How powerful is a shack fire?

In order to understand the power of an uncontrolled fire it is useful to know the typical output powers of controlled sources. To do this, heaters around a typical suburban household in temperate South Africa are considered:

1. Oil Fin Heater: Air Tech OFR-08. Output power 2.5kW.

2. Small three-bar heater, Luxell 2820. Output power 1.8kW.

3. Bathroom heater (two-bar), Goldair 4296DB. Output power 1.2kW.

4. Portable Air Conditioner, Defy ACP12H1. Output power 3.5kW.

Paraffin is an extremely common cooking and heating fuel in poor households. It is effective, burns fairly cleanly and the initial investment in a crude parrafin stove is very small (around US$10.00). Paraffin stoves typically have a fuel reservoir in their base capable of holding several litres of fuel.

These stoves frequently tip or fall over while in use. They spill out their fuel, often coming in contact with the flame as they do so. A liter of paraffin in an uncontrolled fire will typically burn out in around two minutes, determined experimentally.

  • The energy content of paraffin is approximately 45kJ/g.
  • The density of paraffin is approximately 800g/litre.
  • Thus, the energy content in 1 litre of paraffin is 36MJ.
  • If 36MJ of fuel is burned in 120 seconds, the average output power is 36MJ/120=300kW.

An uncontrolled paraffin fire produces around 100 times as much output power as a controlled heat source. Because of this disparity it is practical to detect uncontrolled fires using the “rate of rise” principle.

Most rate-of-rise detectors also feature a “static trigger” feature. This feature causes the detector to trigger immediately if a set threshold is exceeded. Sigthsense has a 63 degree Celsius threshold.

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