How to adopt heat exchanger design for refrigeration systems in the era of low Global Warming Potential (GWP) refrigerants
Global warming topics have been around for many decades. The united global effort to phase out ozone-depleting substances has resulted in the healing of the hole in the ozone layer, which will eventually help protect human health, economies and ecosystems. It is therefore imperative for industries to take action now and act responsibly to lower Global Warming Potential (GWP) and ozone depletion for climate protection.
Challenges of new refrigerants
However, with the adoption of natural and low GWP synthetic refrigerants come new challenges, such as increased flammability, reducing the total charge of the system and the requirements to manage hazardous refrigerants in the workspace.
Back at the beginning of the 20th century, the CFC group of refrigerants such as R11, was popular. Over time, the use of CFCs was outlawed by most countries and replaced with HCFC (R22). However, this was still not the best solution as ozone problems still persisted. Hence, many European countries have started to phase out these refrigerants, with Asia also getting in on the trend of striving to reduce GWP.
Today, the refrigeration industry continues the fight for environmentally-friendly refrigerants, replacing older refrigerants with HFCs, namely R404, R410 or R134a. Although HFCs are ozone-friendly substances, they may not be the perfect solution as they still have a high GWP.
Today, the question lies with refrigeration industry players in taking the next steps to reduce greenhouse gases and fossil fuel consumption. Gases with a higher GWP absorb more energy than gases with a lower GWP, thus contributing more to Earth’s global warming.
Among the most commonly known natural refrigerant gases are:
- Carbon dioxide (CO2): Naturally occurring gas that can used as a refrigerant. However, downsides stem from the requirement for higher pressure ratings and the risk of getting a lower total coefficient of performance.
- Hydrocarbons, propane (C₃H₈): This is most commonly used in Europe for small commercial refrigeration equipment such as domestic refrigerators. Propane has a major drawback in that it is highly flammable.
- Ammonia: Beyond its economic advantages, ammonia is a natural refrigerant that is environmentally benign in the atmosphere. Ammonia refrigeration has made significant contributions to daily refrigeration operations. From an operational perspective, ammonia is generally seen as the most efficient and cost effective refrigerants available. Anyway, there is some hesitation, due to its toxicity.
- Synthetic artificial refrigerants (hydrofluorocarbons, HFO): Synthetic refrigerants are substances that do not occur naturally but have been developed by man for industrial purposes. Today, new synthetic artificial refrigerants are gaining much attention as commercial and industrial refrigerants, but there’s still a long way to go.
These natural refrigerant gases have their own advantages, which can help refrigeration operations operate much more economically. However, the important topic here is how to optimize energy efficiency in the systems while reducing environmental impact.
The advantage of transcritical systems for commercial refrigeration and cold storage duties
Commercial refrigeration refers to the cold storage equipment used in commercial settings such as hypermarkets and larger scale supermarkets.
Some natural refrigerants such as ammonia are toxic; while carbon dioxide requires extremely high pressures to operate, especially in transcritical systems. Today, a transcritical system would be much preferable as a substitution compared to a cascade refrigeration system as it works well with a multiple temperature range. This can be explained from the diagram below which shows that there are more heat recovery opportunities for higher gas temperatures. The system in turn reduces energy consumption while being energy efficient and has a lower GWP.
*A compressor rack includes different types of Brazed Heat Exchangers exposed to many different technical challenges such as extreme high pressure, high temperature gradient, temperature cycling (fatigue)
From the schematic, a transcritical system works best with multiple temperatures ranging between medium (above 0° Celsius) and low (below -18° Celsius) which is very common for commercial refrigeration setups. One compressor rack can serve all of the required temperatures, including a medium temperature cabinet, low temperature freezer and multi-temperature cold storages.
The schematic shows opportunities for better heat recovery, leading to an increase in energy efficiency for refrigeration systems. However, the challenges of operating with high pressure and risk of fatigue due to cycling operations are still unavoidable.
Implications for heat exchanger components
Heat exchanger materials are subjected to physical stress from pressure peaks and rapid temperature changes. This may eventually lead to problems that can cause equipment failure. Alfa Laval, supported by our extensive experience in thermal transfer technology, has studied these phenomenon in depth and successfully produced a resilient brazed plate heat exchanger with a long service life under high operating pressure conditions. As a result of this development, our AXP line, for example, has PED-approval for maximum pressures of 167 bar at 90°C and UL-approval for 2233 psi at 400°F.
Moreover, the fatigue performance of AXP82 exceeds even the high standards of previous models. This is largely thanks to the application of new innovations such as an all-new channel plate design built with Alfa Laval’s unique PressureSecure technology.
Alternatively, to lower the need of relying on natural refrigerants, an "indirect system" can be designed whereby a large chiller with a higher cooling capacity that can cool down brine is used. This is explained in the schematic below.
Distribution of indirect system
The schematic below shows an indirect system used in refrigeration where two chillers with medium and low temperatures respectively have their own setting points. Brazed plate heat exchangers would then work well as evaporators, due to their robustness, smaller space footprint and higher thermal efficiency. The chiller will then produce the cold brine and pump it to the cabinets, cold room and freezers.
With the Alfa Laval brazed plate heat exchanger, thanks to DynaStatic technologies, the customer gets to fully tailor the placement, size and number of inlets to fit their specific application, ensuring an optimal refrigerant distribution system. Alfa Laval’s patented asymmetrical FlexFlow plate design, another one of our many unique innovations, enables improved thermal efficiency to optimise pressure drops and increased turbulence which helps reduce refrigerant volumes and raw materials. The result is higher efficiency with the flexibility to use low-GWP refrigerants.
Advantage of indirect system
- Large chillers are used to cool down brine (1)
- Air coolers keeps the warehouse cold storage areas at medium or low temperature (2)
- Cooling capacity is usually high
- Ammonia and CO2 refrigerants are widely used
- Brazed plate heat exchangers should are in parallel. Gasketed plate heat exchangers can be used as alternatives
Relentless innovation to help industry with transition
In conclusion, while natural refrigerants may be substances that are highly efficient for refrigeration operations and environmentally friendly, there are some technical challenges that need to be addressed and overcome.
Alfa Laval is committed to REFuture, a future-proof investment for tomorrow’s refrigerants. As an innovative driver in the development of solutions for natural refrigerants, we bring to the table years of experience working with products that enable the use of new-generation and low-GWP refrigerants to help our customers meet their sustainability goals and legislative requirements.
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Webinar on-demand: Learn how to design your heat exchanger for new refrigerants
The shift to low Global Warming Potential (GWP) refrigerants is gaining momentum globally. Join this webinar to upskill yourself on the design requirements for GWP refrigerants - to build systems that are energy-efficient, durable and compliant, even under the highest pressure.