A while back, I wrote a column on fine-pitch soldering. Several commenters noted that a hot-air soldering iron was the hand tool of choice when it came to fine pitch soldering by hand. So I decided to look up the tools and found that they were out of my price range. I still wanted one. After more research, I found this site which describes how to build such a tool from a soldering iron. Based on that, I came up with my own set of instructions for putting together a hot-air iron. The instructions are complete and include where to buy the iron. I’d like to give special thanks to Nelly Cabrera of the Stimpson Eyelet Companywho sent me samples of the final piece required to complete the design. The nozzles (eyelets).
The development of the hot-air iron was not smooth sailing despite what seemed like adequate instructions.
Final testing of the hot-air iron had me pulling my hair out because I couldn’t make solder melt with it. I was using the smallest pump I had, an AQT3001, which was a remnant from previous acquarium adventures – my thinking was that if another builder has to buy a pump, a small one will cost less. It didn’t work. I tried different eyelets (nozzle tips), and nothing seemed to help. The narrow steel nozzle (Stimpson A1735) had a fairly high temperature output, but so did a more open brass nozzle (Stimpson GS5-12). Well, I had theories. One was: “too much air flow”. So I used an air valve to decrease the air flow. It made things worse. Was the steel losing less temperature because it was less thermally conductive? I tried using the brass eyelet as a guard around the steel eyelet in the hopes that I’d get some gain. Bigger losses.
I was at my wits end. I thought, “I’ll try something that shouldn’t work, but maybe I’ll learn something.” Did I ever. What I tried was adding a second pump, a Whisper 10, that I had on hand to my setup. I used a “T” and some tubing to connect it to the setup. In theory, more flow should have decreased the temperature output. But it didn’t. The temperature went up. And now I could melt solder. Quite nicely with less than a ten second dwell.
Under what conditions does increasing the flow through a heat exchanger increase the heat transfer? That happens if the increase in flow causes a change from a laminar flow regime to a turbulent flow regime. Increasing the flow causes the flow to became turbulent. A very good thing in a heat exchanger. It is my opinion that the real purpose of stuffing the heat transfer tube tube of the iron with a metalic foil (see construction article) is not to directly increase heat conduction from the heater walls but to increase the turbulence of the airflow through the soldering iron body to increase the heat transfer. If I hadn’t started my experiments with an inadequate air pump because I wanted to keep costs down I would never have learned that.
To get deeper into this aspect of heat transfer, you can look up Reynolds number. In fact, don’t fly without it.
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Filed Under: Rapid prototyping