into the first CMS tank, which contains roughly 78
percent nitrogen, 21 percent oxygen, and 1 percent
trace gases. As the air enters the tank, the oxygen
is trapped in the CMS material. Under pressure, the
nitrogen molecules react in such a way that prevents them from adsorbing into the small pores
of the CMS. The nitrogen bounces off the CMS and
passes through the tank vertically and out to a low-pressure nitrogen storage tank.
As the first CMS tank becomes saturated (
meaning that material cannot adsorb any additional
oxygen), a pressure swing occurs, hence the name
“pressure swing adsorption.” The second CMS tank
starts to pressurize and begins a separation cycle
while the first CMS tank goes into an exhaust mode.
During the exhaust mode, the pressure is released, which also releases the oxygen from within
the CMS. It is then purged using pure nitrogen, making it ready for the next pressure swing cycle.
After the nitrogen has been separated from the
compressed air and concentrated in a low-pressure
nitrogen storage tank, it passes through a high-pressure booster and into a final high-pressure storage
tank before being sent to the laser. The high-pressure booster is typically sized to take the desired
flow rate up to a storage pressure between 75 and
100 PSI higher than the required pressure at the laser inlet. This provides a healthy buffer between the
storage pressure and the point-of-use pressure.
“High-pressure gas-assisted laser cutting typically
requires 300 to 400 PSI at the inlet to the laser,” Messick said. He added that laser cutting applications
often use what are known as oil-free boosters—and
for good reason. “Any oil carryover could damage the
optics on the laser, resulting in a huge expense of replacement as well as production downtime,” he said.
The sizing of a nitrogen generator is based on the
required purity, hourly flow rate, and pressure. As
you go up in purity, you lose flow rate out of the nitrogen generation system. Conversely, if you go up
in flow rate, you lose purity. To achieve a higher flow
rate and purity, you need a larger nitrogen generator, which has more CMS material with more surface
area, allowing for more nitrogen separation to occur.
“When you talk about efficiencies of a nitrogen
generator,” Messick explained, “you’re considering
how much compressed air it takes to make one cu-
bic foot of nitrogen at a specific purity.”
Higher-purity nitrogen requires a greater volume of
compressed air, lower purity requires less. The high-
er purity also requires more CMS material. Accord-
ing to Messick, “To achieve the greatest cost savings
and efficiency, it’s very important to size the nitrogen
generator to the correct purity for your materials and
the thicknesses you will actually be cutting.”
One issue is that the highest purity a custom fab-
ricator is likely to need may call for an expensive ni-
trogen generation system. One simple solution has
been for fabricators to size a nitrogen generation
system for the vast majority of their nitrogen needs,
then supplement that with liquid nitrogen dewars
for jobs that require it. As OnSite’s Montesi said, “If
you have the occasional thick stainless job, you can
bring in liquid nitrogen dewars just for that point in
Today some systems allow fabricators to adjust
purity levels for what they need, using what Messick
called a “purity exchange” valve. For instance, say
a shop cuts thick stainless only occasionally. This
would call for nitrogen assist gas that’s 99. 99 per-
cent pure. But for the majority of its thin-gauge mild
steel work, a consistent flow of 99.90-percent-pure
nitrogen would suit.
Considering a shop’s beam-on time during a typi-
cal shift, the operation needs a system capable of
producing a volume of nitrogen at 1,000 cubic feet
per hour. This is the instantaneous flow rate the gen-
erator can produce, not the amount of nitrogen ac-
tually consumed throughout the course of an hour.
This amount would provide enough of a buffer to
ensure the generation system’s storage tank always
has the nitrogen the laser cutting machines need.
For most of that time, the lasers need 99.90-per-
cent-pure nitrogen assist gas, but that occasional
run of thick stainless steel ups the nitrogen purity
requirement to 99. 99 percent. To maintain 1,000
CFH at that higher purity level would require larger,
more expensive components in the nitrogen gen-
But there is another way, and here is where the
purity exchange valve comes into play. As Messick
described, a system now can be designed to pro-
duce 99.90-percent-pure nitrogen at a flow rate of
1,000 CFH through the CMS, which is suited for most
of the fab shop’s mild steel cutting work.
But when the occasional thick-stainless job comes
up, the fabricator adjusts the purity exchange valve
to the high-purity setting. This raises the purity to
99. 99 percent, which in turn reduces the flow rate
out of the nitrogen generator. Sure, the lower CFH
wouldn’t be sufficient to support all the cutting the
laser does, but as long as the CFH is lowered on an
as-needed basis (again, for the occasional thick-stainless job), it’s sufficient.
How Much Nitrogen?
Messick added that this shows how nitrogen generation systems have evolved to suit the complex
demands of the custom fabricator, which leads to
another factor: sizing a nitrogen generation system.
If a laser cutting operation cuts consistently day
after day, hour after hour, it’s probably a good idea
to size a generation system based on the peak flow
rate—that is, the highest nitrogen flow needed plus
A carbon molecular sieve, or CMS, adsorbs oxygen and other molecules and allows nitrogen to pass through. Image courtesy of South-Tek Systems.
A 16-pack bank of high-pressure cylinders (on right) serve as a buffer, allowing the nitrogen generation system to
adapt to varying levels of demand.