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Seven burning questions about mildly-flammable refrigerants

First they depleted the ozone layer, then they contributed to global warming, and now they might exacerbate fires. That is the history of refrigerant substances, in brief. And now the flammability of different refrigerants needs to be taken care of.

The current phase-down of hydrofluorocarbons (HFC) refrigerants is due to their high global warming potential, i.e., for environmental reasons. This is not a big problem as long as they stay inside the device. But in many cases they don’t. They slowly leak into the air, eventually contributing to the effects leading to global warming. As an example, residential air conditioning systems on average leak 8.5 % annually. Therefore, a replacement is needed.

This replacement is often spelled halogen refrigerants. Unfortunately, they are mildly flammable, and this is the main reason why they are not yet widely deployed. There are four safety classes of refrigerant flammability: 1, 2L, 2, and 3, in order of increasing flammability. During the next decade it is expected that new residential heat pumps transition to Subclass 2L halogen refrigerants, which are mildly flammable. The main issues related to flammability are listed below.

How are lower flammability limits (LFLs) measured? The lower flammability limit is one of the requirements needed to be met for 2L classification. It is normally measured using ASTM E681. There are indications that this method could be improved in order to increase the accuracy of the tests. Suggested changes include choice of material and orientation of the electrode.

Can non-flammable (Class 1) refrigerants burn? Not at room temperature. But there are two scenarios where sufficient external heat is applied for them to burn, emitting a large amount of toxic hydrogen fluoride (HF). 1) Many refrigerant leaks are accompanied by aerosolized lubricating oil. In the presence of a viable ignition source, this oil can ignite the surrounding refrigerant. 2) An external fire impinging a refrigeration system, causing a leakage. The resulting refrigerant jet will ignite if it encounters the flames.

Is it hard to ignite a mildly flammable (Class 2L) refrigerant? This classification has a large quenching distance – on the order of 8-25 mm. And the typical minimum ignition energy is 10 J. As a comparison, a typical static electric discharge releases 0.1 J. This means that they can be ignited by open flames or unenclosed yellow-hot heating wires, but not by typical motors, electrical switches, or resistive heating devices such as toasters, hair dryers, and space heaters.

Can Subclass 2L refrigerants suppress some flames? Yes. For example, when a smoldering cigarette is introduced into a stoichiometric 2L/air mixture, the cigarette quickly extinguishes. When a 2L halogen fills a chamber with good mixing, this extinguishes a candle flame before the LFL is reached. These observations are consistent with other findings showing that halogens can act as either fuels or suppressants depending on the conditions.

What can we learn from past experience with ammonia fires? Ammonia is a 2L refrigerant with flammability characteristics similar to 2L halogen refrigerants. Its flammability is well understood, and there is extensive knowledge regarding safety and risk. For example, large ammonia leaks have ignited and resulted in blasts powerful enough to cause structural damage. Ammonia may be a good surrogate for 2L halogen refrigerants in tests since the products of combustion are far less toxic.

Do reliable area monitoring leak detectors exist? Technologies for detecting 2L halogen leaks are normally optimized for either leak pinpointing or area monitoring. For pinpointing, reliable and affordable detectors exist. This is not the case for area monitoring. Such detectors will operate continuously, preferably without service for many years. Standards are being drafted, but there are no detectors that meet the needed requirements. This gap may delay the release of these standards.

Is hydrogen fluoride (HF) a major hazard?
HF can be produced upon heating even with no flame present. Hydrogen fluoride is toxic. For residential heat pump systems it could be a greater hazard than the other three flame hazards combined: secondary ignition of furnishings, blasts, and thermal injuries. HF volume fractions can be up to 50 % following a 2L halogen refrigerant fire.

Peter B. Sunderland, Univ. of Maryland, USA

This text is shortened by HPC.

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