This is intended to provide a brief synopsis of the primary types of evaporators which are implemented in various industries today. Every evaporator design will have a means of transferring heat energy through a heat transfer surface as well as a means to effectively separate the vapors from the residual liquid or solid. Differences in how these are achieved distinguishes one type of evaporator from another.

As their name depicts, these evaporators depend on natural physical forces in lieu of pumps for their operation. There must be a balance between the two-phase friction and acceleration losses in the flow loop, and the static head developed by the liquid in the main body of the evaporator. The heating surface can be horizontal or vertical, and can be totally immersed or partially submerged, or outside of the evaporator body. Natural circulation systems offer a moderate range of operation (2:1 turndown) and are not recommended for services where wide load fluctuations are expected.

Since the liquid in the evaporator is recirculated and, thus, repeatedly contacts the heat-transfer surface, natural-circulation evaporators are unsuitable for heat-sensitive materials. Moreover, since the liquor entering the heat-transfer surface is at a higher concentration than the feed, its density, viscosity and boiling point are high. Accordingly, heat-transfer coefficients tend to be low. The advantages are that these evaporators can operate over a wide range of concentrations and loads and are well suited for single-effect evaporation.

Also, such evaporators are unsuitable for crystalline products, unless a propeller is used to produce forced circulation.

Industrial applications: These are the same as for short-tube evaporators. Basket types can also be used when the liquor may result in scale.

Basically, these units consist of a single-pass vertical shell-and-tube heat exchanger discharging into a relatively small vapor head. Units may be once-through or recirculating, depending upon the application; the heating surface may be internal or external to the main body of the evaporator.

These are made in a variety of arrangements for services where the feed and/or product liquor has a tendency to salt or scale, and where the viscosities of the solutions are so high that natural circulation is not feasible. Thermal and flow characteristics of the process liquor are so poor that use of forced circulation is necessary.

Forced circulation is achieved by various means, such as locating pumps outside of the evaporator, or by using propellers as in propeller calandria units. Forced circulation leads to high tubeside velocities (6-18 ft/s), and hence higher heat-transfer coefficients and smaller heating surfaces. Positive circulation renders this unit relatively insensitive to variations in physical properties or lards, making it suitable for crystallizing solutions or slurries.

Forced-circulation evaporators enjoy the widest variety of applications. The heating surface may be inside or outside of the evaporator; this is also true for the

device that creates the forced circulation. The tubes can be horizontal or vertical. Boiling can take place, or be suppressed due to the hydrostatic head maintained above the top tubesheet. In the latter case, the liquor is superheated and flashes into a liquid-vapor mixture. The type of vapor head used, ranging from a simple centrifugal separator to a crystallizing chamber, is selected on the basis of product characteristics.

Forced-circulation evaporators offer the highest operational flexibility, since heat transfer, vapor-liquid separation and crystallization can take place in separate components by locating pumps outside of the evaporator or by using propellers as in propeller calandria units. Forced circulation leads to high tubeside velocities (6-18 ft/ s), and hence higher heat-transfer coefficients and smaller heating surfaces. Positive circulation renders this unit relatively insensitive to variations in physical properties or loads, making it suitable for crystallizing solutions or slurries.

These devices are ideal for crystallizing, and for concentrating thermally degradable materials and viscous solutions.

The rising-film evaporator is the original version of the long-tube vertical evaporator. Steam condenses on the outside surfaces of vertical tubes. The liquid inside the tubes is brought to a boil, with the vapor generated occupying the core of the tube. As the fluid moves up the tube, more vapor is formed, resulting in a higher central-core velocity that forces the remaining liquid to the tubewall. This leads to a thinner and more rapidly moving liquid film. As the film moves more rapidly, heat-transfer coefficients increase and residence times drop.

Since the vapor and liquid both flow in the same direction, the thinning of the liquid film is not as pronounced as in a falling-film type of evaporator, and the possibility of tube dryout is less. This makes the rising-film evaporator particularly suited to services having mild scaling tendencies.

The hydrostatic head may create a problem with heat-sensitive products. There is a tendency to scale. Also, the units are sensitive to changes in loads and feed

conditions, and turndown is limited to 2:1.

Falling-film evaporators evolved as a means to solve the problems associated with the rising-film types. Specifically, the hydrostatic head necessary for the operation of rising-film units leads to problems with some heat-sensitive products.

In falling-film evaporators, the feed liquor is introduced at the top tubesheet, and flows down the tubewall as a thin film. Since the film is moving in the direction of gravity rather than against it, a thinner and faster-moving film results, yielding higher heat-transfer coefficients and reduced contact times. There is no static head to affect the temperature driving force. This allows use of a lower tem-perature difference for units to operate in the film regime, and hence yields superior performance in handling heat-sensitive materials.

Flow of vapor and liquid may be either co-current, in which case vapor-liquid separation takes place at the bottom, or countercurrent (the liquid is withdrawn from the bottom and the vapor from the top). For co-current flow, the vapor shear-forces thin the liquid film, and yield higher heat-transfer coefficients. Moreover, since the vapor is in contact with the hottest liquid at the point of withdrawal, stripping is more efficient.

In countercurrent flow, shear forces increase the liquid-film thickness, and reduce the heat-transfer coefficient. If the vapor flow rate is high enough, it may lead to flooding of the tubes, with liquid carried upward beyond the point of injection, resulting in decreased performance and unstable operation. Countercurrent operation is used where it is necessary to evaporate a liquid at a low temperature under vacuum conditions, or where an inert gas (e.g., nitrogen or air) is injected into the tubes at the bottom of the unit to reduce the partial vapor pressure, and hence boiling point, of the liquid.

Another phenomenon common to falling-film evaporators is dry-patch formation, which reduces thermal performance. The dry patches may be caused by a liquid flowrate insufficient to maintain a continuous liquid film or by the evaporator's not being exactly vertical.

The major problem with falling-film evaporators is non-uniform distribution of the feed liquor as a film inside the tubes. The importance of uniform feed distribution cannot be overemphasized. To maintain a continuous liquid film, the feed liquor must be uniformly distributed around the periphery of each tube, and the flow to each tube must be uniform. A variety of devices such as perforated plates, spider distributors with radial arms, spray nozzles, and weir-type distributors have been developed for feed distribution. For selecting a distributor, information on merits and limitations of the various types is scanty.

Industrial applications: In the fertilizer industry, these evaporators are used to concentrate urea, phosphoric acid ammonium nitrate, etc. Falling-film evaporators are also employed for processing food and dairy products, and for desalting

seawater.

These evaporators combine the advantage of the ease of feed distribution of the rising-film with the usual advantages of a falling-film unit. Vapor-liquid separation takes place at the bottom of the unit; the flow of liquid and vapor is always co-current.

These are essentially large-diameter jacketed tubes, in which the product is vigorously agitated and continuously removed from the tube wall by scraper blades (or wipers) mounted on a shaft inside the tube. Thus, the material to be processed is continuously spread as a thin film on the tubewall by a mechanical agitator. This permits processing of extremely viscous and heat-sensitive materials, as well as of crystallizing and fouling products.

Units may be horizontal, vertical or inclined. The heat-transfer tube ranges from 3 to 48 in., with lengths from 2 to 24 ft. The heating medium may be steam or suitable hot oil on the jacket side. The geometry of the unit limits heat-transfer surface area available to about 280 ft2 per effect, and process and economic considerations limit operation to a single effect. However, due to short contact times, very high temperature driving forces can be used effectively without product degradation.

Industrial applications: Agitated thin-film evaporators are used for concentrating, fractionating, deodorizing and stripping in a broad variety of industrial applications, including processing of food and meat, dairy products, pharmaceuticals, polymers (such as various types of latex resins), and organic and inorganic chemicals.

employed with foods. Examples: concentration of fruit juices, milk, soup stocks, tea and coffee extracts, corn syrup, dextrose, etc.

These evaporators are especially suited to the dairy, brewery and food-processing industries since there are no dead zones in which undesired bacterial growth could occur, and frequent and efficient cleaning can be done to meet stringent hygiene requirements. Maximum protection is provided for product flavor and quality since liquid holding-volume is low, and exposure to high temperature is short.

Disadvantages: Maximum design conditions are only about 150 psig and 400°F, due to limitations of gasketing materials, which are usually elastomers such as styrene-butadiene rubber, etc. Multiple gaskets make maintenance time-con-suming. The probability of fluid leakage is higher than for tubular types. However, in food, dairy and brewery plants, this may not be a factor since spills are usually not hazardous. Gaps between the plates limit particulates to 0.25-3 mm.

Industrial applications: As noted, plate evaporators are

a shaft rotating inside the tube (Fig. 12). Thus, the material to be processed is continuously spread as a thin film on the tubewall by a mechanical agitator. This permits processing of extremely viscous and heat-sensitive materials, as well as of crystallizing and fouling products.

Also known as mechanical vapor recompression, thermal recompression or vapor recompression evaporators, these units have gained widespread acceptance in a variety of applications including foods, drugs, dairy products, and pulp and paper as well as for desalting brackish water or seawater. The high cost of energy spurred development initially and with the continuing increase in the cost of energy, the economics of using vapor recompression evaporators has become increasingly favorable, compared with multiple-effect devices.

Such evaporators differ from tubular evaporators mainly in the shape and form of the heating surface, which consists of an assembly or assemblies of corrugated plates. These evaporators are available in four configurations: rising/falling-film, falling-film, forced-circulation with suppressed boiling, and agitated thin-film. For the last type, film thinning is achieved by a combination of fluid hydrodynamics and plate geometry, rather than by a mechanical device. The heating surface consists of similar or different types of plates. The corrugations on the plates and the gaps between them are based on the particular application. Special proprietary designs such as spiral plates have been developed for handling slurries and very large evaporative capacities.

In their most elementary and popular form, these units consist of a single-effect evaporator in which process vapors are compressed to a higher pressure (to increase the saturation temperature) and are used as a heating medium in the same effect. In more elaborate arrangements, the unit may consist of multiple effects, with vapor recompression applied to the first one. The evaporator condensate and intereffect vapors are used for feed preheating to conserve energy. Typically, a single-effect evaporator with vapor recompression provides a steam economy of 1.7 (1.7 Ib of vapor produced/lb of steam used), or approximately that of a double-effect unit.

Vapor recompression is accomplished by mechanical compressors or steam-jet ejectors, depending on the volume and quality of vapors to be handled, and the pressure level required in the steam chest. Inherently, water vapor has a high specific volume of 26.8 ft/lb at 14.7 psia. Thus, the main difficulty in selecting a compressor is the large volume of vapor to be handled. Compressors generally are quite large and expensive, and the choice is limited to centrifugal or axial flow machines. This further sets the requirements for process vapors to be: (a) free of entrained solids as solids carryover may build up on the rotor blades, leading to compressor malfunction or failure; and (b) free of impurities that may cause corrosion or otherwise adversely affect the materials of construction.

Moreover, the inherent limitations of the centrifugal or axial flow machines have to be reckoned with. The compression ratio must be small. Most large capacity single state machines handle up to 300,000 actual ft3/min of vapors, with a compression ratio ranging from 1.2 to 1.5.

Higher discharge pressures can be achieved by resorting to multistage centrifugal or axial machines. However, multistage compressors in this capacity range tend to be quite complex and expensive, since special consideration must be given to machine design, and sealing and lubricating between stages. Maintenance requirements are more demanding, and total installed costs, as well as operating/ maintenance costs, may make installation uneconomical.

Compared with centrifugal or axial compressors, thermal recompression using steam-jet ejectors offers many advantages. These ejectors are simple in construction and have no moving parts. This allows fabrication from any corrosion-resistant material, and, since there are no moving parts, units give long service life, without maintenance requirements. Steam-jet ejectors can handle large volumes of vapor load at low operating pressures.

The major disadvantage of such ejectors is that they generally operate at maximum efficiency under only one condition; they do not function well at off-design conditions. Application is limited to where wide fluctuations in plant loads and/or operating variables (e.g., temperature, pres. sure and fouling) are not expected. Another drawback is that steam is needed for operation; it may not be readily available.

Vapor recompression is therefore widely used not only when a small MTD is essential, but also when it offers distinct advantages over multiple-effect evaporators, such as the ability to:

• Process heat-sensitive materials, e.g., fruit juices and similar applications in dairy and pharmaceutical plants.

• Crystallize solids having inverse solubility curves (solubility decreases with increasing temperature), such as sodium sulfate and sodium carbonate.

• Produce potable water from brackish or salt water in remote locations where either electric power is unavailable (engine-driven compressors must be used) or where considerations other than cost have a higher priority (for example, scarcity of freshwater resources).

Also, vapor recompression is favored in locations where electric power is cheap (hydroelectric power), and steam costs are high due to high fuel costs.