Notes at the time of setting up a process filter for filtering gas. Filter's Company Yamashin

There are many uses of filtering gas in each process, dry air used at the factory to produce electronics, absorpt and exhaust filter of the tank used in the food industry etc., nitrogen gas for purge, carbon dioxide in process of producing carbonated drink and gas for fuel etc. The notes when examining the filter for filtering gas were summarized below.

1.When fluid is gas, compared with a liquid, the filtration efficiency and characteristic of a filter change a lot.
The greatest factor is that gas differ greatly from a liquid about the density and viscosity. It is as follows if air is compared with water. Consequently, if the same filter is used, the filtration efficiency becomes very high in the case of filtering gas rather than the case where a liquid is filtered.

Density Air at 20 degrees : Water = About 1.166 : 1000 kgf/m³
Viscosity Air at 20 degrees : Water = About 0.00156 : 1 cSt

Be careful not to become exaggerated specs in the selection of the filter for filtering gas.

2. Factor that the filtration efficiency becomes very high in the case of filtering gas rather than a liquid if the same filter is used.
- The difference of specific gravity with a subject to be separated is large. ...... Air : Iron = About 1.166 : 7860 kgf/m³
If the flow velocity becomes early, the separative effect by inertia will become large. If the flow velocity becomes loose, the separative effect by diffusion will become large. (The difference of density with gas and a liquid and the lowness of fluid viscosity make high the separative effect by inertia and diffusion.)


Inertia The effect is so large that the flow velocity is quick. The effect is so large that the specific gravity difference of fluid and a particle to be caught is large. The effect is small when the fluid density is high. The effect is small when the fluid viscosity is high. Diffusion and adsorption The effect is so large that contact time with media is large. The effect is small when the fluid density is high. The effect is small when the fluid viscosity is high.


3. In filtration of gas, the conditions of filtration change by the change of temperature and pressure. Volume changes with temperature and pressure.
<The law of Boil Shalle>
The value which divide the product of pressure and volume by the absolute temperature is fixed. It is expressed with a formula below. PV/T = Regularity PV/T = P'V'/T' P: Pressure V: Volume T: Absolute temperature

The volume will become half if Absolute pressure doubles. Keep in mind that it differs from pressure of gage in the case of examination of a filter. The amount of the filter used decreases when operating pressure becomes high. The amount of the filter used increases when the fluid temperature is high.

4. A setup of the amount of the filter used changes by the gaseous density.
In the case of gas, the difference of molecular amount turns into the difference of density. When using the same filter, the flux becomes so high that the gaseous density is low. For example, in hydrogen and carbon dioxide, there is a 22 times difference about their density. There is a 14.5 times difference about density of hydrogen and air. In filtration of gas, the average molecular amount is calculated from the gas composition, and the setup of a filter is performed. The amount of the filter used decreases when the gas has low density.

5. Use of water-repellent media
Water-repellent media are selected especially on the line as which high filtration accuracy is required. The highly precise filter using a film has a risk of filtration resistance going up rapidly when a film is damp.

Hydrophile property media If a film is damp, a blockade will arise with surface tension of liquid. - If it gets wet, filtration area will fall by the blockade of a hole and resistance will go up. - If it gets wet completely, the wetting cannot be destroyed to the B.P. value. - If it gets wet, a particle and bacteria will tend to adhere to a film. - The possibility of the contamination on the secondary side by bacteria breeding is large.
Water-repellent media It is separated on the surface of film by water-repellent media. - If it is hard to get wet, the blockade of a hole doesn't progress so the rise of resistance is loose. - Moisture is separated on the surface and it is hard to carry out the getting blocked of the media. - A particle and bacteria are easy to be taken into moisture. - The possibility of the contamination on the secondary side by bacteria breeding is small.

In the filtration of gas in food and the medicine industry, there are many cases of super saturated steam and please examine a water-repellent filter.

7. Separation of oil mist and super saturated steam
Unlike filtration, the separation of oil mist contained in compressor air etc. and super saturated steam is made to the drop of liquid, and is divided into the secondary side of the filter using gravity(specific gravity). Fluid makes it pass from inside of the filter to the outside contrary to the usual filtration. Although mist is caught by the filter once, it is made the drop of liquid by passing the media. Keep in mind that it differs from filtering separation of dust or microscopic particles