ASHRAE Journal - January 2023 - 46

TECHNICAL FEATURE
FIGURE 4 Closed-cell, cellular glass insulation is impervious to water vapor and
nonabsorbent.
chillers, increasing energy costs and reducing the overall
cooling capability to the building in question.
Pipes in Vaults/Tunnels
When chilled water pipes for district energy systems
For all chilled water systems, it remains important to
consider the specific environment of the facility and
to account for worst-case scenarios during the design
phase, rather than average conditions.
Direct Buried Piping
District energy systems, which provide climate control
for multiple buildings from a single facility, are increasingly
common. This practice can call for pipes to be
direct buried or in underground vaults or tunnels.
When chilled water pipes will be encased in soil,
weight or compressive strength becomes an additional
concern. Any insulation used must have enough
compressive strength to support the weight of soil
backfill, which increases with soil density and depth.
Additionally, pipes and insulation must cope with
any weight on top of the soil-either live load or stagnant
load-such as foot and vehicle traffic or permanent
structures. Without the necessary compressive
strength, insulation cannot continue functioning
optimally.
Direct buried systems also must address hydrostatic
pressure within the soil. Hydrostatic pressure is the
pressure exerted by a fluid at equilibrium at any given
point within the fluid. It is based on the force of gravity
and increases with soil depth. Hydrostatic pressure can
drive moisture into permeable insulation systems, as it
can be particularly damaging to weak or poorly installed
joints. The pressure is of particular concern in areas
with a high water table or in locations prone to seasonally
high precipitation.
Additionally, soil is not a great insulator. When buried,
chilled water lines may be buried near warmer
processes such as hot water or steam lines. In this scenario,
should the insulation on either system fail, heat
would be able to rapidly transfer from the hot pipe to
the chilled lines. This would raise the temperature of the
chilled water in the pipe, adding additional load to the
46
ASHRAE JOURNAL ashrae.o rg
J A N U A RY 2023
travel through tunnels or in vaults, it is also not uncommon
for them to be located next to hot pipes within the
same space. These shared spaces can be big enough for
people to walk through or small enough to serve as a crawl
space, which raises issues of durability. Using an insulation
with more compressive strength can protect it from
incidental damage of nearby workers, while an inorganic
material can help protect against burrowing vermin.
Another challenge with pipes in this situation is the
potential for flooding to occur. Although the installation
of a sump system can help, these devices are prone to
clogging. When flooding does occur, insulation may be
submerged and remain covered by water for indefinite
periods. If a permeable insulation system is used, it
can then absorb and retain flood water, compromising
its thermal performance. This potential for flooding to
occur highlights the need for an impermeable insulation
system and specified accessories, like sealants, to be
applied to underground chilled pipes.
Cellular Glass Insulation
Closed-cell, cellular glass insulation (Figure 4) has several
attributes suitable for insulation material. It is a lightweight,
rigid material with a high compressive strength
that will not flex or warp even when exposed to weight
and a range of temperatures for long periods.* Being
made of millions of completely sealed glass cells, the
material is nonabsorbent, non-wicking and impermeable
to liquid and water vapor according to testing to ASTM
E96.12 This helps the material retain its thermal performance
throughout the life of the system. The insulation
has a service temperature range from -450°F to 900°F
(-268°C to 482°C) and is dimensionally stable, with a
thermal expansion coefficient similar to carbon steel's.†
Conclusion
Keeping moisture away from chilled pipes and out of
insulation helps protect the long-term thermal performance
of the insulation, enables proper function of the
*As established by testing to ASTM C165, ASTM C 240, ASTM
C552 and ASTM C203
†As established by testing according to EN 1604 and ASTM E 228
http://ashrae.org
ASHRAE Journal - January 2023

Table of Contents for the Digital Edition of ASHRAE Journal - January 2023
