Associated Fire Protection

To understand how the different fire extinguishants are stored and expelled, we must review some very basic physics. From physics, we know that all substances can exist in either solid, liquid or gaseous form.

Solid Extinguishants: The storage of solids is quite easy. Solids, however, require some force to remove them from the fire extinguisher onto the fire. The solid dry chemical powders require pressure from nitrogen gas to expel them from the tank.

Liquid Extinguishants: Liquids, such as water can also be contained easily. Water extinguishers use pressure from either pressurized air or nitrogen to discharge the water onto the fire.

Gaseous Extinguishants: Halon, Halotron, Carbon Dioxide (CO2), and FE-36 are somewhat more difficult to store since at normal atmospheric pressure they are gases. Unless a gas is totally contained, it will dissipate.

From basic physics we know that the state of a material will change when it is heated or cooled. Water freezes (turns to a solid) at 32°F and boils (turns to a gas) at 212°F. Pressure also has an effect on the state of a substance. If the pressure is reduced, as at high altitudes, water will boil at a lower temperature. That is why it takes longer to cook things in the mountains. Conversely, if the pressure is increased, water will boil at a higher temperature. That's why foods cook quicker in a pressure cooker. This also means that if you have water under pressure, it will remain water at temperature higher than 212°F.

The same principles apply to gaseous extinguishants. However, different materials boil (change from a liquid to a gas) at different temperatures. For example, Carbon Dioxide normally turns into a gas at -50°F, Halon 1211 boils at 20°F, and FE-36 boils at 29.5°F. Therefore, if we want to keep these gases in a liquid form at normal temperatures, the liquid must be held at an elevated pressure. This has advantages and disadvantages. In the case of Carbon Dioxide, the equilibrium pressure is very high, between 700 and 1000 psi, depending on temperature. This high pressure requires a very strong and expensive cylinder, but provides force to propel to extinguishant onto the fire. In the case of Halon, the equilibrium pressure is much less, about 50 psi. Therefore, the Halon 1211 fire extinguisher requires added pressure from nitrogen to expel the extinguishant easily and effectively.