heat exchanger pinch

8/9/2019 Heat Exchanger Pinch

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Heat Exchanger Network Design:

The Pinch Design Method

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Design of Individual Processesfor Maximum Energy Recovery

Divide the process at the pinch



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The feasibility of heat transfer

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Cross-pinch heat transfer: Actual = Target + XP



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Design RuleDo Not Transfer Heat Across the Pinch

➢ Do not use steam below

➢ Do not use cooling water above

➢ Do not recover process heat across

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Typical Grid Diagram



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Typical Grid DiagramRules for Construction

➢ Hot streams run left to right

➢ Cold streams run right to left

➢ Hot streams on top; Cold streams on bottom

➢ Hot utility =

H

➢

Cold utility =

C

➢ Heat exchanger between streams =

—

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Where Is The Pinch ?



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Pinch Is Easily Shown

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Separate Above/Below-Pinch Regions



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Number of Heat Exchanger Units

➢ Graph any collection of points in which some pairs of points are

connected by lines

➢ Path a sequence of distinct lines which are connected to eachother

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Number of Heat Exchanger Units

➢ A graph forms a single component if any two points are joined by

a path

➢ Loop a path which begins and ends at the same point (CGDHC)

➢ If two loops have a line in common, they can be linked to form

a third loop by deleting the common line (BGCEB + CGDHC →BGDHCEB)

➢ The number of independent loops for a graph:

N UNITS = S + L− C

N UNITS = # of matches or units (lines in graph theory)

S = # of streams including utilities (points in a graph)

L = # of independent loops

C = # of components

➢ A single component and loop-free: N UNITS = S − 1



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The Pinch Design MethodKnown

➢ No exchanger should have a temp diff. smaller than ∆T min

➢ No heat transfer across the pinch by

☞ process-to-process heat transfer☞ inappropriate use of utilities

➢ Compositecurves:

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The Pinch Design MethodStart at the Pinch

(∆T min exists between all hot/cold streams, the most constrained region)



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The Pinch Design MethodDivide at the pinch

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The Pinch Design MethodCP Inequality for Individual Matches

Above Pinch: if CPH >CPC ⇒ infeasible!

T h = 162o (suppose)

∆H h = 0.25(162−150)

= 3 MW

T c = 140 + 3 MW0.2 MW/oC

= 155oC

∆T min > T h − T c= 162− 155

= 7oC



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Above Pinch: if CPH ≤CPC ⇒ feasible

T h = 162o (suppose)

∆H h = 0.25(162−150)

= 3 MW

T c = 140 + 3 MW0.3 MW/oC

= 150oC∆T min < T h − T c

= 162− 150

= 12oC

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Below Pinch: if CPH <CPC ⇒ infeasible!

T c = 125o (suppose)

∆H c = 0.2(140−125)

= 3 MW

T h = 150− 3 MW.15 MW/oC

= 130oC

∆T min > T h − T c= 130− 125

= 5oC



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Below Pinch: if CPH ≥CPC ⇒ feasible

T c = 125o (suppose)

∆H c = 0.2(140−125)

= 3 MW

T h = 150− 3 MW.25 MW/oC

= 138oC∆T min < T h − T c

= 138− 125

= 13oC

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The Pinch Design MethodCP Inequalities: Summary

for temperature differences to increase moving away from the pinch

Above Pinch: CPH ≤ CPC Below Pinch: CPH ≥ CPC



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The Pinch Design MethodThe CP Table

Cold utility must not be used above the pinch

⇒ hot streams must be cooled to pinch temp. by recovery

hot utility can be used on cold streams above the pinch

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The Pinch Design MethodThe ”Tick-Off” Heuristic (above pinch)

Now we have identified feasible matches

⇒ How big should we make them ?



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Maximize loads to ”tick off” streams

⇒ to keep capital costs down

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Then fill in the rest



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The Pinch Design MethodThe ”Tick-Off” Heuristic (below pinch)





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Note: one match violates CP rules

But, it is away from the pinch and therefore feasible



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The Pinch Design MethodThe ”Tick-Off” Heuristic: Summary

To tick off a stream, individual units are made as large as possible⇒ the smaller of the two heat duties on the streams being matched

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The Pinch Design MethodThe Completed Design



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The Pinch Design MethodNetwork Design Using Two Steam Levels

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The Pinch Design Method: Summary

➢ Divide the problem at the pinch into separate problems

➢ Design separate problems, started at the pinch, moving away

➢ Temperature feasibility requires constraints on CP values to besatisfied for matches between streams at the pinch

➢ Loads on individual units are determined using the kick-off heuristic

to minimize # of units

➢ Away from the pinch: more freedom, use judgment and process

knowledge



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Stream Splitting: # of Streams

Cold utility must not be used

above the pinch

⇒ All hot streams must be cooled topinch temperature by heat recovery

⇒ Splitting cold streams

Above Pinch: S H ≤ S C

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Stream Splitting: # of Streams

Hot utility must not be used

below the pinch

⇒ All cold streams must be heated to

pinch temperature by heat recovery⇒ Splitting hot streams

Below Pinch: S H ≥ S C



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Stream Splitting: CP Inequality

Above Pinch: CPH ≤ CPC

Hot stream with larger CP values⇒ Split into smaller parallel

hot streams

(opt flow rates ?)

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Stream Splitting: CP Inequality

Below Pinch: CPH ≥ CPC

Cold stream with larger CP values

⇒ Split into smaller parallelcold streams

(opt flow rates ?)



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Stream-Splitting AlgorithmsAbove the Pinch

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Stream-Splitting AlgorithmsBelow the Pinch



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Pinch Design RuleDo Not Transfer Heat Across the Pinch

➢ Divide at the PINCH

➢ Start at the PINCH and move away

➢ Observe the PINCH rules:

☞ Do not use steam below☞ Do not use cooling water above

☞ Do not recover process heat across

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Pinch Analysis and Process Integration

Case Study

Crude Preheat Train



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PI Case Study: Crude Preheat TrainOriginal Process

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PI Case Study: Crude Preheat TrainContractor’s Design

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PI Case Study: Crude Preheat TrainHot/Cold Streams



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PI Case Study: Crude Preheat TrainNetwork Grid Diagram: Contractor’s Design

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PI Case Study: Crude Preheat TrainNetwork Grid Diagram with Increased HEs



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PI Case Study: Crude Preheat TrainComparison of UA Values for Diff. Network Designs

Contractor’s Inc. area MER Evolved Evolveddesign design design design 1 design 2

Energy 81.9 68.0 61.1 68.0 68.0Max split 2 2 4 4 3

UA for N1 - 0.380(new) 0.393(new) 0.332(new) 0.380(mod 5)N2 - 0.230(new) - - 0.210(new)3 0.288 0.393(mod) 0.714(new) 0.337 0.3374 0.159 0.512(mod) 0.549(mod) 0.412(mod) 0.476(mod)5 0.152 0.150 0.286(mod) 0.147 -6 0.462 0.462 0.462 0.462 0.5067 0.196 0.195 0.293(mod) 0.198 0.1938 0.132 0.115 0.454(mod) 0.241(mod) 0.234(mod)9 0.022 0.022 0.022 0.022 0.022

10 0.111 0.165(mod) 0.180(mod) 0.111 0.111UAAll HEs 1.522 2.624 3.353 2.262 2.421UAAir cooler 0.550(mod) 0.550(mod) 0.550(mod) 0.550(mod) 0.392UATotal 2.072 3.174 3.903 2.812 2.813

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PI Case Study: Crude Preheat TrainComposite Curves and Grand composite Curve



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PI Case Study: Crude Preheat TrainNetwork Grid Diagram: Contractor’s Design

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PI Case Study: Crude Preheat TrainContractor’s Network Showing Pinch Temperature



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PI Case Study: Crude Preheat TrainStream Grid Above The Pinch

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PI Case Study: Crude Preheat TrainComplete MER Network Above The Pinch



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PI Case Study: Crude Preheat TrainStream Grid Below The Pinch

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PI Case Study: Crude Preheat TrainComplete MER Network Below The Pinch



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PI Case Study: Crude Preheat TrainElimination of Four-way Stream Split

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PI Case Study: Crude Preheat TrainSecond Evolution of Network Design



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PI Case Study: Crude Preheat TrainComparison of UA Values for Diff. Network Designs

Contractor’s Inc. area MER Evolved Evolveddesign design design design 1 design 2

Energy 81.9 68.0 61.1 68.0 68.0Max split 2 2 4 4 3

UA for N1 - 0.380(new) 0.393(new) 0.332(new) 0.380(mod 5)N2 - 0.230(new) - - 0.210(new)3 0.288 0.393(mod) 0.714(new) 0.337 0.3374 0.159 0.512(mod) 0.549(mod) 0.412(mod) 0.476(mod)5 0.152 0.150 0.286(mod) 0.147 -6 0.462 0.462 0.462 0.462 0.5067 0.196 0.195 0.293(mod) 0.198 0.1938 0.132 0.115 0.454(mod) 0.241(mod) 0.234(mod)9 0.022 0.022 0.022 0.022 0.022

10 0.111 0.165(mod) 0.180(mod) 0.111 0.111UAAll HEs 1.522 2.624 3.353 2.262 2.421UAAir cooler 0.550(mod) 0.550(mod) 0.550(mod) 0.550(mod) 0.392UATotal 2.072 3.174 3.903 2.812 2.813

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PI Case Study: Crude Preheat TrainContractor’s Design

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PI Case Study: Crude Preheat TrainFinal Selected Design



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PI Case Study: Crude Preheat TrainConclusions

➢ The Pinch Design Method generated a network which was

substantially better than that obtained by any previous methods of

heat exchanger network design.

➢ The targeting stage gives a rapid initial assessment of the scope

for change and the likely difficulties which will be encountered in

obtaining a solution.

➢ The network design method can be used systematically to produce

good “revamp” designs, even where the existing heat exchanger

network is complex.

It allows a productive interaction with the engineer’s experience

(a good example is the use of the pump-around in the preferred

solution).

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PI Case Study: Crude Preheat TrainConclusions

➢ Designs produced by proper use of the method are elegant,

sometimes yielding both energy and capital savings.

➢ A higher degree of process integration does not necessarily

cause control problems. If the integration is well balanced thecontrollability can be enhanced.

➢ Parallel stream splitting is a practical tool for improving energy

recovery and operability.

➢ The Pinch Design Method can be employed to give good designs

in rapid time and with minimum data.

heat exchanger pinch

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