Air & Gas Membrane Filtration Solutions
As a leader in the manufacture of filtration materials for healthcare applications for over 70 years, Pall understands the dynamic science of air and gas membrane filtration solutions. In applications for air venting and gas delivery, hydrophobic membranes incorporated into finished devices may be required for equipment and healthcare worker protection as well as easing patient comfort and protection from environmental contamination. Filtration of air and service gases, as well as in equipment venting, serves to purify them of harmful particulate matter. It is critical that in the process of selecting filter materials for these applications that the following aspects be considered: membranes with high air flow rates with little pressure drop across the membrane surface, excellent water intrusion pressure rating, and sterilization compatibility methods. Membranes meeting the requirements of high efficiency particulate air (HEPA) and ultra low penetrating air (UPLA) standards for air and gas delivery help move your finished device from design to market.
Selecting Air and Gas Membrane Filtration
It may be surprising, but the removal of particulate matter from gas streams is more easily accomplished than from liquids because of the added mechanisms of interactions between particles and pore surfaces. Particles may be arrested by the sieve retention mechanism when trapped in pores that are smaller than the particles themselves. Gravitation settling and electrical attraction forces are both involved in this adsorptive capture by direct interception.
A second mechanism, adsorptive particle capture, is due to inertial impactions and to diffusional interceptions which are heightened in gas and air streams because of the lower viscosities that pose little impediment to the trajectory of a particle. As a result, large particles entrained in an air stream have a greater inertia and travel in a relative straight-line trajectory even if the air flow itself curves. This is especially noted in particles that have a larger mass since the potential for sticking to the solid filter surface is greater. Particles whose size is from 0.3 to 1 µm typically undergo inertial capture, particularly in high velocity streams. These may come to impinge upon a fiber rather than a pore opening. Once captured, the particles are not released back into the air stream.
On the other hand, there may be particles suspended within a gas stream that are too small in mass to undergo inertial impacts. Nevertheless, they may be small enough to come into contact with a filter surface as a result of diffusional movements. As with inertial impact, the low viscosity of a gas stream allows suspended particles to be trapped by diffusive capture. It is this mechanism of diffusional interception that is most responsible for the high filtration efficiencies of air filters. The rate of particle capture is inversely proportional to the square root of the particle diameter.
Due to the large variety of filtration media that is available, understanding the factors that govern the air and gas filter efficiencies will guide you to the correct selection for your specific use. There are four major material specifications that you need to consider when selecting a material for air and gas filtration. These include water intrusion pressure, differential pressure, air flow rate, and bacterial retention. Of additional concern may be oleophobicity, chemical and temperature resistance, and sterilization methods.
Water Intrusion Pressure
Water intrusion pressure is the procedure that is used to confirm the pore size of a hydrophobic membrane in lieu of the bubble point test that is used with hydrophilic membranes. This test is typically performed by placing water on one side of the membrane filter and gradually increasing the pressure until the water is forced through the filter structure and is seen coming through the opposite side of the membrane. When selecting a hydrophobic membrane for your application, it is recommended that you ensure that the pressure that the membrane will encounter while in use is not higher than the rated intrusion pressure. This will ensure that the membrane acts properly by retaining liquid droplets from the filtrate.
Differential pressure (∆P) is the difference between the pressure in the system before the fluid reaches the filter (upstream pressure) and the system pressure after the fluid flows through the filter (downstream pressure). It is imperative that this aspect be analyzed in the design phase when choosing a filter media to ensure that any flow rate requirements of the finished device are met.
Air Flow Rate
What are the requirements for flow rate that your application demands? Is it a venting application where the movement of air through the membrane will be minimal or does it require large volumes of air to pass in a short amount of time? Air flow rates of hydrophobic membranes are driven by factors such as differential pressure, porosity, pore size, and effective filtration area (EFA), which ultimately should steer the design of a device to maximize the capability of the filter
To measure retention efficiency of hydrophobic membranes, the media is typically exposed to aerosolized particles of a standard size. The Bacterial Filtration Efficiency test method is a standardized test to measure this retention efficiency. By definition, this is the measure of the efficiency of a filter based on an aerosol challenge of 0.3 µm Staphylococcus aureus that penetrate a filter at a predetermined flow. It is usually expressed as a percentage of retention.
One of the most common misconceptions about air filtration is that an efficient, sterilizing-grade, hydrophobic filter must have a pore size of 0.2 µm. This belief is incorrect because due to the nature of gas filtration through microporous membrane media, particles smaller than the pore size, usually on the order of 10-1, are retained in venting applications. Membranes with a pore size as high as 3 µm significantly reduce the risk of contamination for equipment and instruments. When identifying the bacterial retention efficiencies of hydrophobic membranes, it is important to note the efficiency ratings as pore sizes larger than 0.2 µm may provide the required aerosol in air and gas filtration applications.
Thermal stability is the ability of the filter media to maintain integrity and functionality at elevated temperatures. Thermal stability is important when considering filter sterilization, such as autoclaving. Certain filters cannot be autoclaved because of insufficient thermal stability. Keep in mind that there is a relationship between chemical compatibility and thermal stability; many types of filter media may be compatible with a chemical at room temperature, but not at high temperature. Thermal stability can be characterized by determining the maximum operating temperature under specified conditions.
Chemical compatibility is defined as the ability of the media to resist chemicals so that the filter’s function is not adversely affected, and the filter material does not shed particles or fibers, or add extractables. It is important to remember that compatibility is specific for a particular chemical or combination of chemicals, at a particular temperature. To select the proper filter, you must determine its compatibility with the fluid. Temperature, concentration, applied pressure, and length of exposure time affect compatibility. The materials used in the manufacture of filtration products are carefully chosen for their resistance to a wide range of chemical solutions. Still, understanding the compatibility between the fluid to be filtered and the filter elements under actual conditions of use is essential.