The Pinch Design Method

Start at the pinch. The pinch is the most constrained region of the problem. At the pinch, A Tmin exists between all hot and cold streams. As a result, the number of feasible matches in this region is severely restricted. Quite often there 

Note that the CP inequalities given by Equations 18.1 and 18.2 only apply at the pinch and when both ends of the match are at pinch conditions. 

The CP-table. Identification of the essential matches in the region of the pinch is clarified by use of the CP-table1,2. 

In the CP-table, the CP values of the hot and cold streams for the streams at the pinch are listed in descending order. 

Having decided that some essential matches need to be made around the pinch, the next question is how big should the matches be  

Having decided that no exchanger should have a temperature difference smaller than ATmi, two rules were deduced. If the energy target set by the composite curves (or the problem table algorithm) is to be achieved, there must be no heat transfer across the pinch by 

Start at the pinch. The pinch is the most constrained region of the problem. At the pinch, exists between all hot and cold 

---

This design procedure is known as the pinch design method and can be summarized in five steps... 

The philosophy in the pinch design method was to start the design where it was most constrained. If the design is pinched, the problem is most constrained at the pinch. If there is no pinch, where is the design most constrained Figure 16.9a shows a threshold problem that requires no hot utility, just cold utility. The most constrained part of this problem is the no-utility end. Tips is where temperature differences are smallest, and there may be constraints, as shown in Fig. 16.96, where the target temperatures on some of the cold... 

Figure 16.10 shows another threshold problem that requires only hot utility. This problem is different in characteristic from the one in Fig. 16.9. Now the minimum temperature difference is in the middle of the problem, causing a pseudopinch. The best strategy to deal with this type of threshold problem is to treat it as a pinched problem. For the problem in Fig. 16.10, the problem is divided into two parts at the pseudopinch, and the pinch design method is followed. The only complication in applying the pinch design method for such problems is that one-half of the problem (the cold end in Fig. 16.10) will not feature the flexibility offered by matching against utility.

The pinch design method developed earlier followed several rules and guidelines to allow design for minimum utility (or maximum energy recovery) in the minimum number of units. Occasionally, it appears not to be possible to create the appropriate matches because one or other of the design criteria cannot be satisfied. 

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. 

The design method used so far, the pinch design method, creates an... 

The pinch design method is a step-by-step approach which allows the designer to interact as the design progresses. For more complex network designs, especially those involving many constraints, mixed equipment specifications, etc., design methods based on the optimization of a reducible structure can be us d. ... 

Linhoff, B. and Hindmarsh, E., 1983. The pinch design method for heat exchanger networks. Chemical Engineering Science, 38, 745. 

The pinch design method, as discussed so far, has assumed the same A Tmin applied between all stream matches. In Chapter 16, it was discussed how the basic targeting methods for the composite curves and the problem table algorithm can be modified to allow stream-specific values of A Tmin. The example was quoted in which liquid streams were required to have a A Tmin contribution of 5°C and gas streams a ATmin contribution of 10°C. For liquid-liquid matches, this would lead to a ATmin = 10°C. For gas-gas matches, this would lead to a ATmin = 20°C. For liquid-gas matches, it will lead to a ATmin = 15°C 2. Modifying the problem table and the composite curves to account for these stream-specific values of ATmin is straightforward. But how is the pinch design method modified to take account of such A Tmin contributions Figure 18.9 illustrates the approach. Suppose the interval pinch temperature from the problem table is 120°C. This would correspond with hot stream pinch temperatures of 125°C and 130°C for hot streams with ATmin contributions of 5°C and 10°C respectively. For... 

The pinch design method creates a network structure based on the assumption that no heat exchanger should have a temperature difference smaller than ATmin. Having now created a structure for the heat exchanger network, the structure can now be subjected to continuous optimization. The constraint that no exchanger should have a temperature... 

Linnhoff B and Hindmarsh E (1983) The Pinch Design Method of Heat Exchanger Networks, Chem Eng Sci, 38 745. 

Research on the synthesis of economically optimal HENs has been performed by various investigators for over 15 years (Nishida et al., 1981). Several powerful synthesis methods have evolved, including the pinch design method (Linnhoff et al., 1982 Linnhoff and Hindmarsh, 1983) and methods based on structural optimization [to predict a minimum set of stream matches (Papoulias and Grossmann, 1983) and to determine the most economical network structure (Floudas et al., 1986) from the predicted matches]. However, these methods synthesize networks only for fixed, assumed nominal values of any uncertain supply temperatures and flow rates and uncertain heat transfer coefficients. 

Linnhoff, B.. and Hindmarsh. E. "The Pinch Design Method for Heat Exchanger Networks, Chem. Eng. Sci. 38, 745 (1983). 

How to use the pinch design method to optimize process heat recovery... 

Fig. 15 The pinch design method. (A) CP tables follow feasible matches to be identified at the pinch. (B) Maximising the heat duties on the matches. (C) The completed design.

The systematic procedure for designing the Heat Exchanger Network based on Pinch principle is designated as the Pinch Design method (Linnhoff et al., 1978, 1983). The methodology consists of the following steps ... 

The points 1 to 5 belong to the targeting procedure already presented. The points 6 to 9 form the core of the Pinch Design method that will be explained in the subsequent subchapters. Particularly the point 7 deserves more attention. 

As explained, the Pinch Design method generates a network with a certain degree of redundancy. The reduction of units can contribute to major saving in capital costs. However, more energy is necessary to inject into the network to restore infeasible driving forces. This task can be treated elegantly by optimisation. 

Linnhoff, B., B. Hindmarsh, 1983, The Pinch design method of heat exchanger networks, Chem. Engng. Sci., 38, 745... 

---