Questions and Answers on Filter Technology of Compressed Air Purification System

1 Why is the most harmful of oil in the air?

Answer: In some places with strict requirements, such as pneumatic control systems, a drop of oil can change the condition of the air holes. Paralyze the production line that was originally running normally and automatically. Sometimes, the oil will expand the sealing ring and cylinder of the pneumatic valve, causing slow operation, serious or even blockage. In processes completed by air, such as blowing shaped parts, oil can also cause product appearance defects or surface pollution.

2 What are the main sources of oil pollution?

Answer: Since most compressed air systems use lubricating oil compressors, the machine vaporizes oil into oil droplets during operation. It is formed in two ways: one is the so-called "dispersed droplets" produced by the compression of the piston or the shear of the blade rotation. Its diameter ranges from 1 to 50μm. The other is the "condensed droplets" formed by the vaporization of lubricating oil when cooling the high temperature body. Its diameter is generally less than 1μm. This kind of condensed oil droplets usually account for more than 50% of the total oil weight. It accounts for more than 99% of the actual particles of all oil pollution.

3 What is the commonly used measurement standard for oil volume?

Answer: The amount of oil pollution generally adopts a very small measurement standard to clearly indicate its accumulation status. 100 scfm (standard cubic feet per minute) air contains 1ppmW/W (parts per million by weight) oil flow is equivalent to 150ml per month.

4 Can oil-free compressors completely eliminate the production of pollutants?

Answer: No. Under the most ideal working conditions, this type of compressor will also produce no less than 0.5ppmW/W of hydrocarbons. Even with a gas volume of 100scfm, the vaporized condensate produced per month is more than 15mL.

5 What is a compressed air filter?

Answer: The compressed air filter is a device for filtering and purifying compressed air. We generally refer to high-efficiency precision filters in compressed air system pipelines.

6 What is the working principle of the filter?

Answer: Generally, the filter element is composed of fiber media, filter mesh, sponge and other materials. The solid and liquid particles (drops) in the compressed air are intercepted by the filter material and condensed on the surface (inside and outside) of the filter element. 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 through an automatic drain or manually.

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

Answer: The adsorption type (activated carbon) filter that uses the surface to generate attractive force has a limited service life, and the adsorption capacity of the adsorbent decreases after the oil is absorbed.

The main material absorbents of absorption filters, such as wool, linoleum, and cotton, will lose their structural advantages and quickly fail after absorbing the liquid inside and flooding it.

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

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

Answer: The glass fiber can effectively separate the lubricating oil droplets with the diameter from 50 to 0.01μm. It does not need to be adsorbed or absorbed during filtration. And it is very effective, better than other materials.

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

Answer: Air flows in from the middle of the filter element and collects oil droplets through four mechanisms: gravity, inertial collision, direct interception and penetration.

Gravity effect: When the airflow velocity in the filter is low, most of the oil droplets with a diameter of 20-50μm will be collected by gravity before reaching the filter layer, and the airflow will continue to fall and collect as it flows through the filter. The greater the air velocity, the lower its efficiency.

Inertial collision: Suspended particles with a diameter greater than 1μm usually have a great impulse. Not always consistent with the air flow path. Therefore, it will hit the fiber layer inertially. The greater the air velocity, the greater the collision rate.

Direct interception: particles with a diameter of 0.3-1μm move with the airflow, and most of them will be intercepted and separated by the fiber layer at 1/2 of the filter element. The smaller the particle, the lower the interception rate.

Penetration: particles with a diameter less than 0.3μm. Because its mass is too small, it does not have the usual characteristics of liquid. They are moving in an irregular Brownian motion. It is inconsistent with the air flow path. It is precisely because of this movement that it can be captured by a finer filter layer. The smaller the particle. The more intense the Brownian motion, the greater the probability of capture.

l0 What are the main features of high efficiency filters?

Answer: The particles with a diameter of 0.3μm can neither be captured mechanically nor effectively. Evaluating whether a filter is efficient is to see whether it has the ability to capture particles of this size to the maximum.

11 What is the simple working process of an efficient coalescing filter?

Answer: After the compressed air enters the middle of the filter element, it is collected by the filter layer through gravity, collision, interception and penetration. After the oil droplets are removed by the filter layer, they must be collected first. The small oil droplets first aggregate into large oil droplets, and when the aggregated large oil droplets are large enough, they will settle to the bottom of the filter layer. Then it flows into the filter tank and is removed from the system by manual or automatic oil draining device.

12 What are the design requirements for efficient coalescing filters?

Answer: Glass fiber is waterproof but not oil-proof. Oil will form a film on the surface of the fiber, affecting collection and increasing the functional diameter of the screen core. In this regard, the design must choose more slender fibers.

The design of the filter layer mainly considers the control of airflow speed and the physical environment of the filter layer. Only to ensure that the material has a large enough surface area and make the air velocity as low as possible to make the interception, collision and diffusion more effective. On the other hand, the designed filter bed must be thick enough to allow the particles to have sufficient residence time. Finally, there should not be too many fiber layers in the filter element, which will hinder drainage, increase pressure loss, and reduce the efficiency of the filter.

The collection of oil droplets is a physical process, and the physical characteristics of pressure, flow rate, humidity and impurities themselves will affect the aggregation result. Therefore, the configuration, arrangement, size and type selection of the filter layer are also critical.

13 Does the filter element of the coalescing filter have a service life?

Answer: Yes. In theory, the filter layer can remove liquid indefinitely and maintain its high efficiency. In fact, the increase in pressure drop due to the continuous collection of liquid in the filter screen will result in a certain effective service life of the filter layer.

14 Why does the filter element of coalescing filter use a porous foam layer (or cover)?

Answer: Generally, in the filter element design of coalescing filters, porous foam layers or covers are set inside and outside the filter layer.

When the oily air flows through the filter center layer, it flows radially into the flexible inner porous foam cover. On the one hand, it acts as a diffuser, pre-filter and space stabilizer for the airflow; on the other hand, it adjusts itself to change the airflow (expansion or contraction), so that the airflow generates soft pressure on the filter layer, ensuring its integrity and effectiveness during work .

When the air flows through the filter layer, the oil droplets are intercepted and collected. After the oil accumulates, it flows through a layer of hard metal pipe to the outer porous foam cover (covered outside the metal pipe). The outer porous foam cover has a large non-absorbent surface, which can quickly cover the surface of the oil and flow to the bottom under the action of gravity, thereby preventing the oil from returning to the airflow. When oil accumulates at the bottom of the cover (no air flow dead zone), the foam holes are blocked and the oil drops into the collection tank.

15 What is the basis for the classification of filter elements?

Answer: According to the ISO 8573. 1 quality level, the level that the filter can handle compressed air is generally classified according to the size of the solid pollutant particles after the filter element is processed, the content of liquid water, and the content of oil droplets, oil mist, and oil vapor. .

16 How are the levels of filters divided?

Answer: The general filter grades can be divided into pre-filtration, preliminary filtration, fine filtration and activated carbon filtration. The pre-filter generally filters out particles with a diameter of 3~5μm, the primary filter generally filters out particles with a diameter of 0.5~1μ and the remaining oil mist content is 1ppm w/w, and the fine filter generally filters out particles with a diameter of 0.01μm and the remaining oil mist content is 0.0 lppm w/w, activated carbon filter is mainly used to remove odor and oil vapor (the remaining oil mist content is only 0.003ppm w/w).

17 What are the applicable occasions for different grades of filters?

Answer: The pre-filter is generally used in the downstream of the compressor (after cooler), and the requirements for use are not high. Primary filters are generally used for tools, motors, cylinders, etc. Fine filters are generally used for painting, injection molding, instrumentation, control valves, transmission, stirring, electronic component manufacturing, nitrogen separation, etc. Activated carbon filters are generally used in food and medicine manufacturing, breathing air, gas processing, etc.

18 Why should filters be matched and purchased?

Answer: The misunderstanding of ordinary people is that they believe that selecting a single filter corresponding to the processing accuracy according to the required air quality can meet the requirements and save money. In fact, it is not. Although the required air quality is determined by the processing accuracy of the selected single filter, there is no pretreatment protection of the pre-lower-level filter, and the high-precision filter element will soon be clogged due to excessive load. Speed ​​up the replacement frequency of the filter element, which will increase production costs in disguise.

19 Can the filter reduce the air dew point?

Answer: Filters generally can only remove solid and liquid particles (drops), while water vapor and oil vapor can pass through the tortuous diameter of the filter material without any obstruction. Therefore, mechanical filters cannot filter them out (except for activated carbon filters). To fundamentally remove water vapor and oil vapor, only use a dryer to lower the dew point temperature of the air.

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

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

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

Answer: Due to raw materials and equipment, domestic filter elements have been lagging behind imported filter elements in terms of filter materials and processing technology. The backwardness of testing methods and equipment makes domestic filter elements unable to improve their quality due to lack of quantitative authoritative analysis. Compared with imported ones, domestic filter elements are generally rougher and bulkier.

22 What are the options for the filter?

Answer: The filter options generally include: internal automatic drain, external automatic drain, differential pressure gauge, differential pressure gauge, electronic differential pressure indicator and liquid level indicator.

23 What is the purpose of the filter option?

Answer: The internal automatic drain and external automatic drain in the filter option are used to automatically discharge the mixture of oil, water and dust filtered by the filter element to reduce the influence of human factors on the filtration efficiency of the system. The differential pressure gauge, differential pressure gauge, and electronic differential pressure indicator are used to guide the time to replace the filter element. The liquid level indicator is used to indicate the amount of mixed contaminants such as oil, water, dust, etc. inside the filter (it can monitor the working condition of the internal automatic drain and guide manual manual drainage).

24 How to determine the replacement cycle of the filter element?

Answer: The replacement cycle of the filter element is determined by its pressure drop. Generally speaking, the pressure drop exceeds 0.68kgf/cm2, and the pointer of the filter differential pressure gauge points to the red area, or it needs to be replaced after working for 6000~8000 hours (one year). The activated carbon filter element is replaced when the smell is detected downstream.

25 Why should the filter element be replaced regularly?

Answer: After the filter element continues to be polluted, the gas flow in the system will become smaller and the pressure drop will increase. At the same time, the energy and electricity consumption will also increase. As a result, the operation and production costs will increase, and the burden on the environment will increase.

26 What aspects should be paid attention to when installing the filter?

Answer: (a) The working pressure cannot exceed the maximum pressure marked on the filter.

(b) The filter is generally installed after the aftercooler and the gas 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 and prevent backflow and impact.

(d) The filter should be installed vertically with enough space below to replace the filter element.

(e) The larger filter should be properly supported in the pipeline.

27 What are the precautions for replacing the filter element?

Answer: (a) Isolate the filter, close the inlet valve or compressed air supply system, and then close the outlet valve after completely depressurizing (or close the relevant valve and completely depressurize through the filter drain hole).

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

(c) Clean the filter housing.

(d) Replace with a new filter element (don't miss the sealing ring, the filter element should be installed tightly).