What types of filters are generally used in laminar flow transfer Windows?

The filter type of the laminar flow transfer window needs to be configured according to its cleanliness level requirements and functional design. Usually, a multi-stage filtration combination (primary efficiency + medium efficiency + high efficiency) is adopted to achieve efficient interception of pollutants of different particle sizes. The following are the common types and characteristics of filters:
I. Types and Applications of Core Filters
1. Primary filter (pre-filter)
Material:
The filter materials are mostly nylon mesh, non-woven fabric, metal wire mesh, etc. They have a simple structure and low cost.
Common forms: plate type, folding type, bag type.
Filtration efficiency
It mainly intercepts large particle contaminants ≥5μm (such as dust, hair, debris, etc.).
Efficiency class: ISO coarse efficiency (G1-G4), among which the laminar flow transfer window commonly uses G3-G4 (filtration efficiency: 50%-90%@5μm).
Function:
As the first-stage filtration, it reduces the load on the subsequent medium and high-efficiency filter elements and extends their service life.
Application scenarios: Preliminary filtration of the surrounding air outside the transfer window, or pre-filtration in low-grade clean areas (such as Class 10,000).
2. Medium-efficiency filter (intermediate filtration)
Material:
The filter material is ** glass fiber, synthetic fiber (such as polyester fiber) **, or non-woven fabric, and the structure is mostly foldable or bag-type.
Filtration efficiency
Intercept particles ranging from 1 to 10μm (such as pollen, fungal spores, and some bacteria).
Efficiency class: ISO medium efficiency (F5-F9), laminar flow transfer Windows commonly use F7-F8 (filtration efficiency: 80%-95%@1-5μm).
Function:
Further, filter out medium-sized particles in the air to protect high-efficiency filters.
Application scenarios: Intermediate filtration in medium and high-grade clean areas (such as Class 1,000 and Class 100), or scenarios where strict control of microorganisms is required (such as biological laboratories).
3. High-efficiency filters (HEPA/ULPA, terminal filtration)
Material:
The filter material is superfine glass fiber or polypropylene fiber. The structure is foldable, and the frame is sealed with metal (aluminum alloy) or plastic.
Filtration efficiency
HEPA (High Efficiency Filter) :
Intercept particles ≥0.3μm with an efficiency of ≥99.97% (DOP method), corresponding to grades H10-H14 of ISO 14644-1 standard. Laminar flow transfer Windows commonly use H13-H14 (applicable to clean areas from grade 100 to grade 10).
ULPA (Ultra-High Efficiency Filter) :
It intercepts particles ≥0.12μm with an efficiency of ≥99.999% and is suitable for ultra-high cleanliness levels (such as ISO 5 and below, semiconductor lithography workshops).
Function:
As a terminal filter, it provides clean airflow to ensure that the interior of the transfer window meets the designed cleanliness standards.
Application scenarios: High-grade clean areas (such as aseptic rooms, GMP workshops, and hospital operating rooms) or biosafety laboratories (BSL-2/3).
Ii. Typical Filtering Combination Modes
Laminar flow transfer Windows usually adopt a three-stage filtration system, and the combination mode is as follows:
Filter level, filter type, installation location, function
The front end of the air inlet of the first-stage primary filter filters out large particle contaminants to protect the medium-efficiency filter element.
The second-stage medium-efficiency filter filters medium particles between the primary and high-efficiency filters, further reducing the load on the high-efficiency filter element.
The front end of the outlet of the third-stage high-efficiency filter (behind the static pressure box) provides clean airflow to ensure that the air inside the transfer window reaches the target cleanliness level.
Iii. Filter Configuration in Special Scenarios
Biosafety scenarios (such as laboratories, and GMP workshops)
Requirements: Additional control of microorganisms (bacteria, viruses) and aerosol leakage is needed.
Configuration:
The high-efficiency filter adopts H14 grade HEPA and is equipped with an aerosol leakage detection interface (such as a PAO test port).
Activated carbon filters (for adsorbing chemical pollutants or odors) or antibacterial-coated filter elements (to inhibit microbial growth) may be added.
2. High humidity or corrosive environments
Requirement: Prevent the filter element from getting damp and clogged or the material from corroding.
Configuration:
The primary and medium-efficiency filters are made of moisture-resistant materials (such as polypropylene non-woven fabric).
The frame of the high-efficiency filter is sealed with stainless steel or epoxy resin to prevent air leakage caused by metal corrosion.
3. Ultra-clean environment (such as semiconductors, and precision electronics)
Requirement: Control sub-micron particles (such as below 0.1μm) to prevent product contamination.
Configuration:
The terminal filtration adopts ULPA filters (efficiency ≥99.999%@0.12μm) and is combined with ** fan filter units (FFU) ** to achieve a laminar air supply.
Iv. Key Points for Filter Selection and Maintenance
Selection basis:
Select the filter element with the corresponding efficiency based on the cleanliness level requirements of the transfer window (such as Class 100, Class 1000), air volume (m³/h), and the characteristics of the contaminants (dust, microorganisms, chemical substances).
For example, a 100-level clean area needs to be equipped with H13-level HEPA, while a biosafety laboratory needs to take into account both efficient filtration and sterilization functions.
Maintenance precautions:
Primary and medium-efficiency filters: They can be cleaned regularly (for non-disposable models), but the cleaning frequency should not exceed three times to avoid a decline in the strength of the filter material.
High-efficiency filter: Non-washable and needs to be replaced as a whole. Before replacement, the equipment needs to be disinfected (such as by ultraviolet irradiation or chemical fumigation) to prevent the spread of contamination.
Compliance requirements:
Industries such as medical care, food, and pharmaceuticals need to follow relevant standards (such as GMP, and ISO 14644), and filters need to pass integrity tests (such as aerosol scanning) and keep records.
Summary
The filter of the laminar flow transfer window is mainly composed of a three-stage combination of primary, medium, and high efficiency (HEPA/ULPA), and its core function is to gradually improve air cleanliness through multi-stage filtration. In practical applications, the material, efficiency, and special function filter elements should be selected according to the scene requirements, and the filtration performance should be ensured through regular maintenance (such as pressure difference monitoring and leakage testing) to meet the strict requirements of different industries for clean environments.