and age of the compressors used. The contamination of the ambient intake air and the humidity, which varies depending on weather conditions, must also be considered, just as the impurities that are introduced by the compression process.
Especially in existing compressed air systems, it is recommended to call in filtration experts to measure the contamination that may be caused by the compressors (Fig 2). The contamination by the lubricating oil alone amount up to 40 mg/m3 depending on the type of oil-lubricated compressors; this may increase to more than 100 mg/m3 in older compressors. Even in oil-free compressors, abrasion, moisture and contamination by water injection and contamination through the lubricants of the gears and bearings can add to this depending on the model.
In older compressed air systems with compressed air pipework that can be several kilometres long and disregarding leaks, can also be susceptible to corrosion, the use of modern energy-efficient compressed air systems frequently falls far short of expectations. The pressure losses through leakages are high and the compressed air pressure in the piping system is contaminated. When the compressed air filters at the points of use are heavily contaminated, the differential pressure rises faster. This problem significantly clarifies the complex interactions of the components in compressed air supply systems.
Experienced engineers know that the condition of a compressed air system can be seen on the filters. They are the best indicators of compressed air quality without considering complex equipment. However, these monitoring functions can only be met by coalescence filters with a differential pressure indicator (Fig 3).

 

The potential savings are at several thousands of Euros
The Fraunhofer Institute for Systems Technology and Innovation Research (ISI) discovered the following in a study in the course of the German Compressed Air Campaign “Druckluft effizient” in 2003: “In the field of compressed air treatment, it is primarily important to achieve the compressed air quality required for the application. It is best for energy and operating costs to meet the requirements of the applications. Exceeding or falling below the air-quality requirements results in increased operating and energy costs. The current potential savings per component part are thereby up to several thousand Euros (Fig 4). Regularly changing the filter elements within the recommended intervals achieve significant savings and thereby minimize the operating costs. „This statement has not lost any of its topicality against the backdrop of rising electricity prices. This means for the practice of compressed air filtration: Compressed air filter elements must be replaced to improve the energy balance:
if they are not validated according to ISO 12500 and an objective performance comparison is impossible or
when the differential pressure of coalescence filters for the separation of aerosols and liquid oil/water contents exceeds the limit or
if they have been used for more than twelve months or 8,000 operating hours.
The specification of value “350 mbar” within the scope of the above-mentioned campaign “Druckluft effizient” is a reference value, and reflects the differential pressure curve of compressed air filters. The differential pressure of a new filter element initially increases very slowly. With increasing operating time, the differential pressure rises more rapidly and then reaches the point at which the filter element replacement is necessary. The recommendations for differential pressure limits vary depending on the filter manufacturer and may well be at 500 mbar.

The Best Filter for each Use
Why is it necessary to monitor each compressed air filter in a compressed air supply system? Contamination of the compressed air can occur at any point of the compressed air purification system. That is why it is so important that the filters have reliable differential pressure gauges. The Economizers, which indicate when the differential pressure reaches the limit value and an element replacement is advisable for reasons of operational safety and energy efficiency, so that the operating costs caused by the filter do not exceed its cost, are easy (Fig 5).

Is the differential pressure alone important?
Although long-term experience shows that an element replacement in the coalescence filters is useful when the differential pressure limit is reached, it is not the sole decision criterion. A sudden steep increase in differential pressure, for example, is a warning sign for increased contamination by particles. Therefore, one cannot solely rely on the differential pressure. Even quality filter elements with their great contamination retention capacity and the high resistance of their filter media against the constant exposure to corrosive substances should be replaced at the latest after 8,000 operating hours or twelve months.
Basically, this replacement interval based on operating hours applies also to activated carbon filters that are used to separate oil vapours and odours. These filters are a standard for operations in sensitive areas such as in the food and pharmaceutical industries and in the generation of process air, in order to achieve a residual oil content of air pressure of 0.01 mg/m3, as referred to in ISO Standard 8573-1 for compressed air quality class 1.
Even the use of particularly high quality activated carbon adsorbents that are used as a bed or as an integrated component of filter media, lose effect, despite their high adsorption capacity, which is not reflected in the differential pressure. The recommendation for the filter element replacement is at a maximum of 2,000 operating hours or three months. Depending on the prevailing oil vapour content and at inlet temperatures above 20 °C, a replacement is recommended at an earlier time.
This is how damages can be prevented
In compliance with the replacement intervals and differential pressure limits, the user can be sure that the productivity and efficiency of his systems, which rely on compressed air, are maintained or increased and the optimum protection of the workers is ensured.
A “worst case scenario” with a production loss or unusable products can result in unpredictable consequences for the company. Good advice and a reliable service, the monitoring of functions in the central control systems and ultimately a clear responsibility for the compressed air supply and the timely replacement of filter elements prevent economic damages.

Filter element replacement made easy
Replacing a filter element is easier today than ever. In a cross reference table (http://b2b.emea.donaldson.com/public/Ultrafilter/prd/xrefifs.htm), the user will find energy-saving filter elements validated under ISO 12500 for most filter housings, the quick availability of which is guaranteed by Donaldson by the “European Distribution Center” (Fig 6). Orders received by 1 pm will usually be shipped the same day. This eliminates the necessity of own warehousing for compressed air users and compressed air resellers. The filter replacement is also offered by a nationwide service network by the Donaldson Service technicians.

Author: Donaldson Filtration Deutschland GmbH in 42781 Haan

Donaldson Filtration (GB) Ltd
Humberstone Lane, Thurmaston,
Leicester, LE4 8HP
Tel: +44 116 269 6161
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Web: www.donaldson.com