ting pressure that requires a flow rate of 4,000 SCFH with the same 60 percent
beam-on time. This means it would consume 2,400 SCFH. With the same 1,800-
SCFH generator, the fabricator could cut for more than 6 hours.
“You are consuming nitrogen at a rate of 2,400 SCFH and producing it at a rate
of 1,800 SCFH. That’s a shortage of 600 SCFH,” Messick explained. “However, you
have a 16-pack containing 4,000 SCF that will serve as a buffer for the shortage.”
To run at a higher flow rate for even longer, a fabricator could add a second
or third 16-pack of storage cylinders. “Many shops only cutting one shift per day
may elect to go with this configuration or even a smaller system and allow it to
refill after hours,” Messick said.
This buffer also helps when maintaining the system. Nitrogen generators require clean, dry compressed air, so fabricators need to follow a preventive maintenance (PM) schedule on the booster and air compressor. But when they do so,
the system can’t produce nitrogen. During these times, lasers can rely on that 16-
pack cylinder buffer to keep the lasers cutting during those nitrogen system PMs.
Why Not Oxygen?
The physics behind oxygen cutting with a laser requires that oxygen to be very
pure. Although the standard purity of oxygen is 99. 5 percent, laser cutting benefits from higher oxygen purity levels. According to a publication from Linde,
“Our minimum specification for oxygen purity for laser cutting is 99. 95 percent.”
Various laser machine manufacturers recommend the same.
The high purity level requirement is a little counterintuitive, considering
many shops use shop air to laser-cut thin stock. But the nature of laser cutting
changes when oxygen is the assist gas. While nitrogen (and shop air, which is
mostly nitrogen) evacuates material from the kerf, oxygen assist gas also stimulates the cut with an exothermic reaction, making for efficient cutting of thick
carbon steel. And again, the cost-benefit analysis of gas generation looks much
different, depending on how much oxygen a laser cutting operation really uses.
PSA oxygen generators are available, only in this case they use a different molecular sieve (made of a material called zeolite) designed to separate oxygen
instead of nitrogen. The issue lies with the purity level attained, usually between
92 and 95 percent.
Why does conventional PSA work so well for nitrogen but not oxygen, at least
for the recommended purity levels for laser cutting? Marc Kornbluh, president
of High Volume Oxygen, which produces industrial oxygen generation for glass-blowing and other fields, said it has to do with oxygen and argon molecules being difficult to separate. “The molecular sieve separates the nitrogen, but some
of the argon ends up going with the oxygen,” he said.
Although High Volume Oxygen has looked into the laser cutting market, and
even commissioned a study on ultra-high-purity oxygen generation last year, it
has since refocused toward its existing oxygen generation business. Besides its
current presence in glass-blowing and the veterinary industry, the company is
looking to expand its presence in other metalworking fields, including welding
and flame cutting.
“We’ve been involved with oxyfuel cutting applications using a gas torch,”
said Bob Schleher, vice president of sales and marketing at North Tonawanda,
N. Y.-based Oxygen Generating Systems Intl. (OGSI). “The purity is still an issue
with those applications, but it depends a lot on the fuel you’re using. In some
cases, 93 percent [pure oxygen] can certainly work. Plenty of people who use
propane [as a fuel gas] swear by [the oxygen generation] technology.” Generating oxygen when using gasoline as a fuel gas has shown particular promise. He
added that OGSI has sold oxygen generation systems for brazing applications
Certain oxygen generation systems that achieve 99 percent purity utilize mul-
tiple stages—but again, they aren’t designed for laser cutting. According to On
Site Gas Systems’ website, “The 99 percent PSA oxygen generator is a multistage
system that starts with a 95 percent oxygen generator. The 95 percent oxygen
is then processed through a second stage to produce 99 percent oxygen, and
again through an oxygen booster in the final stage. The gas is then available in a
tank at the desired pressure.”
Could one PSA system generate both nitrogen and oxygen? According to sourc-
es, this isn’t likely. “A PSA system is optimized for one gas,” Schlehr said. “In oxy-
gen generation, could you use the nitrogen that’s separated out? Well, as soon as
you do that, you alter the production of oxygen.” If a fabricator does wish to gen-
erate both nitrogen and oxygen for various applications—flame cutting, welding,
laser cutting, and more—it probably will be doing it with two different systems.
Oxygen generation for laser cutting may not have taken off (at least not yet),
but nitrogen generation certainly has, and that trend doesn’t look like it will be
changing anytime soon.
Senior Editor Tim Heston can be reached at email@example.com.
High Volume Oxygen, 402-476-0555, www.highvolumeoxygen.com
On Site Gas Systems, 888-748-3429, www.onsitegas.com
Oxygen Generating Systems Intl. (OGSI), 800-414-6474, www.ogsi.com
South- Tek Systems LLC, 888-526-6284, www.southteksystems.com