How should the filter of a 100-level laminar flow hood be replaced?

The replacement of filters in a 100-level laminar flow hood is a crucial step in maintaining cleanliness. Particularly, the replacement of high-efficiency filters (HEPA) must strictly follow the operating procedures to prevent contamination and damage to the equipment. The following are the detailed replacement steps, precautions, and subsequent verification processes:
I. Preparations before Replacement
Confirm the replacement timing
The differential pressure gauge indicates: When the differential pressure increases by 50% compared to the initial value (for example, when the initial differential pressure is 20Pa and rises to 30Pa), or when the equipment has been in operation for more than 1-2 years (conventional usage cycle).
Abnormal performance: Wind speed is lower than 0.3m/s, and the detection of dust particles exceeds the standard (Class 100 standard: the number of particles ≥0.5μm > 3520 per cubic meter).
2. Preparation of tools and consumables
Tools: Screwdriver, wrench, sealant gun, anemometer, dust particle counter (for subsequent verification).
Consumables: HEPA filter of the same model (confirm filtration efficiency ≥99.97%@0.3μm), sealing rubber strip (silicone rubber material), 75% ethanol, lint-free cloth.
3. Environmental and Safety Protection
Operation area cleaning: Wipe the interior of the equipment with 75% ethanol in advance, turn off the fan, and cut off the power to prevent dust from spreading during replacement.
Personnel protection: Wear dust masks (N95 or above), latex gloves, and dust-free suits to avoid contamination by human hair and dander.
Ii. Steps for Replacing High-Efficiency Filters (HEPA)
1. Remove the old filter
Open the inspection panel: Locate the filter installation compartment on the top or back of the equipment (the position varies by model, for example, vertical flow laminar flow hoods are mostly at the top), use a screwdriver to remove the fixing screws, and slowly take off the panel.
Disconnecting components: If the filter has a sealed joint with the air duct or fan, first loosen the clamp or bolt, and be careful to support the filter to prevent it from falling.
Remove the old filter element: Hold both sides of the filter with both hands and slowly pull it out horizontally (avoid tilting to prevent dust from falling). Put it in a sealed bag and discard it (if it comes into contact with biological samples, sterilization treatment is required first).
2. Clean the installation cavity
Cavity disinfection: Use a lint-free cloth dipped in 75% ethanol to wipe the installation slot, inner walls, and surrounding areas to remove dust and residual sealant.
Check the sealing surface: Check if the installation groove is flat. If there is any deformation or damage, it needs to be repaired before installation (a level can be used to calibrate the flatness).
3. Install the new filter
Pre-installation positioning: Place the new HEPA filter smoothly into the installation slot. Pay attention to the airflow direction marking (the arrow points to the operation area. Vertical flow is usually "from top to bottom", and horizontal flow is "from back to front").
Sealing treatment: Evenly apply sealant (silicone sealant, about 2-3mm thick) along the edge of the filter, or install new sealant strips (with a compression of 50% after pressing), ensuring there is no gap.
Fixation and sealing inspection: Fix the filter frame with screws, apply force evenly to prevent deformation, and then press the sealing rubber strip with your fingers to confirm there are no air leakage gaps.
Iii. Replacement of the Primary Filter (If replaceable)
Disassembly and cleaning
Open the air inlet grille, take out the primary filter (mostly made of non-woven fabric or metal mesh), and if it is washable, rinse it with clean water and dry it. If it is a disposable filter element, discard it directly.
Clean the inner wall of the air inlet to prevent dust accumulation from affecting the air intake efficiency.
2. Install the new filter element
Install the new filter element in the direction indicated by the arrow (air intake direction), ensuring it fits tightly against the frame without any looseness or gaps.
Iv. Performance Verification After Replacement
1. Sealing test
Smoke leak detection method: Turn on the fan and use a smoke generator to release smoke at the edge and seal the filter. If there is no leakage or deviation of the smoke, it indicates that the seal is good. If there is smoke escaping, the sealant needs to be reapplied.
Differential pressure test: After running the equipment for 30 minutes, check if the reading of the differential pressure gauge is within the normal range (usually 10-15Pa higher than the initial value. If it is too high, it may be due to overly tight installation or clogged filter elements).
2. Wind speed and airflow detection
Wind speed uniformity: Use a hot sphere anemometer to measure at multiple points in the operation area (at least 5 points), and the wind speed at each point should be within the range of 0.3-0.6m/s, with a deviation of no more than 20%.
Airflow direction: Use a smoke generator to observe whether the airflow is unidirectional (vertical flow should be from top to bottom, horizontal flow from back to front), and there should be no vortex or reverse flow phenomenon.
3. Cleanliness verification
Entrust a third-party institution to test the operation area with a dust particle counter. The requirements of the 100-level standard are:
The number of particles ≥0.5μm is ≤3520 per m³ (ISO level 5);
The number of particles ≥5μm is ≤29 per cubic meter.
V. Precautions for Replacement in Special Scenarios
Biosafety type laminar flow hood
Before replacement, the interior of the equipment and the filter should be sterilized (such as by formaldehyde fumigation or ozone disinfection). When disassembling the old filter element, it should be placed in a biosafety bag and treated as medical waste.
After the exhaust air filter is replaced, the negative pressure system needs to be tested to ensure that the exhaust air is filtered efficiently and without leakage.
2. Laminar flow hoods for semiconductors/precision electronics
The replacement process must be carried out in a dust-free workshop. Tools and personnel need additional anti-static treatment (wearing anti-static wristbands and using dust-free tools).
After replacement, the static voltage of the countertop (≤100V) needs to be tested to prevent static damage to the precision components.
Vi. Key Points of Safety and Record-keeping
Operational safety: Electrical work is strictly prohibited during replacement. If high-altitude work is involved (such as installing filters on the top), a stable ladder must be used and someone should supervise.
Record archiving: Fill in the replacement record, including:
Filter model, batch number, installation date;
Sealant brand, curing time;
Performance test results (wind speed, pressure difference, cleanliness);
The operator signs for easy traceability.
Summary
The core of replacing the 100-level clean bench filter is "aseptic operation, sealing priority, and verification compliance", especially the installation quality of the HEPA filter directly affects the cleanliness level. It is recommended that non-professionals avoid replacing it by themselves. Instead, they can contact the equipment supplier or a professional maintenance team to ensure the safety and effectiveness of the replacement process.