United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-92/203 December 1993
EPA Project Summary
Literature Review: Heat Transfer
Through Two-Phase Insulation
Systems Consisting of Powders
in a Continuous Gas Phase
David W. Yarborough
This review of the literature on heat
flow through powders was motivated
by the use of fine powder systems to
produce high thermal resistivities (ther-
mal resistance per unit thickness). The
term "superinsulations" has been used
to describe this type of material, which
has thermal resistivities in excess of
20 ft2.h.°F/Btu (3.52 K.m2/W) per in.
(2.54 cm) of insulation thickness. The
report is concerned with superinsula-
tions obtained using evacuated pow-
ders.
The literature review shows that the
calculation of heat flow through gas-
powder systems is highly developed.
One major weakness in the calcula-
tional procedures .is the absence of
structural features for the powders,
which are invariably characterized as
regular arrays of spheres or cubes
rather than random irregularly shaped
particles. The effect of particle size dis-
tribution on the shape and size of void
spaces is not modeled, although it af-
fects the thermal conductivity of the
gas. Calculations of thermal perfor-
mance based on simplified descriptions
of the porosity distribution can be used
to show the dependence of thermal re-
sistance on interstitial gas pressure.
The literature reviewed in this report
provides a basis for predicting the in-
terstitial gas pressure at which thermal
conductivity begins to increase. The
objective is to design filler material for
powder insulation systems with ultra-
fine void spaces that will permit pres-
sure increases without dramatic thermal
conductivity increases.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Research
Triangle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
An effort is underway at the Oak Ridge
National Laboratory (ORNL) to facilitate
the use of evacuated powder insulation in
appliances such as refrigerators and freez-
ers and in selected building applications.
The objective of the effort is to produce
and demonstrate performance and dura-
bility of very high thermal resistivity sys-
tems to replace closed-cell foam products
containing environmentally unacceptable
chemicals. Important tasks associated with
the objective include cost effective mini-
mization of the heat transfer through a
layer of fine powder with low interstitial
gas pressure and demonstration that the
low interstitial pressure, which is a major
factor in the thermal performance, can be
maintained for extended periods. In the
case of building insulation, the service life
should be at least 25 years and perhaps
as much as 50 years.
Interest in two-phase dispersed systems
dates back at least 100 years. The early
work focused on solid mixtures, but the
resulting theory is applicable to systems
in which the continuous phase is a non-
condensible gas or, more precisely, a gas
above its critical temperature. The trans-
port of heat by the gas in a gas-solid
composite can be a significant part of the
total heat flow. If the composite is to be
Printed on Recycled Paper
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used as an insulator, the gas phase con-
duction can be reduced by lowering the
pressure (evacuation) and/or reducing the
dimensions of the gas-phase regions (use
line powders).
The motivation for this literature review
was the use of evacuated fine powder
systems to produce high thermal resistivi-
ties (thermal resistance per unit thickness)
or low thermal conductivities. The term
"superinsulations" has been used to de-
scribe this type of material, which has
thermal resistivities in excess of 20
ft2- hr- °F/Btu (3.52 m2- °K/W) per in. (2.54
cm) of insulation thickness.
Tho use of evacuated powders is one
method of producing a superinsulation.
Multi-layer evacuated insulation made from
high-reflQctance/low-emittance foils is an-
other, and evacuated fibrous Insulations"
represent a third. This report is concerned
with superinsulation obtained using evacu-
ated powders.
The transfer of heat through a particu-
late bed containing a stagnant gas is gen-
erally discussed In terms of three
mechanisms: (1) radiation through the void
fraction, (2) conduction through a series
of solid and gas elements, and (3) con-
duction through the solid phase. The three
mechanisms are taken to result in addi-
tive heat flows, although this is not totally
consistent. Radiative heat transport
through the solid particles (as well as the
void space) occurs, and the steady-state
heat flow across planes perpendicular to
the overall heat flow direction is not dis-
tributed among the mechanisms the same
way for every plane. However, most of
the theoretical discussion of heat flow
across gas-solid systems involves a cal-
culation of a heat flow that depends on
the thermal conductivities of the two ma-
terials with an added term for radiation.
The radiative term then must include ra-
diation across voids and radiation through
particles.
The literature review shows that the cal-
culation of heat flow through gas-powder
systemsJs^ highly developed. One major
weakness in the calculational procedures
is the absence of structural features for
the powders, which are invariably charac-
terized as regular arrays of spheres or
cubes rather than random irregularly
shaped particles. Radiative transport cal-
culations are approximate. Particle-to-par-
ticle contact resistance is treated
empirically. The effect of particle size dis-
tribution on the shape and size of void
spaces is not modeled, although it affects
the thermal conductivity of the gas. Calcu-
D.W. Yarbomugh Is with Oak Ridge National Laboratory, Oak Ridge, TN 37831-
6092.
Robert V. Hendrlks is the EPA Project Officer (see below).
The complete report, entitled "Literature Review: Heat Transfer Through Two-
Phase Insulation Systems Consisting of Powders in a Continuous Gas Phase,"
(Order No. DE93-014387; Cost: $17.50; subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research^Laboratory _
U.S. Environmental Protection Agency " , ^ .,i=M.~
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
lations of thermal performance based on
simplified descriptions of the porosity dis-
tribution can be used to show the depen-
dence of thermal resistance on interstitial
gas pressure. The high thermal resistance
that can be achieVed with evacuated pow-
ders has been clearly demonstrated. Cal-
culations of thermal performance based
on simplified descriptions of the porosity
or'void-space distribution can be used to
show the dependence of thermal resis-
tance on interstitial gas pressure. The chal-
lenge for bringing powder-filled evacuated
panels to commercialization lies primarily
with the development of a packaging sys-
tem that will maintain the required low
pressures for many years. The develop-
mental work then should concentrate on
barrier material air-permeability ,rand
vacuum sealing technology.
The development of advanced filler ma-
terials would help provide panels that will
exhibit high thermal resistance even
though air leakage has occurred. The lit-
erature reviewed in this paper provides a
basis for predicting the interstitial gas pres-
sure at which thermal conductivity begins
to increase. The objective is to design
filler material for powder insulation sys-
tems with ultrafine void spaces that will
permit pressure increases without dramatic
thermal conductivity increase.
•U.S. Government Printing Office: 1994 — 550-067/80132
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EPA/600/SR-92/203
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