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The Use on Nitrogen in Autoclave Applications

Updated: Nov 4, 2021

In various industries today the manufacturing of high-performance composites often requires autoclave processing. Autoclaves are of pinnacle use in industries such as composites manufacturing, metal heat treating and aerospace development. Industrial autoclaves are pressure vessels which process parts and cure substances that require exposure to high pressure and temperatures and use this heat and high pressure to cure products or sterilize the machines, appliances, and tools. Autoclaves are manufactured for various forms including rubber bonding, vulcanising autoclaves, etc. Within a variety of industries autoclaves are used to aid in the manufacturing of polymeric composites.

It is the process of autoclaving which allows developers to produce the highest quality of materials. The heat and pressure of an autoclave is used for a variety of purposes, its main functions being aiding the overall quality and strength of these products. As a result of this the machines and aircrafts which are used within the aviation industry are able to withstand several demanding environments.

When these composite components are formed and cured, the pressure in the autoclave’s environment creates a situation which can become highly flammable due to the intense pressure and elevated temperature. However, once the curing is done, these parts become safer, and the risk of probable incineration is entirely removed. During this curing process the composites run the risk of combustion if the correct conditions are met specifically if oxygen we to be introduced. Nitrogen then becomes essential in autoclaves as it has the ability to safely remove these off gases reducing the risk of fire while being an inexpensive and inert option.

Autoclaves tend to be pressurised with either air or nitrogen depending on its requirements. The industry standard for air appears to be temperatures of up to 120°C are acceptable. Above this temperature the nitrogen is generally used to aid in heat transfer and alleviate potential fire hazards. While fires are not common were one to occur, it would significantly damage the autoclave. The losses as a result include the parts themselves and the potential to cut down production time significantly while repairs are being made.

These fires can be a result of localised friction heating from a possible bag leak and resin system exotherm. At higher pressures, more oxygen is accessible to enhance these fires. Nitrogen charging should be considered before a fire as the interior of the pressure vessel would have to be removed in order for the autoclave to be inspected and repaired.

The required pressurisation rates of an autoclave system must be met in order to be usable within industry. The average pressurisation in contemporary autoclaves is 2 bar/min. Today’s autoclaves generally use nitrogen as the pressurisation medium in replacement of air, due to the autoclave cure consumables in the air medium being highly flammable owing to the presence of oxygen. Although the use of nitrogen is an advantage in preventing fires, caution must still be taken in order to avert any potential danger to individual personnel such as asphyxiation due to the lower oxygen levels.

The pressurisation system for Nitrogen gas essentially consists of a primary compressor, a nitrogen plant, a booster compressor, storage tanks and associated piping circuitry. The compressor encompasses air from the surrounding environment and pressurises it to 7 bars(g). While the nitrogen plant obtains the air at 7 bar(g) and uses a PSA System (Pressure Swing Absorption) to isolate the nitrogen from the air in the atmosphere. The use of a PSA System is rapidly spreading as a method in producing nitrogen as the purity of the nitrogen generated is extremely high in quality (99%) which is more than sufficient for the curing needed in aerospace polymeric composites in autoclaves. The booster compressor then further isolates the nitrogen to between 17-22 bar to meet the necessary pressurisation rates.

Why Use On-site Nitrogen Generation

- The production of Nitrogen onsite has proven to be more environmentally friendly due to the lack of emissions from delivery vans

- In comparison to the cost of consistently delivered nitrogen gas cylinders, onsite nitrogen generators provide business considerable savings.

- A continuous and reliable source of Nitrogen

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