Tunnel Sterilizer Validation

Principles as described above for the physical process validation of batch ovens
apply also in the validation of tunnel sterilizers; however, in addition to the
variables affecting batch oven validation, tunnel sterilizers have an extra variable-
belt speed. This variable can be held constant by maintaining the same belt
speed throughout the validation process and not changing it after validation has
been compzleted.
Air Balance Determination
Proper and even air balance is more critical to a tunnel sterile process than a
batch oven process. Since the items being sterilized are moving, they are exposed
to different air systems (e.g., heating zone and cooling zone). Air flow
must be balanced in order to provide a gradual decrease in air temperature as
items move along the conveyor. In the absence of a critical balance of air dynamics,
either the items will not be cooled sufficiently once they exit the tunnel
or they will be cooled too quickly, causing the glass to shatter and contaminate
the entire tunnel area with particles. In fact, the major problem in validating tunnel sterilizers is the control of particles. Not only are the items exposed to
great extremes in temperature, but also the conveyor belt is a natural source of
particulates because metal is moving against metal.
Adjustments in the air source should be made to obtain a controlled flow
of air within the tunnel and across the entrance and exit openings. Air must be
particulate-free as it enters the tunnel area; therefore, all high efficiency particulate
air (HEPA) filters in the tunnel must be integrity tested and certified prior
to validation studies.
2. Heat-Distribution Studies
Thermocouples used in tunnel sterilizer validation must be sufficiently durable
to withstand the extremely high (≥300°C) temperatures in the heating zone area
of the tunnel. Heat-distribution studies should determine where the cold spots
are located as a function of the width of the belt and height of the tunnel chamber.
Trays or racks of ampules or vials should be run through the tunnel and
thermocouples placed at strategic locations among the containers.
Bottle-mapping studies may also be conducted during this phase [21]. The
purpose of these studies is to determine possible locations inside the container
that are most difficult to heat. The loading configuration should be identical to
what will be used in production cycles. The major difficulty in doing these
studies is the avoidance of thermocouple wire hang-ups. Thermocouples must
be long enough to be transported through the entire tunnel. A special harness
for thermocouple wires should be constructed for feeding these wires into and
throughout the tunnel.
Repeatability of the thermal process must be demonstrated during these
studies. Peak temperature readings should remain within ±10°C across the belt
for at least three replicate runs.
3. Heat-Penetration Studies'
Prior to microbial challenge testing of the tunnel sterilization, heat-penetration
studies must be completed in order to identify the coolest container in the entire
load. Results of heat-distribution studies should aid in predicting where the coolest
location within the load should be. Thermocouples should be deposited at or
near the coolest point inside the container as determined previously from bottlemapping
studies. Normally, the coolest point inside the container is at the juncture
of the bottom of the container and the sidewall. The container’s inner surface
should be in contact with the thermocouple tip because the objective is to
sterilize the inner walls of the container, as well as the inner space.
Three to five replicate runs for each commodity size and every loading
configuration should be done using 10 to 20 thermocouples distributed throughout
the load. Careful analysis of the temperature data after each run will be
invaluable in the determination of the cool spot and the repeatability of the
process using the minimum number of replicate runs.
4. Mechanical Repeatability
Tunnel sterilizers must demonstrate mechanical repeatability in the same manner
as batch ovens. Air velocity, air particulates, temperature consistency, and
reliability of all the tunnel controls (heat zone temperatures, belt speed, and
blower functions) must be proved during the physical validation studies.