One phrase, several meanings
“Gas generator” is a loose term. In industrial settings it most often refers to equipment that separates gases such as nitrogen or oxygen from ordinary air at the point of use. In power work, people sometimes use it to mean an engine-driven electrical generator that burns natural gas or propane. In rocketry and some turbine systems, it has a third meaning that is outside this article. Knowing which meaning is intended avoids a good deal of confusion when talking to suppliers.
This article covers the first meaning in more detail and the second briefly. It is general information. Specifying, installing and operating such equipment should involve qualified people and the manufacturer’s documentation.
On-site gas generation: the basic idea
Air is roughly four-fifths nitrogen and one-fifth oxygen, with small amounts of other gases. An on-site generator takes compressed air and separates the components so a plant can produce nitrogen or oxygen on demand, instead of buying cylinders or bulk liquid and having them delivered.
The usual starting point is clean, dry compressed air. Quality of the air feed has a large effect on the equipment, so generators are normally paired with filters and compressed air dryers and supplied by an air compressor. The compressor’s selection and upkeep, covered under air compressor types and the compressor maintenance schedule, strongly influence how well the generator performs.
Common separation methods
Pressure swing adsorption (PSA). Two vessels contain a material that preferentially adsorbs one gas under pressure. While one vessel separates gas, the other is depressurized and regenerated, and they swap in cycles. Depending on the material, the unit can produce nitrogen or oxygen. PSA systems are a common choice for moderate flows and a wide range of purities.
Membrane separation. Compressed air passes along bundles of hollow fibers. Some gases pass through the fiber wall faster than others, leaving a nitrogen-rich stream. Membrane units have few moving parts and are simple, but they are generally better suited to lower purity requirements.
Cryogenic separation. Very large plants cool air until its components separate by boiling point. These are industrial gas company facilities rather than something a typical factory installs.
Purity, flow rate and delivery pressure are the key specifications. Higher purity generally requires more air and more energy for each unit of product, so it is worth defining what the application genuinely needs and not over-specifying.
Where nitrogen generators are used
Nitrogen is chemically inactive in most conditions, which makes it useful for displacing oxygen. Typical uses include:
- Food and beverage packaging, where nitrogen flushing slows spoilage and protects fragile products from crushing, as seen on many food processing lines.
- Laser cutting, where nitrogen assist gas can give clean cut edges on certain metals, as in laser cutting machines.
- Electronics soldering and heat treating, where an inert atmosphere limits oxidation.
- Chemical and oil and gas operations, for blanketing tanks and purging pipes so flammable vapors are not exposed to air, a practice common in oil and gas equipment.
- Laboratories, for analytical instruments that need a steady, clean gas supply.
- Tire and pneumatic filling in some niche uses.
Where oxygen generators are used
Oxygen generators supply processes that need enriched air or oxygen, such as certain combustion and furnace operations, water and wastewater aeration, some chemical reactions and, in suitable medical or laboratory settings, oxygen supply under their own regulations. Medical-grade supply is a regulated area and outside the scope of general industrial equipment.
Advantages and drawbacks of on-site generation
Possible advantages:
- Supply is available whenever the plant is running, without waiting for delivery.
- Less handling of heavy cylinders or cryogenic liquid.
- Cost per unit of gas can be lower at steady, substantial consumption, although this depends on local energy costs and usage.
Possible drawbacks:
- Capital cost and installation effort.
- Ongoing electricity use for the compressor.
- Maintenance of filters, adsorption beds, valves and sensors.
- Limits on the highest purities and flows.
Whether on-site generation pays off depends on how much gas is used, how pure it needs to be, and local prices. Delivered gas can still be the better answer for small or sporadic demand. Any calculation should use your own quotes rather than rules of thumb.
Storage and distribution
Generators often feed a buffer vessel that smooths demand and allows the unit to cycle less frequently. These receivers are pressure vessels and must meet the applicable pressure equipment rules. A buffer tank sized with the generator allows short peaks above the production rate. Piping materials and cleanliness requirements depend on the gas and purity, and oxygen systems in particular need special cleaning and compatible materials.
Safety points
Gases that are harmless in ordinary air can be hazardous when concentrated or leaked.
- Nitrogen is not toxic, but it displaces oxygen. Leaks in enclosed spaces can create an atmosphere in which a person may lose consciousness without warning. Oxygen monitoring, ventilation and discipline around permit required entry are standard safeguards.
- Oxygen enrichment raises fire risk sharply. Materials that barely burn in air can burn vigorously in oxygen-rich air, and oil or grease in oxygen service is hazardous. Equipment must be designed and cleaned for oxygen use.
- High pressure requires properly rated components, relief devices and trained personnel.
Follow manufacturer instructions, local regulations and site procedures, and treat this article as an orientation rather than design guidance.
The other meaning: gas-fueled engine generators
Where people mean an electrical generator, an industrial gas engine generator is an engine fueled by natural gas, propane or biogas driving an alternator. These are used for standby, prime and combined heat and power service. They are attractive where pipeline gas is available, though the gas supply can be affected by the same events that interrupt electricity. Selection, fuel logistics and installation raise the same questions as other engine sets, and they are discussed in the pages on standby and prime power. Mentioning the distinction early in any purchase conversation avoids ordering the wrong equipment.