If your business uses nitrogen — for inerting, blanketing, laser cutting, food packaging or any of the dozens of other applications — you're either buying it in cylinders or generating it on-site. The economics of the two options are quite different at different consumption levels.
Cylinder Supply
Nitrogen cylinders are simple and low in commitment. You call your gas supplier, cylinders arrive, you use the gas, the empties get collected. There's no capital equipment to maintain, no compressed air requirement, no operator training beyond basic cylinder handling.
The cost per cubic metre is relatively high — typically in the range of £3-8 per cubic metre delivered, depending on your contract, location and volume. For low consumption — say, less than 50 cylinders per month — this is almost certainly the right choice.
Pressure Swing Adsorption Generators
PSA (pressure swing adsorption) generators produce nitrogen from compressed air by selectively adsorbing oxygen on carbon molecular sieve. The output is nitrogen at purities typically ranging from 95% to 99.999%, depending on the setting.
The capital cost for a generator serving a medium-consumption site might be £15,000-40,000 including the compressed air treatment required. The running cost is predominantly electricity — compressed air going into the generator.
The produced cost per cubic metre drops to roughly £0.30-0.80 depending on electricity cost, compressed air efficiency and the purity required. At higher purities, the yield from a given volume of compressed air falls, increasing cost per cubic metre.
The Break-Even Calculation
The crossover point depends on your current cylinder cost per cubic metre, the capital cost of the generator, and the ongoing maintenance cost.
As a rough guide: if you're consuming more than 150-200 cylinders per month and your current cost is above £4 per cubic metre, on-site generation will typically pay back in 18-36 months. Below that consumption level, the capital and maintenance cost per cubic metre of generated nitrogen starts to exceed cylinder supply.
The calculation also needs to account for the compressed air cost. PSA generators consume significant volumes of compressed air — typically 3-4 cubic metres of compressed air per cubic metre of nitrogen at 99% purity, rising to 8-10 cubic metres for 99.9% purity. If your existing compressor doesn't have capacity to spare, the generator requires either additional compressor capacity or a dedicated compressor, which changes the capital cost significantly.
Purity Is the Critical Variable
Most general industrial nitrogen applications — laser cutting assist gas, inerting, purging — work well at 99-99.5% purity. Food packaging and some chemical applications require 99.9% or better.
The difference in generator cost and compressed air consumption between 99% and 99.9% is substantial. Make sure your purity specification is driven by what your process actually requires, not by assuming maximum purity is safest.
If you're close to the break-even point at 99.9% purity but comfortably justified at 99%, it's worth investigating whether your process actually needs the higher purity before committing.
Membrane vs PSA
For lower purities (95-99%) and smaller volumes, membrane nitrogen generators are an alternative to PSA. They're simpler, with fewer moving parts, and more compact. The yield per unit of compressed air is lower than PSA, making them less energy efficient, but the lower capital cost can make them attractive for the right application.
For purities above 99%, PSA is generally the better technology choice.