Page 220 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION

5.4 CHAPTER FIVE

both the insulation and the pipe on which it is installed. This impedes the flow of heat. For
this reason, the term apparent thermal conductivity is scientifically correct when referring
to the k values used. With this explanation in mind, please note that the word apparent will
be omitted in future discussions.
The term heat flow is defined as the total heat gain or loss from an entire system or compo-
nent of that system, and is measured in Btu/h. The term heat flux is used to measure the heat
2
2
gain or loss from only 1 ft of a system or component. Heat flux is measured in Btu/h · ft and
is the product of the temperature differential and the conductance C.

Water Vapor Migration

Thermal insulation is fully effective only when completely dry. If water in either the form
of vapor or liquid is present in or on insulation, it will have a serious effect both physically
and thermodynamically. The loss of insulating capacity is well documented. If enough
water is allowed to accumulate, it will cause rotting or other possible corrosive effects in
most types of insulation.
Water vapor present in air has a measurable vapor pressure that is a function of both
temperature and relative humidity. The lower the relative humidity and/or temperature, the
lower the vapor pressure. Applying this fact specifically to an insulated body, the movement
of water vapor is proportional to the difference in temperature between the ambient air and
the wall of the pipe or vessel.
When the temperature of the insulated pipe or vessel is above ambient temperature,
vapor pressure is higher at the pipe wall than on the outside surface of the insulation. This
means that a vapor pressure differential exists between the pipe wall and the surrounding
air, driving the water vapor away from the inner surface and toward the outside.
However, when the temperature of the pipe is below ambient, the opposite is true. The
pressure is lower at the pipe wall than on the outside surface of the insulation, and the direc-
tion of water vapor flow is reversed. It is then possible for the vapor to permeate the insu-
lation, allowing water to be absorbed and retained by the insulating material, and thereby
reducing its effectiveness. It is also possible for water to actually condense as a liquid on
the pipe wall. Certain types of insulation can become saturated over a period of time. If
the insulation material does not absorb water, the air cells may become saturated. Another
possibility is that the water may start corroding the pipe exterior.
The water vapor transmission rate (WVTR) is a measure of water vapor diffusion into
or through any total insulation system. This flow of water vapor, called permeance, is mea-
sured in U.S. perms, or perm. A perm is the weight of water, in grains, that is transmitted
2
through a 1-in (25-mm) thickness of the material in question in 1 h/ft , having a pressure
difference between faces of 1 in of mercury. There are 7000 grains in 1 lb.
In order to restrict the flow of water vapor from the warm to the cold side of the object
being insulated, a vapor barrier is installed on the warm side of the insulation. Since there
is no perfect vapor barrier, the insulation materials used should also have some resistance to
moisture in addition to having a good thermal resistance. The ideal material would absorb
little or no moisture, would allow quick elimination of any that did enter, and would not be
affected by moisture during the time it was present. A generally accepted figure of 0.30 perm
is the maximum rate of an effective vapor barrier.
In addition to the primary purpose of retarding the flow of heat and water vapor, there
are other important secondary factors that must be considered when selecting the type of
insulation or covering for a particular application. They are:
1. Smoke and fire requirements
2. Space limitations
3. Personal protection

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