Parts Per Million Defined

How something so small can cost so much

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Congratulations! You’re the new owner of a laser. With those words ringing proudly in your ears, you are amazed by the speed and accuracy of the cut you can now achieve. You can cut extra-fine details in thin or thick metal. Steel, stainless steel and aluminum are now at your mercy to be shaped into functional parts with great speed and accuracy.

After delivery and installation of the laser, you open up the owner’s manual. You’re comfortable with the mechanics of the laser and understand the cooling needs. But when you look at the gas related terms, like CFH vs SCFH, MG/M3, PPM, vapor pressure dew points, purity percentages and others, it looks like a language that you have not seen since high school chemistry. So, you call the local gas supplier for help. Surely he’ll know what all of this means.

Fortunately, your gas supplier was able to meet the minimum gas requirement in the owner’s manual. Unfortunately, he wasn’t able to help with the compressed air requirements for the beam pathway purge for the CO2 laser. The easy solution? Let your air compressor tech address the concern.

For example, the requirement in the laser owner’s manual is to provide air dried to 40OF pressure dew point and to have less than 5 ppm of hydrocarbon in the compressed air, and the airflow rate for the beam pathway purge is 25 cubic feet per minute, or cfm. 

So what does this mean in easy-to-comprehend terms?
Well, 25 cfm would equal 1,500 cubic feet per hour, so after 667 hours you would have consumed 1 million cubic feet of air. Knowing this foundational concept will help you and your maintenance team understand what is really being defined in the owner’s manual.

At +40OF pressure dew point, the water concentration will be about 2,000 parts per million. Your refrigerated drier is well suited for this application, as it has a 37OF pressure dew point, satisfying the manufacturer’s requirements.

It’s implied that this laser is very sensitive to oil. The requirement is for less than 5 ppm of Total HydroCarbon, or THC. Your air compressor tech declared that the air compressor blow-by oil concentration was only 4 ppm, which is good.

So you get everything hooked up and you’re running at ideal speeds listed on the laser PLC. Cuts are clean and dross free with shallow striations and crisp clean corners. Life is good. Now, on to the next project, which happens to be getting your press brake up and running.

You spend the next month bringing in the press and working on the foundation, electrical and parts flow to limit unwanted costs in your product. It is up and running in 30 days. A month has passed and you realize … the laser-cut parts are not as dross free or perfectly shaped as they were a month ago. What happened?

While your laser operated for 667 hours, you consumed 1 million cubic feet of air in the beam pathway purge. Along with the air, 2,000 ppm of water and 5 ppm of oil also passed through the beam pathway. The water and oil fogged the mirrors, top-side of the focusing lens and the beam pathway side of the output coupler. Wasn’t the air clean?

If you used 1 million cubic feet of air last month, that is 1 million parts per million. With the conversion from cubic feet to parts per million discussed above, we know we can convert the unit of measure for the water and oil from parts per million to cubic feet. What that means is that during the 667 hours, you delivered not only 1 million cubic feet of air but also, the air contained 2,000 cubic feet of water and 5 cubic feet of hydrocarbon, both as a vapor. But because these numbers are so small and the air was clean, according the owner’s manual, you call the air compressor service provider and his answer is to install a desiccant drier.

The air compressor service provider recommends the desiccant drier because it manages water down to -100OF with a sweep air of 17 percent. Sweep air is exhausted air that contains a high percentage of water. This keeps the media or desiccant dry. Water content is decreased to 1.53 ppm. Victory! You think. Water is decreased to 1.53 cubic feet per 667 hours, however, air consumption increased by 17 percent due to the sweep air.

Further discussion with the service provider brings up the fact that the desiccant needs to be replenished or replaced every 6 to 12 months. Replaced is easy to understand, but replenished is a strange word to use.

The discussion continues and you learn that over time the desiccant fractures, like potato chips in a bag. Soon all you have are crumbs. So these ‘crumbs’ take up less room and some of the ‘crumbs’ flow through your filters and end up in the beam pathway of your laser. As desiccant effectiveness decays, water is once again in the beam pathway, if you should overlook the maintenance requirements.

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There has to be a better way
Praxair’s Dehydration Series systems are designed with many applications in mind in the medical, scientific and industrial markets. The Praxair system removes water down to -100OF vapor pressure dew point, oil to 1 ppm or less and particulate to sizes smaller than a germ or virus without a sacrificial desiccant and with a sweep air of only 10 percent. While there are no moving parts that require maintenance, it does contain pre-filters that require semi-annual replacement. These filters are designed to remove and protect the heart of the system.

Field tests with this family of devices ranging from 6 cfm to 44 cfm proved that the best ROI path occurs with drying the air from about -40OF to -45OF vapor pressure dew point and decreasing the THC, or oil, from 5 ppm to 1 ppm or less. These tests were performed with oil concentrations in compressed air as high as 25 ppm on older piston compressors with similar, favorable results.

This is good news. The sweep air is reduced and now, the compressor size does not need to be increased. A smaller dehydration unit can also be used, reducing the overall investment requirement.

So with a -40OF and 1 ppm of THC, how much will water and hydrocarbon pass through the beam pathway?

The refrigerated air drier allows 2,000 ppm or in 667 hours at 1,500 cfh, 2,000 cubic feet of water. The dehydration system from Praxair reduces the water content 100 cubic feet of water over the 667 hours – a 20 to 1 improvement! Hydrocarbon was at 5 ppm and with a Dehydration Series system, THC is 1 ppm or less – a 5 to 1 improvement!

While drying the air down to -40OF to -45OF, vapor pressure dew point proves to support a healthy ROI, striving for lower vapor pressure dew points tends to be more harmful than favorable. Working for results below -40OF to -45OF further drives your costs for compressed air higher and will likely provide no real benefit. This is because the laser mirrors and output coupler need to be cleaned, inspected and aligned about every six months. So driving water content, as an example, to -100OF only adds to your compressed air expenses, but your laser will still require normal maintenance cycles.

Establishing when service is needed for other reasons as well as the design of the drying and THC removal system so they work together seamlessly better supports a healthy bottom line, which is so important in today’s competitive world of metal fabrication.

Praxair

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