From: Food Quality & Safety magazine, October/November 2009

Processed Air Ensures Food Quality

Purifying compressed air and steam may improve food processing

by Peter Froelich and Jack Mahan

Compressed air and culinary steam are commonly used in the food industry in a variety of applications including controlling devices used in the processing operation (e.g., using compressed air to open/close a pneumatic valve), handling the product and or packaging (e.g., ingredients handling, product transfer, a drying operation), or coming into contact with a surface in direct contact with the product (e.g., clean in place, using culinary steam to sanitize a component of the processing system).

The presence of materials like water (in compressed air), particulate matter, compressor oils, or microbial contamination in the air or steam employed in these operations may lead to significant problems. Typical difficulties include the following:

  • The presence of water in compressed air used to dry a product might damage a product, or the product may not meet quality assurance requirements;
  • The presence of particulate matter in compressed air used for controlling a pneumatic valve could cause the premature failure of the system, leading to the need to stop the production line and lost product and/or production time; and
  • The presence of particulate matter in the direct processing of a foodstuff might lead to an out-of specification product (e.g., taste, color, odor, or other quality assurance product issue may result) or to issues related to operational regulatory requirements.

To avoid any of the above situations, the process engineer must determine the characteristics of the air or steam required for each operation and develop the optimum methodology to obtain the gas in the most economical manner. In the same vein, the maintenance manager, facilities manager, and various engineering personnel must continually ensure that the compressed air and steam used in the facility meet the requirements of the manufacturing processes.

Applications for Clean in Place

Clean-in-place (CIP) procedures are commonly employed to ensure the appropriate level of hygiene in the pipes, vessels, fittings, and related components in food processing systems. These procedures are faster, more reliable, less expensive, and less likely to expose workers to chemical risk than the traditional approach of system disassembly for cleaning. The air and steam filters are integrated into an overall system for the CIP process with appropriate valves, drains, pumps, pressure gauges, pressure relief valves, and associated controls, typically on a CIP skid.

The precise configuration of the system, as well as the steam and air filtration requirements, are, of course, dependent on the nature of the process, the materials that are being processed, and the size of the vessels to be cleaned. Typically, steam flow rates as high as 1,000 lbs/hr at pressures as high as 60 psi are employed. Steam filters that will filter up to 3,000 lbs of steam per hour at pressures as high as 125 psi are available; these filters include traps that dispose of the condensate before it reaches the filter.

Removing Materials from Compressed Air

A number of techniques have been used to remove moisture, particulate matter, and oils from compressed air used in food processing activities. In many facilities, the air is simply compressed and then allowed to expand to remove moisture. While this pressure regulation removes some of the moisture, a considerable amount remains in the air, and this air is not sufficiently dry for most purposes. To remove additional moisture, separators, chemical dryers, refrigerators, filters, and desiccant dryers have been employed to obtain air with the appropriate dew point and moisture content.

Filter systems are employed to remove the deleterious substances from compressed air and steam and are included in accepted practices. For example, accepted practices that describe filtering systems for air under pressure in contact with milk, milk products, and product contact surfaces, as well as culinary steam, have been developed by 3-A Sanitary Standards Inc. in collaboration with the U.S. Public Health Service and the European Hygienic Engineering & Design Group.1-2

The accepted practice for supplying air under pressure for contact with milk, milk products, and product contact surfaces involves the use of a coalescing filter.1 The practice requires that the final filter efficiency, as measured by the dioctyl phthalate fog method, be >99%; when commercially sterile air is required, the final filter efficiency should be >99.999%.

In addition to the accepted practices, a number of other standards have been developed regarding the level of allowable contaminants in compressed air, including DIN ISO 8573-1, which defines various classes for solid matter, water content, and oil content.

Using a Coalescing Filter

A coalescing compressed air filter provides a simple, powerful, and reliable method of removing moisture, particulate matter, and oils from compressed air with a minimum of maintenance. The filter consists of a matrix of borosilicate glass hollow fibers in a fluorocarbon resin binder and is resistant to water, hydrocarbon lubricants, and synthetic lubricants. The filter(s) are housed in an assembly in which the liquids are continuously trapped and are directed to a drain via a passive design so that the filter can continue to remove liquids for an unlimited time without loss of efficiency or flow capacity. The filter system is easy to use on a continuous basis, requires only an annual filter element change, and is extremely reliable because it contains no moving parts. A coalescing air filter is typically employed as part of a compressor installation .

A broad range of coalescing filters is available (Parker Hannifin Corporation; Haverhill, Mass.). The filters are designed to withstand a maximum pressure of 250 psi and cover the flow range from one standard cubic foot per minute (SCFM) to 65,000 SCFM with an initial pressure drop of <2 psi. These filters can be used over a temperature range of -150ºF to 300ºF (-100ºC to 149ºC).

Several grades of coalescing filters are available, beginning with a single stage DX filter, which is able to remove >93% of 0.01 µm particles and droplets and is commonly used for general purpose applications such as purifying plant compressed air. For instrument grade air, two-stage filtration is used (a DX filter is followed by a BX filter, which can remove 99.99 % of the 0.01 µm particles and droplets); a three-stage filter system (a DX filter followed by a BX filter and a CI filter) is normally used to remove trace compressor oil vapor. The filter(s) are self-gasketing, which negates the need for end caps and means element replacement is less expensive. The filter system requires minimal maintenance and is extremely reliable and quiet in operation.

Coalescing filters are often used in conjunction with other devices, like chillers, to reduce the workload of the filter. In many facilities, for example, a DX filter is placed upstream before a refrigerated dryer, desiccant dryer, or membrane dryer to prevent most of the oil and water from entering the dryer.

Benefits of Coalescing Filter

A coalescing compressed air filter purification system provides a number of significant benefits:

  • A coalescing compressed air filter does not require any external power. Many techniques used to dry compressed air require considerable power, increasing the cost of the process. When a chiller or refrigerated dryer is employed, for example, the compressed air is cooled to condense the water vapor and then brought back to the desired temperature. A refrigerated air chiller can be quite large, requires routine maintenance on a periodic basis, and uses valuable floor space. In addition, because a fan is used to cool the refrigerant, refrigerated air chillers may be quite noisy. Refrigerator-based dryers cannot achieve dew points below freezing; the optimum level that can be attained is approximately 36ºF (2ºC). It should be noted that the dew point may be difficult to control without sophisticated automatic control systems.
  • The use of the coalescing compressed air filter is a continuous process and requires a minimum amount of maintenance. Once a coalescing compressed air filter is installed with an automatic trap valve, the operator won’t need to perform any maintenance activities—other than an annual change of the filter element—or constantly monitor the performance. When a desiccant dryer is used, however, timed valves are needed to switch from one desiccant vessel to another, and heat is required to drive the adsorbed water during the recycling process. If heat is used to regenerate the desiccant, a considerable amount of energy is expended.
  • A coalescing compressed air filter system is small and does not require any floor space in the facility. A variety of filter housings are available, and they can be line mounted as part of the overall piping system, saving considerable space in comparison to a refrigerator or chiller.
  • A coalescing compressed air filter eliminates the need for the use of refrigerants. Many refrigerants are environmental hazards and are not compatible with food processing facilities. In addition, the fluid level of the refrigerant must be checked from time to time.
  • A coalescing compressed air filter operates silently. Refrigerated dryers have been used at point-of-use applications; however, their physical size, noise level, and electrical requirements often contribute to equipment operator complaints.

Sterile Air Filters

Sterile air (SA) filters, used for direct contact with food and food surfaces, are designed to remove all viable organisms from the air at 99.9999+% efficiency for 0.01 µm particles. These filters are at least 30 times better than the accepted standard for SA filters. A recent study by D.A. Evans, PhD, at the University of Massachusetts, indicated that a Balston SA filter produced commercially sterile air, and, to the limits of detection, no viable colonies of microorganisms were found.

In this study, a DX filter, a BX filter, and an SA filter were autoclaved, dried, and cooled. Air from the normal compressed air supply was passed through the filters and allowed to impinge on a Gelman membrane filter for 20 minutes. The filters were subjected to recovery methods for fungi (yeast and mold), mesophilic aerobic and anaerobic bacteria, and thermophilic aerobic and anerobic bacteria. In all instances, the filtered air displayed no presence of microbial air contaminants downstream of the grade SA filter. These filters have been accepted by the U.S. Department of Agriculture for use in federally inspected meat and poultry plants. They are easy to use, are readily sterilized, and provide the same ease of operation as the coalescing air filter described above.

Steam Filters

Direct steam injection using culinary steam in food processing is an effective method to warm food and reduces the speed of production because the product absorbs the heat more rapidly. Culinary steam “should be free of entrained contaminants (e.g., excess condensate, rust, pipe scale, and boiler feed water additives) and relatively free of water in liquid form.”2

Accepted practices require that the filters used should be nontoxic and should not release media, toxic volatiles, or other contaminants into the steam. The bonding materials should be nontoxic, non-volatile, and insoluble under the intended conditions of use.2 Accepted practices require the filter to be capable of removing 95% of particles that are 2 µm or larger with an associated condensate trap. Presents an overview of a system for direct steam injection.2

Cahoon Farms (Walcott, N.Y.), a manufacturer of private label applesauce, recently described an example of the use of a coalescing filter for the purification of steam for direct steam injection.3 This filter system provides steam with an efficiency of 98%, removing all particles of rust, pipe scale, and other contaminants.

These filters are in full compliance with the 3A accepted practices (609-04) for producing culinary quality steam. In addition, they meet the regulations for Indirect Food Additives used as Basic Components for Repeated Use Food Contact Surfaces as specified in 21 CFR 177, Current Good Manufacturing Practices 21 CFR Part 110, and have been accepted by the U.S. Department of Agriculture for use in federally inspected meat and poultry plants.

Powerful, Efficient, Effective

The filters meet the requirement of accepted practices and the requirements of regulatory agencies and have been accepted for use in a variety of production processes ranging from milk, meat, and chicken to apple sauce. Use of these filters provides a continuous process and requires a minimum amount of operator interaction. In addition, because external power is not required, operating costs are low.

These specifically designed filters, which consist of a matrix of borosilicate glass microfibers in a fluorocarbon resin binder, provide a powerful, efficient, and effective method of removing impurities from compressed air and steam as well as microorganisms in sterile air.

Froehlich works for Peak Media and Mahan is from Parker Hannifin Corporation, Filtration and Separation Division. For more information, contact Mahan at or (978) 858-0505.


  1. 3-A Sanitary Standards, Inc. 3A 604-05: Accepted practices for supplying air under pressure in contact with milk, milk products, and product contact services. McLean, Va.: 3-A Sanitary Standards, Inc.; 2004.
  2. 3-A Sanitary Standards, Inc. 3A 609-03: Accepted practices for a method of producing culinary steam. McLean, Va.: 3-A Sanitary Standards, Inc.; 2004.
  3. MAP Sterile Air Filters-A, Parker Hannifin Corporation, Filtration and Separation Division, Haverhill, Mass., November 2007.



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