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Lecture 8
Lecture 8
Heat Transfer, Thermodynamics
Heat Transfer, Thermodynamics
&
HVAC For Smaller Buildings
HVAC For Smaller Buildings
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Heat Transfer
(Sensible Heat Loss & Heat Gain)
• Windows, doors, skylights, wall cavity, wall
framing, ceiling cavity, ceiling framing,
framed floor cavity, framed floor framing.
•
Transfer (Btu/h) = Area (A) X U
factor (U) X
factor (U) X
Δ
T
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Heat Transfer
Heat Transfer
(Sensible Heat Loss & Heat Gain)
• Slabongrade.
•
Transfer (Btu/h) =
Ffactor
factor
X Perimeter X
X Perimeter X
Δ
T
Ffactor no edge insulation
factor no edge insulation
=
.73
.73
Ffactor with R
factor with R
10 edge
10 edge
insul
.
=
.54
.54
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Heat Transfer
Heat Transfer
(Sensible Heat Loss & Heat Gain)
• Infiltration air
Infiltration air
•
Transfer (Btu/h) = CFM X 1.08 X
Δ
T
• Ventilation air
Ventilation air
•
Transfer (Btu/h) = CFM X 1.08 X
Δ
T
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Therm
Therm
odynamics
dynamics
The study of the effects of
•
work,
•
heat, and
•
energy
energy
on a system.
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3 Laws of Thermodynamics
3 Laws of Thermodynamics
Simplified
• These laws may seem remote to most people,
but they can be applied to every aspect of
science, from chemistry, to meteorology, to
physics, to pizza!
–
First Law
says energy is conserved – it cannot be
created or destroyed.
–
Second Law
says everything moves toward, but
never achieves, equilibrium because of something
called
*entropy
.
–
Third Law
says that there is a lowest temperature,
called absolute zero, where entropy is zero.
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Entropy and Disorder
If you assert that nature tends to take things from order to disorder and give an example
or two, then you will get almost universal recognition and assent. It is a part of our
common experience. Spend hours cleaning your desk, your basement, your attic, and it
seems to spontaneously revert back to disorder and chaos before your eyes. So if you say
that
entropy
is a measure of disorder
, and that nature tends toward maximum entropy
for any isolated system, then you do have some insight into the ideas of the
second law of
thermodynamics
.
Some care must be taken about how you define "disorder" if you are going to use it to
understand entropy. A more precise way to characterize entropy is to say that it is a
measure of the "multiplicity" associated with the state of the objects. If a given state can
be accomplished in many more ways, then it is more probabable than one which can be
accomplished in only a few ways. When "
throwing dice
", throwing a seven is more
probable than a two because you can produce seven in six different ways and there is
only one way to produce a two. So seven has a higher multiplicity than a two, and we
could say that a seven represents higher "disorder" or higher entropy.
For a glass of water the
number of molecules is
astronomical. The jumble
of ice chips may look more
disordered in comparison
to the glass of water which
looks uniform and
homogeneous. But the ice
chips place limits on the
number of ways the
molecules can be arranged.
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 Spring '09
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