Outlet temperature of flue gas

[PCal]

Homework Statement

If the flue gases exiting the boiler are used to preheat the water fed to the boiler from a temperature of 28 C to 90 C 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 300°C 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

Relevant Equations

Enthalpy at 5 bar = 2018kJ/kg

Flame temp= 1997°C

Heat in =(sensible heat in fuel and air + latent heat released by combustion)

Heat out=Σ(mass×mean specific heat capacity×Tf) for each of the products of combustion.

This is the final question on my assignment and I'm clueless as to where I should even start as the lesson seems to give no guidance. Help would be massively appreciated!

 

[Chestermiller]

Are you trying to do the calculation just for the pre-heater, for the combination of preheater and boiler, or for each of them separately?

 

[PCal]

If I'm correct, just the outlet temperature from the pre-heater. The question isn't particularly clear but that's my assumption!

 

[Chestermiller]

Do you know the flue gas temperature coming out of the boiler? Do you know the mass flow rates of the water and the flue gas? or, Do you know the flue gas temperature going into the boiler?

 

[PCal]

I think the flue gas coming out of the boiler is 300° C but tha's not very clear, The flow rate of the flue gas = 1400 kmol h-1, Enthalpy at 5 bar = 2018kJ/kg, the heat capacity at 90°=4.208. As far as I can tell that's all I've got

 

[Chestermiller]

The enthalpy of what at 5 bars is 2018 k/kg? Do you know the water flow rate?

What is the full exact problem statement?