Discussion on the Characteristics and Selection Principle of Air Filter
Filtration is an indispensable means of treatment for the purification of fluids, primarily for the removal of particulate matter or other suspended matter in the fluid. The principle of an Air Filter is to use a porous medium to remove contaminants from the fluid (liquid or gas) to bring the fluid to the desired cleanliness level. Air Filters are often considered to be a simple mesh or sieve, and filtration or separation is carried out on a surface. This is the way to the past, and now most of the Air Filter filter wall is a certain thickness, that is to say the Air Filter equipment with the depth of the "curved channel" in the form of removal of pollutants played a supporting role. The Air Filter is a device that removes a small amount of solid particles from the liquid. When the fluid enters the cartridge with a certain size of the filter, the impurities are blocked and the clean filtrate is discharged from the Air Filter outlet. When cleaning is required, Branch pipe at the bottom of the plug, drain the fluid, remove the flange cover, remove the filter cartridge, after handling can be reloaded. It is well known that the core of the Air Filter is a membrane, which is a thin film filled with tiny pores on the microporous support layer (support). There are many materials for the production of filters, divided into organic film (such as polysulfone hollow fiber membrane) and inorganic film (such as ceramic film). Membrane Air Filter filtration accuracy is higher, particle size control is relatively stable, and backwash easy to restore performance. Therefore, the use of maintenance is very convenient.
1 filtration mechanism and influencing factors
1.1 filtration mechanism
There are two kinds of fluid filtration mechanism. One is based on the size of the particles to separate, such as interception, screening and surface capture, etc .; the other is adsorption, that is, particles in the chemical / charge effect on the filter. This requires the pharmaceutical companies according to their actual needs to choose a different filter membrane.
1.2 Factors that affect filtration
1.2.1 Characteristics of the fluid
And the characteristics of the fluid. For example, the viscosity of the fluid and the chemical / ionic composition, the greater the viscosity of the fluid in the same pressure conditions, the slower the flow rate, fluid and membrane have more contact between the filter effect is better; another example, fluid and membrane mixing / Contact time on the filter effect also has a greater impact, the longer the mixing / contact time, the better the filtration effect. In addition, it should be noted that the characteristics of the fluid only affect the membrane on the adsorption of fluid retention effect without affecting the particle size of the exclusion.
1.2.2 Operating conditions
With the actual operating conditions, such as particle flow rate and filtration pressure. To get a good filter effect, generally choose a lower flow rate, the lower the flow rate the better the effect of interception. Practice has shown that the movement of the membrane is detrimental to the filtration. Once the structure of the membrane has changed during the filtration process, the particles and fibers can be precipitated from the deep Air Filter, affecting the filtration effect. However, the speed / differential pressure only has a significant impact on the adsorption traction, the size of the exclusion of the effect is quite small.
1.2.3 Particle type
Particle type and filtration effect is also a great relationship, the particles are divided into two kinds of deformable particles and non-deformable particles. Under certain pressure, the deformable particles will enter the filter membrane and cause more filter holes to clog, thus affecting the filtration effect, such as gel filtration. However, immutable particles are filtered to form a layer of cake-like objects on the filter.
1.2.4 Filter membrane type
In contrast to the type of filter membrane, the pore size and structure of the different membrane are different. Some of the membrane structure is rigid, and some membrane structures are movable. The pre-filter membrane does not have a uniform national standard, and the different manufacturers have their own definitions and methods. Therefore, the selection and replacement of the business requires a high degree of attention, the same 0.22μm pre-filtration membrane, the choice of different manufacturers of filtration The effect will be very different. The sterilization filter of the common aperture is defined by the law, each business is the implementation of the same standard, in the selection and replacement of the relatively simple.
1.2.5 filter material
And the filter material, the filter material according to the relationship with the water is divided into hydrophilic (water can be infiltrated) and hydrophobic (water can not be infiltrated) two. Hydrophilic Air Filters are mainly used in water or water / organic solution mixing filtration and sterilization filtration, such as cellulosic materials (regenerated cellulose, mixed cellulose ester), PVPP polycarbonate, PVDF modified polyvinylidene fluoride Ethylene; Hydrophobic Air Filter is interrupted by water or "guided" into the filter, mainly used in solvent, acid, alkali and chemical filtration, tank / equipment respirator, process gas, fermentation inlet / exhaust filter Such as PTFE polytetrafluoroethylene, PVDF polyvinylidene fluoride, polypropylene, polysulfone, polycarbonate and the like.
2 Air Filter characteristics and structure
Air Filters are generally divided into deep Air Filter (primary filter), surface Air Filter (middle filter) and membrane Air Filter (depth filter) 3 kinds. Deep Air Filter fiber shedding, can not give a precise pore size, the thickness is generally 3 ~ 20mm, usually adsorption, and a greater capacity of the sewage; surface Air Filter fiber generally used thermal bonding (1 mm), the adsorption capacity is small; membrane Air Filter is characterized by hard texture, not broken, there are twists and turns of the channel and a very high internal surface area , Have a certain opening rate, can do integrity testing, commonly used in deep-level filtration, such as sterile Air Filter.