Pure steam is the key utility needed for sterilisation; it is also used for humidification. The following characteristics are necessary:

Biological Purity: When condensed, pure steam must meet the same standards as Water for Injections and show endotoxin levels below USP standards of 0.25Eu/ml. As with WFI, the accepted method of achieving this uses evaporation. Impurities remain entrained in water vapour droplets; these are then separated from the dry steam in the gaseous phase, leaving it pure.

Freedom from non-condensable gases: NCG’s can compromise the sterilisation performance of saturated steam autoclaves. The commonest non-condensable gas is dissolved Carbon Dioxide. The acceptable limit is set at 3.5% v/v (EN285); reliably achieving this entails de-gassing the feedwater prior to evaporation.

Physical condition. Saturated steam sterilisation relies on pure steam condensing on the load, releasing its latent heat of evaporation.  Steam that is too dry can become superheated and not condense readily enough; steam that includes too much water vapour risks wetting the load and creating a barrier between it and the dry steam.  Dryness needs to be controlled between the fractions of 0.95-1.00 at the point of application for metal loads. This usually means using steam traps in the distribution pipework to correct for condensation.

Responsiveness. Pure steam generators supplying autoclaves have to meet a highly fluctuating demand – from zero to maximum within a few seconds - without appreciable pressure drop.