Compressed air is an utility often used throughout the food industry. As compressed air may not be as clean as it appears, it is essential to be aware of the composition of the compressed air used in the food manufacturing process. This is key to avoiding product contamination.
Untreated compressed air contains many potentially harmful or dangerous contaminants which must be filtered out, or reduced to acceptable levels, in order to protect your product and the consumer.
Along with moisture, oil and particulate matter, inlet air to a compressor generally carries 170 to 1,700 bacteria per m³. Therefore, a 55kW compressor with a capacity of 510 m³/hour takes in 100,000 to 1,000,000 bacteria each hour, depending on the quality of the inlet air. These bacteria are compressed along with the air and begin their journey through the compressed air system. Introducing this type of microbial contamination to food products is very risky and should always be considered as a control point in the HACCP plan.
Untreated compressed air is always saturated with moisture and will always carry particulate matter and oil (maybe in vapour form). At common temperatures of 25 to 40ºC, compressed air therefore offers the ideal conditions for bacteria to grow and multiply.
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But these unwanted contaminants don’t necessarily only come from the atmospheric air, there are actually 4 different potential sources:
1. Atmospheric Air
Compressors draw in atmospheric air which continuously introduces contaminants such as water vapor, atmospheric dirt (particles), oil vapor and micro-organisms (bacteria).
2. The Air Compressor
In addition to the contaminants drawn in from the atmosphere, oil lubricated compressors will introduce additional oil from the compression process into the system. The oil will be in the form of liquid oil, oil aerosols or oil vapor.
3. & 4. Compressed air storage devices & distribution piping
The air receiver (storage device) and the system piping which distributes the compressed air around the facility can store large amounts of contamination. Additionally, they can act as cooling device for the warm compressed air causing water condensation, adding more liquid water into the system and promoting corrosion and microbiological growth. Over time it will introduce rust and pipe scale, especially with the commonly used mild steel material.
In order to operate a safe and cost-effective compressed air system, the contamination must be removed or reduced to acceptable limits. If this isn’t done sufficiently it can result in microbiological contamination, corrosion within storage vessels and the reticulation system, damaged production equipment, blocked or damaged pneumatic cylinders, air motors and tools as well as unplanned desiccant changes on desiccant dryers.
In addition to problems associated with the compressed air system itself, allowing contamination such as water, particulate, oil and microorganisms to exhaust from valves, cylinders, pneumatic equipment, can lead to contaminated food products and an unhealthy working environment.
So to summarise, compressed air contamination can lead to inefficient production processes, spoiled or contaminated food products with a reduced shelf life, reduced production efficiency and increased manufacturing costs.
In order to group the different contaminants according to their treatment solutions we can look at them as follows:
1. Dirt (particles)
Micro-organisms, atmospheric dirt & solid particulate, rust and pipe scale.
2. Water
Water vapor, condensed liquid water and water aerosols.
3. Oil
Oil vapor, liquid oil and oil aerosols.
How are these contaminants removed or reduced in a compressed air system?
Each group of contaminants requires a different approach. Dirt needs to be filtered out or reduced with the help of coalescing filters, water will be removed or reduced with the help of water separators and compressed air dryers and oil will be removed or reduced with the help of adsorption filters.
Dirt removal with coalescing filters
When considering filtration equipment, coalescing filters are vital for the air quality of any compressed air system, regardless of the type of compressor installed. A filtration system will normally consist of two coalescing filters installed in series to remove water aerosols, atmospheric dirt, micro-organisms, rust, pipe scale and even some oil aerosols. The first filter is usually a 1 micron filter and the second stage filter a 0.1micron filter. So basically, the first filter should remove any particles of size 1 micron or bigger, the second filter will reduce that to 0.1micron or bigger. However, this highly depends on the particle load and is usually stated on the specification sheet of the filter. Therefore it is always recommended to measure the particle load after the filtration process.
Water removal with compressed air dryers
Bulk water is usually removed in the aftercooler of the compressor. But the compressed air will always still be saturated with water vapor when it leaves the compressor. Therefore, a compressed air dryer is required in order to dry the air further. The water vapor removal efficiency of a dryer is measured as a pressure dewpoint or ºCtpd. Dewpoint refers to the temperature at which condensation will occur, where pressure dewpoint or ºCtpd refers to the dewpoint of air in a pressurised system. For example, compressed air with a pressure dewpoint of +3°Ctpd would need to be cooled down below +3°C for any water vapor to condense into a liquid.
A pressure dew point of -40°Ctpd is recommended for all food applications where air is in direct contact or indirect contact (non-contact high risk) with production equipment, ingredients, packaging or finished products. This will prevent corrosion and will also inhibit the growth of microorganisms. This can only be achieved with the use of a desiccant dryer (or the not so popular membrane dryers).
Desiccant dryers
Water vapor is removed from compressed air using an adsorption/ desiccant dryer. These dryers remove moisture by passing air over a regenerative desiccant material which is hydroscopic and will draw the moisture from the air. Depending on the type of desiccant material, different grades of dryness can be achieved. The recommended pressure dewpoint for the food industry is -40°Ctpd as it not only prevents corrosion, but it also inhibits the growth of micro-organisms. But for critical applications this can go down to a pressure dewpoint of -100°Ctpd. The performance of the dryer should always be monitored with a pressure dew point sensor after the dryer.
Refrigeration dryers
In non-contact applications during the food manufacturing process it is adequate to make use of a refrigeration type dryer.
Refrigeration dryers work by cooling the air, so are limited to positive pressure dewpoints to prevent freezing of the condensed liquid. Typically used for general purpose applications, they should generate a pressure dewpoint of +3 to +5°Ctpd. Again, the dryers performance should always be monitored with a dew point sensor.
Oil removal with activated carbon (adsorption) cartridge type filters
Oil vapour is oil in a gaseous form and will pass through a coalescing filter just as easily as the compressed air. Therefore, oil vapor removal filters must be employed which provide a large bed of activated carbon adsorbent for the effective removal of oil vapor, providing the ultimate protection against oil contamination. In critical applications this should be monitored with an oil vapor measuring instrument after the filtration process.
Sterile filters
Sterile filters are used in critical applications (at point od use) or when going from mild steel pipe work into stainless steel. Absolute removal of solid particulates and micro-organisms is performed by a sieve retention or membrane filter. Filter housings are manufactured from stainless steel to allow for steam sterilization of both the filter housing and element.
To summarise, it is important to treat the compressed air with the right combination of dryers and filters in order to achieve the required air quality. Furthermore, it is important to monitor all the critical parameters with measuring equipment as dryer and filter performance can fail at any point in time.
About the Author
Patrick Dolz, the Managing Director of CS Instruments
Cobtact him at: https://www.cs-instruments.com/za/
