CO2 Freeze: Up Why it happens, what you can do
Under certain condition, users of carbon dioxide gas (from high pressure cylinders), experience "freeze-up" problems on valves, regulators and other compressed gas equipment. The term, "freeze up", refers to a pressure regulator becoming clogged with dry ice snow or crystals, which restrict the flow of gas through the regulator or other pressure control valve. The following explains this phenomenon in an effort to help users avoid problems in CO2 distribution systems.
Why this happens
When high pressure CO2 gas expands through a regulator seat or other flow control orifice, it appears downstream of the orifice on the low pressure side of the regulator as a mixture of gas with solid (snow) or liquid CO2. If the downstream pressure is below 60 PSIG, the mixture is gas and snow; above 60 PSIG, the mixture is gas and liquid.
The amount of solid (snow) or liquid can vary from 0%, at inlet pressures under 800 PSI when the cylinder is cool, to more than 20% under severe freeze up condition when the pressure is above 1100 PSI resulting from a warm cylinder. Contrary to what one might expect, the most severe freeze-up condition with CO2 exists on warm days when a full cylinder is at 90°F or higher and the cylinder pressure is at least 1100 psig. At normal room temperature, and full cylinder pressures at 700-900 psig, the problem exists, but not as severe as under the conditions above.
Solid CO2 cannot form at pressures above 60 psig. It occurs when the gas undergoes the pressure drop at the regulator valve from inlet pressure to a delivery pressure below 60 psig, emerging as a mixture of gaseous and solid CO2 at a temperature in the range of –70°F at 60 psig to -100°F at the lowest pressures. Under the most severe freeze-up conditions, a significant percentage of the mixture can be solid, and under those conditions, about 200 watts of heat /100 scfh of CO2 would be required to vaporize the solid and raise the gas to room temperature.
Why Use A Heated Regulator
Compressed gas regulators normally operate over a range of delivery pressures above and below 60 psig. Unheated regulators, operating at delivery pressures below 60 psig, are therefore subject to classic freeze up with solid CO2. The CO2 snow and dry ice particles may pass through a regulator if the outlet is wide open. If an orifice or flow control valve is used, a filter is needed to prevent the solid CO2 particles from clogging the orifice; and this can result in the low pressure chamber in the regulator becoming completely filled with solid CO2. The severity of the problem depends upon the flowrate of CO2, the inlet conditions, the duty cycle (percentage of time that the gas is flowing) and upon the size of the regulator.
Small single stage regulators are limited to low flow rates and/or low duty cycles. Larger single stage regulators may transfer more heat to vaporize the solid CO2 accumulated inside the regulator, which may handle more intermittent flow, however, under continuous duty, can still experience the freeze-up condition. Unheated two-stage regulators, operating at delivery pressures below 60 psig, exhibit more resistance to freeze up than single stage regulators because the CO2 in the first stage is in the liquid-gas phase (above 60 psig), which absorbs heat more readily from the regulator body than does solid CO2; and any liquid vaporized in the first stage reduces the solids which can be formed in the second stage.
Unheated regulators, even if they avoid the classic problem of freeze up, cannot avoid the refrigerant effect of the CO2. When the pressure drops at the regulator valve, the CO2 temperature drops sharply to the levels stated above, and at normal flow rates, frost can cover the entire regulator and extend to the downstream system. This frost is a result of the moisture in the air freezing and accumulating on the exterior surface. It is not related to the CO2 effects described here and typically have no effect on the performance of the valve.
Heated regulators can relieve or eliminate freeze-up problems. The new Harris model HP 705 has 200 watts of heat to provide a continuous 100 scfh of CO2 under the most severe freeze-up conditions and higher flowrates under normal (intermittent) conditions. The regulators are two-stage, to include the advantages of the two-stage regulators discussed above. The first stage cavity serves as a boiler to vaporize CO2 liquid and eliminate or minimize any CO2 solids in the second stage. The second stage chamber is then available to heat the CO2 vapor before it reaches the outlet.
David Gailey is the manager for Specialty Products for The Harris Products Group, A Lincoln Electric Co. He has been with Harris for 27 years and served as past chairman of the CGA Industrial Gas Apparatus Committee.