HEAT EXCHANGER NETWORK REPRESENTATION.pdf

8/9/2019 HEAT EXCHANGER NETWORK REPRESENTATION.pdf

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HEAT EXCHANGER NETWORK REPRESENTATHEAT EXCHANGER NETWORK DESIGN FOR MAXIMUM ENERGY RECOVERY



WHERE IS THE PINCH?

Pinch is hard to see on a conventional flow sh



The pinch is much clearly shown on:

THE GRID DIAGRAM

Which only shows heat transfer operation



TYPICAL GRID DIAGRAM

Hot stream 1

Hot stream 2

Cold stream 1

Cold stream 2

C

H

Heat

Exchanger



TYPICAL GRID DIAGRAM (CONT’D)

Utilities can be presented as

hot/cold streams

Can use this diagram to represent

multiple utilities Hot stream 1

Hot stream 2

MP Steam

HP Steam



RULES FOR CONSTRUCTION

Hot streams run left to right

Cold streams run right to left

Hot streams on top; cold stream below

Hot utility =

Cold utility =

Heat exchanger between streams =

C

H



WHERE IS THE PINCH?

Pinch is easily shown: Pinch

Hot stream 1

Hot stream 2

Cold stream 2H

100 oC 100 oC

100 oC 100 oC

90 oC 90 oC

90 oC

90 oC



WHERE IS THE PINCH? (CONT’D)

Can easily assess the implication of the pinch

Allowed match Forbidden match

(heat exchange across the pinch)

Hot 1

Hot 2

Cold

Cold

C

H

Hot 1

Hot 2

Cold 1

Cold 2

C

H



CONVENTIONAL HEN FLOWSHEET

E11

2

3 4

C

H

E2

E3

E4



CONVENTIONAL HEN FLOWSHEET (CONT’D)

Change position of exchangers 2 and 4

Must re-route the process stream

E

11

2

3 4

C

H

E

2

E

3

E

4

E

11

2

3 4

H

E

2

E

3

E

4







RULES SO FAR :

No exchanger has a temperature difference < ΔTmin

No process to process heat transfer across the pinch

No heat transfer across pinch by inappropriate use of utilities



EXAMPLE 2

CP(KW

Hot stream 1

Hot stream 2

Cold stream 1

Cold stream 2

250 oC 50 oC

200 oC 80 oC

180 oC 20 oC

240 oC 140 oC

Qcmin =

1000kWQHmin =

750kW



COMPOSITE CURVES FOR EXAMPLE 2



GRID DIAGRAM

REPRESENTATION

Divide at the pinch

140 oC

150 oC

150 oC

CP(KW

Hot stream 1

Hot stream 2

Cold stream 1

Cold stream 2

250 oC 50 oC

200 oC 80 oC

180 oC 20 oC

240 oC 140 oC

Qcmin = 1000kWQHmin = 750kW

140 oC



Start the network design at the pinch and move away from it.

Go from more constraint to less constraint.

Utilities creates degree of freedom

T

Qcmin

P



CP INEQUALITIES: ABOVE PINCH

if CPH < CPC



CP INEQUALITIES: ABOVE PINCH (CONT’D)

Above pinch:

The CP inequalities (only apply at the pinch when both ends of the m

pinch conditions) – away from the pinch, temperature difference incr

longer essential to obey the CP inequalities

Above pinch: CPH ≤ CPC

If the hot stream is at pinch condition the cold stream it is to be m

must also at pinch conditions

Cold utility must not be used above the pinch



CP INEQUALITIES: BELOW PINCH

if CPH > CPC



CP INEQUALITIES: BELOW PINCH (CONT’D)

Below pinch:

The CP inequalities (only apply at the pinch when both ends of the match are at pinch conditions) awa

temperature difference increase; no longer essential to obey the CP inequalities

Below pinch: CPH ≥ CPC

If the cold stream is at pinch condition, the hot stream it is to be matched with must also at pinch cond

Hot utility must not be used below the pinch



PROCEDURES HEN

1. Develope CP’s TablesCPH CPC C



PROCEDURES HEN (CONT’D)

Now we have identifiedfeasible matches

How big should we make




2. Maximise loads to "tick off" streams

(keeps capital costs down)√

√




3. Then fill in the rest



DESIGN BELOW THE PINCH





DESIGN BELOW THE PINCH

Fill in the rest

√

√

√



But it is away from the pinch and therefore feasible

(Must check temperature though!)

√

√

√



COMPLETED DESIGN.



SUMMARY - PINCH DESIGN METHOD

Divide at the pinch.

Start at the pinch and move away.

Above pinch CPH < CPC

Below pinch CPH > CPC

CP table identifies essential matches.

Use the “tick -off” heuristic to maximize loads.

Then fill in the rest.



NUMBER OF HEAT EXCHANGER UNIT

NUNITS = [SABOVE PINCH – 1] + [SBELOW PINCH – 1]

S = number of streams including the utility stream















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HEAT EXCHANGER NETWORK REPRESENTATION.pdf