Food processors maintain a social responsibility for upholding the quality of their products and that accountability begins with the selection of compressed air system components. In most cases, end users select compressed air system components by comparing technical data from various air treatment manufactures. In 1991, the International Standards Organization (ISO) established the 8573 compressed air quality standard to facilitate compressed air system component selection, design and measurement.

ISO 8573 is a multi-part standard, with Part 1 classifying contaminant type and assigning air quality levels, and Parts 2 through 9, define testing methods to accurately measure a full range of contaminants within the end user’s facility.

ISO 8573.1 identifies three primary contaminant types as prevalent in a compressed air system. Solid particulates, water and oil (in both aerosol and vapor form) are recognized.  Each is categorized and assigned a quality class ranging from class 0, the most stringent, to Class 9, the most relaxed. The end user-user is responsible for defining the air quality required for their particular application or process.

Air treatment manufacturers present technical data in reference to ISO 8573.1. An easy to understand ISO 8573.1: 2001 table defines the various air quality classes. The standard also determines that air quality shall be designated by the following nomenclature:

Compressed Air Purity Classes A, B, C:
Where:
A= solid particle class designation
B= humidity and liquid water class designation
C= oil class designation

Contaminants originate from three general sources.

1. Contaminants in the surrounding ambient are drawn into the air system through the intake of the air compressor. Ingested contaminants appear in the form of water vapor, hydrocarbon vapors, natural particles and airborne particulates.
2.   As result of the mechanical compression process, additional impurities may be introduced into the air system.  Generated contaminants include compressor lubricant, wear particles and vaporized lubricant.
3. A compressed air system will contain in-built contamination. Piping distribution and air storage tanks, more prevalent in older systems, will have contaminant in the form of rust, pipe scale, mineral deposits and bacteria.

Water vapour enters the system through the intake of the air compressor.  In total volume, condensed water vapor represents the majority of liquid contamination in a compressed air system. On a typical summer day of 80(F (21(C) and 70% relative humidity, approximately 19.5 gallons (73.8 liters) of water enters a 100 scfm (170 nm3/hr) system in a 24 hour period. This moisture will spoil food products, cause pneumatic machinery failure and promote bacterial growth in the compressed air piping. Compressed air systems serving the food processing industry must maintain dry, moisture free conditions mitigating the risk of micro-organism growth.

Since compressed air used in food processing operations may come in direct contact with the food, a compressed air dryer producing a sub-zero pressure dew point is required.  Dew point, specified as temperature, is the point at which the water vapor held in the compressed air is equal to the compressed air’s capacity to hold water vapor. Desiccant dryers- using activated alumina- will adsorb water vapor from the air most effectively, delivering ISO 8573.1 Quality Class 2 (-40( F/-40( C) pressure dew point), ideal for the food processing industry.  At this level of dryness, bacteria will cease to grow.

The most scrutinized and often discussed contaminant classified by ISO 8573.1 is oil. Compressed air “free from oil” is a requirement in a food processing environment.  

End users are given the choice of selecting from several air compressor technologies, some of which require lubrication in the compression chamber for cooling and sealing purposes, and others that operate less lubricant in the compression chamber. The end user determines which compressor design best meets the desired requirements. The purpose of this discussion is not to tip the scale toward either technology, but to address air treatment requirements in food processing applications.

Lubricated compressors are typically less expensive to purchase and have a lower cost of ownership. Dependent on the age of the compressor and preventative maintenance programs performed, a lubricated rotary screw air compressor will introduced 2 to 10 ppm/w of oil into the air system. A well maintained 250 scfm lubricated air compressor, with a conservative 4 parts per million carry-over, will add up to 4.8 gallons (18.2 liters) of oil into the air system over an 8000 hour operation.

Lubricant free compressors generally have a higher initial cost and greater maintenance costs over the life cycle of the equipment.  Lubricant is only required for the bearings and timing gears, which is segregated from the compression chamber. This compressor technology presents no risk of lubricant migrating into the process air.  

Both air compressor technologies are subject to the inherent challenges presented by quality of the intake air. Ingested contamination in the form of water vapor, solid particulate and hydrocarbon vapor must be addressed regardless if the compressor is lubricated or free from lubricant. Depending on the location of the compressor intake, oil vapor levels in industrial areas may contain 20-30 ppm of airborne hydrocarbon aerosols. Hydrocarbon vapors, the primary component of fossil fuel combustion, will condense in a piping system when cooled forming a liquid contaminant.

Because compressed air may come in direct and indirect contact with food processing, an elevated level of filtration is required. A high efficiency coalescing filter capable of removing solids and liquids is recommended. It should be capable of removing solid and liquid aerosols 0.01 micron and larger. The remaining oil content should be 0.007 ppm, or less. An activated carbon filter, installed in series, is also recommended downstream of the coalescing filter. The adsorption filter will remove trace odors and oil vapor to 0.003 parts per million by weight. This filter combination will ensure specified filtration levels achieve ISO 8573.1 Class 1 for oil and vapor removal.

In a general industrial area, there are nearly 4,000,000 airborne particles per cubic foot of air.  When this ambient air is compressed to 100 psig, the concentration of solid contamination will reach significant proportions. Most air compressor intake filters are rated to capture sold particles 4 to 10 microns in size and larger and are rated at 90-95% efficiency. Approximately 80% of airborne particles are 10 micron or less. Spores, pollen and bacteria are less than 2 micron in size. This may seem like a lot of particulate matter, but keep in mind, a solid particle 40 um in size is barely visible to the naked eye. Even a well maintained and routinely changed intake filter will allow solid particles to enter the air system.

Solid particulate must be removed from process air serving the food industry. In pneumatic control circuits, solids particles plug control valve orifices, affect accuracy of gauging and score air cylinders walls, causing leaks. Particles may restrict flow through air jet nozzles used to clean food preparation surfaces or adversely affect the consistency of spray coatings applied on food products.

To achieve the recommended ISO 8573.1 Class 2 classification for solid particulate removal, a 1.0 micron particulate filter is recommended. The particulate filter will also enhance the service life of high performance coalescing filters by minimizing solid loading.