9. *The following data apply to a steam boiler fired with natural gas: Mass flow

ID: 1002319 • Letter: 9

Question

9. *The following data apply to a steam boiler fired with natural gas:

Mass flow rate of natural gas =0.9kg/s

Mass flow rate of air =19.1 kg/s

Ambient temperature =25 °C

Flue gas exit temperature =300 °C

Enthalpy of water to boiler tubes =84 kJ/kg
Enthalpy of steam from boiler tubes =3,024 kJ/kg

Heat lost to atmosphere from boiler =3,150 kJ/s

Heat of reaction at 25°C =52,100 kJ/kg (water product is in liquid phase)

Heat of reaction at 25°C =46,660 kJ/kg (water product is in vapour phase)

Mean specific heat of flue gas =1.05 kJ kg/K
Draw a block diagram for this process and determine the following:

(i) the mass flow rate of flue gas. (Ans: 20 kg/s)

(ii) the rate at which useful heat is supplied to generate steam. (Ans: 33,069 kW)

(iii) the boiler efficiency (based on water product is in liquid phase) (Ans: 70.5%)

(iv) the mass flow rate of steam produced. (Ans:11.2 kg/s)

i) Flue gas flow rate = Air flow rate + Fuel flow rate

= 19.1 + 0.9 = 20 Kg/s

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