Types Of Evaporators

Types of evaporators 1. Natural circulation type – a) Vertical short tube or Calandria evaporator b) Long tube vertical (LTV) rising film type c) Long tube vertical (LTV) falling film type 2. Forced Circulation type Calandria Evaporator It has a vertical tube bundle consisted of short tubes (usually less than six feet) integral with the shell. This is called a calandria. There is a vapour space above the tube bundle. The calandria is of annular construction i.e. there is an open cylindrical region at the center called the downcomer.. Feed is supplied through a nozzle above the upper tube sheet and steam to the shell or the steam chest of the calandria. In once through operation (useful for heat sensitive material), the feed liquor passes through the tubes only once, releases the vapour and leaves the unit as thick liquor i.e. all the evaporation is accomplished in a single pass. The ratio of vapourization to feed is limited in single pass units; thus these evaporators are well adapted to multiple effect operation. In recirculation type of operation, a pool of liquid is held within the equipment. Incoming feed mixes with the liquid from the pool and the mixture passes through the tubes. The solution is heated and partly vapourized in the tubes. The vapour-liquid mixture flows up through the tubes due to the prevailing density gradient. Vapour-liquid disengagement occurs above the upper tube sheet. Unevaporated liquid discharged from the tubes returns to the pool down through the central downcomer. Thus a continuous natural recirculation of the solution occurs. Thick product liquor is withdrawn from the bottom through a thick liquor outlet pipe. The steam condensate leaves through a drain nozzle connected to a steam trap. A bleed or vent line is provided in the shell for the release of the non-condensable in steam. The vapour leaves through an outlet at the top of the evaporator body through an entrainment separator or mist eliminator which arrests the liquid droplets in the vapour. They are used to concentrate a variety of solutions – a common example is concentration of the sugar solution. However, these are not suitable for solutions in which precipitation or salting out a solid may occur. The natural circulation velocity in the evaporator is not sufficient to keep the solid particles in suspension. The problem may be overcome by installing an agitator in the downcomer pipe to increase the circulation rate. For most applications, however, the lower equipment costs for other designs has prompted the replacement of calandria evaporators with LTV rising-film, LTV falling-film and forced-circulation evaporators.

Fig. 1. Calandria Evaporator In certain calandria evaporators the tube bundle is not integral to the shell and is removable. The tube bundle with fixed tubesheets is bolted to an internal bracket that is welded to the shell for ming a basket. Therefore this type of evaporator is also called Basket type Evaporator. By removing the bolts the tube bundle can be removed for maintenance and cleaning.

LTV RISING-FILM EVAPORATOR A long tube vertical rising film evaporator consists of a long vertical tube bundle fitted within a shell. The shell is projected into a larger diameter chamber or vapour head at the top. liquor is fed into the bottom liquor chamber and enters the tube bundle at the bottom. It is heated with condensing steam or any other suitable heat-transfer medium flowing outside the tubes and rises upwards due to density gradient. For cold feed, the lower portion of the tubes is used to preheat the liquor to its boiling point. Vaporization then begins at that height within the tubes where the liquor temperature exceeds the boiling temperature at the prevailing pressure. As the liquor climbs up the inside of the tubes, the liquor undergoes vigorous boiling and additional vapor is generated and the velocity of the liquid-vapor mixture increases. The outlet mixture impinges upon a deflector, mounted above the top tubesheet of the heat exchanger, where gross, initial separation of the liquid from the vapor occurs. Additional liquor is separated from

the vapor by gravity as the vapor rises in the vapor body. An entrainment separator can be installed near the top of the vapor body to remove most of the remaining traces of liquid from the vapor. The exit vapor is conducted either to the next effect of a multiple-effect evaporator, or to a condenser. A vertical-tube surface condenser is shown in fig 2. The concentrated liquor is discharged from a connection near the bottom of the vapor body. The major advantage of this type of evaporator is its high heat transfer rate. In any evaporator, the liquid side heat transfer resistance virtually controls the rate of heat transfer as the resistance of the condensing steam on the shell-side is very small. Liquid side heat transfer coefficients are enhanced in the non-boiling section by surface or local boiling and in the boiling section by nucleate boiling. As expected, the heat-transfer rates in the boiling zone are several times greater than those in the non-boiling zone, so it is important to reduce the non-boiling zone to a minimum. Different two-phase flow schemes are created in the boiling zone, including slug flow, where a slug of liquor is followed by a slug of vapor, annular flow, where a ring of liquor encases a center core of vapor and liquid mist; and mist flow, where vapor blankets the tube surface. Mist flow should be avoided because poor heat transfer results when there is not enough liquid present to wet the tube walls. To avoid mist flow, it is sometimes necessary to recycle concentrated product from the vapor body to the bottom liquor chamber so as to supplement the feed liquor. (This recirculation line is shown in fig 2.) Other advantages of LTV rising film evaporator are low cost, low liquid hold up and less floor space requirement. The vapor body shown in fig 2 is integral with the heat exchanger. When the heat exchanger is too large to use the integral configuration, when quicker access to the tubes is desired for maintenance, or when surge volume in the vapor body is required for level control, the vapor body is separated from the heat exchanger, generally as shown in fig. 3. A skirt-type baffle replaces the deflector as the initial separator. Disadvantages of this type of unit are high headroom requirement, unsuitable for viscous and scale forming material.

Figure 2 LTV RISING-FILM EVAPORATOR

LONG TUBE VERTICAL FALLING FILM EVAPORATOR It consists of a long vertical tube bundle heated by condensing steam or any other hot liquid on the shell side. Liquor is fed into the top liquor chamber of the heat exchanger where it is distributed to each tube and flows down the inner walls of the tubes as thin film in once through mode. The liquor accelerates in velocity as it descends inside the tubes because of the gravity and drag of the vapor generated by boiling. Liquid is separated from the vapor in the bottom liquor chamber of the heat exchanger and with a skirt-type baffle in the vapor body. Concentrated liquor is discharged from the bottom liquor chamber and cone bottom of the vapor body. The vapor body can be provided either as a separate component (fig 3) or as an integral component of the heat exchanger, similar to that shown in fig 2, except the heat exchanger would be located above the vapor body in the falling-film configuration. The vapour collected in the vapour body goes to an entrainment separator installed in the upper portion of the vapor body to reduce liquid entrained with the vapor to a minimum. The falling-film evaporator is particularly useful in applications where the driving force in temperature difference between the heat-transfer medium and the liquid is small (∆ T's of less than 150F). The retention time for liquor in this evaporator is less than that for a rising-film evaporator. The combination of short liquid retention time and the ability to operate at a low Delta-T makes the falling-film evaporator ideal for concentrating the most heat-sensitive materials. High heat-transfer coefficients are attained in falling-film evaporators when a continuous film of liquid, preferably at its boiling point, flows down the inside tube wall with a vapor core in the tube center. For some applications, however, it is necessary to supplement an insufficient quantity of feed liquor with product liquor pumped to the top liquor chamber to avoid vapor blanketing of the inside tube surface. The major problem in a falling film evaporator is difficulty in distributing the liquid uniformly as a film inside the tubes. Liquid distribution is carried out by providing a liquid distributor at the top (distribution plate shown in fig3).

Fig. 3 Falling Film Evaporators

FORCED-CIRCULATION EVAPORATOR Natural circulation evaporators are economical, but they cannot be used in quite a few situation: 3. If the solution is viscous, the circulation velocity remains low which greatly reduces the heat transfer coefficient 4. If the solution contains suspended solid particles (it may occur in a crystallizing evaporator) or if there is a possibility of salting out of a solute, settling of particles is very likely to occur because of too low a velocity attainable in a natural circulation unit. 5. While handling the solution of a heat sensitive material, the time of contact of the solution with the hot surface of a tube should be low. This is possible only by maintaining a high velocity of the liquid in the tubes which is not possible in a natural circulation evaporator. The velocity through the tubes is be enhanced in a forced circulation unit by employing a pump. It has a tubular exchanger, either vertical (fig 4) or horizontal (fig 5) for heating the solution without boiling. Slurry is pumped from the bottom cone of the vapor body through the tubes of the vertical heat exchanger, where heat is added. The superheated solution flashes in a vapour or flash chamber. Vapour and liquid separates due to density difference. The concentrated liquor collects at the bottom cone to be recirculated to the heat exchanger. The flash chamber is mounted with a direct contact condenser at the top where vapour is condensed (Swenson design). A short piece of vertical pipe connects the vapor body with the condenser to minimize piping and pressure drop. This design also eliminates structural steel for support of a separate condenser. The heat exchanger may be placed after the pump discharge as well. A part of the concentrated liquor is continuously taken out as product. The feed is introduced at a point before the pump suction. Heating steam is introduced to the shell side of the heat exchanger. A forced circulation unit with a horizontal floating head allows removal of the tube bundle for maintenance or cleaning. The combined total head (static + frictional) must remain sufficiently high so that the liquid does not boil in the heat exchanger. The static head in the exchanger may be increased as desired by placing it sufficiently below the flash vessel. The vapor body is conservatively designed both in diameter and height. It is important to have an adequate free space above the liquor level to allow the liquor droplets entrained in the vapor leaving the boiling surface to reach equilibrium and return by gravity to the circulating slurry. Large diameters result in low vapor velocities which minimize entrainment. A mesh-type entrainment separator may be installed in the upper portion of the vapor body to reduce solids carryover. A low-pressure-drop liquid cyclone is sometimes used (Swenson design) to clarify liquor discharged from the evaporator. The driving force is the pressure drop across the circulating pump. Thickened slurry is returned through a wideopen cyclone underflow connection to the circulating piping before the pump

suction. Forced- circulation evaporators are used for handling viscous or heat sensitive material and also in the crystallization operation. Typical applications are in the evaporation of brine or concentration of caustic soda solutions. Due to high heat transfer coefficient attainable, the area of the heating surface can be small. As a result, these evaporators are preferred for corrosive solutions that demand special material of construction. Disadvantages of these types of evaporators are high capital cost, high headroom requirement and high floor area requirement if a horizontal heat exchanger is used. A Swenson designed forced-circulation evaporator with horizontal heat exchanger and a top-mounted stripping column is shown in fig 5. Reflux liquid is introduced on the top tray of the column to strip one or more compounds from the water vapor. Stripping columns are used for special applications and are provided either integral with the evaporator or as a separate column. The columns are for the recovery of valuable components from the water vapor and for the reduction of volatile pollutants.

Figure 4 FORCED-CIRCULATION EVAPORATOR WITH VERTICAL HEAT EXCHANGER

Figure 5 FORCED CIRCULATION EVAPORATOR WITH HORIZONTAL HEAT EXCHANGER

Evaporator Auxiliaries 1. 2. 3. 4.

Condenser Vacuum Devices Steam Traps Entrainment Separators

1. Condensers Condenser is required to condense the vapour from the evaporator. Mostly contact condenser with barometric leg (Barometric condenser) is used. Since cooling water in a contact condenser has to be discarded (due to mixing of the condensed vapour and cooling water due to direct contact), sometimes due to shortage of cooling water, water cooled surface condensers in which the cooling water can be reused are also used. 2. Vacuum Devices These are used to maintain vacuum within the evaporator and also to remove the non-condensables. They may be broadly classified into two categories – (i) Vacuum Pumps and (ii) Steam jet ejectors Vacuum Pumps – May be either reciprocating or centrifugal (suitable for higher capacities) Steam jet ejector and barometric condenser – A steam jet ejector combined with a condenser with a barometric leg is a simple and cheap method of maintaining vacuum. Water vapour with non-condensables, if any, from the evaporator first enters the barometric condenser where vapour condenses by direct contact with water fed to the condenser. In order to prevent build up of non-condensables and uncondensed vapour within the condenser, the barometric condenser is connected to the suction of a steam jet ejector to withdraw the non-condensables and vapour continuously. In this way the vacuum in the condenser and in the evaporator is stabilized. The vapour and noncondensables sucked by the ejector are ultimately discharged to the atmosphere. 3. Steam Traps This device is used to remove the steam condensate without allowing the steam to blow out. It is an automatic valve that allows the condensate to discharge between traps (or prevents the flow of) steam. It distinguishes between the condensate and steam on the basis of (i) density difference (ii) temperature difference (iii) difference in flow characteristics 4. Entrainment Separators

These are used to remove small droplets of liquid being carried away with the vapour so that loss of liquid from the system can be prevented. The most common type of entrainment separators is a Demister. A demister is a pad made up of several layers of wire-mesh and is installed below the vapour exit pipe. As the vapour flows through the pad, the liquid droplets impinge on the wires of the pad and get separated by impaction mechanism. The retained liquid drips down to the boiling liquid. Another type on entrainment separators are in the form of a set of baffle plates kept fixed in a vessel. As the vapor changes flow direction while flowing through the space in between the plates, the entrained droplets hit the plates and are thus separated from the vapour. Also entrained droplets can be separated by centrifugal separators, however it is more expensive.