Power plants chasing single‑digit ppb oxygen lean on deaerators that heat feedwater to near saturation. The showdown between spray‑type and tray‑type designs comes down to load range, materials, and how tightly operators hold pressure and temperature.
In the steam cycle, the deaerator is the quiet enforcer. Heat the feedwater to near its boiling point at the operating pressure, and dissolved oxygen (O₂) and carbon dioxide (CO₂) all but vanish. As one engineering note puts it, “if the water temperature is raised to boiling point at the prevailing pressure… the partial gas pressure is zero,” so virtually all oxygen is expelled (Stork).
In practice, plants spray or cascade feedwater into a steam atmosphere until it is within a few °C of saturation (Veolia). Well‑designed spray/gas‑contact units vent roughly 97–98% of incoming oxygen simply by heating the water almost to saturation (Veolia), driving feedwater O₂ down toward 0.005–0.01 mg/L (5–10 ppb) in high‑pressure service (Asia Pacific Energy Policy; Veolia). Advanced plants often guarantee ≤7 ppb (≈0.005 mg/L) O₂ in feedwater (Veolia; Asia Pacific Energy Policy). By contrast, an atmospheric feedtank at ~85–90 °C holds on the order of 2 mg/L O₂ (Spirax Sarco), far above modern utility targets.
Any residual O₂ is typically removed with chemical scavengers (“trace amounts… may cause corrosion damage”) (Veolia). Plants often finish that last mile with targeted chemicals such as oxygen scavengers when needed.
Deaerator function and oxygen stripping
The deaerator is a pressurized feedwater heater that removes dissolved gases by direct contact with steam. By Henry’s law and Raoult’s law, water at saturation contains essentially no dissolved gas; heating to the boiling point at the prevailing pressure drives the partial gas pressure toward zero (Stork).
In operation, units spray or cascade inflow to maximize surface area and residence time in a steam atmosphere, targeting water temperatures within a few degrees of saturation (Veolia). Well‑tuned systems vent ~97–98% of oxygen at this stage (Veolia), with the final traces policed chemically downstream.
Spray-type versus tray-type designs
Spray‑type deaerators inject feedwater through nozzles or baffles, creating a fine spray in a compact, single‑vessel layout that responds quickly to load changes. A modern spray unit can deliver less than 7 ppb oxygen from 10–110% load (Stork). These systems typically require only carbon steel construction because the steel is exposed to a steam blanket with little O₂ (Stork).
Tray‑type designs cascade water over multiple perforated trays—often 10–24 tiers—so each step renews liquid film area while crossflowing steam strips gases. The multi‑tray arrangement maintains a large transfer surface independent of flow rate and preserves O₂ removal even at low loads; as flow falls, the film on each tray gets thinner, which sustains efficiency (Deaerator.com).
Trade‑offs show up at turndown. Spray units often need a large temperature rise (often ~30 °C) and robust steam flow; their degassing efficiency “seriously decreases” below ~25% load (Deaerator.com). Yet vendors report spray systems that meet <7 ppb across 10–110% load and “~100% turndown” in practice (Stork). Many engineers opt for tray or hybrid (spray‑→tray) designs in large plants with wide turndown, while spray‑only units fit steadier‑duty or space‑limited installations.
Economics tilt the decision. Spray systems typically need no oxygen scavenger to hit the <7 ppb spec—saving chemical costs (Stork)—whereas tray units often still dose scavengers to lock in performance. Trays also tend to require stainless internals (higher capital cost) and more complex supports (Stork).
Operating pressure and temperature setpoints
O₂ solubility collapses as temperature rises: at 20 °C it’s ~9 ppm, at 60 °C ~5 ppm, at 90 °C ~2 ppm, and at 100 °C (saturation) essentially zero (Spirax Sarco; Stork). A feedtank at 85–90 °C sits around 2.0 mg/L O₂ (Spirax Sarco), hence the push to run slightly pressurized.
Because 100 °C at 1 atm risks cavitation, deaerators are commonly held at 0.1–0.3 bar g, bringing saturation to ~100–105 °C (Spirax Sarco). At 0.2 bar g (≈1.2 bar abs, sat ~105 °C), dissolved O₂ drops to ~0.005 mg/L (Asia Pacific Energy Policy). Many large boilers therefore aim at ≈0.2 bar g—Spirax notes a “saturation temperature of 105 °C” with manageable venting (Spirax Sarco).
The payoff is big: holding setpoint pressure/temperature can cut O₂ by a factor of 100—raising feedwater from 85 °C to 105 °C drops O₂ from ~2 mg/L to ~0.005 mg/L. Even a few degrees below design can spike residuals as gases re‑dissolve. Plants use fast‑acting steam control—“A modulating control valve regulates” vessel pressure so temperature “is guaranteed to be fairly constant”—to stabilize operation (Spirax Sarco), which helps “make use of steam…and improve fuel economy” (Spirax Sarco).
Operationally, an atmospheric hotwell at 70 °C instead of 85 °C would see O₂ rise by a multiple, while a well‑controlled deaerator at 105 °C pushes residuals into the ppb range (Asia Pacific Energy Policy). Because even tiny O₂ traces accelerate pitting at high pressure (Veolia), operators invest in monitoring—often with online O₂ analyzers—to verify the <7 ppb target is consistently met.
Sources and reference notes
Figures and guidelines—including O₂ solubility vs. temperature, “7 ppb guarantee,” “30 °C spray requirement,” and 105 °C setpoints—come from steam‑engineering handbooks and manufacturer data: Veolia; Stork; Asia Pacific Energy Policy; Stork (spray vs. tray); Deaerator.com; Spirax Sarco (O₂ vs T); Spirax Sarco (pressurised operation); Spirax Sarco (control).
