A fuel gas consists of 75% butane (C4H10), 10% propane (C3H8) and 15% butene (C4H8) by volume.
It is to be fed to the combustion chamber in 10% excess air at 25oC, where it is completely burnt to carbon dioxide and water. The flue gases produced are to be used to generate 5 bar steam from water at 90oC.
1 If the flue gases exiting the boiler are used to preheat the water fed to the boiler from a temperature of 28oC to 90oC and assuming:
• a mean specific heat capacity for water over this temperature range to be 4.2 kJ kg–1 K–1
◦ a mean molar heat capacity for the flue gases up to 300oC to be 31 kJ kmol–1 K–1
◦ 10% of the heat required to heat the water is lost in the heat exchanger
◦ all water entering the system is converted to steam
◦ determine the final outlet temperature of the flue gas and state if the dew point will be reached in both of the cases given in part (j)
heat in=heat out (Enthalpy balance)
The Attempt at a Solution
On my previous attempt I got an outlet temperature of 288.58 degrees C. But my tutor marked that as completely wrong.
I attempted the question again using
Heat in = heat out.
I have calculated the heat in by using the cv calculated in previous question plus the (mass x Cpmolar(31) × 62) - The heat lost due to additional water content in the air. Multiplied all this by 0.9 as 10% of heat is lost. I get roughly 2380 mj
heat in = (2580+(mass*Cpmolar(31)*62)-0.3695 mj) *0.9 62 is change in temp
I then calculate the heat out by calculating the enthalopy of each product minus the heat of vapour caused by the 5 bar. I then used the grapgh to get a temp of between 1700 and 1800.
heat out=sum of enthalpy of products-heat of water at 5 bar
Is this the right process
I feel I am completely wrong
This is my last question and I have completed my mechanical engineering studies