Cold feedwater enters a 200-kPa open feedwater heater of a regenerative Rankine cycle at 70°C with a flow rate of 10 kg/s. Bleed steam is available from the turbine at 200 kPa and 160°C. At what rate must bleed steam be supplied to the open feedwater heater so the feedwater leaves this unit as a saturated liquid?

The rate of bleed steam Ms is 0.93 kg / s.

Explanation:

This can be solved by using an energy balanced equation. There are three contributions to the equations naming the cold feedwater, bleed steam, and the exciting water contribution.

cold water flows at a rate Mc = 10 kg / s

pressure p = 200 kPa

temperature Tc = 70 degree celsius

now determining the enthalpy of the cold feed water

Hc = 293 kJ / kg

Bleed steam is supplied at the rate Ms, pressure p and the temperature Ts = 160-degree celsius.

Now determining the enthalpy of the steam

Hs = 2790 kJ / kg

Atlast the existing water leaves at the combined rate Mc + Ms in saturated liquid at a pressure p

enthalpy of exiting water table He = 504.71 kJ / kg

Energy balance equation for calculating mass flow rate of steam Ms is given by,

Ee = Ec + Es

(Mc + Ms) * He = (Mc * Hc) + (Ms * Hs)

(10 + Ms) 504.71 = (10 * 293) + (Ms * 2790)

Ms = 0.93 kg / s.