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.
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
|
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