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7/27/2019 Heat 2bExchanger 2bNetwork 2bDesign 5b1 5d
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Heat Exchanger Network Designone aspect of process integration
J. M. Shaw
Instructor CHE 465
I would happily credit the authors who providedthe example but am unable to do so.
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Introduction Process integration provides a discipline which
allows designers to establish
The minimum energy to operate a process
A process design with the lowest energy intensity An optimal investment strategy
Design decisions are made consciouslyandconsistently. Global capital and operating cost
implications and trade-offs become visible. We will focus on the first bullet only and will
introduce the terminology of the discipline as wego along!
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Start with a flow sheet without heat
exchangers installed
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Identify streams that require heating called
cold streams and streams that require cooling
hot streams.
Without heat recovery we require 750 units for heating cold streams
(steam?) and 660 units for cooling hot streams (cooling water?).
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Next compute the minimum energy
required to operate the process
Compute summary heating
and cooling needs for the
process. Do this step by
step to avoid errors!
These are calledcomposite curves
for heating and
cooling requirements.
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What is the theoretical minimum energy
required to operate the process as designed?
Think of your process as asingle giant counter current heat
exchanger with an infinite
surface area!
The minimum approach
temperature of the hot and cold
composite streams is 0 C! The
temperature at which this occurs
is called the pinch 70 C in
this case.
Energy that cannot be supplied
by exchange must be supplied
by utilities: ~ 200 units for
heating and ~ 110 units for
cooling.
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How closely can this minimum energy
requirement be approached in practice?
Our analogy with a heat exchanger still holds and we establish a minimum approach
temperature for the composite streams, in this case 20 C, that arises around the pinch.
300 units for
heating and210 units for
cooling!
Pay twice for
inefficiencies!
Heating and
cooling
requirements
rise together!
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How do we translate these concepts into
practical heat exchanger network designs?
Golden Rules
1. Avoid exchanging heat between streams where one isabove and one is below the pinch.
2. Avoid cooling streams above the pinch using utilities.
3. Avoid heating streams below the pinch using utilities.
Violating the golden rules may be convenient and smaller heat exchangers
will certainly result but because of the excessive entropy generated you
will pay twice for this violation for as long as the plant operates!
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Separate the network design task at the pinch
and treat the two designs separately
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Design task above the pinch.
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Stream Data
Stream 1 is heated from 60 C to 120 C
Stream 2 is cooled from 100 C to 80 C
Stream 4 is heated from 60 C to 80 C
Heat Exchanger Network Design Data
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Option #1: one heater for the ingredients (1) + two heat
exchangers [product (2)(1) and containers (4)(2)]
60
300
40
92 C
70 C
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Option #2: one heat exchanger [ingredients (1)products]
+ two heaters [ingredients (1) and containers (4)]
100
77 C
40
260
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Summary
Follow this up with
a reality check!
Iterate until an acceptable
design is obtained!
Install heat exchangers,
coolers, and heaters onthat none or few of the
constraints are violated.
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