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2
L E S S O N
Heat Transfer
(Part 2)
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THE ROLE OF INSULATION IN ENERGY CONSERVATION
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he previous Lesson defined the basic refrigeration cycle as the process
of removing heat from one area, object, or material, and releasing it
somewhere else. This process consumes one or more forms of energy.
Continued population growth, necessary antipollution measures, ecology
protection requirements, and fuel shortages all have made the conservation
of energy—especially electrical energy—a matter of high priority.
The phrase “energy conservation,” when it is used in the context of refrigeration,
refers primarily to a reduction in electrical energy consumption. One way to cut
down on energy consumption is to reduce the amount of heat to be transferred
by the refrigeration cycle. Doing so means that equipment “ON” cycles will
be shorter, and that smaller, more energy-efficient equipment can be used.
Insulation plays an important role in reducing the heat load on a refrigeration
application, regardless of whether the heat transfer occurs via radiation,
conduction, or convection.
TYPES OF INSULATION
Insulation is used to retard (slow down) the flow of heat from a warm space to
a cooler one. Recall that reflective types of insulation block the flow of radiant
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heat. Insulation materials that restrict the flow of heat by conduction are the
subject of this Lesson. Materials vary in their ability to conduct heat. Most
metals, for example, are good conductors of heat—in fact, they conduct heat
so well and so rapidly that there is little difference between the temperature of
the “hot” side and that of the “cold” side.
Other materials are very poor conductors of heat. Because they conduct heat
very slowly, these materials make good insulators. Some well-known materials
that have been used as insulation over the years include cork, mineral wool,
wood, paper, corn or cane stalks, and cotton. Today many insulation materials
are synthetic products. Some of the more common modern insulators include
fiberglass, polyurethane foam, phenolic foam, polystyrene, flexible elastomeric
foam, and cellular glass foam.
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“Dead” air—that is, air that is not in motion—is a very good insulator. It has a
theoretical K-value of 0.17. Why theoretical? Because it is very difficult to keep
air absolutely still. Convection currents and radiation of heat across the air space
decrease its insulating value. But the more a single space of open air can be
separated into many smaller spaces, the more the convection currents are broken
up—and the greater the insulating value. Convection currents in the tiny air cells
of insulation materials are very small. These tiny, enclosed air cells are what
give certain materials the ability to act as good insulators—that is, to impede
the flow of heat by conduction.
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Insulation may take various forms:
ª
Fibrous insulation is composed of small-diameter fibers that separate
the available air space into tiny areas. Silica, rock wool, slag wool, glass
fiber, and aluminum silica fibers are used.
ª
Cellular insulation is composed of small, individual cells separated
from each other. These closed-cell materials, as they are called, include
glass, foamed elastomerics, and foamed plastics (such as polystyrene
and polyurethane).
ª
Granular insulation is composed of small nodules that contain voids or
hollow spaces. Vermiculite is an example of a granular insulation.
These materials are manufactured in a variety of forms suitable for specific
functions or applications. Some are rigid boards, blocks, or sheets. Some are
made in preformed shapes so that they can be used as curved segments or for
covering pipes. Flexible sheets, flexible blankets, and cements are also forms
that are widely used. When a fibrous or granular material is pressed or formed
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L E S S O N
2
into boards or sheets to give it more strength and rigidity, a binder of some kind
must be used to make the loose material stick together. However, as little binder
as possible is used, since the binder itself is often not a very good insulator, and
tends to reduce the insulating value of the board. So-called “foamed-in-place”
insulation is used for filling cavities that are hard to get at, or difficult to
insulate by other methods.
TEMPERATURES WITHIN INSULATION
Do not assume that the high temperature on the hot side of a sheet of insulation
suddenly becomes a low temperature on the cold side. Remember that insulation
is not a positive barrier to heat flow—it merely retards the rate of heat transfer.
The rate of heat flow through the insulation is reduced gradually. As a result,
the temperature within the insulation also drops gradually, from the temperature
of the warmer outer surface to that of the cooler inner surface.
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Figure 2-1 below shows the gradual change of temperature—the temperature
gradient—within the insulation itself. These temperatures shift position if either
the outer, warm-side temperature or the inner, cool-side temperature changes.
For example, if the outer temperature drops from 90 to 70°F, but the inner
temperature remains at 30°F, then the mean temperature (the halfway point)
within the insulation moves toward the warm side. If the inner temperature
drops but the outer temperature stays the same, the mean temperature within
the insulation moves toward the cool side of the insulation.
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Cool side of
insulation
Warm side of
insulation
Direction of heat flow
30
40
50
60
70
80
Cool side of
insulation
90
40
Temperature, °F
50
60
Temperature, °F
FIGURE 2-1. Sample temperature gradients within insulation
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Warm side of
insulation
Direction of heat flow
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MOISTURE IN INSULATION
Water is an excellent conductor of heat. It has a K-value of about 4.1, assuming
that there are no convection currents in the water. (Convection currents in water
make it a much better conductor.) Thus, if water gets into insulation, the
insulating value of the insulation is greatly reduced. On average, for every
1% increase (by volume) in the moisture content of an insulation material, its
thermal conductivity (rate of heat transfer) increases by 7.5%. It is, therefore,
absolutely essential for insulation to be dry when it is installed, and perfectly
sealed so that it stays dry.
The first step to preventing moisture from getting into insulation is to determine
the source of the moisture. Water can get into insulation either as liquid or as
water vapor in the air, which can condense on or in the insulation. Water getting
into insulation as a liquid is a comparatively rare (and usually accidental)
occurrence. It may result from washing or spillage, but for the most part does
not represent a major problem. A far more challenging problem is to keep the
water vapor in the air out of the insulation—because this is how most water gets
into insulation.
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You have heard it said that air “contains” moisture. This is not strictly true. What
people refer to as “air” in everyday speech is really a mixture of dry air (itself a
combination of gases in relatively fixed amounts) and water vapor. Each exists
independently of the other. Both are at the same temperature, but each has its
own pressure. The pressure of the dry air alone is very large compared to that
of the water vapor. References to atmospheric pressure or barometric pressure
include both the pressure of the air itself and the pressure of the water vapor in it.
Standard atmospheric pressure at sea level is 29.92 inches of mercury (in. Hg).
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The amount of water vapor in the air varies, depending on the temperature,
relative humidity, and atmospheric pressure. The amount of water vapor that
air can hold depends primarily on temperature. Warm air can hold more water
vapor than the same volume of cold air. The separate and combined pressures
of air and water vapor also vary with changes in weather conditions. Table 2-1
shows some of the properties of various mixtures of air and water vapor.
As you can see, warm air has a much greater vapor pressure than cold air, even
when the two are at the same relative humidity. If the total pressure inside a
refrigerator, where the temperature is 40°F, is 29.92 in. Hg, then the total
pressure outside the refrigerator must also be 29.92 in. Hg, even though the
temperature of the air in the room may be 90°F. However, the partial pressures
will differ dramatically. The vapor pressure inside the refrigerator (at 40°F db and
80% RH) is 0.2 in. Hg. The vapor pressure outside the refrigerator (at 90°F db
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L E S S O N
Dry-bulb
temperature,
°F
Wet-bulb
temperature,
°F
Relative
humidity,
%
Dew point
temperature,
°F
Vapor
pressure,
in. Hg
90
86
85
85
1.207
13.4
90
80
65
77
0.924
10.0
80
76
85
75
0.878
9.47
80
71
65
67
0.672
7.32
70
67
85
65
0.629
6.85
70
62
65
58
0.480
5.41
40
38
85
36
0.212
2.46
40
36
65
30
0.161
1.94
25
24
85
22
0.112
1.35
25
22
0
Grains* of
moisture
per ft3
65
16
0.085
1.03
0.5
85
–3
0.032
0.41
0.0245
0.31
0.0187
0.27
0.0142
0.18
0
–1.0
65
–8
–10
–10.3
85
–12
–15
–15.2
85
–18
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*There are 7,000 grains in 1 lb of water.
TABLE 2-1. Properties of typical mixtures of air and water vapor
(at standard barometric pressure of 29.92 in. Hg)
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53⁄4
and 80% RH) is 1.14 in. Hg, or
times as great. The total pressure is the
same inside and outside the refrigerator. But according to Dalton’s Law of
Partial Pressures, the dry air and the water vapor act independently of each
other. The pressure of each reacts to changes in volume and temperature in
its own way.
The higher vapor pressure outside the refrigerator can actually push the moisture
from outside through the insulation unless a tight barrier is placed outside the
insulation to stop it. This vapor barrier (or vapor seal, as it is sometimes called),
must be made of some material that moisture cannot penetrate, such as metal
foil or polyethylene. The edges must be tightly sealed where the barrier overlaps
at the sides, top, and bottom of the refrigerator. The vapor barrier must be
continuous—that is, there must be no breaks of any sort. Even a pinhole can
defeat the purpose of the vapor barrier.
In a refrigerator, cold storage room, or air conditioned house, the warm side
normally is the outside of the insulation. But this is not always the case.
An insulated house in winter is warmer inside. The same may be true of
a refrigerator operating in a very cold room (colder than the inside of the
refrigerator), or a refrigerated truck or refrigerator car in very cold weather.
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