[b Air is heated from 25°C to 150°C prior to entering a combustion furnace. The

Question

[b Air is heated from 25°C to 150°C prior to entering a combustion furnace. The change in specific enthalpy associated with this transition is 3640 J/mol. The flow rate of air at the heater outlet is 1.25 m/min and the air pressure at this point is 122 kPa absolute. Assuming ideal gas behaviour and that kinetic and potential energy changes from the heater inlet to the outlet are negligible. i Calculate the heat requirement in kW. 5 marks) i] If the kinetic energy change is not negligible in part [i], what additional information would be needed? 2 marks

Explanation / Answer

Given data :

Temperature at the inlet = 25 C = 298 K

Pressure = 122 kPa = 122000 Pa

Volume flow rate = 1.25 m3/min

Universal Gas constat = 8.314 J/molK

Fom ideal gas law,

PV = nRT

n = PV/RT

On substituting,

n = 122000*1.25 / 8.314 * 298 = 61.55 mol/min

Given that the specific enthalpy change during heating air from 25 C to 150 C is 3640 Joules per mole of air. For 61.55 mol/min of air, the heat requirement will be,

Heat requirement = Specific enthalpy per mole * Molar flow rate

= 3640 J/mol * 61.55 mol/min = 224042 J/min= 224042/60 J/sec = 3734.03 J/sec = 3734.03 W = 3.734 kW

b) In order to include kinetic energy term we need information motion of the air. This includes velocity of the air at the inlet and at the exit, A part of the energy supplied o air will be diverted to change the kinetic energy.

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