Capture and drainage must work together
Droplets are captured in the fibrous medium by different mechanisms depending on size and flow. The coalesced liquid drains into the collection chamber. If the drain fails or the allowable flow is exceeded, liquid may be carried back into the system. Flow direction and filter orientation depend on the design.
What to assess when selecting a filter
Check residual aerosol content at the stated inlet conditions, pressure, temperature and flow. Bear in mind the difference between dry media and media wetted during operation. Oil vapours pass through differently from droplets, so another treatment stage may be required to meet a total oil limit.
What to check in practice
- Size the filter for the actual flow and pressure, not just the thread size.
- Check liquid drainage and differential pressure.
- Replace the element according to the documentation and operating conditions, rather than waiting for complete blockage.
Example scenario
Two coalescing filters in series may reduce aerosol content, but do not by themselves establish oil vapour removal. A total oil requirement must consider both forms and any liquid oil; the result is verified by measurement.
What to watch out for
- Confusing coalescence with vapour adsorption.
- Operating the filter with a failed condensate drain.
- Treating low pressure drop as conclusive evidence that the element is intact.
Does a coalescing filter remove moisture?
It separates liquid droplets under specified conditions, but does not substantially reduce water vapour as a dryer does. Further cooling can therefore produce new liquid water even downstream of a correctly working filter.