ASHRAE Journal - May 2023 - 80

REFRIGERATION APPLICATIONS
COLUMN
Andy Pearson
Nonflammable?
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE
Last month's column took a light-hearted look at the complicated and serious issue of
toxicity. This time around the spotlight is on the other side of the safety classification
coin: flammability.
With flammability, dictionary definitions are not
much help when it comes to the specifics of refrigeration.
They generally talk about substances that " can
burn easily " or are " capable of being easily ignited, "
but we need to turn to refrigeration standards to put
this in context. Flammability class 1 is often described
colloquially as " nonflammable, " but ASHRAE Standard
34-2022 doesn't define flammability explicitly and the
definition of class 1 in the standard is actually " no flame
propagation. " The standard does define
flame propagation as " any combustion that
moves upwards and outward from the point
of ignition. " This leads us further down the
rabbit hole: combustion is defined in the
dictionary as " the act or instance of burning "
and burning is defined as " consuming
fuel and giving off heat, light and gases. " My
take on this is that anything that is being
oxidized in a process that produces heat,
light and oxides is being burned; however,
this includes a lot of substances classified as having " no
flame propagation " by Standard 34. In fact, I only found
seven substances listed in the standard that could truly
be described as " nonflammable. " They were helium
(R-704), water (R-718), neon (R-720), argon (R-740),
carbon dioxide (R-744), nitrous oxide (R-744A) and sulphur
dioxide (R-764). These are all either products of
combustion that have already been oxidized (for example
R-718, R-744, R-744A and R-764) or are noble gases
(R-704, R-720 and R-740), which, like the nobility, don't
mingle with anything. All the other flammability class 1
substances, even the perfluorinated ones like R-125 and
R-218, are capable of being oxidized given an ignition
source that is hot enough, so in that sense, even they
can be burned.
80
ASHRAE JOURNAL ashrae.o rg M AY 2023
Eddie didn't see it coming.
Of course, the hazards that various refrigerants present
when they are oxidized vary enormously. Some, like
R-152a, R-290 or R-600 will ignite easily and burn so
rapidly that they cause a very rapid rise in air pressure,
producing shock waves that can blow down walls and
send debris flying for long distances. Others, like R-32,
R-1234yf and R-717 are more difficult to ignite and the
flame front spreads more slowly. This often means that
the pressure rise is more gradual and so the destructive
power is much less, but given the right
fuel to air mixture the flame front will still
spread once ignited and typically has the
power to blow out windows and dislodge
ceiling tiles. Refrigerants that burn but
don't show flame propagation under laboratory
test conditions (those in flammability
class 1) present a different sort of hazard.
The combustion does not usually produce
rapid, destructive pressure rises (although
it can in some circumstances), but the
oxidation frequently produces highly toxic products
of combustion. These substances were frequently the
cause of illness suffered by technicians using halide leak
detection torches, and they have also been linked to sick
building syndrome.
The hard-pressed designer of the system needs to do
a competent risk assessment of all foreseeable hazards
in order to complete a demonstrably safe design. In this
case it is necessary to remember another favorite saying
of Professor Joachim " Joe " Paul, whom I quoted last
month on toxicity. Joe was fond of citing Murphy's Law:
" If it can go wrong, it will go wrong, " adding what he
called Joe's Law- " Murphy was an optimist. "
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in
Glasgow, UK.
CC BY 3.0 DAVID SHANKBONE
http://www.ashrae.org
ASHRAE Journal - May 2023

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