Food Grade Compressed Air

By Mark White, Product Manager, Parker

The introduction of the British Compressed Air Society (BCAS) Food Grade Compressed Air Code of Practice, which is aligned with ISO8573.1 Class 2.2.1, has at last highlighted water as being the major and most problematic contaminant found in compressed air. Water not only leads to corrosion and damage to the compressed air system and finished product, more importantly it provides the ideal environment for the growth of micro-organisms.

How much water is actually in a compressed air system?
If we measured the amount of water in a small compressed air system, the volume is staggering. A small 100 cfm compressor & refrigeration dryer combination, operating for 4000 hours can produce approximately 2600 gallons of liquid condensate per year and with a lubricated compressor, oil would in fact account for less than 0.1 percent of the overall contaminant volume. The volume of condensate increases significantly for larger compressor systems, during longer periods of operation and operation on hot, humid days.

So why is oil seen as the major contaminant
Oil is only perceived to cause the most problems as it is can be seen emanating from open drain points and exhausting valves. In the majority of instances, it is actually oily condensate (oil mixed with water) that is being observed.

How do I achieve the purity levels shown within the code of practice?
To achieve the recommended purity levels stated within the BCAS code of practice you will require the use of several technologies regardless of what type of compressor is installed.

Coalescing filters
Coalescing filters are probably the most important purification equipment in a compressed air system. They are designed to remove aerosols (droplets) of oil & water using mechanical filtration techniques, but also have the additional benefit of removing solid particulate to very low levels (as small as 0.01micron in size). Installed in pairs, most users believe one to be an oil removal filter and the other to be a particulate filter, when in fact, the pair of filters both perform the same function with the first filter, a general purpose filter being used to protect the high efficiency filter from bulk contamination. This dual filter installation ensures a continuous supply of high quality compressed air with low operational costs and minimal maintenance time.

Adsorption (desiccant) dryers
Water vapor is removed from compressed air using a dryer. A dryer’s efficiency is measured as pressure dewpoint. Adsorption or desiccant dryers remove moisture by passing air over a regenerative adsorbent material which strips the moisture from the air. This type of dryer are extremely efficient and typical pressure dewpoints for adsorption dryers are -40°F or -100°F, which means for water vapor to condense into a liquid, the air temperature would have to drop below -40°F or -100°F respectively (the actual air temperature after an adsorption dryer is no the same as it’s dewpoint). Beneficially, a pressure dewpoint of less than -15°F will not only prevent corrosion, it will also inhibit the growth of micro-organisms within the compressed air system.

Refrigeration dryers
Refrigeration dryers work by cooling the air, so are limited to positive pressure dewpoints to prevent freezing of the condensed liquid. Ideal for general purpose applications, they provide pressure dewpoints of +37°F, +45°F or +50°F. Refrigeration dryers are not suitable for installations where piping is installed in ambient temperatures below the dryer dewpoint, like systems with outside piping.

Important note regarding compressed air dryers
As adsorption & refrigeration dryers are designed to remove only water vapor and not water in a liquid form, they require the use of coalescing filters to work efficiently.

Adsorption (activated carbon) filters
Oil vapor is simply oil in a gaseous form and as with water vapor will pass through a coalescing filter just as easily as the compressed air itself. Therefore, oil vapor removal filters must be employed as these provide a large bed of activated carbon adsorbent for the effective removal of oil vapors and provide ultimate protection against oil contamination.

Dust removal filters
Dust removal filters are used for the removal of particulate when no liquid is present. They usually provide identical particulate removal performance to the equivalent coalescing filter and use the same mechanical filtration techniques to provide up to 99.9999 percent particle removal efficiency. For absolute particulate retention (100 percent at a given size), a sieve retention membrane filter must be used.

Microbiological (sterile) filters
Absolute removal of solid particulate and micro-organisms is performed by a sieve retention or membrane filter. They are often referred to as sterile air filters as they also provide sterilized compressed air. Housings are manufactured from stainless steel to allow steam or chemical sterilization of the filter and element. It is important to note that the between the sterile filter and the application must also be cleaned & sterilized on a regular basis.

Are all compressed air filters & dryers the same?
Today, many manufacturers offer products for the filtration and purification of contaminated compressed air, with many being selected based only on their initial purchase cost, with little or no regard for the air quality they provide or the cost of operation throughout their life. Compressed air purification equipment is vital for the removal of system contamination, therefore when purchasing this type of equipment air quality, energy efficiency & lifetime costs must always be considered.

Air quality
This is the reason for installing the purification equipment in the first place. Purification equipment should designed to provide or exceed compressed air quality in accordance with the recommendations shown in ISO8573.1:2001, the latest edition of the international air quality standard. Additionally product performance should be independently verified and backed up by a performance guarantee.

Energy efficiency
Pressure loss is the major contributor to operational costs of filtration products. Filters should be designed to ensure pressure loss and thus energy consumption is kept to an absolute minimum. By considering pressure losses after 12 months of operation and not just at start-up, energy savings in excess of 60 percent are not uncommon between manufacture. Adsorption dryers should be optimized to ensure regeneration costs are minimized and energy management systems are often available to further reduce operational costs during periods where the water vapor entering the dryer is reduced be it due to weather conditions, shift patterns or a variable air demand.

Low lifetime costs
Equipment with a low purchase price may turn out to be a costly investment in the long run. Always consider the initial purchase costs, plus the cost of operating and maintaining the purification equipment. “What is the cost to your business of poor air quality?” should be a factor in the process

Achieving the proper compressed air for food grade service requires several pieces of equipment working together for purification and treatment. By making the appropriate selection based on air quality and ensuring energy consumption is kept to a minimum, meeting the proper ISO8573.1 Class 2.2.1 and BCAS food grade air requirements can reduce the total cost of ownership and improve your bottom line through improved manufacturing efficiencies.