CH-3. Single Effect Evaporation

7/27/2019 CH-3. Single Effect Evaporation

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Chapter 3Single Effect

Evaporation (SEE)Processes


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Objectives

To be familiar with main component of SingeEffect Evaporation (SEE) system.

To understand the basic mass and energybalances. To understand and evaluate the main design

parameters: PR; sA; and sMcw


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Expected Outcomes

By the end of this class, you should be able to:

Carry out mass and energy balances on the main

components of SEE system. Calculate the main design parameters.


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Has very limited industrial applications

The is because of the thermalperformance ratio < 1.0

Used mainly in marine vessels

HOWEVER, Understanding this process isessential for analyzing other processessuch as multiple effect evaporation andvapor compression SEE

SEE: Introduction


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Single effect evaporation process

Distillate, Md , Td

Feed & Cooling

Water.

(Mf+Mcw), Tcw

Heating steam

Ms, Ts

Reject Brine

Mb, TbCondensed

Steam

Ms, Ts

Cooling Seawater

Mcw, Tf

Feed Seawater

Mf, Tf

Product vapor

Md, Tv

Evaporator

Condenser

Demister


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Process component

The main components are The evaporator

The feed pre-heater or the condenser

The evaporator consists of Evaporator/condenser heat exchangers tube

Vapor space

Un-evaporated water pool

A line for removing the un-condensed gases

A water distribution system

Mist eliminator


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Process components

The feed pre-heater has a shell and tubeconfiguration and operates in a counter

current mode where the latent heat ofcondensed vapor is transferred to theintake seawater.


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Process description

The intake seawater (Mcw+Mf) at a temperature(Tcw) and salt conc. (Xf) is introduced into the

tube side of the preheater where its temp.increases to (Tf).

The cooling water (Mcw) is dumped back to thesea.

The function of the cooling water is to to removethe excess heat added to the water in theevaporator by the heating steam.


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Process description

The heat needed to warm the sea water insidethe condenser tubes is supplied by condensing

the vapor formed by boiling in the evaporator Within the evaporator, the feed water is sprayedat the top where it falls in the form of a thinlayer down the succeeded rows of tubes

arranged horizontally.


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Process description

Condensation of the saturated heating steamand the release of its latent heat provides the

required sensible and latent heat for waterevaporation from the feed sea water.

As a result, the feed sea water temp. increasesfrom (Tf) to (Tb)

Vapor is generated at a rate of (Md) at temp.(Tv)


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Process Modeling

The model is is divided into five parts Material balance

Evaporator & condenser energy balance Boiling point elevation

Evaporator & condenser heat transfer area

Performance parameters


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Material Balance

Overall Material and salt balanceMf = Md + Mb (1)

Salt balanceMfXf= Mb Xb (2)

From 1 & 2Mb = Md . xf/ (xb xf) (3)

Mf= Md . xb / (xb xf) (4)


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Evaporator Load, Qe

In the evaporator, saturated steam flowingfrom a steam boiler at a rate of Ms is used to

raise the temperature of feed water (Mf) fromTfto Tb.

In addition, it supplies the latent heat required

to evaporate a specified mass of vapor, Md

Qe = MfCp (Tb Tf) + Mdv = Mss (5)


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Condenser Load, Qc

The condenser works on condensing the vaporformed, Md by transferring its latent heat ofcondensation to heat the feed seawater (M

f+ M

cw).

Qc = (Mf+ Mcw) Cp (Tf Tcw) = Mdv (6)


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Overall energy balance

Assume Tcw is the reference temperatureHeat in = Heat out (7)

Heat in = MssHeat out = Mb Cp (Tb Tcw) + Md Cp (Tv Tcw)

+ Mcw Cp (Tf Tcw)

Here, Tref is Tcw and Cp is constant ataverage T


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Performance ratio, PR

It is the kgs of distillate water producedper kg of steam used.

PR = Md / Ms (8)


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Evaporator heat transferarea, Ae

It is calculated from theAmount of heat to be transferred, Qe

The overall heat transfer coefficient, Ue The difference between the condensing

temperature of the steam Ts, and boilingtemperature of the boiling water, T

b

Qe = Ue Ae (Ts Tb) (9)


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It is calculated from theAmount of heat to be transferred, Qc

The overall heat transfer coefficient, Uc The logarithmic mean temperature difference,(LMTD)c

Qc = Uc Ac (LMTD)c (10)

LMTD = (TfTcw)/ln ((Tv - Tcw)/(Tv - Tf)) (11)

Condenser heat transferarea, Ae


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Boiling Point Elevation, BPE

The rise in sea water boiling temperaturedue to salinity.

Tb = Tv + BPE (11) Tv = Vapor phase Temp.


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Cp &

Cp is calculated at the average temperature ofthe stream

is either calculated or from steam tables at thesaturation temperature.


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Overall Energy Balance

Use Eqn(6) to eliminate [Mcw Cp (Tf Tcw)] in Eqn(7) Ms ls = Mb Cp (Tb Tf) + Md Cp (Tv Tf) + Md lv (12)

Subs. Eqn(11) into (12):

Ms ls = Mb Cp (Tv + BPE

Tf) + Md Cp (Tv

Tf) + Md lv (13)Or:


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Cont.

Ms ls = Mb Cp (Tv Tf) + Mb Cp BPE + Md Cp (Tv Tf) + Md lv (14)

Use Eqn(3) to eliminate Mb, simplify:

(15)

Write Eqn(15) in terms of ratio of flow rates:

v

Fb

FFv

Fb

Fdss

XX

XBPECpTTCp

XX

XMM .1

v

Fb

FFv

Fb

Fs

s

d

XX

XBPECpTTCp

XX

X

M

M .1

Obt i i P f


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Obtaining PerformanceRatio (PR)

(16)

________________________________

v

Fb

FFv

Fb

bs

s

d

XX

XBPECpTTCp

XX

X

M

MPR .

ifi l f li


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Specific Flow Rate of CoolingWater (sMcw):

Rearrange Eqn(6):

Mcw

Cp (TFT

cw) = M

d

d M

FCp (T

FT

cw) (17)

Use Eqn(4) to eliminate MF, rearrange:

cwFFb

bcwFd

d

cwcw

TTCp

XX

XTTCp

M

MsM

ifi f f


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Specific Heat Transfer Area ofthe Evaporator (sAe):

EvaporatorAe = Qe/[Ue (Ts Tb)] (9)

Subst. Qe from Eqn(5) into Eqn(9):

Ae = [MF Cp (Tb TF) + Mdv]/[Ue (Ts Tb)] (19)

Eliminate MF using Eqn(4), Rearrange:

(20) bse

FbbFbv

d

ee

TTU

XXXTTCp

M

AsA

ifi f f


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Specific Heat Transfer Area ofthe Condenser (sAc):

Ac = Qc /[Uc LMTDc] (22)

Qc can be obtained from Eqn(6)

sAc = Ac/Md = ld /[Uc LMTDc] (23)

Td

Tcw

TF

Fd

cwd

cwF

c

TT

TT

TT

LMTDln

S f SEE P f


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Summary of SEE PerformanceParameters:

Thermal Performance Ratio (PR): Equation (16)

Specific Heat Transfer Area (sA):sA = sAe + sAc (25)

Specific Flow Rate of Cooling Water(sMcw): Equation (23)


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SEE Example

Determine theperformance parametersfor the following SEE

Desalination plant. Carry out a What if?

analysis!

Which of the following

parameters has the leasteffect on PRand Why??

Ts, Tf and Tcw

Excel Solution

Ts 100 C

Tcw 25 C

Xf 42000 ppm

Xb 70000 ppm

Mf

2.49 kg/s

CP 4.2 kJ/kgC

Tf 80 C

Md 86 m3/hr

Ue 2.70 kW/m2.C

Tv 86.00 C

BPE 0.80 C
http://localhost/var/www/apps/conversion/tmp/scratch_2/SEE-Example.xlshttp://localhost/var/www/apps/conversion/tmp/scratch_2/SEE-Example.xls

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CH-3. Single Effect Evaporation

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