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re: pipe insulation
23 mar 1996
william bahnfleth wrote:
>>>the trick to doing the experimental verification, however, is maintaining
>>>constant temperature at the inside surface of the insulation.
that depends on what one wants to determine, no? "is there a critical radius?"
being a different question from "what is the r-value of a wire?"
>>it seems to me that the temp throughout the wire would be fairly constant,
>>given that copper has a much higher thermal conductivity than teflon, etc.
>
>>>it might be better to do this with a copper pipes...
>
>>sounds more complicated... i could see someone doing the first experiment
>>in about 10 minutes, with a 20' piece of that wire, 10' of it stripped,
>>strung across a room in a w, with a constant current going thru the whole
>>wire and a meter to measure the voltage across each 10' section.
>
>what you're describing is most likely a constant heat flux condition at the
>inside surface of the insulation rather than the constant inner surface
>temperature condition, isn't it?
i don't think it's either...
>the volumetric heating due to current flow (it has to be the same
>throughout the entire length of the wire, neglecting changes in r
>due to temperature difference)
aha. no, i wouldn't neglect those changes at all. i would _use_ that change
in electrical resistance to measure the temperatures of the wire sections,
or to make the two sections have the same temperature (which may be more
difficult) by measuring the voltage across each wire and dividing by the
current (using "ohm's law for heatflow," with different units :-), and the
well-known fact that the electrical resistivity of copper wire is
rho(t) = 1.8 x 10^-8 (1+3.9 x 10^-3(t-20)) ohm-m, where t is in degrees c.
for instance awg #30 wire has a diameter of 10 mils, (0.01"), so in si units,
the diameter is 2.54 x 10^-4 meters and the cross-sectional area is
5.07 x 10^-8 m^2, and the resistivity at 100 c is
rho(100) = 1.8 x 10^-8 (1+3.9 x 10^-3(100-20)) = 2.36 x 10^-8 ohm-m,
so a 30 gauge 1 ohm wire at 100 c would have a length
l = r x a/rho(100) = 1.0 x 5.07 x 10^-8/2.36 x 10^-8 = 2.15 meters.
the resistance of this wire at a 20 c room temp would be
r(20) = r(100) x rho(20)/rho(100) = 1 x 1.8/2.36 = 0.763 ohms.
ees often measure the temperature of internal copper windings of motors and
transformers by simply measuring their resistance or change in resistance.
>...the one that's insulated would change in temperature to accomodate
>the change in overall resistance.
i guess the insulated wire would have a lower temperature in this case,
in series with the dull bare wire, which would make for a lower electrical
resistance and a lower electrical power dissipation, so it seems the effect
we are looking to detect would be enhanced in this sort of experiment.
>you can determine whether the resistance drops when insulation is added by
>doing this experiment, but you still need to measure the surface temperature
>of both the covered and uncovered sections of wire.
it seems to me that that would be a more complicated and more difficult to
control and less sensitive and different experiment...
nick (speaking ex-cathedra on matters electrical, for once :-)
nicholson l. pine system design and consulting
pine associates, ltd. (610) 489-0545
821 collegeville road fax: (610) 489-7057
collegeville, pa 19426 email: nick@ece.vill.edu
microprocessor hardware, memory, asic, and computer design. telecommunication
system design. computer simulation and modeling. high performance, low cost,
residential solar heating and cogeneration system design. bsee, msee. senior
member, ieee. registered us patent agent. fluent in french.
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