Why it happens, what you can do (2024)

David Gailey

Undercertain condition, users of carbon dioxide gas (from high pressure cylinders),experience "freeze-up" problems on valves, regulators and othercompressed gas equipment. The term, "freeze up", refers to a pressureregulator becoming clogged with dry ice snow or crystals, which restrict theflow of gas through the regulator or other pressure control valve. Thefollowing explains this phenomenon in an effort to help users avoid problems inCO₂distribution systems.

Why this happens
When high pressure CO₂gas expands through aregulator seat or other flow control orifice, it appears downstream of theorifice on the low pressure side of the regulator as a mixture of gas withsolid (snow) or liquid CO₂. If the downstream pressure is below 60PSIG, the mixture is gas and snow; above 60 PSIG, the mixture is gas andliquid.

Theamount of solid (snow) or liquid can vary from 0%, at inlet pressures under 800PSI when the cylinder is cool, to more than 20% under severe freeze upcondition when the pressure is above 1100 PSI resulting from a warmcylinder. Contrary to what one might expect, the most severe freeze-upcondition with CO₂exists on warm days when a full cylinder isat 90°F or higher and the cylinder pressure is at least 1100 psig. At normalroom temperature, and full cylinder pressures at 700-900 psig, the problemexists, but not as severe as under the conditions above.

SolidCO₂cannot form at pressures above 60 psig. It occurs when thegas undergoes the pressure drop at the regulator valve from inlet pressure to adelivery pressure below 60 psig, emerging as a mixture of gaseous and solid CO₂ata temperature in the range of –70°F at 60 psig to -100°F at the lowestpressures. Under the most severe freeze-up conditions, a significant percentageof the mixture can be solid, and under those conditions, about 200 watts ofheat /100 scfh of CO₂would be required to vaporize the solidand raise the gas to room temperature.

Why Use A Heated Regulator
Compressed gas regulators normally operate over a range ofdelivery pressures above and below 60 psig. Unheated regulators,operating at delivery pressures below 60 psig, are therefore subject to classicfreeze up with solid CO₂. The CO₂snow and dry iceparticles may pass through a regulator if the outlet is wide open. If anorifice or flow control valve is used, a filter is needed to prevent the solidCO₂particles from clogging the orifice; and this can result inthe low pressure chamber in the regulator becoming completely filled with solidCO₂. The severity of the problem depends upon the flowrate of CO₂,the inlet conditions, the duty cycle (percentage of time that the gas isflowing) and upon the size of the regulator.

Smallsingle 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 CO₂accumulatedinside the regulator, which may handle more intermittent flow, however, undercontinuous duty, can still experience the freeze-up condition. Unheatedtwo-stage regulators, operating at delivery pressures below 60 psig, exhibitmore resistance to freeze up than single stage regulators because the CO₂inthe first stage is in the liquid-gas phase (above 60 psig), which absorbs heatmore readily from the regulator body than does solid CO₂; and anyliquid vaporized in the first stage reduces the solids which can be formed inthe second stage.

Unheatedregulators, even if they avoid the classic problem of freeze up, cannot avoidthe refrigerant effect of the CO₂. When the pressure drops at theregulator valve, the CO₂temperature drops sharply to thelevels stated above, and at normal flow rates, frost can cover the entireregulator and extend to the downstream system. This frost is a result ofthe moisture in the air freezing and accumulating on the exteriorsurface. It is not related to the CO₂effects describedhere and typically have no effect on the performance of the valve.

The Solution
Heated regulators can relieve or eliminate freeze-up problems. Thenew Harris model HP 705 has 200 watts of heat to provide a continuous 100 scfhof CO₂under the most severe freeze-up conditions and higherflowrates under normal (intermittent) conditions. The regulators are two-stage,to include the advantages of the two-stage regulators discussed above. Thefirst stage cavity serves as a boiler to vaporize CO₂liquidand eliminate or minimize any CO₂solids in the second stage.The second stage chamber is then available to heat the CO₂vaporbefore it reaches the outlet.

David Gailey is the manager for Specialty Products for The HarrisProducts Group, A Lincoln Electric Co. He has been with Harris for 27years and served as past chairman of the CGA Industrial Gas ApparatusCommittee.