Kosky, Balmer, Keat, and Wise: Exploring Engineering, Fourth Edition
Chapter 14, Mechanical Engineering
A number of these exercises have tricky round off significant figures. Check your
answers against the recommendations of http://sigfigscalculator.appspot.com/
14-1 If the heat conduction rate through a 3.00 m2 wall 1.00 cm thick is 37.9 kW when
the inside and outside temperatures are 20.0 and 0.00 °C, respectively, determine the
thermal conductivity of the wall.
Need: Find the thermal conductivity of the wall, kt, = ?.
Know: The wall surface area is A = 3.00 m2, its thickness is Δx = 1.00 cm =
0.010 m, the conduction heat transfer rate is
cond plane
Q
37.9 kWatts, and the
inside and outside temperatures of the wall are Thot = 20.0 °C and Tcold = 0.00 °C.
hot cold
TT
Kosky, Balmer, Keat, and Wise: Exploring Engineering, Fourth Edition
Solve:
o
6
inside outside
outside inside
122 118 F
2 2 (35.0 ft)(223.0 Btu / h·ft·R ) 1.22 10 Btu/hr
ln( / ) ln(1.08 / 0.92)
t
TT
Q Lk rr
This rounds down to 1 × 106 Btu/hr since 122 – 118 = 4 ºF correct to 1
sig.fig.
Instructor’s Notes: Per ºF is the same as per degree R. The student
should also note that the area orthogonal to the direction of heat flow
varies. This is why the conduction equation has the logarithmic ratio of the
inner and outer radii in it.
