June 17th, 2011 | Newsletter 1/2011

Licentiate Thesis

Airborne Contamination Risks in Autoclaves and Freeze-Dryers when Chamber Doors are Open

Abstract
Autoclaves and freeze-dryers are common process equipment used in the pharmaceutical industry. This type of process equipment can cause temperature differences relative to ambient air. During loading and unloading autoclaves and freeze-dryers, entrainment of room air into the loading chambers may occur creating contamination risks. To minimize these risks, high-efficiency particulate air filter units with unidirectional flow (UDF) can be used to provide the chamber openings with clean air protection.

The purpose of this thesis is to increase the understanding of the air movements through openings of autoclaves and freeze-dryers, establish a basis for dimensioning the UDF-units required for protection of the openings and to increase the knowledge of proper placement of the units. The methods used in the studies include visualisation of air movements, temperature measurements, air velocity measurements, particle measurements and computational fluid dynamics analysis.

The result showed that inflowing air covers a three times larger area of the chamber opening compared to the outflow when the chamber door is open to the autoclave/freeze-dryer and there is a temperature differences between the air in the chamber and ambient air. This means that the air velocity of the outflow is higher compared to the inflow. Based on the results from the experimentally studies, the discharge coefficient has a value of 0.5 when the temperature difference to the ambient room air is 20-80ºC. By using the value of the discharge coefficient it is possible to calculate the airflow through the opening of the autoclaves and freeze-dryers and also to design a well-functioning UDF-unit in order to protect the openings of this type of process equipment. An estimated value of the airflow from the UDF-unit is 10-20% greater than the airflow through the opening. Depending on the process (for example manually or automatic loading and unloading, the size and the design of the chamber door) even higher airflow can be required. For chamber openings with outflow in the lower part the UDF-unit should be placed above the opening with air velocities according to regulatory demands (0.45m/s or higher when the process so requires). Chamber openings with outflow in the upper part is preferably protected by horizontal airflow (0.45m/s or higher when the process so requires). However, vertical airflow with high air velocity (0.90m/s) could be used. 

Catinka Ullman
Building Services Engineering
Royal Institute of Technology, Stockholm, Sweden

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