African Petrochemicals- Mar/April Edition 15_2 {2018}
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one side of the triangle, mainly heat in the<br />
case of water based systems, and oxygen in<br />
the case of gas based systems. Water mist has<br />
the advantage that due to the small particle<br />
size and distribution means, it beaves like a<br />
gas suppression agent and has the cooling<br />
properties of a water suppression system.<br />
Water mist is divided into primary and<br />
secondary extinguishing mechanisms.<br />
Primary Mechanisms:<br />
Heat extraction<br />
• Cooling of the fire plume<br />
• Wetting and cooling of the fuel surface<br />
Displacement<br />
• Displacement of oxygen at the flame front<br />
• Dilution of fuel vapour<br />
Secondary Mechanisms:<br />
Radiation attenuation<br />
Kinetic effects<br />
When comparing water mist particle size<br />
to traditional sprinkler and spray systems,<br />
the heat extraction effect that mist presents<br />
is magnitudes larger than conventional<br />
systems. As the water particle decreases,<br />
the surface area exposed for heat transfer<br />
increases. Water mist is therefore able to<br />
absorb/remove more heat per volume of<br />
water than conventional systems. Water has<br />
a sensible heat capacity of 4.2 kJ/kg.K and<br />
a latent heat of vaporization of 2442 kJ/kg.<br />
As the surface area of the droplet increases,<br />
more area is exposed for vaporization and<br />
therefore optimal heat extraction. Water<br />
mist therefore drastically cools the fire<br />
plume, primary mechanism, and prevents<br />
escape of radiation to other fuel sources,<br />
radiation attenuation which is a secondary<br />
mechanism.<br />
As the fine water droplets are rapidly<br />
converted to steam at the flame front, the<br />
oxygen and fuel vapour is displaced due<br />
to the volumetric expansion ratio of water<br />
to steam, the primary mechanism. Water<br />
expands 1700 times when it evaporates to<br />
steam. This expansion effect is seen as the<br />
secondary mechanism and enables the fire to<br />
“suck” in more water mist. These properties<br />
enable mist to be extremely effective on fires,<br />
especially Class, A, B, C and F fires.<br />
The Danfoss-Semco SEM-SAFE® water<br />
mist system is a high-pressure water mist<br />
system that comprises of state of the art<br />
technology. Water mist has many advantages<br />
compared to conventional sprinklers. One<br />
of the key benefits is that it often uses only<br />
one tenth as much water as traditional<br />
sprinklers. In addition, the majority of the<br />
water evaporates, causing much less water<br />
damage to buildings, furniture, electrical<br />
installations, etc. In industrial applications,<br />
the dramatic reduction in direct damage is<br />
of great value, but it is usually even more<br />
important that the production line or factory<br />
will be able to resume manufacturing<br />
very quickly after the fire. A long period of<br />
down-time in production means a high risk<br />
of losing market share, which often results<br />
in a dramatic long-term negative economic<br />
effect, far in excess of the original fire<br />
damage. Statistics show that only 4 out of<br />
10 industries are able to start manufacturing<br />
goods again after a major fire.<br />
Fire catastrophes are a threat to human<br />
life, cultural and industrial assets, private<br />
and corporate property and even our<br />
environment. From complex fire suppression<br />
systems for museums and heritage sites, to<br />
industrial applications over office buildings,<br />
universities and wind turbines, Danfoss<br />
Semco has a successful track record within<br />
commercial and industrial applications using<br />
the SEMSAFE® water mist system. Why?<br />
Because the SEM-SAFE® system provides<br />
better protection than existing water systems<br />
while being 100% environmentally friendly,<br />
harmless to users and with far less impact<br />
on buildings and interiors. It significantly<br />
reduces lifecycle costs, asset damage and<br />
operational downtime in all ‘business critical’<br />
applications, and offers the best protection of<br />
irreplaceable valuables. This offers a greatly<br />
enhanced cost/benefit ratio when compared<br />
to traditional systems.<br />
For more info contact Sperosens<br />
Tel: +27 (086) 177 3767<br />
Fax: +27 (086) 562 6511<br />
E-mail: marihette.hattingh@spero.co.za<br />
Web: www.spero.co.za<br />
It’s all good<br />
when your process<br />
is efficient<br />
Every project is unique and therefore deserves a unique<br />
approach. Afrox offers tailor-made process solutions for<br />
its customers. This is supported by a range of industrial<br />
gases as well as a team of dedicated specialists<br />
addressing the need for safer and more efficient<br />
operations.<br />
Africa’s leading gases and welding solutions partner.<br />
Customer Service Centre: 0860 020202 Shop online: www.afroxshop.co.za www.afrox.co.za<br />
Industrial gases ad 190x130.indd 1<br />
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