- #1
philipwhit
Homework Statement
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)
Homework Equations
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.
Heat
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
= 2380j
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
= 2266-1.18
roughly=2265
Is this the right process
I feel I am completely wrong
This is my last question and I have completed my mechanical engineering studies
Thank you