| :: Gases, Purity & Volume Requirements |
|
Gases, Purity & Volume Requirements
When discussing purity, pressure and
volume requirements it is important to distinguish between the
different gases that are used in a laser system,
namely:
1) resonator
gases
2) process
gases
3) beam path purge gas
|
Gas lasers need gas in order to
generate the laser beam. The laser gas mixture required to run
a CO2 laser contains 60-85% helium, 13-55% nitrogen and 1-9%
carbon dioxide. The exact composition varies depending on the
type of laser and the manufacturer. Some CO2 laser types also
require small additions of other gases such as oxygen,
hydrogen or xenon.
The laser gas
needed for an Excimer laser consists of 0.05-0.3% halogen
(fluorine or hydrogen chloride), 1-10% rare gas (krypton,
xenon or argon) and 90-99% buffer gas (helium or neon). For
safety reasons, the halogen is always diluted with helium or
neon.
Today,
laser manufacturers demand relatively high-purity laser gases.
Impurities in the laser gas impede the laser performance by
lowering the output power, disturbing the uniformity of the
electrical discharge and requiring frequent service and
maintenance for the laser optics. Contaminated optics and
mirror reduce the reflectivity inside the laser cavity and
thereby the amount of amplification. The most harmful
impurities have been identified as water vapor and
hydrocarbons.
High quality
laser resonator gas can still become contaminated on its way
to the resonator by substandard gas supply equipment.
Permeation through rubber and plastic components allows
harmful contaminations to penetrate into the laser gas. In
addition to wasting money on the resonator gas, the
contamination may cause resonator damage and extended
downtime. All supply equipment used to feed the resonator
gases into the laser cavity must be able to maintain the
purity of the gas. With regards to pressure the resonator
gases do not present any challenges. Pressure requirements are
usually moderate in the range of 30psi to 80psi. |
|
|
In laser cutting oxygen and nitrogen are
commonly used as cutting gases. Since it is the task of the
cutting gas to expel the molten metal out of the cut kerf, a
certain amount of kinetic energy is needed. Therefore, volume
and pressure requirements are high, especially when cutting
with nitrogen. In this case supply pressures go up to 435psi
and volume requirements can be as high as 5000scf/h, therefore
requiring sophisticated supply modes illustrated
below.
High pressure bulk
systems, typically rated at 400 or 500 psi are the optimal
solution for customers who predominantly perform nitrogen
cutting or use nitrogen in multiple shift operation. High
pressure tank solutions must however be equipped with a backup
supply system (cylinder banks in the picture below) in order
not to lead to machine downtime during the tank filling
process.
A solution that is gaining in popularity is
the use of pressure booster systems shown below, which allow
the use of 235 or 250psi bulk storage tanks. This PLC equipped
device controls the flow of low pressure liquid product into
two high pressure liquid dewars used to build pressure up to
500psi at flow rates in excess of 10,000sch/h. Although it
requires a higher upfront capital investment, the advantages
of no filling downtime, no handling of backup supply and
reduced filling losses all work in favor of the booster
solution.
In laser welding helium,
argon and mixtures of these gases are used. Flow rates and
supply pressures are similar to traditional welding
technologies such as TIG or MAG welding. For budgetary
purposes assuming a consumption of 60scf/h is
reasonable.
|
| |
|
|
