University of San Simón

Faculty of Sciences and Technology

Chemical Engineering Degree Transport Phenomena A

PRACTICE #3
HEAT TRANSFER

1. In the Huanuni mines, there is a copper wall (K=375 W/m k), 1 cm thick.
of water vapor that condenses (K=10000 W/m2k) to a T=2000C. The other surface is
in contact with ambient air (K=5 W/m k), at a T= 250C Calculate: a) The heat per unit
of area transferred through the plate. b) The temperatures at both surfaces of the wall.
c) The overall heat transfer coefficient.

2. The rear window of a car functions by the passage of hot air over
its inner surface. If the hot air is at Ti= 400C and the convection coefficient
corresponding is hi30 W/m2K. What are the temperatures of the internal surfaces?
and external to the 4 mm thick glass window, if the ambient air temperature
The outside is T = -100C and the associated convection coefficient is h = 65 W/m2K? Yes K
glass1.4 W/m K.
2

3. Determine the heat transfer through the composite wall, which is outlined.
assuming a unidirectional heat flow.

4. The hot combustion gases from a furnace are separated from the ambient air and their
surroundings or enclosure (where the ambient air and walls are at 250C, through a
brick wall of 0.15 m thickness. The brick has a thermal conductivity of 1.2
W/m k and an emissivity of 0.8, the temperature of the outer surface of the wall is measured.
the brick is 1000C, under steady state conditions. If the heat transfer by

Instructor: Gonzalo Alfaro Denus Esp.I&D, M.Sc., Ph.D., PosD.AA Management II/2015
Assistant: Univ. Yhoset A.B. García Montaño
San Simón University
Faculty of Science and Technology

Chemical Engineering Degree Transport Phenomena A

free convection in the air adjacent to the surface of the brick wall is characterized
for a convection coefficient of 20 W/m2K. What is the surface temperature?
interior of the brick wall, that is, the surface in contact with the gas?

Last summer, in Crispín's room, the temperature of the inner wall was 27.0C and
In winter, the temperature was 14.0C, the average temperature of Crispín us
it allows to consider that the temperature of the surface of your skin is at 320C, to one
ambient temperature considered constant at 200C. Under these conditions, take into account
that the heat transfer coefficient from the body to the air is 2 W/m2K, the emissivity is
0.9. Estimate the energy loss of the body in summer and winter.

A bakelite coating will be used with a conductive rod of 10 mm in diameter.

whose surface is maintained at 2000C by passing an electric current. The rod
is in a fluid at 250C and h = 140 W/m2K. What is the critical radius associated with the
Coating? What is the heat transfer per unit length for the rod?
naked and for the rod with a Bakelite coating corresponding to the radius
critical? How much Bakelite should be added to reduce heat transfer by 25%?
associated with the bare rod?

7. The behavior of a freezer consists of a cubic cavity that has 2m on each side.
Assuming that the floor is perfectly insulated, what is the minimum thickness of the insulation?
polyurethane foam (K= 0.03 W/m K) that must be applied to the upper walls and
lateral to ensure a heat load of less than 500 W when the interior surfaces and
the exterior is at -100C and 350C?

8. In steady state, hot air flows at 0.05 Kg/s through a metal sheet duct.
Isolated with a length of 5 m and a diameter of 0.15 m. Consider that the hot air flows through a
constant inlet and outlet temperature of 770C. Also consider the coefficient of
heat transfer between the outer surface of the duct and the surrounding air at a T=
00C is uniform and equal to 6 W/m2K. Estimate the temperature by calculating the
internal surface of the duct.

9. Insulators are used to reduce or prevent heat loss and save energy.
Calculate:
a) The critical insulation radius of asbestos. (K = 0.17 W / m K) to insulate a pipe that is
exposed to air at 200C. b) The heat loss from the tube that has a diameter of 5 cm

Teacher: Gonzalo Alfaro Denus Esp.I&D, M.Sc., Ph.D., PosD.AA Management II/2015
Assistant: Univ. Yhoset A.B. García Montaño
University of San Simón
Faculty of Sciences and Technology

Chemical Engineering Degree Transport Phenomena A

external is 200 C, when it is not isolated and when it is isolated with the calculated value of
0

critical asbestos radio.

10. Air must be pumped through a straight tube with a 2-inch inner diameter.
tube at 700At 1 atm pressure with a speed of 70 lb/h to raise the temperature of the
air at 2300The tube must be heated uniformly from the outside until the temperature
about the internal wall of the tube being 2500F. What should be the length of the tube to achieve
the stated objective.

11. A flat solar collector without a covered plate has a surface of absorption
emissivity 0.1 and solar absorptivity of 0.95. At a given moment of the day, the given temperature
from the absorption surface Ts = 1200C, when the solar irradiation is 750 W/m2the
the temperature of the sky is -100C and the ambient air temperature is T = 300C, assuming
a h (for air conditions can be estimated h= 0.22[Ts- T ]1/3;[W/m2Calculate the
speed of useful heat removal.

12. A reinforced concrete chimney with an inner diameter D2= 800 mm, diameter
exterior D3= 1300 mm, it must be lined on the inside with refractory. Determine the thickness.
of the coating and the temperature T3from the outer surface of the chimney, starting from the
condition that the heat losses of one meter of the chimney do not exceed 2000 W/m,
and that the temperature T2does not exceed the interior surface of the reinforced concrete wall
2000The temperature of the interior surface of the lining is T.1= 4250C; the coefficient
The thermal conductivity of the coating is K.10.5 W/m0C; the conductivity coefficient
thermal of concrete is K21.1 W/m0C.

13. Calculate the heat losses of 1m of a non-insulated pipe with diameter d1/d2 =
150/165 mm was outdoors when water with a temperature runs through the interior of it.
media T1 = 900C and the ambient temperature Ta = -150C. The thermal conductivity coefficient
the material of the tube is K = 50 W/m0C. The heat transfer coefficient for water and
the tube is 1000 W/m2 0C and the tube and the environment is 12 W/ m2 0C. Also determine the
temperatures on the interior and exterior surfaces of the tube.

14. A copper rod and a steel rod of equal length and cross-section are
solders with a common end. The free end of the copper rod is maintained at 100°C
and the free end of the steel one at 0°C. The rods are insulated laterally, the coefficient
The thermal conductivity of copper is 8 times that of steel. Once it reaches the regime
stationary, we can affirm that: a) The temperature at the junction of both rods is lower
a) What 50°C, b) The temperature at the junction of both rods is greater than 50°C, c) The differences

Teacher: Gonzalo Alfaro Denus Esp.I&D, M.Sc., Ph.D., Postdoctoral AA Management II/2015
Assistant: Univ. Yhoset A.B. García Montaño
San Simón University
Faculty of Sciences and Technology

Chemical Engineering Degree Transport Phenomena A

the temperature between the ends of both rods is equal, d) The amount of heat that,
per unit of time, it crosses any cross-section of the copper rod is greater
that which crosses any cross section of the steel rod, e) The amount of heat
that, per unit of time, passes through any cross-section of the copper rod is
less than that which crosses any cross section of the steel rod, f) Does not transfer heat
through the rods. Explain your answer

15. A metal rod, with one end at 250°C and the other at 40°C, conducts 75.4
cal/s. If its length and diameter are reduced by half, and its ends are placed at the same...
temperatures than before, the bar will conduct (in cal/s):

16. If the surface temperature of the Sun were one fifth of its current temperature
(both expressed in K, the heat that the Earth would receive by radiation from the Sun would be, with respect to
currently, it will approximately be: a) 16 times smaller, b) the same, c) one fifth, d) the
half, e) 25 times smaller, f) more than 600 times smaller

17. The concrete wall of a refrigerator measures 3 m high, 5 m wide, and 20 cm of

thickness. The temperature is maintained at -10ºC and the outside temperature is 20ºC. The wall
the interior is covered with a layer of wool to reduce heat flow through the wall
for 90%. The thermal conductivities of concrete and wool are 0.8 and 0.04 W/m K,
respectively. Calculate: a) What is the temperature difference of the wool layer? b)
What is the required thickness of the layer of wool?

18. Calculate the heat loss rate per unit length of a steel pipe of 2
1 inch internal diameter and 2 3/8 inches external diameter covered with insulation for high temperatures
temperatures with a K= 0.065 Btu / h- foot, with a thickness of 0.5 inches. Steam flows through the tube.
which has a quality of 99% and is at 3000F, the wall has an internal resistance of 0.015
h-pie2 0F /Btu, with h = 3.0 Btu / foot2 0F on the outer surface, at a temperature
environment of 600F

19. Which materials can be good conductors of energy,

There are insulating materials and for what cases they are used, c) in a dairy plant, which uses
a vertical plate pasteurizer, carries steam through a pipe that comes from the
boiler at 100°C, what type of insulation will I need for the steam to reach the plates?
at a T= 90°C if the length of the pipe is 5 m?

Professor: Gonzalo Alfaro Denus Esp.I&D, M.Sc., Ph.D., Postdoc AA ManagementII/2015

Assistant: Univ. Yhoset A.B. García Montaño