How to determine whether the filter at the liquid tank supply air outlet needs to be replaced?

To determine whether the filter at the supply air outlet of the liquid tank needs to be replaced, it is necessary to combine the monitoring of operation parameters, appearance inspection, functional testing, and the management of the usage cycle. The following are the specific judgment methods and operation points:
I. Differential Pressure Monitoring Method (Core Basis)
1. Comparison of differential pressure gauge values
Principle: After the filter accumulates dust, its resistance increases. By monitoring the pressure difference (ΔP) before and after it, the degree of decline in filtration efficiency can be determined.
Operation steps:
Record the initial pressure difference: After the new filter is installed, wait for the system to operate stably (about 30 minutes), and record the initial pressure difference (P0) (i.e., "resistance in a clean state").
Set the replacement threshold:
High-efficiency filter (HEPA): It is usually set at 2×P0 (i.e., the resistance reaches twice the initial value) or the manufacturer's recommended value (for example, some manufacturers suggest a final resistance of 400-600Pa).
Medium-efficiency filters: The final resistance is generally 1.5× p ₀ or 200-300Pa (specifically subject to design parameters).
Regular inspection: Check the differential pressure gauge daily or per shift. If the value is close to or exceeds the threshold, a replacement plan should be made.
2. Abnormal pressure difference early warning
Sudden drop in pressure difference: It may be caused by a damaged filter, loose installation, or cracked sealant. The machine needs to be stopped immediately for inspection (there may be a risk of excessive cleanliness).
If the pressure difference remains unchanged for a long time: If the pressure difference is always lower than the initial value during the system operation, it may be due to a malfunction of the pressure difference gauge or the filter not being installed correctly (such as not sealed with the liquid tank). It is necessary to check the instrument and its installation status.
Ii. Visual Inspection Method (Auxiliary Judgment)
Observe the status of the filter directly
High-efficiency filter
If there is obvious damage, holes, or deformation on the surface, or the color of the filter paper darkens abnormally (such as black dust accumulation), it needs to be replaced immediately.
For filters without separators, if the hot melt adhesive separator line breaks or the filter material collapses, it may cause a short circuit in the airflow and needs to be replaced in time.
Medium-efficiency filter
When a large amount of dust can be seen on the surface of non-woven fabric or filter material (such as obvious dust falling off when gently tapped by hand), and the pressure difference is close to the threshold, it needs to be replaced.
2. Inspection of the connection between the liquid tank and the sealing part
If the sealant of the liquid tank cracks or there is a gap between the edge of the filter and the liquid tank, even if the pressure difference does not reach the threshold, the filtration efficiency may decrease due to leakage. The integrity of the filter needs to be checked simultaneously.
Iii. Cleanliness Testing Method (Ultimate Verification)
Particle concentration detection
Applicable scenarios:
The pressure difference has not reached the threshold, but it is suspected that the performance of the filter has declined (such as frequent over-limit of the cleanroom level).
As the design life of the filter approaches, the filtration efficiency needs to be verified in advance.
Operation method:
Use a laser particle counter to detect the concentration of suspended particles on the downwind side of the supply air outlet (30-50cm away from the air outlet surface of the filter).
Judgment criterion:
If the test results exceed the corresponding level standards of the cleanroom (for example, the number of particles ≥0.5μm in an ISO grade 5 cleanroom is > 3520 per cubic meter), and other pollution sources (such as dust from personnel and equipment) are excluded, it may be that the filter has failed.
2. Microbial testing (Biological Cleanroom)
In scenarios such as pharmaceuticals and hospital operating rooms, regular tests for airborne or settled bacteria are required:
If the colony count in the petri dish exceeds the standard (for example, the airborne bacteria in an ISO 7-level clean room are greater than 100CFU/m³), and the disinfection measures are ineffective, it is necessary to check whether the filter has been penetrated or bred by microorganisms.
Iv. Management of Operating Time and Usage Cycle
Estimated based on the designed service life
High-efficiency filter
General cleanrooms (such as electronics factories, and food factories): 2-3 years (depending on the degree of fresh air pollution).
Highly polluted environments (such as cement plants and chemical workshops): It may be shortened to one year or even less.
Medium-efficiency filter
It is generally replaced every 3 to 6 months (this should be combined with pressure difference monitoring. If the dust content in the fresh air is high, the cycle should be shortened).
2. Accelerated replacement in special scenarios
The fresh air inlet is adjacent to major traffic arteries, industrial areas, and other pollution sources, or the clean room has recently undergone renovation, equipment grinding, and other dust-generating operations, which may cause the filter to clog prematurely. Therefore, the inspection cycle needs to be shortened.
After use in a biosafety laboratory, if high-risk samples (such as live bacteria and viruses) are processed, it is recommended to disinfect and replace them regularly even if the pressure difference threshold is not reached (to avoid the reproduction of microbial residues).
V. Auxiliary Judgment Signals and Operation Suggestions
1. Abnormal system operation prompt
The continuous increase in fan frequency (to maintain air volume) and the significant rise in energy consumption may be caused by excessive resistance of the filter. A comprehensive judgment should be made in combination with the pressure difference data.
If the positive pressure value in a clean room is difficult to maintain (such as in an operating room or isolation ward), it may be due to excessive filter resistance or leakage, and an investigation is needed.
2. Pre-verification before replacement
If there are doubts about the pressure difference data, a bypass test with a backup filter can be temporarily adopted:
Short-circuit the current filter, replace it with a spare filter, and then test the pressure difference and cleanliness. Compare the data to confirm whether the original filter has failed.
It is prohibited to decide on replacement based on a single indicator alone. It is necessary to confirm in combination with pressure difference, appearance, and test data to avoid accidental or missed replacement.
Vi. Precautions and Risk Avoidance
Avoid over-reliance on the pressure difference threshold
Some filters (such as ultra-high efficiency filters ULPA) may experience microbial growth or chemical contamination penetration before reaching the final resistance due to the dense filter material. It is necessary to increase the detection dimensions in combination with the scenarios.
For small systems that cannot monitor the pressure difference in real-time (such as FFU fan filter units), the installation date can be marked and a replacement plan can be formulated based on the cumulative operating hours (such as 2000 hours).
2. Key points of safe operation
Before replacement, it is necessary to confirm whether the filter has come into contact with harmful pollutants (such as radioactive particles, and biological hazard factors), wear corresponding protective equipment (such as gas masks, and chemical protective suits), and carry out harmless treatment on the old filter (such as autoclaving, sealed incineration).
During the disassembly process, avoid vigorous shaking of the filter to prevent secondary pollution caused by dust accumulation. High-efficiency filters should be transported vertically and must not be placed horizontally.
By comprehensively judging through the above methods, the timing of filter replacement can be accurately grasped. This not only avoids the risk of cleanroom contamination caused by too late replacement but also prevents the waste of consumables due to premature replacement. For key clean areas (such as chip factories and sterile preparation workshops), it is recommended to establish an online monitoring system to track the performance of filters in real-time and achieve predictive maintenance.