Examining Dry Heat Sterilizers and The Depyrogenation Process

Due to the reduced efficiency of the micro-organism destruction rate, the utilization of dry heat for sterilization is less common than steam. Yet, dry heat is a suitable alternative for some materials which are sensitive to the presence of moisture.

In addition to sterilization, dry heat can also be employed to destroy pyrogens, however, this means that the temperatures required are a lot higher. This process is known as depyrogenation. A dry heat sterilizer can either be designed as an oven or a heat tunnel.

In both instances, hot, filtered air is circulated around or blown vertically down onto the objects which are being sterilized. The dry heat ovens differ in size but can easily be over several m³ in volume.

The hot section is typically much bigger (a number of meters long) for heat tunnels, and a conveyor belt moves the articles through the heating and cooling zones at a defined speed, making it a continuous process.

The temperature and exposure time are critical parameters. Usually, dry heat cycles will be performed at 160 to 190 °C with corresponding exposure times that range from 30 to 120 minutes.

For depyrogenation cycles, the minimum temperature is 200 °C for at least one hour when operating in ovens. As depyrogenation tunnels are operated at much higher temperatures, usually 325 °C or even 400 °C, the holding times are normally only a few minutes long.

Regular Calibration to Ensure the Most Accurate Data Possible

Calibration of the temperature sensors is required prior to use and for post verification in order to ensure that the sensors supply the most accurate data possible. For dry heat sterilizers, sensors must have an accuracy of ±0.5 °C, whilst an accuracy of ±1 °C is considered appropriate for depyrogenation ovens and tunnels. Some systems also utilize vacuum and so require pressure measurements.

Image Credit: Ellab

Challenges

As a regulatory requirement all dry heat sterilizers undergo qualification to verify specifications. This includes temperature and the holding time performance. The temperature influence on the measuring system must be considered due to the relatively high temperatures in the process.

Surface materials and even embedded electronic components must be resistant to heat exposure. However, for cable-based systems, the moving parts found in tunnels and the distances could prove challenging.

There are a number of other critical factors in addition to maintaining the minimum and maximum temperatures for set time periods. These include the spread of temperature across the belt during the sterilization period and the deviation of individual temperature sensors over the sterilization period.

Continuous Calibration of the Sensors

The continuous calibration of measurement sensors is another challenge when validating heat tunnel and oven processes. It is vital that users can document that the sensors are within an acceptable accuracy when they were utilized.

This means calibrating shortly before the validation run (Pre-Calibration) and then checking the accuracy afterwards to ensure that they are still within the predefined tolerances (Post Calibration). The validation study is invalidated if the post calibration check shows that the accuracy is outside the required limits.

Suitable Solutions from Ellab

As previously discussed, the temperatures being employed are highly dependent on whether the goal is dry heat sterilization or depyrogenation, but in both cases levels are relatively high.

Ellab provides two different solutions, a wired and a wireless system; whether to choose the E-Val™ Pro thermocouple system or TrackSense^® Pro wireless data loggers is dependent on the exact conditions.

Historically, wired thermocouple systems like the E-Val Pro were employed for dry heat sterilization and depyrogenation, supplying the accuracy and large number of channels required for validating sterilizers. Yet, the use of multiple cables may be a problem when introducing them through the door seal or along the tunnel conveyer belt.

Additionally, the type of cable is vital, as they must be flexible and able to withstand high temperatures, which is why only the mineral insulated cables or Kapton cables are suitable.

When discussing depyrogenation tunnels specifically, adding in the fact that moving parts are present in the tunnels, there is a further challenge for cable-based systems. TrackSense wireless data loggers are recommended because of this, as they present considerable benefits over wired systems.

Data Logger Advantages

• Loggers are easy to place within applications, reducing the set-up time significantly.
• As loggers are placed directly in applications, they follow the conveyor belt movements.
• Through the use of a thermal barrier around the logger/ battery, loggers are provided with enough protection to permit them to be employed at temperatures of up to 400 °C.

It is crucial to remember the duration of the heat period in addition to the maximum temperature in the tunnel in order to ensure that the barrier supplies enough protection for the logger. It can cause irreparable damage to the equipment if a logger is exposed to too high temperatures.

Ellab Calibration Equipment and Software

The two validation solutions offered by Ellab are slightly different in regard to measuring technology. The TrackSense wireless logger system provides increased accuracy and stability by using Pt-1000 based wireless sensors as opposed to the E-Val Pro thermocouple sensors.

This additional accuracy and long-term stability means less regular calibrations because of lower drift over time. For calibrating dry heat sterilizers, an accuracy of ± 0.5 °C is needed and for depyrogenation, an accuracy of ±1 °C is acceptable.

This is verified easily by employing Ellab calibration equipment in the shape of various dry block calibrators, operated directly by the ValSuite™ software, storing the offset values in a database. For traceability, all wireless loggers/sensors and thermocouples have a unique serial number which is engraved on the body and embedded in the firmware.

This ensures that the ValSuite software, which is utilized to handle both the E-Val Pro and TrackSense Pro, can carry out the required statistical analysis and Fh calculations for validation studies. These results can then be presented as PDF reports or printed, supplying a large amount of flexibility when it comes to meeting the necessary requirements.

Advantages of Selecting Ellab for Qualifying Dry Heat Sterilizers and Validating Depyrogenation Processes   

• FDA 21 CFR part 11 compliance
• Choose between wireless data loggers or wired thermocouple system that are operated in the same software
• Calibration facility embedded in software
• High measuring accuracy for the entire temperature range  
• Reports for full documentation, including Fh calculations

Image Credit: Ellab

This information has been sourced, reviewed and adapted from materials provided by Ellab.

For more information on this source, please visit Ellab.