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Topic: Heater question (16 msgs / 356 lines)
1) From: Alchemist John
OK, you electrical types :-)
I found an inexpensive heater coil (nichrome cast element) rated 
220v, 1000 watts.  If I use it for on 110 v, is that just simply 500 
watts I get?
John Nanci
AlChemist at large
Zen Roasting , Blending & Espresso pulling by Gestalthttp://www.chocolatealchemy.com/

2) From: Michael Wascher
No, you'll get about 250 Watts.
P = V * I = I^2 * R = V^2 / R
Nichrome is non-linear, which is why I put the about in the first line.
On 7/25/06, Alchemist John  wrote:
<Snip>
-- 
"There is nothing new under the sun but there are lots of old things we
don't know." --  Ambrose Bierce

3) From: David B. Westebbe
John - 
This site has a bunch of simple explanations and SAS-driven calculators for
figuring out stuff having to do with Nichrome wire heaters:http://www.nicrome.com/<Snip>

4) From: M. McCandless
Not likely.
The heat to voltage ratio is non-linear.
I've surfed the web for info, but it's a little scarce,
at least for specific types of lamp.
McSparky
At 05:26 AM 7/25/2006 -0700, you wrote:
<Snip>

5) From: Michael Wascher
Based on the RW80 heater alloy tables, resistance nonlinearity is
1:1.08over 20:1200 C range. Now, I don't know what temperatures the
heating
element will be at for either 220 V or 110 V applied, other than it is safe
to say that it will be colder for the 110 V case. So let's just take the
entire range to get an idea of the effect. Some back of the envelope
scribbles indicate that a 1:1.08 resistance variation would translate into
less than 20 Watts.
So at 110 Volts applied you'd get more than the 250 Watts out but less than
270 Watts.
On 7/25/06, David B. Westebbe  wrote:
<Snip>
-- 
"There is nothing new under the sun but there are lots of old things we
don't know." --  Ambrose Bierce

6) From: Alchemist John
Many thanks folks.  Good deal at 1/2 power, not 1/4 :-)
At 19:34 7/25/2006, you wrote:
<Snip>
John Nanci
AlChemist at large
Zen Roasting , Blending & Espresso pulling by Gestalthttp://www.chocolatealchemy.com/

7) From: David B. Westebbe
This is a multi-part message in MIME format.
It is impossible to know the temperature, I think, because the volume =
and
mass which will be heated will have a direct impact on the temp.  All =
you
can know, maybe, is the BTU output.
 
As an example, I have a variable fan speed on my popper.  The faster the
fan, the lower the air temp  But the temp of the nichrome is not =
directly
affected.

8) From: Justin Marquez
On 7/26/06, David B. Westebbe  wrote:
<Snip>
The heat output (wattage consumed) of the nichrome may only be
minimally affected, but the surface temp of the nichrome element
certainly is affected by the air flow.  Carry it to the limit point -
NO AIR FLOW - and the element overheats and burns out.
Safe Journeys and Sweet Music
Justin Marquez (Snyder, TX)

9) From: Michael Wascher
Not impossible, but not trivial. You end up with a thermodynamic model
that's coupled with an electrical model. In days past you'd be solving
simultaneous partial differential equations. Today you'd have modeling
software iterating a solution on a computer.
However, using the max temp range that the site provided data for, which I
presume is the usable temperature range of the material, there's only 20
Watts of change. So using Ohm's law & assuming linear resistance is a good
enough for most applications.
On 7/26/06, David B. Westebbe  wrote:
<Snip>
-- 
"There is nothing new under the sun but there are lots of old things we
don't know." --  Ambrose Bierce

10) From: David B. Westebbe
<Snip>
Good point.  This makes my previous point, that one might be able to =
easily
calculate the heat output, but one would have a much harder time =
calculating
the temperature of the system.

11) From: David B. Westebbe
This is a multi-part message in MIME format.
However, using the max temp range that the site provided data for, which =
I
presume is the usable temperature range of the material, there's only 20
Watts of change. So using Ohm's law & assuming linear resistance is a =
good
enough for most applications.
 Can you expand on that?  I'm thinking of swapping out the little =
Nichrome
wire in my 'Pumper for one that will produce more heat (in the stock =
popper,
there are 2 different nichrome coils - a big beefy one and a little =
wimpy
one).  What I don't know is (a) will it make lots more heat if I use a
different hunk of coiled Nichrome (resulting in a higher temp for any =
given
airflow) and (b), will I draw so much current that the circuit breaker =
will
trip? 
 
 

12) From: Douglas Strait
This is a multi-part message in MIME format.
MessageDavid,
Don't do it. That little wimpy coil limits voltage to the fan which has =
a low voltage motor. If you replace it with a heavier [thicker] gauge =
the fan motor will be overpowered. If you want more heat, just shorten =
the existing beefy one a bit. A 10% reduction in length will give you =
roughly a 10% increase in power and will draw roughly 10% more amperage =
which will still be within limits for even a 15A circuit. Most modern =
households now have 20 amp breakers supplying wall outlets.
Doug

13) From: raymanowen
I like your analysis, Michael. Is it so that "it will be colder for the 110
V case" ?
No more so, I think, than you could say "A 480 volt motor is always higher
powered than a 240 volt motor." OK, a 480 on 240 would be...
If you are saying that a 220 volt (or higher) heater operating on the rated
supply at a 100% duty cycle will be hotter than feeding it 110 volts, I
certainly agree. 50% voltage will give 25% power in a resistive load-
tungsten, nichrome or carbon.
According to a Mozilla reference
1 kilowatt-hour (kWh) = 3413 British Thermal Units (Btu)
So, 1 watt-hour          = 3.413 BTU = 1kW for 3.6 seconds
Lower power = Lower heat,
But a particular heater may be designed for lower voltage and higher current
for the same power, same heat capacity.
Cheers -RayO, aka Opa!
-- 
"When the theme hits the bass, I dance the Jig!" - -Virgil Fox at the Mighty
Wichita (ex- NYC Paramount) WurliTzer- 1976

14) From: Steve Hay
Its not all that bad, is it?  As long as you have symmetry, it becomes a
problem in 1-D and therefore an ODE.  If I remember my thermo, you should be
able to come to an estimation using some algebraic methods.  Wish I
remembered those equations.  I'll blame a lack of TeX support in Google
instead.
Damn Google and their lack of TeX support.
On 7/26/06, Michael Wascher  wrote:
<Snip>
-- 
Steven Hay
hay.steve -AT- gmail.com
Barry Paradox: Consider k to be the greatest element of the set of natural
numbers whose description require maximum of 50 words: "(k+1) is a natural
number which requires more than 50 words to describe it."

15) From: Michael Wascher
As the temperature of the heater changes the resistance changes. That means
the current changes (voltage is fixed in this case). Current change causes
power dissipation to change. This causes a change in temperature. Back to
the top.
So you iterate as above, or you write the equations. Your electrical
equations have resistance that is a function of temperature, and heat into
the thermodynamic equations that are a function of electrical power, and
everything is also a function of time.
But, changes in the nichrome resistance are reallly quite small. Smaller
than I had thought they were. So the quick calculations are close enough for
most applications.
On 7/26/06, Steve Hay  wrote:
<Snip>
-- 
"There is nothing new under the sun but there are lots of old things we
don't know." --  Ambrose Bierce

16) From: David B. Westebbe
This is a multi-part message in MIME format.
Thanks for the advice, but I've got the motor covered already.  I use a
transformer fed by a triac to regulate motor speed.
 
Currently, the two existing coils are in parallel.


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