# Heating a tube of aluminium electrically

I need to regulate the temperature of the surface of a tube of aluminium. The tube is used as a mandrel for composite molding/curing, so the target temperature is below 100C.

Currently, there is no type of heating element attached to the tube. I have done a little bit of research, and these are some of the possibilities I found:

1. Peltier module. Its hot side would be glued with thermal grease to some areas of the tube, the cold side would be brought to room temperature with a heatsink + fan.
2. Resistive heating element. Nichrome wire would be wrapped around the tube and heat it by thermal conductivity. However, it seems to be challenging to electrically isolate the wire from the aluminium tube.
3. Joule-heating the tube. Voltage between the extremities of the tube would heat it by Joule effect. The problem is the current requirement is huge, since the tube resistance is low.
4. Hot air. It is simple to setup up with a fan and heating elements, but the heating would not be uniform due to convection currents.

• The Peltier module is widely used to cooling applications, do they have a down-side when used the opposite way, to heat things up?
• Could the nichrome wire be isolated electrically but not thermically easily?
• Would it be easier to switch to a steel tube and heat it by induction? By coiling the tube and applying AC current?
• can't you have a thin tube, fill it with oil and sink a heating resistor + temperatue sensor into the oil? – user287001 Aug 14 at 0:35
• How uniform do you need the heating to be? – Ale..chenski Aug 14 at 1:08
• RF induction with eddy current heating is possible – Tony EE rocketscientist Aug 14 at 1:25
• as a mandrel: so the work contacts the outside and you want to heat it from inside? – Jasen Aug 14 at 1:34
• A thin walled stainless steel tube could possibly be heated by Joule heating (in other words, resistive heating where the tube is the resistor). You might still need pretty high current, but it will certainly work better than schedule 40 aluminum pipe. – mkeith Aug 14 at 5:43

If you need temperature uniformity along the tube surface, your most reasonable choice is #2 - wirewound resistive heater. Peltier cooler is absolutely out of question, mostly due to their standard geometry is not tubular. Hot air will have entry and exit points and there will be always an inherent temperature gradient.

In case of resistive heater, the insulation is not a problem in this temperature range, even any kapton tape will do the job. Temperature sensor could be the wire itself, if proper material is used.

Direct tube heating is also doable.

ADDENDUM: I was under a false assumption that the material to be heated is inside the tube. It appears that the hot tube (1000 mm x 70mm dia.) is used to form and cure some plastic sheeting outside the tube. In this case the tube is better to be heated from inside with any shape of resistive wires or other electric heat elements, and maybe filled by sand to provide an even distribution of heat. The details will depend on expected heat losses across the external surface. Obviously a layer of wires over the external tube surface will be prone to mechanical wear, and the tube surface would be a good protector if the heater elements are placed inside.

• The resistive element wound around the tube seems to be a good idea, but it seems to me that using the wire itself as a temperature sensor may result in tube underheating due to the insulation barrier. WDYT? – gstorto Aug 14 at 7:26
• @gstorto, see, I don't know what is cooking inside your tube, is there flow-through of something or not, but if you encapsulate the whole thing into thick low-density Styrofoam insulation, the coil will reflect the temperature of equilibrium pretty well. If you have a heat flux from inside your tube, then all bets are off in any case, and thermal sensor(s) must be placed wherever you need your temperature defined/controlled. – Ale..chenski Aug 14 at 7:34
• Not a bad idea. The material to be heated is actually laying on the outside of the tube: sheets of carbon fiber with epoxy. The objective is to heat both to the recommended temperature to cure the epoxy, which is not that precise anyway. – gstorto Aug 14 at 7:38
• @gstorto, how would this work? If you neat the tube from the outside, will you wrap the work piece around the outside of the heater? Maybe you should reconsider. It seems like circulating hot oil through the tube might work better. Just heat the oil to the desired mandrel temperature and pump it through the tube. – mkeith Aug 14 at 15:45
• "The material to be heated is actually laying on the outside of the tube" Don't you think this information belongs to the question? How on earth were you going to heat it by "nichrome wire wrapped around the tube"?! – Maple Aug 15 at 17:39

Because your tube temperature is expected to be less than 100 C, resistive heating should be quite simple. Use a single wrap of something like Sil-pad silicone sheet around the tube, then wind your resistive coil. Then another layer of Silpad, and finally a layer of thermal insulation. You can improve performance by going with heater ribbon rather than standard wire, since this will increase contact surface area.

The inner Sil-pad layer will do two things. First, it will electrically insulate the windings from each other and from the tube. Secondly, it will improve thermal coupling from the wire to the tube. The outer layer will serve to help immobilize the windings, and the layer of insulation will increase heater efficiency.

This approach won't work for temps much above 100 C, since the maximum temp for the stuff is about 180 C. YMMV, of course, since you haven't specified the heat flux you need into the tube.

• To the OP: This is a much better plan than my comment. But you do still need to ground the aluminum tube unless you energize the heating element from 12V or something. – mkeith Aug 14 at 4:20
• It seems to be a good idea. Any problem using Kapton tape instead of the Sil-Pad silicone between aluminium and wire? It seems to be cheaper and easier to find. – gstorto Aug 14 at 7:28
• @gstorto - You can use something like Kapton, but it will not conform to the windings, so it will not have as good thermal transfer from the wire to the tube. Also, (again since it will not conform to the wire) it will not immobilize the windings against lateral movement, and you run the risk that at some point the windings will shift enough to start shorting windings. You could always do something like apply a thin layer of RTV over the windings, but you MUST use electical-grade (non-acid) RTV, and that stuff is expensive. – WhatRoughBeast Aug 14 at 13:47

I'd suggest hot air as safest. I do not see why it could not be made uniform, unless your tube is longer than a meter or so. Aluminum has extremely good thermal conductivity, you'd be pressed hard to create local hot spot, actually.

Put your tube inside another one with bigger diameter and send hot air through tangent nozzle to create vortex in the space between the tubes.

UPDATE

Have your thought about using commercial tape heater or drum heater? This is exactly an application they are made for. All the safety/isolation issues already taken care of and the manufacturers also sell control boxes for them.

• I think your idea would work, but a little bit complex for this application. Encasing the smaller tube inside of another is not that simple, and if the fan fails, the heating wires would become a fire hazard. FYI the mandrel can be 1m~1.5m long. – gstorto Aug 14 at 7:32
• OK. See an update. – Maple Aug 14 at 8:59
• How is hot air safest? With air, you get a pretty bad coupling, so the air needs to be substantially hotter than you want the tube. That's not only inefficient, it also poses the risk of melting the insulation of something else nearby and possibly causing a short. – leftaroundabout Aug 14 at 10:43
• @leftaroundabout If your insulation is melting at 100 deg C you need to change it ASAP. And high thermal conductivity of aluminum means the air does not need to be "substantially" hotter. In fact, with right air flow it can be almost exactly the same as required temperature. – Maple Aug 14 at 10:57
• @gstorto Any heating method can become a fire hazard if you do not include safety features. But if you think it is too complex then consider commercial heating tapes I mentioned in update. Those can be bought with controllers that do have safety features. All you need is wrap your tube and set required temperature. – Maple Aug 14 at 11:02

A tube can be heated by putting a hot wire down its axis; long quartz lamps are used thus in laser printers to heat the fuser rollers. This works best if the inner tube surface is darkened.

1. Peltier is relatively expensive and complicated to implement. It requires current limiting. The longevity can be less than ideal.

2. A resistive heating wire seems to be the most straightforward option by far. It may be the simplest type of electrical component there is. Yes, it can be electrically isolated relatively easily. And if you thermally insulate it from the surroundings (also easy), the thermal conductivity between the wire and the tube becomes a bit less critical.

3. Induction again is more complicated than a resistive wire. I suppose you would need to design or find a suitable AC driver, and design and make an induction coil. Not necessarily trivial.

I am assuming that quick heat-up time is not of the essence, but that you probably want some degree of temperature control.

With a heating wire, this can be done as simple as with a thermostat to switch the heating wire on and off. If you want to upgrade later, it would also not be very complicated: You could for example replace the thermostat with a temperature sensor connected to a PID controller that outputs slow PWM to switch the heating wire on and off with a MOSFET. In other words, you can make is as simple or sophisticated as you want.

• In case someone comes here looking for information about peltiers, I'm just going to put here what they ARE good for in terms of heating, although I agree they don't suit this application. In cases where there is a heat source available by default, they can operate at >100% electrical efficiency due to their ability to transfer existing waste heat, with the "heating" the module itself does only being its resistive losses. – K H Aug 14 at 2:01
• I agree. I am planning to use a microcontroller to PID control the MOSFET and gather temperature data. – gstorto Aug 14 at 7:34

Inductive heating only needs conductance. You can certainly heat it inductively. You have to find a frequency that is high enough but not too high. Somewhere around a couple kHz I would assume. But you haven't said anything about the thickness, which would affect the answer (Assuming a helix coil through the tube, or outside the tube.)

Heating aluminium by wrapping it in a heater is a little bit tricky to do precisely and efficiently - aluminium has a massive reflectivity, which means that anywhere there is not perfect contact between the heater and the tube, radiation will not transfer much power - so you can get cold spots. Thermal paste would help a lot.

Have you considered heating it with steam? I mean, 100 degrees - you could have a fairly simple setup to just condense steam uniformly on the tube - or in the tube.

• Don't forget that $\mathsf{ Al + 3H_2O(steam) \rightarrow Al(OH)_3 + 3H_2 }$. – Andrew Morton Aug 14 at 10:15
• @AndrewMorton You probably mean: $$\mathsf{ Al + 3H_2O(steam) \rightarrow Al_2O_3 + 3H_2 }$$ ? That only happens - IFF the aluminium tube has never ever been exposed to air. Which I think it has, either at the foundry, the manufacturer or OP. And if it has, not a thing will happen, the alumina already covering it is passivating. – Stian Yttervik Aug 14 at 12:07
• The outside layer is already oxidized if it's been in contact with air – laptop2d Aug 15 at 15:42

Assuming it's a mandrel, the outside is what counts. Turn it from solid aluminum rod. Drill two holes lengthwise.

Insert a standard cartridge heater in one hole. Buy a spare. Heaters burn out eventually, even at 100°C.

Insert a standard thermocouple or RTD in the other hole. Preferably both holes are blind. Spring-loaded with a bayonet adapter is best for the temperature sensor and follow the manufacturer's recommendations for minimum hole depth.

Attach a standard temperature controller (1/16 DIN or 1/8 DIN) with a suitable SSR and mains power.

Set the setpoint to 100°C (or whatever you need) and start the auto-tune procedure and wait until it completes.

Fin.

Wrap some insulated heating wire round the pipe. This stuff is used in the manufacture of electric blankets and also to heat (plant) nursery beds.

For example, Heat Cable and Resistance Wire

UPDATE

For some reason I was thinking the tube was square tubular aluminum that would have a flat surface. I'm leaving the answer becasue I believe LEDs are a very good and efficient heat source. Not so much without a flat surface to mount to.

END OF UPDATE

Use a strip of inefficient high power green LEDs with a Tj of 150°C. Green becasue they are the least efficient when measured in radiometric units rather than luminous. Lower efficiency means more of the electrical power is dissipated as heat than light.

Would work best if the LEDs were enclosed or covered.

Conductive heat transfer would be very efficient.

If kept under 110°C LEDs will last a very long time. They will lose luminous intensity but that will improve the heat generating efficiency.

You could use CoB LEDs and mount them directly to the aluminum. Very easy to mount and wire. Depending on the thickness (thicker is better) of the aluminum the heat will likely spread fairly evenly.

CoBs are a very inexpensive heat source. I use them a lot for a heat source in thermal experiments.

Use an LED driver with thermal foldback to set the temperature.

Easy solution for insulating a resistive wire electrically - fiberglass sleeves for wire, that would work a treat. Also glass is a good conductor of heat so it wouldn't thermally insulate the NiChrome wire if you used that (thats why we use it in our kitchens). Also filling the tube with a non-conductive oil that has a high ignition temperature would be a good idea too.

• This is very wrong: "glass is a good conductor of heat". That's like saying glass is a good conductor of electricity. – Misunderstood Aug 15 at 15:09
• Glass has a thermal conductivity of 1W/mK which is the best of the non-conductive materials that won't cost a fortune. Air is at best 0.04W/mK, its all relative. I didn't compare glass to the thermal conductivity of diamond, silver or copper, I compared it to things like silicone and other high heat insulating materials like ceramics for this exact application. Also I will add that fiberglass has a thermal conductivity of 0.04W/mK so exactly the same as air, except it conducts no electricity WOW! – SvenIronhand Aug 30 at 12:30
• It is subjective and relative. If you think 1W/mK is a "good" conductor of heat go with it. You did not compare glass to anything. BTW fiberglass and air both conduct electricity, but I would not say they are good conductors. – Misunderstood Aug 30 at 17:38