Evaporator and Its Types

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Evaporator and its types

Awais Ahmad 41

Irfan Shafiq 09


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Evaporator

It is a equipment that is used to turn the liquid in anychemical or substance to gaseous state.

The basic example is Air conditioning

in this phenomena or process evaporator consist ofa liquid that takes heat of air and convert itself into

gaseous form, allowing cold air to circulate.


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Uses

Food and drink industry ( transportation of icecream)

Pharmaceutical industries(removes excess moisture

which makes a more easily handled product andimproves product stability).

As a heat exchanger in industry

Sugar industry


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Working of evaporator

The solution containing the desired product is fedinto the evaporator and passes a heat source.

The applied heat converts the water in the solutioninto vapor.

The vapor is removed from the rest of the solutionand is condensed while the now concentratedsolution is either fed into a second evaporator or isremoved. The evaporator as a machine generally

consists of four sections. The heating section consists of parallel tubes but

others have plates or coils.


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Working of evaporator

The concentrating and separating section removesthe vapor being produced from the solution.

The condenser condenses the separated vapor, thenthe vacuum or pump provides pressure to increase

circulation.


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Types of evaporators

Natural circulation evaporator Natural circulation evaporators are based on the

natural circulation of the product caused by thedensity differences that arise from heating.

After the water begins to boil, bubbles will rise andcause circulation, facilitating the separation of theliquid and the vapor.

The amount of evaporation that takes place depends

on the temperature difference between the steamand the solution.


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Natural circulation evaporator

If tubes are not well immersed then circulation will becompromised.

So forced circulation is introduced by a pump due to

which good circulation is achieved at hightemperature.

But steam consumption is increased.

Circulation evaporators can operate over a widerange of concentration and thick liquor in single unit.


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Falling film evaporator

This type of evaporator is generally made of longtubes (48 meters in length) which are surrounded

by steam jackets.

The uniform distribution of the solution is important

when using this type of evaporator. The solution enters and gains velocity as it flows

downward. This gain in velocity is attributed to thevapor being evolved against the heating medium,

which flows downward as well. (video)


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Climbing film evaporator

These vertical evaporator are with upward flow ofliquid

Steam in the shell and liquid in pipe.

Separator to remove entrained liquid from the vapor

Inlets for steam and liquid are provided

Outlets are provided for vapor and non condensablegases

They are 25-50mm in dia

3-10m long


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Climbing film evaporator

Feed is entered into the system Concentrated liquor is withdrawn from bottom of

heater

The mixture of liquid and vapor from top of the tube

flows into separator, where its velocity is greatlyreduced

This aids in eliminating liquid droplets


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Multiple-effect evaporators

Unlike single-stage evaporators, theseevaporators can be made of up to sevenevaporator stages or effects.

The energy consumption for single-effect

evaporators is very high and makes up most ofthe cost for an evaporation system.

Putting together evaporators saves heat andthus requires less energy. Adding one evaporator

to the original decreases the energyconsumption to 50% of the original amount.


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Adding another effect reduces it to 33% and soon. A heat saving % equation can be used toestimate how much one will save by adding acertain amount of effects

The number of effects in a multiple-effectevaporator is usually restricted to seven becauseafter that, the equipment cost starts catching upto the money saved from the energy requirement

drop.


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types of feeding

There are two types of feeding that can be usedwhen dealing with multiple-effect evaporators.

Forward feeding Backward feeding.


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Forward feeding takes place when the productenters the system through the first effect, whichis at the highest temperature.

The product is then partially concentrated as

some of the water is transformed into vapor andcarried away. It is then fed into the second effectwhich is a little lower in temperature.

The second effect uses the heated vapor createdin the first stage as its heating source (hence thesaving in energy expenditure).


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The combination of lower temperatures andhigher viscosities in subsequent effects providesgood conditions for treating heat-sensitiveproducts like enzymes and proteins.

In using this system, an increase in the heatingsurface area of subsequent effects is required.


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backward feeding.

In this process, the dilute products is fed into the lasteffect with has the lowest temperature and is

transferred from effect to effect with the temperatureincreasing.

The final concentrate is collected in the hottest effectwhich provides an advantage in that the product ishighly viscous in the last stages so the heat transferis considerably better...


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FORCED-CIRCULATION EVAPORATOR

Natural circulation evaporators are economical,but they cannot be used in quite a few situation:

If the solution is viscous, the circulation velocityremains low which greatly reduces the heat

transfer coefficient If the solution contains suspended solid particles

(it may occur in a crystallizing evaporator) or ifthere is a possibility of salting out of a solute,settling of particles is very likely to occurbecause of too low a velocity attainable in anatural circulation unit.


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While handling the solution of aheat sensitive material, the time of contact of thesolution with the hot surface of a tube should below. This is possible only by maintaining a high

velocity of the liquid in the tubes which is notpossible in a natural circulation evaporator. Thevelocity through the tubes is be enhanced in aforced circulation unit by employing a pump.


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Evaporator and Its Types