On 22/01/13 3:10 PM, Rohan McLeod 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 3/ the likely duration in the event of a bush-fire ? 4/ Can't one then calculate the minimum spray flow in litres / sec and the total volume of water required ? 5/ Can't one then make an informed decision about, whether such a system is physically and financially practical in a particular situation? I can only comment on the system I know, which is the protection system used on a typical tanker (a lot smaller area to protect than a house!). For that space, the water usage is 150 litres/minute, and additional passive shielding is added, such as the vehicle body and reflective curtains for the windows, as well as individual protective clothing.
I suspect that a house would require considerably more water. Having some means to cover windows (e.g. shutters) would help reduce the total radiant heat load. Also, as a house is not mobile and covers greater length, the time of exposure would be potentially greater. However, you can use additional protection such as appropriate garden plantings (e.g. succulent plants are highly fire resistant, due to the high water content in their leaves/stems). The CFA have information on planting for fire prone areas that might help with the garden planning side of things. The bottom line is "defence in depth", a bit like network security - having multiple defences which work together to achieve the desired level of protection. I think it is feasible to build an active protection system, but it would be most effective (in both performance and cost) as part of an overall fire resistant house and garden design. And no, I don't have any papers to quote on this... yet! Of course, YMMV, and consult someone with more knowledge before going down this road. -- 73 de Tony VK3JED http://vkradio.com