Lecture 7
Cooling water system design
L07-1 Energy System Design Update
H2
C1
C2
C
C
H
H1
HENRefrigeration
Distillation
Cooling Water Systems
T2
T3
T1
Effluent TemperatureReduction
L07-2 Energy System Design Update
Research Focused on Cooling Tower Systems
• Cooling tower design
• Makeup minimisation
• Process interactions: refrigeration, air compression
• Environmental protections
• Cooling water treatement: fouling, corrosion, biological fouling
L07-3 Energy System Design Update
Makeup Cooling waterBlowdown
Recirculating water
EvaporationDrift / Windage
CoolingTower HEN
• Open re-circulating cooling water system
?HE 1
HE 4
HE 3
HE 2
ConstraintsCW network design
&CT performance
• Process changes on cooling water system
new HE
Processchanges
L07-4 Energy System Design Update
Design Problem of Cooling Systems
HE 1
HE 3
HE 3
HE 4
Processchange
Network Design ?
Constraints
Constraints
Performance
Target condition ?
Return Hot CW
Fresh CW
L07-5 Energy System Design Update
CW Network
new HE
HE 1
HE 3
HE 2
CT isBottlenecked
HE 1
HE 3
HE 2
HE 4
Is an additional CT necessary ?
Traditional Method for Debottlenecking
Parallel Design
L07-6 Energy System Design Update
Cooling Water System Model
T1 = f (F2,T2,Fair,TWBT)
F1 = f (F2,T2,Fair,TWBT)
QHEN = F2 CP (T2 - T0)
E = f (F2,T2,Fair,TWBT)
B = ECC -1
F0T0 = (F1 - B)T1 + MTM
CC = CB FM
CM FB= M = E CC
CC -1
F0 = F1 - B + M
• Cycles of concentration (CC)
• CT Model
• Makeup / blowdown
• Heat Load of HEN
CWNetwork
QHEN
CT
Makeup
Evaporation
B
F2T2
E
M TM
F1T1
F0T0
ColdBlowdown
L07-7 Energy System Design Update
( ) ( ) ( ){ } ( )( ) ahdzdWGC
dzdWTTCTTCGdzdTGCTTLPL
OOLPLOGPAGSLi −
+−−−+=−
λ
Cooling Tower Model
Water
Ti
L+dLTL+dTL
GTG+dTGH+dHW+dW
G,TGH,W
dZAir
LTL
water
z
dz
G2TG2H2W2
L2TL2
L1TL1
G1TG1H1W1
air
Interface temperature (Ti)
( Details given in Kim and Smith, 2001, Chem. Eng. Sci. v.56(12) pp.3641-3658 )
L07-8 Energy System Design Update
24
26
28
30
32
34
36
3436
3840
4244
4648
5052
0.5
1.0
1.5
2.0
Wat
er O
utle
t Tem
pera
ture
Water Inlet Temperature
Water Inlet Flowrate
Water Outlet Temp. vs Water Inlet Conditions[Merkel's Theory ]
0.6
0.7
0.8
0.9
1.0
3436
3840
4244
4648
50
0.5
1.0
1.5
2.0
2.5
Effe
ctiv
enes
s
Water Inlet Temperature
Water Inlet Flowrate
Effectiveness vs Water Inlet Conditions[ Merkel's Theory ]Water Outlet Temperature Effectiveness
high temperature & low flowrate of inlet conditionshigher heat removal of CT can be obtained
CT Modelling Result
L07-9 Energy System Design Update
HE 1
HE 2
HE 1
HE 2
Parallel Series
New Insights on Cooling Water Systems
IncreaseCW return temperature
DecreaseCW recirculation flowrate
Heat removalof cooling tower
is increased
• Series Arrangements
• Not all of cooling duties require CW at the CW supply temperature
L07-10 Energy System Design Update
FeasibleRegion
HotProcessStream
Q
T
Tcw, inmax
Tcw, outmax
∆∆∆∆Tmin
∆∆∆∆Tmin
Limiting CoolingWater Profile
55
40
20
T( C)
400 1400 3200 3400Q(kW)
75
200 1800 2400 3400Q(kW)
T( C)
Cooling WaterComposite Curve
55
40
20
75
20
Representation of Cooling Water Networks
Targeting forMaximum Re-use
Construction ofComposite Curve
L07-11 Energy System Design Update
Q(kW)
T( C)
55
40
20
75
45.7 kW/ oC75 oC
44.3 kW/ oC 40 oC
90kW/ oC20 oC
45.7 kW/ oC 40 oC
1
90kW/ oC20 oC
45.7 kW/ oC75 oC
45.7 kW/ oC40 oC
Cooling Water Mains
4
3
2
44.3 kW/ oC40 oC
0 kW/ oC75 oC
0 kW/ oC20 oC
• Adaptation of Kuo and Smith’s method for water system design( Trans. IChemE, vol 76, part A, March 1998, p 287 )
L07-12 Energy System Design Update
Constraints on CW Return Temperature
• CW treatment problem limits the CW return temperature.
Q
T CW Composite Curve
No Re-use
Maximum Re-use
Temperature limitation
• Maximum re-use does not guarantee the optimal condition.
L07-13 Energy System Design Update
T
New Pinch
ModifiedCW composite
Q
With Pinch
EnergyPenalty
2.TemperatureShift
T
Q
T
Q
1. Heat LoadShift
CW Composite Curve Modification
L07-14 Energy System Design Update
Q[kW]
T[ C ]
Feasibleregion
for modification
T[ C ]
Q[kW]
T*
Increase CPTemperatureLimitation
ModifiedOriginal
Modified
Original
Q[kW]
T[ C ]
T*
∆∆∆∆Tshift
TemperatureShift
Limiting CW Profile Modification
IncreaseCP ∆∆∆∆Tshift
L07-15 Energy System Design Update
Air 732.24 t/h
HE 1
HE 2
CW NetworkCT
15.1 t/h
Blowdown Makeup7.6 t/h 22.7 t/h
732.24 t/h43.3 oC
28.8 oC
TDBT = 29.4 oCCycles of
Concentration = 3
Evaporation
10 oC
TWBT = 23.9 oC
HE 4
HE 3
• Base Case
Debottlenecking Example
• Limiting Cooling Water Data
2542.1
488.9
635.5
52.7
37
36
33
4*
2
1640.22003728.81
8166.6
Q[kW]
CP[kW/oC]
Tcw, in[oC]
HeatExchanger
4835 3250250
Tcw, out[oC]
3
- ∆Tmin = 10 oC
L07-16 Energy System Design Update
CW from CT
Heat Load of HEN = 15.6 MW
CoolingSystemModel
HE 4
HE 1
HE 3
HE 2
Too Hot !
Tin = 30.4 oC
Tin = 28.8 oC
CP = 1020.9 kW/oCTout = 44.1 oC
Results of Parallel Design Method
CW to CT
CP = 1020.9 kW/oCTout = 44.1 oC
CP = 1020.9 kW/oC
Heat Removal ofCooling System = 14.6 MW
L07-17 Energy System Design Update
Targeting of Cooling Systems for Debottlenecking
Q
CW Supply linefor Parallel Design[No Re-use]
Cooling WaterComposite Curve
T
MaximumRe-use
A
B
FeasibleCW Supply
LineInitial
condition Isothermal line ofcooling systemoutlet temperature
C : Target condition
CT water inlet flowrate
A : Parallel design [no re-use]
B : Maximum re-use
Target (C)Heat removal of
cooling water system
Parallel(A)
15.60 MW
Case
14.61 MW
Max. re-use(B)
15.69 MW
L07-18 Energy System Design Update
T
T*
52.7 C
35 C
36 C
48 C
T
37 C
51.8 C
34.1C
37C
47.1 C
TemperatureShift
PinchMigration
35.1C
Cooling Water Network Design Without a Pinch• Pinch migration • Limiting CW profile modification
Q(MW)15.6
50.3
T( C)
13.3
28.8
T* ∆∆∆∆Tshift
Targetsupply line
∆Tshift = 0.9 oC
L07-19 Energy System Design Update
15.6
Target CW Supply lineCP = 725 kW/oC
28.8
50.3 oC
CW Composite Curve
T( C) Modified CWCompositeCurve
Q(MW)
CW Network Design for Debottlenecking
CP = 725 kW/oCTin = 28.8 oC
Tout = 50.3oC
CW from CT CW to CT
CP =110.7
HE1
HE2
HE4
HE3CP =310
CP =104.3
CP =200
CP =488.9
CP = 68.2
CP = 236.1
L07-20 Energy System Design Update
New CW network design
HE 1
HE 3
HE 2
HE 4
Parallel Design
Debottlenecked Design of Cooling Systems
CT capital cost penalty can be avoided
HE1
HE2
HE4
HE3
L07-21 Energy System Design Update
HE1
HE2
HE4
HE3
New CW Network
new HE
HE 1
HE 3
HE 2
Minimise penalty oncooling water system
Cooling water pinch analysis
Process changesof cooling system
Summary• Retrofit analysis gives design guidelines for debottlenecking
of cooling water systems.
L07-22 Energy System Design Update
Working Session 7Cooling water system design
WS07-1 Energy System Design Update
Evaporation
CW NetworkBlowdown Makeup
623.4 t/h
50.3 oC
28.8 oC
Cycles ofConcentration = 3
HE1
HE2
HE4
HE3
Too High !
CT
Previous problem....
Temperature limitation [47 oC] to CW return temperature
WS07-2 Energy System Design Update
Air 732.24 t/h
TDBT = 29.4 oCHumidity = 0.0165
Q(kW) 15599
CW Supply linefor Parallel Design
28.8
44.1 oC
47 oCCooling WaterComposite Curve
TemperatureConstraint
T( C)
Task 1:
1. Calculate new cooling water supply conditions with temperature constraint to the return temperature.
2. Fill the Answer 1
New conditions of CW supply to CT
Flowrate [t/h]
Temperature [oC]
Flowrate [kW / oC]
47
Answer 1:
WS07-3 Energy System Design Update
* Assume that the heat capacity of cooling water is 4.2 kJ/kg oC.
Task 2 :
1. Open the file WS07.exe
2. Input the CW supply conditions with temperature constraint
input parameters
Air flowrate : 732.24 t/hAir temperature : 29.4 oCAir humidity : 0.0165 kg water / kg airNumber of cycles: 3First assumption of CT water outlet temperature : 32 oC
3. Check cooling water exit conditions with new supply conditions and fill the Answer 2
WS07-4 Energy System Design Update
Target conditions
Heat removal of CT [kW]
47
Answer 2 :
CW exit temp. from CT[oC]
50.3
15599
28.8
CW inlet temp. to CT[oC]
New conditions
WS07-5 Energy System Design Update
2. Air heat exchanger
• Heat load distribution
• Change of CT operating conditions
New Design Option
Increase the air flowrate of cooling tower
decrease the temperature of hot return cooling water
decrease the flowrate of hot return cooling water1. Hot blowdown extraction
WS07-6 Energy System Design Update
CWNetworkCold
Blowdown
Makeup
QAHE
Evaporation T2F2
T1F1AHE
Flowrate beforeand after AHE
doesn’t change.
T1 > T2
∆T = T1-T2
CT
Now, we consider the case for the introduction of AHE
CoolingSystemModel
CPCT,in = 857.1 kW/oCTCT,in = ?
TCW,IN = 28.8 oC
• Find a CT inlet temperature
F2 = F1
WS07-7 Energy System Design Update
Task 3 :
1. Open the file WS07.exe
2. Input different CW supply conditions with reduced temperature less than 47 oC and same CW flowrate
3. Check CW exit conditions with new supply conditions and fill the Answer 3
4. Find the target temperature of CW supply to CT when AHE is introduced. And, calculate the required heat removal of AHE
WS07-8 Energy System Design Update
Answer 3 :
Temp. of CWsupply to CT [oC]
47
::
Heat removal ofAHE [kW]
46
45
::
::
Temp. of CWexit from CT [oC]
0
45
28.8Target ? Target ?
WS07-9 Energy System Design Update
CWNetwork
• Design with Air Heat Exchanger
734.65 t/h
28.8 oC
? oCAHE
47 oC
QAHE = ? MW
QCWN =15.6 MW
CT
WS07-10 Energy System Design Update
Working Session 7Solution
SOL07-1 Energy System Design Update
Q(kW) 15599
CW Supply linefor Parallel Design
28.8
44.1 oC
47 oCCooling WaterComposite Curve
TemperatureConstraint
T( C)
New conditions of CW supply to CT
Flowrate [t/h]
Temperature [oC]
Flowrate [kW / oC]
47
Answer 1:
SOL07-2 Energy System Design Update
857.09
734.65
Target conditions
Heat removal of CT [kW]
29.256
With 47 oC return temperature conditions,the cooling systems cannot satisfy the desired heat removal andCW exit temperature.
CW exit temp. from CT[oC]
50.3
15208 kW = (47 oC - 29.256 oC) X (857.09 kW/oC)
28.8
CW inlet temp. to CT[oC]
New conditions
SOL07-3 Energy System Design Update
47
15599 15208
Answer 2 :
Answer 3 :
Temp. of CWsupply to CT [oC]
47
::
Heat removal ofAHE [kW]
46
45
::
::
Temp. of CWexit from CT [oC]
0
43.2 28.8
Target conditions
SOL07-4 Energy System Design Update
29.256
29.033
3256.94
45
29.145 857.09
1714.18
3256.94 kW = (47 oC - 43.2 oC) X (857.09 kW/oC)
CWNetwork
• Design with Air Heat Exchanger
734.65 t/h
28.8 oC
43.2 oCAHE
47 oC
QAHE = 3.26 MW
QCWN =15.6 MW
CT
SOL07-5 Energy System Design Update
Air heat exchanger with the capacity of 3.26 MWis required for heat load distribution.