SKKK 2313
Assignment 1
1. The composite wall of an oven consist of three materials, two
of which are of known thermal conductivity, kA = 20 W/m.K and kC =
50 W/m.K, and known thickness, LA = 0.3m and LC=0.15m. The third
material, B, which is sandwiched between materials A and C, with a
known thickness of LB = 0.15m, but an unknown thermal conductivity,
kB.
Under steady-state operating conditions, measurement reveal an
outer surface temperature of To = 20oC and inner surface
temperature of Ti = 600oC, and an oven air temperature of T =
800oC. The inside convection coefficient h is known to be 25
W/m2.K.
(a) What is the value of kB?
(b) Calculate the overall heat transfer cofficient, U.
(c) Determine the power required to maintain the oven at 800oC,
if the outer layer( C) is removed and the outside surface
temperature increased to 40oC.
2. Steam at 320oC flows in a cast iron pipe (k = 80 W/m.oC)
whose inner and outer diameters are 5 cm and 5.5 cm, respectively.
The pipe is covered with 3 cm-thick glass wool insulation with
k=0.05 W/moC. Heat is lost to the surroundings at 5oC by natural
convection and radiation, with a combined heat transfer coefficient
of 18 W/m2 oC. Taking the heat transfer coefficient inside the pipe
to be 60 W/m2.oC, determine the rate of heat loss from the steam
per unit length of the pipe. Also, determine the temperature drops
across the pipe shell and the insulation.
3. A steam pipe 170/160mm in diameter is covered with two layers
of insulation. The thickness of the first layer is 30 mm and that
of the second layer 50 mm. The thermal conductivities of the pipe
and the insulating layers are 50, 0.15 and 0.08 W/m.K,
respectively. The temperature of the inner surface of the steam
pipe is 300oC and that of the outer surface of the insulation layer
is 59oC. The convective coefficient between the steam and the inner
surface of the pipe is 500 W/m2.K, while that between the outer
surface of the insulation and surrounding is 25 W/m2.K. Determine
the quantity of heat lost per metre length of steam pipe and layer
contact temperatures. 4. A 3m high and 5 m wide wall consists of
long 16 cm x 22 cm cross section horizontal bricks (k=0.72 W/m.oC)
separated by 3cm thick plaster layers (k=0.22 W/m.oC). There are
also 2cm thick plaster layers on each side of the brick and 3cm
thick rigid foam (k-0.026 W/m.oC) on the inner side of the wall.
The indoor and the outdoor temperatures are 20oC and -10oC,
respectively, and the convection heat transfer coefficients on the
inner and the outer sides are h1 = 10 W/m2.oC and h2 = 25 W/m2.oC,
respectively. Assuming one-dimensional heat transfer and
disregarding radiation, determine the rate of heat transfer through
the wall.
