Study of Technology for Detecting Pre-Ignition Conditions of ... - NIST

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CPSC-IAG95-1145
primary effect of range type was the amount of time required to bring food to ignition. Although
sugar very quickly ignited on the gas range, oil and bacon needed about-50 ‘/0 more time to reach
ignition conditions on the gas range compared to that needed on the electric range. Sugar
required about 50 ‘/0 less time to ignite on the gas range than it required on the electric range,
but the reason for this is explained in Section 3.7.2. While the gas burner flame is at a higher
temperature than the electric element, the heat transfer rate to a pan is less for this particular gas
range compared to this electric range. One reason for this is that the flame contact area on the
pan is less than the open-coil element contact with the same pan.
The measurements of velocity and temperature in the plume are higher at earlier stages
for the gas range because of the instantaneous manner of burner ignition compared to the gradual
temperature rise of an electric element. The burner post-flame gases themselves contribute to
heating of the area above the burner more than the natural convection from an electric element.
3.7.2 Food Type
Oil and bacon require several minutes to reach boiling, and then smoking, conditions. The
bacon fat must first liquify, as well, before boiling can begin, yet the lesser mass of liquid
produced by the bacon compared to the oii requires less time to heat, vaporize, and produce
smoke. Sugar requires less time than bacon and oil to begin generating smoke, and the
production of smoke is more abrupt and less gradual than that of the other foods. Sugar and
bacon seem to produce darker smoke than oil. Sugar has a unique behavior in that it melts,
resolidifies, and then continues to pyrolyze as an intumescent (bubbling and swelling) char.
A reason for sugar’s faster ignition behavior on the gas range as compared to the electric
range is related to the ring-shaped gas flame and the failure of the heat to spread across the entire
sugar mass equally. As liquids, oil and grease tended to conduct and convect heat more readily
than sugar, and they equalized the temperature distribution. Sugar retained the heat unevenly in
the local area above the burner flame which allowed the lesser amount of sugar to progress to
ignition in less time than all of the sugar would have required. The aluminum clad bottom of
the pan was not completely successfi.d in distributing the heat evenly. It is possible that the test
of sugar on the gas range with the range hood on faiied to ignite because the remaining sugar
acted as a heat sink and a barrier to plume generation which prevented the local area of sugar
transformation from developing enough flammable vapor and high temperature.
3.7.3 Range-Hood Status
The range-hood capacity seemed to be overwhelmed by the plume behavior of the heated
foods. When smoke generation was at steady, high levels, most of the smoke seemed to bypass
the range hood. The largest hood available in our survey had about 75 YO greater flow capacity
than the hood used, but it is likely that such an increase would only partially alleviate the
accumulation of smoke. The high-output gas-range test on sugar with the range hood active did
not produce ignition conditions, while sugar cooked on the same range with the hood inactive did
ignite. It is not known whether the hood played a significant role in this case, if the explanation
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