459
Chapter
18
Engineering Fundamentals:
Part 3
Heat Transfer
18.1 Introduction
This chapter presents a basic overview of heat transfer fundamentals,
particularly as they apply to HVAC. For a detailed, rigorous treat-
ment, the reader should refer to a good college-level text on heat trans-
fer or to the ASHRAE Handbook.
1
18.2 Heat Transfer Modes
Heat is transferred between any two bodies by one or more of three
modes: conduction, convection, and radiation. Thermal conduction re-
fers to the direct transfer of energy between particles at the atomic
level. Thermal convection may include some conduction but refers pri-
marily to energy transfer by eddy mixing and diffusion, i.e., by fluids
in motion. Thermal radiation describes a complex phenomenon which
includes changes in energy form: from internal energy at the source
to electromagnetic energy for transmission, then back to internal en-
ergy at the receiver. Radiation transfer requires no intervening ma-
terial, and in fact works best in a perfect vacuum. In accordance with
the second law of thermodynamics, net heat transfer occurs in the
direction of decreasing temperature. In this text, the Fahrenheit (ЊF)
scale is used, or for absolute temperatures the Rankine (ЊR) scale:
ЊR ϭ ЊF ϩ 460Њ.
Source: HVAC Systems Design Handbook
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hour per square foot per degree Fahrenheit). The tests are made in a
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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 ϭ (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
ϭ R ϩ R ϩ R ϩ ⅐⅐⅐ϩ R (18.4)
123 n
U
Because resistance is the reciprocal of conductance or conductivity, a
more specific form of the equation is
11xx111
1 n
ϭϩ ϩ⅐⅐⅐ϩϩ ϩ⅐⅐⅐ϩϩ (18.5)
Uf k k C C f
o 1 n 1 ni
where f

place, such as on inside window surfaces. To avoid problems, extra
insulation, double glazing, or surface heating may then be used.
In HVAC practice, steady-state conduction seldom, if ever, takes
place, because the outside air temperature and inside load conditions
are constantly changing. The transient heat flow effects which result
are functions of several variables, including the mass (storage effect)
of the barrier. The sensible heat gain and cooling load factors dis-
cussed in Chap. 3 are approximations which allow the designer to
compensate for these transients.
18.4 Thermal Convection
Thermal convection refers to heat transfer by eddy mixing and diffu-
sion, as in a flowing airstream. In the typical airstream heating or
cooling process, heat transfer takes place as a result of mixing with,
and diffusion through, the air in the conditioned space. The final
transfer is by conduction between air particles. Convection may be
natural or free convection, due to differences in density, or it may be
forced by mechanical means such as fans or pumps.
An HVAC process illustrating almost pure convective heat transfer
is the mixing of two airstreams such as return air and outside air. If
complete mixing takes place, the mixed airstream has a temperature
(and humidity) resulting from a weighed average of the properties and
masses of the two original air-streams. This is a result of convective
eddy mixing and diffusion plus conductive heat transfer between par-
ticles.
A major HVAC application involving a combination of convection
and conduction is heat exchange between two fluids such as refriger-
ant, water, steam, brine, and air, in many combinations. In general,
the two fluids are separated by a barrier, usually the wall of a tube or
pipe. Typical examples are the shell-and-tube heat exchanger (see Fig.
Engineering Fundamentals: Part 3

where D ϭ conduit diameter, ft
V ϭ average fluid velocity, ft/s
␮
ϭ fluid viscosity, lb/(ft ⅐ s)
␳
ϭ density, lb/ft
3
The transition value of the Reynolds number is in the range of 2100
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