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Chapter 1
Basics of Heat Transfer
130
A room is heated by the radiator, and the warm air is distributed by a fan. Heat is lost from the room.
The time it takes for the air temperature to rise to 20°C is to be determined.
Assumptions
1
Air is an ideal gas since it is at a high temperature and low pressure relative to its critical
point values of 141
°
C and 3.77 MPa.
2
The kinetic and potential energy changes are negligible,
.
3
Constant specific heats at room temperature can be used for air, C
ΔΔ
ke
pe
≅≅
0
p
= 1.007 and C
v
=
0.720 kJ/kg·K. This assumption results in negligible error in heating and airconditioning applications.
4
The local atmospheric pressure is 100 kPa.
Properties
The gas constant of air is
R
= 0.287 kPa.m
3
/kg.K (Table A1). Also, C
p
= 1.007 kJ/kg·K for air
at room temperature (Table A15).
Analysis
We take the air in the room as the system. This is a
closed system
since no mass crosses the
system boundary during the process. We observe
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Mass, Heat

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