Fluid Dynamics – Laminar Flow – Turbulent Flow – Transitional Flow & Particle Reduction

In physics and engineering, fluid dynamics is used to describe the flow of fluids – liquids and gases. For the sake of this discussion, it includes the subset of aerodynamics, which is the study of air and other gases in motion. Many clean room consultants chagrin at the idea of using compressed air movement inside of a clean room that was developed using Laminar Flow techniques. Laminar air flow by design is intended to be slow, smooth regular paths of an air pattern traveling from entrance to exit. The air then travels back through the pre-filters, to the laminar flow filters and back into the room as part of the air change rate per hour. Laminar air low patterns are important to keeping particulate moving out of the clean room, but what if products being process inside of the clean room are already contaminated with particles that could come from people, other items that were brought into the space or that were created by the process itself.

In order to clean particulate from a device, component or packaging material, compressed air devices are necessary. The air devices are typically in the form of ionizing air guns, blowers, nozzles or air knives. These air tools cause turbulent flow, which is fluid motion that agitates the parts and creates eddies, which are violent swirling motions caused by the position and direction of turbulent flow. Eddies can transport mass, momentum and energy across different regions of the flow, with a result being clean, static-free parts. Heat transfer also happens in turbulent flow. So why is heat important? With heat, the flow resistance decreases, making it easier to clean parts. The process of laminar flow becoming turbulent is known as laminar-turbulent transition. It is also known as transitional flow.

Is there a happy medium between using compressed air and maintain an acceptable level of laminar flow? Can we agree that compressed air is a requirement and that compressed air is turbulent? Static Clean believes in the idea of “Controlled Turbulence”. The placement of Static Clean Particle Trap® Systems, in conjunction with compressed ionizing air devices means that the turbulence is localized, particles are captured and removed from the process and the products and parts are clean. By using an ionizing air gun or similar device in front of a Particle Trap®, the debris is directed into the flow of these source capture systems and delivered into the filter media and not back into the clean room to re-contaminate cleaned parts.

Particle Size, Particle Retention & Process Practicality

Particle collection efficiency by a filtration device usually brings a common question. What is the ISO number that is associated with HEPA filters used in the Static Clean Particle Trap® series products? A recent request from a customer asked this exact question. Our answer in response was that the ISO rating on our HEPA Filter is ISO 40E-99.99% at MPPS. Obviously, the customer with the questions knew what to ask and was technically astute, but to a novice it may seem confusing, so first let’s establish what MPPS mean. It is the Most Penetrating Particle Size. Larger particles are unable to avoid the special filter media in a HEPA filter and they become embedded in the filter material. The smaller particles become the MPPS, which gives the HEPA their rating. For more critical filtration needs, ULPA filters are available and could have an efficiency of 99.99995% at MPPS.

For the world of static control, and filter efficiency of the Static Clean Particle Trap® Systems, we are mostly talking about HEPA filtration. But filtration only tells a part of the story. Although you can perform tests to validate HEPA filters and modern particle counters can provide information on airborne particulate, it doesn’t tell the story on how clean a medical injection molded plastic part may be or how many particles are on a catheter or the tray or package that is going to house the medical device. Yes, there are liquid particle counters that can verify all particle sizes, but real time production of high-volume parts means that, at best a visual inspection on the fly is the standard.

Most of the Medical Device Manufacturing is done in an ISO Class 7 or ISO Class 8 cleanroom, with an emphasis on ISO Class 8. Federal Standards FS 209E and ISO 14644-1 require specific particle measurements to verify the cleanliness of the clean room or clean area. When talking about an ISO Class 8 environment, it does mean that the maximum/particles/m3 allows for almost 30,000 particles in the 5 micron or smaller range. It also means two other things as well. There will be particles greater than 5 microns in an ISO 8 space and that total reliance on a cleanroom is not the complete answer. The use of additional filtration methods at key points in the manufacturing process will improve yields by reducing particles on products and in single use packaging that may finds its way to the hospital or clinic. The fact that ionization is used to control static on medical devices, optics and industrial environments is common knowledge, but, source capturing debris at critical stages in the process is less understood but becoming more accepted as the right tool at the right time.

Particle Trap® products are small, benchtop or floor level source-capture systems, that incorporate both pre-filters and HEPA-filters in series, whereby the pre-filter catch the larger particle and the HEPA, (the same used in the clean room construction), captures the smallest debris. What this means for the customer is that particles are taken out of the room at the source and by source, I mean either where they are created or where they can do the most harm and end up inside of a finished package. Regardless of what ionizing blow off device is used in your process, you can rely on Static Clean to make things cleaner and your customer smile.

Where do the particles go?

Where do the particles go?

Did you ever rub a balloon on your hair and stick it to the ceiling? The balloon sticks because you’ve created static electricity on the surface of the balloon. This energy is non-moving static charge. Every material is made up of atoms and they are the basic building blocks of ordinary matter and they can join to form molecules, which is a basic ingredient of most of the objects around us. An atom can hold a positive charge that is called a proton or a negative charge that is called an electron. Atoms with the same charge or polarity repel each other, while those with the opposite charge are attracted to each other. Just like the balloon scenario, static is created by the contact and separation of two materials. The same is true of when you walk across a carpet and touch the metal door knob and get a shock. We call these electrostatic forces, tribo-charging, which renders a plastic material in a state where it can attract dust and other particulates.

Let’s face it, we are using more plastic in our every day lives, from cars to single use medical devices that may end up inside of the human body. Plastic, being highly insulative, can store huge amounts of static electricity. If we just look at the medical device sector, one of the biggest reasons for rejects, rework and potential device failure, is from foreign particles that end up in the finished device. These particles could be in the form of airborne contaminants, plastic flash, skin flake, human hair and other debris that is found in the manufacturing process. Static eliminators in the form of ionizing air guns, nozzles and blowers are used to negate the ill effects of electrostatic forces that pull particles right out of the air and hold them to a device or components. The use of ionized air is absolutely a good practice, but the problem is “where do the particle go”? Typically, they hang around and end up on the worksurface to be a source of re-contamination or the particles end up downstream on already cleaned products.

Enter the Particle Trap® 6000. The Particle Trap® 6000 (PT6000) is the solution to getting rid of particles in the assembly and packaging areas of the medical device manufacturing process. The PT6000 is a source capturing system with a HEPA filter on the exhaust. You can still use the conventional ionizing air blow-off devices, but when working in front of the opening of the PT6000, dislodged particles now are delivered through a pre-filter and then through the HEPA filter, ensuring only clean air is let back into the room. The PT6000 is used not only to clean medical components, but it is especially helpful when used at the packaging level, just prior to the heat sealing of a lid stock to the thermoformed tray. The same would be true for pouching of products such as a catheter on a die cut card being slid into a long plastic bag and then sealed at the end. Normally, most medical device manufacturers do a 100% inspection for foreign matter/particles inside of the seal trays. If a particle is discovered, the lid is ripped off, the product taken out, recleaned and then repackaged. This reject rate is also called the tear down rate, which translates into poor yields, time and money along with customer dissatisfaction, when a product gets through that is not totally cleaned.

Who would benefit from the Particle Trap® 6000? The Particle Trap® 6000 and its sister products, the PT Mini, Particle Trap® CUBE and Medical Cleaning Systems. While the medical device sector has endorsed these products, they also have application in the optics, food and electronics industries for the same reason why all companies are looking to lower their tear down rates, which translates to higher profits. If you want to learn more about how Particle Trap® products can help improve your process, please visit the Static Clean Website, www.staticclean.com or call us direct for expert technical advice.