Selection and Application of Filter Cartridges for Air Intake Filters

1. Working Principle of Air Intake Filters

[Compressor Network] An air intake filter can be likened to a "face mask". It has no driving power of its own; 

the driving force comes from the negative pressure generated at the suction inlet of equipment such as compressors and gas turbines.

Driven by suction negative pressure, the intake filter draws ambient air. As air flows through the filter cartridge, 

relatively large dust particles settle at the bottom of the filter under gravity and inertial force. 

Fine dust particles are captured via combined mechanisms including Brownian diffusion and interception by fibres on the filter cartridge, 

resulting in dust deposition on the surface of the filter media.

After dust accumulates on the cartridge over time, flow resistance rises. When resistance reaches the preset value, the pulse jet cleaning system activates. 

Reverse-pulse gas impacts the filter cartridge from inside, shaking off dust attached to the cartridge and lowering resistance.

Filtered gas enters the clean air chamber and is then delivered through the outlet duct to compressors, steam turbines and other equipment.
2. Current Challenges of Filter Cartridges for Air Intake Filters

For high-speed rotating machinery such as compressors and steam turbines, dust particles are one factor impairing long-term stable operation. 

Expensive aerodynamic equipment is critical plant required to run year-round. To ensure trouble-free, high-efficiency operation and minimize downtime and maintenance workload, 

air intake filters must be installed at air suction ports to mitigate equipment damage caused by dust.

Despite the vital role of intake filters, many problems persist regarding the selection and application of filter cartridges in practical operation, mainly as follows:

Absence of pre-filtration: Large dust particles (particle size ≥5 μm) strike filter cartridges and airflow channels, causing surface abrasion. 

This shortens cartridge service life and makes cartridges prone to rupture.

Consequences include reduced equipment operating efficiency, drift of the surge line, and sometimes destruction of dynamic balance.

Insufficient filtration precision: Fine dust particles (particle size ＜5 μm) form hard scale inside equipment. Such scale disrupts airflow, lowers equipment efficiency, 

shifts the surge line, and may damage dynamic balance.

Poor resistance to corrosive dust: Corrosive dust triggers chemical corrosion on equipment, shortening the service life of both filter cartridges and host machinery.

Inadequate moisture resistance: Filter cartridges fail to recover performance promptly after getting damp. The intake filter runs under persistently high resistance, 

leading to reduced equipment efficiency and higher energy consumption.

Low air permeability: For a given filtration grade, many poorly permeable cartridges cause rapid resistance build-up in intake filters. This results in high energy consumption, 

short cartridge lifespan and elevated overall operating costs.

3. Selection Considerations for Air Intake Filter Cartridges

The selection of intake filter cartridges involves multiple aspects, extending beyond the cartridges themselves to the overall system design. Therefore, 

cartridge selection shall take handling airflow, ambient conditions and pre-filtration into consideration.

Actual operating requirements

Key considerations cover operating conditions of different types of compressors and gas turbines, acceptable operating pressure drop, filtration velocity and efficiency of cartridges, 

target size of particulate pollutants, dust holding capacity, and filtration mode (surface filtration or depth filtration). All these parameters shall be fully evaluated during design.

Design of handling airflow

From the perspective of operational performance, higher handling airflow is preferred. Conservative design is common in intake filter design for air compressors and gas turbines. 

The airflow matching capacity is generally designed at 2 times the nominal demand, and may be further enlarged for better performance. However, 

some asset owners prioritize upfront capital investment. Complaints about short filter cartridge service life often stem from insufficient airflow matching.

Filter cartridge design
Cartridge design mainly covers mounting mode, filtration area, cartridge support mesh, sealing gaskets and media selection, each with distinct requirements.
Mounting mode affects space layout, maintenance and replacement;
Filtration area determines available airflow capacity;
Support mesh influences back-blow performance and cartridge structural strength;
Sealing gaskets govern the stability of sealing performance;
Filter media directly impact cartridge service life and filtration precision.
Every selection requires thorough assessment; no single cartridge type suits all working conditions.
Surrounding environment
Two categories of environmental factors shall be evaluated:
① Natural climatic conditions: seasonal variations, desert, coastal, urban, frigid and other environments all affect media selection.

② Man-made and geographic factors: nearby industrial facilities (thermal power plants, cooling towers, chemical plants, etc.), agricultural activities, wind direction (windward / leeward), 

height of air intake above ground, and local airflow patterns induced by buildings and terrain. All shall be addressed in design and selection.

Multi-stage filtration
Multi-stage filtration includes pre-filtration and downstream high-efficiency filtration. In many cases, a single-stage cartridge cannot meet filtration requirements.

For conditions involving catkins, insects, wind-blown sand, high humidity, fog, rain, salt, snow, smoke, etc., optional pre-filter components can be added, 

such as weather hoods, screens, inertial separators, moisture coalescers, metal protective meshes and non-woven cotton.

If intake air quality needs to reach Class H11–H13 or higher, three-stage high-efficiency filters can be deployed.
Multi-stage filtration effectively protects main filters and cartridges, extends cartridge service life, and represents the most practical solution in modern intake filter design.
4. Selection of Air Intake Filter Cartridges
Filter cartridges for intake filters are available in multiple mounting types: small-hole hanging type, vertical chuck mount type, vertical three-claw rotary type, and horizontal type.
Small-hole hanging type: simple equipment structure but complicated installation and maintenance;
Vertical chuck mount type: easy installation and maintenance yet relatively bulky;
Vertical three-claw rotary type: convenient mounting but demanding sealing performance;
Horizontal type: compact size with easy servicing, yet dead zones exist on cartridge surfaces.

Each mounting configuration has advantages and drawbacks. There is no universal best solution; optimal performance is achieved only by selecting the most suitable mounting method. 

Hence, mounting configuration selection is critical for intake filtration systems.

By media material, cartridges fall into three main categories: glass fiber, polyester fiber and wood pulp fiber.
Glass fiber features high filtration precision, excellent moisture resistance and good air permeability. Nevertheless, its low burst strength (only 3 kPa) disqualifies it as pulse jet cartridge media.
Polyester fibre delivers good burst resistance but poor air permeability, so it is rarely adopted for intake filtration.
Currently, intake filter cartridges mainly adopt wood pulp fiber, blends of wood pulp fiber and other fibres, and nanofiber media.
High-efficiency media dedicated to air intake filtration are not yet domestically manufactured in China; major suppliers are US-based HV Corporation and Ahlstrom Corporation.
Cartridge media shall be selected according to specific working conditions:
Media must be hydrophobic under humid operating conditions;
Cartridges are prone to blockage in harsh environments with sticky synthetic dust such as hydrocarbons (oil mist and fume);
Sufficient pressure recovery is required after pulse cleaning cycles;
Under salt-corrosive conditions, media must prevent salt ingress into host equipment to avoid corrosion.
A series of media grades are specified for intake filtration to match diverse operating scenarios.
4.1 Desert Regions
Typical features: hot climate, dry environment, frequent strong winds, wide variety of sand particles and high dust concentration.
Required media characteristics:
High filtration efficiency against fine particles;
Good hydrophobicity;
Compatible with pulse jet cleaning.
Recommended filter media:
Wood pulp fibre + fine fibre (layered or coated technology): Grade RHFD/234, RHFD/234FR
Wood pulp fibre + synthetic fibre: Grade RHPE80/20
Nanofiber media: Grade RHNF/123
4.2 Offshore Platforms

Typical features: seawater with high salt concentration leads to severe salt corrosion and heavy damage to equipment components;

high ambient humidity and occasional heavy fog impose stringent requirements on intake cartridges.

Required media characteristics:
Medium / high efficiency grade;
Excellent hydrophobicity;
Support pulse jet cleaning.
Recommended filter media:
Nanofiber media: Grade RHNF/123
4.3 Coastal Areas
Typical features: high humidity places stringent demands on cartridge moisture resistance, accompanied by salt corrosion.
Required media characteristics:
Medium efficiency grade;
Good hydrophobicity;
Compatible with pulse jet cleaning.
Recommended filter media:
Wood pulp fibre + high-hydrophobic synthetic fibre: Grade RHPE80/20
Nanofiber media: Grade RHNF/123
4.4 Urban Environments
Typical features: complex airborne contaminants including industrial flue gas and vehicle exhaust, with high and diversified dust loads.
Required media characteristics:
High dust holding capacity for hydrocarbons;
Good hydrophobicity;
High efficiency for fine dust particles.
Recommended filter media:
Wood pulp fibre + fine fibre (layered or coating technology): Grade RHFD/234
Wood pulp fibre + synthetic fibre + glass fibre: Grade RHPE80/20
Nanofiber media: Grade RHNF/123

The above media grades are well suited for intake filtration under corresponding working conditions. 

Targeted media selection for different environments not only guarantees intake filtration efficiency, but also extends filter cartridge service life.