AES provides sterile filters from Zander, Inc. which have been validated by the FDA as efficient in trapping particles to .01 micron @ 100% efficiency. The filters can be used for in-line sterilization where they can be sterilized at various temperatures from 250°F to 284°F. The lower the temperature, the longer the sterilization time and the higher the temperature, the fewer cycles possible. The Zander Process Filter range provides safe use of compressed air, gases and steam for the food and beverage industries, through the removal of particulate and microorganisms.

Coalescing filters, charcoal filters, and prefilters (also made by Zander and sold by AES) are located on the cool raw side of air/gas systems where they will not be subjected to the in-line sterilization produces which may use steam up to 300°F. They remove large particles, moisture, oil, and oil fumes.

Sterilizing-grade gas filters normally use borosilicate-type elements that can pass steam through the element without damage. Borosilicate sheet used in sterile filters is a hygroscopic material, meaning it will not pass water; therefore, the filter must be arranged in the system so that as steam contacts the element it is on the outside of the cartridge rather than on the inside.

If steam contacts the inside of the element, condensate will be formed and it cannot move through the element. Therefore, the cartridge will fill with condensate, be exposed to sterilizing temperatures and pressures, and fail prematurely. If the steam is on the outside of the filter, condensate will form, and can be trapped off without doing damage. The temperature of the condensate must be adequate for sterilization.

The gas can be either on the outside or inside the cartridge as it will pass through the element without damaging it.

The sterilizing-grade element can be formed in either a pleated or round circular version. The pleated version has the advantage in that as the temperature increases during sterilization, the pleats expand, much as the pleats in an accordion. This results in minimal stress. However, if the cartridge is not pleated, and only a round circular element is used, expansion must be accommodated in the element and the metal bands holding the cartridge at the top and bottom.

A design that should be avoided is to pass all of the steam used to sterilize a large piece of equipment, such as an aseptic tank, through the sterile filter. A large tank, in particular during the initial sterilization cycle, will consume huge quantities of steam. Passing all this through a sterile filter can be damaging. A better approach is to use a steam filter (sintered stainless steel) to filter the steam that enters the tank. The sterile filter then should be at the discharge of the tank and filter the spent steam (which will be much less than the steam entering the tank) that is discharged from the tank. After the tank has been brought to, and maintained at, the desired temperature for an adequate period of time, air that enters the tank during cool-down should be filtered with two sterile-grade filters.

Steam filters are normally in stainless steel housings and are made of elements formed from sintered stainless steel or woven stainless steel mesh. When the element becomes clogged, it can be removed, hosed off, or cleaned manually, and placed in the housing. It can be reused often in this manner.

Sterile-grade elements, coalescing elements, and charcoal-impregnated elements cannot be reused. Therefore, they are often used as prefilters for sterile gas systems, as well as steam filters.

Prefilters remove most of the large particles from the gas, so the fine-grade filters do not have as large a job to do. They not only remove particles, but they can also remove a certain amount of moisture, oil, and even odors. Systems may be designed with a steam-grade filter as a prefilter followed by finer-grade filters to remove smaller particles that exist in the gas system. If the gas is air and the air is produced by an oil-type compressor, the air filtration system should include a coalescing filter to remove moisture, a charcoal-type filter or absorber to remove oil and oil odors, and two sterilizing-grade filters capable of removing particles. As indicated, all the filters except the sterilizing-grade filter that contains a borosilicate-type element should be in the low-pressure temperature side of the system.

The size of the particle which will pass through a filter is important. The figure here indicates relative sizes of certain molecules from egg albumin molecules to human red corpuscles. Most bacteria are in the range of 2 to 8 microns (2000 to 8000 millimicrons) and can be effectively removed with a filter. Viruses, which include bacteriophage, are in the size range of .012 to 0.4 millimicrons. By arranging two sterilizing filters in series, which is normal, the majority of viruses can be removed.