Heating up glass with Resistance wire

The best wire I know of for heating things is called nichrome wire. I think it is a nickle chrome alloy It is used to ignite rocket motors. Anything else probably too expensive.

250x300x4mm about 10° Celsius above room temperature. I've made some plans on how to do this, however I need some more guidance, especially choosing and obtaining correct resistance wire.

Electricity will be supplied @ 12V DC @ 1A, or 5V DC @ 500mA if possible.

See this diagram:

for proposed/designed layout.

As pictured in the diagram - 240mm of wire every 10mm for 290mm =

240x29 + 10x29 = ~7,300mm. That's 7.3m of resistance wire. Assuming room temperature is 20°, the wire will temperature will need to be ~35°, to keep the glass at just under 30°.

The wire will be attached to one side of the glass surface using self-adhesive clear plastic wrapper (similar to those used to wrap books).

1. Are these calculations/assumptions correct?
2. What kind of wire (thickness) should I be looking for?
3. What will my power consumption be (will 12V @ 1A be enough)?

I'll be very thankful for any advice on this matter.

Ummmm. Maybe you could just use a ready-made "heat tape" which is made to wrap around water pipes to keep them from freezing? Or a waterbed heater?

250x300x4mm about 10° Celsius above room temperature. I've made some plans on how to do this, however I need some more guidance, especially choosing and obtaining correct resistance wire.

Electricity will be supplied @ 12V DC @ 1A, or 5V DC @ 500mA if possible.

See this diagram:

for proposed/designed layout.

As pictured in the diagram - 240mm of wire every 10mm for 290mm =

240x29 + 10x29 = ~7,300mm. That's 7.3m of resistance wire. Assuming room temperature is 20°, the wire will temperature will need to be ~35°, to keep the glass at just under 30°.

The wire will be attached to one side of the glass surface using self-adhesive clear plastic wrapper (similar to those used to wrap books).

1. Are these calculations/assumptions correct?
2. What kind of wire (thickness) should I be looking for?
3. What will my power consumption be (will 12V @ 1A be enough)?

I'll be very thankful for any advice on this matter.

snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@g14g2000cwa.googlegroups.com:

You could paint or silkscreen a grid on the glass like an automotive rear- window defroster,using copper or carbon conductive(resistive) paint. You will get better thermal tranfer. The paint comes in different resistivities.(ohms/sq.cm)

Attaching hookup wires will probably have to be done with silver conductive epoxy.

--
Jim Yanik
jyanik
at
kua.net

Use nichrome wire ripped from an old toaster. If you want 1 amp at 12 volts, cut a piece that will give you 12 ohms. The hard part is connecting to it. You will have to weld it it. If you try to solder, it may get hot enough to melt the solder under normal operation. You might get away with crimping to it. Good luck!

Al

Crimping or clamping is the usual method. Used wire is often brittle and new wire isn't really that expensive.

Here's a great source with much info to answer your heating questions.

--
Jack

Plonked by Native American

bobo1148atxmissiondotcom


	http://photos.yahoo.com/bc/xmissionbobo/

The main problem will be knowing how much power you need to keep the glass at the right temperature. To predict this by calculation you need to know a lot more details most of which you probably won't know.

Two questions:

1) How well insulated is the set up? eg What is the thermal resistance between the glass and where you measure "room temperature"? If you knew this the sums would be simple but my guess it you won't know this. If you build it and find it doesn't get hot enough then measure how hot it does get and the power and you can work it out.

2) How fast do you want the temperature to rise? Does it matter how long it takes to go from room temp to room temp +10C ? If you know the specific heat of glass and the mass of the glass etc you could estimate how much power you need to achieve the warm up time you need.

The only easy answer is to build something, test it and then have another go if it doesn't perform quite right.

My gut feeling is that 12 W might be the right order of magnitude but could be on the low side.

I'm away from my PC for the next week.

250x300x4mm about 10° Celsius above room temperature. I've made some

..... As pictured in the diagram - 240mm of wire every 10mm for 290mm =

240x29 + 10x29 = ~7,300mm. That's 7.3m of resistance wire. Assuming room temperature is 20°, the wire will temperature will need to be ~35°, to keep the glass at just under 30°.

The wire will be attached to one side of the glass surface using self-adhesive clear plastic wrapper (similar to those used to wrap books).

1. Are these calculations/assumptions correct?
2. What kind of wire (thickness) should I be looking for?
3. What will my power consumption be (will 12V @ 1A be enough)?

The general idea seems ok, but I wonder about heat losses and acceptable temperature variation. If the losses are significant, than the wire spacing of 10mm for 4mm thick glass sounds a bit high - glass is not a good conductor of heat. One approach might be to use a piece of printed circuit board (unetched), and place a continuous copper plane next to the glass, with heating elements attached to the other side (nichrome wire is good). The thin copper sheet will spread the heat to give a more uniform temperature. Based on experience with heaters for telescopes, 12W will not be enough unless the losses are absolutely minimal.

Dave

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Your main problem will be control. If the wire gets to 35C, and you have it well insulated, then unless there is a fixed heat flow into the tube (are the tube contents moving?), then the tube will also eventually get to 35C. Why the need to only use 12V? I would have wrapped a lab heating tape round the tube, with a K thermocouple underneath, and connected to a small PID controller - just dial the temp you want - then if the tube is too hot or too cold then change the tape temperature accordingly.

As an aside to another poster, nichrome wire can be successfully crimped. I once made a 1 metre x 100mm gas phase reactor (in glass), with 8 x 250V circuits of nichrome wire, running at 4 amps each, generating some 8000W. Glass fibre tape was wrapped round the tube, and then the wire was laid, with a glass fibre tape laid over the top. Centre of packing reached 350°C, and you could feel the heat at least 10 feet back... Somehow I feel that our current laws would not allow this anymore... [but it did work, and made lots of product]

--
-- 
Ron Jones

Don\'t repeat history, see unreported near misses in chemical lab/plant
at http://www.crhf.org.uk
Only two things are certain: The universe and human stupidity; and I\'m
not certain about the universe. ~ Albert Einstein

"Dave" wrote in news:1122738906 snipped-for-privacy@spool6-east.superfeed.net:

Why try to bond wire to glass?

Paint or silkscreen resistive paint in a grid or other pattern like a auto's rear window defroster. You get better thermal transfer,it's far simpler. Bond connnecting wires with silver conductive epoxy.

--
Jim Yanik
jyanik
at
kua.net

Had you thought of buying one of those do-it-yourself kits they sell for car window demisting? Included the wire/tape etc and instructions

David

snipped-for-privacy@gmail.com wrote:

Hard to say on the calculations, It may be easier to cut and try. You might also find Nichrome tape easier to use. For instance, I have some Nichrome 5 tape, 1.6 x .22 mm that is 3 Ohms per meter. Joining it is a mission, and spot welding is the best if you have access to a suitable machine. Failing that, crimping with a copper sleeve might be the best.

Alternatively, a long time ago I saw a similar application that used a fine serpentine track etched into a piece of FR4 PCB. That may be too expensive for a one-off. You could do a fabricated version of it by using a sheet underlay of tin-plated steel, or copper, with several resistors attracted. Hook them up in series/parallel as required to get the temp up.

Barry Lennox

You don't need NiCr wire for this application as your temps are very low. #37 AWG enameled magnet wire has a resistance of 0.5 ohms/foot and it has a handy insulating coat (NiCr is bare)to reduce shorting during layout and it can be soldered!

24 feet @ 0.5 ohms/foot = 12 ohms which is the resistance you will need to take advantage of the 12v @ 1A available for 12 watts heating. Radiant loss from the glass surface (1 side) assuming you will insulate the bottom is 5.67e-8*0.075m2*303.15K = 36J/s ambiant radiation absorbtion @ 20C = 5.67e-8*0.075m2*293.15k = 31.5J/s total radiation loss with these parimeters is 4.5 watts at 30 C I am sure the 7.5 watts left over will be more than enough to compensate for convection loss, depending on what you are trying to heat of course. I would also recomend a coat of silicone rubber over the heating wires to provide a thermal path as well as keep everything in place. Also cover the bottom with foam insulation to reduce the thermal sink to one surface. A temp control can be implimented using a string of 4 or 5 1n914 diodes in series located at intervals across the heating side of the glass. Diodes have a nearly linear temp relationship of 1mv/K.Using 5 averages the temperature deviations across the glass and gives 5 mv/C output. This signal can be fed into an lm311 noninverting input with the inverting input to a reference pot for temp setpoint. The output will feed a power pass transistor through a 2k2 resistor as an example. The high thermal mass and low power input should give reasonably stable temperature control.

You don't need NiCr wire for this application as your temps are very low. #37 AWG enameled magnet wire has a resistance of 0.5 ohms/foot and it has a handy insulating coat (NiCr is bare)to reduce shorting during layout and it can be soldered!

24 feet @ 0.5 ohms/foot = 12 ohms which is the resistance you will need

to take advantage of the 12v @ 1A available for 12 watts heating. Radiant loss from the glass surface (1 side) assuming you will insulate

the bottom is

5.67e-8*0.075m2*303.15K^4 = 36J/s ambiant radiation absorbtion @ 20C = 5.67e-8*0.075m2*293.15k^4 = 31.5J/s total radiation loss with these parimeters is 4.5 watts at 30 C I am sure the 7.5 watts left over will be more than enough to compensate for convection loss, depending on what you are trying to heat of course. I would also recomend a coat of silicone rubber over the heating wires to provide a thermal path as well as keep everything in place. Also cover the bottom with foam insulation to reduce the thermal sink to one surface. A temp control can be implimented using a string of 4 or 5 1n914 diodes

in series located at intervals across the heating side of the glass. Diodes have a nearly linear temp relationship of 1mv/K.Using 5 averages

the temperature deviations across the glass and gives 5 mv/C output. This signal can be fed into an lm311 noninverting input with the inverting input to a reference pot for temp setpoint. The output will feed a power pass transistor through a 2k2 resistor as an example. The high thermal mass and low power input should give reasonably stable

temperature control.

5V @ 500 mA? That's not a whole lot of power (2.5W). You know what, I wonder if the step-down transformer alone will generate more heat than that. Insulated box, step-down transformer and glass enclosed, extension cord (plus fuse) to the wall outlet (fuse in case of a leak or short of some kind).

Hey, why not just use a small light bulb for the heat, if it's going to be plugged in anyway.

An even better (safer) idea:

1. Get a Celeron 2.7 GHz computer
2. Overclock it to 3.2 GHz. You might need to swap out the motherboard; I recommend an Asus P4S800-MX.

2b. Install an AGP video card

1. Install a cooling fan in the case (; most computers don't have a cooling fan on the case, *ahem*, hp a305w, *ahem*)
2. Run a program that makes the computer sweat (Prime95, Unreal Tournament 2004, RSA Factoring Challenge code)
3. You now have a nice source of heat. In fact, I did all of the above (cooling fan installed last night), and the room (the ROOM!) increased in temperature by about 5 degrees F above ambient (er, other rooms in the house).