ASHRAE Journal - January 2023 - 43

TECHNICAL FEATURE
Common Issues, Damage From Failed Systems
One of the most noticeable issues with failed insulation
on chilled water pipes is dripping water from the
insulation. Dripping water could be an indicator that
condensation is forming on the outer surface of the
insulation or that water has penetrated and saturated
the insulation, allowing moisture to re-escape the system.
When dripping occurs on indoor piping, it can
create a slip hazard, damage ceiling tiles or stain walls
and equipment located under the pipes. Additionally,
the presence of collected moisture provides one of
the elements necessary for the growth of mold, which
needs moisture, oxygen and a food source to grow.
While most types of insulation do not provide a food
source, the material can collect dust, dander or skin
cells, meaning only moisture is needed for mold to
grow.
The presence of mold indoors negatively alters air
quality and has been linked to an increase in many
health problems including asthma, allergic rhinitis and
long-term illnesses.5 Additionally, about 6% - 10% of the
general population is allergic to mold, and the presence
of toxic mold has become a topic of increased legal focus
in recent years.6
Corrosion under insulation (CUI) is a less obvious
form of damage that can occur when moisture
penetrates pipe insulation. Once moisture enters an
insulation system, it can collect against the outer edge
of the chilled water pipe and start to degrade the pipe
itself. This corrosive damage can take years to develop.
However, the hidden damage to pipe integrity can continue
until a catastrophic failure of the entire piping
system occurs.
In addition to CUI, the presence of moisture within
permeable insulation can greatly reduce the insulation's
thermal performance, which leads to a range of problems.
Water is an excellent conductor of heat, with studies
showing that water can transmit heat or cold 10 to 30
times faster than air.1,7 - 9 When insulation absorbs additional
heat or sees a decline in thermal performance, it
can lead to a loss of process control and create a strain
on the chilled water system because it will require more
energy for the system to reach the necessary temperatures.
Increased energy expenditure raises the cost of
running the system. Warmer supply lines also mean that
climate-controlled areas may not get to their intended
temperature.
FIGURE 1 Humidity levels vary across regions, and those differences must be
considered when designing insulation systems for chilled water pipes.
Low
Medium
High
Preparing for Moisture Vapor
Several key elements should be considered when preparing
to address moisture challenges to chilled water
pipes, including humidity, vapor pressure, dew point
and vapor pressure drive.
Humidity is the concentration of water vapor present
in the air. Relative humidity is a percentage ratio that
describes the relationship between the actual amount
of moisture in the air compared to the greatest amount
it could hold at that pressure and temperature. At 100%
relative humidity, air has no more capacity to hold additional
water as vapor. Humidity varies regionally, making
it important to know the conditions of the specifi c
location where the system will be installed (Figure 1).
Vapor pressure is defi ned as the force exerted by a
vapor that is in equilibrium with its condensed phase
at a given temperature. Vapor pressure increases with
temperature, meaning warmer air spaces of higher
vapor pressure will move vapor toward colder areas of
lower vapor pressure. This tendency of vapor in the air
to move from warm to cold areas is commonly referred
to as " vapor drive. "
Vapor pressure can be viewed as the tendency of a
material to transition into its gaseous state under ambient
conditions, which also will increase with temperature.
As temperature falls, water becomes less able to
transition into vapor under ambient conditions. In any
air space at less than 100% relative humidity, a dewpoint
temperature exists below which the air will be too
cold for water to further transition into a gas. Below this
dew point, water will instead begin to freely condense
from the air into liquid water. If objects or surfaces in
this air space are lower than this temperature, they will
cool the local air in contact with them to below the dew
point, causing surface condensation, or " sweating, " to
occur. This mechanism, along with the " vapor drive "
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ASHRAE Journal - January 2023

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