Feedwater Heaters: TTD, DCA and What They Tell You About Plant Health
Feedwater heaters are among the most heat-rate-sensitive items in a steam plant, and two numbers describe how well each one is doing: terminal temperature difference (TTD) and drain cooler approach (DCA).
Why heat the feedwater with extraction steam?
In a regenerative cycle, small amounts of partly expanded steam are extracted from the turbine and used to heat the feedwater before it enters the boiler. That steam has already done some work, and its latent heat is transferred to the feedwater instead of being rejected in the condenser. Raising the average temperature at which heat enters the cycle improves cycle efficiency, which is why large units carry six to eight heaters.
Types and zones
Closed heaters keep steam and feedwater separate, with the feedwater flowing inside tubes. The deaerator is an open (direct-contact) heater that also removes dissolved oxygen. A high-pressure closed heater typically has up to three zones: a desuperheating zone that removes superheat from the extraction steam, a condensing zone where most of the heat is transferred, and a drain cooling zone that sub-cools the condensate before it cascades to the next lower-pressure heater.
TTD and DCA defined
Terminal temperature difference (TTD) = saturation temperature of the extraction steam at the heater shell pressure β feedwater outlet temperature. It measures how closely the feedwater approaches the steam temperature.
Drain cooler approach (DCA) = drain outlet temperature β feedwater inlet temperature. It measures how far the drain is cooled toward the incoming water.
Design values are on the heater data sheet; they are typically a few degrees Celsius. Use the Feedwater Heater TTD and DCA calculator to compute both from measured temperatures and the extraction pressure.
Reading the numbers
Rising TTD at constant load means the heater is delivering less heat for the same steam condition. Common causes are tube fouling or plugging, non-condensable gases accumulating in the shell (a vent problem), a flooded condensing zone from a high drain level, or feedwater bypass leakage.
High DCA usually points to a low drain level or lost drain-cooler seal so that hot drain or steam leaves the shell, wasting energy and eroding the downstream piping. Very low DCA with a high TTD often indicates flooding.
These are indications, not proofs; confirm against level readings, vent flow and the heaterβs own performance curves. The main value of TTD and DCA is trending: a slow drift over months matters more than a single reading.
Extraction flow from an energy balance
Extraction steam flow is rarely metered directly, but it follows from an energy balance on the heater: the heat released by the steam (and any cascaded drain from the heater above) equals the heat absorbed by the feedwater. The extraction flow calculator solves this with exact IAPWS-IF97 enthalpies. Using real compressed-liquid enthalpy at feedwater pressure matters: at 200 bar the enthalpy of water differs noticeably from saturated-liquid values at the same temperature, which changes the duty by a visible fraction of a percent.
Why it matters for heat rate
Taking a high-pressure heater out of service forces the boiler to heat colder feedwater and reduces the work extracted from the extraction steam, so heat rate rises. Operators therefore track TTD and DCA of every heater, and a heater that is bypassed or degraded is one of the first places to look when unit heat rate drifts.
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