Nichrome Wire Heating Element for Tube Reactor

[gpsimms]

Hey all,

First post, thanks in advance for any help. I see there is a lot of discussion on this site and others about nichrome heating elements. I apologize if my question is very similar to others, but I have not exactly been able to figure out what I need to learn/lookup to answer my questions from the other posts I have read. I'd appreciate anyone pointing me in the right direction.

OK, so I am a new graduate student working in a combustion lab. We want to preheat some fuel in a length of thin quartz tube, before it mixes with hot air. The tube has ID 1mm and OD 1/8". We want to heat the fuel (dodecane in some cases, but other gas fuels as well in other cases) at various pressures. For now, we are running atmospheric pressure but have plans to go to high pressure later.

With our mdot/cp and desired Temperature change (700K-300K) I have calculated the desired power to be ~13W.

The design is to be as follows: Wrap nichrome wire around a few inches of tubing. This would be best to be as short a length as possible. Use ceramic paper around the nichrome so it binds when it heats and it prevents shorts and provides an insulator.

My question is how do I exactly figure the length/thickness of the wire I need? I have been to http://www.cecs.wright.edu/balloon/images/2/22/Nichrome_Wire_Heating_Element_Design_Basics.pdf and a few websites like it.

Here are a few problems I am having with answering my question:

The link above says I need about 15 amps with 14AWG to get to the desired wire temp. It says to cut that amount in half for coiled wire, so 7.5 amps. Doesn't the length matter in that calculation? It seems like it should. Also, I should note we have thermocouples and temp controls. Does this mean I should just use any wire that can go above the desired temp and if connected to thermocouple/temp control it will be safe?

Sorry if these are really stupid questions. The problem is my background is only in math. I've never taken an engineering course before this year so I get overwhelmed a bit with these "real world" problems.

Thanks so much!

simms

 

[Carl Pugh]

Most of the charts on the www are wrong for what you are trying to do.
Get a chart that gives the wire size (for nichrome wire) and the reistance per foot (or meter or centimeter).
Select a wire size say AWG #20. Calculate the length need for 13 watt. (13 watt should include heat loss + heat required to heat the fuel to desired temperature + some safety factor)
Do a layout to make sure wire turns are not touching.
Go to the chart you had previously and make sure nichrome wire doesn't burn out in air. If nichrome wire burns out in air, go to a larger diameter wire.

There used to be some sort of ceramic(?) paste that was thinned with water that could be used to hold the nichrome wire in place and provide better thermal conductivity between the nichrome wire and the ceramic tube. The paste might be easier to use than ceramic paper.

 

[gpsimms]

Hey Carl,

Thanks so much for the input. I am definitely looking into the paste idea. You don't have to check my work here, but I just thought I'd let you know what I am looking at and pick your brain on one or two more things.

OK, so with AWG 20 and 115V, I need a very long length of wire to get the desired resistance. The solution, then, is to choose the length and thickness I want, and then reduce the voltage, right? So with the short length I want to work with (about 2 inch length of tube, wrapped in ~8 turns of thin wire), I find that I only need about 1.5 Volts.

What I plan to do is fold the wire over itself for the leading ends to reduce the resistance in those areas and then connect them to the power supply. Using the chart from this site (http://hotwirefoamcutterinfo.com/_NiChromeData.html), I use the Amps I calculated (8.84) to see that the wire could reach a temp of up to 900K.

I'm pretty sure that's a good result, because it's above the temp we'd like to heat things to, but also not so high that burnout will be an issue, since it will be in good contact with a tube of running fluid and insulated on the other end. Also, like I mentioned we have temperature controllers.

Obviously, I'll run this by other folks in the lab, but I feel like none of us are electricity experts, and nobody has built a heater before. Is there anything you want to add so I don't blow us all up? : ) Thanks again for your help!

simms

 

[Carl Pugh]

Looks like you are using the correct approach; however I would like to make the following comments.

The 900k is for the wire under specific conditions. The actual average wire temperature under the conditions you are operating at will probably be quite a lot lower.

It is usually easier to go to a lower temperature, so maybe instead of 13 watt, design for 100 watt. (100 watt is not that much; look at a 100 watt light bulb)

The easiest way to test the heating element is probably using:
A variable transformer, such as Staco 3PN1010B available from Digi-Key
A stepdown transformer such as Signal Transformer A41-175-12 available from Digi-Key
Don't worry about trying to control the temperature during preliminary tests.

 

[gpsimms]

Thanks again Carl. I think I am good for now. I'll try to post an update, but you know how things can move slowly in lab.

simms