• In falling film evaporators the liquid product usually enters the evaporator at the top. In the top head the product is evenly distributed into the heating tubes. A thin film enters the heating tube and flows downwards at boiling temperature and is partially evaporated. In most cases steam is used for heating the evaporator. The product and the vapors inside the heating tubes 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 from its vapor takes place in the vapor separator.
• Falling film evaporators can be operated with low temperature differences between the heating media and the boiling liquid, and they also have short product contact times, typically measured in 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 carefully for each operating condition; sufficient wetting (product film thickness) of the heating surface by liquid is important for trouble-free operation of the plant. If the heating surfaces are not wetted sufficiently, dry patches and scaling will occur; at worst, the heating tubes will be 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 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 low energy consumption.

• These operate on a “thermo-siphon” principle. Feed product 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 upward movement against gravity has the beneficial effect of creating a high degree of turbulence in the liquid. This is advantageous during evaporation of viscous products and products that have a tendency to foul the heating surfaces.
• Usually there must be a high temperature difference between the heating and boiling sides in this type of evaporator. Otherwise the energy of the vapor flow is not sufficient to convey the liquid and to produce the rising film.
• 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.

• 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 on the heat transfer surface. 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 vapor separator. The liquid product is typically heated only a few degrees for each pass through the heat exchanger. To maintain good heat transfer within the heat exchanger it is necessary to have a high recirculation-flow rate.
• This type of evaporator is used in crystallization applications. Evaporation occurs as the liquid is flash evaporated in the vapor separator. Special separator designs are used to separate crystals from the recirculated crystal slurry.
• The heat exchanger 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 connected to the plate packages with short interconnecting pipe-work. Thus, space requirements are low and the height normally does not exceed 20 to 25 ft (6-8 meters). This means that plate evaporators can be installed in most buildings.
• 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.

In multiple-effect evaporators with TVR, the heating medium in the first effect 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 effect. Vapor from the final effect is condensed with incoming product, supplemented by cooling water. The condensate is high quality water that can be used as boiler feedwater, CIP liquid, for preheating the drying air of an associated spray dryer, or any other application.

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 fan or blower (MVR).

Energy Consumption
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. Generally a TVR has lower CAPEX but higher OPEX, whereas an MVR has higher CAPEX but lower OPEX which makes it attractive. Both systems produce the same product quality as long as certain critical design parameter requirements are met.

Features

1. External straight tube preheaters give short residence time, better deaeration of the calandrias, and easy inspection and cleaning.
2.  Pasteurizing systems (indirect or direct) meet the most stringent product specifications.
3.  Static liquid distribution system ensures that all tubes in the calandrias receive equal amounts of liquid at all times and can accept wide variations in liquid flow and flash vapor.
4. Freestanding design reduces floor space requirements and building costs, and is flexible in arrangement for installation in existing buildings.
5.  Efficient liquid-vapor separation results from controlled vapor velocities with tangential inlet and outlet ensuring minimum pressure losses.
6. Cleaning costs are minimized by CIP procedures, which may be fully automated.
7.  Instrumentation is per customer requirements, including PLC that can optimize product output and quality.

Compact Evaporator Design Has Multiple Advantages:
Alaqua’s multi-purpose compact evaporators have numerous advantages that make them excellent choices for dairy, beverage, specialty food, and industrial fluid applications.

1. Ideal for heat-sensitive products, up to 20,000 lbs/hr (~9,000kg/h) water removal and boiling temperatures as low as 115°F (46°C).
2.  Flexible, simple-to-operate process with full automation allows handling of a variety of products.
3. Low maintenance costs, with stainless steel components and minimum moving parts.
4.  Tubular design with few gaskets minimizes the possibility of vacuum leaks.
5.  Sanitary 3A standards, in compliance with dairy processing requirements.
6.  Complete incorporation of CIP provisions as necessary.
7.  Modular design with low headroom and small footprint fits easily into existing buildings.
8.  Fully skid-mounted, pre-assembled unit allows easy site installation without extensive foundation work.
9. The compact evaporator operates by the simple falling film principle, in which fluid is pumped into the top of each heater 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 heater 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.

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