Chapter hvac engineering Fundamentals: Part 1 Introduction

Engineering Fundamentals: Part 3
461
‘‘guarded hot box,’’ designed so that heat transfer through the edges
of the material is essentially eliminated. The results of these tests are
tabulated and presented, with discussion, in the ASHRAE Handbook.
2
The thermal conductivity
k
of any material is the reciprocal of its
resistance
R
:
1
k
⫽
(18.3)
R
For barriers with material combinations which are not tabulated, the
U
factor may be calculated from the sum of the individual resistances.
The general form of the equation is
1
⫽
R
⫹
R
⫹
R
⫹
䡠 䡠 䡠
⫹
R
(18.4)
1
2
3
n
U
Because resistance is the reciprocal of conductance or conductivity, a
more specific form of the equation is
1
1
x
x
1
1
1
1
n
⫽
⫹
⫹
䡠 䡠 䡠
⫹
⫹
⫹
䡠 䡠 䡠
⫹
⫹
(18.5)
U
f
k
k
C
C
f
o
1
n
1
n
i
where
f
o
⫽
outside film conductance
f
i
⫽
inside film conductance
x
⫽
thickness of homogeneous section with conductivity
k
See Ref. 2 for a more detailed discussion. The incremental tempera-
ture drop through each element of the barrier is proportional to the
resistance of the element. For example, in Fig. 18.1 if the wall is 6-in-
thick perlite concrete with a
k
value of 0.93 per inch, and if the outside
and inside film conductances are 4.00 and 1.46, respectively, then the
overall
U
factor is
1
1
6
1
⫽
⫹
⫹
U
4.00
0.93
1.46
⫽
0.25
⫹
6.45
⫹
0.68
⫽
7.38
1
U
⫽
⫽
0.136
7.38
If a temperature difference of 42
⬚
F is assumed, based on 72
⬚
F inside
and 30
⬚
F outside, then the temperature gradient can be determined
as shown in Table 18.1. This type of calculation is useful in determin-
ing the location where moisture condensation or freezing will take
Engineering Fundamentals: Part 3
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