Understanding Precision Filters and PTFE Filter Elements for Sterile Air Filtration

Understanding Precision Filters and PTFE Filter Elements for Sterile Air Filtration

To avoid wasting labor, financial resources, and time, it is essential to fully understand the system's contamination status and reasonably configure purification facilities—such as dust removal, oil removal, and drying units—within the compressed air system to optimize the supply setup.

After compression, the gas passes through an aftercooler, where the temperature of the hot compressed gas drops to 40–50°C. Approximately 60% of the oil and water content condenses and is drained away; however, residual trace amounts of oil, water aerosols, and solid particulates must be removed through filtration.

Modern filtration processes involve specialized roles; filtration methods vary based on specific requirements and application locations, contrasting with traditional approaches where a single piece of equipment performed all separation tasks at once.

Purification systems requiring high filtration precision often employ multi-stage filtration, progressively increasing the filtration accuracy. This approach meets the user's efficiency and precision requirements while protecting and extending the service life of the fine filters; it also significantly reduces equipment size, facilitating installation and maintenance.

Gas-liquid separation is generally used as a pre-treatment step in the filtration process to remove solid particulates and large liquid droplets entrained in the compressed gas, thereby reducing the load on downstream filters.

Gas-liquid separation typically combines inertial forces (centrifugal force, directional changes, and gravity settling) with wire mesh demisters. Featuring a simple structure and ease of manufacturing, it serves as an excellent pre-treatment method. It generally removes particles larger than 5 μm with an efficiency exceeding 98%.

The primary purpose of coarse filtration is to remove residual solid particulates from the gas to protect downstream high-precision filters; a secondary purpose is to separate oil droplets of a certain size and water aerosol particles. Coarse filter ratings include 1 μm, 5 μm, and 10 μm, with efficiencies exceeding 90%. These filters are often used independently in systems with less stringent requirements, such as general workshop air supply or purging operations.

Fine filtration is designed to thoroughly remove residual oil and water aerosol particles from the gas. High-precision filters available on the market offer filtration accuracy down to 0.01 μm and efficiencies exceeding 99.999%, resulting in a residual oil content of less than 0.01 mg/m³. Fine filtration is typically used in conjunction with coarse filtration to protect the fine filter and extend its service life.

Additionally, in specific sectors such as food processing, fermentation, chemicals, and natural gas, activated carbon is employed to adsorb odors, while absorption methods are used to remove substances like hydrogen sulfide and carbon dioxide.

For clean air supplies used in food processing and microbial fermentation, a multi-stage gradient purification system is mandatory; relying solely on a terminal sterilization filter is insufficient.

In practice, however, most small and medium-sized factories use oil-lubricated or micro-oil air compressors. Due to high procurement and maintenance costs, oil-free equipment is rarely used in liquid fermentation for edible fungi, meaning the filtration systems installed often lack some of the recommended stages.

Oil-injected/micro-oil models:

Must be equipped with Class H filter elements as a mandatory purification reinforcement step; this cannot be omitted.

Shorter replacement cycles:

The mandatory replacement interval for Q/P/S precision filter elements and activated carbon filters has been reduced from 3,000 hours to 2,000 hours to prevent oil mist from breaking through once the filter becomes saturated.

Upgraded compressor room management:

Mandatory mechanical ventilation is required to keep the ambient temperature at or below 32°C. Large activated carbon pre-filters can be installed at the air compressor intake to block workshop odors and reduce the intake of volatile impurities.

Dual-layer terminal sterilization protection:

Two PTFE sterilization filters are installed in series upstream of the fermentation tank. These undergo in-situ steam sterilization at 121°C for each batch, providing dual-layer interception of microbial spores that may carry oil mist.

PTFE filter elements used on fermentation equipment typically offer a 99.999% filtration efficiency for 0.01 μm particles.