When it comes to heat exchangers, chemical processors have a lot to consider. The most common concerns include fouling, heat-transfer efficiency, and the ability of the equipment to handle the high temperatures, high pressures and corrosive materials associated with the manufacturing of chemicals. And heat-exchanger manufacturers are continuously tweaking existing technologies to better suit the unique needs of the users in the chemical process industries (CPI).
Many processes in the CPI require heating or cooling, or both at certain points, which can consume a lot of energy. Heat exchangers play an important role in maximizing the use of energy in many of these heating and cooling applications by transferring heat efficiently and recovering heat for reduced energy use and environmental impact. Heat exchangers are used for heating, cooling, condensing and evaporation duties in many processes.
“The challenge,” says Carl T. Kozacki, APV industrial sales director with SPX Flow Technology (Charlotte, N.C.; www.spx.com), “is to find the best heat transfer technology for the specific application. This will depend upon process parameters, such as operating temperatures and pressures, cleanliness of the fluids being handled, and concerns over fouling, to name a few. The right balance needs to be found to cover process performance, thermal efficiency, accessibility, cleanability and productivity, all while keeping lifecycle costs and initial capital investment low.”
Read on to learn about how the technology is evolving to include new twists on existing designs that make the latest generation of heat exchangers suitable for the toughest applications the CPI can dish out.
“Fouling is by far the toughest problem for any heat exchanger, but it is especially so in the high-temperature, corrosive environment associated with the chemical processing industry,” says Klas Abrahamsson, regional product manager with Alfa Laval (Richmond, Va.; www.alfalaval.com). “The nature of any heat exchanger is that there will be fouling, so the question becomes, What type of heat exchanger and what design can withstand high temperatures and pressures and retain energy efficiency even when fouled?”
Abrahamsson says that although they have been around for many years, spiral heat exchangers were not considered for use in chemical processing applications until recently. A spiral heat exchanger is a circular heat exchanger with two concentric spiral channels, one for each fluid. The curved channels provide optimum heat transfer and flow conditions for a wide variety of fluids, while keeping the overall size of the unit to a minimum (Figure 1).
“Spiral heat exchangers were introduced decades ago for use in the pulp-and-paper industry, which couldn’t use shell-and-tube exchangers because they became fouled too easily and often. While they are finding more use in chemical processing and petroleum refining, there are still many process engineers that haven’t come across or considered using them,” Abrahamsson continues.
“They are not low cost, but we consider them problem solvers in these heavy-duty applications because they solve the fouling problem, which prevents downtime, saves money and easily provides a return on investment. Spiral heat exchangers are easy to maintain and are workhorses.”
Abrahamsson says spiral heat exchangers are versatile and can handle everything from dirty fluids to high-vacuum condensation. A self-cleaning design reduces operating and maintenance costs. The design offers high heat-transfer efficiency and the ability to handle two highly fouling fluids.
Modern plate-heat-exchanger designs, including gasketed, semi-welded pairs, and fully welded units, provide yet another solution to conquering fouling on the heat-transfer surface and provide end users with a variety of options to better meet applications requirements, according to APVs Kozacki.
For example, Kozacki explains, an all-welded hybrid unit is a cross between shell-and-tube and plate technology (Figure 2). On one side, the plate is pressed into a tube-like form so it can handle contaminants, and on the clean side, a plate channel is used to increase thermal efficiency. The unit covers can easily be removed and the tube-side plate can be high-pressure cleaned. This marrying of technologies gives the benefit of the increased thermal efficiency of plate technology along with the tube capability to handle contaminants or solids,” he says.
And no matter the heat exchanger type or design, Emerson Process Management (Austin, Tex.; www.emersonprocess.com) recently launched the pre-engineered Heat Exchanger Monitoring Solution that detects accelerated fouling and identifies the best cleaning time to optimize a facility’s energy usage, capacity and maintenance costs. As part of Emerson’s new suite of Essential Asset Monitoring applications, the heat exchanger solution embeds process and exchanger best practices into software that provides automated, 24/7 monitoring and allows maintenance personnel to schedule an optimal time to clean exchangers. As a result, optimal heat transfer can be achieved, and facilities can reduce energy and capacity loss due to fouling by up to 10%.
“Fouling is the number one issue associated with heat exchangers,” says Nikki Bishop, senior application consultant, R&D/engineering with Emerson Process Management. “And associated with fouling is knowing when to clean exchangers and which ones need to be cleaned. A facility may have a schedule that cleans all the exchangers every six months or all of them every two years or some of them now and some of them later. It may end up that they are cleaning exchangers that don’t need it and neglecting ones that do, which leads to exchangers that fail or lose efficiency and slow the plant down.”
To combat this loss of productivity and efficiency due to fouling, Emerson developed a method of intelligent interpretation of data that allows temperature, pressure, and flow around the heat exchangers to be checked, and based upon that, a fouling rate and fouling percentage can be calculated. This provides realtime information based on the actual performance of the heat exchanger so that the processor can make informed decisions about which heat exchangers to clean and when.
“This provides a lot of benefits. First, it has a great impact on turnaround because you know which exchangers to target and when,” says Emersons Bishop. “You may end up cleaning some exchangers every six months and others every four years, but you know that is what you should be doing based upon the analysis. It also saves time and money spent on unnecessary cleaning and maintenance and increases uptime and productivity, because neglected heat exchangers are receiving maintenance when needed.”
As with fouling, there are designs in existence that can be applied in CPI applications to help improve heat transfer efficiency and new materials applied for use in higher temperatures and corrosive environments.
“Another difficult issue is the efficiency of the heat recovery in the very large process flows associated with the chemical industry,” says Alfa Lavals Abrahamsson. “In the past, process engineers might have used banks of multiple shell-and-tube units because of the low cost and the ability to handle high temperatures, but today’s compact technologies can do the same job, more efficiently and in a smaller footprint.”
Abrahamsson says gasketed- or welded-plate heat exchangers are now being made suitable for such applications. Previously, he says, this type of exchanger might not have found use in the CPI because it couldn’t withstand the high pressures and temperatures, but today’s technologies are pushing the envelope of temperature and pressure capabilities. “The gasket material can withstand many hundreds of degrees higher than years ago. Pressures that were not previously possible in gasketed heat exchangers are now being achieved. It’s not just new materials, but also how the gaskets fit in the heat exchangers and the design of the actual gasket profile that are allowing these efficient and compact heat exchangers to find use in high temperature and high pressure chemical applications today,” says Abrahamsson.
New materials provide other benefits, as well. “There are some developments that allow higher design pressures and temperatures to be available for special geometries than what was available in the past,” agrees Bennant J. Drazner, EPC sales manager with PHE Systems/GEA Heat Exchangers, Inc. (York, Pa.; www.gea.com). “And in addition to these benefits, the overall result is better usage of materials, creating longer successful operating run times between required shutdowns for maintenance, as well as more compact, smaller-footprint solutions that lead, in some cases, to higher yields, faster reaction times through lower hold up volumes, and reduced surfaces to maintain.”
In addition, although many processes in the chemical industry require highly corrosion-resistant materials, the use of these materials was often cost prohibitive in traditional heat-exchanger solutions, such as shell and tubes. But the new compact heat exchangers, such as welded- (Figure 3) and gasketed-plate products can be significantly less expensive in these advanced materials [because they require less of the material than shell-and-tube heat exchangers would] and provide a higher degree of heat recovery, says Drazner.
However, Ron Herman, director of sales and marketing with Enerquip (Medford, Wisc.; www.enerquip.com) says that as the number of options for corrosion-resistant materials continues to grow every year, the pricing for alternative materials used in shell-and-tube exchangers, relative to traditional carbon- and stainless-steel, has been declining. He adds that improvements in the technology behind exchanger tubing are helping as well.
“Tubing suppliers are developing varieties of tubing that enhance the flow and turbulence of fluids flowing through the exchanger, improving heat transfer by as much as 20 to 40%. Corrugated tubing and twisted tape turbulators (static mixers) are both examples of these technologies,” he explains. “Although these can increase pressure drop through the tubes, the result is improved throughput for plants, and can often reduce the size of an exchanger compared to a more traditional design. Tests are being performed to validate the cleanability of these tubing surfaces, since opponents of these technologies claim that they can create possible areas where product buildup can occur, while proponents claim the increased turbulence actually cleans the tube naturally compared to smooth wall tubing.”
Additionally, the use of enhanced surfaces, be it in tubular, plate or other geometries, allows for the more efficient and effective use of materials and possible economic advantages of metallurgy upgrades, says Drazner. “There is also an increase in the possible use of reduced surface-energy materials, such as coatings (both polymer and nanoparticles) or by surface treatment (ion implantation and nitriding) to minimize fouling deposition on heat-transfer surfaces and corrosion of the heat-transfer wall.”
Obviously there are a lot of choices when it comes to heat-exchanger type, design, and material selection. Each heat-transfer technology offers certain advantages, and the key is to understand the need, then match the right technology to the application.
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