This article has attempted to cover the basics of evaporators to give potential evaporator users some background knowledge of what evaporators are all about.The following description outlines the basics of evaporators and offers some guidance in choosing a suitable evaporator configuration

Evaporators come in many different shapes and sizes. Selecting the best evaporator for a givenapplication can sometimes be a confusing and even intimidating task. Technical terms like falling film, forced circulation and multiple effects can add to the challenge. In this brochure we will take a brief, not-too-technical look at the most common types of evaporators, how they work, and some of their applications.


The use of a multiple effect evaporator is key in implementing the Zero Liquid Discharge (ZLD) norms of pollution control board.

Evaporators :

The basic task of an evaporator is simple: to remove water from a solution or slurry by evaporation. Evaporators are distinct from dryers in that the concentrate discharged from an evaporator is always in liquid form. The feed to an evaporator is always in liquid form and remains in liquid form even after the water is evaporated

The physical process of evaporation requires the input of energy in the form of heat to convert a liquid into vapor. Since all evaporators use the process of evaporation to remove water, every evaporator requires a source of heat to operate. The heat source for almost all evaporators is water vapor, either in the form of boiler steam or waste vapor from another process.

A second requirement for all evaporators is a means to transfer heat energy from the heat source into the evaporator liquid. Most evaporators use a tubular heater called a shell and tube heat exchanger for this purpose. In the heat exchanger shell, water vapor condenses on the outside of the tubes thus giving up its heat energy, called latent heat. The evaporator liquid, which is inside the tubes, absorbs the heat given up by the water vapor. This increase in heat causes the water in the evaporator liquid to boil. As the water in the evaporator liquid boils, it forms bubbles of water vapor in the liquid much like a pan cooking on a stove. As these bubbles reach the surface of the evaporator liquid and burst, the escaping water vapor carries some of the evaporator liquidwith it.

The final requirement for an evaporator, then, is a means of separating the evaporated water vapor from the evaporator liquid. These two main components, the heat exchanger and the vapor body, are connected together to form an evaporator. Almost all evaporators operate in the same way. Evaporator liquid is circulated through the heat exchanger tubes to absorb heat and then discharged into the vapor body to give up the water vapor which is boiled off. In most evaporators a centrifugal pump is used to circulate the evaporator liquid through the heat exchanger and vapor body.The circulating rate of the evaporator liquid depends on the type of evaporator and the evaporator liquid. The pressure in the vapor body of an evaporator determines the boiling point of the water in the evaporator liquid. If the pressure in the vapor body is atmospheric, the water will boil at 100oC. This requires the use of boiler steam as a heat source in the heat exchanger shell to achieve proper heat transfer

Types of Evaporators:

Falling film tubular

Forced circulation

Falling Film Evaporator :

Like the rising film evaporator, the heat exchanger in a falling film evaporatoris vertically mounted. In this case, however, evaporator liquid is pumped to the top of the heat exchanger and flows in a downward direction through the tubes.Boiling of water in the evaporator liquid occurs as the liquid flows down through the tubes which help to force the liquid down and out of the tubes.

The falling film evaporator does have the advantage that the film is ‘going with gravity’ instead of against it. This results in a thinner, faster moving film and gives rise to an even shorter product contact time and a further improvement in the value of HTC.


The forced circulation evaporator :

It was developed for processing liquors which are susceptible to scaling orcrystallizing. Unlike the rising and falling film evaporators, this evaporator is specifically designed so that no boiling occurs while the evaporator liquid is in the tubes. The evaporator liquid is pumped by the circulating pump through the heat exchanger tubes where heat is absorbed. Liquid is circulated at a high rate through the heat exchanger, boiling being prevented within the unit by virtue of a hydrostatic head maintained above the top tube plate. As the liquid enters the separator where the absolute pressure is slightly less than in the tube bundle, the liquid flashes to form a vapor. The main applications for a forced circulation evaporator are in the concentration of inversely soluble materials, crystallizing duties, and in the concentration of thermally degradable materials which result in the deposition of solids. In all cases, the temperature rise across the tube bundle is kept as low as possible. This results in a very high recirculation ratio to that of water evaporated. These high recirculation rates result in high liquor velocities through the tube which help to minimize the buildup of deposits or crystals along the heating surface. Forced circulation evaporators normally are more expensive than film evaporators because of the need for large bore circulating pipework and large recirculating pumps. Operating costs of such a unit also are considerably higher.

Evaporator Configurations for Energy Conservation

Conservation of energy is one major parameter in the design of an evaporator system. The larger the evaporation duty, the more important it is to conserve energy

The following techniques are available

Multi-Effect Evaporation

Multi-effect evaporation uses the steam produced from evaporation in one effectto provide the heat to evaporate product in asecond effect which is maintained at a lower pressure. In a two effect evaporator, it is possible to evaporate approximately 2 kgs of steam from the product for each kg of steam supply. As the number of effects is increased, the steam economy increases. On some large duties it is economically feasible to utilize as many as seven effects. Increasing the number of effects, for any particular duty, does increase the capital cost significantly and therefore each system must be carefully evaluated. In general, when the evaporation rate is above 1,000 kg/h, multieffect evaporation should beconsidered.

Thermo Vapor Recompression (TVR)

When steam is available at pressures in excess of 45 psig (3 barg) and preferably around 6 bar(g), it will often be possibleto use thermo vapor recompression In this operation, a portion of the steam evaporated from the product is recompressed by a steam jet venture and returned to the steam chest of the evaporator

A system of this type can provide a 2 to 1 economy or higher depending on the product the steam pressure and the number of effects over which TVR is applied. TVR is a relatively inexpensive technique for improving the economy of evaporation.

TVR can also be used in conjunction with multi-effect to provide evenlarger economies. Shown in Figure are the economies that can be achieved. Thermocompressors are somewhat inflexible and do not operate well outside the design conditions. Therefore if the product is known to foul severely, so that the heat transfer coefficient is significantly reduced, it is best not to use TVR. The number of degrees of compression is too small for materials that have high boiling point elevation.

Combination of Film and Forced Circulation Evaporators

The most economic evaporators utilize falling film tubular or plates, with either TVR or MVR. However with many duties, the required concentration of the final product requires a viscosity that is too high for a film evaporator. The solution is to use film evaporation for the pre-concentration and then a forced circulation finisher evaporator to achieve the ultimate concentration; e.g., a stillage or spent distillery wash evaporator.The material would typically be concentrated from 4% to 20% in a falling film evaporator andthen from 20% to 30% in a forced circulation evaporator. Usually the finisher would be a completely separate evaporator since the finisher duty is usually relatively low. In the duty specified above, almost 98% of the evaporation would take place in the high efficiency film evaporator.For cases where the finisher load is relatively high, it is possible to incorporate the forced circulation finisher as one of the effects in a multi-effect evaporator. However this is an expensive proposition due to the low coefficients at the high concentration.

Skid Mounted Evaporators

  • Skid mounted evaporators are considered to be the most economical product.
  • It is a Pre-Fabricated Evaporator Unit that allows easy site installation without extensive foundation work.
  • As only minimum piping is required, such as input and output connections, it saves time and energy.One of the attractive features is the low maintenance cost. The customer can operate this equipment in any condition and therefore, there is an increasing demand for skid mounted evaporators.

Pan Evaporators

  • Pan Evaporators are the oldest type of industrial evaporators. They have low Heat Transfer Coefficients (HTC) and cycle times are measured in hours. Low surface areas together with low HTC’s generally limit the evaporation capacity of such a system. Heat transfer is improved by agitation within the vessel. In many cases,large temperature differences cannot be used for fear of rapid fouling of the heat transfer surface.
  • In Pan Evaporator, the steam jacketed rectangular vessel is open to theatmosphere. The product is fed into the vessel and heated using steam, until thewater in the product evaporates. The residence time is normally one hour for eachbatch of 100 liters. Then the salt is scraped off from the surface of the vessel.

Agitated thin film dryers

  • Agitated Thin Film Dryer(ATFD) for evaporation of water/solvents to make concentrated liquid to dry powder upto 5% moisture . Based on application either recovered solvent or dry product is important. Agitated Thin Film Dryer design is the ideal apparatus for continuous processing of concentrated material for drying.
  • Agitated Thin Film Dryer is consist of cylindrical, vertical body with heating jacket and a rotor inside of the shell which is equipped with rows of pendulum blades all over the length of the dryer.
  • The hinged blades spread the wet feed product in a thin film over the heated surface. A highly agitated bow wave is formed in front of the rotor blades. The turbulence increases as the product passes through the clearance before entering a calming zone situated behind the blades. As the heat will transfer from jacket to main shell under the smooth agitation water/solvent will evaporate and liquid will convert to slurry, to cake and to dry powder or flakes.
  • He vapors produced rise upwards, counter-current to the liquid and pass through a Cyclone Separator mounted at the vapor outlet of Agitated Thin Film Dryer (ATFD). Further these vapors will be condensed in a condenser and recovered as condensate.The system will be operated under vacuum for temperature sensitive products and atmospheric conditions for normal drying.

Bag lifting centrifuges

  • Bag Lifting Type Centrifuge is the heavy duty centrifuges with fast acceleration and smooth running. After centrifuging process the solids are unloaded by lifting the basket top along with filter bag and solids. The filter bag is fitted with removal basket top with a bag holding ring. The ring holds the filter bag firmly while filtering and lifting. The bottom opening of the filter bag allows the drainage of solids so as to make the discharge of solids simple and swift. It involvesleast manual labor and facilitates for effortless loading and unloading along with superior production capacity per batch.

Basket centrifuge

  • Basket Centrifugesare the simplest and inexpensive centrifuges available.They are primarily used for solid/liquid separation but some liquid/liquid separation is possible with some units.
  • It require the use of a filter media or filter bag in the rotating basket.Feed material is dumped into the rotating basket and liquid flows through the filter media and perforated basket.Solids are collected in the basket.Solids may also be washed in in the basket and dried to a very low moisture content.Removal of solids must be done for each batch either manually or with an automated discharge scraper system.

Pusher centriguge

  • With simple robust design the Pusher Centrifuge gives high throughputs under the toughest conditions. Operates purely on with mechanical drive the Pusher Centrifuge is suited for all fields for the separation of slurries with solid which filters easily. The average particle size of the solids must be at least 0.1 mm. Pusher Centrifuge utilizes continuous Filtration for the suspended, fast draining, crystalline and granular or fibrous solids from the liquid phase. These solids can be washed as they are transported though the basket.

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