Steam-Air Retort

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Steam Air Retorts

Steam-air retorts are normally batch type static or rotary, end-over-end, retorts.

The steam-air retort uses a mixture of steam and air, which is added to create an over-pressure in the retort.

The air over-pressure allows thermal processing of a wide variety of containers including: glass, metal cans, rigid plastic, and flexible pouches.
Steam-air mixtures, although in a gaseous state similar to 100% steam, when used as a heating medium behave more like water. The heat capacity of the steam-air is very small when compared to pure steam. As the steam condenses and gives off heat the air remains.

Because the retort uses air in the steam mixture there must be a method of mixing the air and steam to prevent the formation of air pockets in the retort and to provide for the rapid movement of the heating medium over the container surface.

Air may provide insulation around the container preventing the heat from reaching the container walls in an efficient manner. A large fan is normally used to rapidly mix the steam-air mixture and to force this mixture to flow through the retort and containers during processing.

It is very important to maintain the correct steam-air ratio in this type of retort. The steam-air ratio should be the same as that used during studies to determine temperature distribution in the retort.

Steam-air retorts are normally operated at steam-air ratios ranging from 75% steam/25% air to 95% steam/5% air, depending upon the air over-pressure in the retort. The steam-air ratio is normally determined by the processing temperature and the type of package being processed

The pressure in a steam-air retort must be maintained at the correct pressure to prevent container distortion and to maintain the correct steam-air ratio. If the amount of steam in the steam air ratio drops to too low a level, the energy available to heat the product is reduced. If the temperature is maintained by the steam controller the partial pressure of the steam remains constant throughout the process