Some factors on which the safety of refrigerants are judged include toxicity, carcinogenicity, mutagenicity, and flammability. Recommendations as to where various refrigerants should and should not be used and specification of toxic levels and flammability limits are available from several sources. The data from the different sources do not always agree, although general conclusions can be drawn about, for example, their relative toxicity and flammability.
One safety classification of refrigerants that has been in a state of flux as the writers of the standard attempt to keep pace with the development of new refrigerants is ANSI/ASHRAE Standard 34–92, Number Designation and Safety Classification of Refrigerants. The safety classifications established in Standard 34–92 are carried over into ANSI/ASHRAE Standard 15–94, the premier standard for industrial refrigeration in North America. Standard 34–92 establishes a matrix formed by levels of flammability and toxicity, as shown in Table 12.7. The table also lists an example or two of refrigerants classified in each category.
Underwriters Laboratory has classified some of the industrial refrigerants with respect to their hazardous effects on life. Several of the groups are:
• Group 2. Gases or vapors which in concentrations of about 1/2 to 1% (5,000 to 10,000 ppm) for durations of exposure of about one-half hour are lethal or produce serious injury: ammonia.
• Group 5. Gases or vapors which in concentrations somewhere between 2% and 20% by volume for durations of exposure of about 2 hours are lethal or produce serious injury: HFC-22 and carbon dioxide.
• Group 6. Gases or vapors which in concentrations of 20% by volume for durations of exposure of about 2 hours do not appear to produce injury. It is hoped that HFC-134a will fit into this category.
The U.S. Occupational Safety and Health Administration (OSHA) lists maximum permissible exposures, as shown in Table 12.8.
The American Conference of Governmental Industrial Hygienists defines two Threshold Limit Values (TLVs). One is the TLV-TWA, which is the time-weighted average concentration for a normal 8-hour day, 40-hour week, to which
nearly all workers could be exposed without adverse results. The TLV Short-Term Exposure Limit (TLV-STEL) is the maximum exposure for a 15-minute period repeated no more than four times a day. Adopted TLVs are shown in Table 12.9.
Threshold limit values for the new HFC refrigerants are not yet established, but tentatively HFC-134a will have TLVs that are similar to HCFC-22.
An older set of exposure standards for ammonia that is reproduced in Table 12.10 was not quite so stringent as the two standards mentioned above.
Table 12.11 presents results of tests of 1- to 3-minute duration conducted on seven volunteer subjects are presented in Table 12.11.
Some refrigerants are flammable. The ranges of percentages by volume in air constituting flammable limits of several industrial refrigerants are
Carbon dioxide, HFC-134a, and HCFC-22 are not considered to be flammable refrigerants, although the combustion of a 50% mixture of air and HCFC-22 at pressures higher than 1380 kPa (200 psi) can be triggered by high temperatures.
Many halocarbon refrigerants will decompose when exposed to high temperatures from flames or electric resistance heaters. The products of the decomposition, such as hydrogen chloride and hydrogen fluoride, may be irritating and even toxic.
In summary, all refrigerants should be handled carefully. Even the halocarbon refrigerants, which are the safest, can be injurious and even fatal in high concentrations. The hydrocarbons are quite flammable and should be
used only in plants already outfitted to avoid flames and electrical sparks. Ammonia is the most toxic of the industrial refrigerants and has flammability limits somewhere between those of the hydrocarbons and the halocarbons. The flammability limits of 16% to 25% by volume in air correspond to 160,000 to 250,000 ppm, which is 500 to 1000 times the concentration considered toxic. No human could function in an environment where ammonia could burn. Some refrigeration professionals suspect that many accidents reported as ammonia explosions were actually ruptures of pipes or vessels. Leaks of natural gas may also cause an explosion that results in discharges of ammonia.