Why Are Coalescing Filters Installed in Pairs?

Why Are Coalescing Filters Installed in Pairs?_Compressed Air Lines_Parker GSFEIn an industrial manufacturing plant, coalescing filters are probably the most important piece of purification equipment found in a compressed air system. They treat six of the ten main contaminants found in compressed air (atmospheric particulate, rust, pipe scale, micro-organisms and aerosols of oil and water). But more importantly, they are also used to protect refrigeration and adsorption (desiccant) air dryers from contamination.

This blog compares the benefits of installing a pair of coalescing filters in series versus a 2 in 1 filter in terms of differential pressure, dirt holding capacity, and total cost of ownership.

Coalescing filters are usually installed in pairs but why is this so? 

Typically, coalescing compressed air filters are installed close to where the compressor is located (either in the compressor room on larger installation or on the compressor itself for smaller fixed or portable compressors). 

In order to effectively reduce the aerosols of oil and water, micro-organisms and particles to a level that will protect the compressed air dryer requires the use of a fine filter (treating contaminants down to 0.01 micron).  

Why Are Coalescing Filters Installed in Pairs? Clogged Filters_Parker GSFE DivisionThe particulate found in a compressed air system is of varying sizes and as this filter is very fine, it will block rapidly with large particles (especially rust and pipe scale).

As the filter blocks, the differential pressure across the filter increases. This not only reduces the available pressure downstream, it also requires the compressor to generate the compressed air at a higher pressure, resulting in higher operating costs.

Why Are Coalescing Filters Installed in Pairs?_Pressure-Drop-Graph_Parker GSFE DivisionOn average, it is found that for every 1 bar additional generation pressure there is a loss of 7% in specific energy, therefore keeping pressure losses low helps reduce operating costs.

Running a compressed air filter with high differential pressure is therefore very costly and keeping pressure losses as low as possible is imperative. One way to keep the pressure losses low is to change the filter element on a frequent basis (every 3-6 months). Another way is to oversize the filter; however, making a filter too large has its own issues in terms of filtration performance, purchase cost and installation. Neither way is a cost-effective compressed air treatment solution. 

Why Are Coalescing Filters Installed in Pairs? domnick hunter Coalescing Filters_Parker_GSFE DivisionIf the fine filter could be protected, the pressure losses could be reduced, therefore the most cost-effective solution is to install a pair of coalescing filters in series. 

Each filter will reduce the same 6 contaminants but to differing levels of purity. The first filter, a general purpose filter protects the second, a high-efficiency filter from bulk contamination. This not only improves filtration performance but more importantly, reduces pressure losses and operational costs. Additionally, it also extends the service life of the element from 3-6 months to 12 months.


Are there alternatives to the 2 filter installation?

Yes, there are single filter alternatives, but care has to be taken with this type of filter as they do not always provide the perceived benefits. 

2 in 1 filters

In an attempt to reduce the pressure losses associated with compressed air filters, a number of manufacturers are now offering 2 in 1 filters. These are claimed to reduce the pressure losses associated with having two filter housings (and therefore energy consumption) whilst providing the same level of purification (i.e. particulate retention & oil carryover down to 0.01 micron / 0.01 mg/m3 or lower). In theory, the thought process is a sound one, however, these types of filter do not always deliver in practice.

Understanding differential pressure (dP) 

In a compressed air filter, pressure losses are a combination of fixed pressure loss and incremental pressure loss. Fixed pressure losses are designed into the filter from the beginning and come from the filter housing and element endcap designs whereas incremental pressure losses come from the filter element as it starts operating. Pressure losses for compressed air purification equipment are stated as dP or differential Pressure.

Literature dP is often used to select one filter brand over another, however many users are unaware that this data is only indicative of a filter in a clean, “as new” condition and does not indicate how a filter blocks as it operates.

When selecting a filter, its blockage characteristics must also be considered as this is an indication as to the filters dirt holding capacity (and true operational cost). 

Therefore, to show the real performance of the 2 in 1 type of filter and the true benefits of their new OIL-X filter range, a comparative test between a pair of Parker domnick hunter OIL-X coalescing filters (Grades AO + AA) and a single 2 in 1 filter was undertaken.

As the filters on test are coalescing filters, they were wetted out with oil aerosol (in accordance with ISO12500-1, the international standard for coalescing filter testing) to give an initial saturated dP representative of a new filter as it enters the first days of service. Oil carryover performance was also recorded.  

Results of the initial ISO12500-1 testing showed that whilst the OIL-X AO + AA combination achieved the claimed literature performance for oil carryover and initial wet dP, the 2 in 1 filter oil carryover performance was 87% higher than literature claims and initial saturated dP 5% higher. 

The second test determines the dirt loading characteristics of the two offerings by injecting and diffusing equal amounts of test particulate into the air stream and measuring the dP (this is done 12 times to simulate monthly particulate loading).

So whilst the initial performance of the two filters may look similar in literature, actual dirt load testing indicates otherwise as can be seen in the graph. Testing confirms that a pair of Parker domnick hunter OIL-X filters have a much higher dirt holding capacity than the 2 in 1 filter and will, therefore, have significantly lower operational costs. 

Why Are Coalescing Filters Installed in Pairs?_Dirt Holding Capacity Chart_Parker GSFE Division

Financial implications 

From the test data, true operational costs can now be calculated and the table below shows the financial savings available by installing a pair of Parker OIL-X filters over a 2 in 1 filter.

Why Are Coalescing Filters Installed in Pairs? Energy and Operational Costs OilX Filters_Parker GFSE Division

Based upon 37kW compressor / Cost of electricity £0.10. Parker OIL-X savings will be greater with larger filter/compressor combinations.

Total cost of ownership

The table highlights operational costs, however, when selecting compressed air purification equipment, the total cost of ownership (TCO) should always be considered (purchase price / operational costs/maintenance costs). The initial purchase price for the two Parker OIL-X filters is only 26% higher than the 2 in 1 filter whilst a pair of Parker OIL-X filter elements is 42% lower than a single element for the 2 in 1. As the 2 in 1 filter has a lower operating lifetime than OIL-X, it may require 2 element changes per year in which case the pair of OIL-X elements is 183% lower cost than a pair of 2 in 1 elements. What seems like a low-cost alternative may turn out to be a costly investment. 

Why Are Coalescing Filters Installed in Pairs? OILX Filters_ Parker GSFE DivisionParker domnick hunter OIL-X filter range

The new OIL-X filter range is the latest addition to Parker's comprehensive line of compressed air and gas treatment product solutions. The new OIL-X technology has been designed to carefully balance the need for precise compressed air quality with the need for low dP, low energy consumption and low lifetime cost.

Parker domnick hunter OIL-X filters incorporate unique flow management devices to significantly reduce the pressure losses associated with poor housing designs whilst their filter elements use airflow management technology, specially selected filtration media, energy efficient coatings and unique deep pleated element construction. This not only ensures air quality, it also provides a high dirt holding capacity, culminating in a filter element dP that starts low and remains low for the 12-month lifetime of the filter element.

All Parker domnick hunter OIL-X filtration grades have performance 3rd party validated by Lloyds register in accordance with international standards and are backed up by an air quality guarantee.


Why Are Coalescing Filters Installed in Pairs?_Focused On Compressed Air Treatment Brochure_Parker GSFE DivisionFor more information on Parker's compressed air treatment solutions, download the brochure.







Why Are Coalescing Filters Installed in Pairs? Mark White, compressed air treatment applications manager_Parker GFSE DivisionThis blog was contributed by Mark White, compressed air treatment applications manager, Parker Gas Separation and Filtration Division, EMEA.





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Comments for Why Are Coalescing Filters Installed in Pairs?

Ed Bickel
Well written and very well presented.
Ed Bickel,
Pneumatic Division Product Manager (retired)

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