When evaluating refrigerant options, it is crucial to look at all aspects of the refrigerant, the refrigeration system, and the application. For example, a particular refrigerant may have a lower flammability, but be less efficient and more environmentally damaging.

Too often, limited analysis results in oversimplified judgments – declaring one refrigerant as the universal best choice while dismissing all alternatives. The reality is more nuanced, requiring a comprehensive evaluation of several key factors, which we will explore in detail below.

1. Safety: A Primary Consideration

While the ideal scenario is for refrigerants to remain sealed within their systems throughout their operational life, leaks can and do occur.  This reality demands a thorough evaluation of each refrigerant's safety profile, potential risks, and necessary mitigation measures for specific applications.

Toxicity: Claims of a refrigerant being either "non-toxic" or "toxic" oversimplify a complex issue.  Most refrigerants carry some degree of toxicity.  Air stands as a rare exception and it can function as a refrigerant, though its use is limited due to other factors to specialized applications like aircraft cabin cooling.

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) assesses toxicity in Standard 34 – Designation and Safety Classification of Refrigerants.  This framework categorizes refrigerants into two classes based on their occupancy exposure limit (OEL):

• Class A: "Lower toxicity"

• Class B: "Higher toxicity"

  (The threshold between these classifications is set at 400 ppm.)

  
  

Here is the toxicity classification of several refrigerants, as per ASHRAE:

If the quantity of refrigerant in a system is sufficient that the toxicity or flammability concentrations can exceed the acceptable limits, then the ability to detect a leak is crucial. Most refrigerants are odorless, which presents a safety risk, but ammonia (NH₃) has a unique advantage as a self-alarming refrigerant due to its sharp, irritating, and pungent odor. With an average odor threshold of 5 ppm, well below harmful concentrations, ammonia's distinctive smell ensures that even small leaks are quickly detected and repaired. 

Flammability: Much like toxicity claims, declarations of "non-flammable" refrigerants can be misleading.  Carbon dioxide (R-744) stands as a notable exception – not only does it not burn, but it actually acts as a fire suppressant when released near flames.

ASHRAE Standard 34 provides a systematic classification of refrigerants based on their flame propagation properties, tested under specific conditions at a temperature of 140 °F (60 °C) and a pressure of 14.7 psia (101.3 kPa).

Most current air-conditioning refrigerants fall into Class 1, designated as "No Flame Propagation." However, this classification comes with an important caveat; many Class 1 refrigerants can become flame propagating at higher temperatures and pressures. More concerning is that when exposed to flames, many can decompose into highly toxic substances, including chlorine, hydrochloric acid, hydrofluoric acid, phosgene, and carbonyl fluoride.

For flame-propagating refrigerants, ASHRAE Standard 34 establishes three additional classifications:

1. Class 2L for “lower flammability,”

2. Class 2 for “flammable,”

3. Class 3 for “higher flammability.”

The ASHRAE standard also includes the Lower Flammability Limit (LFL) – the minimum concentration in air required for flammability – though this measurement is included for reference only as it does not affect classification.

Critical reminder: The designation "non-flame propagating" should not be confused with "non-flammable" or "non-combustible."

Here are the safety classes as per ASHRAE of some commonly used refrigerants:

Detection: If the quantity of refrigerant in a system is sufficient that the toxicity or flammability concentrations can exceed the acceptable limits than the ability to detect a refrigerant leak is crucial.   Unfortunately, most refrigerants are odorless, which presents a safety risk.  Ammonia (NH₃) has a unique advantage in this regard – its distinctive pungent odor makes it detectable even at very low concentrations.  Ammonia has been referred to as a self-alarming refrigerant.

For commercial and residential direct cooling applications, safety standards limit govern the acceptable quantity of refrigerant allowed in a space.  These standards also mandate the installation of refrigerant leak detectors in certain applications. These devices provide essential early warning and can automatically trigger other system safety measures to mitigate the risk.

Installation: Comprehensive safety standards restrict when and how a refrigerant can be used in a system as well as the necessary safety requirements to ensure safe operation. It is important to evaluate these restrictions and requirements in the process of making a refrigerant decision.  In the United States, ASHRAE Standard 15 Safety Standard for Refrigeration Systems serves as the widely adopted framework for most refrigerant installations while ANSI/IIAR 2 American National Standard for Design of Safe Closed-Circuit Ammonia Refrigeration Systems applies for ammonia (NH₃, R-717). In Canada, CSA B52 Mechanical Refrigeration Code is adopted for all refrigerants except water and air. 

2. Environmental Impact: A Complex Balance

The relationship between safety and environmental impact presents an unexpected paradox: refrigerants with lower immediate safety risks often carry greater long-term environmental consequences. This reality demands evaluation across several critical environmental factors.

Ozone Depleting Potential (ODP): The 1990s marked a turning point with the phase-out of chlorofluorocarbon (CFC) refrigerants due to their high ODP when released into the atmosphere. Eventually Hydrochlorofluorocarbons (HCFCs) faced the same fate. While current regulations prohibit new systems from using refrigerants with any ODP, this introduced new environmental challenges.

Global Warming Potential (GWP): The hydrofluorocarbon (HFC) refrigerants that replaced CFCs and HCFCs revealed an unexpected drawback – when they leak into the atmosphere they act as global warming agents, typically thousands of times more potent than Carbon Dioxide.

Recent developments have introduced refrigerant blends combining HFCs with mildly flammable Hydrofluoroolefins (HFOs) to reduce GWP while still maintaining the non-flame propagating Class 1 status.  These blends are frequently marketed as "low" GWP refrigerants, however with GWP ratings between 600-1,400 they are far from a low GWP solution.

Even newer HFC-HFO blends classified as mildly flammable 2L refrigerants, while representing progress, still demonstrate GWP ratings hundreds of times higher than CO₂.

In contrast, natural refrigerants have emerged as environmental leaders: NH₃ with GWP=0, CO₂ with GWP=1 (the benchmark), and hydrocarbons with GWP=3.

Contamination: When certain HFC and HFO refrigerants leak into the atmosphere, they break down and produce trifluoroacetic acid (TFA), a "Forever Chemical" that persists for thousands of years. The accumulation of TFA raises growing concerns about long-term environmental and health impacts on ecosystems and living organisms.

Production and disposal: The environmental impact of refrigerants extends throughout their lifecycle.

Energy requirements for production and disposal vary significantly among refrigerants. CO₂ stands out favorably in this regard, as it is a byproduct from other industrial processes, requiring minimal additional energy input.

Manufacturing processes have historically witnessed instances of chemical releases and exposure incidents, with some chemicals used during fluorinated refrigerant production posing even greater concerns than the refrigerants themselves.

3. Efficiency: Beyond Simple Metrics

Energy efficiency in refrigeration systems presents a complex interplay of factors. Energy usage varies significantly based on application, refrigerant choice, and system design. Further, a refrigerant that is very efficient in one situation may not be so efficient in another. 

Refrigerant properties: The vapor compression refrigeration cycle relies on a compressor to maintain a low refrigerant pressure in one part of the system to allow heat absorption into the system at a low temperature while simultaneously increasing the refrigerant pressure in the other part of the system to facilitate heat rejection from the system at a higher temperature. The compressor functions as the system's heart, circulating refrigerant and driving the entire process.  To accomplish this most refrigerants leverage phase change – the transition between liquid and vapor states – to facilitate heat absorption and rejection. Within this fundamental process, refrigerants exhibit vastly different physical properties that significantly influence system performance.

Key physical properties create distinct performance characteristics:

System design: System design plays a crucial role in optimizing efficiency. Performance improvements can be achieved through various engineering approaches: implementing heat exchangers with closer approach temperatures, minimizing refrigerant pressure drops, employing multi-stage compression or economizing, the type of compressor, and even motor cooling means. However, the effectiveness of these design strategies varies depending on the refrigerant choice, as these measures are not equally effective with different refrigerants.

4. Cost and Availability: Scale Matters

The impact of refrigerant costs varies dramatically with system size. In residential or commercial air-conditioning systems, where refrigerant charges are relatively small, the cost of recharging after a leak remains manageable. However, the equation changes significantly for larger commercial and industrial refrigeration systems, which can contain hundreds or thousands of pounds of refrigerant.

While modern system design has reduced typical refrigerant charges, these larger systems present unique challenges. Their complexity – often incorporating hundreds of components and thousands of feet of pipe – increases leak potential. This risk is particularly concerning with odorless refrigerants, where small leaks can result in significant losses before detection.

The cost differential between refrigerant types adds another layer of consideration. Fluorinated refrigerants (F-gases) typically cost dozens of times more than natural alternatives like NH₃ and CO₂. Historical trends have shown that as older fluorinated refrigerants (F-gasses) face regulatory phase-outs or chemical manufacturers cease production, costs tend to escalate sharply as availability becomes increasingly uncertain.

Making the Right Choice

No single refrigerant is perfect for all situations.  Selecting the best refrigerant requires careful consideration of safety requirements, environmental impacts, system efficiency, costs, and availability.

Finally, with evolving refrigerant regulations and advancing technology, staying updated is also crucial.