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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 />

<strong>2018</strong>/02/<strong>15</strong> 2:41 PM<br />

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