Small size particles prompt regular filter replacement

1st December 2017

Owing to typical commercial primary filters in a pneumatic circuit being designed to filter out particles as small as 5 μm from the air, motion and control technologies diversified manufacturer Parker Hannifin says companies need to change or clean dirty elements as a way of optimising air flow efficiency for pneumatic systems.

Parker Hannifin European air preparation product manager Hermann Störk explains that, in the twenty-first century, pneumatics can be found everywhere in manufacturing as one of the most universally adopted utilities. Compressed air is used in everything from cooling to controlling automated machinery on assembly and packaging lines.

Pneumatics can be very efficient but wherever air is compressed, the cost can be disguised as part of the general electricity cost. Owing to this, many manufacturers may be unable to count the exact running cost of their pneumatic technologies.

He says the average cost of compressed air in Europe ranges from 1.5 c/m3 to 2.7 c/m3, depending on various factors such as the local cost of electricity per kilowatt, the output of the compressors used, size of the system and the density of the air being drawn into the compressors.

Reducing energy consumption is a hot topic among original-equipment manufacturers today, with legislative demands and the rising cost of electricity putting pressure on manufacturers to be as energy efficient as possible. Every year, the UK industrial sector, for example, uses over 10 TWh of electricity to compress air, which is the equivalent of over five-million tonnes of carbon dioxide emitted into the atmosphere.

“Since pneumatics makes up a sizable proportion of a plant’s energy consumption, taking the steps to ensure the minimisation of waste and leaks, while also increasing air flow in the system, can only result in a win-win,” Störk states.

He explains that a standard airline filter uses centrifugal action in the head to prevent water and other particles from contaminating the line. If the filter is sized too large – according to the pipe size, as is usually the case – the centrifugal action is not robust enough to remove the water, whereas sizing it too small will result in a noticeable pressure drop.

Finding the exact size is critical for the overall efficiency of the system and avoiding pressure drops. Another factor to consider is to check whether the piping itself is correctly sized.

“We are generally a lot smarter today when it comes to applying the right sized components than we were 40 years ago. In the ‘old days’, the saying was ‘pipe size is right size’ and it’s taken many years to get to the point where engineers realise that large, pipe-sized components are no longer the default choice when designing pneumatic systems for plants,” he explains.

Initially, the particulate filter might be sized for two or three pounds of pressure differential. This provides enough back pressure for the correct amount of air flow in and water to be filtered out. However, Störk points out, when contamination starts to coat not only the outside of the element but also the inner surfaces, part of the orifice is bound to get closed up, effectively dropping the three-pound differential steadily. Finally, it might get to the point where between 5 pounds per square inch (psi) and 10 psi is lost simply because the element is dirty.

A lot of users will try to take the element out and give it a clean to get the system back in service. Störk states that this is not a good idea because a lot of the contaminants are on the inside and simply cannot be reached. Instead of simply replacing the relatively inexpensive part, users are running the system on a clogged filter that costs more to run every month because the compressor is labouring to make up the extra pressure drop.

“One of the main reasons Parker moved from a sintered bronze or fibre paper type of filter to a white sintered plastic element was to give a clear visual indication of, when they should be replaced,” Störk says.

One of the main principles of compressing air is to take the surrounding air and jam it into a small container, but this air also brings with it a multitude of water, dirt and everything else that is in the environment around the compressor. When air is compressed, it is also heated up. This results in the air wanting to hold on to water until it cools again.

As the air cools, water condenses in the system’s pipes. There are many different mechanisms that reduce the amount of water going downstream into the pneumatic system, but one of the main starting points would be a dryer, which will cool the air and allow the water to condense and fall out of the system.

“Putting a dryer into the system won’t eliminate all the water, but will remove the bulk of the water in the system. Particulate filters then come into play to clear out the remainder of the water and dust in the system,” he concludes.