Autoclaves are in use today in several industries, such as composites manufacturing and metal heat treating. An industrial autoclave is a heated pressure vessel with a quick opening door that uses high pressure to process and cure materials. It uses heat and high pressure to cure products or disinfect machines, devices, and instruments. Several types of autoclaves are manufactured like rubber bonding / vulcanizing autoclaves, composite autoclaves, and many other types of industrial autoclaves. Autoclaves are used in several industries to help in the manufacturing of polymeric composites.
The process of auto claving allows manufacturers to produce materials of the highest quality. The heat and pressure in an autoclave is used to a variety of products, helping in improving the overall quality and strength of these products. Hence, the machines and aircrafts used in the aviation industry are able to handle demanding environments. Autoclave manufacturers can help in producing composite autoclaves that can produce quality products.
When composite parts are created and cured, the pressure in the autoclave environment puts them into a situation where they become highly flammable due to increased pressure and temperature inside the autoclave. However, once curing is completed, these parts are safe and the risk of combustion is nearly eliminated. During the curing process these composites could combust if the right conditions prevailed – namely, if oxygen were introduced. Nitrogen is used in autoclaves since it is inexpensive and is inert, thus will not catch fire. Nitrogen can safely remove these off-gases and reduce the risk of fire in an autoclave.
Autoclaves can be pressurized with air or nitrogen, depending on customer requirements. Industry standard seems to be that air is OK up to temperatures of around 120 deg C. Above this temperature, nitrogen is usually used to assist heat transfer and mitigate the potential for fire. Fires are not common, but they can cause a lot of damage to the autoclave itself. Losses could include a full load of parts and production down time while repairs are made. Fires can be caused by localized frictional heating from a bag leak and resin system exotherm. At higher pressures, more oxygen is available to feed the fire. Since the entire interior of the pressure vessel must be removed to inspect and repair an autoclave after a fire, nitrogen charging should be considered.*1
An autoclave system must ensure that the required pressurization rates in the autoclave are met. The average pressurization rate in modern autoclaves is 2 bar/min. Nowadays, many autoclaves use nitrogen as the pressurization medium instead of air. This is because the autoclave cure consumables are highly inflammable in the air medium due to the presence of oxygen. There have been several reports of autoclave fire resulting invariably in the loss of the component. Though the nitrogen medium ensures fire-free autoclave cure cycles, care must be taken to avoid danger to personnel (possibility of asphyxiation) in nitrogen environments due to the lower oxygen levels.
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The nitrogen gas pressurization system consists of a primary compressor, nitrogen plant, booster compressor, storage tanks, and associated piping circuitry. The primary compressor takes in the air from the atmosphere and pressurizes it to 7 bar(g). The nitrogen plant receives the air at 7 bar(g) and by a process known as Pressure Swing Adsorption (PSA) isolates nitrogen from the atmospheric air. The PSA has emerged as one of the popular methods of producing nitrogen. The nitrogen purity produced by this method is of the order of 99%, which is adequate for curing of aerospace polymeric composites in autoclaves. The nitrogen, thus, isolated is further pressurized using a booster compressor to higher pressures, typically 17–22 bar. Higher pressure is required to create sufficient pressure differential in order to meet the required pressurization rate. The nitrogen storage tanks are sized in such a way that the free-air delivery (FAD) of these storage tanks is 2.5 times the FAD of the autoclave.*2
*1 https://www.compositesworld.com/cdn/cms/whitepaper_southbrook_autoclave_design.pdf
*2 A. R. Upadhya, G. N. Dayananda, G. M. Kamalakannan, J. Ramaswamy Setty, and J. Christopher Daniel, “Autoclaves for Aerospace Applications: Issues and Challenges,” International Journal of Aerospace Engineering, vol. 2011, Article ID 985871, 11 pages, 2011. https://doi.org/10.1155/2011/985871.
Why On-Site Nitrogen Generation
- Providing vast savings in comparison, on-site generation of nitrogen is preferred over bulk nitrogen shipments.
- Nitrogen production on-site is also environmentally friendly as trucking emissions are avoided where nitrogen delivery was being done before.
- Nitrogen Generators offer a continuous and reliable source of nitrogen, ensuring the customer’s process never comes to a standstill due to want of nitrogen.
- Nitrogen generator return on investment (ROI) is as little as 1-year and makes it a lucrative investment for any customer.
- Nitrogen generators have an average life of 10-years with proper maintenance.
