Lesson
34
Cooling And Heating
Load Calculations

Heat Transfer Through
Buildings  Fabric Heat
Gain/Loss
Version 1 ME, IIT Kharagpur
1
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View Full DocumentThe specific objectives of this chapter are to:
1. Discuss the general aspects of heat transfer through buildings (
Section 34.1
)
2. Discuss onedimensional, steady state heat transfer through homogeneous, non
homogeneous walls, through air spaces and through composite walls of the buildings
(
Section 34.2
)
3. Discuss unsteady heat transfer through opaque walls and roofs (
Section 34.3
)
4. Discuss onedimensional, unsteady heat transfer through opaque walls and roofs
with suitable initial and boundary conditions (
Section 34.4
)
5. Describe the analytical method used to solve the 1D, transient heat transfer
problem through building walls and roofs (
Section 34.4.1
)
6. Briefly discuss the numerical methods used to solve the transient heat transfer
problem (
Section 34.4.2
)
7. Discuss the semiempirical methods based on Effective Temperature Difference
or Cooling Load Temperature Difference, discuss the physical significance of
decrement and time lag factors and present typical tables of CLTD for walls and roof
(
Section 34.4.3
)
At the end of the chapter, the student should be able to:
1. Calculate the steady heat transfer rates through homogeneous and non
homogeneous walls, through composite walls consisting of a combination of
homogeneous and nonhomogeneous walls and air spaces
2. Explain the need for considering transient heat transfer through buildings
3. Derive onedimensional, transient heat conduction equation for building walls and
roof and indicate suitable initial and boundary conditions
4. Discuss the general aspects of the analytical, numerical and semiempirical
methods used to solve the transient building heat transfer problem
5. Use the ETD/CLTD methods to estimate heat transfer rate through opaque walls
and roof of the buildings
34.1. Introduction:
Whenever there is a temperature difference between the conditioned indoor
space of a building and outdoor ambient, heat transfer takes place through the
building structure (walls, roof, floor etc.). This is known as fabric heat gain or loss,
depending upon whether heat transfer is to the building or from the building,
respectively. The fabric heat transfer includes sensible heat transfer through all the
structural elements of a building, but does not include radiation heat transfer through
fenestration. Exact analysis of heat transfer through building structures is very
complex, as it has to consider:
a) Geometrically complex structure of the walls, roofs etc. consisting of a wide
variety of materials with different thermophysical properties.
b) Continuously varying outdoor conditions due to variation in solar radiation,
outdoor temperature, wind velocity and direction etc.
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 Spring '12
 NPTEL
 Heat Transfer, IIT Kharagpur

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