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Introduction
One of the myths perpetuated with radiant cooling
systems is condensation risk on the cooling surface. This would
be a real risk if there was no moisture control. Without
moisture control numerous risk factors develop such as
deterioration of materials. According to
The Image Permanence
Institute (IPI), a university-based non-profit research
laboratory devoted to preservation research, the
Preservation Index (PI) represents the overall rate of chemical
decay in organic materials based on a constant temperature and
RH. A higher number indicates a slower the rate of chemical
decay. The Image Permanence Institute developed the Time-Weighted Preservation Index, or
TWPI, to illustrate chemical decay as the environment changes
over time.
Clearly you can see that at a nominal temperature
of 73F, a no risk conditions exists for organic materials between 25% RH and 35% RH
for a dew point condition between 45F and 59F. Now this range of
space conditions is well below what a radiant cooling system
operates at with a very large safety factor - in fact it is so
large that the system could easily operate up to 60% RH albeit
in the presence of a progressively greater risk to the natural
aging of preserved materials. If the space doesn't hold
materials to be preserved then 73F @ 60% RH space conditions
with a cooling panel above 61F is perfectly acceptable. For
other operating conditions visit the
The Image Permanence
Institute.
The
American Museum of Natural History regarding preservation
and temperature and relative humidity (RH) says this;
"Different types of collections have
substantially different relative humidity requirements and so it
is hard to give specific set-points. Specimens with metal
components may benefit from RH levels that are as low as
possible. Organic artifacts require more moderate RH
levels to prevent desiccation or embrittlement. Most
specimens benefit from RH levels that are moderate and stable to
prevent physical damage that can be caused by wide climatic
shifts. Generally, recommendations for museum environments
are given as to 50% while attempting to minimize dramatic swings
to between 40-60%, even if broad seasonal trends are hard to
avoid."
Again, if one is not prepared to control moisture
regardless of the HVAC system then cooling should not be done.
Full stop. If moisture control is going to be implemented then
it also enables the use of radiant cooling systems. There is no
logic in stating, "don't use radiant cooling because of moisture
concerns" then proceed to design an air based HVAC system which
provides moisture control. Do you now see why this a Huh?
moment?
This content is a key component from our course,
"Integrated HVAC Engineering: Mastering Comfort, Health,
and Efficiency."
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