Design for Multiple Pinches

Example 18.3 The stream data for a process are given in Table 18.3  

Stream Supply temperature (°C) Target temperature (°C) Heat capacity flowrate (MW-r1) 

It has been decided to integrate a gas turbine (GT) exhaust with the process. The exhaust temperature of the GT is 400°C with CP = 0.05 MW K-1. Ambient temperature is 10°C. 

By interpolation from the problem table cascade, heat flow at T = 150°C 

Following the pinch rules, there should be no heat transfer across either the process pinch or the utility pinch by process-to-process heat exchange. Also, there must be no use of inappropriate utilities. This means that above the utility pinch in Fig. 16.17a, high-pressure steam should be used and no low-pressure steam or cooling water. Between the utility pinch and the process pinch, low-pressure steam should be used and no high-pressure steam or cooling water. Below the process pinch in Fig. 16.17, only cooling water should be used. The appropriate utility streams have been included with the process streams in Fig. 16.17a. 

The network can now be designed using the pinch design method.The philosophy of the pinch design method is to start at the pinch and move away. At the pinch, the rules for the CP inequality and the number of streams must be obeyed. Above the utility pinch and below the process pinch in Fig. 16.17, there is no problem in applying this philosophy. However, between the two pinches, there is a problem, since designing away from both pinches could lead to a clash where both meet. 

More careful examination of Fig. 16.17a reveals that between the 

Following this approach, the design is straightforward, and the final design is shown in Fig. 16.176. It achieves the energy targets 

It is rare for there to be two process pinches in a problem. Multiple pinches usually arise from the introduction of additional utilities causing utility pinches. However, cases such as that shown in Fig. 16.18 are not uncommon, where there is, strictly speaking, only one pinch (one place where occurs), but there is a near-pinch. This 

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