TYPES OF EVAPORATORS

Industrial Operations

Submitted to: Sir. Kamran KhalidSubmitted by: Ms. Pakeeza Bukhari Roll no 17 B.Sc Industrial engg Institute of Quality & Technology Management, University of Punjab

EVAPORATION:

Evaporation is the process by which the moisture content of a dilute liquid product is reduced to obtain a more concentrated product. Evaporators are classified according to the mechanism they dry the product. The following evaporators are being discussed:

1. Horizontal tube evaporator2. Short tube vertical evaporator3. Long tube vertical evaporator4. Forced-circulation evaporator

1. HORIZONTAL TUBE EVAPORATORS:The first evaporator that got recognized was a design utilizing horizontal tubes.

1.1 CONSTRUCTION & WORKING:The simplest evaporator design is a shell and horizontal tube arrangement with heating medium in the submerged tubes and evaporation on the shell side.

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Tubes are usually 7/8 inch to 1-1/2 inches in diameter and 4 to 16 feet long. The maximum area in a feasible design is about 5,000 sq. ft. The tube bundle is not removable. Above the heating section there is a cylindrical section in which separation of liquid from vapour takes place. The vapour leaves through some de-entraining device in the separator so the vapour leaving the tube doesnot carries liquid droplets. The thick liquor is obtained from the bottom. With the passage of time, Modifications were made in its design. One of them is the use of U-bend tubes to facilitate bundle removal for inspection and cleaning. Another modification is the kettle-type re-boiler. This design permits the use of longer tubes which means more heat transfer surface can be provided. The tube bundle can also be removed for inspection and cleaning in this re-boiler modification. Another alternative is the use of bent-tubes.

1.2 ADVANTAGES:The advantages of horizontal tube evaporators include:

relatively low cost in small-capacity applications easy mechanical cleaning of the outside tube surface and possibility of tube

bundle dismounting (for limited heat exchange surface) the space within the tube bundle allows the evaporated vapors to flow around

the tubes at low velocity and enables a very low evaporation temperature large vapor-liquid separation area relatively good heat transfer with proper design the potential for easy semiautomatic de-scaling

1.3 DISADVANTAGES:Disadvantages include: Limitations for use in salting or scaling applications, generally Bent-tube designs are relatively expensive Liquid circulation is poor so it is unsuitable for viscous liquors

1.4 APPLICATIONS: They are well adapted for non-scaling, low viscosity liquids The short tube variety is seldom used today except for preparation of boiler

feed water The kettle-type re-boiler is frequently used in chemical plant applications for

clean fluids

2. SHORT TUBE VERTICAL EVAPORATORS:Although the vertical tube evaporator was not the first to be built, it was the first type to receive wide popularity. The first was built by Robert and the vertical tube evaporator is often called the Robert type. It became so common that this evaporator is sometimes known as the standard evaporator. It is also called a calandria.

2.1 CONSTRUCTION & WORKING:

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It is similar in construction & working to horizontal tube evaporator except that vertical tubes are used i-e tubes 4 to 10 feet long, often 2 to 3 inches in diameter, are located vertically inside a steam chest enclosed by a cylindrical shell. The first vertical tube evaporators were built without a downcomer. These were never satisfactory, and the central downcomer appeared very early. There are many alternatives to the center downcomer; different cross sections, eccentrically located downcomers, a number of downcomers scattered over the tube layout, downcomers external to the evaporator body. Circulation of liquid past the heating surface is induced by boiling (natural circulation). The circulation rate through the evaporator is many times the feed rate. The downcomers are therefore required to permit liquid flow from the top tubesheet to the bottom tubesheet. The downcomer flow area is generally approximately equal to the tubular flow area. Downcomers should be sized to minimize liquid 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.

2.2 Basket Type Evaporators:

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In the basket type evaporator construction and operation is much the same as a standard evaporator except that the downcomer is annular. This construction often is more economical and permits the evaporator to be removed for cleaning and repair. Also that crystal formed in the downcomer donot breakup. As the circulation of the liquors in the tube is better, it is widely used in the sugar and salt industries where throughput is large.

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2.3 APPLICATIONS: 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

2.4 ADVANTAGES:The short tube evaporator offers several advantages:

Low headroom High heat transfer rates at high temperature differences ease of cleaning low initial investment

2.5 DISADVANTAGES: Disadvantages include: High floor space and weight Relatively high liquid holdup Poor heat transfer at low temperature differences or high viscosity. Natural circulation systems are not well suited for operation at high vacuum

3. LONG TUBE VERTICAL EVAPORATORSMore evaporator systems employ this type than any other because it is versatile and often the cheapest per unit capacity.

3.1 CONSTRUCTION & WORKING:The essential parts are (1) a tubular exchanger with steam in shell and liquid to be concentrated in the tubes, (2) a separator or vapor space for removing entrained liquid from the vapor, (3) when operated as circulation unit, a return leg are provided for the liquid from the separator to the bottom of the exchanger. Inlets are provided for feed liquid and steam, and outlets are provided for vapor, thick liquor, steam condensate, and noncondensable gases from the steam.Long tube evaporators normally are designed with tubes 1 to 2 inches in diameter and from 12 to 30 feet in length. Long tube units may be operated as once-through or may be recirculating systems. If once through, no liquid level is maintained in the vapor body, tubes a deflector plate is often provided in the vapor body, and tubes are 12 to 20 feet long. Recirculated systems can be operated batchwise or continuously. Circulation of fluid across the heat transfer surface depends upon boiling. 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.

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3.2 Rising or Climbing Film EvaporatorsThe long tube evaporator described above is often called a rising or climbing film evaporator.

3.2.1 Theory:The theory of the climbing film is that vapor traveling faster than the liquid flows in the core of the tube causing the liquid to rise up the tube in a film. This type of flow can occur only in a portion of the tube. When it occurs, the liquid film is highly turbulent and high heat transfer rates are realized. Residence time is also low permitting application for heat sensitive materials.

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3.3 Falling Film EvaporatorsThe falling film version of the long tube evaporator eliminates the problems associated with hydrostatic head.

3.3.1 Working:Liquid is fed at the top of long tubes and allowed to fall down the walls as a film. Evaporation occurs on the surface of the highly turbulent film and not on the tube surface. This requires that temperature differences be relatively low. Vapor and liquid are usually separated at the bottom of the tubes. Sometimes vapor is allowed to flow up the tube counter to the liquid. Pressure drop is low and boiling point rises are minimal. Heat transfer rates are high even at low temperature differences.

3.3.2 Uses: The falling film evaporator is widely used for concentrating heat sensitive products because the residence time is low. Falling films are also used in fouling services because boiling occurs on the surface of the film and any salt resulting from vaporization is swept away and not deposited on the tube surface. They are also suited for handling viscous fluids.Falling film units are also easily staged.

3.3.3 Problems:The main problem associated with falling film units is the need to distribute the liquid evenly to all tubes. All tubes must be wetted uniformly and this may require recirculation of the liquid unless the ratio of feed to evaporation is relatively high. Recirculation can only be accomplished by pumping. Distribution can be achieved with distributors for individual tubes, with orifice plates above the tubes and tubesheet, or by spraying. Updraft operation complicates the liquid distribution.

3.4 APPLICATIONS: They are best applied when handling clear fluids, foaming liquids, corrosive

fluids, large evaporation loads. Falling film units are well suited for heat sensitive materials or for high vacuum

application, for viscous materials, and for low temperature difference.

3.5 ADVANTAGES: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 services.

3.6 DISADVANTAGES: Disadvantages include:

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High headroom 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 fluids, large evaporation loads Falling film units are well suited for heat sensitive materials or for high

vacuum application, for viscous materials, and for low temperature difference.

4. FORCED CIRCULATION EVAPORATORS:The basic principle of forced circulation evaporator is that the liquid-film transfer coefficient is increased by increasing the flow of liquor through tubes.

4.1 CONSTRUCTION & WORKING:In forced-circulation evaporator and pump is mounted outside the evaporator body. The liquor is introduced at the bottom and pumped straight through the calandria, or it is introduced in the separating section. The liquid enters the bottom of the tubes and is heated as it rises and at the same time pressure falls. 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 prevent boiling. The heating element may be installed vertically usually single pass. Heating elements may also be installed horizontally often two pass Sometimes the pumped fluid is allowed to vaporize in the tubes. This often provides higher heat transfer rates but increases the possibility of fouling. Consequently this type of evaporator is seldom used except where headroom is limited or the liquids do not scale, salt, or foul the surface.

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The forced circulation system is the easiest to analyze and permits the functions of heat transfer, vapor-liquid separation, and crystallization to be separated. Forced circulation systems are generally more expensive than natural circulation systems and are therefore used only when necessary. A choice of forced circulation can be made only after balancing the pumpingenergy 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. These are therefore suited for vacuum operation.

4.2 APPLICATION: Forced circulation evaporators are best applied: viscous products and corrosive products solutions with suspended solids solutions close to crystallization

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They are also well adapted for vacuum service and for services requiring a high degree of concentration and close control of product concentration

4.3 ADVANTAGES:Forced circulation evaporators offer these advantages:

high rate of heat transfer positive circulation relative freedom from salting, scaling, and fouling ease of cleaning; and a wide range of application

4.4 DISADVANTAGES: Disadvantages include:

High cost and relatively high residence time Necessary pumps with associated maintenance and operating costs.

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