Question

Air is heated from 25 C to 140C prior to entering a combustion furnace. the change in specific enthalpy associated with this transition is 3349 j/mol. The flow rate of air at the heater oulet is 1.65 m^3/min and air pressure is 122Kpa apsolute.

a) Calculate the heat requirements in kW, assuming ideal gas behavior and that the kinetic and potential energy changes from the heater inlet to the outlet are negilible.

b) would the value of the cahnge in KE (wich was neglected in part a) be positive or negative, or would you need more info. If the latter what more info would you need.

Answer 1

Sol :-

(a). Using ideal gas equation :

Molar flow rate (n) = PV/RT

= 122 x 10³ Pa x 1.65 m³/60 s / (8.314 m³Pa.K^-1mol^-1 x 413 K)

= 0.9771 mol/s

As,

ΔH + Δ (K.E) + Δ (P.E) = q - w

Because,

Δ (K.E) = Δ (P.E) = w = 0

Therefore,

q = ΔH

q = n x Δh

q = 0.9771 mol/s x 3349 J/mol

q = 3272.3 J/s

q = 3272.3 W

q = 3.2723 KW

(b).

Change in kinetic energy (Δ.K.E) = 3.k_B.ΔT / 2

k_B = Is Boltzmann constant

ΔT = Change in temperature. Because temperature increases therefore value of ΔT will be positive.

Hence, value of change in kinetic energy that is Δ (K.E) will be positive.