The Watermist fire suppression system, in use in packaging manufacturer Nampak’s paper mill, in Rosslyn, Pretoria, uses a fine mist to suppress fires, resulting in little to no water damage to equipment.
The small water droplets in mist evaporate more readily than larger drops and the resulting steam displaces other gases, such as oxygen, suppressing the fire, says mechanical and industrial engineering firm MK Designs co-owner Martin Kruger.
Fires are common in paper mills where excess paper pulp sometimes lands on motors and drive shafts and ignites from heat caused by friction. Fire suppression systems in mills are typically deluge systems that bathe an area in water to suppress fires, he says.
The Nampak paper mill uses steam-filled rollers that would be damaged by conventional sprinkler systems. The mist system sprays small droplets that have a large combined surface area. The large surface area, coupled with the even distribution of the mist throughout an area, means that the rollers in the mill cool down evenly when the system is in operation. A conventional sprinkler system could result in uneven cooling of the surface of the rollers and cause the metal to contract unevenly, leading to cracks and bends, Kruger explains.
“The smaller droplets fall more slowly and evaporate faster. This process absorbs more energy, resulting in fewer or no droplets reaching the floor as water. As each droplet evaporates, it expands more than 1 700 times, and, in the process, displaces oxygen and combustible gases from the fire,” says MK Designs mechanical engineer Mark Adam.
High-pressure electric pumps draw water from a stainless steel water tank for the mist system and force it through a nonreturn valve to a high-pressure manifold. The tank is not pressurised and can be supplied by municipal water reticulation systems.
The water is then distributed to the relevant sections through the main valves. Water is pumped through the 15-mm stainless steel distribution pipes and out of the nozzles at a pressure of 90 bar. The high pressure and nozzle design create a fine mist, Adam says.
“The system effectively combines the advantages of a gas suppression system with a traditional sprinkler system. The mist removes heat up to seven times more effectively than a water sprinkler system,” he says.