According to the different discharge methods of cold air, pressure steam sterilizers are divided into two categories: down-exhaust pressure steam sterilizers and pre-vacuum pressure steam sterilizers.

The lower exhaust pressure steam sterilizer is also called the gravity displacement pressure steam sterilizer. Its sterilization is based on the principle of gravity displacement, so that the hot steam flows from top to bottom in the sterilizer, and the cold air is discharged from the lower exhaust hole. The cold air is replaced by saturated steam, and the latent heat released by the steam is used to sterilize the items.

The sterilization principle of the pre-vacuum pressure steam sterilizer is to use the method of mechanical vacuum to form a negative pressure in the sterilization cabinet, and the steam can quickly penetrate into the interior of the item for sterilization. According to the number of times of vacuuming, it is divided into two types: pre-vacuum and pulsating vacuum. The latter is more thorough in removing air and more reliable in effect due to repeated vacuuming.