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Air is heated from 25C to 140C prior to entering a combustion furnace. The change in
Chapter 7, Problem 7.21(choose chapter or problem)
Air is heated from \(25^{\circ} \mathrm{C}\) to \(140^{\circ} \mathrm{C}\) prior to entering a combustion furnace. The change in specific enthalpy associated with this transition is \(3349 \mathrm{~J} / \mathrm{mol}\). The flow rate of air at the heater outlet is \(1.65 \mathrm{~m}^{3} / \mathrm{min}\) and the air pressure at this point is \(122 \mathrm{kPa}\) absolute.
(a) Calculate the heat requirement in \(\mathrm{kW}\), assuming ideal-gas behavior and that kinetic and potential energy changes from the heater inlet to the outlet are negligible.
(b) Would the value of \(\Delta \dot{E}_{\mathrm{k}}\) [which was neglected in Part (a)] be positive or negative, or would you need more information to be able to tell? If the latter, what additional information would be needed?
Questions & Answers
QUESTION:
Air is heated from \(25^{\circ} \mathrm{C}\) to \(140^{\circ} \mathrm{C}\) prior to entering a combustion furnace. The change in specific enthalpy associated with this transition is \(3349 \mathrm{~J} / \mathrm{mol}\). The flow rate of air at the heater outlet is \(1.65 \mathrm{~m}^{3} / \mathrm{min}\) and the air pressure at this point is \(122 \mathrm{kPa}\) absolute.
(a) Calculate the heat requirement in \(\mathrm{kW}\), assuming ideal-gas behavior and that kinetic and potential energy changes from the heater inlet to the outlet are negligible.
(b) Would the value of \(\Delta \dot{E}_{\mathrm{k}}\) [which was neglected in Part (a)] be positive or negative, or would you need more information to be able to tell? If the latter, what additional information would be needed?
ANSWER:
Step 1 of 4
The energy balance is the first law of thermodynamics in which energy can neither be created nor be destroyed. A system is said to be closed and open according to no mass across the boundary of the system during the time period by the energy balance. Energy balance of the closed system is defined as the net energy transferred to the system.
a) From the given,
Temperature at inlet;
\(\left(T_{1}^{\circ} \mathrm{C}\right)=25^{\circ} \mathrm{C}\)
Convert degree Celsius to kelvin as follows:
\(T_{1}(K)=25+273\)
\(=298 K\)
The temperature at outlet ;
\(T_{2}^{\circ} \mathrm{C}=140^{\circ} \mathrm{C}\)
Convert degree Celsius to kelvin as follows:
\(T_{2}(K)=140+273\)
\(=413 K\)
The energy balance is given as follows:
\(\Delta H+\Delta E_{k}+\Delta E_{p}=Q+W \cdots(1)\)
\(\Delta H\)= The change in enthalpy
\(\Delta E_{k}\)= The change in potential energy
\(Q\)= The heat
\(W\)= The work