I am building a compact air heater as a hobby project where the air is blown between fine fins of several aluminum heat sinks. The temperature needs to be raised from ~0C to ~37C and needs to handle air flows of up to ~100 L/min. I did an envelope calculation taking into account heat capacity of dry air, and it appears that I would need ~100W, 300W to be safe, for that.
So I want to run a nichrome wire in a spiral pattern along the surface of the aluminum heat sinks. But wouldn't that cause a short? Can I get insulated nichrome which would conduct heat, but not conduct electricity? Any suggestions?
You might look for "water glass". It's a silicon compound (???) that's water soluble. Evidently, it is used as an electrical insulator at hi temperatures.
The resistor idea by itself - would not work. The more surface there is in contact with air of unit volume per unit time, the faster the heat transfer, isn't it? Yes, I can run the heat wire in a complex 3D spiral pattern to get the same contact surface. But running a wire in a spiral on the base of a heat sink and have the fins take care of the
No. Adding the thermal mass of the heat sinks would substantially slow things down.
Using a heatsink might make sense if you need non-stratified heating (not guaranteed *with* a heatsink, and do-able other ways anyhow) or if you want to avoid scorching dust particles in the air.
For very little cash outlay, you can go buy a hair dryer that has the fan and a (presumably) safe heating element, all in one compact case. I'd guess the fans in those are capable of providing your desired 100 L/min airflow.
If you INSIST on using a heatsink, why not just mount two or three cheap power semiconductors to it and use them to generate the heat? This is done in some crystal ovens. Or get some of the power resistors designed to mount to heat spreaders. Caddock and others make them in cases similar to power semiconductors, or you can find ones in aluminum cases with a flat side and mounting holes that bolt easily to a heatsink. Another alternative would be something like a hot water tank heater of the sort that's a metal tube bent in a tight "U" shape with terminals at one end. The tube is already insulated from the heating element. They are typically rated a couple kilowatts or more, but that's in water for cooling; if you control the power so the case doesn't get too hot you should be OK. You're still left with the problem of getting it to make good thermal contact with the heat sink. -- But as others have pointed out, you can make nichrome wire hot enough that it will have no trouble getting rid of the heat you want just into the forced airstream, as is done in a hair dryer (or, alternatively, a heat gun; Harbor Freight has one on sale at the moment for under $10).
Or, go get one or two tubular ceramic-core 200W resistors and arrange a fan to blow through their open centers. Since they are designed to dissipate 200 watts in free air, you should be able to use just one to get the effect you want, keeping it a bit cooler by blowing air through it.
In short, don't mess with trying to use nichrome wire directly; use a heating source someone else has already done for you. It will be cheaper and much safer in the long run. (Though there are still plenty of ways you can make it un-safe, of course.)
I am blowing cold air with an air pump through a tube of ~ 2 inch cross section. The air needs to come out of the tube heated. The tube needs to be as short as possible. The simplest design is to run a wire around the tube's wall and make the tube heat conducting (e.g. Al). But I figured I could heat the air more efficiently if there are fine metal fins across the tube's cross-section to expose as much surface of the heating element to the air as possible inside the tube. This is where the "heat sink" came from - it works not as a "sink" per se, but as a heat exchanger. Heat sinks happen to be easily accessible and have a large surface which I can use experiments
What I have described probably indeed looks like a hairdrier. However, hairdriers quickly heat air in a small package because of heating the wire to a very hot temperature (don't know how hot, but the wire glows with infrared). In my case, I want to avoid using high localized temperatures due to safety concerns. So the idea was to re-distribute the heat over a large surface, and thus being able to use lower temperatures.
An insane idea would be to solder together lots chip resistors to make a pipe 3D structure with the max surface area. Each resistor can contribute a little heat. Would be fun to see. Send a picture if you do :)
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.