Chapter 15 heat exchanger networks - ii - min dt

Chapter 15 – Pinch Technology Design of Heat Exchanger

Networks – Optimum DTmin

Department of Chemical Engineering

West Virginia University

Copyright - R. Turton and J. Shaeiwitz, 2012 1

Optimal DTmin

Algorithm for Finding DTmin

• Estimate Qhot and Qcold and cost of utilities

• Estimate the installed cost of all heat exchangers – “design the network”

• Take into account the materials of construction (MOC)

• Estimate EAOC or NPV of network

• Repeat for different DTmin

• Plot EAOC vs DTmin


Optimal DTmin


EAOC, $/yr

DTmin

Exchanger cost

Energy costs (utilities)

Total cost

DTmin, opt

Optimal DTmin

Approach

• Estimate areas without designing network

– Cover details of hand calculations

– Show how HENSAD can be used to solve this problem


Optimal DTmin

Exchanger Cost


Purchased Exchanger Cost, npC KA

Installed Exchanger Cost, ( , , )TM pC f C MOC P

Find A

Find MOC, and P

Optimal DTmin

Consider a Single Heat Exchanger


DT1

DT2

Q

T 1 2

1 2ln( / )lm

T TT

T T

D DD

D D

1

1 1

i o

U

h h

-1

lm

QA

U T F

D

Include fouling

1

2

3

Number of shells = 3

n

shellsshells

APurchased Costof Exchanger n K

n

Optimal DTmin

Now Consider Cumulative T-Q Diagram


Q

T

Q

T hot Streams 1 and 3 (say)

cold Streams 4 and 5 (say)

We can match Streams 1 with 4 and 3 with 5 or

Streams 1 with 5 and 3 with 4 or

Stream 1 with some other stream ……..

Optimal DTmin

Stream Matching

• Not necessary to match hot and cold streams within a given interval

• Results by matching streams within intervals are not too much different than by matching streams outside interval (~20%)

• We match streams within intervals since it is easy and results are reasonable


Optimal DTmin

Area required per interval, Ai

Total Area required in Network, Anetwork


hot coldstreams streams

, ,

,

i i

hot i cold ii ii

lm i

Q Q

h hA

F T

D

Assume F =0.80

all intervals

1

network i

i

A A

Optimal DTmin


sum up all contributions from each interval to get an estimate of total area

Use minimum number of exchangers to determine the average area per exchanger

Ntotal = (Nmin, above pinch + Nmin, below pinch)

, ( )

n

networkp network total

total

AC N K

N

Optimal DTmin

1

1 2

1 2

( )(13.14)

( )

nM P

bc

B B F Fh

B Bh


What about MOC and P ?

1 21.18( ) (13.12)nTM M PC B B F F KA

1 2 1 21.18( ) 1.18( )n nM P bcB B F F KA B B KA

1

1 2

1 2

( )(13.13)

( )

nbc M PA B B F F

B BA

Base case conditions

P = 1 atm and MOC = CS

For non-base case conditions adjust h downwards

Optimal DTmin


What about MOC and P ?

Optimal DTmin


HENSAD – Heat Exchanger Network Synthesis, Analysis, and Design

HENSAD performs the following calculations:

• Temperature Interval Diagram

• Cascade Diagram

• Composite Enthalpy Diagram

• Minimum Number of Exchangers

• Optimum DTmin

• Design of Network

• Above pinch

• Below pinch