Fundamentals of Heat and Mass Transfer
7th Edition
ISBN: 9780470917855
Author: Bergman, Theodore L./
Publisher: John Wiley & Sons Inc
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Chapter 3, Problem 3.31P
A commercial grade cubical freezer, 3 m on a side, has a composite wall consisting of an exterior sheet of 6.35-mm-thick plain carbon steel, an intermediate layer of cork insulation, and an inner sheet of 6.35-mm-thick aluminum alloy (2024). Adhesive interfaces between the insulation and the metallic strips are each characterized by a thermal contact resistance of
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A dormitory at a large university, built 50 years ago, has exterior walls constructed of L, = 25-mm-thick sheathing with a thermal
conductivity of k, = 0.1 W/m-K. To reduce heat losses in the winter, the university decides to encapsulate the entire dormitory by
applying an L; = 25-mm-thick layer of extruded insulation characterized by k; = 0.029 W/m-K to the exterior of the original
sheathing. The extruded insulation is, in turn, covered with an Lg = 5-mm-thick architectural glass with kg = 1.4 W/m-K. Determine
the heat flux through the original and retrofitted walls when the interior and exterior air temperatures are T = 22°C and To =
O°C, respectively. The inner and outer convection heat transfer coefficients are h; = 5 W/m?-Kand h,
00,
25 W/m2-K, respectively.
The heat flux through the original walls is 1.46
W/m?.
The heat flux through the retrofitted walls is
0.92
W/m?.
Homework
O H.W. 1:
The walls of a refrigerator are typically constructed by sandwiching a layer of
insulation between sheet metal panels. Consider a wall made from fiberglass
insulation of thermal conductivity k; = 0.046 W/m.K and thickness L, = 50 mm
and steel panels, each of thermal conductivity k, = 60 W/m.K and thickness L,
= 3 mm. If the wall separates refrigerated air at T = 4 C from ambient air at T,.
= 25 C, what is the heat gain per unit surface area? Coefficients associated
with natural convection at the inner and outer surfaces may be approximated
as h, = h, = 5 W/m?.K.
%3D
%3D
L; = 0.050 m K
K Lo = 0.003 m
Refrigerated
air
Ambient air
Too.i = 4°C
hi = 5 W/m2-K
To,o = 25°C
ho = 5 W/m2-K
%3D
Panel (2)
kp = 60 W/m-K
Insulation
k; = 0.046 W/m-K
22
Warith Alanbiyaa (Dr. ALI M)
Heat Transfer
Page 11 of
Steel pipe (outer diameter 100 mm) is covered with two layers of insulation. The inner layer, 40 mm thick, has a thermal conductivity of 0.07 W / (m K). The outer layer, 20 mm thick, has a thermal conductivity of 0.15 W / (m K). Pipes are used to deliver steam with a pressure of 600 kPa. The temperature on the outer insulation surface is 24 ° C. If the pipe is 8 m long, determine the following: (assuming that the conduction heat transfer resistance of the steel pipe and the vapor convection resistance are negligible).
a. Heat loss per hour. = kJ / hour.
b. Temperature between insulation layers. = ° C
Chapter 3 Solutions
Fundamentals of Heat and Mass Transfer
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