The saturator engine of a DAF: Henry's-law air solubility, the air-to-solids (A/S) ratio equation, packed vs unpacked saturation efficiency, and why ΔP across the release device sets bubble size.
The saturator is the engine of a DAF. By dissolving air into a side-stream of recycled clarified water at 4–6 bar and then releasing it into the atmospheric flotation cell, it converts pressure energy into a cloud of microbubbles. Two numbers govern everything downstream: how much air the water carries out of the saturator (set by pressure and saturation efficiency), and the pressure differential ΔP across the release device (which, with the orifice geometry, sets the bubble size).
The key design parameter is the air-to-solids (A/S) ratio — mass of released air per mass of solids to be floated, typically 0.005–0.06 kg/kg. For a recycle-pressurised DAF:
A/S = 1.3 · sa · R · (f·P − 1) ⁄ (Sa · Q)
where sa is air solubility (mg/L), R the recycle flow, f the saturator efficiency (0–1), P the absolute saturator pressure (atm), Sa the influent solids concentration and Q the influent flow. The term (f·P − 1) is the heart of it — the fraction of dissolved air actually released when pressure drops to atmospheric. Raising pressure or efficiency, or increasing recycle ratio, all raise the available air.
Below ~3 bar there is too little dissolved air to float typical industrial solids loads at a sensible recycle ratio. Above ~7 bar the gains flatten (Henry’s law is linear, but pumping and saturation energy rise) and bubble coalescence increases. 4–6 bar is the practical optimum for the great majority of industrial duties; the recycle ratio (typically 8–30 % of forward flow) is then trimmed to hit the target A/S.
The release device — needle valve, fixed orifice or proprietary nozzle — drops the full saturator pressure to atmospheric across a single sharp restriction. The larger and sharper that ΔP, the finer the bubbles (see the nucleation barrier ΔG* ∝ 1/ΔP²).
A pressure vessel filled with packing media that maximises gas–liquid contact area. Reaches 90–100 % saturation efficiency in a compact footprint — the modern standard for high-rate DAF.
Air sparged into an empty pressure vessel. Simpler and less prone to fouling, but only 50–80 % efficient, so it needs a larger vessel or higher recycle for the same air.
Needle valves, fixed orifices and engineered nozzles each give a characteristic bubble distribution. Wear, scaling and fouling open the gap and coarsen bubbles — a key maintenance item.
| Saturator pressure (bar g) | Packed (f≈0.95) released air (mg/L) | Unpacked (f≈0.65) released air (mg/L) | Typical recycle ratio for A/S≈0.02 |
|---|---|---|---|
| 3 | ~64 | ~42 | 20–30 % |
| 4 | ~87 | ~58 | 15–22 % |
| 5 | ~109 | ~73 | 10–18 % |
| 6 | ~131 | ~88 | 8–15 % |
Released air ≈ sa(f·P − 1); recycle ratio depends on influent solids and target A/S. Use the DAF sizing calculator for a duty-specific figure.
Reynolds & Bauhm can review your saturator, release and contact-zone design, run the CFD and propose a sized, validated solution. Send us your flow, stream analysis and target effluent.
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