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Heat Transfer

Steam Condenser Sizing

Preliminary steam condenser sizing logic and the difference between sensible cooling and latent heat condensation. Covers duty, condensing temperature, cooling-water flow, LMTD, vacuum/pressure context, and why detailed condenser design requires specialist methods.

TypeEngineering guide — concept explainer

Definition

Steam condenser sizing is the preliminary sizing of a heat exchanger that condenses a saturated or near-saturated steam stream against a cooling-water utility. The dominant duty is latent heat (Q = ṁ_steam × h_fg), not sensible cooling. The condensing-side temperature is set by the operating pressure of the condenser through the saturation curve, which means the hot-side stays at one temperature across most of the exchanger. This makes the LMTD calculation simpler than a liquid–liquid exchanger, but introduces a different set of design issues — non-condensables, vacuum, condensate sub-cooling, and the strong dependence of duty on condensing pressure — that make detailed condenser design a specialist scope.

Why it matters

Steam condensers appear in turbine exhaust service, process condensers on distillation overheads, vent condensers, and reboiler/condenser-driven utility schemes. Treating one like a liquid–liquid sensible exchanger leads to two common errors: under-counting the latent heat (which can be 5–10× larger than the sensible cooling at the same flow) and ignoring the impact of operating pressure on condensing temperature. A preliminary sizing pass that uses the right latent-heat duty, an appropriate condensing temperature for the design pressure, and a sensible LMTD approach gives a defensible starting area for budgeting and plot layout — but the final design always sits with the vendor and the condenser specialist.

Formula

Latent heat duty (pure saturated steam, no sub-cool)
Q = ṁ_steam × h_fg
Total duty with desuperheat and sub-cool
Q = ṁ_steam × (h_steam,in − h_condensate,out)
Cooling-water flow
ṁ_cw = Q / (Cp_cw × ΔT_cw)
LMTD — pure condensing (hot side isothermal)
ΔTₘ = (ΔT₁ − ΔT₂) / ln(ΔT₁ / ΔT₂), with ΔT₁ = T_sat − T_cw,in, ΔT₂ = T_sat − T_cw,out
Heat transfer area
A = Q / (U_dirty × F × ΔTₘ)

Units involved

  • •Q — heat duty in kW, W, or BTU/h
  • •ṁ_steam — steam mass flow rate in kg/s, kg/h, or lb/h
  • •h_fg — latent heat of vaporisation in kJ/kg or BTU/lb (about 2257 kJ/kg at 1 atm, varies with pressure)
  • •T_sat — saturation temperature corresponding to the condenser operating pressure
  • •ṁ_cw — cooling-water mass flow rate in kg/s or lb/h
  • •Cp_cw — cooling-water specific heat ≈ 4.18 kJ/(kg·K)
  • •ΔT_cw — cooling-water temperature rise
  • •U — overall heat transfer coefficient in W/(m²·K) or BTU/(h·ft²·°F)
  • •A — heat transfer area in m² or ft²

Worked example

Preliminary sizing for a pure-steam condenser. Saturated steam at 0.12 bar absolute (≈49.4 °C saturation) at 1.5 kg/s, condensing fully with no sub-cooling. Cooling water enters at 28 °C, leaves at 38 °C. Assume h_fg ≈ 2380 kJ/kg at this pressure, U_dirty = 1800 W/(m²·K) (clean steam condensing on water-cooled tubes), F = 1. Apply 25% design margin to reflect non-condensables and condensing-side uncertainty.

  1. 01Q = 1.5 × 2,380,000 = 3,570,000 W = 3,570 kW
  2. 02ṁ_cw = 3,570,000 / (4180 × 10) = 85.4 kg/s
  3. 03T_sat (condensing) = 49.4 °C (from saturation table at 0.12 bara)
  4. 04ΔT₁ = 49.4 − 28 = 21.4 °C
  5. 05ΔT₂ = 49.4 − 38 = 11.4 °C
  6. 06LMTD = (21.4 − 11.4) / ln(21.4/11.4) = 10 / 0.629 = 15.9 °C
  7. 07A = 3,570,000 / (1800 × 1 × 15.9) = 124.8 m²
  8. 08A_design = 124.8 × 1.25 = 156 m²
Result

Required area ≈ 156 m² with 25% design margin. The condensing-side area, vent system for non-condensables, vacuum equipment design, and condensate hot-well are all specialist scope.

Common mistakes

  • •Treating a steam condenser as a sensible-cooling exchanger — latent heat dominates, and a sensible-only Q underestimates the duty by an order of magnitude.
  • •Using the steam supply temperature instead of the condensing temperature — the hot side stays at T_sat across most of the area; the upstream superheat is a small fraction of the total duty.
  • •Picking T_sat without picking the operating pressure — condensing temperature is fixed by pressure through the saturation curve. A 0.1 bara vacuum condenser sits near 46 °C; a 1 bara condenser sits near 100 °C.
  • •Ignoring non-condensables — even a small fraction of air or inerts dramatically depresses the apparent U-value and changes the temperature profile near the vent. Real condensers always need a vent and a vacuum system.
  • •Assuming the LMTD is high because the steam is hot — the LMTD depends on the difference between condensing temperature and cooling-water temperatures, which can be small in low-pressure condensers.
  • •Forgetting condensate sub-cooling — if the condensate is intentionally sub-cooled, the duty includes both latent heat and sensible sub-cooling, and the LMTD is no longer pure-condenser.
  • •Sizing a turbine exhaust condenser as if it were a process condenser — turbine exhaust design includes air removal, hot-well design, and back-pressure curves, which are entirely specialist scope.
  • •Forgetting that condensing-side U-values are vendor-specific — published U-value ranges are starting points, not final values.

When to use the calculator

Use the Heat Duty Calculator for sensible portions only — for the latent component, calculate Q = ṁ × h_fg outside the calculator and add it to any sensible duty. Use the LMTD Calculator with the condensing temperature on the hot side (both T_h,in and T_h,out set to T_sat for a pure-condensing zone). The Heat Exchanger Area Calculator then handles A = Q / (U_dirty × F × LMTD). Pull U-values from the Typical U-Values Reference (look for steam-condensing on water rows) and a generous fouling resistance from the Fouling Factors Reference.

Heat Exchanger Area Calculator →LMTD Calculator →Heat Duty Calculator →Fouling Factor Selector →

FAQ

Why is steam condenser duty different from a sensible-heat exchanger?
Most of the duty in a condenser is latent heat — the energy released when vapour turns to liquid at constant temperature. Latent heat is typically 5–10 times larger than the sensible cooling of the same mass over a similar temperature range. The condenser duty is Q = ṁ × h_fg for a pure-condensing zone, plus any desuperheat or sub-cool contributions.
What sets the condensing temperature?
The condenser operating pressure. Saturation temperature and pressure are linked through the steam tables. A condenser operating at 0.1 bar absolute condenses around 46 °C; at 1 bar absolute, around 100 °C. The available cooling-water temperature usually drives the choice of condenser pressure, which in turn fixes T_sat.
Why is the LMTD different in a condenser?
On the condensing side the hot stream stays at T_sat across most of the exchanger, so the temperature profile is a vertical line at T_sat with the cooling water warming up underneath it. The LMTD is calculated with the same equation but with both hot-side temperatures equal to T_sat (for a pure-condensing zone with no desuperheat or sub-cool).
Do I need to worry about non-condensables for preliminary sizing?
Yes — at least as an awareness item. Even small fractions of air, CO₂, or other inerts depress the condensing-side film coefficient and create a temperature pinch near the vent. Preliminary sizing uses clean condensing U-values and a larger design margin (often 20–30%) to allow for this. Detailed vent and air-removal system design is specialist scope.
Can this approach size a turbine exhaust condenser?
Only as a very rough first cut. Turbine exhaust condensers (surface condensers) include hot-well design, vacuum equipment, back-pressure curves, expansion joints, and cleanliness factor — all of which sit outside preliminary thermal sizing. Use this only to scope area and cooling-water duty, then hand off to a condenser specialist or vendor.
What U-value should I use for a steam condenser?
For steam condensing on water-cooled tubes, typical fouled U-values are 1000–3500 W/(m²·K) for shell-and-tube and 2000–5000 W/(m²·K) for plate exchangers, depending on water quality and condensate cleanliness. Non-condensables, low-pressure operation, and dirty cooling water push this lower. See the Typical U-Values Reference and confirm with vendor data.

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