Power Plant
Feedwater Heater Performance (TTD & DCA)
Terminal Temperature Difference and Drain Cooler Approach are the two numbers a feedwater heater's performance is actually monitored against — a rising TTD points to tube-side fouling or a flooded heater; a rising DCA points to a degraded drain-cooling zone.
Verified against published standard
Enter extraction pressure and the three temperatures to calculate.
About This Calculation
A feedwater heater is designed with a small, deliberate temperature gap between the extraction steam's saturation temperature and the feedwater it heats (TTD), and another small gap between the drained condensate and the incoming feedwater (DCA) — both gaps exist by design, not by accident, and both are genuinely diagnostic when they change. A stable TTD/DCA at a given load is normal; a RISING TTD or DCA at unchanged load and extraction pressure is what actually signals degrading heater performance.
Frequently Asked Questions
Why does TTD normally have a small POSITIVE value rather than zero?
A finite temperature difference is what actually drives heat transfer across the tube bundle — an infinitely large heat transfer surface would be needed to reach TTD=0, so real heaters are designed with a small, economically-sized TTD (commonly a few °C) as a deliberate design trade-off between heater size/cost and thermodynamic performance, not a flaw to be eliminated.
What does a RISING TTD over time actually indicate?
At unchanged load and extraction pressure, a rising TTD points to reduced heat transfer across the tube bundle — most commonly tube-side fouling/scaling, or the shell side flooding above its design water level (which reduces the effective heat transfer area exposed to condensing steam), both of which genuinely degrade the heater's ability to close that temperature gap.
Why is DCA a separate check from TTD, not just the same information twice?
TTD checks the condensing (steam) zone's performance; DCA checks a physically DIFFERENT zone — the drain cooler section, where already-condensed drain water is subcooled against incoming feedwater before leaving the heater. A heater can have a healthy TTD while its drain cooler zone is uncovered or bypassed, which DCA specifically catches and TTD alone would miss.