Page 219 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION 5.3

Solids transfer heat by conduction. Whenever the molecules move about, their constant
vibration or oscillation causes a collision between them and other adjacent molecules. The
heat does not remain constant because faster-moving molecules strike the slower-moving
ones with a resulting decrease in the speed of the faster molecule. This will eventually
equalize the temperature throughout the solid unless heat is added or removed.
There are several methods used to calculate the flow of heat through material, or the
material’s resistance to such flow.
The ability of a specific solid to conduct heat is called thermal conductivity, and is
measured in Btu/h. This is referred to as the k factor. The standard that is used to find k
determines the amount of heat that will flow in 1 h through material measuring 1 in thick
2
and 1 ft in area and having a temperature difference of 1°F (0.5°C) between the faces of the
2
material being measured. The total heat flow in Btu/ft /h through any material is calculated
by the following formula:
k = KI × Δ T × A (5.1)
t
2
where k = heat flow in Btu/h/ft /°F
KI = k factor of the insulation material
ΔT = temperature difference, °F
A = area in square feet, at insulation exterior
t = thickness of insulation, in

As the k factor increases, so does the flow of heat. Conductivity (conductance) measures
heat flow through a standard arbitrary thickness sample of a specific material. Conductance (C)
2
is measured in Btu/h/ft /°F, calculated with the following formula:
C = thermal conductivity (5.2)
thickness (in)
The resistance to the flow of heat through any individual solid is called thermal resis-
2
tance R, and is measured in Btu/h/ft /°F. It is the reciprocal of the conductance value.
R = 1 (5.3)
Conductance
As the resistance increases, the heat flow is reduced.
Total thermal resistance, RT, of a system measures heat flow in several materials layered
together in series. It is found by adding together all the individual resistances R to obtain
the total resistance of the entire system.
Thermal transmittance U is the rate of heat flow through several materials layered
2
together, and is measured in Btu/h/ft /°F. It is the reciprocal of the total thermal resistance,
and is calculated from the following formula:

U = 1 (5.4)
Total thermal resistance
For insulation, conduction is the primary method in the transmission of heat. True ther-
mal conductivity takes place only in homogeneous materials. In the range of materials
used for thermal insulation, a homogeneous material is defined as a substance whose ther-
mal conductivity does not change within the range of thickness normally used. For the
most part, building materials such as brick and lumber are considered to be homogeneous.
However, most thermal insulation is porous and actually composed of solid material sur-
rounding small pockets of air. Therefore, conduction is not the sole means of heat transfer.
In addition, a surface film of air, liquid, or even solid matter is almost always present around

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