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Radiant Based Cooling Systems
and Equipment - Part I
Introduction
One of the biggest
misunderstandings in heating, ventilation and air
conditioning
(HVAC)
is how comfort cooling works.
First we need to remind
ourselves that HVAC is not about cooling the building, its
about
cooling you and I and controlling humidity.
When do we feel cool? Well
when our body loses more heat than it can generate we
experiences a cool sensation. If the heat loss from our body
is excessive we can go into hypothermia...if we can't get
rid of excessive heat we go into hyperthermia - both
scenarios - not a good thing.
To cool our body just enough
to be comfortable we can lower the mass temperature in our
home a few degrees cooler than our skin temperature which
creates the same experience you get when you walk into an
underground garage on a hot summer day or into the frozen
food section at the super market. Why do we feel cool in
the underground garage or freezer section? It's because the
temperature of the concrete or glass in relation to our skin
is lower so our body releases its heat via radiation to the
cooler surface - thus the reason we feel cool. The best
part of it all, is a body at rest, transfers over 50% of its heat
via radiation so radiant cooling makes perfect sense. In
fact the transfer of our body heat to the cooler surface via
radiation is called "sensible" heat transfer. So now you
know that radiant (sensible) cooling works because you have
experienced it a thousand times before just like the cave
dwellers of years past…you just never thought about it.
Where most uneducated voices
speak up and against radiant cooling has to do with
condensation. You see they don't know about sensible vs.
latent cooling.
The solar heat inside a
greenhouse is a great example of sensible heat. In
 a
building, sensible cooling is the absorption of the radiant
energy from the sun and other heat sources such as your
body, lights, motors, compressors, stoves, ovens, dryers,
computers and home theatre systems. All of these
loads which raise the mass temperature of the home can be
removed by absorbing the excess heat with a cool surface.
Think of a cooled surface as 'sponge' for sensible heat
What can't be absorbed by a
cool surface is the heat being held in the moisture content
of the air. This heat is called the "latent" load.
Latent means dormant, hidden or concealed. Latent heat shows
up every time you sit outside on a hot humid day with a cool
drink in your hand...water actually forms on the glass
surface. Where did the water come from? It
 was
actually hidden in the air and the amount is based on an
every day term that you are familiar with called relative
humidity. The visible presence of moisture or condensation
occurred because the ‘humid’ air (latent heat) met a chilled
surface (sensible). Can
you guess where some of the heat in the moisture went to?
That's right...it went into the glass via conduction.
It's one of the reasons why you need to keep putting ice
into the glass to keep the drink cool. The rest of the heat
went back into vapor or dripped all over your shirt or
shorts.
This is very important to
understand because is some parts of North America the
relative humidity in the summer is very low almost desert
like. In other parts of the country the humidity is very
high. In other parts it changes based on how much rain
falls in the region and for how long. In all cases the
latent load must be controlled either for comfort or to
prevent moisture damage in buildings
So the key to cooling with
radiant is to control the surface dew point and relative humidity and that is easy
to do.
go to part II -
click here., Controlling the Relative
Humidity
go to part III -
click
here,
Heating and Cooling Comfort
Systems
go to part IV - click here, Radiant Cooling Educational Movie Clips
go to part V - click here, Featured Project, Manitoba Hydro Office Building
People who visited these radiant cooling pages have also visited our Radiant Design Guide,
Psychrometrics - Made Easy and Introduction to Indoor Environmental Quality.
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