Concept explainers
(a)
The power rating of the electric heater.
(a)

Answer to Problem 99P
The power rating of the electric heater is
Explanation of Solution
Write the formula to calculate the volume of room
Here, length of room is
Write the ideal gas equation to calculate the total mass of air in the room
Here, initial pressure of air is
Assume the entire room as the steady-flow system that is a control volume as mass traverses the boundary.
Write the energy balance for system in the rate form
Here, rate of net energy transfer into the control volume is
At steady state, rate of change in internal energy of the system is zero. Thus rewrite the energy balance equation for the system.
Here, power rating of the electric heater or electrical work input is
Conclusion:
Substitute
Refer Table A-1, “Gas constant of common gases”, obtain the gas constant of air as
Substitute
Refer Table A-2, “Ideal – gas specific heats of common gases”, obtain the constant volume specific heat of air as
Substitute
Thus, the power rating of the electric heater is
(b)
The temperature rise of air as it passes through the heating duct.
(b)

Answer to Problem 99P
The temperature rise of air as it passes through the heating duct is
Explanation of Solution
Write the mass balance equation for the flow of air.
Here, mass flow rate of air at the inlet is
Assume the heating duct as the steady-flow system that controls the volume as mass traverses the boundary.
Write the energy balance for system in the rate form as follows:
Re-write the energy balance equation for the system as follows:
Here, initial specific enthalpy of air is
Conclusion:
Refer Table A-2,“Ideal – gas specific heats of common gases”, obtain the constant pressure specific heat of air as
Substitute
Thus, the temperature rise of air as it passes through the heating duct is
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Chapter 5 Solutions
THERMODYNAMICS(SI UNITS,INTL.ED)EBOOK>I
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