Condensate recovery
Condensate Recovery & Flash Steam Calculator
Condensate leaving a steam trap is hot, clean, treated water that you have already paid to heat and paid to treat. Most plants send a large part of it to drain. This works out what that is costing, how much of the loss is flash steam rather than water, and what returning it would be worth.
Your condensate
Roughly equal to your steam consumption, less any steam injected directly into product.
The pressure the condensate is at before it discharges.
0 is a vented atmospheric receiver. A pressurised flash vessel makes the flash steam usable but produces less of it.
What you would otherwise be feeding the boiler.
How much of the available condensate actually gets back to the feed tank.
Used only to price the heat saved.
Worth recovering, per year
₹8,41,153
in fuel alone, plus 6,062 m³ of treated water you do not buy
What happens at the trap
- Flashes to steam
- 13.3%
- Stays as water
- 1,735 kg/h
- Flash volume
- 444 m³/h
- Condensate arrives at
- 169.8 °C
265 kg/h
at 100 °C
what the discharge line must pass
7 kg/cm²(g)
Only 13.3% of the mass flashes, but it occupies 444 m³/h — hundreds of times the volume of the water it came from. That is why discharge lines are sized on the flash, and why a plume at the vent is usually flash steam rather than a trap blowing through.
What recovery is worth
- Heat recovered
- 85,028 kcal/h
- Indian steam coal saved
- 25.9 kg/h
- Treated water saved
- 6,062 m³/year
- Feed tank rises to
- 72.5 °C
1,29,408 kg/year
from 30 °C
That feed temperature is the number to carry across to the boiler capacity calculator. Feedwater temperature is the single biggest lever in sizing, so recovery makes the boiler you need smaller as well as cheaper to run.
Blowdown, the saving people forget
Blowdown rate
7.1% → 2.7%
Returned condensate is essentially distilled water, so it dilutes the feedwater to 78.6 ppm and the boiler needs to blow down less to stay within its TDS limit. That is heat, water and treatment chemical saved on top of everything above.
Is your drain line big enough?
Velocity
1.26 m/s
limit 1.5 m/s
At 1.26 m/s this bore is within the 1.5 m/s guideline for a 8 m run.
This is the liquid line up to the trap. Downstream of the trap the flow is two-phase and the flash dominates — a separate and much larger sizing problem.
Sizing the trap itself
Size the trap for 2× the running load — 4,000 kg/h
Drain point on a constant-pressure application. Twice the running load is the usual selection factor.
Want this checked on your actual plant?
Condensate recovery is usually the shortest payback in a steam plant. Our engineers can survey what you have and come back with a scheme and a budgetary offer.
Talk to an engineerFuel figures are indicative. Indicative industry figure — edit for your own fuel and contract price. Not a GyanTosh performance guarantee. The heat and water savings above are physics and do not depend on them; only the rupee figure does.
How the calculation works
Condensate at trap pressure is saturated water — it is as hot as water can be at that pressure. Drop the pressure and it cannot stay liquid, so part of it boils off instantly. That is flash steam, and understanding it is what separates a working condensate system from one that waterlogs.
- 1Flash steam fractionThe sensible heat the water can no longer hold, divided by what it costs to boil a kilogram at the new pressure: (h_f at trap pressure − h_f at receiver pressure) ÷ h_fg at receiver pressure. At 7 kg/cm²(g) discharging to atmosphere, about 13% of the condensate flashes off.
- 2Why a small mass is a huge volumeThat 13% by mass occupies hundreds of times the volume of the water it came from. This is why trap discharge lines are sized on the flash, not on the condensate, and why a vent plume looks far more alarming than it is.
- 3Heat recoveredReturning the remaining hot condensate to the feed tank saves the heat difference between it and cold make-up water. Capturing the flash steam as well saves its latent heat too — but flash used as steam is not water back in your tank until it is condensed somewhere else.
- 4The knock-on savingsReturned condensate is essentially distilled water, so it dilutes the feedwater and cuts the blowdown rate. It also raises feedwater temperature, which is the single biggest lever in boiler sizing — so recovery makes the boiler you need smaller as well as cheaper to run.
flash fraction = (h_f at trap pressure − h_f at receiver pressure) ÷ h_fg at receiver pressure
Flash steam at a glance
How much of your condensate flashes off when it discharges to a vented atmospheric receiver. It depends only on the two pressures, not on how much condensate you have. Computed from IAPWS-IF97.
| Pressure at the trap | Flashes to steam | Stays as water | Per 1,000 kg/h of condensate |
|---|---|---|---|
| 1.0 kg/cm²(g) | 3.8% | 96.2% | 38 kg/h flash, 962 kg/h water |
| 2.0 kg/cm²(g) | 6.3% | 93.7% | 63 kg/h flash, 937 kg/h water |
| 3.0 kg/cm²(g) | 8.2% | 91.8% | 82 kg/h flash, 918 kg/h water |
| 5.0 kg/cm²(g) | 11.0% | 89.0% | 110 kg/h flash, 890 kg/h water |
| 7.0 kg/cm²(g) | 13.3% | 86.7% | 133 kg/h flash, 867 kg/h water |
| 10.0 kg/cm²(g) | 15.9% | 84.1% | 159 kg/h flash, 841 kg/h water |
| 14.0 kg/cm²(g) | 18.7% | 81.3% | 187 kg/h flash, 813 kg/h water |
| 17.5 kg/cm²(g) | 20.7% | 79.3% | 207 kg/h flash, 793 kg/h water |
| 21.0 kg/cm²(g) | 22.5% | 77.5% | 225 kg/h flash, 775 kg/h water |
Flash steam or a failed trap?
This is the most common misdiagnosis in a steam plant, and it costs money in both directions — good traps get replaced, and genuinely failed ones get ignored because “that is just flash”.
- Flash steam
- A lazy, billowing plume that forms slightly away from the discharge and dissipates. It comes in surges as the trap cycles, and it is white because it has already begun condensing in air.
- Live steam blowing through
- A sharp, continuous, high-velocity jet that is nearly invisible at the outlet before it condenses. It does not surge with the trap cycle because the trap is no longer closing.
If the plume is continuous and violent rather than intermittent and lazy, the trap needs attention. If it surges with the trap cycle, it is flash — and the table above tells you roughly how much to expect.
Common questions
What is flash steam, and is it a sign my steam trap is leaking?
Almost always no. Condensate leaving a trap is saturated water at trap pressure; when it discharges into a lower-pressure line it carries more sensible heat than water can hold at that lower pressure, so part of it boils off instantly. That is flash steam, and it is normal and unavoidable. A trap blowing live steam and a trap discharging flash look similar at the vent, which is why traps get replaced unnecessarily. The give-away is that flash forms a lazy plume that dissipates, while live steam blows through sharply and continuously.
How much flash steam will I get?
It depends only on the two pressures, not on how much condensate you have. Discharging to atmosphere, condensate at 3 kg/cm²(g) flashes about 8% by mass, at 7 kg/cm²(g) about 13%, and at 14 kg/cm²(g) about 19%. The lower you can keep the receiver pressure the more of the condensate stays liquid, but the less useful the flash steam is when you do capture it.
Is condensate recovery actually worth doing?
It is usually the shortest payback available in a steam plant, because it saves four things at once: fuel, because the feedwater arrives hot; treated water, because you buy less make-up; water treatment chemicals; and blowdown, because condensate is nearly free of dissolved solids and dilutes the boiler water. A plant returning 70% of its condensate typically lifts feedwater temperature from around 30 °C to 70–80 °C, which is roughly a 10% cut in fuel for the same steam output.
Why does my condensate line hammer or back up?
Most often the discharge line is sized for the condensate but not for the flash steam, which occupies a far greater volume. The flash then chokes the line, back pressure rises, and traps cannot discharge against it. Other common causes are a rising line where the trap cannot lift, a common line shared with traps at very different pressures, or a temperature-controlled application that has stalled — where the control valve has throttled the coil pressure below the back pressure so condensate stops draining at all.
How big should the drain line to a steam trap be?
Size it on velocity. As a working guideline, keep condensate below about 1.5 m/s on runs under 10 metres and below about 1.0 m/s on longer runs, with pressure drop under 200 Pa/m and 100 Pa/m respectively. Note this applies to the liquid line up to the trap — downstream of the trap the flow is two-phase and the flash dominates, which is a separate and much larger sizing problem.
How much bigger should a steam trap be than the running load?
A trap sized on the average running load cannot clear a startup. For drain points on constant-pressure applications, two times the running load is the usual selection factor. For temperature-controlled applications such as storage tank or bulk liquid heating coils, three times is normal, because on a call for heat the coil floods and the trap must clear it. Steam main drainage should be sized on the warm-up condensing rate of a cold main rather than on the steady-state running load.
How do you do a condensate return calculation?
Take the steam flow into the process, subtract what is lost to venting and leaks, and the balance is condensate available to return. This tool works the value side of that: the fuel, treated water and blowdown a given return rate saves, with flash steam kept separate because it saves fuel only.
How to calculate condensate return percentage?
Condensate returned to the feed tank divided by steam generated, over the same period. Measure it at the tank rather than estimating from trap counts — the gap between the two is usually the finding.