Condenser Performance and Cooling-Water Flow in a Steam Power Plant
The condenser is where a steam plant rejects most of its heat, and its pressure sets the low end of the whole thermodynamic cycle. Small changes in condenser performance show up directly in output and heat rate.
What the condenser does
Exhaust steam from the low-pressure turbine condenses on the outside of tubes carrying cooling water. Because steam occupies enormously more volume than liquid water, condensation creates a vacuum, and the pressure in the shell is the saturation pressure at the condensing temperature. Water boils at 41.5 Β°C at 0.08 bar absolute and at 45.8 Β°C at 0.1 bar, so the achievable back-pressure follows the temperature of the available cooling water.
Heat load and cooling-water flow
The heat to be removed is Q = steam flow Γ (exhaust enthalpy β condensate enthalpy). For wet exhaust steam the enthalpy is hf + xΒ·hfg. Cooling-water flow follows from the water-side enthalpy rise: flow = Q Γ· (hout β hin). As an example, 400 t/h of exhaust at 0.08 bar and dryness 0.92 rejects about 245.6 MW; with cooling water warming from 25 to 35 Β°C the required flow is about 21,200 mΒ³/h, or roughly 53 kg of water per kilogram of steam. The condenser cooling-water calculator performs this balance with exact IAPWS-IF97 enthalpies instead of a constant specific heat.
TTD, range and LMTD
Three temperatures describe the heat exchange: the saturation temperature of the steam, the cooling-water inlet, and the cooling-water outlet. The terminal temperature difference (TTD) is the saturation temperature minus the water outlet. The range is the water temperature rise. The log-mean temperature difference (LMTD) = (ITD β TTD) Γ· ln(ITD Γ· TTD), where ITD is the saturation temperature minus the water inlet. For a given duty and area, a lower cooling-water inlet temperature or higher water flow reduces the back-pressure.
What raises condenser pressure
- Warm cooling water in summer or from an undersized cooling tower.
- Tube fouling that cuts heat transfer and raises TTD at the same load.
- Air in-leakage, which blankets the tube surface; the air-removal system must keep non-condensables low.
- Low cooling-water flow from pump or screen problems.
- Tube plugging or leaks reducing surface area.
Because turbine output and heat rate depend directly on back-pressure, a sustained rise of even a few millibar is worth chasing.
Using the numbers for diagnosis
Trend TTD against load and cooling-water inlet temperature. At a given load and water temperature, a rising TTD indicates fouling or air in-leakage; a rising range with constant TTD simply means the load or the water flow changed. The cooling tower side of the system is covered by the Cooling Tower Performance calculator.
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