Fluorocarbons & Sulphur Hexafluoride: Safety Aspects

When selecting a refrigerant, safety issues cannot be overlooked. The major drawbacks of HFC alternatives are the safety characteristics. Ammonia and some hydrocarbons, whilst they are excellent refrigerants, are toxic, and incase of hydrocarbons extremely flammable. They therefore should be used with care, in conditions of safety demanding maximum attention and strict precautions.

The Table hereunder illustrates some of the characteristics of different refrigerants from a safety perspective.

The toxicity rating is given in terms of the Occupational Exposure Limit (OEL). The 1000 ppm limit on non-toxic substances is to prevent any danger of suffocation due to oxygen displacement.

Flammability properties are characterised by the concentration limits in which combustion can take place and by the auto-ignition temperature.

Toxicity and Flammability Characteristics of Certain Refrigerants
Compound Refrigerant Number Boiling point (° C) AOLE (ppm) AOSE (ppm) Cardiotoxic LOAEL (ppm) Flammability Limits (Vol % in Air) NFPA
AOEL : Acceptable Occupational Long-term exposure Limit (ppm)
AOSEL : Acceptable Occupational Short-term exposure Limit (ppm)
NFPA - National Fire Codes, 2000
HFC-32 R-32 -51.8 1000 3000 350 14.4-28.9
HFC- 125 R-125 -48.9 1000 3000 100 none
HFC-134a R-134a -26.2 1000 3000 80 none
HFC-143a R-143a -47.2 1000 3000 300 8.2-17.4
HFC-152a R-152a -25 1000 5000 150 3.9-16.9
Carbon dioxide R-744 -78.4 5000 30 15 none
Propane R-290 -42.1 1000 / / 2.1-9.5
Propylene R-1270 -47.7 / / / 2.4-11
n-Butane R-600 -0.5 800 /
/ 1.9-8.5
Iso-Butane R-600a -11.7 / / / 1.8-8.4
Ammonia R-717 -33.3 25 35 / 15.5-28

AMMONIA

Ammonia is toxic and inflammable. The use of ammonia is now restricted to the industrial sector and to some specific applications such as refrigerated storage facilities, dairies and breweries. In such situations the technology requires supervision by skilled personnel. However this set-up involves a 30% to 40% cost premium compared to using less dangerous fluids, for additional safety measures, and can also lead to lower efficiency and increased energy consumption.

CARBON DIOXIDE (CO2)

In several standards (EN, ASHRAE) the use of CO2 refrigerant is limited to 0.1 kg/m3. This limitation helps to limit individual risks of using CO2 .

For instance in a Research Report1 Professor J. Berghmans (Katholieke Universiteit Leuven, Belgium) concludes that “It can be concluded that the risks related to the use of CO2 in heat pumps are low and acceptable if the charge is limited”.

Considering CO2 new developments in refrigeration technology, its toxic effects were documented by the US EPA.

Limits for passenger exposure

  • 3 % limit CO2 short-term exposure limit (STEL) averaged over 15 minutes (US EPA – SAE 7th Alternate Refrigerant Systems Symposium)
  • Suffocation in the case of lasting concentrations of more than 8% (short term exposure not critical)

Human Exposure Data

  • Functional impairment (5% for about an hour)
  • Dizziness, muscle twitching unconsciousness (7-10% exposure for a few minutes)
  • Unconsciousness / Death (18% for less than 1 minute)

Conclusion from the Presentation : “Proposed charge of CO2 could expose occupants to unacceptable risk unless mitigated. CO2 acceptable if engineering solutions limit potential passenger compartment concentrations to safe levels”

HYDROCARBONS

Hydrocarbons are very inflammable, explosive, and thus dangerous. A French Decree forbids the use of flammable refrigerants in Public Areas ("Etablissements recevant du Public"). (Arrêté du 14 février 2000 - Article CH 35: Production, transport et utilisation du froid - "L'emploi des fluides inflammables et explosifs est interdit").
Reference: (article CH 35)

 

The mostly used Hydrocarbons are:
In the refrigeration and air conditioning :

  • butane (R-600) and isobutene (R-600a)
  • propane (R-290)or propylene (R-1270)

As foam blowing agents :

  • pentane isomers – n, iso or cyclo

The very low concentration that is required to sustain combustion is an indication of their high level of flammability. Their use therefore requires additional safety measures, and can also lead to lower efficiency and increased energy consumption.

More information on the use of Hydrocarbons in refrigeration :

HYDROFLUOROCARBONS - HFCs

Most of the HFCs in practical use are non-flammable, and are unquestionably the safest refrigeration fluids to use. They can be used in a large number of applications and present minimum risk, even in the case of an accidental leak.

HFCs are practically non-toxic and non-flammable, providing the only perfectly reliable option applicable without risk in any situation, be it industrial, in public areas, in residential complexes or in transport systems.

HFCs are thus the only refrigerants that do not present safety problems and significant risk when used in public areas at large scale.

Most HFC refrigerants are non-flammable, as illustrated by HFC 134a. However, HFC 32 is flammable, but therefore it is practically always used in a mixture with other HFCs, carefully formulated to ensure zero flammability throughout their full operating range and in leakage situations.

HFC-152a which has been proposed for Mobile Air conditioning is flammable, and can produce the highly toxic fluorhydric acid (HF) when burning.

1 Energy efficient and environmental friendly heat pumping system using CO2 as working fluid – COHEPS