6. heat transfer equipment evaporator

8/19/2019 6. Heat Transfer Equipment Evaporator

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Evaporator

ERT 216 HEAT & MASS TRANSFER


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1. Introduction &

2. General Types of Evaporators3. Methods of Operation of

Evaporators4. Overall Heat-Transfer

Coefficient In Evaporators

5. Calculation Methods for SingleEffect Evaporators


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5.1 Effects of Processing Variables

on Evaporator Operation5.2 Boiling-Point Rise of Solutions5.3 Enthalpy-Concentration Charts

of Solutions6 Evaporation of biological

materials


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Evaporation:The vapor from a boiling liquid solution

is removed and a more concentratedsolution remains.

A separation process of removing water

from an aqueous solution.Examples: Concentrated aqueous solutionsof….

Sugar

Sodium

chloride Glycerol MilkOrange juice


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Evaporation processes: to evaporate seawater to provide

drinking water have been developed andused.

to concentrate a solution so that upon

cooling, salt crystals will form andseparated (crystallization).


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Physical & chemical properties of the

solution being concentrated and of thevapor being removed are influenced bythe type of evaporator used and thepressure and temperature of the process.

Some of the properties which affectingthe processing methods:i. Concentration in the liquid.

Liquid feed to an evaporator isrelatively dilute (its viscosity is low),relatively high heat transfer

coefficients are obtained.


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As evaporation proceeds, the solutionmay become very concentrated & quite

viscous, causing heat transfercoefficient to drop markedly.

Adequate circulation &/or turbulence

must be present to keep the coefficientfrom becoming too low.ii. Solubility As solutions are heated and the

concentration of the solute or saltincreases, the solubility limit ofmaterial in solution may be exceeded

and crystals may form.


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This may limit the maximumconcentration in solution which can beobtained by evaporation.

Fig 1:Solubility

curves for sometypical salts in

water.


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iv. Temperature sensitivity of materialsFood & biological products may be

temperature sensitive & degrade athigher temperatures or afterprolonged heating.

The amount of degradation is afunction of the temperature and thelength of time.

v. Foaming & frothingCaustic solutions, food solutions (e.g.

skim milk), & fatty acid solutions

form a foam or froth during boiling.


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This foam accompanies the vaporcoming out of the evaporator andentrainment losses occur.

vi. Pressure & temperature The boiling point of the solution is

related to the pressure of the system. The higher the operating pressure of

the evaporator, the higher the

temperature at boiling. As the concentration of the dissolved

material in solution increases, the

boiling temperature may rise.


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vii. Scale deposition & materials ofconstruction

Some solutions deposit solid materialscalled scale on the heating surfaces.

Scale could be formed by

decomposition products or bydecreases in solubility.

The overall heat-transfer coefficient

decreases, the evaporator musteventually be cleaned. Material of construction of the

evaporator should be chosen tominimize corrosion.


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Evaporator:The heat is generally provided by thecondensation of a vapor (e.g. steam) onone side of a metal surface, with theevaporating liquid on the other side.

The type of equipment used dependsprimarily on the configuration of theheat-transfer surface & on the means

used to provide agitation or circulationof the liquid.


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1. Open kettle or pan:The simplest evaporator (inexpensive).It consists of an open pan

(or kettle) in which theliquid is boiled.

The heat is supplied bycondensation of steamin a jacket or in coils

immersed inliquid ordirect-fired.


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2. Horizontal-tube natural circulationevaporator

The horizontal bundle of heating tubes issimilar to the bundle of tubes in a heatexchanger.

The steam entersthe tubes, where itcondenses.

The steam condensateleaves at the other endof the tubes.


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It is relatively cheap & is used fornonviscous liquids with high heat transfercoefficients and liquid that do not depositscale.

It is not suitable for viscous liquids due to

poor circulation.Usually, the feed enters at a constant rate

& the concentrate leaves at a constant

rate (most types of evaporator).


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3. Vertical-type natural circulation evaporator

Vertical rather than horizontal tubes are

used; the liquid is inside the tubes and thesteam condenses outside the tubes.

The liquid rises in

the tubes by naturalcirculation due toboiling and decreasesin density.

The liquid flowsdownward througha large, central open

space or downcomer.

Short

tubeevaporator


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This natural circulation increases theheat-transfer coefficient.

It is not used with viscous liquids.A variation is the basket type,the vertical tubes

are used but theheating element isheld suspended in the

body so there is anannular open spaceas the downcomer.


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It differs in such a way where it has a

central instead of annular open space asthe downcomer.

It is used in the sugar, salt & cautic soda

industries.


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4. Long-tube vertical type evaporatorSince the heat-transfer

coefficient on the steamside is very highcompared to that on

the evaporating liquidside, high liquidvelocities are required.

In this type, the liquidis inside the tubes(3 – 10 m long).

Climbing film

evaporator


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Vapor bubbles are formed iside the tubescauses a pumping action, which gives quitehigh liquid velocities.

Generally the liquid passes through thetubes only once and it is not re-circulated.

Contact times can be quiet low.In some cases (e.g. condensed milk), when

the ratio of feed to evaporation rate is

low, natural recirculation of the productthrough the evaporator is affected byadding a large pipe connection between the

outlet concentrate line & feed line.


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5. Falling-file type evaporatorA variation of long-tube type evaporator.The liquid is fed to the top of the tubes &

flows down the walls as a thin film.Vapor liquid separation usually takes place

at the bottom.It is widely used for concentrating heat

sensitive materials (e.g. fruitjuice)

because the holdup time is very small &heat transfer coefficients are high.


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Falling film

Evaporator Agitated-film Evaporator


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6. Agitated-film evaporatorHeat transfer in an evaporator is limited

on the liquid side.By actual mechanical agitation of the

liquid film, turbulence in film and hence the

heat transfer coefficient is increased. This is done in a modified falling-film

evaporator with only a single, large,

jacketed tube containing an internalagitator.


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Liquid enters at the top of the tube & as itflows downward, it is spread out into aturbulent film by vertical agitator blades.

The concentrate leaves at the bottom &vapor leaves through a separator and out

the top.It is very useful with highly viscous

materials and heat-sensitive viscous

materials (e.g. rubber latex, gelatin,antibiotics, fruit juices).It has a high cost & small capacity.


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7. Forced circulationtype evaporator

The liquid film heattransfer coefficientcan be increased by

pumping to causefood circulation ofthe liquid insidethe tubes.


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The long tube vertical type is modified

by adding a pipe connecting a pumpbetween the outlet concentrate line andthe feed line.

The vertical tubes of this type are usuallyshorter than the long-tube type.This type is very useful for viscous liquids.


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1. Single effect evaporators:The feed enters at T F (K) and saturated

steam at T S enters the heat exchangersection.

Condensed steam leaves as condensate or

drips.Simplifieddiagram of

singleeffect

evaporator


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Solution in the evaporator is completelymixed

: Concentrated product & the solution inthe evaporator = T 1 (~ boiling point).

Vapor = T 1 = boiling solutionPressure (P1) is the vapor pressure of the

solution at T 1.An overall heat transfer coefficient is

used, the rate of heat transfer in anevaporator:

)T T ( UAT UAq S 1Δ


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Single effect evaporators are ofen usedwhen the required capacity of operation isrelatively small and/or the cost of streamis relatively cheap compared to theevaporator cost.However, for large-capacity operation,

using more than one effect willmarkedly reduce steam costs.


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2. Forward-feed multiple effect evaporators

A single effect evaporator is wasteful ofenergy because the latent heat of thevapor leaving is not used but is discarded. This latent heat can be recovered &

reused by employing multi effectevaporator.

Simplifieddiagram offorward-

feed tripleeffect

evaporator


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The first effect operates at atemperature that is high enough that theevaporated water serves as the heatingmedium to the second effect. Almost 3 kg of water will be evaporated

for 1 kg of steam in a three-effectevaporator.

The steam economy is increased.

used steamkg

evaporated vapor kg economySteam


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Forward-feed operation:The fresh feed is added to the first

effect and flows to the next in the samedirection as the vapor flow.

It is used when the feed is hot or the

final concentrated product might bedamaged at high temperature.

The boiling temperatures decreases

from one effect to another effect.If the first effect (P1) is 1 atm, then

the last effect (P3) will be under

vacuum.


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3. Backward-feed multiple-effect evaporatorsThe fresh feed enters the last & coldest

effect and continues on until theconcentrated product leaves the firsteffect - ”Reverse feed”.

Simplified diagram of backward-feed

triple-effect evaporator


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It is advantageous when the fresh feed iscold. Liquid pumps must be used in each

effect, since the flow is from low tohigh pressure.

It is used when the concentrated productis highly viscous.


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In making preliminary designs or costestimation, it is helpful to have availableoverall heat transfer coefficients usuallyencountered in commercial practice. Refer to Table 8.3-1

However, detailed calculation is neededfor actual evaporator design and/or for

evaluating the effects of changes inoperating conditions on the coefficients.


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5.1 Heat & Material Balances for Evaporators

Basic equation for solving for the capacityof a single effect evaporator:

q could be determined by making a heat &material balance on the evaporator.

Material balance: F = L + VFor a balance on the solute (solids) alone,

T UAq Δ The difference in temperaturebetween the condensing steam andthe boiling liquid in the evaporator.

L F xLxF


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Fig 1: Heat & Mass balance for single effectevaporator

Enthalpy

Enthalpy

Enthalpy

Enthalpy

Enthalpy

Mass fraction


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The condensed steam leaving of S kg/h isassumed usually to be at T S (saturationtemperature) and enthalpy hS.

The steam gives off only its latent heat:

For heat balance,total heat entering = total heat leaving

S S h H λ

Heat in feed +Heat in steam

Heat in concentratedliquid + Heat in vapor +

Heat in condensed steam


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Assumes no heat by radiation orconvection:

Hence,

The q transferred in the evaporator,

S V LS F ShVH LhSH Fh

V L F VH Lh λS Fh

λS h H S q S S Note: The latent heat (λ) of steam at the

saturation temperature TS can be obtained

from steam tables.


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5.1 Effects of processing variables onevaporator operation:

1. Effect of feed temperature:The inlet temperature of the feed (T F)

has a large effect on the operation on the

evaporator.Example 8.4-1:The feed temperature was cold (311.0 K)

as compared to the boiling temperature(373.2K).About ¼ of the steam was used to heat

the cold feed to the boiling point.


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Only about ¾ of the steam was left forvaporization of the feed.

Preheating the feed can reduce the sizeof evaporator heat-transfer area needed.

2. Effect of pressure:

In many cases a large ∆T (>10K) isdesirable, since, as ∆T increases, theheating surface area A and cost of the

evaporator decrease.To reduce the pressure (< 101.32kPa – to

be in vacuum), a condenser and vacuum

pump can be used.


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when P ↓, boiling point ↓, ∆ T ↑

3. Effect of steam pressure:Using higher pressure, saturated steam

increased ∆T , which decreases the size

and cost of the evaporator.Although high-pressure steam is more

costly, more valuable power source.

Overall economic balances are reallyneeded to determine the optimum steampressures.


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5.2 Boiling point rise of solutions: In most cases, the thermal properties of

solution being evaporated may differconsiderably from those of water.The processed solutions are not dilute

solutions.The concentration of the solutions are

high enough that the heat capacity &

boiling point are quite different fromthose for water.The boiling point rise for strong solutions

of dissolved solutes cannot be predicted.


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A useful empirical law (Duhring’s rule)can be used.

Fig 2: Duhringlines foraqueous

solutions of

sodiumhydroxide(at constant

pressure)


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5.3 Enthalpy-concentration Charts ofsolutions:

Heat of solution - It is found aconsiderable temperature rise occur, heatis evolved when pellets of NaOH are

dissolved in a given amount of water. Enthalpy-concentration chart for NaOH:It is not made for solutions having

negligible heats of solution, since theheat capacities can be easily used tocalculate enthalpies.

Such charts are available for only a few

solution.


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Fig 3:Enthalpy-

concentrationchart for the

systemNaOH-water.


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Evaporation of biological materialsfrequently differs from the evaporation ofinorganic materials and organic materials. Inorganic: NaCl, NaOH Organic: Ethanol, acetic acid

Biological: pharmaceuticals, milk, citrus juices, vegetable extracts.

The degradation of biological materials onevaporation is a function of temperature,

and time length.

Heat sensitive, & often contain fine

particles of suspended matter in solution.


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To keep the temperature low, theevaporation must be done under vacuum,

which reduces the boiling point of thesolution.

To keep the time of constant low, the

equipment must provide for a low holduptime (contact time) for the material beingevaporated.

Examples: Long tube vertical evaporator:condensed milk.

Falling film evaporator: fruit juices


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Agitated-film (wiped-film)evaporator: rubber latex,gelatin, antibiotics, fruit juices.Heat-pump cycle

evaporator: Fruit juices,milk, pharmaceuticals.

Fig 4: A rising film cassette evaporator

for orange juice production.


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6.1 Fruit JuicesFruit juices are heat-sensitive and the

viscosity increases greatly asconcentration increases.Solid suspended matter in fruit juices

has a tendency to cling to the heatingsurfaces, thus causing over-heatingwhich leads to burning and spoilage of

the matter.To avoid this tendency of stick and toreduce residence time, high rates ofcirculation over the heat-transfer

surface are necessar .


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A fruit juice concentration plant usuallyemploys a single and not a multiple

evaporation unit.Vacuum is used to reduce the

temperature of evaporation.

A typical fruit juice evaporation systemusing the heat-pump cycle employs low -temperature ammonia as the heating fluid.


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6.2 Sugar solutionsSugar (Sucrose) is obtained by primarily

from sugarcane and sugar beets.Sugar tends to caramelize if kept at high

temperatures for long periods.

The tendency is to use short-tubeevaporators of the natural circulationtype.

The feed is first preheated by exhaustsystem and then typically enters a six-effect forward-feed evaporator system.


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6.3 Paper-pulp waste liquorsIn the manufacture of paper pulp in the

sulfate process, wood chips are digestedor cooked and spent black liquor isobtained after washing the pulp.

This solution contains primarily sodiumcarbonate and organic sulfidecompounds.

It is concentrated by evaporation in sixeffect system.

6. heat transfer equipment evaporator

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