(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

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)

2. Relevant equations

heat in=heat out (Enthalpy balance)

3. 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

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# Homework Help: Heat transfer and combustion

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