In filter manufacturing, filter testing is essential for ensuring high product quality, equipment efficiency, and regulatory compliance. While offline batch testing is acceptable for certain products, devices such as the P100, FFP3/P3, or similar respiratory filters and cartridges require quality testing of each and every product directly on the production line. Filters play a critical role in removing contaminants that could compromise products, protecting machinery from wear and tear, and maintaining a safe environment for workers.
Factors in Filter Testing
Several factors can significantly affect filter test results, highlighting the need for precise and controlled testing conditions:
- Particle Size and Distribution: The size and distribution of particles being filtered are critical parameters. Filters must be tested against a range of particle sizes to ensure they can effectively capture contaminants of all sizes, especially the smallest and most challenging particles.
- Aerosol Type and Composition: Different filters may perform variably depending on the type and composition of the aerosol. It's important to test filters with the specific types of aerosols they will encounter in their intended applications.
- Particle Charge and Neutralization: The charge of particles can influence how they interact with the filter media. Neutralizing the particle charge during testing can provide a more accurate measure of the filter's performance in real-world conditions.
- Face Velocity: This refers to the speed at which air passes through the filter. Testing filters at various face velocities helps determine their efficiency and effectiveness under different airflow conditions.
- Detector Type: The type of detector used in testing can impact the results. Advanced detectors, such as optical particle counters, provide precise measurements of filter efficiency across a range of particle sizes.
- Media Type: The material and construction of the filter media are crucial for performance. Testing should account for different media types to ensure they meet the required standards and perform effectively.
- Mass Loading Over Time: As filters collect particles, their performance can change. Evaluating how the mass loading of particles over time affects filter efficiency helps in understanding the filter's lifespan and maintenance needs.
Key Equations in Filter Testing
Understanding key equations is fundamental to interpreting filter test results accurately.
- Penetration = Downstream / Upstream = 1 - Efficiency
- Face Velocity = Flow Rate / Area
Effect of Aerosol Type, Face Velocity, and Media Type
The results of filter testing are significantly influenced by the type of aerosol, face velocity, and media type used during the tests. For a given media, different aerosol types can produce varying results due to differences in particle characteristics. Diffusion and electrical attraction, two primary mechanisms by which particles are captured, are dependent on the velocity of the particles through the filter.
Electrostatic media, which relies on electrical attraction to capture particles, shows a larger change in performance with varying face velocities compared to mechanical media, which relies on physical barriers. Additionally, there is more charge on sodium chloride (NaCl) particles compared to oil aerosols, resulting in different filtration efficiencies. These variables must be carefully controlled and considered during filter testing to ensure accurate and reliable results.
| Media Type | Aerosol Type |
|---|---|
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Types of Devices Used
The follwing table shows different types of devices relevant for filter testing, their typical appearance, relevan standards related, adequate aerosol generators and recommended filter testing equipment:
| Device Name | Respirators (Filtering Facepiece e.g. N95 or other types of respirators) | Barrier Face Coverings | Medical Masks | |||
|---|---|---|---|---|---|---|
| Typical Appearance | Filtering Facepiece Respirator (e.g. N95, FFP) | Elastomeric Mask with Cartridges (e.g. P100) | ||||
| Relevant Standards |
European: |
NIOSH (USA) 42 CFR Part 84 |
China GB2626 |
ASTM F3502 | ASTM F2100 | |
| Aerosol Generator | NaCl | NaCl and distilled water | ||||
| Oil | Paraffin, Oil | PAO-4 (i.e. Emery Oil) or DOP, as desired | N/A | |||
| Filter Test Equipment | Automated Filter Tester such as Model 8130A | |||||
Effect of Particle Size Distribution and Face Velocity
Particle size distribution and face velocity are critical factors in filter performance testing. For a given filter media, particle penetration varies based on particle size and the face velocity of the air passing through the filter. Larger particles, which often carry a higher charge, are more effectively captured by electrostatic (ES) media due to the smaller Most Penetrating Particle Size (MPPS). Conversely, small particles are efficiently removed through diffusion, particularly at low face velocities, where the slower airflow enhances both diffusion and electrostatic capture. This dynamic underscores the importance of testing filters under varying conditions to accurately assess their performance.
Effect of Loading and Aerosol Type
Penetration can be loading-dependent, with loading impacting electrostatic and mechanical media differently. When using salt aerosol, loading mechanical media decreases penetration due to increased surface area from salt dendrite formation. Conversely, loading charged media initially increases penetration because charged aerosols are more attracted to the charged filter media until charge sites are covered. With oil aerosol, coating filter fibers increases inter-fiber velocity, raising penetration for both media types. Oil also efficiently shields charge sites on electrostatic media, further increasing penetration. Neutralizing oil by adding charge can slightly reduce penetration, highlighting the complex interplay of loading effects.
Effect of Detector Type and Media Type
Detector Effects: Detector type significantly influences filter test results. Photometers measure the total light scattering of all particles collectively, with a response proportional to the sixth power of the particle diameter (Dp6), making it highly nonlinear. In contrast, condensation particle counters (CPCs) count individual particles, with a response usually independent of particle size. These differing detection techniques, combined with the influence of particle size on photometer response, mean that filtration results can vary for a given media depending on the particle size distribution and the type of detector used.
Media Effects: Electrostatic media typically has a smaller Most Penetrating Particle Size (MPPS) than mechanical media, resulting in higher penetration with CPCs, which are more sensitive to smaller particles. Oil aerosols, having lower charge than salt aerosols, tend to show higher penetrations through electrostatic media. Mechanical media, on the other hand, exhibits a flatter penetration curve, making the difference in penetration less pronounced. Photometers, which are biased towards larger particles, indicate higher penetration for electrostatic media due to their sensitivity to larger particles where penetration is greater.
Conclusion
The method of testing filters significantly impacts the results, underscoring the importance of precise testing protocols. Fractional penetration tests reveal varying penetration curves across different filter types, influenced by factors such as detection technology (photometer vs. CPC), aerosol type, and face velocity. Additionally, oil and salt aerosols affect filters differently, particularly electrostatic media, and flow rate can alter overall penetration and shift the penetration curve. These variables highlight the need for careful consideration of testing conditions to accurately assess filter performance and ensure they meet required standards.