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If you would like to learn more about Alaqua's Evaporators Systems, Solvent Recovery Systems, Crystalizers, Heat Exchangers, Distillation Equipment, and Spray Dryers, contact our office today.
In the field of thermal separation / concentration technology, evaporation plants are widely used for concentration of liquids in the form of solutions, suspensions, and emulsions.
The major requirement in the field of evaporation technology is to maintain the quality of the liquid during evaporation and to avoid damage to the product. This may require the liquid to be exposed to the lowest possible boiling temperature for the shortest period of time.
This and numerous other requirements and limitations have resulted in a wide variation of designs available today. In almost all evaporators the heating medium is steam, which heats a product on the other side of a heat transfer surface. The following list contains links to descriptions of the most common types of evaporators, and the ones that we most frequently design:
Falling Film Evaporators Print Send link Home / Evaporation & Crystallization / Evaporator Systems / Falling Film Evaporators In falling film evaporators the liquid product (A) usually enters the evaporator at the head (1) of the evaporator. In the head the product is evenly distributed into the heating tubes. A thin film enters the heating tube are it flows downwards at boiling temperature and is partially evaporated. In most cases steam (D) is used for heating the evaporator. The product and the vapor both flow downwards in a parallel flow. This gravity-induced downward movement is increasingly augmented by the co-current vapor flow. The separation of the concentrated product (C) form its vapor (B) is undergoing in the lower part of the heat exchanger (3) and the separator (5).
In falling film evaporators the liquid product (A) usually enters the evaporator at the head (1) of the evaporator. In the head the product is evenly distributed into the heating tubes. A thin film enters the heating tube are it flows downwards at boiling temperature and is partially evaporated. In most cases steam (D) is used for heating the evaporator. The product and the vapor both flow downwards in a parallel flow. This gravity-induced downward movement is increasingly augmented by the co-current vapor flow. The separation of the concentrated product (C) form its vapor (B) is undergoing in the lower part of the heat exchanger (3) and the separator (5).
Falling film evaporators can be operated with very low temperature differences between the heating media and the boiling liquid, and they also have very short product contact times, typically just a few seconds per pass. These characteristics make the falling film evaporator particularly suitable for heat-sensitive products, and it is today the most frequently used type of evaporator.
However, falling film evaporators must be designed very carefully for each operating condition; sufficient wetting (product film thickness) of the heating surface by liquid is extremely important for trouble-free operation of the plant. If the heating surfaces are not wetted sufficiently, dry patches and incrustations will occur; at worst, the heating tubes will be completely clogged. In critical cases the wetting rate can be increased by extending or dividing the evaporator effects, keeping the advantages of single pass (no recirculation of product) operation.
The proper design of the product distribution system in the head of the evaporator is critical to achieve full and even product wetting of the tubes.
Because of the low liquid holding volume in this type of unit, the falling film evaporator can be started up quickly and changed to cleaning mode or another product easily.
Falling film evaporators are highly responsive to alterations of parameters such as energy supply, vacuum, feed rate, concentrations, etc. When equipped with a well designed automatic control system they can produce a very consistent concentrated product.
The fact that falling film evaporators can be operated with small temperature differences makes it possible to use them in multiple effect configurations or with mechanical vapor compression systems in modern plants with very low energy consumption.
These operate on a "thermo-siphon" principle. Feed product (A) enters the bottom of the heating tubes and as it heats, steam begins to form. The ascending force of this steam produced during the boiling causes liquid and vapors to flow upwards in parallel flow. At the same time the production of vapor increases and the product is pressed as a thin film on the walls of the tubes, and the liquid rises upwards. This co-current upward movement against gravity has the beneficial effect of creating a high degree of turbulence in the liquid. This is advantageous during evaporation of highly viscous products and products that have a tendency to foul the heating surfaces.
Usually there must be a rather high temperature difference between the heating and boiling sides of this type of evaporator. Otherwise the energy of the vapor flow is not sufficient to convey the liquid and to produce the rising film. The length of the boiling tubes will typically not exceed 23 ft (7m).
This type of evaporator is often used with product recirculation, where some of the formed concentrate is reintroduced back to the feed inlet in order to produce sufficient liquid loading inside the heating tubes. A number of different designs have been developed using this basic principle. A good example is the Roberts evaporator, which is the oldest type of circulation evaporator. This type of evaporator has a wide circulation tube in the center of the heating tube bundle through which concentrate flows back to the bottom of the tube bundle. The Roberts evaporator is still widely used in the sugar industry.
Forced circulation evaporators are used if boiling of the product on the heating surfaces is to be avoided due to the fouling characteristics of the product, or to avoid crystallization. The flow velocity in the tubes must be high, and high-capacity pumps are required.
The circulating product is heated when it flows through the heat exchanger and then partially evaporated when the pressure is reduced in the flash vessel (separator). The liquid product is typically heated only a few degrees for each pass through the heat exchanger. To maintain a good heat transfer within the heat exchanger it is necessary to have a high recirculation flow rate.
This type of evaporator is also used in crystallizing applications because no evaporation, and therefore no concentration increase, takes place on the heat transfer surface. Evaporation occurs as the liquid is flash evaporated in the flash vessel/separator. In crystallizer applications this is then where the crystals form, and special separator designs are used to separate crystals from the recirculated crystal slurry. More information about crystallization is available in the crystallization section.
The heat exchanger (in evaporator parlance sometimes called the "calandria") can be arranged either horizontally or vertically depending on the specific requirements in each case.
Instead of tube and shell heat exchangers, framed plates can be used as an heating surface. These plate assemblies are similar to plate heat exchangers, but are equipped with large passages for the vapor flow. In these units a product plate and a steam plate are connected alternately. The product passage is designed for even distribution of liquid on the plate surfaces and low pressure drop in the vapor phase.
Plate evaporators are of compact design. Separators are flanged directly to the plate packages with short interconnecting pipe-work. Thus, space requirements are low and the building height normally does not exceed 10-13 ft (3-4m). This means that plate evaporators can be installed in most buildings.
5-effect plate evaporator, directly heated, with plate condenser, aroma recovery and concentrate cooling system. Product: apple juice at 11% TS. concentrated to 72% TS. Evaporation rate: 25,000 lbs/hr (~11,340kg/h).
They are designed for single pass rising film operation. This results in even and gentle evaporation of the product. Depending on the duty, the evaporator can be operated with product circulation as well.
Since the plate package can be opened easily, surfaces can be inspected, individual plates changed if necessary, and the evaporation rate can be altered by adding or removing individual plates. The units can be designed to meet USDA Dairy sanitary requirements.
Alaqua's multi-purpose compact evaporators have numerous advantages that make them excellent choices for dairy, beverage, specialty food, and industrial fluid applications.
The compact evaporator operates by the simple falling film principle, in which fluid is pumped into the top of each calandria through a liquid distributor and flows down the inside of each tube as a thin liquid film. Hot vapors condense on the outside tube walls, releasing heat that is transferred to the fluid. Vigorous boiling takes place as the fluid is falling vertically at turbulent conditions. Concentrate plus product vapors exit the tube bottoms, to be separated in the calandria bottom and integral separator. Fluid may be concentrated in one or more effects in series, flowing parallel to or counterflow with the vapor flow. All condensed vapors are removed from the calandria shells as liquid condensate. Waste heat is absorbed in a surface condenser.
Heating of the evaporator is typically by live steam or by thermal vapor recompression (TVR), with some vapors recycled for improved efficiency.
This evaporator design is functionally identical to the time-tested and broadly accepted long-tube falling film evaporators that are so extensively used for food, pharmaceutical, and many industrial applications.
In multiple-effect evaporators with TVR, the heating medium in the first calandria is the product vapor from one of the associated effects, compressed to a higher temperature level by means of a steam ejector (TVR). The heating medium in any subsequent effect is the vapor generated in the previous calandria. Vapor from the final effect is condensed with incoming product, supplemented by cooling water if necessary. The condensate can be used a boiler feedwater, CIP liquid, or for preheating the drying air of an associated spray dryer.
In evaporators with MVR, the heating medium in the first effect is vapor developed in the same effect, compressed to a higher temperature by means of a high-pressure fan (MVR). Any excess vapor from the high heat section is condensed or may be utilized in a high concentrator. The condensate temperature is, however, too low for further beneficial waste heat utilization.
Since prices for steam and electricity vary by region, the choice between MVR and TVR (and in the case of TVR, the number of stages) depends on local prices, possible utilization of hot condensate, and depreciation of the capital cost. Both systems produce the same product quality as long as certain critical design parameter requirements are met.
Evaporation Process Principles:
Evaporation is an operation used to remove a liquid from a solution, suspension, or emulsion by boiling off some of the liquid. It is thus a thermal separation, or thermal concentration, process. We define the evaporation process as one that starts with a liquid product and ends up with a more concentrated, but still liquid and still pump-able concentrate as the main product from the process. There are actually a few instances where the evaporated, volatile component is the main product, but we will not discuss that here.
In most cases it is essential that the product is subject to minimal thermal degradation during the evaporation process, requiring that temperature and time exposure must be minimized. This and other requirements brought on by the physical characteristics of the processed product have resulted in the development of a large range of different evaporator types. Additional demands for energy efficiency and minimized environmental impact have driven development toward very innovative plant configurations and equipment design. Our technology is supported by several test and development facilities, where the technology is being continually refined, improved, and applied to new products.
During the design of evaporation plants, numerous, sometimes contradictory, requirements have to be considered. They determine which type of construction and arrangement is chosen, and the resulting process and economic data. The most important requirements are as follows:
To learn more about our evaporator systems & crystallizer systems, please contact us today.