Pinch Design in Practice: Balancing Energy Efficiency with Real-World Constraints
Pinch analysis is widely used for designing heat recovery systems within the process industries. However, it is not a panacea. The best pinch projects are designed and implemented in combination with good engineering practices and a large dose of common sense.
I provided a brief introduction to pinch analysis in my previous blog, Energy in a Pinch: How Did We Get Here? That article focused on pinch targets. Today, I’d like to move on to pinch design.
First, a quick review of some terminology: In pinch jargon, process streams that are cooled down, releasing heat, are called “hot streams.” Those that are heated, absorbing heat, are called “cold streams.” The terms “hot” and “cold” do not refer to the temperature of the streams. Rather, they denote the direction of heat transfer.
When we examine heating and cooling demands in most processes, we find a “pinch,” characterized by a “pinch temperature,” that divides the process into two distinct regions (see Figure 1):
- The region above the pinch temperature has a net heat deficit. An external utility heat source (e.g., steam or a furnace) completes the energy balance.
- The region below the pinch temperature has a net heat surplus. An external utility heat sink (e.g., air or cooling water) removes the excess heat.
Figure 1: The pinch divides a process into two distinct heat flow regions.
The objective of most pinch work is to minimize the utility heating and cooling requirements in a process, and thus achieve the pinch “energy target,” which includes both the hot utility target, Qh, min, and the cold utility target, Qc, min. To accomplish this goal, we must design independent heat exchanger networks for the two regions. This ensures that we do not violate the pinch principle: Do not transfer heat across the pinch. If you don’t transfer heat across the pinch, you are guaranteed to achieve the energy target for the process.
There is an established “pinch approach” for designing heat exchanger networks [1]. It starts by separating the streams into two groups: those above the pinch temperature and those below it. A systematic procedure is then followed, whereby heat transfer matches are created between the hot and cold streams in the above pinch group, starting from the pinch and moving to progressively higher temperatures, until all available heat from the process streams is consumed. At this point, at least one cold stream is usually short of heat; the deficit must be satisfied by a utility heat source (e.g., steam or fired heater).
Following an equivalent approach for the group of streams below the pinch will usually result in at least one hot stream with excess heat. This has to be removed by a utility heat sink (e.g., cooling water, air, or chiller).
However, designing heat exchanger networks isn’t always easy, even for seemingly simple systems. In order to achieve the pinch target, we often have to split streams into parallel branches, and/or place multiple heat exchangers in series. There are systematic methods for designing these heat exchanger networks, but the added complexity and the increased number of heat exchangers can raise costs and adversely affect operability. These designs generally require simplification to ensure operability and/or to reduce costs. Additional systematic methods have been developed to simplify these designs in ways that reduce cost and complexity while still retaining most – but not all – of the energy savings. Consequently, most practical designs use somewhat more energy than the theoretical pinch target.
Pinch analysis was initially developed to improve new, “greenfield” plant designs. In these situations, there are relatively few constraints on plant layout and equipment selection, and we can usually implement designs with energy use close to the pinch target.
Pinch analysis is also applied to revamps. In these cases, the design approach needs significant modifications, for several reasons. In most revamps, strong incentives exist to maximize the use of existing heat exchangers and other equipment, even if they are not ideal when viewed from a pinch perspective. This often results in revamp designs that differ markedly from new plant designs. Revamps must also account for existing equipment and plot space. Lack of space can limit the opportunity to add new heat exchangers and other equipment. Moreover, revamps typically occur during turnarounds, when time is at a premium. This makes it hard to justify complex projects that are difficult or time-consuming to execute.
Many different retrofit design procedures have been proposed. Most approaches start by identifying the existing heat exchangers in which heat crosses the pinch. The various approaches then use different methods to correct these local inefficiencies. However, as in new plant designs, these methods can to lead to overly complex designs, and compromises have to be made. The resulting revamps typically include modifications to existing heat exchangers (e.g., tube bundle replacements to increase heat transfer capability) and changes in the way existing heat exchangers are interconnected, as well as installing new heat exchangers.
The pinch design approach focuses on heat flow. This allows us to identify design options that maximize heat recovery and eliminate unnecessary energy losses. However, it has important limitations. We have already seen that it can lead to excessively complex heat exchanger network designs. More generally, good engineering practices must be followed, and pinch designs should never be considered in isolation. This includes, among other things:
- Safety: In some situations, there are incompatibilities between the streams that are ideally matched using the pinch approach. In an ideal heat exchanger network, these streams are paired against each other in a heat exchanger. If there is a large pressure difference between streams, a leak in the heat exchanger could over-pressurize equipment and cause damage on the low-pressure side of the process. In other cases, leaks may lead to mixing of incompatible materials. This could result in explosive compositions or other dangerous conditions. Hazard analyses should be carried out to screen for these and other potentially dangerous scenarios. When identified, these problems can often be addressed by appropriate design options, but in some cases there is no economically viable way to pair the streams.
- Operability: Most designs for continuous processes assume steady flow rates. However, there are some streams that are either variable or intermittent. Incorporating these into a heat exchanger network can create instabilities that make process control difficult.
- Metallurgy: Different process streams often require different metallurgies. This often means that the materials needed for heat exchangers, piping, and other equipment in pinch designs are different – and more expensive – than those needed in non-pinch designs.
- Economics and Return on Investment: Solving design challenges costs money, and cost-benefit analyses should be carried out. Some parts of an aggressive pinch design may be uneconomic and should be excluded from the final design.
In summary, pinch analysis is a powerful tool, though its use must be tempered with realism. Properly applied, it can often reduce process heating and cooling requirements by 15% or more.
Looking to improve energy efficiency at your facility and understand how to practically apply pinch principles into your analysis? Becht’s team of experts can help you uncover hidden savings. Contact us to get started.
Like what you just read? Join our email list for more expert insights and industry updates.
Reference
[1] B. Linnhoff, D. W. Townsend, D. Boland, G. F. Hewitt, B. E. A. Thomas, A. R. Guy, and R. H. Marsland, A User Guide on Process Integration for the Efficient Use of Energy, pp. 14–88, I.Chem.E., Rugby, U.K. (1982)
