Link: .
Name: Dan Bemi
Email: dbemi@megtec.com
Title: Regional Sales Manager
Company: MEGTEC Systems, Inc.
Address: 830 Prosper Road
City: De Pere
State: WI
ZIP: 54115
Country: U.S.A
Phone: 262-255-3275
Fax: 262-255-3578
Post Title: Contamination Control in Process Dryers
Most web-based products require some minimum level of contamination control to meet customer requirements and/or expectations. The level of cleanliness and the type of controls that may be required vary depending upon a number of factors including; aesthetics, product functionality, customer-driven quality control demands and regulatory-driven requirements. Any discussion of contamination control must begin with a clear definition of what the customer means by “clean.”
For many applications the term “clean” simply means, “No visible contamination.” Usually, this means free of any dirt or particulate that would result in the loss of aesthetic appeal of the end product whether it is advertising, product packaging or a graphic arts product. Dryers can be a source of visible contamination when the make-up-air (MUA) sources (both web slot infiltration and forced make-up-air) are themselves laden with airborne material. Particles larger than 50 um are normally accepted as “visible” to the naked eye. For these applications, simple furnace type filters (80% effective is typical) located at the inlet to the make-up-air openings will provide a clean enough MUA source. If dirt is still being introduced into the dryer through web slot infiltration, the air near the web openings must be clean since the dryer will be operating under negative pressure and will draw in whatever is contained in the ambient room air. Another potential source of such particulate can be the heat sources, particularly direct-fired, natural gas or fuel oil burner systems. Fuel oil burners are seldom used because they are well known contributors of particulate matter in the form of “products of incomplete combustion” including carbon fines, sulfur and other potentially reactive chemical components. Gas-fired burners, if improperly tuned, can also be a source of ‘products of incomplete combustion’.
Where product aesthetics are the primary concern, some combination of routine cleaning of the surrounding process room, MUA filtration and regular heat source maintenance will most likely be adequate to meet typical cleanliness requirements.
Another key driver of clean processing is product functionality. For instance, a release liner cannot function properly if the coating contains too much particulate matter since this contamination can affect its release properties. Similarly, window film producers must supply an optically clear film to meet automotive and industrial application standards.
For the release coating example, simple furnace type filtration along with routine cleaning of the surrounding process area will be most likely be adequate for maintaining product functionality. However, for tinted window films and other protective window film products, HEPA filtration is the standard for cleanliness along with stainless steel internal construction to prevent particulate from forming through the process of metal oxidation. HEPA filter banks will usually consist of a pre-filter section containing disposable furnace type filters, followed by high-efficiency filters that are rated for up to 99.97% removal efficiency on 0.3 micron-sized particles. Also, dryer and air-handling internals, including MUA ductwork, must be thoroughly vacuumed and solvent wiped after installation, in order to insure a clean starting point.
In some cases, customer quality control requirements will be the primary driver of cleanliness requirements. In these cases, it is important that the supplier gets a clear understanding of the customer needs and concerns in the form of a specification. This will allow the supplier and customer to conduct a cost /benefit analysis and work together to identify the best equipment features and configurations to meet the specific needs. For instance, dryers that feature split enclosures with retraction systems offer the best access for cleaning dryer internals, but they add to equipment cost and may be overkill for any given application.
When cleanliness requirements are driven by regulatory bodies such as the Food & Drug Administration (FDA), contamination control can be a primary driver of both dryer design, and fabrication practices. Some medical, pharmaceutical, and food packaging processes require high levels of cleanliness, but do not require compliance with any specific cleanness standard. However, clean-room standards (Federal Standard 209E/ISO 14644-1 and -2.) are the primary tools used to quantify “cleanliness” and the use of these standards is becoming more and more common. It is important to note that US Federal clean-room standards do not specify particular equipment design or fabrication requirements whereas ISO-14644-4 Cleanroom Design & Construction standard published in 2001 does offer some guidance. But, the main purpose of both standards is to specify the amount of particulate matter of a specific size (ranging from 0.1 um to 5.0 um) that is allowed within a given volume of ambient air contained within the process room or within the equipment operating inside the room. In other words, for the most part, these standards tell us where we need to go, not how to get there.
Some of the recommended manufacturing practices used to meet clean-room and FDA requirements include:
∑ Use of HEPA filtration for make-up and return air.
∑ Stainless steel materials for any surface that comes into contact with process air. NOTE: Often carbon or aluminized steel can be used for internal surfaces upstream of HEPA filters, sometimes including fan internals.
∑ Welds need to be ground smooth so as to remove any burrs that could pick off remnants of the rags or wipes used in the routine cleaning of internal surfaces.
∑ Use of fasteners should be limited to those areas of construction that require parts removal for maintenance and/ or operation.
∑ Internal surfaces may need to be polished to mirror finishes for some applications.
∑ Internal air handling designs should minimize sharp corners, indentations and crevices that can collect dust and debris.
∑ Dryers should include maintenance features like removable nozzles and supply and return air-header access to facilitate cleaning between product changes, or in the event of a web break.
∑ Carefully consider the design of any moving parts inside the dryer, such as retraction components or web threading systems, if utilized. These can be a source of contaminants. Avoid exposed lubricants and sources of debris from mechanical abrasion.
FDA Current Good Manufacturing Practices (CGMP’s) also offer guidance for companies interested in supplying goods or services used in the production of an FDA regulated process or product. For the most part, FDA cGMP regulations define the requirements of an overall quality system needed to manufacture pharmaceuticals and medical devices. Of particular importance is determination of the key parameters, specification limits on these parameters, written validation protocols, and validation of the process. FDA CGMP’s do not provide specific design and fabrication guidelines. Rather, they help insure that the design and methods of fabrication used to produce a “qualified” product are repeatable, and can be documented and tracked from the point of purchase of materials to delivery and start up and validation of the equipment.
In summary, contamination controls for process dryers can range from simple good house-keeping practices to rigorous cleanroom design and fabrication requirements. The level of controls that are required will depend upon product aesthetic requirements, functionality, customer specified needs and potentially, regulatory-driven needs. In all cases, it is of utmost importance that the customer specify a clear definition of “cleanliness” and establish a method of cleanliness validation that is based on key performance parameters.
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