There have been a few rare but potentially fatal accidents involving exploding compressors due to an effect known as diesel effect. If the gas mixture being compressed contains enough air, the heat of compression can ignite the refrigeration oil in the cylinder, much the same way diesel oil is ignited in a diesel engine. The heat of compression plus the heat of combustion from the refrigeration oil then ignites the refrigerant in the cylinder, creating a dramatic increase in pressure which blows the compressor apart. Note that this is not just a phenomenon limited to flammable refrigerants, but can happen with A1 rated refrigerants such as 134a or 410A. How?

Refrigerants are rated for flammability according to ASTM E681 at a temperature of 60°C. Many refrigerants that will not burn under ASTM E681 conditions will burn at higher pressures and temperatures, including R-22, R-134a and R-410A. It is worth noting that in tests where they were trying to create diesel explosions, the University of Tokyo found no significant difference between the behavior of A1 refrigerants (R-22, R-410A) compared to A2L refrigerants (R1234yf, R32). They also found that compressing refrigerant and air mixtures without refrigeration oil did not create a diesel explosion. The refrigeration oil had to be present.

So how can we avoid compressor diesel effect explosions? Simply put: keep the air out. With no oxygen you cannot have an explosion. Air is never good for any refrigeration system anyway. It should not be news to anyone who has studied refrigeration at all that air does not belong in a refrigeration system. However, you may not realize that leaving air in the system not only hurts system performance and reduces the equipment life, it can create a real hazard to service technicians. Here are a few precautions you can take to avoid the specter of a diesel effect explosion in your refrigeration system.

• Check new installations for leaks using nitrogen and repair any leaks in the system.
• Never use compressed air or oxygen for leak testing refrigeration systems.
• Thoroughly evacuate the lines and coil of new split system installations AFTER verifying they don’t leak.
• Never pump a system down into a vacuum. Reducing the low side pressure to a vacuum can suck in air through leaks or incorrectly positioned service valves.
• Never jump out safety controls such a low-pressure switches. Forcing a system to run when it is low on refrigerant creates the possibility of sucking in air through leaks on the low side.
• Don’t simply add charge to systems that are low on refrigerant. This is especially true for systems that are significantly low. You should find and repair the leaks.

You can read the results of the University of Tokyo study here. https://hpc2017.org/wp-content/uploads/2017/06/o324.pdf