Practicing Pinch-Point Analysis What Can Be Learned from Composite Curves?

Ivana Lukec, Ph.D.
Practicing Pinch-Point Analysis

Improving energy efficiency of the process in many cases starts with a heat exchanger assessment. Very often heat exchange performance deviates from the optimum. Pinch method is a good and efficient way to

1. Evaluate the existing heat exchanger network,

2. To analyze opportunities for improvement, and

3. Improve heat efficiency of the unit by determining required actions to improve the performance.

By using mathematical modeling and simulation to analyze existing and improved cases, exact information about the potential heat recovery can be determined.

Step 1 in the analysis is a development of composite curves which graphically represent a potential for improvement of heat exchanger network.

There are specialized software tools that make the analysis simple, but it can be done using basic chemical engineering knowledge and simple tools such as MS Excel.

Basic chemical engineering knowledge is a must in both cases.  

What can be learned from composite curves?

Briefly, the curves can reveal very important insights for a heat recovery problem: maximal process heat recovery, pinch point, and hot and cold utility targets, which can be visualized, as on the Figure.

The explanations for these basic concepts are:

  • Minimum Temperature Approach DTmin: For a feasible heat transfer between the hot and cold composite streams, a minimum temperature approach must be specified, which corresponds to the closest temperature difference between the two composite curves on the T/H axis. This minimum temperature approach is termed as the network temperature approach and defined as DTmin.
  • Maximal Process Heat Recovery: The overlap between the hot and cold composite curves represents the maximal amount of heat recovery for a given DTmin. In other words, the heat available from the hot streams in the hot composite curve can be heat-exchanged with the cold streams in the cold composite curve in the overlap region.
  • Hot and Cold Utility Requirement: The overshoot at the top of the cold composite represents the minimum amount of external heating (Qh), while the overshoot at the bottom of the hot composite represents the minimum amount of external cooling (Qc).
  • Pinch Point: The location of DTmin is called the process pinch. In other words, the pinch point occurs at DTmin. When the hot and cold composite curves move closer to a specified DTmin, the heat recovery reaches the maximum and the hot and cold utilities reach the minimum. Thus, the pinch point becomes the bottleneck for further reduction of hot and cold utilities. Process changes must be made if further utility reduction is pursued.