In large steam power plants, the feed water is frequently heated in closed feed

Question

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

Explanation / Answer

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

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