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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