We are called to investigate coalescer underperformance perhaps twenty times a year. In roughly seven cases out of ten, the element is in perfect condition, the differential pressure is low, and liquid is still reaching the compressor.
The cause is almost always the same, and it is not a filtration problem. It is a fluid mechanics problem that was decided when the vessel diameter was chosen.
How coalescence actually works
A coalescer does not strain aerosols out of a gas stream. Sub-micron droplets are far too small for that; they follow the gas around any obstacle. What the media does is capture droplets by interception and diffusion, then merge them into progressively larger droplets that migrate to the downstream face of the element.
Those enlarged droplets — now perhaps 500 µm to 2 mm — drain down the outside of the element under gravity into the sump. That drainage is the second half of the process, and it is entirely dependent on gas velocity.
The Souders-Brown limit
The maximum gas velocity at which a droplet of a given size can settle against upward drag is given by the Souders-Brown expression, where velocity scales with the square root of the density difference between liquid and gas divided by gas density.
The practical consequence: gas density matters enormously. A vessel sized correctly at 20 bar becomes badly undersized at 60 bar, because higher gas density reduces the settling velocity while the volumetric flow through the same cross-section stays similar.
Above the critical velocity, coalesced droplets are stripped off the element face and carried downstream. The element is working perfectly. The vessel is not letting it finish the job.
Sizing that survives contact with the plant
Size on actual operating density, not standard conditions. Then check three cases, not one: design flow at design pressure, maximum flow at minimum pressure, and the start-up condition where pressure is low and flow is erratic.
- Target a design velocity of 0.6 to 0.75 times the Souders-Brown critical value, leaving margin for process upsets
- Verify the sump has sufficient residence time and level-control response for the worst-case liquid slug, not the steady-state rate
- Confirm drainage geometry is unobstructed at the element base — a poorly placed support ring will hold a liquid film and re-entrain it
- Re-check the sizing whenever operating pressure changes, because the same vessel behaves differently at a different density
None of this is exotic. It is standard separation engineering that gets skipped when a coalescer is purchased as a component rather than designed as a vessel.