In today’s eco-conscious world, companies everywhere are taking a close look at their processes to determine just how green they are. Many find there are at least a few areas where they can improve.
Companies may take a number of routes on their journey to greater energy efficiency. Sometimes, this includes incorporating new technology to cut costs, such as integrating a shell and tube heat exchanger to recover wasted heat. Another strategy is to re-evaluate fuel types used and adopt more renewable energy sources.
While these are worthwhile efforts in many cases, there’s one method of increasing power efficiency and decreasing energy costs that’s often found to be the most effective, according to Oil and Gas Business. That method is ensuring your current equipment is working and being utilized to the best of its potential. A pinch analysis is a highly effective way to study the efficiency of current equipment and to design a new heat exchange system if the results show that improvement is needed.
“A pinch analysis helps to determine the best locations for each piece of equipment.”
Thermodynamics and the pinch point
A pinch analysis uses the principles of thermodynamics; specifically, the first and second laws of thermodynamics, according to Mukesh Sahdev, writing for ChE Resources:
The first law of thermodynamics
The first law of thermodynamics states that heat is a form of energy, and energy cannot be created nor destroyed. Therefore, in all cases, including in a heat exchanger, heat cannot be created nor destroyed; only transferred.
Sahdev writes that, in a pinch analysis, the first law relates to enthalpy changes in a heat exchanger system.
The second law of thermodynamics
The second law of thermodynamics is also known as the law of increased entropy. It states that even though the amount of heat (or energy) will always stay the same, the quality will change. Entropy relates to unusable energy inside a closed system.
Sahdev explains that the second law relates to the direction of heat flow in an exchanger – it can only go from hot to cold.
In a heat exchanger, the cold fluid will never be able to reach a temperature higher than the warmest point of the hot fluid stream. Likewise, a hot fluid stream can never be cooled to a colder temperature than the lowest point of the cold stream.
Further, the hot stream can only be cooled to the “temperature approach,” or the minimum allowable temperature difference. This figure – the minimum allowable temperature difference – is also called the “pinch point.”
What is a pinch analysis?
A pinch analysis identifies the pinch point, which helps in determining the optimal size of a heat exchanger for a particular use. An analysis can also help companies recognize reasonable energy and capital cost targets. The whole idea is to determine the best locations for each piece of equipment; it’s kind of like solving a puzzle in your operation.
There are a number of resources and technologies that companies can use to conduct a pinch analysis or to hire a professional to perform one in their facilities. Companies can also purchase pinch analysis software that will collect the necessary information. The company can then send that information to a third party to analyze it.
Writing for Chemical Processing, Gary Faagau describes a do-it-yourself version of a pinch analysis. This is a good first step, as it avoids the time, labor and money required to conduct a formal pinch analysis.
To begin, open a spreadsheet:
- In column A, list all streams in your plant that need to be heated, ordered from the lowest final target temperature to the highest.
- In column B, list all streams that need to be cooled, ordered from the lowest initial temperature to the highest.
- In column C, list all available utility heat (include steam pressures, fired heating and cost).
Next, calculate your average heat capacity over the range of temperatures needed (as shown in columns A and B). You can use a heat exchanger simulation program to do this step.
Then, you can begin analyzing which areas make the most sense for heat recovery. When doing this, prioritize heating or cooling capacity for your most important streams. If you don’t have a defined pinch point, you can assume an approach of 20 degrees Fahrenheit for a standard shell and tube heat exchanger, or a 10 or 15 degree approach for a more complex set of exchangers, Faagau noted.
Benefits of a pinch analysis
When formulating a process without a pinch analysis, engineers need to determine the design of the core process first, then the heat recovery system and finally the utility system. Each aspect of the overall design is created independently of each other.
The pinch method, on the other hand, works with defined targets for each of the three components of the design. By taking into account the unique features of each part of the design as well as reasonable targets for each, engineers can identify opportunities for greater heat integration. Not only can this reduce energy costs, but can also contribute to a more useful heat exchanger design.
“A pinch analysis can also contribute to a more useful heat exchanger design.”
The pinch method was developed in the 1980s for the petrochemical industry. Its use has shown to be extremely helpful both in the planning of new facilities and the retrofitting of existing operations in various chemical engineering applications.
Sahdev pointed out that companies have reported that pinch analyses have led to a:
- 15 to 40 percent decrease in energy costs.
- 5 to 15 percent reduction in capacity bottlenecks for retrofits.
- 5 to 10 percent decrease in capital costs for new designs.
Pinch analyses may be particularly useful for existing facilities, especially if they’ve been in operation for many years or have never been evaluated for efficiency. Considering that many petrochemical plants have been in operation since the 1960s or 1970s, as noted by Oil and Gas Business, a pinch analysis could be worth the time.
Despite the many benefits of a pinch analysis, there are a few obstacles companies will need to overcome to make the most of them. First, they’re time-consuming and involve complex calculations. If a company decides that it would be best to conduct a formal pinch evaluation, it can be a high upfront expense. However, the savings and reduced energy expenditure may justify the cost.
If you find that there are areas where a new or redesigned custom shell and tube heat exchanger would benefit your process, reach out to the helpful engineers at Enerquip. They’re experts at solving problems and making the best use of their exchangers.