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A Closer Look
Coalescing Filters...The Real Story
How does a coalescer work?
Pick up any coalescing filter brochure and you will find verbiage extolling the benefits of low water and oil carryover. In many cases, vendors make these
claims by stating residual carryover in parts per million by weight (PPMw). In other cases, vendors make fantastic efficiency claims about their ability
to remove particles of .01 micron in size. The purpose of this paper is to help determine what these ratings mean and whether the information provided is
relevant and to show how a coalescer works.
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Figure 1. Coalescer
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The main function of coalescing filters is to trap and subsequently remove liquid oil and water from a compressed air or gas stream. The secondary purpose
is to remove particulate matter.
Figure 1 shows a typical coalescing filter in operation. When the amount of liquid challenging a filter becomes too great, the liquid tends to burst out
of the filter media instead of dropping quietly out of the bottom of the filter. With this bursting or spraying through the media, some of this liquid
can be re-entrained into the compressed air and carried downstream.
A low release point is critical to the success of any coalescing filter. If oil and water permeate through the media anywhere in the upper 70% of the
cartridge, re-entrainment in the air stream is almost certain to occur.
At Fluid Energy, experience has shown that all coalescing filters can have low release points, provided that the challenge rate is not excessive.
For example, we have tested many coalescing filters that are highly efficient as long as the liquid challenge rate does not exceed 3 PPMw. The
most efficient coalescing filter we have found can handle liquid challenge rates up to 50 PPMw and provide a residual carryover of .0014 PPMw.
For example, consider three different brands of coalescing filters all rated for a capacity of 1000 SCFM at 100 PSIG with challenge rates of 50,
10 and 3 PPMw respectively. To put the term PPMw in perspective, the following chart converts “PPMw” into “ounces/hour” assuming that the
liquid contaminant is a typical mixture of water and compressor lubricant:
Filter Capacity
(100 PSIG &
100şF) SCFM
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Challenge Rate (Ounces/Hour)
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50 PPMw
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10 PPMw
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3 PPMw
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1000
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3.5
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.7
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.2
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In other words, the best 1000 SCFM coalescing filter we have found can remove 3.5 ounces/hour of liquid and provide a residual carryover of .0014 PPMw.
Coalescing filters rated at 3 PPMw can only remove .2 ounces/hour and still remain efficient.
What this means is it does not matter what the residual carryover rating of a coalescing filter is, provided it is low. This information is useless
without knowing the associated challenge rate.
Please note, knowing the efficiency of stopping .01-micron particles is irrelevant information when evaluating coalescing filter performance.
Contact Fluid Energy for further information or a product demonstration.
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