Types of Evaporators
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.
Natural Circulation Evaporators (Calendria type)
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 turn down) and are not recommended for services where wide load fluctuations are expected.
Single Pass service passes the feed liquor through the tubes only once, and the two-phase mixture is released into the main evaporator body where vapor and liquor are separated. Since all evaporation is accomplished in a single pass, these units are especially useful in handling heat-sensitive materials, due to their short residence times.
Recirculating units maintain a pool of liquid within the evaporator. The feed liquor mixes with the liquid in the pool and passes over the heat-transfer surface. The two-phase mixture returning to the evaporator is separated into vapor and liquid. This liquid mixes with the liquid in the pool. The product is withdrawn from this pool so that all liquor in it is at maximum concentration.
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.
Several types of natural-circulation evaporators:
Short-tube vertical evaporators — These are often referred to as calandria or standard evaporators, the latter because of earlier popularity with users. Units consist of short tubes, 4-6 ft long, and 2-4 in. dia., set between two horizontal tubesheets that span the evaporator-body diameter. The tube bundle contains a large circular downcomer that returns concentrated liquor above the top tubesheet to below the bottom tubesheet for product withdrawal. The driving force for flow of liquid through the tubes is the difference in density between the liquid in the downcomer and two-phase mixture in the tubes.
Advantages: these evaporators can be used with scaling liquids, since evaporation takes place inside the tubes, which are accessible for cleaning. Fairly high heat-transfer coefficients are obtained with thin liquors (i.e., water or dilute solutions of 1-5 cP). Units are relatively inexpensive, provided that they are made of carbon steel or cast iron.
Disadvantages: a large area is required since the units are squat. Heat-transfer coefficients are sensitive to the temperature difference and liquor viscosity, and, due to large liquid holdup, these evaporators cannot be used with heat-sensitive materials. Turndown and flexibility are low — turndown being <2:1.
Also, such evaporators are unsuitable for crystalline products, unless a propeller is used to produce forced circulation.
Industrial applications: these short-tube vertical evaporators are suitable for noncorrosive (e.g., cane sugar), clear and noncrystallizing liquors.
Basket-type evaporators — These are similar to calandria-type units except that the tube bundle is removable and the liquor downtake occurs between the bundle and the shell, instead of in a central downcomer.
Advantages: The heating surface is removable, allowing easy cleaning and maintenance. Also, due to the construction, differential thermal expansion is not a problem.
Disadvantages: These are the same as for short-tube evaporators.
Industrial applications: These are the same as for short-tube evaporators. Basket types can also be used when the liquor may result in scale.
Long-tube vertical evaporators — The three types of long-tube vertical evaporators are the most popular evaporators used today. More evaporation is accomplished in these units than in all other types combined. While they are natural-circulation evaporators, they are also categorized individually as rising-film, falling-film, and rising/falling-film types.
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.
Advantages: This is the most economical design, since a large heat-transfer surface can be packed into a given body; these evaporators occupy little floor space. Heat-transfer coefficients are high, and the units are ideal for substantial evaporation duties. Highly versatile, they are used in various industries. they are especially suited for foaming or frothing liquors, as the foam is broken due to the liquid/vapor mixture striking an impingement baffle.
Disadvantages: these vertical units require high headroom. Generally, they are unsuitable for scaling or salting liquors, and are sensitive to changes in operating conditions.
Industrial applications: the once-through type is used in pulp-and-paper plants for concentrating black liquor. Other versions of this evaporator are discussed later.
These evaporation systems 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 the use of forced circulation is necessary.
Forced circulation is achieved by various means, such as locating pumps outside of the evaporator. 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.
Advantages: Forced-circulation evaporators are the most versatile of all evaporators. This is because they do not depend on a natural thermosyphon effect that limits the heat-transfer coefficient. High heat-transfer coefficients can be achieved for problem liquors, and hence required surface area is kept to a minimum. The economics are especially favorable for applications that require the more expensive alloys such as stainless steels, high-nickel alloys, etc. Also since material is pumped around the unit, fouling can be controlled well. Operation is not limited by the liquid/vapor ratio, and turndown can be as low as 5% of capacity.
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.
Disadvantages: These evaporators are usually less economical than other types, due to operating and maintenance costs for the pumps. Corrosion-erosion can occur, due to high circulation velocities. Also, plugging of tubes where liquor enters can be a problem in salting services where the salt deposits detach and accumulate at the bottom.
Industrial applications: These systems are used in producing common salt, caustic soda and other crystalline products.
Rising Film Evaporators
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 tube wall. 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.
Advantages: Since feed enters at the bottom, the feed liquor is distributed evenly to all tubes. Other advantages are those of the long-tube vertical unit, described before.
Disadvantages: Heat transfer is difficult to predict; pressure drop is higher than for falling-film types. Performance is extremely sensitive to the temperature driving force. Heat transfer falls off at low temperature differences (less than 25°F) or at low temperatures (about 250°F).
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.
Industrial applications: Major uses of rising-film evaporators include concentrating black liquors in pulp-and-paper mills, and concentrating nitrates, spin-bath liquors, electrolytic tinning liquors, etc.
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.
Advantages: Falling-film evaporators offer all advantages of rising-film units, plus higher heat-transfer coefficients satisfactory operation at low temperature driving forces (10-1 25°F), and concentration of heat-sensitive and viscous chemical products.
Disadvantages: These are the same as for rising-film types, except that, in addition, feed distribution is a major problem. However, temperature driving force is not limiting and a broader range of applications is possible.
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.
Agitated thin-film evaporators
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 75 ft. The heating medium may be steam, suitable hot oil, molten salt, or tempered water on the jacket side. The geometry of the unit limits heat-transfer surface area available to about 300 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.
Advantages: These devices can process extremely viscous (to 100,000 or even 1 million cP), heat-sensitive or crystallizing liquids, as well as slurries. In some applications, agitated thin-film evaporators are, in fact, the only evaporators that will work. Continuous scraping of the tube wall allows processing of severely scaling or fouling liquids. Applications include services in which liquid loads are so small as to cause dry-patch formation in falling-film units.
Disadvantages: Agitated thin-film evaporators are the most expensive of all evaporators. Also, due to the moving parts, operating and maintenance costs can be higher than for some of the other types. The heat-transfer surface area is limiting, which may require use of a high-temperature heating medium to achieve higher capacities. Heat-transfer coefficients are usually low, due to the inherent characteristics of the materials being processed, and the thicker tube walls (1A-Vz in.) necessary to meet structural and mechanical requirements.
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.
Advantages: Plate-type evaporators have low installation costs. Thus, they are economical for the more costly materials (e.g., stainless steels, high-nickel alloys, titanium, etc.). Large Heat-transfer areas can be packed into a smaller volume, and heat-transfer coefficients are usually higher than for tubular evaporators. Capacity can be changed by simple addition or removal of plates. Fouling and scaling are less, since the fluid motion imparts a scouring action on the corrugated plate surface. Headroom is low.
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.
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.
Industrial applications: vapor recompression evaporators are characterized by low temperature driving forces across heat-transfer surfaces, due to low compression ratios used for mechanical or thermal compressors. This leads to larger heat-transfer areas and, hence, higher capital costs. The low available mean temperature difference (MTD) limits application to single-effect evaporators, in which vapor from the same effect is used as the heating medium after compres-sion. Obviously, vapor recompression evaporators are unsuitable where there is a high boiling-point rise or fouling tendency, conditions that necessitate a high MTD for satisfactory operation.
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.
Advantages: These units are economical for processes handling heat-sensitive materials. There is a distinct economic advantage in applications that require both multiple-effect operation and use of more expensive alloys. These evaporators can be used in remote locations where utility steam is unavailable. In existing facilities undergoing expansion, vapor-compression evaporators may be the only choice, due either to plot-plan limitations or to shortages of utility steam.
Disadvantages: Vapor-compression evaporators are difficult to justify where low-pressure steam is readily available. These evaporators cannot be used when process liquors are fouling or show a high boiling-point rise, situations that demand a high MTD for sustained operation. Maintenance is significant with mechanical compressors.