Choosing a gas delivery mode to improve efficiency
September 14, 2004
A new 4-kilowatt laser just hit the floor. The technician will arrive on Monday to install it. It's your responsibility to make sure the gases arrive. The preinstallation manual was faxed to your local gas supplier, and all is well.
Not so fast. Other gas matters can affect cost and efficiency. The laser requires controlled purity for the resonator gases, moisture and oil restrictions for the beam purge, and minimum pressure and flows for the assist gas. Supplying the assist or process gas can present challenges that can cause the initial capital outlay to vary as much as $40,000.
The decision to purchase a $500,000 machine tool is based in part on the premise that its enhanced capabilities and lower throughput costs will increase market share. Success or failure partially depends on selecting the appropriate assist delivery mode that offers flexibility and quality at a competitive rate. From start-up to lights-out production, assist gas system requirements vary based on material type, thickness, part frequency, and the desired level of quality.
The rest of the story begins with Figure 1, which graphs the oxygen pressure and flow requirements for various mild steel thicknesses. As you can see, cutting mild steel with oxygen typically requires between 8 and 70 pounds per square inch (PSI) and 25 to 250 cubic feet per hour (CFH) at the nozzle. However, depending on the manufacturer, the minimum pressure required at the back of the laser can vary between 110 and 290 PSI.
The delivery system should be able to meet the minimum pressure requirement throughout the life of the supply source, such as a liquid can or high-pressure cradle. If the supply source can't sustain the pressure threshold, the laser will trip the low-pressure alarm and terminate the cut sequence. Productivity and cut quality can suffer in the form of fewer parts produced and more dross adhesion.
Oxygen pressure and flow requirements for mild steel vary based on the thickness. This information is based on CO2, diffusion-cooled, and Nd:YAG 1,000-multiwatt resonators.
With a start-up monthly oxygen consumption of 40,000 cubic feet, manifolded 350-PSI relief liquid cans supplied via automatic switchover or microbulk vessels can be cost-effective. The deciding factors are mode availability and future growth potential. A typical setup for either choice must include a relief valve and particulate filter equipment to ensure safe delivery and laser protection.
The oxygen delivery system should include a quick-response regulator that precisely controls pressure to ensure edge quality and piercing. With 350-PSI relief liquid cans that deliver 80 to 320 PSI, the automatic switchover should be able to supply the 110- to 290-PSI pressure required. This has been a challenge for the traditional pressure differential switchover, which uses multiple preset regulators to obtain the 40- to 60-PSI automatic swing function. Whether used in a job shop or an OEM production environment, the automatic switchover should have economizing features and provide low-return loss to be competitive with stationary microbulk vessels.
Beyond the 40,000-CFH monthly consumption plateau, a standard bulk tank can be cost-effective. However, if the laser requires a minimum inlet higher than the effective operating pressure of a standard bulk vessel, then a high-pressure, 1,500- to 2,000-liter microbulk vessel or standard bulk tank with a pressure-boosting system is an alternative. It's important to analyze the system's total cost of ownership, which includes delivery charges, downtime during fill, vent loss, product cost, and monthly facility fees.
Different aluminum and stainless steel thicknesses have different nitrogen pressure and flow requirements. This information is based on CO2, diffusion-cooled, and Nd:YAG 1,000-multiwatt resonators.
Nitrogen typically is used to cut aluminum and stainless steel to prevent oxidation of the cut edge. Figure 2shows the nitrogen pressure and flow requirements for various aluminum and stainless steel thicknesses. Once again, with nozzle requirements of 75 to 360 PSI and 300 to 5,500 CFH, the minimum pressure at the back of the laser will be between 150 and 450 PSI, depending on the manufacturer. Again, the assist gas delivery system must be able to meet the minimum requirements.
You should determine the specific amount of edge oxidation acceptable for painting, welding, or final assembly operations. In some cases, a nitrogen generator can supply up to 1,800 CFH at 190 PSI and 95 percent pure, clean-cut assist gas at a reduced cost. Ninety-seven percent purity levels are available, but at reduced flow rates of 1,200 CFH. A nitrogen generator is an option if a majority of material is 0.250 inch and less. If a generator meets the demands of your product portfolio, determining the compressor's power consumption and the amount of moisture content at the rated flow is recommended. Moisture levels greater than 100 parts per million (PPM) can reduce productivity and increase dross adhesion.
For job shops and OEMs, a nitrogen assist gas delivery system can be difficult to configure at start-up if 50,000 cubic feet is consumed monthly. Sporadic nitrogen cutting and varying flow requirements can make it difficult to use 500-PSI relief liquid cans because of flow restrictions and evaporation losses due to inactivity.
Microbulk delivery systems are suitable when service is available in the area. If it isn't, an alternative is an electronic automatic switchover system that can deliver 400 PSI at flow rates up to 5,000 CFH from liquid or high-pressure cylinder bundles.
Another type of system lets the user select the delivery source for each application remotely via an RS-232 communication port or by pushing a button. For 1/4-in. and thinner aluminum and stainless steel, the operator can select the 500-PSI liquid can source. A vaporizer sized 1.5 times the desired flow rate can ensure that the gas temperature doesn't harm the laser internals. For thicker materials, the operator can select the high-pressure bundle source to optimize the pressure and flow required to obtain a quality cut.
If your monthly nitrogen consumption exceeds 50,000 cubic feet, you may opt for a high-pressure bulk tank or a standard bulk tank with an auxiliary pressure-building system. Both systems can meet pressure and flow requirements for thicker aluminum and stainless steel, but with two major differences:
1. The high-pressure bulk tank must be vented before being filled, thereby creating losses of up to 12 percent of the bulk tank contents. For a 3,000-gallon, high-pressure tank operating at 400 PSI, losses can amount to $300 to $500 per fill, because few high-pressure delivery trucks are available through the U.S.
2. A vented tank pressure may need up to an hour to recover the minimum required operating pressure to the laser. However, an auxiliary pressure-building system uses a standard bulk tank that can be filled without shutting off the supply of assist gas to the laser. In this case, productivity is maintained for about the same monthly cost.
Whether it's your first laser or your third, selecting the right gas delivery system can improve your laser's profitability and prevent your company from being burdened with potentially noncompetitive hourly rates. Spending some time to define your target market by material type, thickness, and job order frequency can help you optimize a laser gas delivery system to maximize both productivity and the quality of a product.
The following companies also contributed to this article:
TRUMPF Inc., www.us.trumpf.com
Liquid Cylinder. A liquid cylinder is a double-walled, vacuum-sealed DOT-4L 230- to 500-PSI-rated vessel designed to transport cryogenic liquid product. The liquid cylinder incorporates separate pressure-building and gas-vaporizing copper coil circuits to supply the product in either liquid or gaseous form at a constant pressure. A typical liquid cylinder is 20 in. in diameter, 60 in. tall, and has a storage capacity of 3,600 to 7,000 cubic feet and a withdrawal rate of 350 to 400 CFH of gas.
High-pressure Cradle. A high-pressure cradle is a portable gas manifold cart that carries up to 12 to 18 DOT 3 or DOT-E cylinders with a common outlet valve. The bundled cylinders are filled with product in the gaseous state at pressures between 2,200 and 4,500 PSI. A typical 12-cylinder cradle is 36 in. wide, 60 in. long, 72 in. tall, has a storage capacity of 3,600 cubic feet, and has a withdrawal rate of 3,500 CFH at 500-PSI residual pressure.
Microbulk Cylinder. A microbulk cylinder is a double-walled, vacuum-sealed, portable DOT-4L or stationary ASME 230- to 500-PSI-rated vessel designed to store cryogenic product. The more common stationary or on-site ASME vessel has five to eight times more storage capacity than liquid cylinders. Microbulk tanks also incorporate separate pressure-building and gas-vaporizing copper coil circuits to supply product in either liquid or gaseous form. The microbulk vessel may be located inside or outside a building, with the largest footprint being 48 in. in diameter, 117 in. tall, and have a withdrawal rate up to 2,000 CFH without additional vaporization.
Bulk Station. A bulk station is a double-walled, vacuum-sealed, stationary ASME 250- to 500-PSI-rated vessel designed to store cryogenic product. Bulk tanks use external pressure-building and gas-vaporizing circuits for ultrahigh flow rates. Bulk tanks must be located outside a building. Special concrete pads are required to support tanks as wide as 114 in. and as tall as 525 in.