Evaporators G.P MANDI ADAMPUR , HISAR Copyright 2014 Given to Diploma in food Technology

*

What is Evaporators ?EVAPORATION PROCESS:-Evaporation is a heating process in which water is removed from the liquid substance. The goal of evaporation is tovaporizemost of the water from a solution which contains the desired product. Evaporators are widely used in the food processing industry to remove a portion of the water from food products. This reduces bulk and weight for subsequent processing, increases solids content (as for jams and molasses), helps in preservation of the product, provides convenience to the end consumer and concentrates color or flavor.

EXAMPLE OF EVAPORATION PROCESS

What is evaporators Evaporators: - An evaporator is a device used to turn the liquid form of a chemical into its gaseous form. The liquid is evaporated, or vaporized, into a gas. Many types of evaporators and many variations in processing techniques have been developed to various products.

Internal view of Evaporators PROCESS

MULTIPLE EFFECT EVAPORATION The single effect evaporation use steam to supply heat to liquid and provide latent heat of vapourisation. This vapors then taken to a condenser where the latent heat is given up to the cooling water, which commonly then goes to waste. In multiple effects evaporation two evaporators are joined together with piping arrangement so that the calandria of the first effect is heated by steam: the vapor from the first effects used to heat the calandria of second effect. The vapor from the second effect is then taken to a condenser in the ordinary way.

Types Of Evaporators The more common types of evaporators include:- 1. Batch pan Evaporators. 2. Forced circulation. 3. Natural circulation. 4. Rising film tubular. 5. Falling film tubular. 6. Rising/falling film tubular.

Batch pan EvaporatorsThe batch pan evaporator is one of the simplest and oldest types of evaporators used in food industry. Now-a-days it is outdated technology, but it still used in a few limited applications, such as the concentration of jams and jellies where whole fruit is present and in processing some pharmaceutical products. Up until the early 1960's, batch pan also enjoyed wide use in the concentration of corn syrups. In batch pan evaporator the product is heated in a steam jacketed spherical vessel. The heating vessel may be open to the atmosphere or connected to the condenser and vacuum. The heat transfer area per unit volume is in batch pan evaporator is small. Thus, the residence time normally is many hours. Therefore, it is essential to boil at low temperatures and high vacuum when a heat sensitive or thermo degradable product is involved.

Diagram of Batch pan Evaporators

Natural Circulation evaporatorsIn Natural Circulation evaporators, short vertical tubes, typically 1-2 m long and 50-100 mm in diameter, are arranged inside the tubes, and the product is concentrated. The concentrated liquid falls back to the base of the vessel through a central annular section. A shell-and-tube heat exchanger can be provided outside the main evaporation vessel to preheat the liquid feed. The most common application for this type of unit is as a reboiler at the base of a distillation column.

Example of Natural Circulation Evaporators

Forced Circulation EvaporatorThe forced circulation evaporator was developed for processing liquors which are susceptible to scaling or crystallizing. Here the liquor is circulated by means of a pump and as it is under pressure in the tubes, the boiling point is elevated and no boiling takes place. As the liquor leaves the tubes and enters the body of the evaporator, there is a drop in pressure and vapor flashes off from the superheated liquor.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 temperature difference across the heating surface in the heat exchanger is usually 3-50 c.

1)These evaporators are mainly used for thermo labile materials.

2)These can also be used in practice for the concentration of insulin and liver extracts

Advantages 1)These have an great advantage over natural circulation evaporators in that the rapid liquid movement improves heat transfer, especially with viscous liquids or materials that deposit solids or foam readily. 2)The equipment is suitable for operation under reduced pressure due to the capacity of the process to overcome the effect of greater viscosity of liquids. 3)These evaporators possess rapid evaporation rate.4. Both capital and operating costs of these evaporators are very low compared to other.

Example of Forced Circulation Evaporator

These are considered to be the first 'modern' evaporator used in the industry, the rising film unit dates back to the early 1900's. The rising film principle was developed commercially by using a vertical tube heated from the outside with steam. Liquid on the inside of the tube is brought to a boil, with the vapor generated forming a core in the center of the tube. As the fluid moves up the tube, more vapors are formed resulting in a higher central core velocity those forces the remaining liquid to the tube wall. Rising-film Evaporators

Example of Rising-film Evaporators

The falling-film evaporators has a thin liquid film moving downward under gravity on the inside of the vertical tubes. The design of such evaporators is complicated by the fact that distribution of liquid in a uniform film flowing downward in a tube is more difficult to obtain than an upward-flow system such as in a rising- film evaporator. This is accomplished by the use of specially designed distribution or spray nozzles. The falling-film evaporators allow a greater number of effects then the rising-film evaporator. The falling-film evaporator can handle more viscous liquids than the rising film type. This type of evaporators is best suited for highly heat sensitive products such as orange juiceFalling-Film Evaporators

Falling-Film Evaporators

Rising/Falling EvaporatorIn the rising/falling evaporator, the product is concentrated by circulation through a rising-film section followed by a falling-film section of the evaporator. The product is first concentrated as it ascends through a rising tube section, followed by preconcentrated product descending through a falling-film section, there it attains its final concentration.

Example of Rising/Falling Evaporator

*