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The use of carbon dioxide as an extinguisher is closely linked to the spread of electrification. It was in particular the Bell Telephone Company that, since 1914, stimulated the search for an electrically non-conductive chemical to extinguish fires on its telephone switchboards.

The Walter Kidde Company developed, for these requirements, the first portable CO 2 extinguisher and in the 1920s automatic systems that used carbon dioxide were already available.

In 1928 the first NFPA standard for carbon dioxide extinguishing systems began to take shape. Colourless, odourless, non-corrosive, electrically non-conductive, carbon dioxide does not participate in combustion reactions and, therefore, if it is poured onto flames, it moves oxygen (and the vapours that can be ignited) away from these thereby extinguishing them.

Released from refrigerated or compressed storage at high pressure, the gas also expands, cools down, strikes the materials involved in the flames and removes their energy, thereby acting on two of the vertices of the fire triangle (even if the main action is to remove comburent from the reaction).

Typically used in closed rooms, these systems can also be employed in open environments, exploiting localised applications and leveraging on the discharge time and the dynamics of carbon dioxide which, denser and heavier by 50% than the surrounding air, can create blankets that flood the protected spaces.

A CO2 system does not cause damage to structures, furnishings or to protected assets and does not leave residues or decomposition products; naturally present in the air, carbon dioxide has no application limitations and costs that may be related to the use of other gaseous extinguishing agents.

CO2 systems have a good gas penetration rate in the areas to be protected and are effective on a wide range of flammable and combustible materials.

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Saturation or localised extinguishing CO2 systems

CO2 systems, among the most widespread gas fire protection systems, can be assimilated to clean agent systems, in terms of storage and diffusion methods (total flooding or localised application). Virtually the only extinguishing agent used in localised applications, CO2 on board a machine is used on flammable products. These are systems that are able to create, at local level, particular conditions of carbon dioxide concentration but they are only suitable for the extinguishing of superficial fires. Total flooding systems, with longer action times than inert gas or halocarbons, must saturate the atmosphere of the protected space. In fact, with surface fires, extinguishing is fast on fires that have penetrated to the bottom; it is necessary to keep the atmosphere inert for a sufficiently long period to avoid the resumption of flames.

Extinguishing strategies of CO2 systems

Being cheaper than other extinguishing agents, CO2 is used according to a different extinguishing strategy, the so-called prolonged or slow discharge, which accompanies or follows the main discharge, continuing to diffuse a sufficient amount of CO2 to restore the quantity of gas that is dispersed.

Unfortunately the concentration of CO2 necessary for extinguishing is such as to prevent human survival, imposing particular precautions and limitations.

Low and high pressure CO2 systems

Unlike other gas extinguishers, CO2 has a singular chemical-physical behaviour, sublimating from the solid state, without passing through the liquid one. In low-pressure systems, in which carbon dioxide is stored in refrigerated tanks at 18 bar and especially suited to address many medium-to-large hazards, almost half of the discharge, it reaches the flames in the form of dry ice particles. 

This provides greater local application, effectiveness and greater cooling capacity; with a high pressure system, in which the CO2 is stored in cylinders up to 70 bar and in fact only approximately one third consists of dry ice particles. The cylinders of high-pressure systems each have their own valve which, once opened, completely drains the contents; the reserve capacity requires an additional series of cylinders and a distribution system that allows the passage from one cylinder to another.

Low-pressure systems, less flexible and less suitable for minor risks, however, more effectively exploit stored carbon dioxide, have less discontinuity in the transition between discharges and are more easily scalable. The low-pressure system, much more bulky and heavy than a high-pressure system, needs to be kept refrigerated and requires maintenance and checks that are much more frequent than a high-pressure system, which, on the other hand, requires greater attention to the sealing aspects both over time and in the filling phases. In general, a low pressure system shows its advantages when quantities of CO2 greater than one ton are required or where several hazards must be protected against within a single structure or in other particular design solutions.

Typical applications of CO2 systems

Aside from the afore-mentioned dimensional variables, carbon dioxide systems are used for the protection of industrial machinery, data centres, substations and electrical panels, process systems in chemical and pharmaceutical industries but also in museums and where protected assets must be safeguarded from the risk of corrosion and wetting.

The main limitation for use of the system is due to the dangerousness of CO2 which at use concentration is dangerous for the occupants. The main reference standard for this type of system is the NFPA 12 Standard on Carbon Dioxide Extinguishing System.

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