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Condensate return and flash steam

See how much condensate flashes, how much is lost today and what raising the return to the boiler is worth: fuel, water, chemical treatment and the payback.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Preliminary estimate
Steam and condensate
Costs

Include supply, softening or RO and effluent disposal in the water cost.

Economic analysis
—Annual savings from more condensate return
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Flash steam fraction

Share of condensate that flashes when dropping from trap pressure to each flash pressure.

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Where the savings come from

Yearly savings by item: fuel, water and chemical treatment.

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Next step

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Project payback

Cash flow of the improvement: investment in year 0 and annual savings escalated at the rate entered, discounted at the MARR.

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Method, assumptions and references

Equations used

Flash fraction: x = (hf(p1) − hf(p2)) ÷ hfg(p2)
Heat in returned condensate: Q = ṁliq × (h(Tret) − h(Tmake-up))
Recovered flash steam (replaces boiler steam): Q = ṁc × x × (hg(p2) − h(Tmake-up))
Fuel saved = ΔQ × hours ÷ η; cost = GJ × price per GJ (LHV)
Water saved = (ṁliq + ṁflash recovered) ÷ ρ × hours

Assumptions and limits

  • Current case: condensate returned with flash vented to atmosphere. Proposal: higher return and, if selected, flash used from the flash vessel.
  • Saturated condensate at upstream trap pressure; IAPWS-IF97 properties.
  • Reduced blowdown from better-quality feedwater is not included: actual savings tend to be larger.
  • Flash steam only has value if there is a low-pressure user (deaerator, tank heating, process air heating).

Standards and technical documents

  • IAPWS R7-97(2012): Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam
  • U.S. DOE, Steam Tip Sheet #8: Return Condensate to the Boiler; Steam Tip Sheet #12: Flash High-Pressure Condensate to Regenerate Low-Pressure Steam
  • Spirax Sarco, The Steam and Condensate Loop (flash steam and condensate recovery)
  • ASME BPVC Section VIII, Division 1: flash vessels as pressure vessels

U.S. regulations

  • EPA 40 CFR 403.5(b)(5): heat discharged to a POTW must not raise the treatment plant influent above 40 °C (104 °F)
  • ASME BPVC Section VIII and state boiler and pressure vessel laws: flash vessels and pressurized receivers
  • EPA Clean Water Act NPDES permits: temperature limits for direct discharges

Frequently asked questions

How much condensate turns to flash steam?

It depends on the pressures. Condensate at 115 psig vented to atmosphere flashes about 14%; into a 7 psig vessel, about 12%. The chart shows other pressures.

Should I return contaminated condensate?

Only after treatment or with monitoring. Condensate with oil or product can damage the boiler. In that case recover only the heat in an exchanger and discharge the water within limits.

Why is hot condensate to drain a compliance issue?

EPA pretreatment rules (40 CFR 403.5) bar heat that raises POTW influent above 40 °C (104 °F), and local sewer ordinances often set limits at the discharge point.