Droplet separation velocity is the maximum upward vapor velocity through a vessel’s vapor space at which entrained liquid droplets can still fall back out of the flow instead of being carried out with the vapor. It sets a lower bound on vessel diameter (or an upper bound on allowable vapor flow) for any refrigeration vessel where liquid and vapor separate, including suction accumulators, recirculators, and intercoolers. Exceed the separation velocity and liquid starts carrying over into whatever piping or equipment sits downstream of the vessel.
The Souders-Brown idea
The standard way to estimate separation velocity is the Souders-Brown correlation, which relates allowable vapor velocity to the density difference between the liquid and vapor phases: [ELI-REVIEW: v = K × the square root of ((liquid density − vapor density) ÷ vapor density)]. The K-factor is an empirical constant that depends mainly on vessel orientation (vertical vs. horizontal) and whether a mist eliminator is installed — it is not a universal physical constant, and different sources publish somewhat different K values for refrigeration service. Once an allowable velocity is known, the required vapor-space cross-sectional area follows directly from the vapor volumetric flow rate, which sets the vessel diameter.
| Orientation | No mist eliminator | With mist eliminator |
|---|---|---|
| Vertical | [ELI-REVIEW: lower K, roughly 0.1] | [ELI-REVIEW: higher K, roughly double the no-eliminator value] |
| Horizontal | [ELI-REVIEW: somewhat higher than vertical, no eliminator] | [ELI-REVIEW: higher still, with an eliminator fitted] |
Where the low-SST fallback comes from
At very cold saturated suction temperatures, some refrigeration engineering practice uses empirically fit correlations (curves derived from operating data rather than the Souders-Brown formula directly) because the physical property inputs the Souders-Brown correlation needs become less reliable at extreme conditions. A common approach is to take the more conservative (lower) of several available limits [ELI-REVIEW: above roughly −10°F SST, and fall back to the Souders-Brown result below that threshold]. The exact crossover point and correlation coefficients vary by source and should be confirmed against the design basis in use for a given project.
Run it
ColdCalcs' Vessel Sizer runs the separation-velocity check against your refrigerant, SST, and vessel geometry and reports the safety factor on screen, free.
Open the Vessel Sizer →FAQ
What happens if a vessel is sized above the separation velocity limit?
Liquid droplets that would otherwise settle back into the pool instead get carried out of the vessel with the vapor stream, which can flood downstream piping, foul heat exchangers, or, in a suction accumulator, carry liquid toward the compressor.
Does adding a mist eliminator let you use a smaller vessel?
Yes, that is the main reason to add one: a mist eliminator raises the allowable velocity for a given droplet size, which lets a smaller-diameter vessel meet the same separation duty, at the cost of added pressure drop and periodic maintenance.
Is separation velocity the only check that sizes a vertical vessel?
No. Wall thickness, MAWP, L/D proportion, and liquid holdup/surge capacity are all checked independently, and the vessel has to satisfy every one of them; separation velocity is usually the check that governs vapor-space diameter specifically.
Does refrigerant choice change the separation velocity limit?
Yes. The liquid and vapor densities that go into the Souders-Brown correlation are refrigerant- and temperature-specific, so the same vessel geometry can have a different separation-velocity safety factor on ammonia than on another refrigerant at a different saturation condition.