Deepak Vetal, Product Marketing Manager, U.S. Oil Free Air Div. Atlas Copco Compressors
Compressed air is a safe, reliable and essential power source for countless manufacturers. However, compressed air quality—gaged by the quantity of particulates, water and oil the air contains—can vary widely, depending on the application. In packaging, food and beverage, and pharmaceutical manufacturing processes, air cleanliness requirements are almost always more stringent than in other industrial settings
For instance, high-quality compressed air is essential in food production to prevent product contamination and ensure consumer safety, as well as avoid costly headaches like product recalls, litigation and damage to brand reputation.
Surprisingly, there are two schools of thought when it comes to high-quality compressed air, particularly in regard to oil content. Here’s how manufacturers weigh risk versus cost when specifying compressed air systems
Oil versus oil-free
Food and packaging plants rely on compressed air for wide-ranging tasks, including powering actuators, cleaning equipment, pressurizing vessels, transporting and sorting material and blow-molding containers.
Such operations are often classified as contact and noncontact. Contact applications involve air that directly contacts food or in processes where compressed air is used in food production, processing, packaging and transportation. In contact operations, oil content in compressed air should be less than 0.01 mg/m³.
Noncontact compressed air is the air exhausted into immediate surroundings during food preparation, processing, packaging and storage. Oil content for noncontact operations should be less than or equal to 0.01 mg/m³.
Due to the wide range of applications, at least incidental contact of compressed air with products or equipment is often unavoidable. Thus, manufacturers that select compressed-air systems must carefully consider safety on one hand, and productivity and cost on the other.
Unfortunately, FDA regulations only provide broad recommendations when it comes to compressed air quality. That is, guidelines only say that compressed air introduced into food, or used to clean food-contact surfaces or equipment, must not contaminate food with unhygienic or harmful substances. The FDA does not stipulate specific compressor types for the food and packaging industries, so manufacturers can use both oil-free and oil-injected compressors
Probably the main reason manufacturers consider oil-lubricated compressors is that they cost less than
their oil-free counterparts. In the process of compressing air, they introduce oil into the air, which is subsequently removed downstream. Separator elements extract the majority of the oil from the compressed air; coalescing and carbon filters remove most of the rest. The tradeoff is high maintenance costs and constant system performance monitoring. Also, oil-removal filters and separators cause pressure drops, and that increases energy consumption.
Oil-injected compressors with oil-removal filters deliver what is often termed “technically oil-free” air. However, they also risk contamination. One aspect influencing the efficiency and purity of system air is temperature. When using oil-injected compressors with oil-removal filters, oil carryover through filter media increases exponentially according to the temperature at the filtration interface.
Filter performance is often specified at 20° C. If ambient temperature rises to 30° C (not unusual in compressor rooms) compressor outlet temperature could rise to 40° C, causing oil carryover 20x the specified value.
High temperatures can also increase vapor content of the air and shorten the life of activated carbon filters. An increase from 20 to 40° C can cut filter life from 1,000 to about 50 hours. Even worse, there is no warning when an activated carbon filter is saturated—it simply passes oil downstream to the end product.
Failure of the air-oil separator is another risk associated with lubricated compressors. If the separator fails, oil passes into the piping network and leads to contamination. While preventive maintenance can help reduce risk, there is always the potential of oil carryover.
To lessen the risk, compressors lubricated with food-grade oils are often used in food-processing applications. But they’re no panacea, either. Granted, food-grade oil that bypasses the filters tends to pose less risk to the end product. The downside is that such oil typically has a relatively short life and needs to be changed more often, raising costs.
Lubricated air compressors also introduce oil as aerosol droplets, vapor and liquid into the compressed air stream. Material safety data sheets (MSDS) for food-grade oil note that inhalation of aerosols may damage the health of nearby individuals. There is some evidence that such aerosols can cause respiratory irritation. Finally, companies do not always list additives in food-grade oil due to confidentially reasons.
Oil-free screw designs
For these reasons, more and more food, beverage and packaging companies now view oil-free compressed air systems as the best option, as they eliminate virtually all risk with regard to product contamination.
Oil-free compressors certified to ISO 8573-1:2010 Class Zero have a higher initial cost, but have lower maintenance costs because they eliminate the need for oil and filter replacements and reduce energy consumption due to pressure drops in the filters. Oil-free compressors come in several versions, including piston, diaphragm, toothed and scroll designs. Oil-free screw compressors are often specified in food and packaging operations.
Twin-screw compressors are positive-displacement rotary devices originally developed in the 1930s to address the need for rotating compressors that generate high flow. They are constant flow, variable-pressure devices. At a given speed, they always supply the same amount of air, but can do so at different pressures. Because compression takes place in a continuous process—as opposed to a cyclic process like in reciprocating piston compressors—they supply steady, nonpulsating flow with little or no vibration.
Helical-screw shaped rotors are the heart of these compressors. Male rotors have lobes and female rotors have flutes, and they turn in opposite directions. During each revolution, air is trapped in pockets between the two rotors and gradually decreases in volume, increasing pressure and temperature. Cold, low-pressure air enters on one side, is trapped between the rotors, and hot and high-pressure air exits on the other side.
Timing gears synchronize the position of the counter-rotating rotors, and an electric motor typically powers the gears and rotors. Pressure-ratio capacity (the maximum outlet pressure divided by the inlet pressure) depends on the length, screw pitch and geometry of rotors, as well as other factors like the housing design and ports. Bearings on the rotor shafts handle axial and radial forces during operations.
The rotors in oil-free screw compressors do not contact each other or the housing, so no lubrication is required inside the compression chamber. And seals between the gears and rotors prevent oil in the gearbox from contaminating the compressed air. Consequently, the resulting compressed air is completely oil free.
Rotors are made of carbon steel and coated and baked with Teflon. The coating helps protect rotors from corrosion. In the Atlas Copco design, the top layer of coating is Teflon graphite, which allows run-in and provides minimal clearance between the rotors.
Nonetheless, there are trade-offs. The high-precision rotors and housing must be meticulously manufactured to exacting dimensions and tolerances and assembled with minimal clearances. Lower clearances mean low volumetric losses and better efficiency. However, oil-free compressors typically run at higher speeds and are normally two-stage units, due to higher temperatures in each stage.
Also, many oil-free compressors have water-cooling channels built into the housing to help limit thermal expansion. In contrast, oil-type rotary screw compressors inject cold oil into the compressor chamber to cool and lubricate the rotors and air and seal gaps to reduce leakage past the rotors.
While single-stage, oil-injected screw compressors can have pressure ratios around 13, for oil-free types that ratio is around 3.5 for each stage. Two-stage designs also need an intercooler. Air exits the first stage at about 140 to 180° C, and travels to the intercooler where temperature drops to about 25 to 30° C. A moisture trap after the intercooler removes water from the air. Cool air then travels to the second stage for
final compression, and on to an aftercooler and additional moisture trap. Having two compression stages and an intercooler is another reason oil-free screw compressors are more expensive to purchase compared to oil-injected versions.
Tangible benefits of Class 0 air
Nonetheless, for product safety, as well as reasons like energy efficiency, minimal maintenance and lower total cost of ownership, an increasing number of food and packaging manufacturers around the world are turning to oil-free compressors for their compressed air needs.
Take the case of dairy company DMK, based in Bad Bibra, Germany. It processes one million liters of milk into cheese and whey concentrate every day. Atlas Copco’s oil-free, water-cooled Z Series compressors supply the air for this process, while 75% of the energy used to generate the air is captured and reused by a unique heat recovery system.
The company processes 6.6 billion kg (14.5 billion lb) of milk annually, making it the largest milk processing company in the German market. Quality and safety are paramount in milk processing, according to company officials. To ensure high standards, the dairy in Bad Bibra uses Class Zero oil-free compressed air in the cheese production lines. Because compressed air comes into direct contact with the cheese, it is absolutely critical that any oil is completely eliminated during processing.
“In the past, we used oil-injected compressors,” explains Johannes Bechtle, technical manager at DMK. “We filtered oil from the compressed air using activated carbon and other filters.” Ultimately, this method was not reliable enough for the dairy facility, and it was also costly because the filters needed to be changed frequently.
Instead of the laborious process of filtering oil out of compressed air, oil-free air is now produced from the outset. As an added safety measure, coalescing filters installed downstream of the compressors remove any other potential pollutants.
One ZR 55 compressor with variable-speed drive (VSD) and two ZR 45 fixed-speed compressors with load/unload regulators supply air to its production lines. The ZR 55 compressor’s VSD regulates its speed to meet peaks in demand, while the fixed-speed units are alternated at regular intervals.
This configuration has resulted in significant energy savings for DMK. Since the installation of the Atlas Copco machines two years ago, energy consumption has dropped from 12,000 to 10,000 kWh per week, and improved regulation also has cut no-load hours by 1,000 kWh per week. The network’s central controller monitors demand and runs compressors to maintain a stable and narrow compressed air-pressure band between 7.5 and 8 bar.
Cooling water leaves the compressor at nearly 80° C; two heat exchangers preheat water for a 90° C wash system. Of the 10,000 kWh per week required for air compression, 7,500 kWh (75% of the energy used) are reclaimed through the heat recovery system and used for cheese reproduction.
Similarly, as the largest regional beverage company in China, Hangzhou Wahaha Group provides more than 150 varieties of food and beverage products, such as drinks, canned foods, health products and milk powder. Currently, Atlas Copco provides more than 90% of Wahaha Group’s 600 compressors.
“Food safety is the biggest concern for food and beverage manufacturers,” explains Qiu Yijun, Wahaha’s engineering manager. The company has relied on Atlas Copco Class Zero oil-free compressors for more than a decade, because they virtually eliminate compressed-air related food risks during production. For example, compressed air is required to make PET bottles. Any traces of oil in the compressed air can enter the bottles during the PET blowing process and contaminate the end product. This would have a negative impact on the taste and odor of the end products and could even affect consumers’ health.
As an added advantage, compressors equipped with energy recovery systems provide significant savings. The heat recovered from the compression process is used to preheat water for cleaning and sterilizing, greatly reducing steam demand. Savings due to heat recovery is estimated at about 140,000 per year.
Sidebar: ISO 22000 and compressed air
ISO 22000:2005 is an international standard that specifies requirements for a food safety management system based on interactive communication, system management and Hazard Analysis and Critical Control Point (HACCP) principles. It lets companies and organizations all along the food chain demonstrate their ability to control food safety hazards. The goal is to ensure that food is safe for human consumption. Compressed air related to food safety is an important consideration regarding ISO 22000 certification.
The objective of ISO 22000 is to ensure food safety by adequate control over equipment and processes throughout the supply chain. It’s an auditing and certifying quality management system, although it does not define air quality.
Food safety hazards can surface at any point during production. Getting food to a table involves parties ranging from feed producers to food manufacturers, transport and storage operators, and retail and food service outlets. Related organizations include equipment builders and service providers, as well as manufacturers of packaging materials, cleaning agents and food additives.
How does it apply to compressors and other air-quality equipment? When used as intended, oil-free compressors will not generate biological, chemical or physical hazards. For example, during manufacturing of compressors, components (such as piping) exposed to chemical substances must be thoroughly cleaned during assembly and final testing. Likewise, when used as intended, oil-free compressors and related equipment might possibly release some particulates from the interior. In most cases, they are inert and, unless otherwise specified, require no special attention. Filtration to safeguard the user, adhering to ISO 8573, is advised.
ISO certification should be a key consideration for compressor manufacturers because compressed air plays an important role in the food manufacturing process. Working with certified suppliers assures customers that key pieces of equipment are made in a clean, safe environment. Being ISO certified means that a company has a food safety management system in place and has the ability to control food safety hazards.
In May of this year, Atlas Copco was the first compressor manufacturer certified to ISO 22000 for its Z series of oil-free compressors.
Atlas Copco Compressors