Post by Roger Hayter Post by Spike
Time for some arithmetic, and some basic unit definition.
1 Joule (J) = 1 Watt-second (Ws). They are measures of Energy.
A Watt (or 1 Joule per second) is a unit of Power.
Power is the rate at which Energy is expended.
Half a pint of liquid Butane contains 14.5 MegaJoules (MJ) of Energy.
If this is released in 10 milliseconds, typical of a 'gas explosion' in
a small building, the amount of Power released in that time is 14.5 MJ /
.01 second = 1.45 Gigawatts.
Or, if this Energy was released in 10,000 hours (36 Megaseconds), the
Power would be 14.5 MJ / 36 Ms = 0.4 Watts.
Your electricity meter reads Energy consumed in units called
kilowatt-hours (kWh), which is the Energy consumed in 1 hour (or 3600
seconds) by something using 1000 Watts of Power. So 1 kWh = 1000 x 3600
= 3.6 MJ.
Not really. When the fire brigade talk of a flat fire producing 10MW of
power they are clearly talking about somehting happening over minutes or
tens of minutes and therefore causing a distinct nuisance to adjacent
structures, not somehting happening for milliseconds.
Except if it's a gas explosion, which means that the fire crews will be
Firstly, there were enough posts in this sub-thread to suggest that
people are confused with the meanings of the terms 'power' and 'energy'.
Perhaps I should have posted this as a separate thread. Mea culpa.
Secondly, loose terms like '10 MW' are in themselves unhelpful unless it
is clear from the context that the meaning is either 'power' or an
incorrectly used term for 'energy'. If a fire with an energy expressed
as '10 MW' occurs lasting an hour, then the actual power driving it is
only 3kW , which is hardly an unusual figure in a domestic kitchen
 3000W x 3600s = 10.8 x 10^6 Ws = 10.8 MJ or '10MW'
Thirdly, the thing to come out of these calculations is the surprisingly
high energy contents of organic materials, e.g. butane in this case, or
petrol, or plastic 'insulation', or chip fat. The latter is ~38 MJ/kg,
so a two-pint chip pan fire burning for 10 minutes has a power of (38 x
10^6)/600 > 6kW. Add in some furnishings and you've got a '10MW' fire.
Throw in some tons of plastic insulation round a building, and you have
a serious problem.
I once attended a presentation on fire safety. The ex-fireman giving it
described a fire in a Woolworth store. The alarms went off, but the
people in the cafe on the first floor were having their lunch and so
ignored the warnings until it was too late for them to get out of the
area. He was the first fireman to reach the cafe, and he was in tears as
Never fanny about with fire safety, be aware of the energy content of
materials, plan your escape routes, and have an ad-hoc smoke mask in
mind. The clearest part of a smoke-filled room is the inch above the
floor. Use it to breath, avoid smoke, and navigate to your chosen exit.
I also carry a small torch and a Metropolitan Police whistle - the
latter is *far* more effective than shouting.