Leak Detection Method for High Air Filters
First, we must establish a core concept: HEPA mini pleat air filters are not solid, airtight plates. Instead, they are made of extremely fine glass fiber filter paper, folded multiple times and mounted within a metal frame.
This means potential leaks primarily arise from two sources:
1. Damage to the filter paper itself due to aging, vibration, or external impact.
2. Gaps in the seal between the filter and the mounting frame, caused by aging or improper installation.
Our leak detection work is a precise "leak-finding" mission. Currently, the most internationally recognized and scientific method is the DOP/PAO leak detection method. Let's break it down step by step.
Leak Detection Principle
Smoke Generation
We evenly release a very fine, harmless artificial smoke upstream of the filter (on the air inlet side). These smoke particles have a fixed diameter, such as 0.3 microns, which is precisely the size that HEPA mini pleat air filters are most challenged to filter.
Scanning Detection
Downstream of the filter (on the air outlet side), an inspector holds a handheld aerosol photometer. The scanner's probe is held close to the filter's outlet side (covering the entire filter paper surface and the critical frame seal), moving slowly at a speed of no more than 5 cm/second for a comprehensive, "no-blind-spot" scan.
Determining the Leak Point
The principle is simple: If the filter is intact and tightly sealed, the vast majority of particles will be trapped, and the smoke concentration detected downstream will be very low. If the scanner passes over a specific point and the instrument alarms or the readings spike sharply, this indicates a leak at that location.
Detailed Leak Detection Procedure
Step 1: Preparation
Safety First: Ensure the clean room's air conditioning system is operating normally and is stable. Inspectors must wear appropriate protective gear. Although PAO is considered harmless, avoid inhaling it in large quantities.
Instrument Calibration: Before testing, the photometer must be calibrated to ensure data accuracy.
Setting Up the Smoke Generator: Connect the aerosol generator to the air supply duct upstream of the filter and ensure it generates smoke uniformly and stably.
01
Step 2: Baseline Reading
Measure the initial aerosol concentration upstream. This value serves as a benchmark for calculating the extent of any downstream leaks.
02
Step 3: Downstream Scanning for Leaks
When scanning, keep the probe approximately 1-2 cm from the filter surface, overlapping movement paths to ensure full coverage. Pay close attention to the joints between the filter and the mounting frame, the seams between filters, and the filter paper for any obvious damage.
03
Step 4: Data Recording
Upon discovering a leak, immediately mark the location and record the leak rate. After the full scan, categorize the leak: Is it a frame seal issue or damaged filter paper?
04
Step 5: Repair and Verification
The purpose of leak detection is not just to identify problems but to fix them. If the issue is with the frame seal, try resealing it. If mounting screws are loose, retighten them. After repairs, rescan the area to confirm the leak has been eliminated.
05
Other Auxiliary Leak Detection Methods
Particle Counter
This method is more direct. It does not use artificial smoke but instead directly measures the number of naturally occurring dust particles in the clean room air. If the particle count directly below a HEPA mini pleat air filter is significantly higher than in other locations or exceeds the standard, it may indirectly indicate a leak. This method is better for overall environmental monitoring and is not as efficient asthe PAO method for pinpointing specific leaks.
Traditional Visual Inspection
Shine a strong flashlight diagonally across the downstream side of the filter to see if any light passes through. This method is very crude and can only detect large, gross leaks. It is completely ineffective for micron-level leaks and is not recommended as a formal detection method.
Airflow Resistance (Final Resistance)
A new filter is unobstructed. Over time, trapped dust gradually clogs the filter media. This clogging creates a pressure drop, technically known as "final resistance."
It is recommended to monitor this pressure difference with a gauge regularly. When the filter's final resistance reaches twice its initial resistance (the resistance when new), or when the increased resistance causes a significant drop in airflow that fails to meet the cleanroom's air change requirements, this is a strong signal for replacement.
01
Regular Leak Testing
Replacement should be considered if the following conditions are found during regular PAO leak testing:
Multiple points of damage in the filter paper that cannot be simply repaired.
The overall leakage rate exceeds the standard's specified limit (e.g., no more than 0.01%).
02
Cleanliness Level
This is the ultimate test of filter performance. If routine monitoring shows that the airborne particle count consistently exceeds the standard, and after eliminating other causes (such as improper personnel procedures, room sealing issues, or equipment generating dust), the problem points to the HEPA deep pleat air filter, it must be replaced immediately regardless of its age.
03
Service Life and Environmental Load
High-Load Environment: If your clean room is in an area with poor air quality (e.g., high PM2.5), or if the pre-filters and intermediate filters are not maintained properly, the HEPA deep pleat air filter's lifespan may be shortened to less than a year.
Good Environment: With appropriate pre-filter protection, good ambient air quality, and limited operation (not running 24/7), the lifespan can extend to three years or longer.
04
Extending Filter Life
Focus on Primary Protection
Primary and secondary filters act as the "first line of defense," responsible for intercepting the majority of larger dust particles. Strictly adhering to the schedule for replacing or cleaning these filters is the most economical and effective way to protect HEPA deep pleat air filter and extend their service life.
Practice Careful Installation
Avoid damage caused by improper handling. Filter replacement must be performed by trained professionals who handle the filters with care. During installation, ensure the frame is under even stress and that the sealing gasket is intact and properly compressed. Rough installation is a leading cause of frame leaks and filter media damage.
Maintain a Health Record
Implement a digital management system. Create a record for each HEPA deep pleat air filter, documenting its installation date, initial resistance, leak detection results, and maintenance history. Analyzing this data trend allows for more accurate lifespan predictions, enabling proactive, predictive maintenance and avoiding unplanned downtime.
Conclusion
HEPA air filters are a relatively small but crucial part of a clean room investment. To manage them effectively, we should establish a scientific management system centered on regular PAO leak testing, routine final resistance monitoring, and adherence to cleanliness standards. This approach allows us to find the optimal balance between ensuring product quality and controlling operational costs.
We hope this detailed guide proves valuable in your work. If you face other challenges in clean room operations, please continue to follow SYNRUI's website for more original and valuable insights!
