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7676 á1227ñ / General Information USP 41

Table 2. Error as a Percentage of Mean for Plate Counts (Continued)

cfu per Plate Standard Error Error as % of Mean
6 2.45 40.8
5 2.24 44.7
4 2.00 50.0
3 1.73 57.7
2 1.41 70.7
1 1.00 100.0

á1228ñ DEPYROGENATION

Change to read:

INTRODUCTION

The production of parenteral products requires not only that products be sterile, but that they are also free from harmful
levels of pyrogens. Depyrogenation is defined as the direct and validated destruction or removal of pyrogens. For the purposes
of this and subsequent chapters of the á1228ñ series, the term “depyrogenation” refers to the destruction or removal of bacte-
rial endotoxins, the most prevalent and quantifiable pyrogen in parenteral preparations. The chapters in this series discuss de-
pyrogenation procedures that are applicable to product streams, equipment, and drug product containers and closures.
The á1228ñ series builds on the tenets described in Sterility Assurance á1211ñ (CN 1-May-2018), first published in USP 20–NF 15.
In the time since that publication, the science of depyrogenation has advanced, as has control over manufacturing processes.
Manufacturers are very aware of the impact that facility, equipment, and process design has on endotoxin levels and depyro-
genation, and they are mindful of the effects of elevated endotoxin levels in raw materials and water systems. Risk manage-
ment tools, including process evaluation tools such as Hazard Analysis Critical Control Point (HACCP), are helping companies
to identify critical control points (CCP) for the control of endotoxin and other impurities or contaminants in the manufacturing
process. As a result of these advances in science, technology, and philosophy, the á1228ñ series will also offer alternatives to
historical or traditional thinking.

BACTERIAL ENDOTOXIN AND LIPOPOLYSACCHARIDE

Bacterial endotoxin is a component of the outer cell membrane of Gram-negative bacteria. The natural endotoxin complex
contains many cell wall components including phospholipids, lipoproteins, and lipopolysaccharide (LPS), which is the biologi-
cally active component of endotoxin. Purified endotoxin is chemically defined as a LPS. LPS consists of three distinct regions:
1. The hydrophobic lipid A portion of the molecule is highly conserved among Gram-negative bacteria, and is largely re-
sponsible for most, if not all, of the biological activity of endotoxin.
2. A core oligosaccharide links the lipid A to the hydrophilic O-specific side chain or O-antigen.
3. The hydrophilic O-antigen is a highly variable region that confers serological specificity to the organism and is often used
to distinguish strains of Gram-negative bacteria.
General Chapters

Due to the amphipathic nature of the LPS molecule [i.e., having both a polar (hydrophilic) end and a nonpolar (hydropho-
bic) end], purified LPS preparations, such as reference standard endotoxin (RSE) and control standard endotoxin (CSE) tend to
form bilayers, micelles, ribbons, and other conformations when in solution, and they may adsorb, or “stick”, to surfaces, mak-
ing them difficult to extract and detect. The degree of adsorption of LPS to solid surfaces is affected by the composition and
finish of the material to be depyrogenated. The extent of aggregation of LPS in solution is affected by a host of formulation
matrix attributes to which it is exposed, such as temperature, pH, salt concentration, divalent cation concentration, detergents
or emulsifiers, and chelating agents.
When parenteral products are contaminated with endotoxin, the contaminant is not purified LPS, but rather whole cells or
cell wall fragments generated during the normal growth cycle of the bacteria or disruption of bacteria, where the LPS remains
embedded in or associated with other cell wall components. Purified LPS and native endotoxin are dissimilar in many respects,
and the two terms should not be used interchangeably. Depending on the materials of construction or the formulation of the
article to be depyrogenated, the use of native endotoxin as a challenge material in depyrogenation studies may be a consider-
ation because a native endotoxin preparation better reflects operational reality, particularly for the depyrogenation of product
streams, and because LPS molecules in natural endotoxin are embedded in cell wall complexes, they may be much less prone
to the aggregation and adsorption issues seen with purified LPS.

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USP 41 General Information / á1228ñ 7677

For the purposes of the á1228ñ series, the term “challenge material” will be used to generically describe material (endotoxin
or LPS) used as a spiking analyte for depyrogenation studies. “Endotoxin” will refer to the moiety in its natural state, meaning
pieces of Gram-negative cell wall from a well-characterized source. LPS will refer to the purified material.

MEASURING ENDOTOXIN PRE- AND POST-PROCESSING

The primary procedure used for the measurement of bacterial endotoxin is the Bacterial Endotoxins Test á85ñ (BET). A well-
controlled BET assay can provide assurance of accurate readings for the calculation of the reduction in challenge material activ-
ity pre- and post-processing, as well as provide consistent quantitation of levels of native endotoxin in raw materials, at CCP in
the manufacturing process, and in finished products.
There are many variables in study structure and test method that can affect the outcome of a depyrogenation study. Devel-
opment of a test method depends on the material under test, the identification of an appropriate challenge preparation, and
the method used to extract recoverable activity prior to processing and residual activity after processing. Once a test system is
developed that includes the identification of a source for challenge material preparations, inoculation of the articles to be de-
pyrogenated (including drying procedures), extraction or recovery methods, and appropriate BET test methodology and sensi-
tivity, it is recommended that subsequent tests should use the same conditions to ensure the comparability of test results.
Points to consider when constructing a depyrogenation study include the following:
1. The challenge material: Consider the source of the endotoxin (purified or natural). When using purified LPS, choose a
preparation with no fillers, because the presence of these fillers can add to the variability and therefore decrease the accu-
racy of the assay. Once challenge material preparation is chosen, it is recommended to use material from the same source
in subsequent studies to reduce variability.1
2. The characteristics of the material being depyrogenated: It is important to understand the characteristics of the mate-
rial being tested. For example, LPS may adsorb to plastics, and although two objects may be made of the same plastic,
surface finish, surface area, and conformation differences may affect extraction efficiency and LPS recovery. For solutions,
formulation matrices may affect aggregation of purified LPS in that pH, salt concentration, chelating agents, surfactants,
and the presence of divalent cations may all have an impact on the recovery of the challenge material. The use of natural
endotoxin may mitigate some of these recovery issues. For materials that are received with low or undetectable levels of
endotoxin, depyrogenation studies using endotoxin or LPS challenge materials may be unnecessary if control is demon-
strated and decisions are scientifically justified.
3. The level of activity needed for the study: How much pre-processing activity do you need to execute the study? The
current industry standard is to add enough endotoxin to the system so that at least 1000 EU can be recovered prior to
depyrogenation. However, depending on the test system, 1000 EU may be either excessive or insufficient. For example,
when designing a study for the depyrogenation of a product stream that normally contains <1 EU in a certain volume, a
spike of 1000 EU of a CSE in that same volume may be excessive. For solid-surface materials, the level of activity in the
challenge material should be established taking into account the materials of construction and finish as they may contrib-
ute to LPS adsorption. Knowledge of historical levels of endotoxin in or on the surface, the efficiency of the depyrogena-
tion processes, the efficiency of the challenge material extraction or recovery method, and the log reduction or safety lev-
el target acceptance criterion are all important to the setting of a pre-processing activity requirement. If a reduction study
for a product stream is required, it may be more appropriate to add an amount of naturally occurring endotoxin to the
product consistent with the maximum expected endotoxin load (“worst case”), based on known endotoxin contributors
(e.g., raw materials and water) and process capability to demonstrate reduction to safe levels. Whatever the procedure,
the logic and methodology for endotoxin reduction studies should be justified and documented. For those materials that
routinely contain a level of endotoxin, such as fermentation broths where there are high levels of activity to begin with, it

General Chapters
may not be necessary to add additional challenge material.
4. Preparation of test samples: The method used to affix challenge materials to the surface of materials to be depyrogen-
ated may affect its removal or recovery. Air-drying is the most convenient method of affixing challenge material to hard
surfaces, but freeze drying and vacuum drying also have been used. To improve drying efficiency, it is suggested that a
small volume of a highly concentrated activity of challenge material be used. This volume may be added to the surface of
the item in an area that has been defined as “hardest to depyrogenate” or may be dispersed to represent the more likely
natural occurrence. Inoculation methodology must be well defined for comparability across studies. Decisions regarding
the design of studies must be documented and justified.
5. Recovery methods: Although there are standard methods for the recovery of endotoxin from medical devices, there is no
standard method for the recovery of endotoxin dried onto solid surfaces that are used as indicators in depyrogenation
studies. Although it is most convenient to adopt the standard extraction methods described in Medical Devices—Bacterial
Endotoxin and Pyrogen Tests á161ñ and ANSI/AAMI ST72:2011,2 a laboratory may choose to develop and validate a meth-
od that better suits the material under test. The efficiency of the recovery of endotoxin will depend on the attributes of
the material under test, the composition of the challenge material, the concentration of the challenge material spike, and
the method of drying. Recoveries of less than 100% of the challenge material nominal spike in positive controls are not

1 Ludwig JD, Avis KE. Recovery of endotoxin preparations from the surface of glass capillary tubes. J Parenter Sci Technol. 1989;43(6):276–278.
2 ANSI/AAMI ST72:2011. Bacterial endotoxins—Test methods, routine monitoring, and alternatives to batch testing, AAMI, Arlington, VA.

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7678 á1228ñ / General Information USP 41

uncommon. Perhaps more important than the percent recovery in positive controls is consistent recovery across lots of
the same material and across depyrogenation studies.3,4
6. Test method: Lysate formulations differ, and they may be subject to different interferences such as leachables, chelators,
and salts. If interferences are encountered during method development that are difficult to overcome, consider trying an-
other test method (e.g., gel clot, kinetic chromogenic, kinetic turbidimetric, endpoint chromogenic) or a different source
of reagent.
7. Depyrogenation method: Variability in the depyrogenation treatment may impact recovery. Treatments vary in efficien-
cy, both within and between treatment types. For example, a large difference in efficiency will be seen when comparing
the depyrogenation of glass vials using dry heat versus rinsing in water for injection (WFI), and differences may be seen in
the reduction of challenge material by the filtration of a solution depending on the type of filter chosen. Likewise, variabil-
ity can be seen when using a single depyrogenation methodology with two different materials, such as the filtration of
two different solutions using the same type of filter medium.
The efficiency of a depyrogenation process has been historically measured in terms of a logarithmic (log) reduction of a
large “spike” or bolus of purified endotoxin that is added to or dried onto a material prior to treatment. Although log reduc-
tion is a convenient measurement benchmark, the more relevant and pragmatic indicator of depyrogenation efficiency is one
based on process capability and patient safety, which is the reduction of the measured or anticipated worst case natural levels
of contaminating endotoxin to safe levels as defined by reference to calculated endotoxin limits for the material.
With the implementation of the principles of quality by design (QbD) and risk management, the 3-log reduction that was
first introduced in 1984 may be inappropriate as a universal benchmark for modern depyrogenation processes. For example,
materials with a high level of native endotoxin, such as fermentation broths, could require more than a 3-log reduction to
reach a safe level, whereas dry heat depyrogenation of materials with normally low or undetected levels of native endotoxin,
such as washed glass vials, will require substantially less than a 3-log reduction. The appropriate endotoxin log reduction for
the process should be determined by the user based on a full understanding of the product and process capability including
input sources, levels of endotoxin, efficiency of depyrogenation methods, and output (product- or process-specific) endotoxin
requirements. Effective process control requires knowledge of input, in-process (where appropriate), and output endotoxin lev-
els. Under these circumstances, with appropriate process development, justification for reduced endotoxin challenges or the
elimination of endotoxin challenges may be made based on historical data and demonstration of continued control.

CONTROL OF ENDOTOXIN IN PARENTERAL PRODUCTS

The best control of endotoxin levels in parenteral products is the control of Gram-negative bioburden in raw materials,
equipment, process streams, and manufacturing environment and operators. Parenteral manufacturers may exercise three cat-
egories of control to keep endotoxin content in drug products at safe levels.
The first category is “indirect control”, which is comprised of a series of preventive measures that control bioburden, the
potential endotoxin contribution by formulation components (e.g., raw materials, APIs, excipients), water, primary packaging
components, equipment, and the manufacturing environment, including personnel.
The second category is “process control”, in which endotoxin is monitored at CCP during processing to ensure that there is
no increase in endotoxin. These process control elements are subject to validation or qualification.
The third category is “direct control”, or the direct destruction or removal of endotoxins from product streams, equipment,
and primary packaging materials. As with controls on processing, direct measures of endotoxin destruction or removal must
be validated.

INDIRECT CONTROL
General Chapters

Reducing opportunities for Gram-negative microbial proliferation at any stage of manufacturing will reduce the likelihood of
endotoxin contamination in the following ways:
• Exercising control over the endotoxin content of incoming materials, particularly materials derived from natural sources or
those with high water activity, will reduce the opportunity for Gram-negative microbial proliferation and therefore reduce
or eliminate the need for endotoxin removal downstream. Because of their manufacturing processes, glass and plastic
containers as well as elastomeric closures are often received with very low or undetectable levels of endotoxin. Qualifica-
tion of primary packaging suppliers should include an audit that examines and confirms the supplier's consistent and
documented control over applicable manufacturing processes.
• Bioburden and endotoxin control should be a component of a vendor audit and supplier qualification program for formu-
lation materials that could potentially contribute endotoxin to parenteral products.
• Water is the most ubiquitous raw material in the manufacturing of parenteral products, but unless the generation and
distribution of high-quality water is properly validated and controlled, the system will be prone to contamination by
Gram-negative bacteria and the establishment of biofilms that can contribute significantly to the endotoxin load of the

3 LAL Users Group. Preparation and use of endotoxin indicators for depyrogenation process studies. J Parenter Sci Technol. 1989;43(3):109–112.
4 PDA Technical Report 3 (Revised 2013). Validation of dry heat processes used for depyrogenation and sterilization. Bethesda, MD: Parenteral Drug Association;
2013.

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USP 41 General Information / á1228ñ 7679

product (see Water for Pharmaceutical Purposes á1231ñ for a discussion of the types of waters used in pharmaceutical man-
ufacturing and guidance for validation, maintenance, sampling, and testing of systems).

PROCESS CONTROL

Product-specific process control requires the identification of CCP for the introduction or removal of endotoxin. Process con-
trol requires good process and equipment design consistent with QbD using a risk management tool such as HACCP. Process
control measures include but may not be limited to the following:
• Control of manufacturing practices is essential to endotoxin control. Endotoxin control should be a part of validated
cleaning procedures. The use of product contact materials for which endotoxin control cannot be established should be
avoided. Clean product-contact equipment should be stored dry to avoid bioburden and Gram-negative bacterial prolifer-
ation.
• Hold times during manufacturing, particularly for nonsterile bulk in-process materials and drug product, should be valida-
ted to ensure that the hold conditions do not support microbial proliferation and therefore potential endotoxin produc-
tion by Gram-negative bacteria.
• Environmental control, including good manufacturing practices (aseptic, as appropriate), is essential to process manage-
ment of endotoxin. Operators should be properly garbed and trained. Housekeeping and disinfecting practices should be
established to reduce the possibility of microbial proliferation in critical areas. Cleaning regimens should emphasize that
standing water be removed at the end of the cleaning process.
• Endotoxin control and monitoring will be covered in a forthcoming chapter.

DIRECT CONTROL

These processes require validation to ensure that endotoxins are removed or reduced to safe levels. Direct control may be
accomplished by a variety of methods and processes that may be combined to ensure endotoxin reduction to a safe level. The
most commonly used depyrogenation processes and the associated control measures are the subject of the á1228ñ series.

SELECTION OF AN APPROPRIATE DEPYROGENATION METHOD

The basic principles for the control and validation of a depyrogenation process using a life cycle approach include:
• Assessment of manufacturing processes to identify materials and process components that are essential to the control of
endotoxin
• Focused depyrogenation process development that is consistent with the material to be depyrogenated, the resident en-
dotoxin level of the material to be depyrogenated, and the limit of endotoxin for the finished article
• Adequate validation studies
• Ongoing monitoring of process controls to ensure continued efficacy of the depyrogenation process
• Identifying and documenting changes (a change control program) to the depyrogenation processes over time
Known or anticipated levels of endotoxin can be determined so that appropriate indirect or direct measures of control, con-
sistent with the product or materials of construction, will ensure that endotoxin is eliminated or reduced to levels that ensure
product and patient safety.
Should direct methods of depyrogenation be required for the control of endotoxin in or on the article, an important consid-
eration during manufacturing process development is the selection of an appropriate method from the possible alternatives:
dry heat, chemical, filtration, or physical removal. In some instances this selection is limited by the potential effects of the de-

General Chapters
pyrogenation treatment may have on the materials themselves. The choice of the appropriate process for a given item requires
knowledge of depyrogenation techniques and information concerning effects of the process on the material being processed.
The selection of a particular treatment (and the details of its execution) often represents a compromise between those condi-
tions required to destroy the endotoxin or remove it to the desired level and the effect of the process on the materials. Depyr-
ogenation processes should be no more aggressive than required for effective process control to avoid adverse consequences
to material quality attributes.

VALIDATION OF A DEPYROGENATION METHOD

The validation program comprises several formally documented stages to establish that the depyrogenation process is capa-
ble of operating within prescribed parameters for process equipment, that independent measurements of critical parameters
are possible and accurate, and that acceptance criteria for challenge material removal or destruction are met.
• The development stage investigates and establishes the operating parameters that define the controls to be used for the
depyrogenation process.
• The installation qualification (IQ) stage establishes that equipment controls and other instrumentation needed to execute
depyrogenation processes and measure the results of the depyrogenation process are properly designed and calibrated.

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7680 á1228ñ / General Information USP 41

Documentation should be available to demonstrate the acceptability of any required utilities such as steam, water, and
air.
• The operational qualification (OQ) stage confirms that the equipment and other processes components function within
the defined depyrogenation parameters.
• The performance qualification (PQ) stage of the validation program directly evaluates the depyrogenation of materials or
articles. Wherever possible, these studies should employ or simulate the actual conditions of use, including the use of real
or simulated product material. Worst case conditions, for example, might include the bracketing of critical parameters
such as time/temperature and belt speed for dry heat depyrogenation of glass vials, flow rate for depyrogenation by filtra-
tion of solutions, and maximum measured, anticipated, or defined endotoxin loads for any material. “Worst case” should
be defined and justified in the validation protocol. Endotoxin indicators may be utilized to support physical measurements
in the validation of the depyrogenation process. Although the “rule of three” suggests that three consecutive successful
validation runs be executed, perform sufficient replicate studies to demonstrate the capability and efficiency of the depyr-
ogenation process, including the validation of operational ranges of any equipment used in such processes. The number
of replicate studies chosen should be scientifically based and justified. At the end of the PQ, a report is written to establish
operating parameters.

ROUTINE PROCESS CONTROL

Once a depyrogenation process has been validated it must be maintained in that state to ensure the continued acceptability
of its operation. This is accomplished through a number of related practices essential for continued use of the process.
• Physical measurements: Data reported by the equipment sensors and recorders must be verified after the completion of
each depyrogenation cycle.
• Calibration: Any equipment used in the control or quantitative assessment of parameters required for a depyrogenation
process must have its measurement accuracy verified against a traceable standard on a periodic basis.
• Preventive maintenance: There should be a defined maintenance schedule for each piece of process or testing equip-
ment required for depyrogenation that is consistent with the manufacturer’s written recommendation.
• Ongoing process control verification: Depending on the specifics of the particular depyrogenation process, there may
be additional requirements for ongoing confirmation of process efficacy. These can include the testing of raw materials,
water supplies, and in-process sampling. These performance parameters are monitored against assigned limits designed
to ensure that finished products meet acceptable endotoxin levels. Monitoring of operating parameters and controls plays
an important role in maintaining the depyrogenation process in a validated state.
• Periodic reassessment: It is expected that the effectiveness of depyrogenation processes be reconfirmed on a periodic
basis. A reassessment schedule should be formalized to assess the potential impact of de minimis or undetected changes
to maintain the process in a validated state.
• Change control: In order to remain in a validated state, the various material, procedure, and equipment elements im-
pacting the depyrogenation process should be carefully monitored to ensure that changes are properly evaluated for their
potential impact on the process. The scope of the change control program must include materials being processed, proc-
ess equipment, processing parameters, and process holding time limits. The extent of the effort required to support a
change will vary with the potential impact of the change on the process outcome.
• Training: Depyrogenation processes rely heavily on scientific principles for the effective destruction or removal of endo-
toxins. Scientists and engineers well-grounded in the principles of endotoxin removal and testing develop processes to
ensure effective depyrogenation. Individuals involved in the development of depyrogenation processes require a back-
ground in microbiology, physics, chemistry, and engineering, and they must be familiar with good manufacturing princi-
General Chapters

ples and regulations. Depyrogenation is an interdisciplinary activity where the combined knowledge of a group of individ-
uals is generally required for the establishment of a reliable process. In addition to the depyrogenation process develop-
ment team, individuals responsible for the maintenance and operation of depyrogenation processes must also be trained
appropriately to ensure that their actions contribute to success. The operators are often the first to identify changes in
process performance because of their intimate involvement with it. Effective training programs should be established and
documented. Training programs should emphasize depyrogenation principles, adherence to established processes and
procedures, and the importance of documenting deviations from normal operations.

ROUTINE TESTING

Testing is not a control mechanism but rather a tool to assess the effectiveness of control measures. Depending on the spe-
cifics of the particular depyrogenation process, there may be additional requirements for ongoing confirmation of process effi-
cacy.
These requirements can include the testing of raw materials, water supplies, and in-process sampling. These performance
parameters are monitored against assigned limits based on historical data, and are designed to ensure that finished products
meet acceptable endotoxin levels. Monitoring of operating parameters and controls plays an important role in maintaining the
depyrogenation process in a validated state.

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USP 41 General Information / á1228.1ñ 7681

á1228.1ñ DRY HEAT DEPYROGENATION

INTRODUCTION

Dry heat is the method most frequently used for the depyrogenation of heat stable materials. Dry heat depyrogenation is
dependent upon two parameters: time and temperature, which equates to a thermal input. As a result, dry heat depyrogena-
tion processes can be easily monitored/controlled and are highly reproducible. Depyrogenation processes typically operate at a
range of temperatures from approximately 170° up to about 400°.
The most prevalent pyrogenic agents in parental manufacturing that are of concern relative to patient safety are bacterial
endotoxins, found in the outer cell walls of Gram-negative bacteria. The destruction of bacterial endotoxins (depyrogenation)
by dry heat has been studied extensively and has been shown to follow first order kinetics. The well-defined kinetics of inacti-
vation makes it possible to predict the efficacy of dry heat processes operating at different times and temperatures by under-
standing the total thermal input (FD).
The range of temperatures used for dry heat depyrogenation overlaps the upper range of temperatures used for dry heat
sterilization (see Dry Heat Sterilization á1229.8ñ). This is because bacterial endotoxins are more resistant to the effects of dry
heat than the most heat-resistant bacterial spores. This chapter provides an overview of the process of dry heat depyrogena-
tion, its control, and validation.

TECHNOLOGIES USED FOR DEPYROGENATION BY DRY HEAT

Although all dry heat depyrogenation processes rely strictly on time of exposure and temperature to assure effectiveness, the
equipment used typically falls into two categories: the dry heat “batch” oven and continuous tunnel systems. Batch ovens are
routinely used for the depyrogenation of product containers, most often glass, but also other heat stable product contact parts
or laboratory equipment. Continuous tunnels, on the other hand, are used primarily to depyrogenate glass product containers.

Batch Ovens

Circulating heated air is used to heat the load items, which may be individually covered or wrapped in a material that is
unaffected by the temperature used, or placed in a lidded container for protection during pre- and post-process handling.
When depyrogenation and sterilization are to be achieved in the same process, air supplied to the oven is passed through one
or more high efficiency particulate air (HEPA) filter(s) to maintain sterility within the oven after completion of the dwell period.
These forced air ovens typically operate at a positive air pressure differential relative to the surrounding room. This design re-
sults in particulate air quality that can meet ISO 5 requirements to reduce particulate matter and microbial contamination risk
throughout processing.
In order to ensure sufficient lethality and process control, oven control probe(s) must maintain a predefined temperature for
a predefined time period prior to cooling. The limited heat transfer capacity of air requires that items in the oven be placed in
fixed locations confirmed acceptable during the cycle development/validation effort. Caution should be exercised in defining
variable load patterns as minimum load sizes may result in inadvertent slower heating of the load and greater temperature
variability. Smaller facilities may use a single door oven, but the principles of operation and validation are the same as with
larger double door production units. The important batch oven process variables are set-point temperature, duration of dwell
period, load type and configuration, airflow characteristics, and container size.

General Chapters
Continuous Tunnels

The use of tunnels for dry heat depyrogenation of glass containers on a moving conveyor allows for substantially higher
throughput and packing densities than the batch process, reduces handling, and is ordinarily integrated with a washing and
filling system. Tunnels typically use forced heated air systems or radiant IR systems that recirculate air through a battery of
HEPA filters. Load items in tunnels are typically fed directly from an integrated container washing system.
Depyrogenation tunnels have separate zones for heating and cooling, allowing for continuous in-feed and discharge at tem-
peratures appropriate for production purposes. The tunnel is maintained at constant airflow and temperature conditions dur-
ing use, and as glass passes through the tunnel it is heated to depyrogenating temperatures and cooled before exiting. Al-
though the conditions within the tunnel are essentially constant and well controlled, the temperature of the glass as it passes
through the tunnel on the conveyor will change with its location. Dwell time is controlled by adjusting the conveyor speed,1
which in the depyrogenation tunnel is the process parameter that governs exposure time.
The air in the tunnel is most commonly heated using electrical coils but other heat sources, such as infrared or high-pressure
steam, have been used. For energy conservation, heated air in depyrogenation tunnels is often recirculated. The important

1 Not all tunnels have a variable speed capacity.

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