Heat Exchanger Networks Pinch Technology

1. Heat Exchanger Networks & Pinch Technology P M V Subbarao Professor Mechanical Engineering Department I I T Delhi

2. Heat Exchanger Network Heat exchanger network (HEN) design is a key aspect of chemical process design. Typically, 20-30% energy savings, coupled with capital savings, can be realised by improved HEN design. The task involves the placement of process and utility heat exchangers to heat and cool process streams from specified supply to specified target temperatures. The objective is to minimize total costs, i.e. capital and operating costs expressed as annual charges. Development of systematic procedures to meet this objective has been an active area of interest in the chemical engineering literature started in late 1970s.

3. Design Methods for HEN The approach to HEN design problem has been radically altered by two major discoveries: Network performance targets and Network temperature pinch. The network temperature pinch represents a bottleneck to feasible heat recovery in HEN design.

4. Network performance targets Network performance targets: The minimum utility usage/requirement, The minimum overall surface area, The minimum number of “units” (i.e. process and utility exchangers). Calculation of these targets is simple and is possible independent of design. Targets can be used, first to stimulate the designer towards better designs, and second to give the designer confidence that his solution is near-optimal.

5. Target for Minimum Utility Requirement Two procedures to target for minimum utility requirements are available. Both procedures identify the best possible degree of process heat recovery as a function of the minimum temperature difference in exchangers, heaters, and coolers, DTmin. These procedures offer more information about the availability of heat at different temperature levels.

6. Targets for minimum overall surface area Capital costs can be considered through targets for minimum overall surface area and minimum number of units. As far as surface area is concerned it is easily established that widely differing solutions for the same problem generally feature similar overall surface areas as long as their degree of energy recovery is similar. What is important in the early stage of network design is the target for the minimum number of units. Units in excess of the minimum number require more foundations, pipe work, instrumentation, maintenance, etc. and lead to greatly increased capital cost.

7. Minimum Cost Heat Exchanger Network In general, minimum cost networks feature the correct degree of energy recovery and the correct number of units. This is achieved in two stages of a design method: First, the method aims for a minimum energy solution, corresponding to a specified DTmin with no more units than is compatible with minimum energy. This task is achieved through a thorough understanding of the pinch phenomenon, hence the method is called the pinch design method. Second, the method involves a controlled reduction in the number of units. This may require “backing-off from minimum utility usage.

8. Pinch Technology The term “Pinch Technology” was introduced by Linnhoff and Vredeveld to represent a new set of thermodynamically based methods that guarantee minimum energy levels in design of heat exchanger networks. Over the last two decades it has emerged as an unconventional development in process design and energy conservation. The term ‘Pinch Analysis’ is often used to represent the application of the tools and algorithms of Pinch Technology for studying industrial processes.

9. As an added bonus, the

10. The systematic design of chemical processes, commonly called ‘Process Synthesis’, is an important & recent activity in the chemical engineering. The design of heat exchanger network has been subjected to the most concentrated effort in process synthesis.