By Eamonn Ryan
Heat exchangers have evolved significantly to meet the growing demands for energy efficiency and integrated building services.
An example of the various forms of heat exchangers – cooling tower, condensing units and air handling unit. © RACA Journal
These devices, which include cooling towers, condensing units, thermal storage systems, and air handling units, are now pivotal in harnessing and redirecting energy from various sources.
In recent years, heat exchangers have become integral in capturing and repurposing previously wasted energy. Initially, these systems were used primarily to enhance water storage for domestic purposes or cleaning tasks, utilising what was once considered ‘waste heat’. However, their applications have expanded dramatically, now supporting advanced technologies and innovative solutions.
Understanding heat exchangers
At their core, heat exchangers are devices designed to transfer heat between two or more fluids. They are used in a variety of industries, including energy, chemical processing, HVAC, transportation, electronics, and refrigeration. The complexity of heat exchanger design extends beyond mere heat transfer analysis.
Factors such as fabrication and installation costs, material selection, weight and size are crucial in determining the final design. In today’s compact plant environments, footprint considerations are often paramount.
Historically, rudimentary heat exchanging devices can be traced back to Roman baths, where systems known as ‘recuperators’ were used. By the turn of the first millennium, indirect heat exchange principles were applied in alcohol production, laying the groundwork for modern heat exchanger technologies. The formal development of heat exchangers began in the 1880s, with early applications in the food and beverage industry.
The first patent for a plate heat exchanger was granted in 1878, but commercial applications did not emerge until the early 1900s, with tubular heat exchangers being developed for power plants.
Technological advancements
From the late 19th to early 20th centuries, heat exchanger technology progressed rapidly, particularly with plate heat exchangers, which saw numerous patents and innovations, especially in milk pasteurisation. Over the decades, materials evolved to withstand corrosive media, and advancements in viscous flow understanding led to new applications in the oil and gas industry.
Modern heat exchangers are now versatile, catering to everything from simple scenarios to complex industrial systems. Innovations such as magnetic technology that adjusts flow dynamics within micro-channel forms have emerged from research in the US and UK. For instance, during the COVID-19 pandemic, South African engineers developed a novel heat exchanger design for ultra-low temperature vaccine storage, demonstrating the technology’s adaptability and potential.
The heat exchanger market is poised for substantial growth, driven by increasing efficiency demands, emerging economies expanding manufacturing capabilities, and the rise of renewable energy systems and electric vehicles. Micro-technology and electronic developments are also pushing the boundaries of heat exchanger applications, including micro-energy generation systems.
To stay competitive, manufacturers must invest in product innovation and R&D. Advances in heat exchanger technology, including new techniques and materials, are expected to further drive market growth and enhance efficiency.
A plate heat exchanger installed at a manufacturing facility. © RACA Journal
Principles and design considerations
Heat transfer theory dictates that heat moves from a hot medium to a cold one until equilibrium is achieved. Heat exchangers (HEX) facilitate this transfer between fluids separated by a barrier (indirect HEX) or in direct contact (direct HEX). Factors influencing heat exchanger design include flow rates, pressure, temperature parameters, and maintenance needs. Different applications require specific designs, such as tube sizing and fin spacing, to optimise performance and efficiency.
Maintenance is crucial for the longevity and efficiency of heat exchangers. Regular inspections, cleaning, and checks of electrical connections and mechanical components are essential to prevent performance degradation and ensure system reliability. For instance, coil heat exchangers should be visually inspected monthly, with thorough cleaning every three to six months.
Design errors often arise from incorrect pipe sizing, valve fitment, and coil placement. Additionally, assuming all fluids have the same properties can lead to inefficiencies. Accurate information on fluids and system limits is critical for optimal heat exchanger design.
Product footprint remains a significant consideration due to smaller plant room sizes. Innovations such as integrated condensers and compact designs are becoming increasingly popular. New heat exchanger technologies, including advanced gasket plate designs and asymmetric channels, are improving efficiency and reducing fouling, offering long-term benefits and cost savings.
As heat exchangers continues to evolve, ongoing research and technological advancements promise to address modern challenges and push the boundaries of efficiency and performance.
Trans-critical CO2 systems
Marius la Grange, general manager: Thermocoil, explains that few traditional HVAC&R applications remain as unsuited for CO2 as refrigeration but the main industry that has rapidly applied and fine-tuned the CO2 trans-critical technologies would be the retail sector. This is because retail has a big focus on running costs/ energy consumption and life cycles.
CO2 is used in various types of heat exchangers for specific applications. “Similar to HFC systems, evaporators absorb heat energy, while gas coolers reject heat energy. The gas cooler acts as a condenser at low to moderate ambient temperatures and functions as a gas cooler at high ambient temperatures.”
How CO2 compares to traditional refrigerants in terms of efficiency depends on the operating temperatures and the ambient conditions where the system is functioning. “The lower the ambient temperature where the system is installed, the more hours each year a CO2 system will operate in sub-critical mode. A refrigeration system operating in sub-critical mode is likely to be more energy-efficient than a comparable HFC system.
“The main advantage of R744 (CO2 ) is its energy efficiency across a wide range of operating temperatures. In industrial applications, CO2 competes with ammonia. In some cases, CO2 is chosen primarily because of its lower capital cost, even if ammonia is more energy efficient. HFCs are generally not considered in these situations.”
As to how CO2 impacts the design and operation of heat exchangers, La Grange explains that the operating pressures are significantly higher than in traditional HFC systems. An HFC system typically has an operating limit of 31 bar, while a new trans-critical CO2 system has an operating limit of 120 bar.”
A larger process cooling heat exchange. Supplied by HC Heat Exchangers
He lists one of the biggest challenges as being the skills required to design, build and maintain trans-critical systems. “It’s more complex than HFC system and a solid understanding is needed. Other challenges associated with using CO2 in heat exchangers include managing high operating pressures and addressing the difficulties in designing for a wide range of operating loads, which are concerns not unique to CO2 applications.
“CO2 enhances the energy efficiency of heat exchangers primarily through its latent heat of vapourisation. This metric measures the amount of heat absorption possible when the high-pressure liquid refrigerant changes phase to vapour upon exposure to heat energy. For R744 (CO2 ), the latent heat of vapourisation is 350 kJ/kg at 1 bar, which is significantly higher compared to 217 kJ/kg for R134a and 248 kJ/kg for R407C. This higher latent heat of vapourisation allows CO2 to absorb more heat energy per unit of surface area, improving the overall efficiency of heat exchangers.”
He lists an example/case study of where CO2 has significantly improved energy efficiency as including the Proklima ‘Natural Refrigerant’ work by Dr. Volkmar Hasse, which serves as an excellent reference. The scientific metrics presented in this study remain relevant and are a great starting point for those new to working with CO2.
“The long-term energy savings associated with using CO2 in heat exchangers are significant enough to drive the shift toward CO2 systems, although contractors and owners often keep specific figures confidential. Since a refrigeration system operates dynamically, energy savings must be evaluated in context. While a CO2 system might not be as energy efficient as an HFC system during extremely hot afternoons, a year-round review of energy consumption reveals that CO2 systems generally have a small percentage of operational hours where they are less efficient compared to HFC installations.”
He notes that using CO2 in heat exchangers contributes to reducing greenhouse gas emissions in several indirect ways.
“A CO2 evaporator can be smaller than those required for other refrigerants, leading to a reduction in the amount of copper needed and the associated indirect global warming potential (GWP) from raw material production. Additionally, CO2 gas coolers, which are often made with durable stainless-steel tubing, are less likely to develop refrigerant leaks compared to copper tube condensers. Energy savings from CO2 systems also contribute to lower indirect GWP, as less coal is needed for power generation, or if solar power is used, the indirect GWP is further reduced. The TEWI (Total Equivalent Warming Impact) index is a useful tool for combining these environmental factors into a single, comparable metric,” says La Grange.
He says that the lifecycle impacts of CO2 -based systems compared to traditional refrigerant systems are still being assessed, but they appear to be at least on par with, if not better than, traditional HFC systems. “Older trans-critical CO2 systems, which have been in operation for around 10 years, suggest a potential for longer lifespan in the future. The higher operating pressures in CO2 systems lead to more robust design, installation, and commissioning processes, resulting in fewer leaks compared to historically poorly installed HFC systems.
A coil set for an ice storage installation is another example of the heat exchange design. Supplied by Evapco
Well-maintained CO2 systems are likely to last longer, and the significant capital investment in these systems emphasises the importance of doing things correctly from the start. However, direct comparisons are challenging due to the variability in the installation quality of many HFC systems, which often suffer from leaks.
“Current research trends and future developments in CO2 technology for heat exchangers focus on several key areas. Manufacturers are continuously competing to improve the coefficient of performance (COP) of compressors. Additionally, significant development efforts are being directed towards advancing electronic system controls and expansion devices, which are crucial for optimising the efficiency and performance of CO2 systems.”
He notes that potential breakthroughs and technological advancements that could enhance CO2 system performance include improvements in compressor technology. “Manufacturers have successfully increased operating pressures, which has significantly reduced the CO2 temperature limits and expanded the feasibility of CO2 systems in high ambient temperature areas where they were previously considered impractical.
“Analysing both initial investment and long-term operational costs reveals evolving trends in CO2 systems. While past research extensively covered the feasibility of R744 systems, recent studies are increasingly focused on new components and their specific benefits in various applications. Although the capital outlay for a trans-critical CO2 system is higher compared to an HFC system of similar capacity, the premium has been steadily decreasing and continues to shrink, making CO2 systems a more competitive option over time,” says La Grange.
