cycle of the evaporator
This type of evaporator, the solution is in the evaporator as a circular flow. As circulation caused by different reasons, can be divided into two types of natural circulation and forced circulation.
1. The central circular tube evaporator evaporator is also known that the standard evaporator. It's heating chamber formed by the vertical tubes, the middle of a large diameter central circular tube, the rest of the smaller diameter heating pipe called the boiling tube. Because the central circulating pipe larger share of the unit volume of the solution heat transfer surface than the boiling tube Rongye occupied units within the smaller, the central loop heat pipes and other heat pipe Rongye different levels, thus boiling tube vapor-liquid mixture density the central loop than the density of the solution in a small tube, combined with the upward rise in the pumping action of steam, the solution will form the evaporator tube from the central circulation decline, rising from the boiling tube circulation. This cycle, mainly caused by the difference between the density of the solution, it is called natural cycle. This role is conducive to heat transfer within the evaporator increased.
To make the solution a good cycle, the central circular tube cross-sectional area is generally the total cross-sectional area of the other heat pipe 40 ~ 100%; heating height is generally 1 ~ 2m; heating between 25 ~ 75mm in diameter. The evaporator as compact structure, convenient manufacture, transfer and operation of good and reliable, is widely used. However, due to structural constraints, circulation rate is not. Solution in the heating chamber with continuous cycle, it has nearly completed the concentration of solution concentration, the boiling point so high, the effective temperature difference to decrease. This is a common circulation evaporator shortcomings. In addition, equipment cleaning and maintenance are not convenient enough, so that the evaporator is difficult to fully meet the production requirements.
2. Hanging basket evaporator evaporator to overcome the cycle of evaporation and crystallization solution, easy to clean, easy to scale and not the disadvantages of the standard evaporator structure was more reasonable improvement, which is hanging basket evaporator. Heating chamber 4, as a basket, hanging lower in the evaporator shell, and the heated outdoor wall and the evaporation channel devices instead of the ring between the inner wall of the central circular tube. Solution along the central heating up, and then follow the wall hanging basket outdoor heating the evaporator wall and the annulus between the downward movement of the form cycle. Because heating of the annulus area of about 100 the total cross-sectional area to 150% faster than standard so the solution circulation evaporator for large, up to 1.5m / s. In addition, the evaporator heating chamber can be removed for repair or replacement of the top, but also less heat loss. Its main drawback is that complex structure, the unit consume more heat transfer area of the metal.
3. Levine evaporator above the natural circulation evaporator, the circulation rate is not big enough, generally in the 1.5m / s or less. To make the evaporator more applicable to Evaporative viscosity and crystallization or scaling solution of the serious and improve the solution circulation rate to extend the operating cycle and reduce the number of cleanings.
Its structure is characterized by boiling in the heating chamber on the additional room. Solution in the heating chamber by the boiling chamber for additional fluid column and not the role of static pressure in the heating tube boiling, until the room up to boiling reduce the pressure on when it begins to boil before, so the solution boiling vaporization moved from the heating chamber no heat transfer surface of the boiling chamber, thus avoiding the crystallization or dirt in the heating tube formation. In addition, this circular tube evaporator heating pipe cross-sectional area about the total cross-sectional area of 2 to 3 times, the solution circulation rate up to 2.5 至 3 m / s or more, so the overall heat transfer coefficient is also larger. The main drawback is that the evaporator liquid column hydrostatic head effect of temperature difference due to losses (meaning see 6.3.1) is large, in order to maintain the effective temperature difference between the requirements of a high pressure steam heating. In addition, the equipment is large, more material consumption, need tall plants and so on. In addition to the natural circulation evaporator, the evaporation of viscosity and crystallization and scaling of the materials, but also by forced circulation evaporator. In this evaporator, the solution of the loop mainly depends on external power, along with a pump to force it to produce a certain direction of the flow loop. The size of circulation rate can be adjusted to control the flow of the pump, generally 2.5m / s or more. Forced circulation evaporator heat transfer coefficient than the average big natural cycle. But it's obvious drawback is energy consumption, heat per square meter area of about 0.4 ~ 0.8kW.
Second, one-way evaporator
This category is characterized by the evaporator: the solution in the evaporator chamber by heating only once, without a concentrated liquid circulation flow that is discharged. Solution through the heating chamber, the flow in the membranous wall was on, so the habit is also known as liquid film evaporator. According to the material flow in the evaporator is different from the evaporator is divided into one-way type the following.
1. L film evaporator heating chamber of its length by a number of vertical tubes. Commonly used in heating pipe diameter 25 ~ 50mm, tube length and diameter ratio of about 100 to 150. The warm liquid from the bottom of the evaporator after the introduction of the heating tube in the boiling heat and quickly vaporize, the steam generated by high-speed increase in the heating tube, when operated under normal atmospheric pressure suitable for the export of gas velocity 20 ~ 50m / s, vacuum motor speed when operating up to 100 to 160m / s or greater. Solutions were driven by the rising steam, rising along the wall and continue to evaporate into the membrane, vapor and liquid mixture in the separator 2 to separate the bottom of the completion fluid discharged from the separator, the second in the top of the export steam. It should be noted that if the liquid from the small amount of water evaporated, it is difficult to achieve the above requirements of the vapor velocity, the rise thicker film evaporator does not apply to the evaporation of the solution; its viscosity and large, easy to crystallize or easy to scale the material does not apply.
2. Falling film evaporator, falling film evaporator and lift-film evaporator difference is that liquid is added from the top of the evaporator, under gravity along the wall down into the membrane, and increased evaporation in the process thick, are concentrated liquid at the bottom. The formation mechanism is different from the l film evaporator, so evaporation of falling film evaporator to a higher concentration, viscosity (for example, 0.05 ~ 0.45Ns/m2 range), heat-sensitive materials. But not easy to film in the pipe evenly distributed, heat transfer coefficient than the smaller rise film evaporator, is still applicable or easy to crystallize the material scale.
The solution in the evaporator was one-way membrane-type flow, and thus greatly improve the heat transfer coefficient, making the solution of the heating chamber in time through the loop no longer meet the requirements of the concentrations, than the cycle with a larger evaporator advantages. Solution does not benefit cycle are: (1) solution in the evaporator is very short residence time and thus particularly suitable for the evaporation heat-sensitive materials; (2) the concentration of the solution, not as complete recycling solution that is always close to concentration, and thus that the effective temperature difference between evaporator. Its main disadvantages are: the volatility of the feed load is very sensitive time when the design or operation is not difficult to film, this time, the heat transfer coefficient will be significantly reduced.
3. Scraper evaporator evaporator jacket enclosure with the heating steam, and its contents are the scraper blade can be rotated. Scraper with both fixed and rotor type, the former with the housing wall of the gap 0.5 ~ 1.5mm, the latter with the wall of the gap with the rotor revolutions and change. Liquid from the evaporator to join the upper tangent (also added to the coaxial with the scraper on the feed tray rejection). Because of gravity, centrifugal force and rotating scraper scraping with a role in the formation of solution in the device under the rotating wall film, and in the evaporation process is complete the liquid in the bottom of the discharge. Zhezhong evaporator is a film using one-way plus power 型 evaporator, its outstanding advantage is the adaptability of materials, and the stay time is short, usually a few seconds or tens of seconds, it can be adapted to high viscosity (such as tannin, honey, etc.) and easy crystallization fouling, heat-sensitive materials. But its complex structure, power consumption, heat transfer surface per square meter is about 1.5 ~ 3kW. In addition, the treatment is very small and the manufacture and installation of high requirements.
Third, direct contact with the evaporator heat transfer
Actual production, and sometimes also applied in direct contact evaporator heat transfer. It is the fuel (usually gas and oil) and air mixture in the combustion chamber immersed in the solution of the burning flames and smoke produced by high temperature combustion chamber nozzle directly injected into the lower part of the solution evaporated. Direct contact with hot gas and solution, while the heat so that vaporization of water evaporation, water vapor and waste gas produced along with the top of the evaporator discharge. The chamber depth immersion in the solution is generally 0.2 ~ 0.6m, the combustion chamber gas temperature up to 1000 ℃. Is connected by direct contact heat transfer, so it's very good heat transfer, thermal efficiency high. Since the heat transfer does not require a fixed wall, so simple, especially for easy crystal, scaling and corrosive materials evaporation. Ammonium sulfate present in the waste acid treatment and evaporation of the solution, it has been widely used. However, if the evaporation of liquid allowed to be contaminated by smoke, the type of evaporator is generally not applicable. And because of the existence of a large number of gas, limiting the use of secondary steam. In addition the nozzle immersed in the hot liquid, easier to damage. As can be seen from the introduction, many of the structural type evaporator, each has its advantages and disadvantages of the occasion. In the selection, first look at whether it can meet the technical characteristics of the evaporation materials, including materials, viscosity, heat-sensitive, corrosive, and is it easy to mold, or scaling and so on, and then ask for its simple structure, easy to manufacture, metal consumption small, easy maintenance, good heat transfer, etc.

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