The boiler eats what the feed tank serves. Serve it cold, gassy water and it responds with fuel bills and pinhole leaks; serve it hot, de-gassed water and half the chemistry problems of lesson B7 never happen. Feedwater is where the condensate system, the water treatment and the fuel account all meet — one tank, three budgets.

The economics: every 6 °C is a percent

Heat fed to the boiler as hot water is heat the fuel need not supply. The practice plant's arithmetic from lesson B3: raising steam takes hg − hfeed, so every kilocalorie already in the feedwater comes off the fuel bill directly. The working rule: every ~6 °C of feedwater preheat saves about 1% of fuel. The practice plant feeds at 90 °C instead of 30 °C — sixty degrees, roughly ten percent of the fuel account, worth about ₹17–18 lakh a year at its ₹600/t, 6 TPH duty. Where does that 90 °C come from? Free heat, twice over: returned condensate at ~90 °C (lesson D6) and flash steam sparged into the tank (lesson A8). The feed tank is not storage; it is the plant's cheapest heat exchanger.

The chemistry: gases that eat boilers

Raw water arrives with dissolved oxygen and carbon dioxide on board. In a hot, pressed boiler, oxygen turns electrochemical: it attacks steel at points, drilling neat hemispherical pits under blisters of rust — a tube can pinhole in a season while losing almost no average thickness. CO₂ rides out with the steam, dissolves in condensate, and returns as carbonic acid that grooves the bottom of condensate lines (lesson D6 meets the result). Cold water holds these gases tightly. Hot water cannot: solubility of both gases falls towards zero as water approaches its boiling point. That physics is the whole trick of deaeration.

trays / sprays — water rains down make-up + condensate in vent — gases out (a wisp, always) steam in — scrubs upward, holds ~105 °C at ~0.2 kg/cm²g to boiler feed pump — hot, de-gassed storage — minutes of reserve (P&ID-grade drawing SCH-10 to follow)
The pressurised deaerator: rain the water through rising steam, keep it at a gentle boil, and let the freed gases leave through the vent. Physics does the treatment.

Three grades of deaeration

The hot feed tank — condensate return plus a flash-steam sparge holding 85–95 °C, atmospheric vent. Removes most of the gas load and most of the cost; this is the practice plant's arrangement, trimmed with a chemical scavenger. Right up to mid-size shell boilers with good condensate return. The pressurised deaerator — the figure above: spray/tray dome over a storage vessel at ~0.2 kg/cm²g, water held at saturation (~105 °C), oxygen down to a few parts per billion. The standard for water-tube plant, high make-up, or anywhere scavenger chemical bills grow embarrassing. Chemical scavenging — an oxygen-absorbing dose (classically sulphite for process plant) that mops the residual after thermal deaeration. It is the trim, never the main act: scavenging cold, gassy water by chemicals alone is pouring money into a physics problem.

Feed tank craft

A good feed tank returns condensate high and lets it fall (releasing its flash inside, not into the room), sparges flash steam low through a distributor, keeps a loose lid — insulated but vented, never sealed — and holds enough for 20–40 minutes of boiler demand. Watch its thermometer like a gauge: a falling feed temperature means lost condensate return or a failed sparge, and it will surface in the fuel account within the week (lesson B4's 574-kcal arithmetic moves immediately).

At site
  • Read the feed tank temperature daily. Every 6 °C lost ≈ 1% fuel. A ₹300 thermometer guards a lakh-rupee line item.
  • Pitting found in tubes or the shell waterline = oxygen at work. Fix deaeration first, dosing second.
  • Deaerator vent: a steady wisp of vapour is the gases leaving — it must be there. No plume = not venting; a roaring plume = wasting steam. Valve it to a whisper.
  • Feed pump suction from a hot tank needs static head or the pump cavitates — if the pump rattles on hot days, the tank is telling you it sits too low.
Pin this
  • Feedwater heat is fuel: ~6 °C of preheat = ~1% off the fuel bill.
  • Hot water cannot hold gas — deaeration is temperature, applied deliberately.
  • Oxygen pits; CO₂ acidifies condensate. Both are cheaper to remove than to repair.
  • Thermal deaeration does the work; chemical scavenging only trims the residue.
  • The feed tank is the meeting point of condensate, flash and make-up — run it hot, vented and watched.
Steam stories

Marine engineers learnt deaeration before anyone: a ship's condenser-fed boiler recycles the same water for months, and early steamships that topped up with aerated water ate their boilers on long runs. The deaerating feed heater became standard naval practice by the 1920s — and the specification "oxygen: seven parts per billion" that modern power stations chase descends directly from what kept wartime boiler rooms afloat. Process industry inherited the vessel, the trays and the number.