X

Liquid food

Temperature

Figure 7. Flow regimes and temperature distribution in a climbing-film evaporator. Source: Reprinted with permission from Ref. 16.

Figure 5. Calandria evaporator. Source: Reprinted with permission from Ref. 14.

Evaporator Centri Therm

Thin film of liquid

Vapor

Figure 6. Climbing-film evaporator. Source: Reprinted with permission from Ref. 12.

Thin film of liquid

Vapor

Figure 6. Climbing-film evaporator. Source: Reprinted with permission from Ref. 12.

falls short of a minimum value, fewer but longer heating tubes may be used, thus giving more liquid per tube while maintaining the same total heating surface area. The next choice is to subdivide the calandria to create more passes on the product side; recirculation of product within a calandria also improves coverage, however, it increases the residence time. Tube length design shall be a compromise between adequate liquid coverage and pressure drop. Short tubes with larger diameters have smaller pressure loss, whereas small diameter tubes lead to high velocity of the two-phase flow, causing high pressure loss and boiling point elevation. To this effect, it has been recommended that tubes between 8 and 12 m in length and with a diameter between 30 and 50 mm be used (16).

Product distribution in the upper part of the calandria can be done in two ways; dynamic distribution or static distribution. Dynamic distribution may be done by a full-cone nozzle in which the kinetic energy of the liquid is used to atomize the fluid into droplets and distribute them on the upper tube sheet. This distribution device, although simple and economic, does not guarantee even distribution. Static distribution devices (Fig. 9) are generally preferred. Even distribution is achieved by having uniform liquid static pressure over all openings in the base of a predistribution bowl, giving uniform wetting of all tubes. The system adapts readily to varying feed conditions by varying the liquid level. The falling-film evaporator is widely used for sugars and syrups, yeast extracts, fruit juices, dairy products and starch processing.

It has been pointed out that both types of long-tube evaporator are characterized by short residence times, high-heat transfer coefficients, and efficient energy use (steam economy = 2.5 to 3.3) in multiple-effect systems (12).

Forced Circulation Evaporators

Forced circulation evaporators have a pump or scraper assembly to move the liquor, usually in thin layers, to enhance heat-transfer rates and shorten residence times. The forced circulation design is generally used in applications where a solid is precipitated during evaporation or a scaling constituent is present.

Recirculated falling-film configurations are used when insufficient feed liquor is available to use the heat-transfer surface with single-pass operation (natural circulation method). A portion of the product liquor is combined with the feed stream and is pumped to the upper liquor cham-

Condensate

Figure 8. (a) Falling-film evaporator, (b) Falling-film evaporator, with falling and climbing sections. Source: Reprinted with permission from Ref. 15.

Produce feed

Produce feed

Figure 9. Liquid distribution in a falling-film evaporator. Source: Reprinted with permission from Ref. 8.

Predistribution device

Liquid level Vapor tubes Distribution plate

Evaporation tubes

Figure 9. Liquid distribution in a falling-film evaporator. Source: Reprinted with permission from Ref. 8.

ber. Product retention time is greater than for the singlepass evaporator but is still relatively short as the operating volume is small. These evaporators find applications in moderately heat-sensitive foods.

The plate type or AVP evaporator, developed in the UK in the 1950s, is suitable for viscous liquids (0.3-0.4 N • s/m2), because the food is pumped through the plate stack, consisting of a series of climbing-film and falling-film sections. The heating medium is steam. They are compact and are advantageous in having a small amount of product in the system at any one time. They have the additional advantage of ease of maintenance and inspection.

An example of a high-temperature, short-time (HTST or flash) evaporator is one with rotating heating surface and is shown in Figure 10. It is a mechanical thin-film evaporator. Feed flows to the underside of a stack of rotating hollow cones from a central shaft. The feed liquor is kept in rapid motion, and the centrifugal force (750-3,000 N) causes the formation of a very thin film (about 0.1 mm thick) in which the liquid remains for only a very short time of 0.6 to 1.6 s (17). Steam condenses on each cone, and heat is conducted rapidly through the thin metal to evaporate the liquor. High rates of heat transfer are obtained as steam condensate is flung from the rotating cones as soon as they are formed. A high degree of flexibility can be achieved by changing the number of cones. This evaporator is used for coffee and tea extracts, meat extract, fruit juices and enzymes for use in food processing.

A schematic cross section of an agitated-film (also called scraped-surface or wiped-surface) evaporator is shown in Figure 11. The product film near the heat-transfer surface is continuously agitated by a rotor to increase the heat-transfer rate on the product side. The evaporator may also consist of a steam jacket surrounding a highspeed rotor, fitted with short blades along its length (Fig. 12). The liquid enters the wide end and is forced through the precise space between the rotor and the wall (heated surface) toward the outlet for the concentrate. The blades agitate the thin film of liquid vigorously, thus promoting high heat-transfer rates and preventing the product from burning onto the hot surface. If the viscosity increases greatly during evaporation, it is better to feed the liquid at the narrow end. This type of equipment is highly suited to viscous (up to 20 N • s/m2), heat-sensitive foods, or to those that are liable to foam or foul evaporator surfaces (fruit pulps and juices, tomato paste, meat extracts, honey, cocoa mass, coffee, and dairy products). Only single effects are possible, thus giving low steam economy. It is more intended for finishing highly viscous products after preconcentration in other equipment (12).

Feed

Steam

Vapor

Feed

Feed

Steam

Vapor

Condensate

Concentrate

Figure 10. Mechanical thin-film evaporator. Source: Reprinted with permission from Ref. 15.

Condensate

Concentrate

Figure 10. Mechanical thin-film evaporator. Source: Reprinted with permission from Ref. 15.

Feed inlet

Rotor assembly

Vaporizing section

Feed inlet

Rotor assembly

Vaporizing section

Product intake

Figure 11. Schematic cross-section of agitated film evaporator. Source: Reprinted with permission from Ref. 4.

Product intake

Figure 11. Schematic cross-section of agitated film evaporator. Source: Reprinted with permission from Ref. 4.

Energy Efficiency

The energy efficiency of evaporation systems may be improved by the following approaches (19): multiple-effect, vapor recompression, or refrigerant cycle evaporators.

Multiple-Effect Evaporating System. For the evaporation of temperature-sensitive materials it is important to keep the temperature as low as possible. This is achieved by

Heated surface

Heated surface

Vapor

Condensate

Figure 12. Centri-term evaporator. Source: Reprinted with permission from Ref. 18.

Vapor

Condensate

Figure 12. Centri-term evaporator. Source: Reprinted with permission from Ref. 18.

working at a lower pressure. Practically all the evaporators mentioned above can be used at pressures below atmospheric.

To make use of the large amount of heat in the vapor driven off the liquid food, the water vapor can be used as the heating medium in a following evaporator. This second evaporator must then operate at a lower pressure than the first. This is called double-effect evaporation. Repetition of this step produces multiple-effect evaporation. The effects then have progressively lower pressures to maintain the temperature difference between the feed and the heating medium. A multiple-effect evaporating system can be designed in several ways in regard to the direction of flow of the solution. Figure 13 illustrates, with a triple-effect evaporator, the arrangements of forward feed, backward feed, parallel feed, and mixed-feed system.

The liquid in the forward feed system enters the highest temperature evaporator first and exits as a concentrated liquid from the third evaporator that operates at the lowest temperature and pressure. The liquid is usually preheated by steam or vapor from one of the effects before it enters the first evaporator. The liquid leaving effect 1 has a higher temperature than the liquid in effect 2, and in crossing the throttle valve to effect 2, a small amount of the liquid water will flash to water vapor and the temperature of the liquid will drop to the operating temperature of effect 2.

In the backward feed system, the liquid flows in opposite direction of the forward feed evaporator. This system allows the concentrated liquid to exit at the highest temperature, which is advantageous for products having a high viscosity in the concentrated form. Because the liquid leaving effect 3 has a lower temperature than the liquid in effect 2, a pump is needed to provide additional energy to increase its temperature to the saturation temperature

Vapor

Vapor to condenser

Concentrate

Feed

Condensate

Vapor

Concentrate

Cone

Feed

Condensate

Vapor

Steam c=

Vapor to condenser

Condensate

Vapor

Vapor to condenser

Steam

Concentrate

TUi ia

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