energy Bala Saturated steam at 100 degree C is heated to 400 degree C. Use the s

Question

energy Bala Saturated steam at 100 degree C is heated to 400 degree C. Use the steam tables to determine (a) the required heat input (J/s) if a continuous stream flowing at 100 kg/s undergoes the process at constant A fuel oil is burned with air in a boiler furnace. The combustion produces 813 kW of thermal energy, of which 65% is transferred as heat to boiler tubes that pass through the furnace. The combustion products pass from the furnace to a stack at 650 degree C. Water enters the boiler tubes as a liquid at 20 degree C and leaves the tubes as saturated steam at 20 bar absolute. (a) Calculate the rate (kg/h) at which steam is produced. (b) Use the steam tables to estimate the volumetric flow rate of the steam produced. (c) Repeat the calculation of part (b), only assume ideal gas behavior instead of using the steam tables. Would you have more confidence in the estimate of part (b) or part (c)? Explain. (d) What happened to the 35% of the thermal energy released by the combustion that did not go to produce the steam. Liquid water is fed to a boiler at 24 degree C and 10 bar and is converted at constant pressure to saturated steam. Use the steam tables to calculate Delta H (kJ/kg) for this process, and then calculate the heat input required to produce 15,000 m^3/h of steam at the exiting conditions. Assume that the kinetic energy of the entering liquid is negligible and that the steam is discharged through a 15-cm ID pipe.

Explanation / Answer

a) We know that

H + Ek + Ep = Q -Ws all flowrates

Ek , Ep , W =0      U = Q

H(400 degrees C, 1 atm) = 3278 kJ / kg   (table B.7)

H(100 degrees C, sat'd 1 atm) = 2676 kJ / kg   (table B.5)

stream in: 100 kg H2O (v)/s

                100degrees C, sat'd

stream 1 out:   100 kg H2O (v)/s

                      400degrees C, 1 atm

also (into system): Q(kW)

Q = (100kg/s)[(3278-2676)kJ/kg](1000 J/kJ) = 6.02 x107 J/s

b)

U + Ek + Ep = Q -W   (eq. 7.3-4)

Ek , Ep , W =0     U = Q

Table B.5 U(100 degrees C, 1 atm) = 2507 kJ/kg

V(100 degrees C, 1 atm) = 1.673 m3 / kg = V(400degrees C, Pfinal)

Interpolate in table B.7 to find P at which V = 1.673 at 400degrees C, and then interpolate again to find U at 400 degrees Cand that pressure:

V = 1.673 m3 / g Pfinal =1.0 + 4.0[(3.11-1.673) / (3.11-0.617)] = 3.3 bar

U(400 degrees C, 3.3 bar) = 2966 kJ/kg

Q = U = mU = 100 kg [(2966 - 2507)kJ/kg](1000J/kJ) = 4.59 x 107 J

The difference is the net energy needed to move the fluidthrough the system (flow work).

(The energy change associated with the pressure change in Part(b) is insignificant.)

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