User Manual

Siemens Switzerland Ltd 25
Building Technologies Division
4.3.3. Chemical powder
Extinguishing powder relies on interrupting the combustion process by interfering with the chemical reaction and
the associated extraction of energy. Powder is mainly used in fire extinguishers. One disadvantage of this type of
extinguisher is that it leaves a very fine residue which is difficult to remove and is highly corrosive. The powder is
also incompatible with electrical and electronic equipment.
4.3.4. Water
The most commonly used extinguishing agent is water. Water is not only an effective extinguishing agent; in most
cases it is readily available, ecologically acceptable and inexpensive.
The extinguishing effect of water relies not only on the cooling of the burning material but also on the
suppression of oxygen by the steam that is generated. Water has a very high latent heat of vaporization, which is
four times that of all other non-combustible liquids. Water is absolutely non-toxic and can be stored at room
temperature and pressure.
The boiling point of water (100 °C) is considerably lower than the temperature range where the pyrolysis (thermo
chemical decomposition) of solid combustible material can be expected to occur (250 - 400°C). It therefore
permits the surface temperature of the fuel to be cooled to well below these temperatures.
However, water also has a number of disadvantages and can itself cause considerable damage to property. As it
boils at 100 °C and freezes at 0 °C, it can also lead to damage of any water-carrying pipes. Water is a conductor of
electricity, making its unsuitable for electrical fires. It is ineffective in extinguishing certain combustible liquids,
particularly those that are not water soluble or float on water (such as oil, petrol or diesel). Water is also
unsuitable for some hot metals (e.g. magnesium) or for chemicals. For such materials other extinguishing agents
must be used, for example foam, gas or powder.
4.4. Extinguishing systems
4.4.1. Gas extinguishing systems
The main application area for gas extinguishing systems is the protection of closed rooms. They are particularly
suitable for rooms that contain sensitive objects or equipment where water cannot be used. These typically
include all types of electrical equipment and computer rooms.
The gases are stored in pressurized cylinders:
• The non-liquefiable inert gases Ar, N
2 and gas mixtures are stored in gas cylinders at a pressure of 300
bar.
• In high-pressure systems CO
2 (which does liquefy under pressure) is stored in gas cylinders at 56 bar; in
low-pressure systems it is stored in large cooled containers.
• Clean agents such as HFC227ea and Novec 1230 fluid are stored in gas cylinders, pressurized with
nitrogen, which acts as a propellant. The industry’s most common storage pressures are 25 bar
(particularly in the USA) and 42 bar, which allows for higher flow rates, smaller pipe diameters and
longer pipe runs.