Oil-lubricated compressors are the most commonly used technology for any general air compression application. Only in applications where oil is unacceptable, or the need for clean air is paramount, have oil-free compressors replaced them. In less demanding applications, oil may be present, but filtration is usually considered sufficient.
While filters can reduce the level of oil particles, they do not reduce the risk of oil contamination. Plus, it is difficult know exactly when the filters (which use activated charcoal to capture contaminants) are full and will no longer provide protection. Therefore, in many medical applications especially, oil-free compressors are often used.
Oil-free systems use a range of technologies that bypass the need for oil. While oil-lubricated compressors use oil in the compression chamber to cool and seal the compressed air generated, oil-free technology eliminates this risk of contamination while delivering guaranteed air purity.
There are a range of different oil-free compressor technologies available on the market, all of which ensure there is no possibility of any oil coming into contact with the compressed air.
“The water-injected DH series from CompAir does not use a single drop of oil inside the complete compressor package. Water is used instead of oil to seal, lubricate and cool the compression process. The compressor is powered by an energy-efficient IE3 motor, which drives the compressor element directly so there is no requirement for a gearbox, eliminating the need for associated oil-lubrication systems, unnecessary friction, heat and noise, which would all result in wasted energy,” says Dr David Bruchof, product manager for oil-free industrial compressors at CompAir.
Another oil-free technology is a two-stage dry-running system used in its D Series, in which the compression process is divided into two stages, using an intercooler after the first stage to bring the efficiency of the compressor closer to the thermodynamic optimum of isothermal compression.
“Both compression stages in the D Series (pictured p20, bottom) comprise ‘male’ and ‘female’ rotors, which feature a protective coating,” Bruchof says. “There is no physical contact between either the matched rotor pairs, or the rotor and air-end casing. Everything is operated by a precisely-engineered air gap.”
He adds that the ‘male’ rotors of both the first and second stages are driven by a single main gear at the back of the air-end. Helical timing gears at the front of both sets of rotors ensure perfect synchronisation is kept at all times.
This means no oil is needed to seal the compression process, making it suitable for applications where oil-free compressed air is a critical requirement. Oil never enters the compression chambers, but is used to keep gears and bearings lubricated and cool. Seals between the rotors and bearings prevent air from passing into the oil system, and oil from passing into the compression chambers.
These compressors are used in larger environments, such as pharmaceutical manufacturing sites that produce medicines and cosmetics from the milling and micronisation of fine powders and compound mixtures.
The pharmaceutical industry is strictly regulated. Anyone wishing to manufacture medicines must abide by ‘Good Manufacturing Practice’ (GMP1) guidelines, the European Pharmacopoeia, various guidelines from the FDA (Food and Drug Administration), and recommendations from the ICH (International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use).
In all these applications, the requirement for air that contains zero contaminants – including not just oil, but pollen, dust, hydrocarbons, or heavy metals such as lead, cadmium, or mercury as well.
IN HOSPITALS
Air compressors can be used in many applications in hospitals and other medical care facilities. These include the drying, sterilising and preparation of medical devices, such as endoscopes, providing breathing air in ventilators, shockwave therapy, disinfection applications, operating dental and medical tools.
In addition to compressed air systems, low-pressure blower technology can be used in medical applications such as air beds, chair lifts and respiratory devices, and vacuum systems are employed to draw off liquids that might occur during surgical procedures. All of these require no oil to contaminate air quality for the health and safety of staff and patients.
Avoiding process contamination is a necessity for hospital and medical sites using compressors. Fortunately, there are rigorous standards in place governing compressed air performance and provision in these environments, ensuring processes remain free from contaminants – or they are reduced to acceptable levels – while minimising unscheduled downtime and the associated costs.
These regulations include ISO 8573, a group of international standards stipulating compressed air purity and quality. ISO 8573 consists of nine separate parts, with part one identifying contamination levels that can be tolerated within a single cubic metre of compressed air. Specifically, part one identifies permitted particle count, pressure dewpoint and amount of oil allowed within the sample.
The other eight parts detail testing methods for a range of contaminants, including solid particles, oil vapour, organic solvents, microbiological contaminants, liquid water and gaseous contaminants, alongside humidity levels and aerosol content. They also ensure that air quality test results are comparable within a given tolerance of measurement.
“By design, ISO 8573 is very strict and thorough,” Bruchof comments. “However, many decision-makers are going beyond its standards to guarantee high-quality compressed air, instead adopting the principles of the Hazard Analysis Critical Control Point (HACCP). This ensures facilities are complying with hygiene legislation and either eliminating any potential hazards or reducing them to an acceptable level.”
LOW MAINTENANCE
An additional advantage of oil-free compressors over oil-lubricated ones are that service costs are typically lower for oil-free solutions. Thereby, whole life costs are reduced because businesses can save on the cost of oil replacement. Furthermore, they do not need to purchase equipment that would clean and separate oil from air, such as oil separators, filtration equipment and condensate treatment.
“Another benefit to consider with oil-free technology is that, with many businesses faced with ambitious sustainability goals, deciding on an oil-free solution provides the most eco-conscious choice too, helping to contribute towards a facility’s green credentials,” says Bruchof. “For example, while oil-lubricated compressors mean oil or oil-contaminated condensate will need to be disposed of, impacting the environment, this is not a concern with oil-free technology.”
BOX: OIL-FREE SCREW VACUUM PUMP CASE STUDY
SHJ Hospital Pipelines (SHJ) designs, installs and maintains piped medical gas systems for hospitals which incorporate vacuum technology to help draw off liquids that might occur during surgery in theatres, or from patients staying on wards.
To help deliver the medical gas required, Elmo Rietschle has supplied its S-VSI 301 oil-free, screw vacuum pump to SHJ.
The dry-running screw vacuum technology is said to require less maintenance than alternative oil-lubricated models and, because there is no oil at all within the screw technology’s pumping chamber, the risk of process contamination is reduced to zero.
Kevin Witt, service manager at SHJ, explains: “Most hospitals will fit oil-lubricated centrifugal vane vacuum pumps within these systems. We are one of only a few medical gas companies in the UK that are now using screw vacuum pumps too. This is for a number of reasons, but the key one is the improved energy efficiency that the technology offers. Driven by an inverter, these variable-speed vacuum pumps are in line with the NHS’ commitment to reducing its carbon footprint, offering an efficient solution that consumes less energy.”
Offering suction capacities from 100-600m3/hour, end vacuum of 0.01 millibar and quiet operation, the S-VSI screw vacuum pump is described as offering a lower total cost of ownership throughout its lifetime due to the vacuum pump’s contactless technology. This not only reduces potential breakdowns but minimises ongoing maintenance costs too.
Rocco Fanella, sales manager for vacuum and low-pressure solutions at Elmo Rietschle, says: “We approached SHJ some years ago, replacing a number of competitor rotary vane pumps with screw models, which were better suited to meet the needs of SHJ’s customers. SHJ’s units deliver essential medical gas to hospitals across the country, and we are very happy that our screw vacuum pumps play such an important role in these systems.”
BOX: MAINTENANCE TIPS
According to compressed air sales, service and repair organisation Air Supply UK based in York, oil-free air compressors still require regular maintenance, even if to a lesser degree than oil-injected air compressors. Here are its tips.
1. Drain the air compressor tank. Moisture is extracted from the compressed air and will fill the tank and reduce the amount of air stored. Failing to regularly drain the air receiver will result in the stored water spoiling and contaminating the compressed air. This will eventually cause the compressor to become damaged and stop working.
2. Regularly inspect the power lead. Keeping an eye on the cable to see if it has become damaged due to wear and tear.
3. Check the pressure. When the air compressor is running, check the pressure regularly and make sure it is not too high, as this could damage the compressor over time.
4. Monitor compressor behaviour. Irregular behaviour can indicate a fault. In that case, shut it down, unplug it from the mains and check it over or call a maintenance engineer.
5. Look for leaks. Have a look at the area around your compressor for leakage when it is on or off. If you find a leak, it’s important to call in an engineer or fix the air compressor before you use it again.
6. Inspect the air filter. If the air filter inside is made of paper or felt, it will need replacing once it shows signs of wear and tear. If it’s a foam filter, you should be able to clean it out with soap. Make sure you let the filter dry before returning it. Replace any filter that’s damaged or worn.
7. Examine your air compressor. Regularly inspect elements of your air compressor such as the hoses, valves, and seals for cracks or signs of wear and tear and make sure any fittings are still secure. It’s also important to check the air tank for weak spots or rust. If the tank is damaged, it cannot be fixed and will need to be replaced.
8. Test the safety valve. Regularly check safety valve of your air compressor. Put on safety glasses, plug the unit in, allow it to run until it reaches its shut-off pressure and then pull out the safety valve ring so the pressure is released from the tank. If it doesn’t close automatically, or if the air is not released, it’s time to replace the check valve.
9. Replace the seals. Depending on use, compressor seals will need to be replaced after between 2,000-8,000 hours of operation (but some oil free air compressors can last for longer). Seal replacement can take the air compressor out of action for several hours, so it’s better to replace the seals before the compressor’s performance deteriorates to a point that it can no longer function effectively.