2

I am trying to find the most power efficient method for heating a metal tube in order to heat up the airflow within it. I came across the Minco mica heater but when I talked to another engineer they said it would be just as effective to simply wrap the cylinder in heater wire and then put ceramic tape over the top. Is there a good way to determine which route is better without buying and trying?

I am trying to maximize airflow through the tube, hence heating the tube itself, and not heating the air directly inside.

I am not stuck on a cylinder, necessarily, but from a mechanical standpoint, it seemed like the most effective shape to take advantage of the heat, but that was possibly a terrible assumption.

Secundus
  • 121
  • 5
  • Why would you want to cool the cylinder? – EM Fields Mar 04 '16 at 00:56
  • @EMFields Maybe there are typos and misheard lyrics (cool → coil). – Nick Alexeev Mar 04 '16 at 01:00
  • I think that it's a trade off between cost an convenience. What kind of material is the cylinder made of? – Nick Alexeev Mar 04 '16 at 01:00
  • From a thermodynamic standpoint, resistive heating is all the same. But if the object is to heat the air, then I believe blowing air past a hot coil or hot ceramic element will be better. Why heat the cylinder if your goal is to heat the air? But in the end, both the cylinder and the air will become warm. One other option is to wrap transformer wire around the cylinder and run high frequency current through it. This will cause inductive heating in the cylinder. – user57037 Mar 04 '16 at 01:45
  • Also, if efficiency in heating the airflow is important, be aware that, for a given cross-section area, a circular tube is the worst possible choice, as it has the lowest ratio of surface to volume. – WhatRoughBeast Mar 04 '16 at 04:37
  • Sorry @EMFields, that was a typo from my typing the question on my phone. Nick Alexeev was correct, it was supposed to say coil (fixed now). – Secundus Mar 04 '16 at 13:13
  • Summary: You have air flowing through a cylinder. You want to heat the cylinder to heat the air. You want to maximize the heat transfer from the cylinder to the air. Is that correct? – JRE Mar 04 '16 at 13:20
  • @mkeith, the reason I was heating the cylinder/tube rather than the air directly was for the sake of minimizing the impacts on airflow within the cylinder, particularly with respect to velocity. Additionally, the vapors within the tube are relatively corrosive, though that is something of a non-issue since it will affect the tube too, so I'm not saving much in that regard. – Secundus Mar 04 '16 at 13:21
  • Yes, @JRE, the system of which this is a component is pretty space constrained, so I am trying not to upsize the pump that I am using for the air, and I would like to make the most effective use of power to heat the airflow within the cylinder. – Secundus Mar 04 '16 at 13:25
  • OK. I asked because there was some confusion in the "cooling the cylinder" and "heating the air" bits. Maybe you could clarify your question a little? Clear questions get better answers. – JRE Mar 04 '16 at 13:27
  • @NickAlexeev, it is currently an aluminum cylinder, though I am not married to that idea. – Secundus Mar 04 '16 at 13:28
  • "efficient" by what parameter? Minimum energy input? Minimum cost input? Minimum time required? – Phil Frost Mar 04 '16 at 13:37
  • @JRE, I don't know how I ended up typing cool in the question body. The "cool" that was originally in the title was an auto-correct mistake, but I have fixed both now. The question was pertaining solely to heating the cylinder; no cooling involved. – Secundus Mar 04 '16 at 13:37
  • @PhilFrost, thank you for cleaning up the post. I will be sure to edit more cleanly in the future. I clarified the "efficiency" that I was shooting for as power. For the given system, I would like to make the most effective use of the power we are expending on the heater(s). Cost is not particularly a driver (within reason), and [I assumed] if I am getting more efficient heat transfer to the metal tube, it would reduce the time taken to heat the metal tube. – Secundus Mar 04 '16 at 13:48
  • From a power efficiency perspective, it probably does not matter what type of resistive heating element you use. Make sure you electrically insulate the tube from the heating element to avoid shock or short-circuit hazards. There is nothing wrong with heating the tube, especially if the process will run for a long time. Do place insulation on the outside of the heating element to avoid heating the air outside the tube. – user57037 Mar 04 '16 at 21:36
  • Depending on what temperature you are trying to achieve, and the temperature of ambient air, a heat pump may be more efficient than a heater. But if you are trying to heat something up to a high temperature, forget that - it will be more about efficiently transferring and containing the heat from your resistance heater. – Chris Stratton Mar 05 '16 at 04:49

1 Answers1

2

Heating the cylinder is an inefficient way to heat the air inside. This is because the cylinder loses heat to the outside, as well as to the air inside. A better scheme would be a heater that warms the center of the (presumably streamline) airflow, and perhaps a porous plug or baffle at the outflow end of the air stream that conducts heat so that the air becomes more uniform in temperature. As for 'maximizing airflow', that would result in lower temperature for any given heating power. Is that the real goal?

Whit3rd
  • 7,568
  • 22
  • 27
  • Heating the air is the primary goal. After reading your response, I realized that "minimizing impact to the airflow" might have been more appropriate. The pump I am using was already sized for the airflow needed, so I was trying not to make it work harder, but that component might be worth rethinking if I can save some power on the heating. – Secundus Mar 07 '16 at 13:46