Evaporators

TYPES OF EVAPORATORS

A.NATURAL CIRCULATION EVAPORATORS

• It consists of short vertical tubes , typically 1-2m long and 50-100mm in diameter are arranged inside the steam chest.

• The whole calandria is located in the bottom of the vessel.

• The product when heated rises through these tubes by natural circulation while steam condenses outside the tubes.

• Evaporation take place inside the tubes and the product is concentrated.

• The concentrated liquid falls back to the base of the vessel through a annular section.

1.OPEN-OR-CLOSED-PAN EVAPORATORS

ž These consists of a hemispherical pan heated directly by gas or electrical resistance wires.

ž They are also heated by steam passed through internal tubes or an external jacket.

ž They are fitted with a lid for vacuum operation and a stirrer or paddle to increase the rate of heat transfer and to prevent food from burning on to the pan.

ž They have relatively low rates of heat transfer and low energy efficiencies, and they cause damage to heat sensitive foods.

ž They have low capital cost , are easy to construct and maintain and are flexible for applications where frequent changes of product are likely , or when used for relatively low or variable production rate.

ž They have wide application in the preparation of ingredients such as sauces and graves or in the manufacture of jam and other preserves.

2.SHORT-TUBE EVAPORATORS

ž This type of an equipment is an example of a tube-and-shell heat exchanger.

ž It is also used in pasteurization , heat sterilization and freezing.

ž It consists of a vessel which contains a bundle of tubes.

ž The feed liquor is heated by steam condensing on the outside of the tubes. Liquor rises through the tubes, boils and recirculates through a central down comer.

ž The vertical arrangement of tubes promotes natural convection currents and therefore higher rates of heat transfer.

ž In some designs the tubes are fitted as a ‘basket’ which is easily removed for cleaning.

ž These have higher rates of heat transfer than open or closed pans, when used with relatively low viscosity liquids.

3.LONG-TUBE EVAPORATORS

ž These evaporators consists of a vertical bundle of tubes , each up to 5cm in diameter contained within a steam shell 3-15m high.

ž Liquor is heated almost to boiling point before entering the evaporator.

ž It is then further heated inside the tubes and boiling commences.

ž The expansion of stem forces a thin film of rapidly concentrating liquor the walls of each tube.

ž The concentrate is separated from the vapor and removed from the evaporator , passed to subsequent effects in a multiple effect system.

ž Vapor is reused in multiple effect system.

4.EXTERNAL CALANDRIA EVAPORATORS

ž These are tube-and-shell heat exchangers which are ,fitted with an external pipe for recirculation of the product.

ž In this way, convection currents are established to product relatively high rates of heat transfer.

ž The calandria is easily accessible for cleaning.

ž They are suitable for concentrating heat sensitive foods, including dairy products and meat extracts, when operated under partial vacuum.

B.FORCED CIRCULATION EVAPORATORS

ž In forced circulation evaporators a pump or scraper moves the liquor, usually in thin layers, and thus maintains high heat transfer rates and short residence times.

ž This also results in more compact equipment and higher production rates but increases both the capital and the operating costs of the equipment.

1.PLATE EVAPORATORS

ž Plate evaporators are similar in construction to the heat exchangers used for pasteurization and ultrahigh- temperature sterilization .

ž In this case the heating medium stem and the climbing-and falling-film principle is used to concentrate liquids in the spaces between plates.

ž Feed liquor enters at the base of each climbing film section, boils are rises to the top pf the plates. It then enters a falling film section where boiling continues.

ž The number of climbing or falling film section fitted with a single machine depend on the production rate and degree of concentration required.

ž The mixture of vapor and concentrate is separated outside the evaporator, and the vapor may be reused in vapor recompression system.

ž These type of evaporators have high rates of heat transfer, short residence time and high energy efficiencies.

ž They are more suitable for liquors of high viscosity than long-tube evaporators are, because the food is pumped through the plate stack.

ž They are widely used for heat sensitive foods, including yeast extract, dairy products and meat extract.

2.EXPANDING-FLOW EVAPORATORS

ž This evaporators uses similar principle to the plate evaporators but has a stack of inverted cones instead of a series of plates.

ž It is similar in appearance to the Centri-therm evaporator .

ž The liquor flows to alternate spaces between the cones from a central shaft and evaporators as it passes through up through channels of increasing flow area.

ž The vapor- concentrate mixture leaves the cone assembly tangentially and is separated by a special design of shell which induces a cyclonic effect.

ž Stem is fed down alternate channels.

ž The reduction in flow area along each channels maintains a high film coefficient for the condensing steam.

ž This evaporator has a number of advantage including compactness ,short residence times and a high degree of flexibility achieved by changing the number of cones.

3.MECHANICAL (OR AGITATED) THIN-FILM EVAPORATORS.

ž Scrapped or wiped surface evaporators are characterized by difference in the thickness of the film of food being processed.

ž Wiped film evaporator have film thickness of approximately 0.25 mm whereas in thin film evaporators it is up to 1.25mm.Boyh types consists of steam jacket surrounding a high speed rotor , fitted with short blades along its length.

ž The design is similar to scraped surface aseptic sterilizer .

ž Feed liquor is introduced between the rotor and the heated surface.

ž Evaporation takes place rapidly as a thin film liquor is swept through the machine by the rotor blades .

ž The blades keep the film violently agitated and thus promote high rates of heat transfer and prevent the product from burning onto the hot surface.

ž This type of equipment is highly suited to viscous heat sensitive foods or to those that are liable to form or foul evaporator for surfaces

(for e.g.: fruit pulp and juices, tomato paste, honey, dairy products etc)

ž It is used for finishing highly viscous products after concentration in other equipment.

ž In this case the volume of food is smaller, there is less water to be removed and benefits of economy are fewer , but the product is valuable and there is a subsequent risk of heat damage.

ž Second design of mechanical thin film evaporator is the Centri-therm evaporator.

ž Here liquor is fed from a central pipe to the underside of rotating hollow cones.

ž Steam condenses on the inside of the cone and heat is conducted rapidly through the thin metal to evaporate the liquor .

ž Centri-therm evaporators employs a centrifugal force to move the liquor rapidly across the heated surface of the cone .

ž A very high overall heat transfer coefficients and short residence time are possible.

ž The equipment produces a concentrate which when rediluted has sensory and nutritional quality .

ž It is used for coffee and tea extracts ,meat extract ,fruit juices and enzymes for use in food processing.