DUST, oil particles, bacteria, hydrocarbons and other pollutants are naturally present in the air we breath. When this air is compressed, the contaminants are concentrated compared with the free air around equipment.
According to CompAir’s air treatment manager for Australasia, Jon Tonkin, this can cause problems for downstream equipment which relies on compressed air.
Tonkin told Manufacturers’ Monthly contaminants such as water and oil in compressed air can block up or erode valves, control devices and other air-operated componentry.
Water is typically removed from compressor systems using refrigeration driers or dessicant driers, Tonkin explained. “You’ve got refrigeration drying whereby you chill the air down to a 3°C [pressure] dew point...or you can do it with a desiccant drier which will take it down to a -70°C [pressure] dew point,” Tonkin said.
Pressure dew point is the temperature at which water vapour in the compressed air will condense to form liquid water. The lower the temperature of the pressure dew point, the drier the air.
Tonkin claims it is difficult to prevent moisture and other contaminants from being taken into the air compressor. “You can stop some of the bigger particles being taken in, which most compressor manufacturers do by putting air intake filters on, but most of them are around the 20μ mark, so downstream you need to take out the smaller particles and the oil and water,” he said.
Champion Compressor spokesman, Dino Alessio claims keeping the air compressor in a cool and well ventilated area can help. “That’s one thing a lot of people overlook,” he told Manufacturers’ Monthly.
“They stick their compressors inside poorly ventilated rooms and it gets very hot in there. Then you’re asking the filtration equipment to do a job it’s not designed to do, and you’re just making it that much harder.
“Get a cool, well ventilated room to start with and then add on your filters and drying to suit your application,” he advised.
Alessio added regular maintenance is also critical to ensuring cost effective supply of quality air.
“For example, a 15kW compressor which was recently checked after 1500 hours of operation revealed a clogged oil filter worth just $50. If left unattended for a supplementary 1500 hours, the clogged filter would have caused an increase in energy needs to compress the oil through the channel.
“Should the oil ultimately be forced to bypass the filter altogether, unfiltered oil would deposit dirt and foreign material directly onto the bearings, coolers and oil separator. The greater restriction in the oil separator could equate to a $100 increase in power costs over the period,” Alessio said.
“This could also lead to the need for an air-end overhaul priced at around $2000 for a compressor of this rating, not to mention the costs associated with downtime,” he added.
Don’t over do it!
Alessio claims matching the level of filtration and drying used in the system to the quality of air required is the most cost effective and energy efficient option.
Tonkin agrees. “If it’s going to be used for instrument air then obviously you’re going to have to remove a little less [contaminant] than if you were going to use it for breathing air,” he said.
“With instrument air you can get away with 0.1 parts per million of oil remaining in the air, but for breathing air you need something like an activated carbon filter in there to remove the odour from the hydrocarbons, and also to drop the remaining oil content to less than 0.003 parts per million.”
Tonkin suggested systems should incorporate 1μ filters for removal of smaller particles, 0.01μ coalescing filters to remove oil, and air driers to remove moisture. He added that catalytic converters were also needed for breathing air to remove carbon monoxide.
“Carbon monoxide can be a problem because it is a poisonous gas...in a concentrated form, it’s going to kill you fairly quickly. You can actually remove it; what they do is use a catalytic converter which converts it to carbon dioxide and you can breath carbon dioxide.”
Tonkin said carbon monoxide was likely to be present in compressed air from petrol driven pumps or compressors, and in highly built-up areas.
Alessio said most contaminants come from the atmosphere and have little to do with the type of compressor used. “Using water injected or dry screw or centrifugal [compressor] isn’t going to get rid of it. You will still always need to have filters and driers,” he said.
Oil injected compressors are often said to reduce air quality, but Alessio claims carry over of oil has less impact on air quality than is commonly believed.
“The oil carry over from an oil injected screw is about one part per million of the actual oil, but in typical air we are breathing in any industrial location around Australia, you’ve got about 10 parts per million...an oil-free screw still has to have all the filters because it’s got 10 parts per million [oil] going in from the atmosphere,” he said.
Alessio claims filters for removing contaminants from the air system are now more reliable and longer lasting than in the past.
Tonkin agrees. “I think where we are at the moment, the only way it’s going to improve to any degree is that you’ll find filter manufacturers and suppliers are going to be providing filtration with less differential pressure, so you’re going to be losing less air...I think the improvements are going to be found in energy efficiency by not losing air rather than by the actual filtration methods,” he said.