The pressure vessel jacket surrounds the chamber of the sterilizer. The jacket is constructed of 304 stainless steel channels and plates unless the customer orders 316L stainless steel as an option. The jacket channels’ width and depth vary with the sterilizer size and are covered by jacket plates.

After construction, the vessel jacket is thoroughly insulated with chloride-free fiberglass insulation. An aluminum or stainless steel cover (the skin) is attached to the jacket prior to moving the pressure vessel to the final assembly area.

Vessel Structure

The jacket provides significant structural support for the overall pressure vessel and allows the sterilizer to operate safely at the elevated pressures experienced during sterilization. This structural strength is attained by welding numerous evenly spaced channels to the outer circumference of the chamber. The channels are covered by stainless steel plates that also add strength to the vessel.

The jacket only keeps the top, bottom, and sides of the sterilizer heated during standby mode and during a cycle. The door(s) and back wall are not heated by the jacket.

Steam Flow

The jacket is always controlled by temperature. Based on the specific sterilization temperature setting, the jacket temperature will be maintained within 2°C or F of the setpoint.

Steam circulates continuously in the jacket. Steam enters through the steam-to-jacket valve, circulates around the chamber, condenses, drops to the floor of the jacket, and ultimately exits the sterilizer through a steam trap. A pressure relief valve (PRV) is mounted on the jacket to prevent any unexpected over-pressure condition.

In a PRIMUS steam sterilizer, the steam from the jacket does not enter
the chamber as is common in several other brands. Independent
chamber and jacket steam flows provide finer temperature control in
the chamber.

Jacket Function During Sterilization

The jacket is kept constantly heated as long as the sterilizer is turned on. This keeps the chamber walls hot between cycles, thereby avoiding excessive condensate forming inside the chamber and severely wetting the load when steam initially enters the vessel.

Steam to the jacket is stopped when the sterilizer is turned off or when in the liquid slow exhaust phase.

During a sterilization cycle, the jacket is maintained within a very tight temperature band. This, in turn, allows the chamber temperature to be maintained in a half-degree C band around the required sterilization setpoint (121°C or 132°C).

In addition to preventing significant condensation from forming at the beginning of a cycle, at the end of a cycle, the jacket stays on during the final vacuum phase. This helps evaporate any remaining liquid, allowing porous loads to be dry when they are removed at the end of the cycle.

Periods of Inactivity

Many facilities only operate their steam sterilizers on one shift, five days a week. In most circumstances, it is advisable to keep the jacket hot during these relatively short periods of inactivity. This will allow for a quick start-up in the morning and can also be energy efficient.

Depending on the model, a PRIMUS steam sterilizer vessel can weigh over 20,000 pounds. If this metal mass is allowed to cool to ambient temperature, it will require a huge amount of energy to return the mass to the required sterilization temperature. By keeping the jacket warm, unnecessary thermal cycles on the metal can also be avoided.

Have a question or having difficulties? Talk to Dave Schall, our PRIMUS sterilizer expert who is here to assist you anytime at 877.679.7800 extension 1212 or email at dschall@spire-is.com.

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