a) How much heat is needed to convert one kg of feed water at 20oC into dry saturated steam at a pressure of 9 bar? Take specific heat of water as 4.187 kJ/kg K.

The heat needed to convert 1 kg of feed water at 20°C into dry saturated steam at a pressure of 9 bar is 2690.19 kJ/kg.

Explanation:

Step 1 : Obtain the enthalpy of staurated steam and enthalpy of evaporation at 9 bar pressure from the steam table:

From steam table, at 9 bar, the enthalpy of saturated water = 742.83 kJ / kg

enthalpy of evaporation = 2031.1 kJ / kg

Step 2: Calculate the enthalpy of dry saturated steam:

Enthalpy of dry saturated steam = enthalpy of saturated water + enthalpy of evaporation

= 742.83 + 2031.1 = 2773. 93 kJ / kg

Step 3: Calculate the enthalpy of 1 kg of feed water at 20°C

Enthalpy of 1 kg of feed water = c * (T2 - T1)

= 4.187 * (20-0)

= 83.74 kJ / kg

Step 4 : calculate the heat needed by 1 kg of feed water at 20°C to be converted to dry saturated steam at 9 bar:

Heat needed = Enthalpy of dry saturated steam - enthalpy of feed water

Heat needed = 2773.93 kJ/kg - 83.74 kJ/kg

Heat = 2690.19 kJ/kg