Deaerator Vent Rate Calculator
A deaerator has one job that protects everything downstream of it: strip the dissolved oxygen and carbon dioxide out of boiler feedwater before that water can pit tubes and eat pipework. The stripped gases leave through the vent, riding on a small amount of steam, and that vent stream is pure cost once it passes the point the gases needed.
This calculator turns the two numbers a plant actually knows, feedwater flow and the vent percentage it works to, into the vent loss in kilograms per hour, with the daily total worked out alongside it so the figure can go straight into an energy audit or a steam balance.
How it was calculated
A screening and audit calculation. It converts a percentage you choose into a mass flow; it does not set the vent valve. The correct setting comes from the deaerator manufacturer’s instructions and your outlet dissolved oxygen results.
Quick Answer
Key takeaways
- The formula is one multiplication: vent rate = feedwater flow x vent percentage / 100. Everything useful about this number comes from choosing the percentage honestly.
- Check the basis before trusting any quoted vent figure. Percentages of feedwater flow (about 0.1% to 0.5% in common practice) and percentages of steam supplied (5% to 14% in US DOE guidance) describe different fractions of different streams.
- Too little venting risks oxygen carryover and corrosion; too much is a continuous steam bill. The calculator prices the second failure so you can weigh it against the first.
- Judge the setting by dissolved oxygen at the outlet, around 7 ppb or less for a pressurized deaerator, not by the size of the plume alone.
- Quick Answer
- How to use the deaerator vent rate calculator
- The vent rate formula, and what each term really is
- Worked example: 50,000 kg/hr at 0.3%
- Vent loss across flows and settings: reference table
- Percent of what? The basis trap behind most vent rate confusion
- Too much venting versus too little: the trade-off in one view
- What competing calculators miss
- Frequently Asked Questions
- Add This Calculator to Your Website
- References
How to use the deaerator vent rate calculator
Enter the feedwater flow through the deaerator in kilograms per hour, then the vent percentage your plant works to, entered as a plain number: 0.3 for 0.3%. Press Calculate and the vent rate appears in kg/hr to two decimal places, with the arithmetic and the implied daily total underneath it. Run it again at the percentages either side of your setting, 0.1% and 0.5% for a typical unit, and you have the range your vent loss can sit in across the settings operators actually use.
Two input traps are worth naming. First, the flow is feedwater flow, not boiler steam output and not makeup flow alone; on a plant with high condensate return, those numbers differ a lot. Second, the percentage here is always a percentage of feedwater. If a manual quotes you a vent figure, ask "percent of what" before you use it. Once you have a kg/hr figure you trust, the cost of energy calculator can translate the lost steam into the fuel energy behind it.
The vent rate formula, and what each term really is
The formula is a definition dressed as arithmetic: the vent percentage is defined as the share of treated feedwater that leaves as vent steam, so the mass flow is that share of the flow. The engineering content sits in the percentage, not the multiplication. Inside the vessel, steam heats the incoming water to saturation and scrubs the dissolved gases out of it; the US Department of Energy's steam tip sheet on deaerators in industrial steam systems describes how those non-condensable gases, plus a small amount of steam, are then released through the vent. The vent stream has to be large enough to carry the gases away continuously, and every kilogram beyond that is boiler steam bought and discarded.
Heating. Steam raises the incoming feedwater and returned condensate to saturation temperature, where dissolved gas solubility falls toward zero.
Stripping. Spray valves or trays break the water into droplets or thin films so the oxygen and carbon dioxide can escape into the steam space.
Carrying. The freed gases collect in the vent condenser section, where some steam condenses against incoming water and returns its heat.
Venting. The remaining steam and gas leave through the vent orifice or valve. That stream, and only that stream, is what this calculator measures.
Worked example: 50,000 kg/hr at 0.3%
The calculator is pre-filled with this case, and the figures below are exactly what it returns.
| Measure | Result | Working |
|---|---|---|
| Vent rate | 150.00 kg/hr | 50,000 x 0.3 / 100 |
| Vent loss per day | 3,600 kg/day | 150.00 x 24 hours |
| Cost at $10 per 1,000 kg of steam | $36 per day | 3,600 / 1,000 x $10 |
| Cost over a continuous year | About $13,140 | $36 x 365 days |
The cost rows are why this small calculation earns its place in an energy audit. Cutting the setting to 0.1% on this plant saves about $24 a day, but if that cut lets outlet oxygen drift upward, the corrosion it invites will cost far more than that when a tube fails. The number informs the decision; the oxygen measurement makes it.
Vent loss across flows and settings: reference table
Every cell below is the tool's own formula applied to that row and column, so you can read a plant's vent loss straight off, or use the table to check a hand calculation.
| Feedwater flow | At 0.1% | At 0.25% | At 0.5% |
|---|---|---|---|
| 10,000 kg/hr | 10.00 kg/hr | 25.00 kg/hr | 50.00 kg/hr |
| 20,000 kg/hr | 20.00 kg/hr | 50.00 kg/hr | 100.00 kg/hr |
| 50,000 kg/hr | 50.00 kg/hr | 125.00 kg/hr | 250.00 kg/hr |
| 80,000 kg/hr | 80.00 kg/hr | 200.00 kg/hr | 400.00 kg/hr |
| 100,000 kg/hr | 100.00 kg/hr | 250.00 kg/hr | 500.00 kg/hr |
The loss scales exactly with both numbers, so a big plant at a modest percentage vents more than a small plant at a sloppy one. Percentage targets should always be quoted with the flow they were measured at.
Percent of what? The basis trap behind most vent rate confusion
Two reputable-seeming numbers circulate for deaerator venting, and they look contradictory until you notice they divide by different things. Industry practice, repeated in deaerator sizing guidance such as the BFS Industries sizing walk-through, sizes the vent at roughly 0.1% of the deaerator's rated feedwater capacity, with many plants operating somewhere in a 0.1% to 0.5% of feedwater band. The US Department of Energy guidance, meanwhile, says a small fraction of the steam supplied to the deaerator, usually 5% to 14%, must be vented to meet the stripping requirement. Both can be true at once, because steam supplied for heating is itself a modest share of the feedwater stream.
The performance test cuts through the terminology. A pressurized deaerator in good order delivers feedwater at about 7 ppb dissolved oxygen or less, the figure cited in Spirax Sarco's guidance on pressurised deaerators, with chemical scavenger handling the last traces. If outlet oxygen sits at target, the vent is doing its job whatever percentage it works out to; if oxygen creeps up, the vent setting, the spray valves, and the water temperature all go on the suspect list together.
Too much venting versus too little: the trade-off in one view
Vent set too high
- Continuous steam and heat loss, priced by this calculator in kg/hr and dollars per day
- Higher makeup and fuel demand for the same boiler output
- Often a symptom, not a choice: a stuck valve, a worn orifice, or a control fault
- No extra protection to show for it once outlet oxygen is already at target
Vent set too low
- Stripped gases accumulate in the vessel (air blanketing) and can return to the water
- Outlet dissolved oxygen rises above the roughly 7 ppb design outcome
- Oxygen pitting attacks boiler tubes, economizers, and feed lines
- Scavenger chemical consumption climbs to compensate, hiding the fault while it grows
The field check between those two failures is disarmingly simple. Operating guidance collected at CleanBoiler.org's deaerator page calls for a continuous visible plume at the vent during normal operation, commonly specified at least about 18 inches long: present, steady, and unspectacular. No plume means look for under-venting; a plume you can hear across the plant house means look for a valve or orifice problem. Then confirm with oxygen testing, because the plume is an indicator, not a measurement.
What competing calculators miss
The top-ranking deaerator vent tools reviewed for this page mostly calculate a different quantity than their titles promise, and the difference is not academic.
Miss 1: the formula runs backward. The leading rivals define their tool as vented steam divided by feedwater flow, times 100, and so compute a percentage from a vent flow you already know. Their own worked examples then report vent rates of 10% of feedwater, a setting that would pour steam off a real plant at a ruinous rate and that no deaerator is run at. This page computes the direction operators actually need: percentage in, kilograms per hour out.
Miss 2: the basis is never stated. Rival pages quote typical ranges from under 1% to 15% without saying percent of what, mixing feedwater-basis and steam-supply-basis figures into one meaningless band. Here the basis is printed on the input, in the formula, and in a dedicated warning section, because a right number on the wrong basis is a wrong number.
Miss 3: no cost, no consequence. A bare kg/hr figure does not change anyone's setting. This page carries the worked example through to kilograms per day and dollars per year, and sets the loss against the corrosion risk of under-venting, so the reader gets the actual decision, not just the arithmetic.
One honest limit in the other direction: sophisticated rivals model vent demand from inlet oxygen, target oxygen, and steam-to-air ratios. That gas-load method answers a design question this screening tool does not attempt. For an existing plant auditing an existing setting, the percentage-of-flow calculation here is the standard first check; for a new vessel, use the manufacturer's sizing process.
Frequently Asked Questions
It is the mass of steam and non-condensable gas discharged from a deaerator vent per hour, expressed here in kg/hr. This calculator computes it as a chosen percentage of feedwater flow: vent rate = feedwater flow x vent percentage / 100. At 50,000 kg/hr and 0.3%, the vent passes 150.00 kg/hr.
Use the basis your plant actually works to, and check which basis a quoted figure uses before typing it. A long-standing industry rule of thumb sizes the vent at about 0.1% of rated feedwater capacity, and many plants work in a 0.1% to 0.5% of feedwater band. A different figure, 5% to 14% in US Department of Energy guidance, is a share of the steam supplied to the deaerator, not of feedwater, and must never be typed into this tool.
The stripped oxygen and carbon dioxide have nowhere to go. They accumulate in the vessel, a fault operators call air blanketing, and can be carried back into the feedwater, where dissolved oxygen pits boiler tubes and economizers. Pressurized deaerators are built to deliver water at about 7 ppb oxygen or less; under-venting puts that performance at risk even when the vessel is mechanically sound.
Every extra kilogram vented is steam the boiler made and the plant paid for, thrown away with its heat. At 150 kg/hr and a steam value of $10 per 1,000 kg, the vent costs about $36 a day, roughly $13,140 a year at continuous operation. Doubling the percentage on the same plant doubles that bill while removing no extra oxygen worth paying for.
Look at the vent itself: operating guidance for deaerating heaters calls for a continuous visible plume, commonly specified as at least about 18 inches long at the vent outlet during normal operation. A plume that has vanished suggests under-venting; a roaring, oversized plume suggests a valve too far open, a failed orifice, or a control fault. Confirm with dissolved oxygen testing at the deaerator outlet, not by eye alone.
Mostly, but not entirely. The vent passes through a vent condenser on most pressurized units, which recovers part of the steam's heat into incoming water before the gases leave. What exits the stack is still lost mass and lost energy, which is why the rate belongs in an energy audit, but the net loss is somewhat smaller than the gross kg/hr figure suggests.
They control different things at opposite ends of the system. Venting discharges gases and a little steam from the top of the deaerator, before the water reaches the boiler, to protect against corrosion. Blowdown drains boiler water itself to control dissolved solids and sludge. A plant can have a perfect vent rate and still need blowdown, and the two figures are never interchangeable.
No. It converts a feedwater flow and a vent percentage into a vent mass flow, for audits, energy balances, and plausibility checks. Setting an actual vent valve or orifice belongs to the deaerator manufacturer's instructions and your outlet oxygen results, because the right setting depends on vessel design, load pattern, and makeup water quality.
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References
- US Department of Energy, Industrial Technologies Program, Steam Tip Sheet 18, "Deaerators in Industrial Steam Systems" - explains deaerator function, venting of non-condensables, and that a small fraction of supplied steam must be vented for stripping. US DOE: Deaerators in Industrial Steam Systems (PDF). Accessed October 2026.
- US Department of Energy, Office of Industrial Technologies, Steam Tip Fact Sheet 18 (earlier edition) - describes the deaeration section, storage tank, and vent, and how vent rate depends on deaerator type and rated capacity. US DOE: Deaerator fact sheet (PDF). Accessed October 2026.
- Spirax Sarco, "Pressurised Deaerators" - covers venting capacity, oxygen targets below 7 ppb for high-pressure plant, and why pressurized deaeration is preferred at higher pressures. Spirax Sarco: Pressurised Deaerators. Accessed October 2026.
- CleanBoiler.org, "Deaerator" - operating guidance including the continuous vent plume check, the 7 ppb oxygen outcome, and storage and scavenger practice. CleanBoiler.org: Deaerator operation. Accessed October 2026.
- BFS Industries, "How to Size a Deaerator for Your Boiler System" - a manufacturer's sizing walk-through that includes evaluating vent rate and the under-venting versus over-venting trade-off. BFS Industries: How to Size a Deaerator. Accessed October 2026.
- saVRee, "Deaerators Explained" - describes spray and tray operation and states the share of steam passing through a deaerator that is typically discharged at the vent. saVRee: Deaerators Explained. Accessed October 2026.