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What is Moist heat sterilization?

In the design of many autoclaves, removing air is a critical factor. An air-steam mixture will have less latent heat than air-free steam ( pure steam) at the same pressure, thus reducing the efficiency  of heat transfer from steam to the load. Vacuum air removal cycles are designed to remove air out of the chamber and load more efficiently than gravity displacement cycles. However, the air in the autoclave may not be all removed depending upon the capability of process. The type or quantity of the autoclave load have a great influence on the quantity of air removed. For example, a load with gowns, filters and hoses will retain more air than a load without these. However, it must be demonstrated through the qualification effort that such low levels of air have no adversely influence on the sterilization cycle.

steam autoclave

autoclave

The heat transferred by the condensation of saturated steam is many times greater than that which would be transferred from steam above its saturation point. The sterilization by super-heated steam is a dry heat phenomenon,and its process is less effective than a saturated steam process. Changing 1 0C in super-heated steam temperature has only 1.5kj/mole of energy. Yet saturated steam releases 39.6kj/mole energy which make it become a more efficient process. Dry steam is the other name of super-heated steam, because once it contact with cold surface, it does not form condensate immediately. Thus, the important hydrating effect will not happen when super-heat is present. Pressure reductions should be avoided in order to maintain a saturated steam condition. Maintaining steam in equilibrium with water at the steam generator can avoid the generation of super-heat. A jacket that steam filled around a sterilizer chamber may be used to reduce heat-up time and increase chamber temperature uniformity, but during the phase of heat-up and exposure, the jacket should be controlled at or below the chamber temperature, and avoid the generation of super-heated steam in the chamber.
For saturated steam cycles, condensation to water will cause a volume decrease in excess of 99%, and if the condensed steam were not immediately replenished with additional steam to the sterilizer, which would normally result in a substantial pressure decrease. It is a condensation or replenishment cycle that allows the steam to rapidly heat the surface of load items until they reach an effective sterilization temperature. The sterilizers and their corresponding cycles are designed to ensure that the sterilizing medium reaches all these surfaces of all items. When a container filling aqueous liquid is terminally sterilized, the sterilization effect inside the sealed container will result from the heating effect of steam or super-heated water on the outside of the container. The steam and super-heated water in the sterilizer chamber is only an important source of energy, and the steam saturation is not a major question as long as heat transfer to the container proceeds at the required rate.

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