A boiler tank with a volume of 112 liters is designed to gen

A boiler tank with a volume of 112 liters is designed to generate saturated steam at 2 bar. The boiler is partially filled with water, and is heated to the operational pressure. The boiler contains 7 kg of saturated steam-water mixture when the operational pressure is first reached. During the next 33 minutes, the boiler is heated at a steady rate, generating saturated steam at a constant rate of 0.080 kg/min.

List your assumptions and determine:

a. draw a schematic showing the system

b. The liquid water and steam in boiler when the operational pressure is first reached.

c. The total heat transferred to the boiler during the 33 minutes operation.

d. The heat transfer rate at 6 minutes after the initiation of the operation

e. Net change of entropy in the boiler. Explain if this change is consistent with the 2nd Law of Thermodynamics.

Solution

b) Volume of saturated steam + Volume of water = 0.112 m^3

     Mass of saturated steam + Mass of water = 7 kg

     density of saturated steam at 2 bar = 1.13 kg/m^3

      mass of steam / 1.13 + mass of water/1000 = 0.112

      mass of steam + mass of water   = 7

      solving above two equations we get

      mass of steam = 0.1188 kg   mass of water = 6.8812 kg

c) heat transferred   = 0.08 * 33 * enthalpy of steam

                             = 0.08 * 33 * 2710   = 7154.4 kJ

d) heat transfer rate   = 0.08 * enthalpy of steam    = 0.08 * 2710 = 216.8 kg/min

e) change in entropy of the boiler   = 0.08 * ( s2 - s1)

                                                  = 0.08 * (7.13 - 1.53) = 0.448   kJ/ K - min

    As change in entropy is greater than zero, yes this change is consitent with 2nd law of thermodynamics.