As the second largest power source after electricity, compressed air is also a process air source with multiple uses. Its application range covers the petrochemical industry, power electronics, automobile manufacturing, food and beverage, chemical pharmaceuticals, machinery manufacturing, medical technology, Glass manufacturing, aerospace, and other industries.
Therefore, preparing clean and pollution-free compressed air and the filtration and purification treatment of compressed air are essential to achieve these goals. Compressed air purification refers to filtering, drying, and degreasing the compressed air generated by the air compressor.
Typical compressed air systems include air compressors, coolers, gas storage tanks, filters, dryers, automatic drainers and gas pipelines, pipeline valves, control instruments, pneumatic tools, pneumatic power machinery, etc.
In this article, I will describe in detail some core knowledge about compressed air filtration and related air filters, which can help you better understand compressed air filtration. Also, I want to explain that this article is about particulate matter or aerosol filtration.
Why does compressed air need to be filtered?
To produce compressed air, the air compressor often sucks air from the ambient atmosphere and compresses it to make compressed air.
Therefore, in principle, compressed air not only contains all the pollutants in the atmosphere, such as particles, dust, pollen, moisture, and vapor, but these appear in compressed form in the compressed air.
In addition, unclean factors in the compressed air network system are generated during the compressed air production process, such as rust, dust, erosion, and impurities from the compressor oil or other pipes entering the compressed air system.
If low-quality compressed air is used in some vital pharmaceutical production processes, the compressed air can significantly reduce the quality of the pharmaceuticals.
In such a critical process, it is necessary to filter the compressed air to remove unnecessary impurities, thereby improving the purity of the compressed air.
What contaminants in compressed air can be removed by filtration?
Pollutants in compressed air can be divided into four categories: moisture (including liquid water droplets and water vapor), oil (oil droplets, oil mist, and oil vapor), solid dust and particles, various gas impurities, and chemical odors.
In addition to water vapor that needs to be removed by the dryer, other pollutants (including gaseous oil vapor and various chemical odors) can be removed by filtration methods. For this purpose, air filters are used in the pharmaceutical industry to remove multiple contaminants.
For most industrial fields, “aerosol” pollution formed by fine particles (including solid and liquid particles) in compressed air is a significant problem, so controlling aerosol pollutants is the most important.
What are aerosols?
Solid particles that can be dispersed (suspended) in a gas for a certain period are called dust. From the perspective of colloidal chemistry, dust is a dispersion system in which the dispersed phase is solid particles, and the dispersion medium is air.
Technically, this dispersion is usually called an “aerosol.” Therefore, aerosols are particles suspended in the air, ranging from a few nanometers (1/10³µm) to a few tenths of a millimeter. Aerosols generally exist in a complex state of motion that is heterogeneous, irregular, and unbalanced.
Aerosol filtration is the main task of compressed air filtration. The aerosol particle size range that filtration methods can remove is between 0.1 and 0.3um. The lower particle size limit is equivalent to the smallest particle instruments can detect. In contrast, large particles are particles that cannot be suspended in the air due to the influence of gravity.
Next, I will write in detail for 9 essential knowledge about air filters in Compressed Air Filtration; as mentioned above, I focus on air filters for particulate matter or aerosol filtration.
1. What are the filtration mechanisms for air filters to remove aerosol impurities?
According to the precision filtration theory, the filtration mechanism for suspended particulate impurities and aerosols in compressed air differs.
Aerosol particles with a wide range of particle size distribution can be separated from compressed air under the combined action of multiple filtration mechanisms.
Generally speaking, there are the following five mechanisms of action: (1) diffusion deposition, (2) direct interception, (3) inertial impact, (4) gravity deposition, and (5) electrostatic deposition.
What is diffusion deposition?
Diffusion deposition is caused by Brownian motion.
Particles or smoke with very small diameters (below 1 μm) can produce an irregular linear motion in very slow airflow, called “Brownian diffusion.”
The movement distance of Brownian diffusion is very short, and it only works at significant airflow speeds and large fiber gaps.
However, in very slow airflow speeds and small fiber gaps, when the particle size is less than 0.1 μm, Brownian diffusion becomes obvious, and the result is that the chance of contact and retention between particles and fibers is significantly increased.
What is inertial sedimentation?
The cross-section of a single fiber with a diameter of df. The air is blocked when the particles move vertically toward the fiber direction at a certain speed with the airflow. It changes the direction of movement, bypassing the fiber and moving forward. However, particles with larger diameters have greater inertia due to their movement. The direction of movement cannot be changed in time to follow the dominant air flow, so the particles rush directly to the fiber surface and remain on the fiber surface due to friction and adhesion.
The ratio of the width interval b in which the fiber can retain particles to the fiber diameter df is called the inertial deposition capture efficiency of a single fiber, that is, η2=b/df.
What is direct interception?
When the airflow speed drops below a certain speed, the particles cannot collide with inertia and remain on the fibers, and the collection efficiency drops significantly.
However, the practice has proved that as the airflow speed gradually decreases, the particle collection efficiency of the fiber increases again, which shows that there is another mechanism at work: direct interception.
In the direct interception mechanism, particles will be intercepted when they approach the collection surface (filter element surface) as long as the particle radius equals the gap on the filter media surface.
For the capture of particles larger than 1 μm, the mechanism of inertial deposition and interception usually accounts for 99.9%.
What is gravity sedimentation?
Gravity sedimentation—This is a relatively stable separation effect. When the gravity on the particles is greater than the drag force of the airflow on them, the particles will settle. When settlement occurs, the movement trajectory of the particles deviates from the main flow line of the air, causing the particles to touch the filter media and settle there.
The larger the particle mass, the more obvious the effect of gravity deposition. Gravity sedimentation improves the capture efficiency of interception sedimentation.
What is the electrostatic adsorption mechanism?
When friction occurs due to relative movement between dry air and non-conductive filter materials, induced charges will be generated. This is especially obvious when synthetic fibers are used as filter materials.
Most of the particles suspended in the air have different charges. These charged particles will be attracted by objects with opposite charges and settle.
2. How to comprehensively describe the filtration mechanism?
I believe that the five mechanisms that filter particles can work independently of each other, but their results are not necessarily additive. Most air filters regard three mechanisms (diffusion deposition, inertial impact, and direct interception) as the most important when designing them.
When discussing particles with very small particle sizes, the effect of gravity is very small and is often ignored. Electrostatic forces are often overlooked because they are considered secondary.
According to my research, for particles with a particle size smaller than 0.2 μm, diffusion dominates; for particles larger than 1 μm, inertial collision or interception dominates;
Particle sizes range from 0.2 μm to 1 μm, for which all mechanisms may be important.
Of course, the sum of the efficiencies of the three mechanisms does not equal the total efficiency.
3. What is filtration efficiency?
Filtration efficiency, particle capture efficiency, refers to the ratio of particles filtered by the filter layer of the filter element to the number of particles before filtration. It is an important indicator to measure the filtration capacity of the filter:
η=（N1-N2）/N1=1-N2/N1(-1)
In the formula, N1 and N2 are the content of particles in the air before and after filtration, respectively (i.e., dust concentration: particles/L)
The most important thing to evaluate whether a filter medium is superior is its filtration efficiency.
The filtration efficiency of an air filter is mainly related to factors such as particle size, type of filter media and their specifications (i.e., fiber thickness), media filling density, media thickness, and flow rate of the passing air.
The parameter indicating the technical performance index of the filter-filtration efficiency is usually for particles of specific particle size. For example, the “DOP efficiency” commonly used in the United States, Europe, and other countries is the filtration efficiency specifically for particles with a particle size of 0.3 μm rather than the filtration efficiency for particles ≥0.5 μm.
For example, if a HEPA filter has a filtration efficiency of 99.91% for 0.3μm particles, the filtration efficiency for 0.5μm particles can reach 99.994%, and the filtration efficiency for ≥0.5μm particles can get 99.999%.
4. What is the air filter penetration rate?
In the formula (-1), N2/N1 is the ratio of the number of particles in the air before and after filtration, that is, the ratio of the number of particles that penetrate the filter layer to the original number of particles, which is called the penetration rate K (%).
Obviously, the smaller the penetration rate K, the higher the filtration efficiency of the filter.
K=(1-η)*100 (-2)
5. What is the logarithmic penetration law of the air filters?
The logarithm of the ratio of the number of particles entering the filter layer to the number of particles penetrating the filter layer is a function of the thickness L of the filter layer. It can be expressed by formula (-3):
N2/N1=10-K1L (10 to the power of -K1L) (-3)
The filtration constant K in the formula is related to many factors, such as fiber type, fiber diameter, filling density, air flow rate, particle diameter, etc.
Generally, specific conditions are selected and obtained experimentally.
6. What is filtration pressure drop?
When air passes through the filter layer, it must overcome the medium’s friction, causing a pressure drop. This is a loss of energy consumed in capturing particles.
The pressure drop varies with the filter layer’s thickness, the filter media’s nature, and the filling conditions. When the above conditions remain unchanged (for a specific air filter), the pressure drop is proportional to the air volume flow through the filter. The relationship is:
ΔP=W Q(-4)
In the formula, ΔP—filter element pressure drop
W—The air flow resistance in the filter (related to the filter layer’s thickness and the filter media’s properties, etc.)
Q—air volume flow through the air filter.
The relationship between pressure drop and flow rate is essential for air filters. For air filters, pressure drop and capture efficiency are equally important. The pressure drop, which is the main technical indicator of the filter, often determines the air filter’s service life.
7. What is the surface speed and filtration speed of the air filter?
Surface velocity refers to the speed of airflow passing through the filter section, generally expressed in m/s. Right now
u=Q/F (5)
In the formula: Q—air volume m³/s
F—Filter cross-sectional area, that is, windward area
The surface velocity reflects the filter’s passing capacity and volume.
The filtration speed refers to the speed of airflow passing through the filter material area, generally expressed in L/cm².min or cm/s.
The filtration speed reflects the passing ability of the filter material, especially the filtration performance of the filter material. The filtration speed of HEPA and ULPA filters is generally 3cm/s, and that of sub-HEPA filters is 5-7cm/s.
8. What is the critical speed of the air filter?
In the single fiber air streamline diagram, when the airflow speed drops to the point where the inertial force of the particles is not enough to cause the particles to break away from the dominant airflow and collide with the filter, that is, at any point in the airflow, the particles also change the direction of movement with the airflow and move around the fiber. , that is, when b=0, the collision efficiency of fibers caused by inertial deposition equals zero. The airflow speed at this time is called the critical speed Vc of inertial collision.
The filter’s critical speed, Vc, is the actual airspeed when it travels between the fibers of the filter layer. It is affected by the airflow velocity Vs of the empty cross-section of the filter container and the filling density α of the filter medium:
Vc = Vs/1-α(6)
At the critical speed, the inertial deposition in the filter medium (fiber) is equal to zero, and its filtration efficiency is mainly affected by interception sedimentation (for large particles) and Brownian diffusion (for small particles).
Generally, the design flow rate of filters is below the critical speed to reduce the filtration pressure drop. The airflow rate through the filter during use should also be controlled below the critical speed.
The critical speed Vc changes with the fiber diameter and particle diameter. Particles of different diameters have different critical velocities for fibers of different diameters.
9. What is the dust-holding capacity of the air filter?
The dust holding capacity of the filter is an indicator directly related to the service life.
Usually, when the final resistance of the filter during operation reaches a value that is twice as large as the initial resistance (if the one-time value is too low, it can also be set as another multiple),
Or (for pre-filters) the weight of dust deposited on the filter when the efficiency drops below 85% of the initial efficiency is used as the dust-holding capacity of the filter.
Last summary
The pharmaceutical industry is one of the industries that use air filters more often, and compressed air filtration is a typical application of filters. Some of the core knowledge about air filters I mentioned above also applies to filtration in other industries. As a leading air filter manufacturer, we have extensive experience and expertise. Likewise, I am very happy to share this knowledge with everyone to obtain better industry applications.
