Heating a tube of aluminium electrically

Question

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.

I am looking to some answers about:

• 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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

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.

Answer 8

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

Answer 9

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 T_j 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.

Answer 10

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.