If you ask a manufacturer: «What function do air filters perform at your food and beverage production facility?» — in response you will usually hear: «Removing dust and contaminants». Some perceptive observers may also mention the removal of water-and-oil aerosols.
Although these answers are correct, the most dangerous contaminants in air that is in direct or indirect contact with food are microorganisms. These microscopic particles can be either completely harmless or — under certain conditions — put human life at risk. An air compressor creates such conditions: moisture, heat and oil in the air are a favorable environment for the development of many types of microorganisms. Constant monitoring of the microorganism composition in a compressed-air system is not feasible, so it is extremely important to organize production processes in such a way as to protect the producer and consumers.
There are a number of organizations that set standards, make recommendations and define the best ways to reduce the risk of product spoilage due to microbial contamination. Donaldson gives manufacturers the following recommendations on protecting products from spoilage, as well as on how to protect themselves and their consumers: filtration, sterilization, monitoring и maintenance.
1. Filtration
Pre-cleaning
The compressor increases the concentration of particles and bacteria that are naturally present in the ambient air. Filtering high concentrations of such contaminants can be very difficult. In addition, the mechanical action of the compressor introduces additional particles and oil aerosols into the air, which must also be removed. When cooled in the receiver, the air is usually 100 % saturated with water vapor, which leads to condensate formation. The condensate is carried downstream in the form of an aerosol of tiny droplets, which, in the absence of an entrained-liquid separator and a coalescing pre-filter, can damage refrigeration dryers or desiccant dryers. Donaldson offers cyclone separators and coalescing filters for pre-cleaning.
Dehumidification
After passing through the pre-filters, water-and-oil aerosols and solid particles larger than 5 microns must be removed from the air.
After this the air will still be saturated with moisture, and any cooling of it will lead to condensate formation.
If everything is left as it is, over time the condensate will entail significant costs, including due to corrosion and icing. In addition, the mixture of water and oil creates favorable conditions for the growth of bacteria.
It should be noted that stopping bacterial growth is much simpler and safer than retaining high concentrations of bacteria afterward. Most often, air cooling in a refrigeration dryer or vapor adsorption in an adsorption dryer is used to reduce the concentration of oil and water vapors. Donaldson recommends using a heatless regenerable desiccant dryer of the Ultrapac™ Smart series. The hygroscopic adsorbent material of this dryer restores its properties during heatless regeneration. This makes it possible to achieve significant cost savings that are not attainable with refrigeration dryers and heated-regeneration dryers. In addition, the Ultrapac Smart dryer is equipped with a pre-filter to protect the desiccant from contamination, as well as a fine filter that protects the equipment located downstream.
Point-of-use filtration
(Donaldson DF, P-EG and PG-EG series filter housings)
After most of the water-and-oil aerosols and coarse solid particles have been removed from the air, and before contact with the product or with surfaces that come into contact with the product, the air should undergo fine cleaning. Point-of-use filtration is necessary, because filtration at the previous stages is not designed to remove bacteria with an absolute retention rate. In addition, deposits and contaminants may be present in the lines between the pre-filters and the point-of-use filters. Since kinetic energy is proportional to the square of velocity, at a high air-flow velocity even fine particles may have enough energy to damage the sterile-air filter. Donaldson recommends using a coalescing filter and an activated-carbon filter of class M or S. The activated-carbon filter absorbs a number of substances (for example, hydrocarbon vapors, perfumes, citrus essential oils, sulfuric acid, toluene and methanol) that could change the smell or taste of the finished product.
The final cleaning stage is the sterile-air filter. The Donaldson sterile-air filter is designed to remove more than 99.99998 % of contaminants the size of a bacterium.
As a test microorganism, the accepted industry standard ASTM F838 uses Pseudomonas Diminuta (number 19146 in the American Type Culture Collection (ATCC)) with an average diameter of 0.3 microns. When 1.0 x 107 bacteria per cm² of filter surface are supplied, the sterile filter produces a sterile filtrate.
This is an important condition, because some filter manufacturers sometimes label their filters as sterile for particles 0.01 microns in size. It is easier for most filters to retain particles 0.01 microns in size than particles 0.2 microns in size, so Donaldson labels its filters as sterile for particles 0.2 microns in size. The Donaldson P-SRF filter element for sterile air has an efficiency of 99.99998 % or higher when removing particles 0.2 microns and smaller.
2. Sterilization
(Donaldson P-GS and P-GSL N filters for steam)
Two ways of controlling microorganisms were described above. Drying removes the water necessary for life. Filtration retains bacteria and the oil they feed on. If bacteria remain in the filter and only dry air is supplied to the point of use, the bacteria will be in the stage of inactive spores. Nevertheless, under certain conditions the bacteria can switch to the active stage. In that case the bacteria can multiply, pass through the filter material and enter the product or the process downstream.
One way to prevent the risk of bacteria growing and passing through the filter is to sterilize the sterile-air filters with hot steam or using an autoclave. The heat generated in the process causes irreversible intracellular reactions that kill microorganisms. Since steam is widely used to sterilize pipelines and vessels, it is also convenient to use it to sterilize filters and housings. Saturated steam is excellent for sterilizing filters because it gives off heat at a constant temperature. This heat changes the properties of intracellular proteins, killing microorganisms. Steam filtration is no less important. Donaldson offers two different types of filters for culinary steam. These are the P-GS filter made of sintered stainless steel and the pleated P-GSL N filter made of stainless steel.
3. Monitoring
Clogging of a sterile-air filter element does not lead to an increase in differential pressure by which the need for its replacement could be determined. To detect microbial contamination of the filter, it is necessary to periodically take measurements and record their results. One of the monitoring methods is checking the integrity of sterile-air elements.
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Food and beverages
Donaldson offers a wide range of products for a variety of applications, from pre-cleaning to sterile filtration in systems with a wide flow range.
The type of test for checking filter integrity depends on the type of sterile-air filter. The integrity of depth filter elements (for example, P-SRF and P-SRF V) can be checked by aerosolized-oil dispersion. The integrity of membrane filters (for example, PP N) is checked by bubble-point or diffusion-flow methods. These methods for checking point-of-use filters make it possible to ensure safety when air is in direct or indirect contact with food.
4. Maintenance
Over time, filters become clogged with contaminants. The filter material is subjected to significant loads, which can ultimately lead to a loss of filter integrity. The correct choice of the filter installation sequence will significantly reduce the tendency to clog, increase the protection of the sterile filter's integrity and make it possible to save money by reducing maintenance costs.
Donaldson has determined recommended filter replacement intervals that will protect the process when the preventive-maintenance schedule is followed. Not only the safety of the process and consumers, but also the company's reputation may depend on this.
