Radiation from Concrete

 Hi all.

If this is not the best forum to raise this issue, then I apologise
and perhaps someone could steer me to somewhere more suitable.

I would like to conduct an evaluation of the effectiveness of the
'ceramic bead' type of isulation that is used by adding to paint and
applying to a surface such as that supplied by http://www.hytechsales.com/
.

Apparently, this type of insulation reduces heat loss through
radiation, rather than heat loss through conduction as per the
'standard' types of insulation materials normally used.

Does anyone know how much energy is radiated from concrete, or where I
might find such information? I presume it would be related to
temperature.

Take care.

Mike

 
there's no way this stuff insulates against anything.

 AC Me wrote:

Radiated energy from a surface is proportional to the emissivity of the
surface and the fourth power of the *absolute* temperature.  The most
emissive surface is a black body.  You can find the theory in any
physics textbook.

You can reduce the emissivity to about 10% or so by painting the surface
white, and to about 1% by applying a mirror finish.

Radiation is a surface effect.  Adding stuff inside a material that is
in conductive contact with the bulk of it will have no effect at all on
radiation. Glass beaded paint is certainly highly reflective, and I
believe the idea of the ceramic beads is that they have low emissivity /
high reflectivity in the near infra-red where most of this radiation
occurs.  So the theory is good but the practice is dubious. The amount
of heat loss through radiation with only a few degrees of differential
at normal temperatures is small.

However radiation is a two way street. Whatever you do to reduce heat
loss through radiation also proportionately reduces the heat absorption
from the sun or anything else hot, as emissivity is the inverse of
reflectivity. As the sun's photosphere is some 20 times the absolute
temperature of your house, you are generally onto a loser unless your
objective is to keep your house cool in a hot climate, or you are in a
very cold and sunless place.

The majority of heat loss through a wall is through conduction and
convection, and evaporation in wet weather (a.k.a. wind-chill).

If you really want to check, I have developed an instrument for directly
measuring the heat flow though walls, doors, ceilings etc.  I note from
this that the flow though my single glazed windows reverses when they
are exposed to the weakest sunshine, and even in hazy bright conditions.


Tim Jackson

 


According to the emissivity table that came with my Wahl instrument, gloss
paint is below 1%, depending on pigment (color doesn't matter much) and mirror
surfaces such as glass and chrome are in the 0.01 to 0.001 range.


Not really.  Radiation emission certainly is NOT a surface effect.  Glass hot
enough to glow is a simple illustration that one can easily visualize.  The
paint very well may be transparent or partially so to the wavelength involved.

If these ceramic pellets/discs are hollow, radiation certainly takes place at
the surface/space interface.

Does this product work?  No idea?  Could it work in theory?  Absolutely.  A
couple of quick questions you can ask them are a) what is the NASA patent
number b) what is the NASA technology transfer contract number. c) who is the
NASA contract administrator and d) what is the article number (normally a NASA
tech briefs article) for the substance.  All this information is easily and
readily available if they really are NASA technology partners, as NASA refers
to companies who license their technology.

At the second stage of evaluation, ask for independent lab test results.  I'm
not sure but I believe that NASA requires such testing before a product can be
tagged with the NASA logo and the "technology partner" moniker applied.

I CAN tell you that ceramic coatings CAN do wonders.  A common use is on
racing engine exhaust headers.  A few micron coating turns the headers from an
engine compartment scorching heater to something that you can touch for short
periods.  There is almost no radiant OR conducted heat.

Absolutely false.  Asymetrical emissivity/absorptive materials have been
commercially available for decades.  Most common use I know of is on thermal
solar collectors.  Even some finishes are asymetrical.  black chrome comes to
mind.  Omega engineering has this info conveniently tabulated for your handy
reference.


May or may not be true but with a concrete wall, if it's cold, it doesn't
matter how warm the room is, one can still be uncomfortable because of
radiative losses from the body to the walls.  Most anyone who's been in a
concrete block building in the winter recognizes the effect, if not the cause.

Since this instrument has been an off-the-shelf item for decades, I can just
imagine what kind of crap you've "improved on".

Back to the OP:  It looks like enough of this stuff to treat a gallon of paint
is about $10.  I suggest getting some, painting a square of surface in a room
and then compare it on a cold or very hot day.  Simply standing in front of
the painted square vs another area will let you feel the difference. Cheap
enough experiment.

BTW, glass microspheres are widely used to reduce the weight of composite
resins.  I cant get to the site right now for some reason but AirCraft Spruce
sells the beads in gallon buckets for very little money.  You might buy some
of them and give them a test alongside this magic stuff.

John



--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
The profligate use of energy is the sign of a healthy, expanding civilization.
Conservation is a leap backward toward the caves.

 Neon John wrote:

Oh true, I can read test books too.  But *exterior* surfaces won't stay
that way for long in practice.  I'm talking about actual building
surfaces and worst case after a few years of weathering and bird shit.
I'd refer you to,
www.omega.com/literature/transactions/volume1/emissivity.html
which would correct me the other way if anything.


I admit I did mean opaque materials.  Paints and walls are generally
designed to be opaque, otherwise they are called varnishes and windows.
  I know we can create exotic materials and situations in the lab that
don't fit my simple model.


On CONCRETE? We are losing the plot a bit here aren't we?
If you paint concrete white you lose heat absorption during the day.
Are you disputing that?  Few building materials are seriously asymmetric.

What is important is that in daytime we are talking about incoming
radiation in the near infrared and emission in the far infrared. Many
materials have emissivity which varies with wavelength.

But this is not really the question here.  We are talking about heat
radiated from the surface of the wall, heat received by the surface of
the wall from the ambient environment, and heat received from the sun.

The first two will be in balance if the surface of the wall is at
ambient temperature and the sky is opaque.  In practice, conducted heat
losses through the wall will elevate it slightly above ambient.  In
still air a solid wall without cavity may be losing say 50W/m² which
will warm the outside surface by no more than 3°C in absence of
differential radiation. The differential radiation on a cloudy day will
be under 20W/m² (the remainder being convection) and lower it by about
1°C. Making the emissivity *zero* would therefore reduce the losses by
about 5%, which is about the same as adding a layer of embossed
wallpaper inside.

In foul weather, forced convection and evaporation will reduce the
differential and the benefit will be less.

On a clear night however you lose a lot more, your roof could lose
400W/m². Of course it gets most of that back by conduction and
condensation from the air, but it means the surface can get several
degrees colder than ambient (hence dews and ground frosts).  By applying
a low emissivity coating you may raise the surface temperature by a
degree or two, and so reduce the conductive losses through the wall by
up to 10%. On the other hand, I severely doubt that the paint coating
will not also increase the reflection of sunlight which at 1000W/m²
times the sine of the sun's elevation, is likely to be more.

My conclusion is that whatever you save (if anything) by applying a
coating, it is always a percentage of what you are losing by conduction
through the wall, so the better insulator the wall, the less advantage.
And almost always it will be cheaper and easier to improve the wall's
insulation.


 >
The radiated heat from the body is around 100W according to Wikipedia.
The corresponding received radiation from an environment at freezing
would be about 60W.  This is independent of the air temperature.

On the other hand, if you are in a room with walls at 0°C filled with
warm air, there will be a considerable convection current off the walls
which is what will make it feel uncomfortable.  (If the air is cold, you
get the same convection off your body.) Also I've noticed that it is
almost always colder inside an unheated bare concrete structure
(specifically a football stadium stand) than outside, because of thermal
inertia.


I didn't say I'd improved on anything. I didn't say it was a product. I
built and calibrated my own instrument for my own use because I couldn't
find one on the market here that would do what I wanted, such as take a
reading on the underside of a 3m high ceiling without damaging the
decoration.  It was made from spares from thermal instruments (eg
bed-quilt conductivity test) that I do build and sell commercially to
industry.  I'd be interested in any references you have though, if my
experience of industrial instrument is anything to go by, my device is
probably an order of magnitude cheaper.


Tim Jackson