In large steam power plants the feedwater is frequently heat

In large steam power plants, the feedwater is frequently heated in closed feedwater heaters, which are basically heat exchangers, by steam extracted from the turbine at some stage. Steam enters the feedwater heater at 1 MPa and 200C and leaves as saturated liquid at the same pressure. Feedwater enters the heater at 2.5 MPa and 50C and leaves 10C below the exit temperature of the steam. Neglecting any heat losses from the outer surfaces of the heater, determine (a) the ratio of the mass flow rates of the extracted steam and the feedwater heater and (b) the reversible work for this process per unit mass of the feedwater. Assume the surroundings to be at 25C.

Solution

The solution is as follows

At 1.0 MPa & 200 C

h1 = 2827.86 kJ/Kg

At 1.0 MPa & saturated liquid

h2 = 762.68 kJ/kg

Temperature at 1 MPa & saturated liquid

T = 179.91 °C

Temperature existing the feed water = 169.91 °C

Heat lost by steam = Heat gained by feed water

ms (2827.86 - 762.68) = mw * 4.187 (169.91 - 50)

ms/mw = 0.243

(b) Reversible work

= Cp (T2w - T1w) - T0 * Cp ln (T2w/T1w)

= 4.187 (442.91 - 323) - 298 * 4.187 ln (442.91 / 323)

= 108.13 kJ/Kg