It has been a year since the Environmental Protection Agency instituted a number of changes to 40 CFR Part 82, Protection of Stratospheric Ozone. These changes are meant to sharply restrict the use of substances that allegedly harm the earth’s protective ozone layer and contribute to climate change. The targeted chemicals are HCFCs primarily used as refrigerants, specifically R-22, -142b, -123, -124, -225ca, and -225cb. These chemicals, if leaked into the atmosphere, deplete the stratospheric ozone layer and are being phased out under the Montreal Protocol. According to scientists, by cutting the production and import of these chemicals and limiting how they can be used, the rules will help the ozone layer heal faster.
As of Jan. 1, 2010, R-22 — what had been the primary refrigerant used in aircraft ground support preconditioned air units, home air conditioners for the building trades, industrial and commercial air conditioners, and other applications — could not be incorporated in equipment for new installations. This was not a surprise; indeed, Europe had banned HCFCs in 2005. In the United States, manufacturers of industrial and commercial air conditioners and the construction industry were ready for the change. They had decided on a new “standard” refrigerant, designed the equipment, and had an inventory of units ready to meet the requirements of new projects as they came along. PCA acquisition documents, however, were still listing R-22 as recently as December 2010, and some PCA manufacturers were struggling with the reality that design work was a necessity and that components for the “legal” refrigerants cost more than those for R-22.
The U.S. ground support industry stood and watched as 2009 passed and 2010 rolled in with new rules for preconditioned air equipment. Neither the airlines, nor the airport authorities, nor the PCA suppliers had come to any consensus as to what direction to take for an application that required economical, effective cooling. Unitron conducted a survey of airlines and airports in 2009. This survey asked which refrigerant their organization had selected to meet the requirements of the new environmental law, or if they had made a selection. Most respondents had made no selection, and some did not realize that a “deadline” loomed. Part of the reason for this was past experience with projected EPA mandates, some that were delayed time and time again. The final guidelines for phasing out R-22 (and similar refrigerants) were not passed until Dec. 28, 2009, but they were passed and the start date remained Jan. 1, 2010. Manufacturers’ PCA brochures and “cut-sheets” contributed to the airline and airport view that nothing needed to be done, because the existing designs could meet both the legislative requirements and also the performance issues.
R-22 PCA brochures and specification sheets have listed R-134a and R-407c as alternative refrigerants for years. What they have not said is that the 60-ton PCA shown on the R-22 data sheet will only provide 40 tons of cooling when operated on R-134a or that an R-134a unit that supplied 60 tons of cooling was about 30% larger and 20% to 30% more expensive. Refrigerants and performance issues related to an industry awakening to a new reality are detailed below.
Understanding Alternative Refrigerants
What alternatives to R-22 are legally acceptable and available? The government’s answer is: “Most are HFCs and include R-134a, R-404A, R-407C, and R-410A. In the United States, R-410A is the most popular choice for home air conditioners.” (http://www.epa.gov/)
Each refrigerant listed above has characteristics that determine its economic impact and technical acceptability for the preconditioned air application, which has relied on R-22 for the past 40 years.
R-134a: The advantage to selecting R-134a is that as a single component refrigerant, it is not subject to any of the difficulties that arise with mixtures. However, R-134a has a 40% lower refrigeration capacity than R-22. In practical applications, using this refrigerant requires larger heat exchanger sizes and bigger compressors to provide the same capacity and efficiency attained with R-22. This requires more sheet metal, a larger equipment footprint, and higher manufacturing costs. This refrigerant cannot be used in existing R-22 systems as a “drop-in” replacement, but requires new, dedicated equipment. R-134a systems with the same cooling capacity as the R-22 equivalent cost about a third more.
R-404a: This refrigerant is a “low-temperature” mixture (three parts, including R-134a) and is most commonly used in food storage reefers or freezers. Low temperature in this case means sub-zero (32°F down to 2°F or lower).
R-407c: The advantage in selecting R-407C is its ability to be used as an alternate refrigerant in systems designed for R-22 for some environments. When retrofitting equipment that is in use, the R-22 refrigerant and mineral oil must be carefully and thoroughly removed before charging, as R-407C is not miscible with the mineral oils typically used in R-22 systems, requiring instead a synthetic lubricant. When used in systems designed for R-22, R-407C performs with about a 5% reduction in efficiency. A primary disadvantage of R-407C is that, as a zoetropic refrigerant, it experiences fractionation (the mixture does not maintain a constant composition across phase changes). Saturation temperatures may vary up to 9°F (a phenomenon known as temperature glide) in the refrigeration cycle because at a given pressure, the blend boils/condenses at different temperatures. This can be a particularly serious issue if the system develops leaks and performance diminishes. When re-charging the system, refrigerant cannot simply be added because the leak rates of the individual components are not equal due to the fractionation effect. The system must be completely evacuated. The previous charge must be recovered and disposed of or recycled. A complete re-charge of unused refrigerant is required to ensure the proper mixture and desired performance.
R-410a: The refrigerant blend R-410A is a nearly azeotropic refrigerant blend and does not experience fractionation upon change of phase, and very little temperature glide (\
Refrigerant Selection: Environmental Considerations
Refrigerant selection goes beyond the legal aspects pointed out in the previous paragraphs. PCA manufacturers share the same environmental responsibilities that are being imposed on the construction and commercial air conditioner manufacturers, including power consumption and “Total Equivalent Warming Impact.” Some European manufacturers are touting the advantage of R134a because of its low “Global Warming Potential.” However, if one is concerned about the environment, then one needs to consider three attributes of any refrigerant. These are:
1) Ozone Depletion Potential: A material’s ozone depletion potential (ODP) is a measure of its ability to destroy, compared to CFC-11 (R11), stratospheric ozone
2) Global Warming Potential: The global warming potential (GWP) of a greenhouse gas is an index of its ability, compared to CO2 (which has a very long atmospheric lifespan), to trap radiant energy
3) Total Equivalent Warming Impact: An air conditioner’s total equivalent warming impact (TEWI) is based on the refrigerant’s direct warming potential and the indirect effect of the energy that the air conditioner uses.
Any new air conditioner design must employ a refrigerant that provides the cooling required for the application, and all three of the refrigerant attributes shown above in order to be both a legal and also a responsible design. For any refrigerant, the ozone depletion potential (ODP) is by definition a discrete number and must be zero for new designs as mandated by national and international law.
The global warming potential (GWP) is also a discrete number by definition. Its primary significance in refrigerant selection is the direct warming potential component of the total equivalent warming impact (TEWI).
The TEWI is the more important attribute to consider when selecting a refrigerant for a new design, even though the number varies with the environment. The TEWI reflects the direct warming potential, and it also reflects the indirect effect of providing energy to achieve the cooling required. Therefore, a more efficient refrigerant requires less energy, which results in a lower TEWI. See the quote that follows: “Gopalnarayanan (1999) examined eight different refrigerants as possible substitutes for R-22. He found that for all eight refrigerants, the direct GWP effect of a refrigerant represented less than 7.5% of the TEWI at performance rating conditions. This means that the indirect effect of a system’s energy efficiency is more than 13 times more important than GWP. Therefore, TEWI, which considers both the direct and indirect global warming effects of a refrigeration system, is a much more accurate indicator.
In addition, Gopalnarayanan (1999) also found that of all eight refrigerants examined, Puron refrigerant (R-410A) had the best performance from the point of view of energy efficiency and TEWI.” (PURON® REFRIGERANT R-410A)
An Argument for R-410a
Some European manufacturers are saying that R-134a is the best refrigerant to select for the PCA due to its GWP (1300 as compared to 2000 for R-410a). I am saying that R-410a is a better refrigerant, because it is more efficient and has a lower TEWI despite its higher GWP. In addition, the coils of an R-410a system can be as much as 20% smaller than an R-134a system. Thus, the R-410a system is physically smaller, is more efficient, and requires less refrigerant for the same cooling.
When legal acceptance, physical size, cooling performance, and cost are all considered, then R-410a is the logical choice for the PCA application. Some may try to argue that components — expansion valves, for example — are available for R-407c but not R-410a, because R-410a is a newer refrigerant. However, components are available. Manufacturers have been developing them since the “worldwide” transition started 11 years ago.
When the acquisition cost for a given performance, the energy efficiency related to the cooling medium, and the environmental impact of the refrigerant are all considered, R-410a is the clear choice for the PCA application.