Why compromise air quality for the cost of a filter element?

Richard Moore, at Parker Hannifin's domnick hunter Industrial division, looks at the importance of selecting compressed air filtration for both air quality and lifetime operating cost, rather than just initial purchase cost

Compressed air is widely used throughout industry, so much so that it is often regarded as the fourth utility. However, unlike electricity, gas and water, it is unique in that it is generated onsite rather than being delivered by an external service provider. Ultimately, this creates a situation where the quality of compressed air and the costs associated with its generation and subsequent use are solely in the hands of the user, including the level of contamination removal.

With companies across industry coming under increasing pressure to minimise costs, a reduction in both energy consumption, through lower compressed air usage, and effective, planned maintenance can play a vital role in protecting operating margins.

Effective filtration is key to the efficient operation of all compressed air systems. To be truly cost effective, however, filtration must be proactively planned and managed as part of an ongoing maintenance strategy, and be properly costed; this means looking beyond the initial purchase price of each element to the total lifetime operating cost.

To realise the full capacity of a modern industrial plant, compressed air quality must be kept consistent using correctly specified, installed and maintained filtration technologies to remove oil and water, airborne particulate, rust and pipescale, micro organisms and oil vapour.

Filter elements form the heart of every compressed air system. However, by their very nature, the filter element and housing restrict the flow of compressed air, thus reducing the available system pressure downstream and affecting the energy consumption of the filter. This system pressure reduction, often known as pressure loss, is typically measured between the inlet and outlet of the purification equipment, and recorded as a pressure drop or differential pressure.

As compressors generally require large amounts of electrical energy to produce a volume of compressed air at a given pressure, these pressure losses will be converted directly into energy losses and consequently to operational costs for the user.

Furthermore, production machinery or processes typically require a fixed volume of compressed air, delivered within a predetermined pressure range to function correctly. Therefore, to overcome filter and other pressure losses in the system, compressors are often operated at greater pressures than would normally be required by the application, increasing compressor operating costs still further.

These pressure losses across a filter fall into one of two categories - fixed or incremental pressure losses. Fixed pressure losses are a direct result of the filter housing design and the element fit or interface, with a poorly designed filter housing and element interface creating a greater resistance to airflow and producing a higher differential pressure, which translates to increased operational costs.

Incremental pressure losses stem from the filter element itself and increase as the element collects more contaminants, however, incremental losses are often variable, depending on the filtration media used, the method of construction and its ability to remove and retain contamination.

Total pressure losses are a combination of fixed and incremental losses; so if fixed losses are high as a result of poor design, then the incremental losses will add to the already elevated operational costs of the filter. Accordingly, in a bid to put a cap on operational costs, companies generally base the decision to replace elements on the pressure drop measured across each filter.

However, what often is forgotten is that the fundamental function of the filter is to improve air quality by removing contamination. In practice, the ability of an element to remove contamination may well have fallen to a critical level long before the point at which the pressure drop indicates an element change may be required.

Perhaps most importantly, it is vital that highly specialised filtration materials are used in order to achieve the stringent air quality levels required both by modern industry and ISO 8573-1:2010, the international standard for compressed air quality.

Yet, regardless of which materials are employed, the filter media in coalescing and dust removal filters is constantly bombarded by high velocity dirt particles, while also having to withstand extremes of pressure, temperature and a variable air demand throughout their operating life. In addition, coalescing filters also have to contend with being submersed in acidic condensate, including oil and chemical additives from compressor lubricants, as well as hot water.

As a result, all filtration media has both a finite life and capacity to retain contamination and once spent the required air quality can no longer be maintained. Therefore, considering that filters are installed to provide contaminant removal to a specific air quality requirement, the sole reason to replace filter elements should always be to maintain air quality and not simply because of a pressure drop across the filter.

However, pressure losses in some systems can still cause significant problems, particularly in heavily contaminated installations where excessive pressure losses may result in the shut down of production processes. In these instances, filter elements should be changed according to the manufacturer's recommendations to maintain air quality unless unacceptable pressure losses result.

Many modern filter housings are fitted with differential pressure gauges, which measure the difference in pressure between the air entering and exiting the filter. It is important to remember, however, that these are not service indicators, do not provide a measurement of air quality, are generally not precise and offer no level of calibration. Often, these gauges have areas of green and red, which misleadingly can indicate that the element does not require replacement if the needle is in the green.

For example, as the media in the filter element degrades, even a tiny hole the size of a pin point can result in the filter media rupturing, allowing all contamination to be carried past the filter and into the compressed air system. Should this occur, the gauge needle would remain in the green area and the filter element would not be serviced until the user eventually detected contamination downstream. Even when the filter element is replaced after such an incident, contamination would still be present downstream for some time.

Considering the importance of system performance and reliability, it is important to analyse the cost of filter element replacement against the cost of not replacing them, taking into account the risk of damaged equipment, increased downtime, emergency repair costs and contaminated or faulty product batches.What sometimes seems like a cost saving in the short term can turn out to be a costly mistake. Ultimately then, the sole reason to change filter elements must always be to maintain air quality and therefore it is always wise to follow the manufacturer's recommendations if this is to be achieved.Why compromise air quality for the cost of a filter element?

Parker Hannifin Ltd, Industrial Division
Dukesway, Team Valley Trading Estate,
Gateshead, Tyne & Wear,
NE11 0PZ, England.

Phone +44 (0)191 4029000, Fax +44 (0) 191 482 6296
Email: [email protected]