NFPA 12 Standard on CO2 extinguishing systems recognizes two types of carbon dioxide systems. The first type is the familiar high pressure CO2 systems, and the second type is the low pressure CO2 system. The basic difference between the two types of systems lies in the method of storing the carbon dioxide.

The high pressure system utilizes DOT spun steel storage cylinders. These cylinders are kept at room temperature and the pressure within the cylinder varies according to temperature. At a 70 degrees F ambient temperature, the internal pressure in such a unit would reach 850 PSI. These cylinders are available in 50, 75 and 100 pound capacities.

On the other hand, the low pressure storage unit maintains the CO2 in a refrigerated pressure vessel. Typical storage temperature is 0 degrees F with a corresponding CO2 vapor pressure of 300 PSI. The refrigerated storage concept uses an ASME coded pressure vessel with a 350 PSI working pressure. Such units are available in standard capacities from 1 1/4 through 60 tons. Larger units have been made for special applications.

CO2-Low Pressure