CHEMICAL ENGINEERING TRANSACTIONS 

VOL. 63, 2018 

A publication of 

The Italian Association 
of Chemical Engineering 

Online at www.aidic.it/cet 

Guest Editors: Jeng Shiun Lim, Wai Shin Ho, Jiří J. Klemeš 
Copyright © 2018, AIDIC Servizi S.r.l. 
ISBN 978-88-95608-61-7; ISSN 2283-9216 

An Enhanced Tool for Heat Exchanger Network Retrofit 

Towards Cleaner Processes 

Yee Qing Laia,b, Zainuddin A. Manana,b,*, Sharifah R. Wan Alwia,b 

aProcess Systems Engineering Centre (PROSPECT), Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia 
bFaculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia 

 dr.zain@utm.my 

The individual stream temperature versus enthalpy plot (STEP) is a graphical tool that can be used to 

simultaneously diagnose and retrofit existing heat exchanger networks (HEN).  The Modified Energy Transfer 

Diagram (ETD) utilises the Grand Composite Curve (GCC) to provide insights on the scope for HEN retrofit and 

to pinpoint retrofit alternatives based on the Bridge Analysis method.  Both the STEP and the Modified ETD 

methods are based on graphical representation of individual process streams that enable users to visually 

identify the potential scope for improvement in an existing HEN. It is found that, using the distance of the 

horizontal gap between the stream pairs of the STEP diagram can simplify the construction of the Modified ETD. 

The Modified ETD can help monitor the progress of HEN retrofit. This paper shows through a case study that, 

combining the use of STEP-ETD methods can simplify, facilitate and enhance the simultaneous targeting, 

diagnosis and retrofit of a HEN toward achieving cleaner processes, while yielding the same final retrofitted 

structure like those produced by using only the Modified ETD. 

1. Introduction

Energy in the form of heat is one of the most extensively-used resources in the process industry.  Heat wasted 

from manufacturing processes into the environment can increase the requirement of utilities, operating 

expenses, and also emission of carbon dioxide into the environment.  Heat exchanger network (HEN) retrofit 

can play a part in maintaining a plant’s resource and contribute toward cleaner process operations by improving 

its energy efficiency.  Efficient heat recovery system is a clean technology option for reducing fuel requirement, 

saving utilities and reducing emissions. HEN retrofit methods based on Pinch Analysis have been used over the 

years to guide users to enhance heat recovery from processes with the aid of graphical tools. 

One of the earliest Pinch Analysis-based was introduced by Tjoe and Linnhoff (1986) who proposed a HEN 

retrofit method that involves the setting of a conservative retrofit investment targets.  Over the years, researchers 

have developed numerous alternative graphical visualisation tools that are aimed at making HEN retrofit easier 

and more convenient.  Lakshmanan and Bañares-Alcántara (1996) represented the heat loads and driving 

forces using the Retrofit Thermodynamic Diagram (RTD).  Nordman and Berntsson (2001) introduced analysis 

methodologies which involves eight different curves to screen retrofit options.  Nordman and Berntsson (2009) 

also introduced the Advanced Composite Curves which can be used for HEN retrofit by solving the problem of 

Pinch rule violations and by identifying retrofit alternatives. Lai et al. (2017) implemented Stream Temperature 

vs Enthalpy Plot (STEP) diagram for HEN retrofit.  The method allows users to perform HEN retrofit directly from 

the temperature-enthalpy graph geometry by observing the Pinch rules and a set of retrofit heuristics.  The 

STEP retrofit method also eliminates the need for repetitive temperature and enthalpy calculations during the 

course of retrofit. STEP was first introduced by Wan Alwi and Manan (2010) for designing grassroots HEN.  

STEP shows the mapping of segments of continuous individual hot and cold process streams on a temperature 

versus enthalpy plot, the maximum heat allocation, the Pinch points and energy targets. 

The Energy Transfer Diagram (ETD) was proposed by Bonhivers et al. (2014) who proposed the Bridge Analysis 

concept.  ETD is a graphical tool used to identify heat transfer bridges in a HEN. The concept of Bridge Analysis 

                               
 
 

 

 
   

                                                  
DOI: 10.3303/CET1863082

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Yee Qing Lai, Zainuddin Abdul Manan, Sharifah Rafidah Wan Alwi, 2018, An enhanced tool for heat exchanger 
network retrofit towards cleaner processes, Chemical Engineering Transactions, 63, 487-492  DOI:10.3303/CET1863082   

487



was used to produce several graphical tools for HEN design and retrofit.  A heat exchanger load diagram was 

introduced to simplify the effort of searching for retrofit solutions (Bonhivers et al., 2015).  Bonhivers et al. (2016) 

combined the Pinch Analysis and Bridge Analysis to solve HEN retrofit problems by correlating the Composite 

Curves (CC), the ETD, and the heat exchanger load diagram (HELD).  Walmsley et al. (2017) introduced a 

Modified ETD which can be used to determine retrofit alternatives and the amount of heat transferred based on 

Bridge Analysis.  The Modified ETD shows the location of the heat surpluses and deficits that are present in the 

HEN.   

Recent developments on HEN retrofit have seen the emergence of the graphical tools representing individual 

streams being increasingly utilised as compared to the conventional CC and Grand Composite Curve (GCC).  

Use of individual curves allows designers to pinpoint the specific network location and the individual heat 

exchanger and streams that need to be retrofitted in a HEN.  Both the STEP diagram and the Modified ETD 

retrofit methods utilise the individual stream concept, each with unique advantages of their own.  This paper 

shows that combining the STEP and Modified ETD graphical tools can enhance the simultaneous targeting, 

diagnosis and retrofit of a HEN.  Application of this new method on a HEN retrofit case study shows that the 

combined graphical tool can help simplify and facilitate the retrofit process while yielding the same final retrofitted 

structure like those produced by using any retrofit method, including the Modified ETD method. 

2. STEP Diagram and the Modified Energy Transfer Diagram 

STEP diagram can be used to represent, diagnose and retrofit an existing HEN based on the individual stream 

approach.  Figure 1a from Lai et al. (2017) is a conceptual example of how this is done. Individual streams are 

plotted in pairs using shifted temperature to indicate the transfer of heat from hot streams to cold streams across 

a heat exchanger. The inlet and outlet temperatures of the streams can be read from the y-axis.  The horizontal 

gap on the x-axis shows the heat load of the exchanger.  The slope of the curve can be obtained from the 

reciprocal of the heat capacity flowrate (FCp) of the stream.  STEP diagram can be used to simultaneously 

diagnose and retrofit existing HEN.  Users can perform stream splitting and heat load division by utilising the 

simple graph geometry. 

The Modified ETD is a plot of GCC for HEN retrofit, as shown in Figure 1b (Walmsley et al., 2017).  Individual 

stream pairs are first translated into individual exchanger GCCs using the temperature intervals of the individual 

stream pairs.  The Modified ETD is then constructed by stacking the GCC for each exchanger.  Based on Figure 

1b, GCC of the individual stream pairs at heat exchanger E1 (hot STEP 1 and cold STEP 2 in Figure 1a) is 

plotted first, follow by stacking the GCC of heat exchanger E2 to the right of the GCC of heat exchanger E1.  

The diagram contains useful information such as the Pinch point, the maximum heat recovery potential, and 

extent of heat recovery for the existing HEN relative to the targeted value. The Pinch temperature for the 

example in Figure 1 is at 160 °C and 145 °C.  In this example the heat is fully recovered.  Identification of 

maximum heat recovery from the Modified ETD is illustrated in the next section using another case study.  The 

extreme right end of the curve in the Modified ETD shows the grassroots GCC for the existing HEN. 

 

  

 (a) STEP diagram     (b) Modified ETD 

Figure 1: STEP diagram for retrofit and the Modified ETD 

0

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Tpinch = 145 °C Tpinch = 145 °C 

Tpinch = 160 °C 

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Some key similarities can be observed between the two graphical tools.  Both tools can be used to determine 

cross-Pinch heat exchangers. This example shows that the cross-Pinch heat exchanger identified is heat 

exchanger E2 which transfers heat from hot STEP 2 to cold STEP 2. In STEP diagram, cross-Pinch heat 

exchangers happen when a hot STEP appears partially or fully above the Pinch and a cold STEP appears 

partially or fully below the Pinch point (see for example Hot STEP 2 and Cold STEP 2 in Figure 1a). In the 

Modified ETD, cross-Pinch heat exchanger can be identified when a heat exchanger area spreads across the 

Pinch (see for example heat exchanger E2 in Figure 1b). Both tools can identify exchangers with problems 

through direct observation. Both diagrams can determine potential matches at the same temperature range.  

For example, hot STEP 1 has the potential to be matched with cold STEP 1 or cold STEP 2 which are both 

below the Pinch point (as shown in Figure 1a). The hot stream of E1 can be matched with the cold stream at E2 

(shown by the horizontal arrow in Figure 1b). 

In this study, it is discovered that the Modified ETD can be constructed by direct association between the STEP 

diagram and the GCC of the Modified ETD.  The horizontal gap between the individual stream pairs in STEP 

diagram is equivalent to the horizontal distance of the GCC curve (relative to the y-axis) for the respective heat 

exchanger in the Modified ETD.  For example, the distance of XE1 in Figure 1a is the same as the distance of 

XE1 in Figure 1b. By taking the horizontal gap of the individual stream pairs at each temperature intervals in 

STEP diagram, the Modified ETD can be constructed easily. A more detailed example for the translation of 

STEP diagram into the Modified ETD is explained in the next section. 

3. Translation of STEP diagram into Modified Energy Transfer Diagram 

This section demonstrates how the STEP diagram can be translated into the Modified ETD using an illustrative 

case study.  A retrofit problem that consists of four streams from Klemeš et al. (2014) is used as an illustrative 

example.  The minimum temperature approach (∆Tmin) for this problem is 10 °C. The Pinch temperature for this 

case is at shifted temperature of 145 °C.  The minimum heating requirement (Qh,min) is 750 kW while the 

minimum cooling requirement (Qc,min) is 1,000 kW.  Figure 2 shows the grid diagram of the existing HEN. 

 

Figure 2: Grid diagram of the existing HEN 

The steps involved for the translation of STEP diagram into the Modified ETD are as follow: 

1. Represent the HEN in the STEP diagram according to the steps described in the work by Lai et al. 

(2017). 

2. Plot the Modified ETD starting from heaters, coolers, and finally the process-to-process heat 

exchangers. The sequence of the curves can be changed according to user’s preference.  Note that 

the curves for heaters have positive slopes while for coolers, negative slopes. 

Begin by drawing the heater HU1. The STEP diagram in Figure 3a shows that the heat load of heater HU1 is 

XHU1. The distance of XHU1 can be directly translated into positive slope in the Modified ETD in Figure 3b using 

the same temperature range. Similarly, plot coolers CU2 and CU1 with the distance of XCU2 and XCU1 (read from 

STEP diagram in Figure 3a) but this time with negative slopes. As the curve for cooler CU1 is stacked on the 

right side of cooler CU2, the heat load need to be accumulated. XCU1 is added on XCU2 instead of starting from 

zero at the y-axis. Note that the heat load accumulation is applied to every temperature interval present in the 

existing HEN. Plot heat exchanger E1 using XE1 and YE1, and for heat exchanger E2 using XE2 and YE2.  The 

right end of the Modified ETD in Figure 3b shows the grassroots GCC of the existing network. X is the maximum 

amount of recoverable heat. 

2,400 kW 

2,350 kW 

15 H1 

30 

40 °C 

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E1 

FCp (kW/°C) 

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C1 

C2 

E2 CU1 

80 °C 

20 °C 

140 °C 

2,700 kW 

250 °C 

200 °C 

180 °C 

230 °C 

196.7 °C 

104 °C 

140 °C 

800 kW 

600 kW 

CU2 

HU1 

E2 CU1 

E1 

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The HEN retrofit problem is solved by using STEP diagram based on the graph in Figure 3a. Above the Pinch, 

there are 2 hot STEPs and 2 cold STEPs. Note that C1E2 has a steeper gradient (and hence, a smaller FCp) 

as compared to H2E1. Matching these two STEPs may result in ∆Tmin violation. To avoid this, C1E2 is moved 

horizontally to the left to match with stream H1. The heat load of C2HU1 is then divided into two portions to 

match with stream H2 and stream H1 to observe the heuristic of matching hot and cold streams of comparable 

temperatures. For below the Pinch, there are 2 hot STEPs and 1 cold STEPs. C1E1 is moved horizontally to be 

paired with stream H2 instead of stream H1 as the FCp of stream H2 is bigger than that of stream C1 while FCp 

of stream H1 is smaller than that of stream C1. Matching streams C1 and H1 can cause ∆Tmin violation.  In 

order to minimise network changes, and hence, the retrofit cost, the high temperature part of C1 is matched with 

the high temperature part of H2 without affecting the existing cooler CU2, and the remaining part of stream C1 

is paired with stream H1. After retrofit, there are total of 8 units with an additional of 3 more heat exchangers to 

recover 1,950 kW. 

The STEP diagram for the HEN after retrofit is shown in Figure 4a. It is then translated into the Modified ETD 

shown in Figure 4b). Note that the extreme right end of the Modified ETD shows the shape of the grassroots 

GCC even before retrofit is performed, and will be maintained throughout the retrofit. This grassroots GCC curve 

will shift horizontally to the left when users make modification to the HEN to improve heat recovery.  When the 

curve eventually touches the y-axis at the Pinch temperature, the maximum heat recovery is achieved. The grid 

diagram of the HEN after retrofit is as shown in Figure 5. 

 

Figure 5: Grid diagram of the HEN after retrofit 

4. Discussion 

Table 1 compares the results obtained from using STEP diagram (this work) with the results obtained using the 

Modified ETD which is based on the Bridge Analysis. 

Table 1: Comparison of results 

  Existing network Modified ETD 

(Walmsley et al., 2017) 

This work 

Number of units  5 8 8 

Cold utility (kW)  2,950 1,000 1,000 

Hot utility (kW)  2,700 750 750 

Percentage reduced for cooling utility (%) 66.10 66.10 

Percentage reduced for heating utility (%) 72.22 72.22 

 

The comparison shows that both the methods achieve the same results and yield the same number of units.  

Combination of the STEP diagram and the Modified ETD can simplify the HEN retrofit procedure to arrive the 

same retrofitted structure as the one produced by the Modified ETD. The construction of the Modified ETD is 

facilitated by using the distance of the horizontal gap between the stream pairs in the STEP diagram. The 

Modified ETD can help monitor the progress of the HEN retrofit. The combination of the two graphical tools is 

able to simultaneously target, diagnose and retrofit existing HEN. 

The combined graphical tool is found to be beneficial in solving HEN retrofit problem which involves several 

steps to achieve improved heat recovery. When STEP diagram is used individually, targeting is performed using 

other Pinch Analysis tool such as CC or Problem Table Algorithm (PTA). As for the Modified ETD, users need 

 

 

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to reconstruct the diagram after making every change to the existing HEN. The combined graphical tool can 

solve the problems faced when the graphical tools are used individually. Users can directly obtain the current 

heat recovery performance from the Modified ETD after alternating the HEN structure using STEP diagram. The 

Modified ETD can be translated directly from STEP diagram easily as compared to when it is used individually. 

It is recommended that the combined graphical tool can be programmed using software in the future work so 

that the monitoring of the simultaneous targeting, diagnosis, and retrofit existing HEN can be performed 

efficiently. 

5. Conclusions 

The STEP diagram and the Modified ETD are useful graphical tools with pros and cons of their own. This work 

shows that STEP diagram can facilitate and simplify the construction of the Modified ETD while the Modified 

ETD can represent the retrofit progress carried out by using the STEP diagram. The combination of STEP and 

the Modified ETD provides users with an enhanced clean technology tool for energy reduction that can assist 

users to simultaneously target, diagnose, and retrofit an existing HEN while yielding the same final retrofitted 

structure like the one produced by the Modified ETD. 

Acknowledgments  

The authors gratefully acknowledge the financial supports from the Universiti Teknologi Malaysia (UTM) 

Research University Grant under Vote No. Q.J130000.2508.17H16. 

Reference  

Bonhivers J.-C., Alva-Argaez A., Srinivasan B., Stuart P.R., 2015, New Analysis Method to reduce the Industrial 

Energy Requirements by Heat-Exchanger Network Retrofit: Part 2 – Stepwise and Graphical Approach, 

Applied Thermal Engineering, 119, 670-686. 

Bonhivers J.-C., Korbel M., Sorin M., Savulescu L., Stuart P.R., 2014, Energy Transfer Diagram for Improving 

Integration of Industrial Systems, Applied Thermal Engineering, 63(1), 468–479. 

Bonhivers J.-C., Moussavi A., Alva-Argaez A., Stuart P.R., 2016, Linking Pinch Analysis and Bridge Analysis to 

Save Energy by Heat-Exchanger Network Retrofit, Applied Thermal Engineering, 106, 443–472. 

Klemeš J.J., Varbanov P.S., Wan Alwi S.R.W, Manan Z.A., 2014, Process Integration and Intensification: Saving 

Energy, Water and Resources. Walter de Gruyter GmbH & Co KG, Berlin, Germany. 

Lai Y.Q., Abdul Manan Z., Wan Alwi S.R., 2017, Heat Exchanger Network Retrofit Using Individual Stream 

Temperature vs Enthalpy Plot, Chemical Engineering Transactions, 61, 1651-1656. 

Lakshmanan R., Bañares-Alcántara R., 1996, A Novel Visualization Tool for Heat Exchanger Network Retrofit, 

Industrial & Engineering Chemistry Research, 35, 4507-4522. 

Nordman R., Berntsson T., 2001, New Pinch Technology Based HEN Analysis Methodologies for Cost-Effective 

Retrofitting, The Canadian Journal of Chemical Engineering, 79 (4), 655-662. 

Nordman R., Berntsson T., 2009, Use of Advanced Composite Curves for assessing Cost-Effective HEN retrofit 

I: Theory and concepts, Applied Thermal Engineering, 29(2-3), 275-281. 

Tjoe T.N., Linnhoff B., 1986, Using Pinch Technology for Process Retrofit, Chem. Eng. (New York), 93, 47-60. 

Walmsley M.R.W., Lal N.S., Walmsley T.G., Atkins M.J., 2017, A Modified Energy Transfer Diagram for Heat 

Exchanger Network Retrofit Bridge Analysis, Chemical Engineering Transactions, 61, 907-912. 

Wan Alwi S.R., Abd Manan Z., 2010, STEP—A new graphical tool for simultaneous targeting and design of a 

heat exchanger network, Chemical Engineering Journal, 162, 106-121. 

 

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