*2.2. Potential Heat Exchanger Type Selection Using SRTGD-STR*

When the heat paths are identified by SRTGD, the next step is to select the feasible and economical heat exchangers used for heat recovery. In the proposed SRTGD-STR, the allowable temperature ranges of different heat exchanger types are coupled in the diagram. The area of hot and cold streams connected by one heat exchanger should be in its temperature range. When determining a retrofit plan, this diagram can help easily identify the boundary of the heat exchangers.

The shifted temperature ranges of heat exchangers are added in the SRTGD-STR to help select the heat exchanger types according to the data given in Table 2. As the temperatures of hot streams are shifted temperature in the SRTGD scale, the upper boundary of temperature ranges of heat exchangers in the diagram should also be shifted by minus ΔTmin of the studied case. The suitability of each heat exchanger type for transferring heat between streams depends on the specifications and requirements of the application. Table 2 lists several commonly used heat exchanger types, their temperature ranges, and normal area ranges. In the retrofit design process, these factors should be considered together with the aim of utility saving.


**Table 2.** Heat exchanger types and their temperature and area ranges.

If the temperature of some potential heat recovery range passes through the temperature range boundary of some types of heat exchangers or temperature ranges of some heat exchangers are in the range of more than one heat exchanger type, then focus should be given to this heat path. If the heat recovery range passes through the temperature range boundaries, then whether to implement more than two heat exchangers on one heat path should also be considered to achieve the minimum retrofit cost.

By implementing the data from Table 2 to illustrative Example 1, the corresponding SRTGD-STR is shown in Figure 2. The working temperature range of these heat exchanger types is shifted by minus ΔTmin, which is 20 ◦C in this example. As seen in Figure 2, stream S2 is within the shifted temperature range of plate and frame, double-pipe, and shell-and-tube heat exchangers, which means these three types of heat exchangers can be applied without the consideration of supply and target temperatures. S2 also comes across the upper-temperature boundaries of the spiral tube and spiral plate heat exchangers. It indicates the possibility of using different types of heat exchangers on a single stream, and these two types of heat exchangers should be examined based on the Pinch Rules.

**Figure 2.** SRTGD-STR of the existing HEN for Example 1.

To fully utilise the heat from the hot stream, four retrofit plans are proposed based on SRTGD-STR. The first retrofit plan is shown in Figure 3 and uses one new heat exchanger between streams 2 and 3.

As can be observed from Figure 3, some parts of the shifted temperature range of the new heat exchanger are higher than 330 ◦C, which is higher than the upper bound of the spiral tube heat exchanger. Double-pipe, plate and frame, or shell and tube heat exchangers can be used. The method to determine which type of heat exchanger should be used is explained in Section 2.3. In this plan, 480 kW of heat is exchanged between the streams S2 and S3. To not violate the Pinch Rule, stream 2 uses E2 and C1 to reach the target temperature. The vertical line indicates the Process Pinch.

**Figure 3.** SRTGD-STR of the first retrofit plan for Example 1.

The second retrofit plan (Figure 4) considers implementing a spiral tube heat exchanger for this HEN. It is easy to identify the retrofit plan based on the SRTGD-STR. To implement a spiral tube heat exchanger to this HEN, the highest shifted temperature on both hot and cold streams could not be higher than 330 ◦C, which is also marked on the diagram. For heat exchanger E3 (spiral tube), considering the hot stream S2 has a relatively lower heat capacity flow rate than the cold stream, the shifted inlet temperature in the hot stream should not be higher than the shifted upper bound of implementing a spiral tube heat exchanger. The inlet and outlet temperatures of heat exchanger E2 as well as E1 can also be determined.

**Figure 4.** SRTGD-STR of the second retrofit plan for Example 1.

Using a spiral plate for heat recovery is considered. However, the upper-temperature boundary of the spiral plate heat exchanger minus the ΔTmin equals the inlet temperature of stream S3. It is not feasible to use a spiral plate heat exchanger.

There is another potential option for implementing two double-pipe heat exchangers. The heat transfer area of the double-pipe heat exchanger has a 20 m2 upper limit. For this case, if only one double-pipe heat exchanger is implemented, then the required heat transfer area is higher than the upper limit for the unit. Considering the relatively low capital cost of double-pipe heat exchangers, it is possible to add two double-pipe heat exchanger units to satisfy both the heat recovery requirement

and the heat-transfer area limitation. The Pinch Point for this option is still 280 ◦C. The range of the normal area of the double-pipe is 0.25–20 m2. By adjusting the inlet and outlet temperatures of heat exchangers E3 and E2 (both are double-pipe heat exchangers), the retrofit plan of implementing two double-pipe heat exchangers is shown in Figure 5.

**Figure 5.** SRTGD-STR of the third retrofit plan for Example 1.

The potential retrofit plan using a plate and frame heat exchanger is considered. As seen in Figure 2, stream S1 passes through the upper-temperature boundary of the plate and frame heat exchanger, which makes implementing this type of heat exchanger possible. In the retrofit plan illustrated in Figure 6, the heat of stream S1 is recovered by two heat exchangers. The one in the higher temperature range is a double-pipe heat exchanger, and the other one is a frame and plate heat exchanger.

**Figure 6.** SRTGD-STR of the fourth retrofit plan for Example 1.

This step identifies the potential retrofit plans considering the temperature range. When the plans are obtained, then further actions are required to examine other factors, including pressure, area, and corrosiveness. After excluding these inappropriate plans, the next step is calculating the utility and capital cost.
