An advanced aerospace-grade autoclave is a key system required for manufacturing aerospace structures using composite materials. The aerospace fraternity could progress in their goal to realize lightweight, high strength, cost-effective, and adaptable composite structures for air vehicles, launch vehicles, and satellites through autoclaves. To meet the ever-increasing demand for the material, autoclave needs to provide an enhanced process control environment involving high temperature and high pressure. This article presents the limitations of using air as an autoclave medium and prescribes the use of inert nitrogen gas to mitigate this and enable a paradigm shift in the processing of autoclave-based composites.
Aerospace autoclave, inert gas autoclave, autoclave fire safety, nitrogen generation, nitrogen asphyxiation, composite processing
An autoclave is a pressure vessel that provides a controlled environment of elevated temperature and pressure for processing materials loaded into it. The details of autoclave sub-systems and the process of curing composites in the autoclave are presented by Upadhya, A. R. et al. .
Autoclaves may use steam, water, or gas as the internal medium to create the required process conditions. The steam-based autoclaves are widely used in the medical and construction industry. Gas or air-based autoclaves are more commonly used for rubber vulcanization and for curing advanced aerospace composite structures that require accurate process control. Given their widespread use in processing advanced aerospace composites, the discussion on inert gas (nitrogen) based autoclaves assumes significance.
Composite materials are processed in an autoclave under an internal vacuum and external pressure and temperature. Vacuum around the composites is achieved by using an encapsulated vacuum bag, which is connected to an external vacuum pump through vacuum hoses. Typical pressure, temperature, and vacuum of an aerospace- grade autoclave are 7 bar, 200°C, and 2 mbar respectively. These process boundaries are adequate for processing or ‘curing’ a wide variety of thermoset polymers used for most aerospace structures. Owing to the non-recyclable nature of thermoset composites, thermoplastic composites are being developed worldwide, particularly in strategic sectors like aerospace. However, thermoplastic composites require higher processing temperatures, typically 300 to 400°C and 7 to 15 bar pressure, with rapid heating and cooling rates. Moreover, certain niche and demanding applications, particularly those used in high-temperature operations, require process temperature and pressure greater than 450°C and 15 bar respectively. As the process temperature exceeds 120°C, the risk of ‘fire inside autoclave’ increases. Autoclave-fire creates a sudden increase in temperature and pressure that burns the expensive composites material, mould, autoclave equipment and entails the risk of explosion and associated damage to life and property  .
Causes of Fire in an autoclave
The ‘Fire Triangle’ consisting of combustible material, oxygen gas, and heat are always available in an air-pressurized autoclave. When these three sides of the fire triangle adequately combine to create a chemical reaction with exotherm, it instigates ‘fire’. Generally, the materials used in the autoclave have a much higher flash point than the operating temperature of the autoclave. However, fire can occur due to excess heat produced because of uncontrolled exothermic reaction during polymerization, electrical short circuit or arcing, faulty heater control system, presence of inflammable material, frictional heating due to the blower, discharge of static electricity from the vacuum bag, etc. 
The compressed air contains oil particles mixed in it, especially when a lubricated compressor without a proper oil particle filter is used. Poor maintenance of the compressors and filters increases the oil content. This oil mist air, coupled with a higher concentration of oxygen can also cause a fire inside the autoclave.
When components with higher thickness are cured, a larger amount of heat is generated at the thicker zones. This sudden increase in temperatures can cause a fire in the surrounding materials such as vacuum bags, breathers, etc.
In the case of vacuum bag leakages at high pressure and temperatures, the gas leakage through the bag causes friction and can raise temperatures locally resulting in fire. The possibility of a fire in the autoclave increases with the use of materials having low flashpoints.
The higher air pressure inside the autoclave provides a large amount of oxygen to aid and sustain fire in all the above cases. While extreme caution is exercised to avoid the above situations, fire accidents in the autoclave are not uncommon  .
Mitigation of fire risk in an autoclave using nitrogen
All the above chances of fire can be prevented, if we ensure that oxygen is not present inside the autoclave. This can be achieved by employing an inert atmosphere like nitrogen gas as the pressurizing medium. The atmospheric air in the autoclave is flushed out by filling it with nitrogen gas to a pressure of about 0.2 bar and then dumping it into the atmosphere. Thereafter the autoclave is pressurized with nitrogen gas. This process helps in ensuring the negligible presence of oxygen in the autoclave.
The inert nitrogen gas enables a substantial increase of operating temperature and pressure without the risk of fire. Higher process temperature and pressure enable the development of high temperature cured polymers in turn resulting in advanced composites with higher service temperature. In addition to fire prevention, nitrogen prevents oxidation of autoclave interior material and the seepage of oxygen into sealant material used in the pressurized fan motor and the aerospace components being processed.
Generation of Nitrogen Gas
In general, large volumes of gas are required due to the large size of the autoclaves necessary for accommodating airframe structures. Aerospace autoclaves’ size range from 3 m to 9 m in diameter and length can be as long as 30 m. To meet such a large demand, on-site gas generators are used.
Cryogenic Pressure Swing Adsorption (PSA) and modern Membrane Technology (MT) are popular methods of producing nitrogen gas. In cryogenic technology, the air is compressed, devoid of impurities such as CO2, H2O, etc., using molecular sieve adsorption. The air is then liquefied and very high purity nitrogen (1ppm of impurities) is separated in a cryogenic distillation column. This is a very expensive process and, in most cases, high purity nitrogen is not required for autoclave processing.
PSA technology utilizes Carbon Molecular Sieves (CMS), which adsorbs oxygen and other trace gases from the compressed air producing nitrogen gas . The impurity levels vary from 0.0001% to 3%. To increase the life of CMS, refrigerator driers are used to remove the moisture and reduce the temperature of compressed air entering into the CMS tanks. This is a fairly cheaper process of producing on-site nitrogen gas .
MT separates gases by the principle of selective permeation – separating nitrogen from air based on permeation rate. It is modular in construction, facilitating part load production of nitrogen gas. The lowest impurity level that can be achieved is 0.5%. It is a relatively expensive way of producing nitrogen gas in comparison with PSA, as it requires a high-pressure difference across membranes.
Among the three methods, PSA technology is best suited for autoclave pressurization as it gives the required purity level and is cost-effective. Typically, the cost of a nitrogen generation plant for autoclaves is recovered in less than two years compared to the use of nitrogen gas tanks.
Safety requirements of inert gas autoclave
The downside of using nitrogen is the risk of human exposure and the resulting asphyxiation by oxygen displacement. Being heavier than oxygen, if excess nitrogen fills the human-occupied area it displaces oxygen from the ground level. As nitrogen is odorless and tasteless, humans do not sense it and continue to breathe the gas without significant biological alarm. The low level of oxygen (less than 15%) makes the person giddy and a further reduction (4 to 6%) can cause coma and death in a few minutes.
To prevent oxygen deficiency, areas where nitrogen is stored and the autoclave is installed should have sufficient ventilation. At least 4 to 6 changes of fresh air per hour are recommended, depending on the room size and the quantity of nitrogen being used. Oxygen monitoring and alarm systems, both hand-held units and stationary area monitors along with the display of appropriate warnings should be provided . It is also very important to route the pipelines from the outlets of the autoclave vent, pressure relief valves, rupture discs, etc. to a safe location outside.
Even in the case of normal operation after the processing and depressurization of the autoclave, the operator should not enter into the autoclave, as large pockets of nitrogen can still be present inside. To remove the nitrogen, the flushing of autoclaves with atmospheric air is carried out either through pressurized air or an exhaust system. In addition to these measures, personnel working around the nitrogen-based systems are given a wearable ‘low oxygen alarm’ device that generates audible alarm and vibration upon low oxygen levels. The facility storing or using nitrogen gas should have an emergency response plan that covers situations such as release and medical emergencies, phone numbers, evacuation procedures, etc.
A typical autoclave with its nitrogen generator is shown in Figure.1 and Figure. 2 respectively. CSIR-NAL has designed and developed several autoclaves of size ranging from desktop (working area: 450 mm x 500 mm) to large autoclaves (working area: 5500 mm x 13000 mm). These autoclaves are operational at the leading aerospace organizations of India, such as ISRO, VSSC, SHAR, DRDO, HAL, and several IITs. To meet the growing demand, autoclave technology has been transferred to Indian MSMEs.
The use of air as the pressurization medium in autoclaves operating at higher temperatures and pressure creates a considerable risk of fire hazard that results in avoidable loss of expensive material, equipment, time, and money and can escalate to injury and loss of life. The use of inert gas, such as nitrogen for pressurizing not only avoids fire but also enables the development of high-performance composites.
Various reasons behind autoclave fires, the risks associated with them, and the ways to mitigate the same were elaborated. Different methods of generating the nitrogen gas for autoclave pressurization were briefly presented. The paper also described the risks associated with the use of nitrogen and the measures to avoid nitrogen asphyxiation. The use of nitrogen gas for autoclave pressurization will continue to grow along with the performance demands on the advanced composite structures be it thermosets or thermoplastics.
Thanks to Dr. G.N. Dayananda, Outstanding Scientist, CSIR for spearheading the autoclave programs at NAL from the stage of design & development to successful commercialization. Thanks to him for motivating us to bring out this paper and for reviewing it. Thanks to Mr. Jitendra J Jadhav, Director, CSIR-NAL, and Dr. Ramesh Sundaram, Head, ACD & CSMST, CSIR-NAL for their constant support and encouragement.
Thanks to autoclave team members at CSMST, ACD, and the Electrical Section of NAL for their valuable contributions to the autoclave developmental activities at NAL.
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G.M. Kamalakannan & J. Ramaswamy Setty, CSIR-NAL, Bengaluru