A Passing Gas?

Little in recent memory has stirred emotions like this issue. Replacement of ozone-depleting refrigerants like R-12 and soon R-134a, presents a serious challenge to the auto manufacturing community and close on their heels, the aftermarket repairer. The fact is, CFC and HFC (chloro-fluorocarbon and hydro-fluorocarbon) based refrigerants were adopted because they work, are relatively cheap and are inert enough to be safely used, stored and handled. Unfortunately, one of the few chemical species they will attack is atmospheric ozone, which, like sunscreen on a Florida beach, is essential to keeping damaging UV radiation at bay. Conventional refrigerants have to be replaced, but by what? Ammonia has been used in commercial refrigeration, but it’s a deadly poison as well as corrosive. Carbon dioxide research is under way, but it involves scary pressures (2000 psi has been described in the literature) in expensive, sealed non-serviceable units.

With a considerable service history in stationary applications, especially in Europe, it was inevitable that hydrocarbon blends would be tried in mobile applications. Normally a blend of propane and a form of butane (the same gas used in cigarette lighters, in a purer form) hydrocarbon refrigerants will produce cold vent temperatures when used as a “drop-in” replacement for R-12 or R-134a. As refrigerants, they work, but the main objection from opponents is the more pressing issue of safety. Are they safe? Like many politically-charged and highly polarizing issues, the answer isn’t a simple “yes” or “no”.

How flammable?

The gases used in hydrocarbon drop-in refrigerant replacement products are ethane, propane, isobutane, normal butane, isopentane and normal pentane, with refrigerant numbers of 170, 290, 600a, 600, 601a and 601 respectively. Propane and isobutane, common blended HC refrigerants, are both common fuel gases. They’re flammable when vented to the atmosphere and ignited through burners such as gas barbeques and hand-held torches. The safety issue in motor vehicle use, however, is how flammable are they inside a vehicle passenger compartment? All flammable substances, whether hydrocarbon refrigerants, motor fuels or campfires, require two additional environmental factors besides the fuel itself: an oxidizer and sufficient heat to start the chemical reaction that we call “burning”. In most cases, the oxidizer is the oxygen in the air, while the heat source can be anything from a burning cigarette to an electrical short circuit. Automotive service technicians are better able to understand the process than the public because of their understanding of vehicle fuel systems, which operate by similar principles. Inside the combustion chamber, getting the correct ratio of air to fuel is so important that a computer is assigned to add the right amount of fuel via the injector pulse width. The injector itself atomizes the fuel to get it into a form with lots of surface area to react with the compressed air in the cylinder. For a hypothetical leak of a flammable refrigerant gas into a passenger compartment, the situation is similar, although the terminology is different. In this case, the lean condition is called the ‘lower explosive limit’ and the rich extreme the ‘upper explosive limit’, both of which are analogous to lean and rich conditions in a gasoline engine. For a leak into a passenger compartment of a car or light truck, the critical measure is the lower explosive limit, since it would be reached first in a ruptured evaporator situation. Since all refrigerants exist as gases at normal atmospheric pressure, the key determinants to a fire or explosion are rate of gas leakage, the extent of gas mixing with the ambient air in the passenger cabin, the rate of air exchange in the cabin, and the presence of an ignition source.

Spectacular video images of car interiors exploding after a hydrocarbon refrigerant leak were provided to the media by several major A/C component manufacturers, and HC refrigerant manufacturers responded with similar graphic evidence from escaping R-134a and lubricant mist. But is this scenario likely?

Two readily available analyses of the risk of hydrocarbon refrigerants in automotive use are by Colbourne and Ritter of U.K.-based Calor Gas Ltd., and Ian Maclaine-cross of the University of New South Wales in Australia. The Colbourne and Ritter paper uses quantitative risk analysis to determine the likelihood of an HC incident, while the Maclaine-cross paper is an engineering analysis of HC combustion in the passenger compartments of popular passenger cars such as Toyota, Nissan and Volvo.

On the engineering side, the Maclaine-cross paper sates that with the rate of air exchange in a modern passenger car and the crack propagation properties of aluminums used in evaporator and refrigerant line construction, gas concentrations up to the explosive limit are highly unlikely. His findings also conclude that electrical sources such as shorting wiring or arcing would not ignite the mixture, with hand-held cigarette lighters as the primary ignition risk. In his closed cabin model, Maclaine-cross determined that cars manufactured after 1985 have a significantly higher rate of air exchange than earlier models, even with vents and windows closed, reducing the risk of high concentrations of flammable gas. He also notes that the catastrophic failure needed to dump large amounts of gas into the passenger compartment quickly, such as a severed high-side line or ruptured evaporator, is unlikely without an accident serious enough to shatter the window glass and windshield. “Unlikely”, however, is not the same as “never”. Statistically, Colbourne and Ritter’s paper cites a U.S. Department of Energy (Dieckmann and Bentley 1991) that studied risks associated with crashes and hydrocarbon refrigerants.

Explosion data was 2-3 x 10-9 for modern cars. This data, interpreted more conventionally, suggests that passenger compartment fires should occur at rates of about one in three million car-years, while the likelihood of explosion is roughly one in three billion car-years. Another study quoted by Colbourne and Ritter quotes the frequency of car crash due to a hydrocarbon refrigerant at one in ten-million car-years, about the same frequency as human lightning strikes. These studies show very small risk numbers, but automotive manufacturers and the U.S. Environmental Protection Agency have shown little enthusiasm for hydrocarbons. Both the U.S. agency and Environment Canada do not recommend HC refrigerants in systems not designed for them and major component manufacturers will not honour warranties if non-R134a refrigerants are used. Some hydrocarbon suppliers have answered with their own warranty programs and even replacement parts like compressors that they certify for HC use. Most also add odour-causing agents that should warn abut leakage before levels become dangerous, as is used with home fuel gas systems.

Why would a technician use hydrocarbon refrigerants? Lower head pressures, lower vent temperatures, low cost and ease of use are major factors and the low impact on the environment is another, as HC’s are not ozone-depleting. Technicians will still need the same certification to handle them however, and all service rules and regulations regarding auto A/C systems still apply. Common sense precautions, such as careful handling and avoiding smoking while servicing the system are a good idea. A safe practice would be to treat a hydrocarbon install like you would a pressurized fuel rail or a tank drop and think safety. Why won’t OEM’s endorse them? Opinions vary and off the record, industry players cite everything from a conspiracy by the fluorochemical industry to hyper-conservative OE engineers scared by their legal departments. Opponents cite safety concerns and the lack of a single, definitive standard for risk, or universally accepted test results. Either way, hydrocarbons are out there, and as a technician, you will encounter them, either flowing into automotive A/C or during recovery. Check SSGM ne
xt month for more on this topic.