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 FMCM FB

= M = E CCCC -1

F0 = F1 - B + M

Cycles of concentration (CC)

CT Model

Makeup / blowdown

Heat Load of HEN

CWNetwork

QHEN

CT

Makeup

Evaporation

B

F2T2E

M TM

F1T1

F0T0

ColdBlowdown

L07-7 Energy System Design Update

( ) ( ) ( ){ }( )( ) ahdzdWGC

dzdWTTCTTCGdzdTGCTTLPL

OOLPLOGPAGSLi

++=

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 Te

mpera

tureWater 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 F

lowrate

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 Smiths 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

CWNetworkColdBlowdown

Makeup

QAHEEvaporation T2

F2

T1F1AHE

Flowrate beforeand after AHE

doesnt 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.