*2.1. Process Visualised Using SRTGD*

The first step is to identify the hot and cold process streams that have the potential for heat exchange. The flow rates, supply and target temperatures, and heat capacities of streams should be used to determine the utility targets. The stream conditions such as medium, pressure, viscosity, individual heat transfer coefficient, and material corrosiveness should also be extracted. Table 1 is

an example to show the data extracted from a process. There are three streams, and an existing heat exchanger is connecting stream 1 and 3 for heat recovery. In this example, ΔTmin is set at 20 ◦C.

The developed SRTGD-STR is the extension of SRTGD proposed by Yong et al. [12]. SRTGD is used to identify the potential retrofit plan regarding heat recovery. The streams extracted are presented on the SRTGD according to their temperature ranges and heat capacity flow rates (CPs). For better visualisation, the cold streams are usually presented on the downside of the graph and hot streams are on the upside of the graph. If the slope of the link between heat exchangers is negative, this indicates an infeasible implementation of heat exchangers. Stream names are shown on the right side of each stream. The heat capacity flow rate of each stream can be observed on the y-axis. Supply and target temperatures of streams and heat exchangers can be observed on the x-axis. Pinch points are identified when the link between one cold and one hot stream is vertical.

SRTGD is capable of providing visual information in choosing heat paths. The temperatures of hot streams are shifted in the SRTGD. It can also help the designers to find the Network Pinch and identify the retrofit plan for maximum heat recovery. The detailed method for drawing an SRTGD and using SRTGD for HEN retrofit can be found in Ref. [12]. Figure 1 shows the SRTGD of illustrative Example 1.

**Figure 1.** Shifted Retrofit Thermodynamic Grid Diagram (SRTGD) of the existing HEN for Example 1.
