Evaporator Types

Most evaporators consist of three main elements or parts: a heating unit (calandria), a region for liquid-vapor separation (sometimes called a vapor head, flash chamber, or settling zone), and a structural body to house these elements and to separate the process and heating fluids. One simple way to classify evaporators is:

1. Heating medium separated from evaporating liquid by tubular heating surfaces

2. Heating medium confined by coils, jackets, double walls, flat plates, etc.

3. Heating medium brought into direct contact with evaporating liquid (e.g., a submerged combustion evaporator)

4. Heating with solar radiation'131

By far, most evaporators used in the process industries fall into the first category, having tubular heating surfaces. In the natural circulation evaporators, movement of liquid across the heating surface is induced by the boiling process itself, the two-phase mixture of liquid and vapor being less dense than the column of liquid behind it, which pushes it forward and upward. For some thicker fluids, liquids with a high solids content, or liquids which have a tendency to react or foul on a heated surface, a forced circulation evaporator may be a better choice; a centrifugal pump circulates liquid through a loop around the heating unit at a much higher velocity than is possible in a natural circulation evaporator.

Evaporators can be designed to operate batchwise, continuously or in a semi-batch or campaign fashion, but once an evaporator system is designed to operate in one of these modes, it is not easy to change from one type of operation to another from the standpoint of available hardware and process instrumentation.

The specialty evaporators make up the second classification of evaporator types. These are generally much smaller and simpler than the tubular evaporation systems, and are often batch or multipurpose evaporators. The third group is a unique classification and the direct-fired, submerged combustion evaporator is the best example of this type.

The last classification includes the solar evaporation system, the oldest evaporation principle employed by man and, in concept, the simplest evaporation technique. Solar evaporators require tremendous land areas and a relatively cheap raw material, since pond leakage may be appreciable. Solar evaporation generally is feasible only for the evaporation of natural brines, and then only when the water vapor is evaporated into the atmosphere and is not recovered.

Evaporators may be operated either as once-through units, or the liquid may be recirculated through the heating elements. In once-through operation, all the evaporation is accomplished in a single pass. The ratio of evaporation to feed is limited in single pass operation; single pass evaporators are well adapted to multiple-effect operation, permitting the total concentration of the liquid to be achieved over several effects. Mechanically agitated thin-film evaporators are generally operated once-through. Once-through evaporators are also frequently required when handling heat-sensitive materials.

Recirculated systems require that a pool of liquid be held within the equipment. Feed mixes with the pooled liquid and the mixture circulates across the heating element. Only part of the liquid is vaporized in each pass across the heating element; unevaporated liquid is returned to the pool. All the liquor in the pool is therefore at the maximum concentration. Circulatory systems are therefore not well suited for evaporating heat sensitive materials. Circulatory evaporators, however, can operate over a wide range of concentrations and are well adapted to single-effect evaporation.

There is no single type of evaporator which is satisfactory for all conditions. It is for this reason that there are many varied types and designs. Several factors determine the application of a particular type for a specific evaporation result. The following sections will describe the various types of evaporators in use today and will discuss applications for which each design is best adapted.

A number of different evaporator designs are illustrated in Fig. 8, and the variations based upon these concepts are many. Often, physical properties and materials handling considerations for the feed or the bottom streams (e.g., solids content, viscosity, heat sensitivity) will indicate that one evaporator type will be better suited for the duty than other types J14]

5.1 Jacketed Vessels

When liquids are to be evaporated on a small scale, the operation is often accomplished in some form of jacketed tank or kettle. This may be a batch or continuous operation The rate ofheattransferis generally lowerthan for other types of evaporators and only a limited heat transfer area is available. The kettles may or may not be agitated.

Jackets may be of several types: dimpled jackets, patterned plate jackets, and half-pipe coil jackets. Jacketed evaporators are used when the product is somewhat viscous, the batches are small, good mixing is required, ease of cleaning is important, or when glass-lined steel equipment is required.

Types Evaporators Food

Figure 8. Evaporatortypes. (a) Forced circulation, (b) Submerged-tube forced circulation. (c) Oslo-type crystallizer. (d) Short-tube vertical, (e) Propeller calandria. (f) Long-tube vertical, (g) Recirculating long-tube vertical, (h) Falling film, (i) Horizontal-tube evaporator. C, condensate; F, feed; G, vent; P, product; S, steam; V, vapor, ENTT, separated entrainment outlet. (From Chemical Engineers'Handbook, edited by R. H. Perry and C. H. Chilton, 5th ed., p. 11-28, ©1973, McGraw-Hill. Used with permission.)

Figure 8. Evaporatortypes. (a) Forced circulation, (b) Submerged-tube forced circulation. (c) Oslo-type crystallizer. (d) Short-tube vertical, (e) Propeller calandria. (f) Long-tube vertical, (g) Recirculating long-tube vertical, (h) Falling film, (i) Horizontal-tube evaporator. C, condensate; F, feed; G, vent; P, product; S, steam; V, vapor, ENTT, separated entrainment outlet. (From Chemical Engineers'Handbook, edited by R. H. Perry and C. H. Chilton, 5th ed., p. 11-28, ©1973, McGraw-Hill. Used with permission.)

5.2 Horizontal Tube Evaporators

The earliest fabricated evaporator designs incorporated horizontal tubes. A vertical tank-like cylinder housed a horizontal tube bundle in the lower portion of the vessel, and the vapor space above the tubes served to separate the entrained liquid from the rising vapors. A later design based on a horizontal body and a removable U-type bundle is illustrated in Fig. 8(i). Another modification, the kettle type re-boiler, is similar and is more often employed as a bottoms heater for a distillation column than as an evaporator.

Initial investment for horizontal tube evaporators is low, but heat transfer rates may also be relatively low. They are well suited for non-scaling, low viscosity liquids. For several scaling liquids, scale can sometimes be removed from bent-tube designs by cracking it off periodically by shock-cooling with cold water; or, removable bundles can be used to confine the scale to that part of the heat transfer surface which is readily accessible.

Horizontal tube evaporators may be susceptible to vapor-binding, and foaming liquids cannot usually be handled. The short tube variety is seldom used today except for preparation of boiler feed water. The kettle-type reboiler is frequently used in chemical plant applications for clean fluids.

The advantages of horizontal tube evaporators include relatively low cost for small-capacity applications, low headroom requirements, large vapor-liquid disengaging area, relatively good heat transfer with proper design, and the potential for easy semiautomatic descaling. Disadvantages include the limitations for use in salting, or scaling applications, generally.

5.3 Short-Tube Vertical Evaporators

The short-tube vertical evaporator, Fig. 8(d), also known as the calandria or Robert evaporator, was the first evaporator to be widely used. Tubes 4' and 8' long, often 2" to 3" in diameter, are located vertically inside a steam chest enclosed by a cylindrical shell. The early vertical tube evaporators were built without a downcomer but did not perform satisfactorily, so the central downcomer appeared very early. There are many alternatives to the center downcomer; different cross sections, eccentrically located downcomers, anumberofdowncomers scattered over the tube layout, downcomers external to the evaporator body.

The short-tube evaporator has several advantages: low headroom, high heat transfer rates at high temperature differences, ease of cleaning, and low initial investment. Disadvantages include large floor space and weight, relatively high liquid holdup, and poor heat transfer with low temperature differences or with high product viscosity. Natural circulation systems are not well suited for operation at high vacuum. Short-tube vertical evaporators are best applied when evaporating clear liquids, mild scaling liquids requiring mechanical cleaning, crystalline product when propellers are used, and for some foaming products when inclined calandrias are used. Once considered "standard," short tube vertical evaporators have largely been replaced by long tube vertical units.

Circulation of liquid across the heating surface is caused by the action of the boiling liquid (natural circulation). The circulation rate through the evaporator is many times the feed rate. The downcomers are therefore required to permit the liquid to flow freely from the top tubesheet to the bottom tubesheet. The downcomer flow area is, generally, approximately equal to the tubular cross-sectional area. Downcomers should be sized to minimize holdup above the tubesheet in order to improve heat transfer, fluid dynamics, and minimize foaming. For these reasons, several smaller downcomers scattered about the tube nest are often the better design.

5.4 Propeller Calandrias

Natural circulation in the standard short tube evaporator depends upon boiling. Should boiling stop, any solids suspended in the liquid phase will settle out. The earliest type of evaporator that could be called a forced-circulation device is the propeller calandria illustrated in Fig. 8(e). Basically a standard evaporator with a propeller added in the downcomer, the propeller calandria often achieves higher heat transfer rates. The propeller is usually placed as low as possible to avoid cavitation and is placed in an extension of the downcomer. The propeller can be driven from above or below. Improvements in propeller design have permitted longer tubes to be incorporated in the evaporator.

5.5 Long-Tube Vertical Evaporators

More evaporator systems employ this type of design than any other because it is so versatile and is often the lowest cost per unit of capacity. Long-tube evaporators normally are designed with tubes 1" to 2" in diameter and from 12' to 30' in length. A typical long-tube evaporator is illustrated in Fig. 8 (f). Long-tube units may be operated as once-through or as recirculating evaporation systems. A once-through unit has no liquid level in the vapor body, tubes are 16' to 30' long, and the average residence time is only a few seconds. With recirculation, a level must be maintained, a deflector plate is often provided in the vapor body, and tubes are 12' to 20' long. Recirculated systems can be operated either batchwise or continuously.

Circulation of fluid across the heat transfer surface depends upon boiling and the high vapor velocities associated with vaporization of the liquid feed. The temperature of the liquid in the tubes is far from uniform and relatively difficult to predict. These evaporators are less sensitive to changes in operating conditions at high temperature differences than at lower temperature differences. The effects of hydrostatic head upon the boiling point are quite pronounced for long-tube units.

The long-tube evaporator is often called a rising or climbing film evaporator because vapor travels faster than the liquid upward through the core of the tube, therefore dragging the liquid up the tube in a thin film. This type of flow can occur only in the upper portion of the tube. When it occurs, the liquid film is highly turbulent and high heat transfer rates are realized. Average residence times are low, so long-tube vertical evaporators can be utilized for heat sensitive materials.

The long-tube vertical evaporator offers several advantages: low cost, large units, low holdup, small floor space, good heat transfer over a wide range of applications. Disadvantages include: high head room is needed, recirculation is frequently required, and they are generally unsuited for salting or severely scaling fluids. They are best applied when handling clear fluids, foaming liquids, corrosive liquids, and large evaporation loads.

5.6 Falling Film Evaporators

Falling film evaporators, Fig. %(h), are long-tube vertical evaporators that rely upon gravity flow of a thin fluid layer from the top of the tubes, where the liquid is introduced, to the bottom of the unit where the concentrate is collected. Evaporation takes place on the surface of the falling liquid film which is highly turbulent. The fluid pressure drop across the process side of a falling film evaporator or re-boiler system is very low and usually negligible, due to the gravity flow.[151 Separation of entrained liquid from the vapor is usually accomplished in a chamber at the bottom of the tubes, although some units are designed so that the volatiles flow upward against the descending liquid film and are removed at the top of the unit.

Feed to a falling film evaporator is usually introduced under the liquid level maintained at the top of the tubes, so that a reservoir of rather low velocity liquid is available for liquid distribution to the many vertical tubes. In falling film evaporator and re-boiler design, equal fluid distribution among the tubes and film initiation are very important factors. For this reason, a number of sophisticated and very effective hydraulic distributing devices have been developed to handle different types of process fluids.'161 In order to achieve uniform liquid loading and evaporation rates in each tube, and to ensure that sufficient liquid is available in every tube to maintain the liquid film (thus avoiding dry or hot spots), particular attention must be paid to liquid distribution. Figure 9 is a cross section of a urea concentrator and Fig. 10 illustrates some of the many tube distributors or ferrules that can be inserted into the flush upper end of the evaporator tubes.

Cross Section Falling Film Evaporator
Figure 9. Falling film evaporator for urea concentration; bottom vapor takeoff. (Henry Vogt Machine Company.)

Figure 10. Tube distributors for falling film evaporators. (Henry VogtMachine Company.)

Heat transfer rates in falling film evaporators are relatively high even at low temperature differences across the liquid film; thus, these evaporators are widely used for heat sensitive products because of uniform temperatures and short residence times. Generally, moderately viscous fluids and materials with mildly fouling characteristics can easily be handled in falling film evaporators in series for heavy evaporation loads, and part of the liquid can be pumped and recycled to the top of the unit.

The least expensive of the low residence time evaporators, falling film evaporators, offer many advantages, particularly for large volumes of dilute material. These advantages include: large unit sizes, low liquid holdup, small floor space, and good heat transfer over a wide range of conditions. Falling film units are well suited for heat sensitive materials or for high vacuum application, for viscous materials, and for low temperature differences. Occasionally, rising and falling film evaporators are combined into a single unit.

5.7 Forced Circulation Evaporators

Evaporators in which circulation is maintained, independent of the evaporation rate or heating temperature, through the heating element are known as forced circulation evaporators. Forced circulation systems are illustrated in Figs. 8(a) and 8(b). Forced circulation systems are more expensive than comparable natural circulation evaporators and are, therefore, used only when necessary.

A choice of a forced circulation evaporator can be made only after balancing the pumping cost, which is usually high, with the increase in heat transfer rates or decrease in maintenance costs. Tube velocity is limited only by pumping costs and by erosion at high velocities. Tube velocities are usually in the range of 5 to 15 feet per second. Sometimes the pumped fluid is allowed to vaporize in the tubes. This often provides high heat transfer rates, but increases the possibility of fouling. Consequently, this type of evaporator is seldom used except where head room is limited or the liquids do not scale, salt, or foul the surface.

The majority of applications are designed so that vaporization does not occur in the heat exchanger tubes. Instead, the process liquid is recirculated by the pump, is heated under pressure to prevent boiling, and is subsequently flashed to obtain the required vaporization. This type of evaporator is often called the submerged-tube type because the heating element is placed below the liquid level and uses the resulting hydrostatic head to elevate the boiling point and to prevent boiling in the tubes. The heating element may be installed vertically (usually, single pass), or horizontally (often, two-pass as shown in Fig. 8b).

The recirculation pump is a crucial component of the evaporation system, and the following key factors need to be considered when establishing the recirculation rate and the pump capacity:

1. Maximum fluid temperature permitted

2. Vapor pressure of the fluid

3. Equipment layout

4. Tube geometry

5. Velocity in the tubes

6. Temperature difference between the pumped fluid and the heating medium

7. Pump characteristics for the pumps being evaluated with the system

A recirculating pump should be chosen so that the developed head is dissipated as pressure drops through the circuit of the system. It is important that the pump and system be properly matched. The fluid being pumped is at or near its boiling point and, therefore, the required NPSH (net positive suction head) is usually critical. The pump should operate at this design level. If it develops excessive head, it will handle more volume at a lower head. At the new operating point, the required NPSH may be more than is available, and cavitation will occur in the pump. If insufficient head is provided, the velocities may not be sufficiently high to prevent fouling; lower heat transfer rates may result; or the fluid may boil in the heating element with subsequent fouling or decomposition.

Forced circulation evaporators offer these advantages: high rate ofheat transfer; positive circulation; relative freedom from salting, scaling, and fouling; ease of cleaning; and a wide range of application. Disadvantages include: high cost; relatively high residence time; and the necessity for centrifugal or propeller pumps with associated maintenance and operating costs. Forced circulation evaporators are best applied when treating crystalline products, corrosive products, or viscous fluids. They are also well suited for vacuum service, and for applications requiring a high degree of concentration and close control of bottoms product concentration.

5.8 Plate Evaporators

Plate evaporators may be constructed of flat plates or corrugated plates, the latter providing an extended heat transfer surface and improved structural rigidity. Two basic types of heat exchangers are used for evaporation systems: plate-and-frame and spiral-plate evaporators. Plate units are sometimes used because of the theory that scale will flake off such surfaces, which can flex more readily than curved tubular surfaces. In some plate evaporators, flat surfaces are used, each side of which can serve alternately as the liquor side and the steam side. Scale deposited while in contact with the liquor can then be dissolved while in contact with the steam condensate. There are still potential scaling problems, however. Scale may form in the valves needed for cycling the fluids and the steam condensate simply does not easily dissolve the scale produced.

A plate-and-frame evaporator, like the one illustrated in Fig. 11, is so named because the design resembles that of a plate-and-frame filter press. This evaporator is constructed by mounting embossed plates with corner openings between a top carrying bar and a bottom guide bar. The plates are gasketed and arranged so narrow flow passages are formed when a series of plates are clamped together in the frame. Fluids pass through the spaces between the plates, either in series or parallel flow, depending on the gasketing which confines the fluids from the atmosphere.

Spiral-plate evaporators may be used instead of tubular evaporators, and offer a number of advantages over conventional shell-and-tube units: centrifugal forces improve heat transfer; the compact configuration results in a shorter liquid pathway; they are relatively easily cleaned and resistant to fouling; differential thermal expansion is accepted by the spiral arrangement. These curved-flow units are particularly useful for handling viscous material or fluids containing solids.

Figure 11. Plate evaporator, rising/falling film type. (APVCompany, Inc.)

A spiral-plate heat exchanger is constructed by winding two long strips of plate around an open, split center to form a pair of concentric spiral passages. Spacing is maintained along the length of the spiral by spacer studs welded to the plates. In some applications both fluid-flow channels are closed by welding alternate channels at both sides of the spiral plate (Fig. 12). In other applications, one of the channels is left completely open and the other is closed at both sides of the place, Fig. 13. These two types of constructions prevent the fluids from mixing.

The spiral heat exchanger can be fitted with covers to provide three flow patterns: (i) both fluids in spiral flow, (ii) one fluid in spiral flow and the other in axial flow across the spiral, (in) one fluid in spiral flow and the other in combination of axial and spiral flow.

Plate type heat exchangers (see Fig. 11) can be designed to operate as rising film, falling film, or rising-falling film evaporators. In some applications the rising and falling films are removed from the plate by the turbulence caused by extremely high vapor velocities. This action reduces the apparent viscosity and tends to minimize scaling.

Rising Film Evaporator

Figure 11. Plate evaporator, rising/falling film type. (APVCompany, Inc.)



Calandria Evaporator Design
Figure 12. Spiral plate heat exchanger, both fluids with helical flow pattern. (Graham Manufacturing Company, Inc.)
Evaporator Fluid Flow
Figure 13. Spiral plate heat exchanger, one fluid in helical flow and one fluid in axial flow pattern. (Graham Manufacturing Company, Inc.)

The volume of product {holdup) in the evaporator is very small in relation to the large available heat transfer surface. Plate-and-frame evaporators can generally handle the evaporation of heat sensitive, viscous, and foaming materials. They permit fast start-up and shutdown and are quite compact, so little head room is required. They are easily cleaned and readily modified.

A major concern is the need for gaskets and the large gasketed area. However, interleakage of fluids cannot occur without rupturing a plate, because all fluids are gasketed independently to seal against the atmosphere. Leakage can be avoided by selecting appropriate gasket materials and following proper assembly procedures.'171

5.9 Mechanically Agitated Thin-Film Evaporators

These evaporators, sometimes called wiped-film or scraped-film evaporators, rely on mechanical blades that spread the process fluid across the thermal surface of a single large tube (Fig. 14), not unlike the wiper on the windshield of a car. All thin-film evaporators have essentially three major components: a vapor body assembly, a rotor, and a drive system.'181

In this thin-film evaporator design, product enters the feed nozzle above the heated zone and is mechanically transported by the rotor, and gravity, down a helical path on the inner heat transfer surface. The evaporator does not operate full of product; the liquid or slurry forms a thin film or annular ring of product from the feed nozzle to the product outlet nozzle as shown in the cross section of Fig. 15. Holduporinventoryofproductinathin-film evaporator is very low, typically about a half a pound of material per square foot of heat transfer surface. The high blade frequency, about 8 to 10 blade passes per second, generates a high rate of surface renewal and highly turbulent conditions for even extremely viscous fluids. A variety of basic or standard thin-film evaporator designs is commercially available, including vertical or horizontal designs, and both types can have cylindrical or tapered thermal bodies and rotors.

The rotors may be one of several zero-clearance designs, a rigid fixed clearance type, or in the case of tapered rotors, an adjustable clearance construction type (Fig. 16). One vertical design includes an optional residence time control ring at the end of the thermal surface to hold back product and thus build up the film thickness. The majority of thin-film evaporators in operation are the vertical design with a cylindrical fixed-clearance rotor shown in Fig. 14.'191

Propeller Calandria Evaporator
Figure 14. Mechanically agitated thin-film evaporator, vertical design with cylindrical thermal zone. (Luwa Corporation.)

Heated wall

Heated wall

Agitated Thin Film EvaporatorAgitated Evaporators
Figure 15. Distribution of liquid in mechanically agitated thin-film evaporator. (Luwa Corporation.)
Agitated Thin Film Evaporator

Zero" Clearance Carbon or Teflon Wipers

"Zero" Clearance "Pendulum" Hinged Blades

"Zero" Clearance "Scraping" Hinged Blades

Zero" Clearance Carbon or Teflon Wipers

"Zero" Clearance "Pendulum" Hinged Blades

"Zero" Clearance "Scraping" Hinged Blades

Hinged Blade Evaporator

F xed Clearance Low Viscosity

F med Clearance Medium Viscosity

F ixed Clearance High Viscosity

Figure 16. Six types of rotors for mechanically agitated thin-film evaporators; cross-sectional views. (Luwa Corporation)

F xed Clearance Low Viscosity

F med Clearance Medium Viscosity

F ixed Clearance High Viscosity

Figure 16. Six types of rotors for mechanically agitated thin-film evaporators; cross-sectional views. (Luwa Corporation)

Mechanically agitated thin-film evaporators are used for four general types of applications:

1. Heat sensitive products

2. Fluids with fouling tendencies

3. Viscous materials

4. Liquids containing a large amount of dissolved or suspended solids

The one-pass, plug flow operation of a thin-film evaporator is an advantage for minimizing thermal degradation of a heat sensitive product in an evaporation step. The mean residence time in the evaporator can be just seconds, rather than minutes or hours in a recirculating evaporation system. For this reason, thin-film evaporators are widely used for heat sensitive food, pharmaceutical, and other chemical products. Also, it should be noted that the thin-film evaporator can be operated at a higher temperature to make a better separation, whereas care must usually be taken to keep the product temperature lower in an evaporation system with longer residence times (see Fig. 21, later in this chapter).

Thin-film evaporators are frequently used for extremely viscous fluids, those in the range of 1,000 to 50,000 centipoise, and for concentrating streams with more than 25% suspended solids. Heat transfer coefficients for these types of materials in a thin-film evaporator are typically much greater than coefficients in any other type of evaporator for the same conditions. Very high temperature difference (e.g., 100 to 200°F) can be maintained to better utilize the heat transfer area by increasing the heat flux, Q/A.

These evaporators are necessarily precision machines and therefore are more expensive than other types, particularly so if compared strictly on equivalent heat transfer area. When the performance for a specific evaporation duty is the basis of comparison, the thin-film evaporator is often the more economical choice because the larger heat transfer coefficient and higher driving force mean much less surface is required than for other evaporators (A = Q/U AT). Thin-film evaporator cost per unit area decreases significantly with unit size, and the largest available unit has 430 square feet of active heat transfer surface.'201

5.10 Flash Pots and Flash Evaporators

The simplest continuous evaporation system is the single stage "flashing" of a heated liquid into an expansion tank orflash pot which is maintained at a lower pressure than the feed. The principle is that of an adiabatic (or iso-enthalpic) expansion of a saturated liquid from a high pressure to a lower pressure, thus generating a mixture of saturated liquid and vapor with the same total enthalpy at the lower pressure.

First applied for production of distilled water on board ships, flash and multistage flash evaporators have been more recently utilized to evaporate brackish and sea water as well as for process liquids. An aqueous solution is heated and introduced into a chamber which is kept at a pressure lower than the corresponding saturation pressure of the heated feed stream. Upon entering the chamber, a small portion of the heated water will immediately "flash" into vapor, which is then passed through an entrainment separator to remove any entrained liquid and condense the water vapor. A series of these chambers can be maintained at successively lower pressures with vapor flashing at each stage. Such a system is called a multistageflash evaporator.

The flashing process can be broken down into three distinct operations: heat input, flashing and recovery, and heat rejection. The heat input section, commonly called a brine heater, normally consists of a tubular exchanger which transfers heat from steam, exhaust gas from aturbine, stack gases from a boiler, or almost any form of heat energy. The flashing and recovery sections consist of adequately sized chambers which allow the heated fluid to partially flash, thereby generating a mixture of vapor and liquid. The vapor produced in this process is passed through moisture separators and directed either to the heat recovery condensers (for multistage units) or to the third section, the reject condensers. Since the evaporator does no work, the heat reject sections receive essentially all of the energy supplied in the heat input section of the evaporator.

Usually, the three sections are combined into one package. In single stage flash evaporators, there are no regenerative stages to recover the energy of the flashed vapor. A multistage system extends the flashing and recovery zone by condensing the flashed vapor in each stage by heating the brine prior to the heat input zone. This reduces the amount of heat required for evaporation. The number of stages or flashes is determined by the economics of each installation. Until recently, flash evaporators were limited to "water poor" areas, where there was an abundance of relatively low cost fuel or energy. The flash evaporator is an extremely flexible system and can be made to operate with almost any form of heat energy. Proper instrumentation must be applied for multistage evaporators which incorporate a large number of stages. The interrelated variables of brine recirculation, makeup and blow down flow rates, brine heater temperature, and final stage liquid level must be properly controlled.

5.11 Multiple Effect Evaporators

The use of multiple effects in series is quite common for evaporation of large amounts of dilute aqueous feed, requiring the evaporation of from thousands to hundreds of thousands of pounds per hour of water. The basic principle is to use heat given up by condensation in one effect to provide the re-boiler heat for another effect. In most multiple effect units, the overhead vapor from one effect is condensed directly in the heating element of the next effect.

Multiple effect evaporators are generally large, complex systems and are normally the most expensive type of evaporator to procure and install, but they can also be the most economical evaporator to operate, thus justifying their high first cost. Perhaps it is best to think conceptually of multiple-effect systems as requiring a higher "up front" investment of total capital in order to significantly reduce the largest variable operating cost, the cost of energy. Simplistically, the addition of a second effect will reduce energy consumption by about 50%; a four effect evaporator installation will use about 25% of the energy of a single effect evaporator performing the same duty. It is not

Vapor to condenser

World Largest Condenser Coil

Thick liquor

Steam Trap

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