Question: Run-around coil heat recovery systems

[Lord Dark]

hey guys ,, how are you all ??

guys , I've got the following problem , Air enters a gas-fired furnace at 20deg at a mass flow rate of 0.2kg/s and is burned with an air-fuel ratio by folume of 12. the gases leave the furnace at 350deg. A run-around coil system is installed to recover some of the energy of the exhaust gases in order to preheat the air entering the furnace. Calculate :

1- the required mass flow rate of the 2nd fluid.
.
.
.etc .

my problem is i don't know how to get the mass flow of the gas exiting , i have the AFR = 12 in volume , how to use it to get the gas out of the furnace ? they gave me the density of the air and the fuel only not the gas out .. should i get it density too ?

 

[philrainey]

The mass rate out is the same as the combined mass flows in of the fuel and air. You know the the density of the air you can work out the volume of air which from the ratio you can work out the volume of fuel. You know the density of the fuel so you can work out the mass flow of the fuel.