Misconceptions About CO2 & N2O Cylinder Manifolds
You’re probably thinking that the tanks connected to cylinder manifolds are gas, not liquid. If that’s what you’re thinking, you are incorrect. Don’t feel bad, most people are not aware that CO2 (carbon dioxide) and N2O (nitrous oxide) cylinders are filled (approximately 70%) with liquid. Because these cylinders contain liquefied gas and most all medical applications require gas, we are dependent on the liquid to boil or vaporize inside these cylinders at a rate sufficient to sustain the usage. How much gas can we withdraw from a CO2 or N2O cylinder in an hour? A rough rule of thumb is 50-scfh (23.6 lpm) per cylinder maximum at 70° F (21° C). So, it’s important to realize that there is a limit to the amount of gas we can hope to withdraw in a given time period.
It’s also important to realize that the number of cylinders manifolded directly relate to the amount of gas which may be withdrawn. Ambient temperature is the third element of the equation. Unfortunately, this author has been unable to locate any flow vs ambient temperature graphs for CO2 or N2O. It is fair to say though that the flow decreases as the ambient temperature decreases. Until the 2012 edition of NFPA 99, CO2 and N2O cylinders were not permitted to be installed in locations where the ambient temperature fell below 20° F (-7° C). This author remains concerned and in opposition to the recent change in the NFPA 99 2012 edition to permit CO2 gas CO2 and N2O cylinders and N2O gas manifolds to be installed in locations where the ambient temperature may be as low as -20° F (-29° C). As stated, the necessary Engineering flow vs ambient temperature documentation does not exist to substantiate this change.
Another common misconception is that heaters installed either in the manifold headers or manifold cabinet will increase the rate of flow. The ambient temperature of the cylinder (where the liquid resides) is the determining factor of flow rate. The heaters serve a different function – which is to prevent the primary regulators from freezing up. When the gas decompresses (from cylinder pressure to the pressure regulated by the primary regulator) it chills, in extreme flow rates as much as 100° F (38° C). So, if the gas enters the primary regulator at an ambient temperature of 70° F (21° C) it could chill to as cold at -30° F (-34° C). The orifices thru which the gas passes from the high-pressure chamber to the low-pressure chamber of the primary regulator are very small and can easily be blocked by ice crystals which will form in high flow conditions. Heater designs vary among manufacturers in efficiency, cost and safety. Some manufacturers heaters heat the manifold piping to such an extreme that the headers connected to the cabinet are too hot to touch. Tri-Tech Medical has a unique, highly efficient, heating design in which ceramic heating elements are fastened to the high-pressure chamber area of the primary regulators.
To this point, we’ve only discussed flow. The pressures of CO2 and N2O gas cylinders are 838-psig and 745-psig (respectively) at 70° F (21° C). There have been many Engineering studies done graphing cylinder pressure vs ambient temperature. As the ambient temperature increases, the cylinder pressure increases. At 100° F (38° C) the pressure in a N2O cylinder is approximately 1,000-psig. We have received many calls from facilities where they did not understand this phenomenon. The requirement for cylinders to be sheltered from direct sunlight is due in part to the high cylinder pressures which may be achieved when a cylinder ‘bakes’ in direct sunlight. It is also true that as the ambient temperature decreases, the cylinder pressure decreases. At 20° F (-7° C) the pressure in a N2O cylinder is approximately 400-psig and at -20° F (-29° C) the cylinder pressure is approximately 200-psig. If you plan to install a CO2 or N2O gas manifold outdoors, you’ll want to verify that the manifold you select will operate properly under these conditions.
What conclusions can we make from all of this information? Probably the best way to ensure trouble free operation from your CO2 or N2O cylinder manifold is to install it in a sheltered, climate-controlled atmosphere. We can also conclude that the number of cylinders manifolded per bank should be sufficient to provide the peak gas flow rates and the duration of days desired between cylinder change-outs.
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