Diesel Effect Compressor Explosions

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 more about the research into compressor diesel effect explosions here. https://hpc2017.org/wp-content/uploads/2017/06/o324.pdf

Flammable Refrigernats

I confess that I have always thought of flammability as an either or question: it either burns or it doesn’t. So the concept of different levels of flammability was a hard one for me to grasp. I wondered: what is the difference between 3,2, and 2L refrigerant designations? What follows is a somewhat lengthy discussion of what I learned.

First off,  found that it is not all that simple. There are several flammability characteristics that can be compared: lower flammability limit, upper flammability limit, auto ignition temperature, minimum ignition energy, heat of combustion, and flame velocity. The table at the bottom of the article shows these different specifications for a small selection of flammable refrigerants. Note that pressure and temperature also play a part. For the ASHRAE safety tests, a temperature of 140°F at atmospheric pressure is specified. You get different results when applying higher pressures and temperatures.

The original three classifications (1,2,3) were determined by the lower flammability limit and the heat of combustion. A refrigerant is classified as highly flammable, Class 3, if  either it requires 3.5% or less less by volume for a flammable mixture or it has a heat of combustion equal to or exceeding 19 kilojoules per gram. Note that EITHER condition will place it in class 3. Class 2 refrigerants require a concentration greater than 3.5% by volume to create a flammable mixture and they must have a heat of combustion less than 19 kilojoules per gram. Note that BOTH conditions must be met in order to be classified as class 2. Later, ASHRAE added a 2L category for refrigerants with burning velocities less than 10 centimeters per second. The table below summarizes the different flammability classifications.

Classification	Lower Flammability Limit % by volume	Heat of Combustion	Burning Velocity
1	Does not support combustion at atmospheric pressure
2L	Greater than 3.5%	Less than 19 kj/g	10 cm/s or less
2	Greater than 3.5%	Less than 19 kj/g	Greater than 10 cm/s
3	3.5% or less	19 kj/g or more	NA

Lower flammability limit (LFL) is the minimum percentage required in air to be combustible. For example propane (R290) has an LFL of 2.1% by volume while ammonia (R717) has an LFL of 15%. Notice that propane only requires 2.1% while ammonia requires 15%. So that is one difference – the amount that must build up before it can burn.

Upper flammability limit (UFL) describes the maximum concentration which will still burn. If the concentration of flammable vapors exceeds the UFL, it will not ignite. It is more difficult to draw a straight line comparison using the UFL. However, you can say that refrigerants whose LFL and UFL are closer together are generally a bit safer simply because the conditions dor a flammable mixture are less likely to occur.

Auto ignition temperature is the lowest temperature at which it spontaneously ignites in normal atmosphere without an external source of ignition. With the exception of 1234yf, the lower flammability refrigerants have higher auto ignition temperatures than the more flammable refrigerants.

Minimum ignition energy is a bit different than the auto ignition temperature. It is the minimum amount of energy required to ignite a flammable mixture, measured in megajoules. Note that in this case R1234yf stands out because the minimum ignition energy is so high compared to the other refrigerants. Also note that the class 2L refrigerants all have minimum ignition energy ratings in the hundreds of megajoules or higher while propane’s minimum ignition energy is a very small 0.25 megajoules. Basically, this means it takes a lot more energy to ignite the 2L refrigerants than a highly flammable refrigerant such as propane. Again, this means that the chance of having the right condition for combustion is much lower for class 2L refrigerants.

Heat of combustion is a measure of the amount of heat created when the refrigerant burns. Note that the class 2L and class 2 refrigerants have a heat of combustion in the single digits per gram while propane jumps to 46 kilojoules per gram. This means that the heat produced by combustion of a class 2L or class 2 refrigerant is far less than a class 3 refrigerant. Indeed, it would be possible for a class 2L refrigerant to burn and not ignite other nearby flammable materials.

Burning velocity is the characteristic which distinguishes 2 and 2L refrigerants. It is the speed with which the flame advances. Note that the 2L class refrigerants have a burning velocity in the single digits while 152a, a class 2 refrigerant, has a BV of 23 cm/sec. Propane’s burning velocity is twice that of 152a. The take home point here is that the flames from higher flammability refrigerants spread faster.

So wrapping it up, my general impression is that lower flammability refrigerants are less likely to burn in the first place and when they do burn, the flames are not as hot and do not spread as quickly as a high flammability refrigerant such as propane.

  

Refrigerant	R1234yf	R32	717 Ammonia	152a	290 Propane
Safety Group	A2L	A2L	B2L	A2	A3
Lower Flammability LImit	6.5%	14.4%	15%	3.9%	2.1%
Upper Flammability Limit	12.3%	33.3%	28%	16.9%	10%
Auto Ignition Temperature	405°C	648°C	651°C	440°C	455°C
Minimum Ignition Energy	5,000 – 10,000 mJ	30 – 100 mJ	100 – 300 mJ	0.38 mJ	0.25 mJ
Heat of Combustion	9.5 kJ/g	9 kJ/g	22.5 kJ/g	6.3 kJ/g	46.3 kj/g
Burning Velocity	1.5 cm/sec	6.7 cm/sec	7.2 cm/sec	23 cm/sec	46 cm/sec

Sorting Out Refrigerant Flammability

Most techs know that ASHRAE Standard 34 originally established three categories of refrigerant flammability: 1,2, and 3. They ranged from 1 – nonflammable, to 3, highly flammable. Category 2 was listed as mildly flammable, or somewhat flammable. Then they added a new category – 2L – for an even lower category of still flammable refrigerant. I admit that I always found that a bit confusing. In my mind, it either burns or it doesn’t.  In order to get a better handle on this I have done some studying.

Two characteristics are used to differentiate category 1 and category 2 refrigerants: the lower flammability limit and the heat of combustion. The lower flammability limit is the lowest percentage concentration of gas in a gas-air mixture that will ignite. Concentrations lower than the lower flammability limit will not burn. Even highly combustible gasses such as gasoline have a lower flammability limit. Refrigerants with a lower flammability limit of 3.5% or less are considered class 3, highly flammable. For comparison the lower flammability limit of gasoline is 1.4%  and propane‘s is 2.1%. Another way for a refrigerant to be considered class 3 is for its heat of combustion to equal or exceed 19 million joules per kilogram. In general terms, it does not take very much class 3 refrigerant to burn and when it does it is very hot.

Class 2  refrigerants have a lower flammability limit greater than 3.5%. It requires more than 3.5% concentration in order to ignite. Class 2 refrigerants do not burn as hot as class 3 refrigerants: their heat of combustion is lower. The lower heat of combustion is important because that is what sets other things on fire. It is possible for a class 2 refrigerant to burn without burning up everything around it.

So where does the 2L come in? Flammability class 2L is really a subclass of 2. Refrigerants with a 2L designation have a burning velocity of 10 centimeters per second or slower. The burning velocity is how fast the flame travels. A burning velocity of 10 cm/s means that the flame will travel about 4 inches in a second. In contrast, the class 2 refrigerant HFC-152a  has a burning velocity of 23 cm/sec.  – a little more than twice as fast.  Propane, a class 3 refrigerant, has a flame velocity of 39 centimeters per second – 4 times as fast.

Why is this important? The flame velocity and heat of combustion are what determine whether or not an explosion can occur. Rapid burning and high heat of combustion expand the air and combustion gasses so rapidly that great pressure is created, blowing things apart. Class 2L refrigerant cannot burn fast enough or hot enough to blow anything up. In many cases, a burning class 2L refrigerant will not even catch other combustible things around it on fire.

To summarize:
Class 3 Refrigerants have a lower flammability ratio of 3.5% or lower and/or a heat of combustion equal to or greater than 19Mj/kg. They burn fast and hot.

Class 2 Refrigerants have a lower flammability ratio exceeding 3.5%. It takes more of them to burn and they do not burn as hot as class 3 refrigerants.

Sub-Class 2L Refrigerants in addition to a lower flammability ratio exceeding 3.5% also have a flame velocity of 10 cm/sec or less. They burn slowly and without releasing as much heat.

Class 1 Refrigerants do not burn.

Substitute Refrigerant Safety

As R22 prices increase and R22 availability decreases, many technicians have at least thought about using a substitute refrigerant in place of R22. Usually, we are looking for a drop-in which can be used to charge a system instead of R22. My primary consideration is safety. Of course I want any substitute refrigerant to work well and be environmentally friendly, but most of all, I want it to not hurt me or my customers. The refrigerant manufacturer should be able to show that their refrigerant is listed on the EPA SNAP list of acceptable replacement refrigerants. If they can’t, I would not use it. Another source of safety information is the required MSDS sheet. Any chemical sold in the United States must have an MSDS sheet. If the seller cannot provide an MSDS sheet, I would not consider using the refrigerant. The MSDS sheet will list important information such as toxicity and flammability. One replacement refrigerant that is advertised on the internet has enough information on the MSDS sheet to let me know I don’t want any part of it. The proper shipping name is “Petroleum Gasses Liquified.” Other statements include “Vapor may ignite if exposed to static discharge", and "Flammable vapor may form if allowed to mix with air. Accumulation of gas is an ignition hazard. Vapors are heavier than air and may travel to an ignition source." R22 systems are not built to operate safely with a flammable refrigerant. There is no way I would put this in a system designed for R22. Last year technicians were killed while working on refrigerated shipping containers that exploded.(See post about it). They were R22 systems which had been charged with a flammable substitute refrigerant. Although the EPA has recently approved the use of flammable refrigerant in limited charge applications, it is important to note that there are many requirements a system must meet to be safe with flammable refrigerants. R22 systems meet none of these. For more information on flammable refrigerants, take a look at my previous post on Flammable Refrigerants. For more information on R22, take a look at my previous post on R22 Conversion. A couple of simple rules to follow will help keep you safe. Can you buy the refrigerant at your local wholesaler? If you can only get it over the internet or from a guy at a flea market, you don’t want it. Is the refrigerant listed on the EPA SNAP list for the specific application? If not, you don’t want it. Does it seem too good to be true? If so, you probably should avoid it like the plague.