Air enters a gas-fi red furnace at 20 C at a mass flow rate of 0.2 kg/s and is burned with an air-fuel ratio by volume of 12. The gases leave the furnace at 350 C. A run-around coil is installed to...

Air enters a gas-fi red furnace at 20C at a mass flow rate of 0.2 kg/s and is burned with an air-fuel ratio by volume of 12. The gases leave the furnace at 350C. A run-around coil is installed to recover some of the energy of exhaust gases in order to preheat the air entering the furnace. Using the data given, neglecting thermal losses, calculate

(a) the required mass flow rate of secondary fluid,

 (b) the effectiveness of the overall heat recovery process,

 (c) the rate of energy recovery and

 (d) the temperature of the gases at exit and the temperature of the air at entry to the burner

Given:

_p
of air = 1.01 kJ/kg K,

_p
of gases = 1.15 kJ/kg K,
 of inlet air = 1.204 kg/ m³,
 of inlet gas = 0.715 kg/m³,

_p
of secondary fluid = 1.6 kJ/kg K, U₀
for each heat exchanger = 60 W/m²
K and heat transfer area for each heat exchanger = 11 m².