When fluids are terminally sterilised in closed vessels, differential expansion of the fluid, the container and the enclosed airspace lead to a considerable (and variable) pressure build up. Particularly during cooling this pressure will often vary significantly from that of saturated steam in the chamber, and the pressure differential can distort flexible containers, burst blister packs, force plunger movement in pre-filled syringes or even break larger glass vessels.  Terminal autoclaves counter this by applying compressed air to equalise the pressures inside and outside the fluid containers.

There are two techniques for terminal sterilisation offered by :

Air-Steam counterpressure units use steam as the heating medium.  After sterilisation, a range of cooling techniques is available: spray cooling, air cooling, fan-assisted plate or jacket cooling.

Superheated water shower autoclaves spray pressurised, superheated water as the heating medium, cooled water as the cooling medium. Heat transfer is faster but the load ends up wet and there may be temporary ‘blooming’ of flexible LVPs.
In both cases exceptionally tight temperature uniformity and F0 control comes from magnetically coupled fans, ducting and spray bar design and advanced process control techniques. Standard chamber sizes range up to 28 m3 for GMP manufacturing facilities. Fedegari provide conveyors and systems for loading and unloading pallets, load rotation mechanisms to accelerate the attainment of stable and uniform temperatures, then cooling. This is particularly useful when sterilising emulsions or suspensions.

Hybrid autoclaves can perform both saturated steam cycles and air-steam counter-pressure cycles.