Pinch principle

The Pinch principle states that any design where heat is transferred across the Pinch will require more energy than minimum requirements. Consequently, the heat recovery problem is divided into two subsystems, above and below the Pinch. 

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 ... 

Figure 10.42 Water Pinch principle concentration composite curve (left), targeting water consumption (right)...

FIGURE 9.10. Pinch principle penalty of cross-pinch heat transfer. 

Thus the appropriate placement of heat pumps is that they should be placed across the pinch. Note that the principle needs careful interpretation if there are utility pinches. In such circumstances, heat pump replacement above the process pinch or below it can be economic, providing that the heat pump is placed across a utility pinch. Such considerations are outside the scope of the present text. 

A refrigeration system is a heat pump in which heat is absorbed below ambient temperature. Thus the appropriate placement principle for heat pumps applies in exactly the same way as for refrigeration cycles. The appropriate placement for refrigeration cycles is that they also should be across the pinch. As with heat pumps, refrigeration cycles also can be appropriately placed across utility pinches. It is common for refrigeration cycles to be placed across a utility pinch caused by maximizing cooling water duty. 

The concept of the appropriate placement of distillation columns was developed in the preceding chapter. The principle also clearly applies to evaporators. The heat integration characteristics of distillation columns and evaporators are very similar. Thus evaporator placement should be not across the pinch. ... 

Rotor Impactors The rotor of these machines is a cylinder to which is affixed a tough steel bar. Breakage can occur against this bar or on rebound from the walls of the device. Free impact breaking is the principle of the rotor breaker, and it does not rely on pinch crushing or attrition grinding between rotor hammers and breaker plates. 

In this section the fundamental principles of the pinch technology method for energy integration will be outlined and illustrated with reference to a simple problem. The method and its applications are described fully in a guide published by the Institution of Chemical... 

If an inappropriately placed distillation column is shifted above the heat recovery pinch by changing its pressure, the condensing stream, which is a hot stream, is shifted from below to above the pinch. The reboiling stream, which is a cold stream, stays above the pinch. If the inappropriately placed distillation column is shifted below the pinch, then the reboiling stream, which is a cold stream, is shifted from above to below the pinch. The condensing stream stays below the pinch. Thus appropriate placement is a particular case of shifting streams across the pinch, which in turn is a particular case of the plus-minus principle (see Chapter 19). 

This simple example illustrates the basic principles of water network design for maximum reuse for a single contaminant. A number of issues need to be considered that would apply to more complex examples. Consider Figure 26.25 involving three water mains and three operations. Operation 2 above the pinch terminates at a concentration less than the concentration for the high concentration water main. The outlet of Operation 2 must not be fed directly into this final water main. The basis of the mass balance from Figure 26.17 dictates that all streams must achieve the concentration of the water mains into... 

Most recent synthesis algorithms are also based upon the principles of the thermodynamic pinch (Linnhoff et al., 1979 Umeda et al., 1978). Recognition of the pinch provided great physical insight into the problem of HEN synthesis. The reader is assumed to be familiar with the principles of the pinch and with general methods for HEN synthesis [e.g., pinch design method (Linnhoff et al., 1982 Linnhoff and Hindmarsh, 1983), structural optimization methods for selection of a minimum set of stream matches (Papoulias and Grossmann, 1983), and determination of the most economical network structure (Floudas et al., 1986) from the predicted matches]. 

---