The closed feedwater heater of a regenerative Rankine cycle is to heat 7000 kPa feedwater from 2608C to a saturated liquid. The turbine supplies bleed steam at 6000 kPa and 3258C to this unit. This steam is condensed to a saturated liquid before entering the pump. Calculate the amount of bleed steam required to heat 1 kg of feedwater in this unit.

the amount of bleed steam required to heat 1 kg of feedwater in this unit is 0.078 kg/s

Explanation:

Given that:

Pressure of the feed water = 7000 kPa

Temperature of the closed feedwater heater = 260 ° C

Pressure of of the turbine = 6000 kPa

Temperature of the turbine = 325 ° C

The  objective is to calculate the amount of bleed steam required to heat 1 kg of feedwater in this unit.

From the table A-4 of saturated water temperature table at temperature  260° C at state 1 ;

Enthalpies:

From table A-6 superheated water at state 3 ; the value of the enthalpy relating to the pressure of the turbine at 6000 kPa and temperature of 325° C  is obtained by the interpolating the temperature between 300 ° C and 350 ° C

At 300° C; enthalpy = 2885.6 kJ/kg

At 325° C. enthalpy = 3043.9 kJ/kg

Thus;

At pressure  of 7000 kPa at state 6; we obtain the enthalpies corresponding to the pressure at table A-5 of the saturated water pressure tables.

From state 4 ;we obtain the specific volume corresponding to the pressure of 6000 kPa at table A-5 of the saturated water pressure tables.

However; the specific work pump can be determined by using the formula;

where;

= pressure at state 4

= pressure at state 5

Using the energy balance equation of the closed feedwater heater to calculate the amount of bleed steam required to heat 1 kg of feed water ; we have:

where;

Replacing our other value as derived above into the energy balance equation ; we have:

Collect like terms

Hence; the amount of bleed steam required to heat 1 kg of feedwater in this unit is 0.078 kg/s

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