Feedwater Heater Temperature Rise
Feedwater Heater Temperature Rise
Feedwater heaters preheat the water entering a boiler using extracted steam or waste heat, improving overall Rankine cycle efficiency by reducing the heat the boiler itself must supply, and sizing one starts from a straightforward sensible-heat balance: the heat duty delivered raises the water's temperature in proportion to its mass flow rate and specific heat. Getting the outlet temperature right matters downstream too — too little preheat wastes the efficiency benefit the heater was installed for, while overshooting risks flashing the feedwater to steam before it reaches the boiler, which can damage feed pumps not designed to handle two-phase flow.
The temperature rise is dT = Q/(m·cp), and the outlet temperature is the inlet temperature plus that rise. where Q is the heat duty delivered to the feedwater, m is the feedwater mass flow rate, cp is its specific heat, and T_in is the inlet temperature.
Inputs
Q_fw := 500 kW=500000 W
m_fw := 50 kg/s=50 kg/s
cp_w := 4.2 kJ/(kg·K)=4200 m^2/K/s^2
T_in := 320 K=320 K
Dividing the heat duty by the mass flow rate and specific heat gives the temperature rise the feedwater experiences as it passes through the heater.
dT_fw := Q_fw/(m_fw·cp_w)=2.38 K
Adding that rise to the known inlet temperature gives the feedwater temperature leaving the heater and entering the boiler.
T_out := T_in + dT_fw=322.38 K
Results
A rise of about 2.4 K on a large feedwater flow is a modest but realistic increment for a single low-pressure feedwater heater stage in a multi-heater regenerative train; real plants stack several heaters in series to build up tens of degrees of total preheat before the boiler. The outlet temperature should always be checked against the local saturation temperature at feedwater pressure — if T_out approaches or exceeds it, the water would begin flashing to steam, which is unacceptable on the pump suction side, so the heat duty or flow rate would need to be revised.
Assumptions & limitations
- Feedwater remains subcooled liquid throughout (no flashing to steam).
- Specific heat treated as constant over this temperature range.
- Steady-state, single-pass heater with no heat loss to ambient.
- Heat duty Q assumed already known from the extraction steam side of the heater.