Basic Coalescing Filter Design
Aerosol coalescing filters are composed of an array of obstacles designed to maximize the effect of the three coalescing processes. Unlike standard in-line filters, coalescing filters carry air flow from the inside out; contaminants are captured in the filter matrix and collect together into larger and larger droplets through collisions with the glass microfibers. These droplets eventually emerge on the outside of the filter tube where they collect and are drained away by gravity. Modern coalescing filters use a graded porosity filter medium with fine glass fibers in the interior and larger fibers on both the inside and outside surfaces. By varying the fiber size distribution in the filter manufacturing process, filters can be tailored to meet specific application requirements. Typical filter elements have 8 to 10 mm pores on the inner surface, reducing to 0.5 mm pores in the interior of the element, and widening to 40 to 80 mm pores on the outer surface.
This figure illustrates a typical coalescer pore in cross section.
The inner element surface acts as a prefilter to remove large contaminants while the internal pores are small enough to remove sub-micronic aerosols and solids from the air stream. The reduced density of the exterior surface enhances drainage. The larger outer pores of the filter element reduce air turbulence, preventing reentrainment of oil or other contaminants due to excessive turbulence. Coalescing filters are typically longer in shape than standard inline filters. This length helps assure filter diameter and the housing’s inside diameter. The spacing between these two surfaces must be sized so that air velocity is minimized, thus reducing the possibility of oil or water vapor carryover. The larger outside pores also allow the air stream to pass freely, minimizing pressure drop. A drain layer conducts collected contaminants from the outer filter element surface to the sump in the bottom of the filter housing where it can be periodically drained by diverting the air stream flow from passing through the filter wet zone – generally the lower 1/2 to 2 inches of the filter. (Air passing through the wet zone could reentrain liquids, carrying them downstream and defeating the coalescing process.) Also important in the design of coalescing filters is the relationship between the filter element outside diameter and the housing's inside diameter. The spacing between these two surfaces must be sized so that air velocity is minimized, thus reducing the possibility of oil or water aerosol carryover.
Media Types
| Type | Description | Type | Description |
| Flow (C, QU, H) - Inside Out | Flow (G, S, 3PU, AU) - Outside to In | ||
| C | Coalescing element composed of an epoxy saturated, borosilicate glass micro-fiber tube with intimate interlocking contact with rigid seamless retainer. Surrounded by a coarsefiber drain layer, retained by a synthetic fabric safety layer. | G | Particulate removal element constructed of the same fiber matrix as type “C,” but with no rigid retainer or drain layer. |
| . | . | S | Particulate removal element like “G” tube, except silicone saturant replaces epoxy. 3PU Pleated cellulose particulate removal element. Includes molded polyurethane end seals. |
| QU | Coalescing element with the same configuration as “C” tube, but with “3P” type pleated cellulose prefilter built-in. Includes molded poly-urethane end seals. | 3PU | Pleated cellulose particulate removal element. Includes molded polyurethane end seals. |
| H | Coalescing element similar to type “C,” however no rigid retainer is used. Typically for lower pressure or higher temperature applications | AU | Hydrocarbon vapor removal element. Ultrafine grained, highly concentrated, activated carbon sheet media. Includes molded polyurethane end seals. |
Media Specifications
Media Dry |
Media Wet With 10-20 wt. Oil |
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| 2 4 6 8 10 |
99.999% 99.995% 99.97% 98.5% 95% |
.001 .003 .008 .2 .85 |
.01 .01 .01 .5 .7 |
1.5 1.25 1.0 .5 .5 |
4-6 3-4 2-3 1-1.5 .5 |
| 3PU AU |
N/A 99%+ |
N/A N/A |
3.0 N/A |
.25 1 |
N/A N/A |
| (1) Tested per BCAS 860900 at 40 ppm inlet. (2) Add dry + wet for total pressure drop. (3) Oil vapor removal efficiency is given for Au media. |
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Filter Efficiency
Filter efficiency is measured by the percentage of contaminants of a particular micron size that are captured by the filter. Filter efficiency is important because it affects not only contaminant removal performance, but also filter life. (Higher efficiency requires greater contaminant- holding capacity.) Filter efficiency ratings for contaminant removal vary from 90% to more than 99.99%, providing a range of capabilities to fit the needs of a variety of systems. Since more efficient filter media may have shorter service lives, it is sometimes desirable to sacrifice some efficiency in the interest of economy. In applications where high efficiency and extended filter service life are critical, a pre-filter is used to remove large quantities of solid particles before they reach the coalescing filter. This can increase the coalescer’s service life by up to six times. For optimum performance, select a prefilter with a 3 mm absolute rating. The table above shows, by fiber grade, typical contaminant removal efficiency and operating characteristics of various coalescing filters. Efficiency ratings are valid for flows from 20% to 120% of rated flow at 100 psig. At flows below 20%, or in non-continuous flow systems, aerosols do not agglomerate as efficiently into larger droplets, allowing more to pass through the filter uncollected. At flows above 120% of rated flow, air velocity is so high that some contaminants can be reentrained into the air system.