On Tue, 22 Jan 2013, Rohan McLeod <rhn@jeack.com.au> wrote:
Assembled cognoscenti; it being the bush-fire season my thoughts turned to the practicability of protecting buildings , using external water-spray systems. My thoughts went something like this 1/ If one knows the air temperature , humidity, wind velocity, distance to, and composition and dryness of surrounding bush . 2/ Can't one calculate the radiant heat in KW/m2 impacting on the building ?; and
I don't think it's the heat on the overall building that matters. It's the heat coming through the windows, to leaves in the gutters, to exposed wooden parts of the building, etc. After bushfires you see chimneys still standing, so it's obviously possible to make bricks that won't be significantly damaged by fire. Are regular brick walls sufficient? If not how much more does it cost to use fire bricks? Will a fire make a galvanised iron roof hot enough to bend or to set fire to wooden beams in the ceiling? If the former then a water spray would help, but I suspect that the latter would be the only problem in which case iron roof beams would be the solution. It seems to me that if a building was constructed to be heat resistant and had solid shutters on all the windows (EG hard wood shutters with gaps too small for embers with a sheet of galvanised iron on the outside) then you probably wouldn't need external water. But if the building has gaps that allow embers in (EG a typical tile root), has wooden window frames, has big windows with flammable curtains, or many other features of typical homes then even a water spray won't necessarily solve the problem.
3/ the likely duration in the event of a bush-fire ?
The radiant heat will be of very small duration. Wind-blown embers will last for a much longer time.
Because such systems don't seem to receive much attention from media or insurance companies ; I assumed the answer must be that they are usually not practical. So I was somewhat surprised on reading : http://en.wikipedia.org/wiki/External_water_spray_system
If I was living in an area that's at risk of bush fire then I'd get have an entirely concrete house built with solid shutters on the windows. -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/