Air Purification System Filters Q&A

1 Why is oil in the air the most hazardous?

   A: In some demanding areas, such as pneumatic control systems, a single drop of oil can change the condition of an air hole. It can paralyze a production line that would otherwise run normally and automatically. Sometimes oil can also swell the seals and columns of pneumatic valves, causing sluggish operation and even blockage in severe cases. In processes completed by air, such as blowing-shaped parts, oil can also cause defects in the shape of the product or contamination of the exterior.

2 What are the main sources of oil contamination?

   A: As most compressed air systems use lubricating oil compressors, the machine vaporizes oil into oil droplets during operation. It is formed in two ways: as a result of the compression of the piston or the shearing action of the rotating blades, so-called "dispersed droplets" are formed. Its diameter is from 1 to 50 μm, the other is in the lubricating oil cooling high-temperature body, vapor formation of "condensate droplets", its diameter is generally less than 1 μm, this condensate oil droplets usually accounted for more than 50% of the total oil weight, accounting for all the actual number of particles of oil pollution more than 99%.

3 What is the common measurement standard for oil volume?

   A: The amount of oil contamination is generally measured using a very small measure to give a clear indication of its cumulative state. 100 scfm (standard cubic feet per minute) of air containing 1 ppm W/W (parts per million by weight) of oil flow is equivalent to 150 ml per month.

4 Can oil-free compressors eliminate the production of contaminants?

   A: No. Under optimum operating conditions, such compressors will produce no less than 0.5ppm W/W of hydrocarbons, and even at 100scfm air volume, the vapourised condensate produced will be more than 15mL per month.

5 What is meant by a compressed air filter?

   A: A compressed air filter is a device for filtering and purifying compressed air. We generally refer to the high-efficiency precision filter in the compressed air system pipeline.

6 What is the working principle of a filter?

   A: The general filter element is made up of fiber media, filters, sponges, and other materials. The solid and liquid particles (droplets) in the compressed air are intercepted by the filter materials and coalesce on the surface of the filter element (inside and outside). The liquid droplets and impurities accumulated on the surface of the filter element are precipitated to the bottom of the filter by gravity and then discharged by an automatic drain or manually.

7 What are the characteristics of several general types of filters?

   A: The adsorption type (activated carbon) filter, which uses the surface to create an attraction, has problems such as limited service life and a reduction in its adsorption capacity after the adsorbent has absorbed the oil.

   The main material absorbents for absorption filters, such as wool, linoleum, and cotton, lose their structural advantage and fail rapidly after absorbing the liquid to the interior and saturating it.

   Mechanical separators and screen air filters, usually classified by 5, 10, 20, and 40 μm, are ineffective for the tiny particles that make up the majority of the oil droplets.

8 What are the characteristics of glass fiber materials used in filtration?

   A: Glass fiber is very effective in separating oil droplets with diameters ranging from 50 to 0.01μm, and it does not require either adsorption or absorption during filtration. It is also very effective and superior to other materials.

9 What is the working mechanism of a coalescing filter with glass fiber as the core of the cartridge?

   A: The air flows in through the middle of the filter element and the oil droplets are collected by four mechanisms: gravitational action, inertial collision, direct interception, and infiltration.

   Gravity action: When the airflow speed in the filter is low, oil droplets of 20~50μm in diameter are mostly collected by gravity-free fall before reaching the filter layer, and the airflow also continues to fall and collect when flowing through the filter. The greater the velocity of the airflow, the lower its efficiency.

   Inertial collision: Usually suspended particles larger than 1μm in diameter have a large impulse. They do not always follow the same path as the airflow. As a result, they will inertially hit the fiber layer. The higher the air velocity, the higher the collision rate.

Direct interception: particles with a diameter of 0.3-1 μm are moving with the airflow and will mostly be separated by the interception of the fiber layer at 1/2 of the cartridge. The smaller the particles, the lower the interception rate.

   Permeation: particles with a diameter of less than 0.3 μm. Because their mass is so small they no longer have the usual characteristics of a liquid. They move in an irregular Brownian motion． They do not follow the same path as the airflow. Because of this motion, they can be captured by a finer filter layer. The smaller the particles are. The more violent the Brownian motion, the greater the chance of capture.


10 What are the main features of a high-efficiency filter?

   A: Particles with a diameter of 0.3 µm can neither be mechanically nor effectively captured. The efficiency of a filter is evaluated by its ability to capture particles of this size to the maximum extent possible.

11 What is the simple working process of a high-efficiency coalescing filter?

   A: Once the compressed air enters the middle of the filter element, it is collected by the filter layer by gravity, collision, interception, and permeation. Once the oil droplets have been removed by the filter layer, they are first collected. The small oil droplets first aggregate into large oil droplets, and when the mass of the aggregated large oil droplets is large enough, they settle to the bottom of the filter layer. They then flow into the filter spoil and are removed from the system either manually or by an automatic oil drain.

12 What are the design requirements for a high-efficiency coalescing filter?

   A: Glass fibers are water-resistant but not oil resistant. Oil forms a thin film on the surface of the fibers, which affects the collection and increases the functional diameter of the screen core. In this regard, it is necessary to choose a finer fiber for the design.

   The design of the filter layer focuses on controlling the airflow velocity and the physical environment of the filter layer. It is only necessary to ensure that the material has a sufficiently large surface area to keep the airflow velocity as low as possible so that the interception, collision, and diffusion effects are more effective and, on the other hand, to design the filter bed to be thick enough to allow sufficient residence time for the particles. Finally, there must not be too many layers of fibers in the filter cartridge, which would impede drainage and increase pressure loss, making the filter less effective.

   The collection of oil droplets is a physical process, with pressure, flow rate, humidity, and the physical characteristics of the impurities themselves all affecting the aggregation outcome. Therefore, the configuration, arrangement, size, and choice of type of filter layer are also critical.

13 Do coalescing filter cartridges have a life span?

   A: Yes. Theoretically, the filter layer can remove the liquid an unlimited number of times and maintain its high efficiency. The increased pressure drop due to the continuous collection of liquid in the filter during use results in the certain effective service life of the filter layer.

14 Why is a porous foam layer (or cover) used in the filter element of a coalescing filter?

   A: Generally, the filter cartridge design of coalescing filters will have a porous foam layer or cover inside and outside the filter layer.

   When the gas flow containing oil is radially flowing through the central layer of the filter, it flows into the flexible inner porous foam cover layer. On the one hand, it acts as a disperser, pre-filter, and spatial stabilizer for the airflow; on the other hand, it changes the airflow by its adjustment (expansion or contraction) so that the airflow exerts a gentle pressure on the filter layer and ensures that it works completely and effectively.

   As the air flows through the filter layer, the oil droplets are intercepted and collected. After the oil has collected, it flows through a layer of rigid metal tubing to the outer porous foam hood (outside the metal tubing). The outer porous foam hood has a large non-absorbent surface which allows the oil to quickly cover its surface and flow to the bottom under gravity, thus preventing the oil from reentering the air stream. When oil accumulates at the bottom of the hood (no airflow dead zone), the foam holes are plugged and the oil descends into the collection tank.

15 What is the standard classification of filter cartridges based on?

   A: According to ISO 8573.1, the grade achieved by the filter for compressed air is generally based on the size of the solid contaminant particles, the content of liquid water, and the content of oil droplets, oil mist, and oil vapor after treatment by the filter cartridge.

16 How are the grades of filters specifically classified?

    A: The general filter grades can be divided into pre-filtration, primary filtration, fine filtration, and activated carbon filtration. The pre-filter generally removes particles with a diameter of 3 to 5 μm, the primary filter generally removes particles with a diameter of 0.5 to 1 μm and the remaining oil mist content of 1 ppm w/w, the fine filter generally removes particles with a diameter of 0.01 μm and the remaining oil mist content of 0.01 ppm w/w, and the activated carbon filter is mainly used to remove odor and oil vapor (the remaining oil mist content is only 0.003 ppm w/w).

17 What is the application of the different levels of filter standards?

    A: Pre-filters are generally used downstream of the compressor (after-cooler) and are not required for demanding applications. Primary filters are generally used for tools, motors, cylinders, etc. Fine filters are generally used for paint spraying, injection molding, instrumentation, control valves, transmission, mixing, electronic component manufacturing, nitrogen separation, etc. Activated carbon filters are generally used in food and pharmaceutical manufacturing, breathing air, gas processing, etc.

18 Why should filters be purchased in combination?

    A: It is a common misconception that selecting a single filter with the corresponding treatment accuracy according to the required air quality will meet the requirements and save money. This is not the case. Although the required air quality is determined by the processing accuracy of the selected single filter, without the pre-treatment protection of the lower pre-filter, the high-precision cartridge will soon be clogged due to excessive load, which speeds up the replacement frequency of the cartridge, thus increasing production costs in disguise.

19 Can the filter reduce the air dew point?

    A: Filters can generally only remove solid, liquid particles (droplets), while water vapor and oil vapor can pass unimpeded through the curved passages of the filter material. Therefore, they cannot be removed by mechanical filters (except for activated carbon filters). The only way to fundamentally remove water vapor and oil vapor is to lower the dew point temperature of the air with a dryer.

20 What is the relationship between filter efficiency and air temperature?

    A: The temperature of the oil and water contained in the compressed air affects the filter efficiency. For example, when the temperature is 30°C, the oil content flowing through the filter is 5 times that at 20°C; when the temperature rises to 40°C, the oil content flowing through the filter is 10 times that at 20°C. Therefore, the filter should generally be installed at the lowest temperature point of the compressed air system.

2l What is the difference between domestic and imported filter elements?

    A: Due to raw materials, equipment, and other reasons, the domestic cartridge has been lagging behind the imported cartridge in terms of filtering materials and processing technology. The backwardness of testing means and testing equipment makes the domestic cartridge unable to improve its quality due to the absence of quantitative authoritative analysis. Compared with imported cartridges, domestic cartridges are generally rough and bulky.

22 What are the optional parts of the filter?

    A: The optional parts of filters generally include: an internal automatic drain, external automatic drain, differential pressure gauge, differential pressure meter, electronic differential pressure indicator, and liquid level indicator.

23 What is the purpose of the filter options?

    A: The internal automatic drain and external automatic drain of the filter options are used to automatically discharge the oil, water, and dust mixture from the filter element to reduce the human factor affecting the filtration efficiency of the system. The differential pressure gauge, differential pressure meter, and electronic differential pressure indicator are used to guide the time of filter element replacement. Level indicators are used to indicate the number of mixed pollutants of oil, water, and dust inside the filter (the working condition of the internal automatic drain can be monitored and manual drainage can be guided).

24 How is the replacement interval of the filter cartridge determined?

    A: The replacement cycle of the filter cartridge is determined by its pressure drop. Generally speaking, if the pressure drop exceeds 0.68 kgf/cm2, the pointer of the filter differential pressure gauge points to the red zone, or the filter has been working for 6,000-8,000 hours (one year) and should be replaced. The activated carbon cartridge is replaced when an odor is detected downstream.

25 Why should the filter cartridge be replaced regularly?

    A: Because continuous contamination of the filter cartridge will result in a lower flow of gas through the system and a higher pressure drop, as well as an increase in energy and power consumption, resulting in higher operating and production costs and an increased burden on the environment.

26 What aspects should be noted in filter installation?

A: (a) The working pressure must not exceed the maximum pressure indicated for the filter.

   (b) The filter should normally be installed after the aftercooler and storage tank, as close as possible to the point of use and the lowest temperature point.

   (c)The filter should not be installed after the quick opening valve to prevent backflow and shock phenomena.

   (d) Filters should be installed vertically with sufficient space below to change the filter element.

   (e) Larger filters should be properly supported in the pipework.

27 What are the precautions for replacing the filter cartridge?

    A: (a) Isolate the filter, close the inlet valve or compressed air supply system, and completely depressurize before closing the outlet valve (or close the valve in question and then completely depressurize through the filter drain hole).

   (b) Unscrew the housing and remove the old filter element.

   (c) Clean the filter housing.

   (d) Replace the filter element with a new one (do not leave out the sealing ring and the filter element should be fitted tightly and correctly).