"In House" Nitrogen GenerationNeil Jackson - MSS Ltd

Introduction

Laser Industry trends indicate that sheet and plate working fabricators are increasingly selecting higher power lasers for not only cutting thicker materials, but also faster cutting of Stainless steels, Aluminium and thinner Carbon Steels with Nitrogen as the assist gas. Some 5 to 10 years ago, the most common power levels ranged between 1500 and 3000 watts, however, surveys show in recent years a huge swing towards installations of 4000 to 5000 watt systems and even 6000 watt units.

Today, the more experienced laser fabricators are purchasing high power lasers to expand their Nitrogen laser cutting capabilities and consequently differentiating themselves from their competitors. However, like all lasers users regardless of laser power available, great consideration needs to be taken in the method and source of Nitrogen supply.

Gas Consumption

When processing Stainless Steel with Nitrogen, large volumes of assist gas are used, with more than 65% of the hourly laser running and maintenance costs being associated with Nitrogen gas usage. In such cases the cost of the gas makes a significant contribution to the overall economics of the laser process. For instance if we take the following example of a 6mm Stainless steel component produced on a 3000 watt laser with Nitrogen supplied via MCP (Bottled Manifold Cylinder Pack).

Bearing in mind that gas costs and overall laser running costs vary greatly dependant on type of laser cutting machine used and also gas volume usage, discounts etc, the above simply serves as a representation for estimating purposes. It is important to point out that for thicker stainless steel, that Nitrogen gas costs could be as high as 90% of the hourly running costs.

Part Size: 126 x 100mmMaterial: 6.0mm Stainless SteelCutting Time: 46 sec.Cutting speed: 1.5 m/min.

Laser Machine Running Cost Per Hour £ 41.60 (ie Electricity, consumables, maintenance, labour, laser gas and BOTTLED NITROGEN)

Nitrogen Cost Per Hour £ 27.50

Nitrogen is 66% of the hourly laser running cost

It would seem logical that in order to make the laser cutting operation more economical, we start by addressing the largest single cost associated with this type of processing - NITROGEN GAS COST !!

How Can Nitrogen Gas Costs Be Reduced ?

Although some improvements can be made through negotiating gas, delivery and rental charges with gas suppliers and even replacing conventional MCP's with bulk liquid storage etc, not all options are available to everyone; in particular, space and access restrictions can make bulk storage impractical. An alternative, which is becoming more common place within the laser cutting industry, is the use of Nitrogen Generation equipment, which effectively allows the laser user to take complete control of the Nitrogen supply process by simply producing the gas "In House".

Nitrogen Generation Principle –Typical Layout

Air compressor Air Receiver

NN2

MAXIGAS

N2 Generator

Buffer Vessel

High Pressure Nitrogen Receiver

Electric Control Panel

6 barg

40 Barg

High Press Booster

Laser Cutting Machine

9 Barg max

Low Pressure Feed Vessel

N2 Compressor

High pressure

filters

domn ick hunter

dh

Crd

Dryer Dryerdryer

Using the pressure swing adsorption principle, a continuous flow of NITROGEN is produced from compressed air. In the case of an MSS Ltd Nitrogen Generator system, extruded Aluminium towers, filled with a Carbon Molecular Sieve (CMS) are used to adsorb the Oxygen from the compressed air. With Nitrogen being 78% of the composition of air at the outset, the adsorption of unwanted Oxygen is relatively quick and simple.

Nitrogen Purity And Flow Rates

The single biggest misconception of Nitrogen Generation relates to the purities, flow rate and pressure of Nitrogen as well as the options for future expansion. We should address all these points individually:

Purity: Although it is the case that some Generators can only produce Nitrogen at a purity of 99.95% (500 ppm Oxygen) this is not the case with all systems. For instance MSS Ltd Nitrogen Generators have variable Nitrogen purities to 99.995% (50 ppm). Another misconception is that higher purities similar to that obtained in bottles or liquid bulk storage are required when laser cutting. For the majority of material thickness this simply is not the case, it can be demonstrated for instance that 3mm Stainless steel cut with Nitrogen at a low purity of 99.95% gives an edge quality with an identical appearance to that achieved with bottle gas. Likewise by setting the Generator to a higher purity of say 99.99% an identical scenario is achieved in 6mm + Stainless steel.

Flow Rates: Flow rates generally are never a problem, with a Generator system able to supply gas to up to THREE laser cutting machines cutting 10mm Stainless Steel simultaneously. Ultimately the flow rate achievable is related to the purity of gas required, for example a typical mid size generator can supply gas with a purity of 99.95% at a flow rate of 61 Nm³/h, but if a higher purity of 99.99% is required the flow rate is reduced to 38 Nm³/h.

Stored Gas Pressure: Typically gas is stored High Pressure at around 40-42 bar which is more than adequate for all major CO² laser system manufacturers.

Future Expansion: It would be fair to say that not all Nitrogen Generators offer a simple means of expansion if usage increases significantly, ie change in work pattern or addition of a new machine.

Laser Cutting Advantages

LOW COST NITROGEN, typically £0.04 per m³ gas*. Suitable for ALL thickness of Stainless steel and Aluminium. Continuous 24 hour operation, NO machine downtime for changing gas. No unexpected running out of gas, especially weekends. No disruptive gas deliveries to accommodate. No gas waste with MCP's returned partially full. No gas rental, delivery charges or restrictive gas supply contracts. Enables high power lasers to cut Mild steel faster and cheaper than Oxygen

cutting, with an "Oxide Free" edge. The laser user takes complete control of the Nitrogen supply process.

*Depending on cost of electricity

Summary

In the past, laser users were restricted to either bottled or Cryogenic Nitrogen supply. However, with an increasing number of laser users taking advantage of the benefits listed above and realising considerable savings on Nitrogen costs, "In House" Nitrogen production is proving to be a viable and cost effective alternative.

Aluminium Towers

In the case of an MSS Nitrogen Generator system this is not the case, the initial system is specified with room for cost effective expansion,