POWER September 2017 - 50

FUNDAMENTALS
Useful Tips for Designing Thermal
Insulation Systems
Thermal insulation is installed on almost every piping system and much of the
plant equipment at power generation facilities. It not only saves energy,
but also protects workers, reduces noise, helps protect against freezing,
and more. However, determining the most cost-effective thermal insulation
solution can be difficult. Completing a few calculations can help ensure
systems and personnel are adequately protected while optimizing
process variables.
S. Zaheer Akhtar, PE
T
hermal insulation is mainly used to
limit heat loss from hot surfaces and
heat gain on cold surfaces. In addition,
thermal insulation also serves to provide condensation
(anti-sweat) control, protection for
personnel, freeze protection, fire protection,
noise control, and process control.
This article explains the basic calculations
associated with the design of thermal
insulation for hot and cold pipes, including
surface temperature and freeze protection
calculations. Energy savings estimates and
the specific requirements for cold insulation
and jacketing are also presented.
Basic Heat Transfer Calculations
Heat flows in the direction of the temperature
gradient and the relationships governing heat
flow depend on whether the heat transfer is
occurring by conduction, convection, or radiation.
These mechanisms for heat transfer are
discussed below with sources of equations,
such as the American Society of Heating, Refrigerating
and Air-Conditioning Engineers
(ASHRAE), appropriately noted.
Conductive Heat Transfer Through a
Flat Slab. The basic equation for conductive
heat loss or gain through a flat slab is
given by the following equation, which can
be found in the 2013 ASHRAE Handbook:
Fundamentals, I-P Edition:
Q = k x A x (T1
- T2
) / X
where Q is the heat loss or gain (Btu/hr); k
is the thermal conductivity (Btu-in./hr-ft2
F); A is the area of heat flow (ft2
-
); T1
- T2
is
the temperature difference (F); and X is the
thickness of material (in.).
Conductive Heat Transfer Through a
Hollow Cylinder. The basic equation for
conductive heat loss or gain through a hollow
cylinder, also found in the 2013 ASHRAE
50
where Q is the total heat loss (Btu/hr); L is
the length of pipe (ft); Ti
fluid inside pipe (F); To
outside environment (F); r1
us of pipe (ft); r2
(ft); r3
hi
is the inside radiis
the outside radius of pipe
is the outside radius of insulation (ft);
is the inside film coefficient (Btu/hr-ft2
-F);
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is the temperature of
is the temperature of
handbook, is similar with a slight change in
the value of X and A, as shown below:
Q = k x A2
x (T1 - T2) / (r2 x ln (r2
/ r1
))
where Q is the heat loss or gain (Btu/hr); k
is the thermal conductivity (Btu-in./hr-ft2
T1 - T2 is the temperature difference (F); A2
the area of outer surface (ft2
of outer surface (in.); and r1
-F);
is
); r2
is the radius
is the radius of
inner surface (in.).
The above equations for heat loss and
gain through a flat slab and a hollow cylinder
are based on conductive heat transfer
only. However, the heat transfer from the
surface to the environment involves convection
and radiation as well. This is shown in
the equation below for heat transfer through
an insulated pipe.
Total Heat Transfer Through Insulated
Pipe. The basic heat transfer equation for
heat loss based on conduction through the
metal pipe and insulation, and convection at
the inside fluid film and the outside insulation
surface air film is given as follows, obtained
from the Mechanical Engineering Reference
Manual for the PE Exam, 12th Edition, written
by Michael R. Lindeburg, PE:
hc is the outside convective film coefficient
(Btu/hr-ft2
-F); kpipe
of pipe material (Btu/hr-ft-F); and kinsulation
is the thermal conductivity
is
the thermal conductivity of insulating material
(Btu/hr-ft-F).
Convective Heat Transfer and Radiant
Heat Transfer from Outer Surface of Insulated
Pipe. The convective heat transfer
coefficient and the radiant heat transfer coefficient
determine the heat loss or gain from
the outer surface of the insulation (or pipe, if
uninsulated) to the ambient atmosphere. The
heat loss equation from the 2013 ASHRAE
handbook is given by:
QCR = (hc
where QCR
+ hr
) x A3
transfer (Btu/hr); Tsurf
x (Tsurf
- Tamb
)
is the convective and radiant heat
is the temperature of
the surface in contact with ambient (F); Tamb
is the ambient temperature (F); hc
vective heat transfer coefficient (Btu/hr-ft2
F); hr
(Btu/hr-ft2
area (ft2
).
The convective heat transfer coefficient
(hc
) for the outside air film can be calculated
by several methods. The most common
method is to use a correlation involving dimensionless
numbers in the Nusselt equation
in the following form:
Nusselt No. = C x (Grashof No. x Prandtl
No.)n
wherein the convective heat transfer coefficient
hc
is a parameter within the Nusselt No.,
and C and n are constants.
However, a more convenient form for
calculating the convective heat transfer
coefficient is given in the 2001 ASHRAE
Handbook: Fundamentals, I-P Edition, as
follows:
POWER | September 2017
is the outer surface
is the con-
is
the radiant heat transfer coefficient
-F); and A3
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POWER September 2017

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