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Reducing Costs for Nitrogen Assist Gas

April 2012

By purchasing a nitrogen generator, fab shops that use laser cutting systems could save money, and the savings could be significant.

“It’s really a matter of economics,” says Joe Moretti, president of Industrial Solutions, LLC. “It just makes all the sense in the world for most companies that are doing any significant amount of spending on nitrogen gas as a consumable for their laser cutter to purchase one.”

“I found over the years that it almost always works out for a laser user that he can buy a generator, depending on the system, at the same cost or less than what he can spend monthly for nitrogen, if it’s financed over a five- to seven-year term.”

He adds, “A laser cutter can reduce his costs for consumable nitrogen gas significantly depending on how much they are currently spending on gas and looking into future usage. Savings also depend on the kilowatt hour rate they are paying for electricity.

“In most cases it’s less than, and in many cases it’s a lot less than buying nitrogen. But if a user is spending this money anyway, he has the benefits of owning the system and its tax depreciation benefits. Over the last few years a company could take a 100 percent depreciation for that year, although today it’s less. But the tax write-off pays for a significant amount of the system’s cost. The rest will be paid for with the money that would have been spent on nitrogen”

Economically it makes sense to own a nitrogen generator remarks Moretti. The problem in the industry is that there are a number of companies selling systems that don’t know how to configure them properly, and they don’t supply the correct type of system that is set up properly for laser cutting.

He adds, “Some laser users have been burned, because their systems weren’t properly configured for them. I have been in the metal fabricating business my whole life. My laser cutting background goes back to 1984. Therefore, I understand what a laser user needs in the way of a generator.”

Steffan Albrecht, vice president and CBO of Liberty Systems, says, “Nitrogen generators are most cost-effective when usage is continuous. The more they are used, the better their return on investment is. Ultimately for our systems, 24/7 usage is the best way to get your greatest return on investment.

“However, when a liquid nitrogen gas user gets into the $.80-per-100-cu.-ft. gas usage range, not including rental, delivery or other fees, this is a good time to start looking at a nitrogen generator, whether it’s a pressure-swing absorption (PSA) or membrane style,” he says. “Typically users with these costs are on a micro-bulk system(s). Bulk delivery usually has a lower cost per hundred cubic feet, because the nitrogen is offered at a better rate at this consumption level by the gas companies.”

How they work
PSA

According to Moretti’s website, PSA is nothing more than filtration. Filtration separates different sized particles for the process. There are no chemicals involved, and no waste or hazardous materials are produced.

The air around us is 20.9 percent oxygen and 79 percent nitrogen. Nitrogen molecules are larger and slower moving than CO, CO2 and oxygen molecules. If the larger nitrogen gas molecules are filtered from the smaller ones, they can be isolated, captured and used.

With PSA, nitrogen and oxygen are forced into a tank under pressure using an air compressor. This tank, or sieve bed, is filled with a filter medium: a porous carbon-molecular sieve (CMS) that generates nitrogen. The pressurized, excited molecules move actively throughout and around the CMS that is like pumice or a sponge. The holes of the CMS are so small that the oxygen molecules pass through them and get captured inside the particles, while the larger nitrogen molecules remain outside.

Once this separation process takes place over time, the free-floating nitrogen molecules are released at low pressure and captured in a surge tank. When the nitrogen has been drawn out of the tank (or sieve bed), the pressure in it is released quickly, allowing the oxygen to escape into the air. This release cleanses the CMS for the next cycle. The sieve bed is then re-flooded with pressurized air and the cycle starts again.

PSA sieve technology is used in all of Moretti’s generators to increase the consistency of the nitrogen delivery. The compressed feed air is fed into one sieve bed, and then the feed swings to pressurize the other sieve bed. The CMS adsorbs oxygen to make nitrogen giving the name Pressure Swing Adsorption.

The PSA will essentially permeate at the same rate and purity for the life of the system for at least 20 years. High purity nitrogen is available from this system with a low percentage of hydrocarbons.

Membrane

A simple explanation of how a membrane system works is that compressed air enters one end of a permeable membrane. The membrane is composed of many hollow fibers. The nitrogen in the compressed air travels the length of the fibers and exits at the other end of the membrane. The oxygen in the air passes through the sidewall of the fibers and exits through the sides of the membrane filtering out the nitrogen for use by the laser.

But what’s important for both of these systems and the laser user, is the purity of the usable nitrogen gas, as it can contain hydrocarbons that can damage a laser’s optics system. Both of these systems can be designed to reduce hydrocarbons trapped in the initial compressed air to levels that won’t damage a laser system. However, a PSA system is required to produce the higher purity generally required for laser cutting.

“There are some differences between the two systems, remarks Albrecht. The purity produced by a PSA system can be at the 99.95 percent purity level. The edge color of the material that we cut is typically silver with a PSA system.

“With a membrane generator, there is always some discoloration on the edge of the cut material – although this doesn’t affect its welding characteristics. It’s purely aesthetics, but some people don’t like the discoloration that is caused by the oxygen content being absorbed into the edge of the material.”

However, mentions Albrecht, “We can actually exceed what is produced by liquid nitrogen with a PSA system. Liquid nitrogen is typically what’s referred to as 99.998 percent pure, which is 20 ppm of oxygen within the nitrogen. With our systems, we can actually drive to 10-ppm oxygen. The issue is that once you get to a certain point, the equipment and electrical costs outweigh the fee of a liquid nitrogen source when you do a cost analysis. There is a breaking point where it is unrealistic for us to make nitrogen at this level. Although, we can do it if needed.

“Most metal fabricators only need 500-ppm purity which is 99.95 percent nitrogen. Technically it’s 500 ppm of oxygen, because there is some argon present. Typically when you drive to 10 ppm, this is for a laboratory setting.”

However, Moretti mentions that “99.95 percent purity is not okay for stainless steel cutting on material that’s thicker than 11 gauge. Membrane-type generators for this application are generally set up to produce 95 to 97 percent nitrogen purity.

“With thicker stainless steel, the oxidation may affect the weldability and other benefits of cutting with N2 might be compromised. Certainly the appearance of the edge will be discolored. The thicker the material, the worse it will look.”

He adds, “We always recommend 99.99 gas purity for stainless steel that is over 11-gauge thick. This is especially important in the job shop where a customer has a certain expectation of what a quality cut looks like.”

Which system is best?

Asked what system would be best for a laser user, Albrecht says, “We would probably go with a membrane nitrogen generator when a company is using one or two lasers and is typically cutting 11-gauge and thinner materials. When a user is cutting metal thicker than 11 gauge with one or more laser system, then a PSA generator is the best choice.

 Albrecht says that membrane systems are typically less expensive. However, delivery pressure is usually lower. A PSA system usually has a booster compressor that adds to the cost of the system. A membrane system might not need a booster compressor, but this is what limits the system to 11-gauge material and thinner.

“We are always trying to drive to the lowest purity level, because the cost of equipment is the least expensive,” says Albrecht. “Just about every system has an air compressor, and the air compressor’s horsepower increases as your nitrogen purity or flow rate increases, adding to the system’s initial cost.”

System requirements

Asked what a company should it be looking for when it purchases a system, Moretti says, “They should be looking for efficiency and a system that is sized properly for their application or for future applications, which constitutes the pressure available from the system and the correct purity. Some companies that laser-cut carbon steel might not require a high level of gas purity compared to other fabricators that might be cutting stainless steel where higher levels of purity are needed.”

Moretti also mentions that a nitrogen generator seller should understand the current and future needs of the laser user to get the correct volume and purity of produced nitrogen for the application.

“Every system that we sell is specifically sized for the particular customer’s needs for their cutting operation. I give my customers a pledge that if their system doesn’t perform as I told them it would, I will buy it back within 90 days after it’s installed and running. But I’ve never had to do this, as everyone knows what they’re getting before I sell it to them.

System maintenance

System size really doesn’t matter, says Moretti, as nitrogen generators all have similar maintenance needs.

“Everyone in the industry understands what it takes to maintain an air compressor,” says Moretti. “This is the first component of a PSA system. These would include some post filters on the air compressor system. The generator has three pre–filters. My systems have controllers and monitors that will tell the operator when these need to be changed.

“Next is the booster pump that brings the nitrogen up to pressure and needs similar maintenance to an air compressor,” he continues. “But my booster pump is an oil-free one. Some companies that sell nitrogen generators are using oil-bathed compressors. What happens with these is that after you make clean nitrogen, you push it through a compressor or booster pump that will contaminate this nitrogen with oil or hydrocarbons that have to be filtered out before they get to the laser system.

“If you do get hydrocarbons into the laser, it can cause havoc with the laser optics, as it will contaminate the lenses and mirrors,” Moretti explains. “This is a critical part of a laser machine and its generated nitrogen. The nitrogen generator company must not use an oil-bathed compressor. Otherwise it can cause expensive problems for the laser.”

A PSA-booster pump will occasionally need valve maintenance, and then after a certain number of hours, the booster will need to be overhauled, mentions Moretti. Basically these items are the only maintenance that his PSA systems require.

Moretti concludes that his company currently has over 80 systems in the field, more generators than any other manufacturer that focuses on laser cutting, he says.

Industrial Solutions LLC
Liberty Systems