[Stoves] flames touching pot

Mon Oct 3 13:24:05 CDT 2011

Thanks for this info. Very informative and helpful in understanding what we
are actually measuring. 

I have a stack of ~12 black metal pipes in a furnace and want to get an
accurate measure of temperature in the range of 300 to 900 deg. The furnace
has a hot top and sides and vented in the back. Taking the temperature up in
steps and slowly it will not over shoot the readings. But the more tools for
measuring the more different readings I get.

The center pipe is fixed with holes inserting probes and pipe picked for
surface having an even color for IR measurements. 

The pipes are situated away (~2.5 cm)from the hot blocks and touch so heat
transfer from pipe to pipe should be good. 

Question: If everything in the oven is 450 deg C and stabilized and the
black surface has emissivity of 1, the reading should be 450 deg in the IR.
What would the reading be if the true emissivity is, say, 0.75?

If I set the IR to have emiss to 0.75 does that bring the reading to 450?
Should I set the IR emiss reading to give the correct reading based on a
probe in the pipe? I want to not have probes but want to take the readings
using IR once I find out the controller setting to give close the reading I
want. 

Thanks for any suggestions

Frank

Frank Shields
Control Laboratories, Inc.
42 Hangar Way
Watsonville, CA  95076
(831) 724-5422 tel
(831) 724-3188 fax
frank at compostlab.com
www.compostlab.com

-----Original Message-----
From: stoves-bounces at lists.bioenergylists.org
[mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
ajheggie at gmail.com
Sent: Saturday, September 24, 2011 4:12 AM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] flames touching pot

Although this was a reply to Frank that got held I'll insert it here 
because it seems relevant AJH:

From: 
Matthew Redmond <mredmond3 at gatech.edu>

2011-09-16 22:23

Frank,

I wanted to add something to this discussion that seems to have been left
out.  In the discussion of the emissivity of a painted surface, it is
important to remember the emissivity is not actually a constant for any
surface.  It actually varies as a function of wavelength. We just often
treat it as a constant (by averaging) to simplify energy balance 
equations.

As you probably know, low temperature objects emit thermal radiation in
primarily the IR range.  Very high temperature objects (i.e. the sun) emit
thermal radiation in primarily the visible light range but also in the IR
range as well.

Now, another simplification often made in radiation heat transfer is that
the absorptivity of a surface and the emissivity of a surface are 
identical.
 *This is true only if the wavelengths of absorbed and emitted radiation 
are
also identical*. That second part is something that we can easily forget.
 This is what causes the "greenhouse" effect and makes your car get really
hot when it is in the sun.  Glass is completely transparent to visible 
light
(i.e. sunlight), but is completely opague to IR light (i.e. thermal
radiation from the inside of your car trying to get out).  This effect
causes cars to get REALLY hot in the sun.

It is similar for painted surfaces.  For example black paint has an
emissivity=absorptivity=~1 for both IR and visible light.  This means that
it absorbs a lot of heat, but also can reject a lot of heat. White paint, 
on
the other hand, has an emissivity=absorptivity=0.1 in visible light, but 
an
emissivity=absorptivity=0.9 in IR wavelengths.  So when you leave a white
object in the sun, it only absorbs about 10% of the sunlight and still 
emits
thermal radiation just as effectively as the black paint. This is why 
white
objects left in the sun don't get that hot (this is the opposite extreme 
as
what happens with glass in cars).

Now, to answer your question about red paint....it all depends on the
wavelength of light that you're concerned about.  In the visible spectrum,
the emissivity=absorptivity of red paint is probably around 0.5 -0.7.  But
what you're asking about is the emissivity in the IR spectrum, because the
painted surface will stay relatively cool.  Most all paints (even white)
have emissivity=absorptivity=0.9 in the IR spectrum.

Generally.....

   - organic materials have emissivity=absorptivity=0.8 - 0.1
   - shiny materials have emissivy = absorptivity < 0.5
   - color, surface roughness, and temperature STRONGLY affect the
   emissivity of a surface.

If you want to know more about this topic, I would suggest the following 
web
page: http://en.wikipedia.org/wiki/Thermal_radiation

Also, if you think you will be doing a lot of heat transfer calculations
(conduction, convection, radiation, etc), the following book is 
SUPERB.  It
is very easy to understand, and is used to teach the introductory heat
transfer course at nearly every engineering school in the US:

$5 version (1980s):
http://www.amazon.com/gp/offer-listing/047142711X/ref=dp_olp_used?ie=UTF8&co
ndition=used
$30 version (2000s): http://product.half.ebay.com/_W0QQprZ1818827

Of course, schools normally use the newer editions, but those are much 
more
expensive.

I hope this helps,
Matthew Redmond

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