Question

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. ​

Answer 1

To convert 1 kg of feed water at 20°C into dry saturated steam at 9 bar, 2595.36 kJ of heat is needed. This can be calculated by determining the heat required to raise the temperature of water from 20°C to its boiling point, and then the heat required to vaporize the water.

Step-by-step explanation:

To convert water at 20°C into dry saturated steam at a pressure of 9 bar, we need to calculate the amount of heat required using the specific heat of water and the heat of vaporization. First, we need to calculate the heat required to raise the temperature of water from 20°C to its boiling point, which is 100°C. The formula for this is Q = m * c * ΔT, where Q is the heat, m is the mass, c is the specific heat, and ΔT is the change in temperature. In this case, the mass is 1 kg, the specific heat is 4.187 kJ/kg K, and the change in temperature is 100 - 20 = 80°C. Plugging in these values, we get Q = 1 kg * 4.187 kJ/kg K * 80°C = 335.36 kJ.

Next, we need to calculate the heat required to vaporize the water. The heat of vaporization of water is 2260 kJ/kg. The formula for this is Q = m * H, where Q is the heat, m is the mass, and H is the heat of vaporization. In this case, the mass is 1 kg. Plugging in these values, we get Q = 1 kg * 2260 kJ/kg = 2260 kJ.

Finally, we can add up the two heats calculated to get the total heat required. 335.36 kJ + 2260 kJ = 2595.36 kJ. Therefore, the amount of heat needed to convert one kg of feed water at 20°C into dry saturated steam at a pressure of 9 bar is 2595.36 kJ.

Answer 2

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.

Step-by-step 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