Drawing No. EH–TH–007 // Thermal Engineering & HVAC
Reviewed September 2026
Calculate condenser heat rejection, cooling-water requirements, terminal temperature difference and heat-exchanger effectiveness for generic industrial and process condensers.
5000 m³/h of cooling water entering at 20 °C and leaving at 30 °C, condensing steam at 40 °C saturation.
Step 1 — the water-side energy balance. Everything the steam gives up appears as a temperature rise in the cooling water:
Q = ρ · V̇ · cp · (Tout − Tin)
V̇ = 5000 m³/h = 1.389 m³/s
× 998 kg/m³ × 4.186 kJ/kg·K × 10 K
= 58.02 MW
Step 2 — range, approach and effectiveness. Three numbers describe the same result:
Range = Tout − Tin = 30 − 20 = 10 K
Approach = Tsat − Tout = 40 − 30 = 10 K
Effectiveness = range / (Tsat − Tin) = 10/20 = 50%
Effectiveness is the fraction of the available temperature difference the condenser actually uses. At 50% the water leaves exactly halfway between its inlet temperature and the condensing temperature — a middling result. A clean, well-sized condenser on this duty would reach 60–75%.
Note that range and approach are not independent. Fixing the duty and the inlet temperature, more water flow gives a smaller range and therefore a smaller approach — which lowers the condensing temperature. That trade, more pumping power for lower backpressure, is the central optimisation in a condenser system.
A condenser is not just a heat sink. The saturation temperature it achieves sets the turbine exhaust pressure, and that sets how much work the steam cycle can extract.
At 40 °C saturation the condenser sits at about 7.4 kPa absolute — roughly 2.2 inHg. Let the condensing temperature rise to 45 °C and the pressure rises to about 9.6 kPa. Nothing has changed except the heat sink, yet the turbine now expands to a higher backpressure and produces measurably less power for the same steam flow. As a working figure, each 1 kPa of additional backpressure costs on the order of 0.5–1% of gross output on a condensing steam turbine, with the sensitivity strongest at the lowest pressures.
This is why a condenser's approach is scrutinised far more than its duty. The duty is fixed by the cycle; the approach is what the condenser design and its condition actually buy you.
What drives the approach up in service:
Fouling. Biofilm, scale and silt on the waterside raise thermal resistance. Condenser performance is tracked as a cleanliness factor — the ratio of actual to design heat transfer coefficient. A drop from 90% to 75% is a routine finding and shows up directly as a higher approach and lower output.
Air in-leakage. The shell runs far below atmospheric pressure, so any leak admits air. Non-condensable gas blankets the tubes, collapses the local heat transfer coefficient and raises backpressure sharply. This is often the single largest recoverable loss on an older plant, and the air-removal system's capacity is the first thing to check when backpressure drifts up with no change in cooling water.
Tube plugging. Plugged tubes reduce area and redistribute flow; a few percent plugged is tolerable, more begins to bite.
It is a water-side balance, not a heat-transfer design. The calculation tells you the duty that is being transferred and the temperatures involved. It does not size the condenser — that needs surface area, the overall coefficient U, and the LMTD, together with tube material, velocity and the cleanliness factor you are prepared to design to.
Single condensing temperature. A real condenser has a desuperheating zone at the inlet and often some condensate subcooling at the outlet. Both are ignored here. Subcooling is a real thermodynamic penalty: every degree of subcooling is heat that must be added back in the feedwater train.
Constant properties. Density and specific heat are taken as fixed. Over a 10 K rise near ambient that is accurate to a fraction of a percent; over a wide range, or with seawater or a glycol mixture, use the correct properties — seawater cp is about 3.99 kJ/kg·K, some 5% below fresh water, which matters directly in the balance.
No non-condensables, no fouling. The result is a clean, air-free condenser. Real performance is worse, and the gap is precisely what condition monitoring exists to quantify.
Steady state. Start-up, load changes and cooling-water temperature swings through the day are outside the model.
Condenser design and performance: the HEI Standards for Steam Surface Condensers is the reference for surface condenser sizing, cleanliness factors and air-removal capacity. ASME PTC 12.2 covers performance testing of steam surface condensers; PTC 6 covers the steam turbine itself.
Tube-side construction: TEMA standards for shell-and-tube mechanical design; ASME BPVC Section VIII where the shell is a pressure vessel.
Water chemistry and biofouling: the cooling-water treatment regime governs the cleanliness factor you can sustain, and it interacts directly with the cycles of concentration chosen on the cooling-tower side.
Next steps: compare the measured approach against the design approach at the same cooling-water inlet temperature and flow — that difference, not the absolute value, is the performance signal. If the approach has grown, separate the causes before acting: check air in-leakage first, because it is cheap to find and expensive to ignore, then cleanliness. Feed the resulting condensing temperature into a cycle calculation to put a number on the lost output, and into the cooling-tower duty to see whether the tower or the condenser is the binding constraint.
Include the inputs and assumptions used.