ALTERNATIVE REFRIGERANT CHOICES: LIMITED BUT PROMISING

A PRIMER FOR UNDERSTANDING THE CHEMISTRY OF REFRIGERANT SELECTION

By:

Jim Parsnow

Director, Environmental Systems Marketing

Carrier Corporation

INTRODUCTION

The current migration away from chlorofluorocarbon (CFC) refrigerants is in direct response to the need to preserve Earth's delicate, protective ozone layer. In fact, Earth is the only planet in the solar system with an atmosphere that supports life. Preserving the ozone is one of the many essential steps we must take to protect life on our planet for future generations. Similarly, the spectrum of alternative refrigerant choices is limited to the basic elements found on Earth.

THE BASIC ELEMENTS

The Periodic Table of The Elements (fig. 1) is a compilation of the basic substances that comprise the planet Earth. Compounds are simply combinations of these basic elements. Not every elements can be used to create a refrigerant compound, however. Elements marked by a single cross-hatch solidify at the temperatures required for refrigeration. The shaded elements are toxic in compound form. Those marked with a double cross-hatch are very rare, manmade or radioactive. Elements marked with a plaid hatch form unstable compounds with boiling points too low for refrigeration applications.

Fig 1.

REFRIGERANT COMPOUNDS

By process of elimination, eight elements remain as candidates for refrigerant compounds: carbon, nitrogen, oxygen, sulfur, hydrogen, fluorine, chlorine and bromine. Focusing on these eight elements, Thomas Migley, Jr.1 formulated the first CFCs used for refrigeration in 1930. Prior to Migley's discovery, the elements used for refrigeration had serious drawbacks. For example, ammonia (NH3) was (and still is) used for isolated industrial process refrigeration. Since ammonia is moderately flammable and toxic, it is not a viable solution for commercial refrigeration. Sulfur dioxide (SO2) was also used as a refrigerant, but its higher toxicity makes it a potential liability in commercial applications (ASHRAE 15 Safety Code for Mechanical Refrigeration2 currently classifies SO2 as a B2 Class, higher-toxicity refrigerant).

One of the most common compounds, water (H2O) is used as a refrigerant (R-718) in absorption chillers. These systems use lithium bromide (salt solution) to quickly absorb and evaporate the water in a vacuum, creating a refrigeration effect. However, the application of water as a refrigerant is limited by several factors: lower efficiency, equipment size, heat sources required, steam, gas-fired heat and first costs.

The elements that hold potential for refrigerant compounds are clearly limited; they are further narrowed by the choices for fluorocarbon refrigerants. Chlorine (Cl) and bromine (Br) have been eliminated as refrigerant elements as they have proven to be catalysts that destroy ozone when discomposed from their compound form.

LIMITED CHOICES

Clearly, the group of elements considered viable presents a limited set of options when applied to refrigerant compounds. The matrix triangle depicted in fig. 2 shows the relationship of the elements Hydrogen, Chlorine and Fluorine combined with a base element Carbon (not shown). This triangle - with each element at an extreme apex - represents all of the compounds that may be formed. Refrigerants located in the left side of the triangle contain either too much hydrogen, and are thus flammable, or too much of both hydrogen and chlorine, and are thus toxic. Today's common refrigerant compounds are found in the right area of the triangle. In figs. 3 and 4, the combinations are shown as both their ASHRAE refrigerant designations (i.e., R-11) and their chemical makeup (i.e.,CFCl3 - one Carbon, one Fluorine and three Chlorine).

fig. 2

fig. 3

For years, chlorofluorocarbon refrigerants - CFC 11 (R-11), CFC 12 (R-12), CFC 113 (R-113) and CFC 114 (R-114) - were preferred for their stability, non-toxicity, non-flammability, thermodynamic properties (ability to cool) and practical use in designing equipment. Now, a new consideration, ozone protection, has forced the elimination of chlorine-bearing refrigerant compounds from the already limited list of viable options. Both international agreements (i.e., Montreal Protocol) and the legislation of individual nations (i.e., U.S. Clean Air Act) mandate the phaseout of all chlorine-bearing or CFC refrigerants.

INTERIM SOLUTIONS: HCFCS

CFC refrigerants, with their long atmospheric life and high chlorine content, are no longer choices for new equipment. The search for alternatives to CFC refrigerants has led to a new focus on the refrigerants found toward the lower center and bottom of the matrix tri-angle (fig. 2). Hydrochlorofluorocarbons (HCFC) refrigerants such as HCFC 123 (R-123) and HCFC 22 (R-22) may be used as interim refrigerant solutions. However, HCFCs are also subject to a phaseout schedule because they, too, contain Chlorine, though in smaller amounts than the CFCs. Global concern over all chlorine-bearing refrigerants has produced legislation mandating the phaseout of all HCFCs through gradual, stepped-down levels of production.3

HCFC 123 is a close replacement for CFC 11 in centrifugal chillers. However, HCFC borders on the toxi-city range, and so must be used with extreme precaution. This compound has undergone numerous toxicity tests in order to qualify for use (it is rated as B1 Class in ASHRAE 152). Far from new, HCFC 123 is more than 20 years old, but until now has remained in the shadows of CFC 11.

HCFC 22 is also more than 20 years old, and is currently the most widely used fluorocarbon refrigerant in commercial refrigeration. It also used in most comfort cooling applications, from room air conditioners to the largest chillers. The large volume of installed HCFC 22 combined with legislation prohibiting HCFC 22 venting should ensure adequate amounts of this refrigerant for extended interim usage.

LONG-TERM SOLUTIONS: HFCS

Given the realities of an accelerated CFC phaseout and a somewhat less accelerated HCFC phaseout, the search for long-term refrigerant solutions must focus on the bottom of the matrix triangle (fig. 2). HFC 134a is the leading choice for a variety of reasons. A close replacement for CFC 12, HFC 134a will grow in demand at a rate that will easily exceed even the quantities of HCFC 22. Given its chlorine-free, phaseout-free status and its extreme stability, HFC 134a is probably the next safest refrigerant to water. The lack of chlorine in HFC 134a did lead to the study of its performance apart from thermodynamic applications. For example, HFC 134a does not mix with mineral oils, a feature that is essential in refrigeration equipment so that lubricating oil can be reclaimed and returned to compression devices. This hurdle has been overcome by the introduction of synthetic refrigerant oils. Today, HFC 134a is being applied in foam blowing, pharmaceuticals, auto air conditioning, refrigeration chillers, residential air conditioning, water fountains, domestic refrigerators and supermarkets.

fig. 4

fig. 5

AZEOTROPES

Although HFC 134a will play a leading role in many applications, other HFC refrigerants found at the bottom of the matrix triangle are also good replacements for CFCs and interim HCFCs. Excluding HFCs that are highly flammable, very high pressure or lack good application thermodynamics narrows the field to five compounds. These compounds have recently been commercialized as azeotropes and zeotropes. Azeotropes are near-compounds - unique mixtures of two or more compounds which have nearly or precisely the same composition in both liquid and vapor states. ASHRAE Standard 344 defines azeotropes as "blends comprising multiple components of different volatilities (evaporating quickly) that, when used in refrigeration cycles, do not change volumetric composition (size) or saturation temperatures as they evaporate (boil) or condense at constant pressure."

Only five base compounds are being used to form azeotropes and zeotropes: HFC 32, 23, 152a, 143a and 125. HFC 134a is also included in these blends, which are being marketed under several different brand names. Understanding that there are only six compounds being used is helpful in sorting through the various refrigerant brands for the future. A partial list of these newly-created mixtures of existing compounds is shown in fig. 5. Allied Signal's AZ20 is a proposed replacement for HCFC 22, although it may not be used as a drop-in due to its nominal 40% higher condensing pressure. AZ50, an azeotrope of HCFC 22 and HCFC 115 (fig. 5), is the replacement for R-502 which was used in the past for low-temperature applications and is itself an azeotrope of CFC 12 and HFC 152. Both AZ20 and AZ50 are azeotropes that match a true compound very closely.

ZEOTROPES AND BLENDS

The remaining refrigerants on the chart in fig. 5 are either zeotropes or blends. Zeotropes are defined by ASHRAE 34 as "blends comprising multiple components of different volatilities that, when used in refrigeration cycles, change volumetric composition and saturation temperatures as they evaporate (boil) or condense at constant pressure. The word is derived from the Greek words zein (to boil) and tropos (to change)." Blends are defined by ASHRAE 34 as "refrigerants consisting of mixtures of two or more different chemical compounds, often used individually as refrigerants for other applications."

The refrigerants from DuPont, HC62 and AC9000, are referred to as zeotropes, as is ICI's refrigerant KLEA 66. As shown in fig. 4, all of the azeotropes and zeotropes are mixtures of the refrigerants from the bottom of matrix triangles in figs. 2, 3 and 4. Since azeotropes perform without volumetric composition or saturated temperature change, they are promising as replacements for new designs.

Zeotropes may fit some limited drop-in applications as well as several replacement design applications. Each application must be reviewed in light of the potential for different boiling and condensing temperature ranges of the zeotropes, commonly referred to as "guide." Guide is defined by ASHRAE 34 as "the absolute value of the difference between the starting and ending temperatures of a phase change process by a refrigerant within a component of a refrigerating system, exclusive of any sub-cooling or superheating. This term usually describes condensation or evaporation of a zeotrope." Simply put, the components of the blend each evaporate and con-dense at different rates. For this reason, zeotropes will not be used in flooded chiller applications where the design requires a constant evaporation/condensation rate.

In addition, as a zeotrope refrigerant leaves a system, its composition may change, leaving behind a mixture that is different from the original virgin blend. This change is called "fractionalization" and is defined by ASHRAE 34 as "a change in composition of a blend by preferential evaporation of the more volatile component(s) or condensation of the less volatile components." The following is an example of fractionalization:

HFC 134a + HFC 125 + HFC 32 ------> LEAK

Nominal: -15.1° F -55.3° F -61.1°F

Boiling Point: -26.2° C -48.5° C -51.7°C

Note: One component of the blend would not leak out singularly, but as a mixture of its next evaporating components, as noted by the plus (+) marks.

FUTURE SOLUTIONS

Testing on azeotropes and zeotropes continues, and these refrigerants will ultimately supersede the HCFC refrigerants now used as interim solutions. A broad range of these substances will be available (see charts at the end of this paper) and brand marketing to target applications will continue.

CONCLUSION

By taking the mystery out of the subject, a basic under-standing of refrigerant chemistry can help decision-makers sort through complex and often confusing claims. The migration away from CFC and HCFC refrigerants makes it important to understand that current options are limited by both the chemistry and the application. Could a "miracle" refrigerant be developed in the future? Perhaps, but the substance will require elements beyond those found here on Earth.

Today's choices are limited, but the HFCs and specifically, HFC 134a, are excellent choices for a future of chlorine-free, non-ozone depleting refrigerant compounds. As HFCs are applied to new technology and continuously improving efficiencies, it is unlikely that they will present other environmental concerns, such as global warming. At this point, any refrigerant selected for long-term use must support an overall system that is safe, environmentally sound, reflecting considerations such as new technologies, waste minimization, containment and energy efficiency. It is clear that today's refrigerant choices must be made with an eye toward protecting and preserving the future of our delicate planet Earth.

REFERENCES

1 Thomas Migley, Jr. directed the research team that developed the first CFC refrigerants in the General Motors research laboratory at the request of Frigidaire Corporation of Dayton, Ohio. In 1937, Migley published "From the Periodic Table to Production: Industrial and Engineering Chemistry."

2 ASHRAE 15-1992 Safety Code For Mechanical Refrigeration, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle NE, Atlanta, GA, 30329.

3 Summary of Montreal Protocol as amended at the fourth meeting of the Parties in Copenhagen, Denmark, November, 1992:

Jan. 1, 1996 Production freeze capped at 3.1% of the ozone depletion potential of the CFCs and HCFCs consumed by a country in 1989.

Jan. 1, 2004 Cap reduced by 35%

Jan. 1, 2010 Cap reduced by 65%

Jan. 1, 2015 Cap reduced by 90%

Jan. 1, 2020 Cap reduced by 99.5%

Jan. 1, 2030 Cap reduced by 100%

4 ASHRAE 34 (currently under change review for new refrigerants), Number Designation and Safety Classification of Refrigerants, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle NE, Atlanta, GA 30329.