Why You Need Gas and Equipment Savvy for a Second-Hand Laser

The beam-purge system is designed to provide a clean, dry atmosphere for the beam delivery optics in the resonator to direct and maintain beam quality to the work piece. Most laser manufacturers highly recommend nitrogen as the beam-purge gas. It can normally be from the same nitrogen gas source as the assist gas being supplied. Operating pressures range from 50 psi to100 psi and flow requirements of 80 scfh to 120 scfh. A high-purity single-stage regulator is the appropriate regulator recommendation for this function.

Assist Gas
High-pressure cylinder “cradles” can be used for startup or low-flow applications, but are inefficient and costly when the application demands higher flows. Nitrogen and oxygen are the two main assist gases required. Average nitrogen pressures required at the back of the laser range from 300 psi to 500 psi, and flow requirements can be in the 2000-4000 scfh range. Oxygen pressures range from 200 psi to 400 psi, with flows between 500-1500 scfh depending on material cut. Recommended regulators for assist gas delivery are high-flow dome loaded or balance-stem design (see Figure 3).

When setting up the nitrogen and oxygen assist gas system, installing 40-micron inline filters and appropriately sized relief valves is also recommended by laser manufacturers. Because of the high-flow requirements for assist gas, the next step would be liquid cylinders for the economical continuous use of these gases. Liquid cylinders are essentially big thermos bottles capable of delivering gas or liquid product to an application and are commonly available in oxygen, nitrogen, argon and CO2 service. One liquid cylinder’s volume is equal to approximately 15 to 20 high-pressure cylinders, and it costs much less to fill one liquid cylinder than it does to fill 20 high-pressure cylinders.

Liquid cylinders have a gas-use port, liquid-use port, and vent-connection port on the top of the cylinder. A pressure-building circuit generates additional pressure and flow to the application and an economizer circuit to keep from generating gas when demand is low. Safety relief valves set at 230 psi, 350 psi or 500 psi are installed on the liquid cylinder to insure internal pressures will never reach the point where the cylinder could rupture or explode. With standard liquid nitrogen and oxygen cylinders, the pressure-building capacity is normally around 300 scfh, with peek flow of 350 scfh for short periods. If the application demands more flow than one cylinder can provide, additional cylinders can be added to supply the application.

When choosing a switchover system, it is good to remember that pressure-differential switchovers do not work efficiently with liquid cylinders supplying the product. If the demand for gas is too high for a liquid cylinder to keep up with, the head pressure in the cylinder will drop below the switchover point and create a false or premature empty condition. Also, pressure differential switchovers tend to leave 15 percent or more of unused product behind, and there comes a point when the gas flow demand to an application is so great that using multiple liquid cylinders’ internal vaporizing capability is no longer practical.

Pusher System
One solution is to extract the liquid product from the liquid cylinder and put it through an external vaporizer to create larger volumes of gas product. Multiple liquid cylinders can be connected to a cryogenic manifold to feed the vaporizer a continuous supply of liquid product. A pusher system can be used to ensure the entire liquid product in a cylinder is used and constant pressure and flow to the vaporizer and downstream application is maintained. With a pusher system, the outlet pressure (gas) of a separate liquid cylinder (pusher) is used to force the liquid product out of the (slave) liquid cylinder(s) through the cryogenic manifold and in to the vaporizer.

An uninterrupted supply of gas can then be supplied to a laser assist-gas system by locating a pusher system on either side of a fully automatic switchover system (see Figure 3). With the pusher system you are only limited by the vaporizers rated capacity to produce gas product and the amount of cylinders hooked up to supply the application  (see Figure 4). Incorporating these principles and pointers into the purchase and installation of a second-hand laser should assist in achieving first-class efficiency and operation.