In a lab, like in politics, there is often a need for hot air; sometimes for a gentle preheating of something, sometimes for
drying paint or other water- or solvent-based substance. A salvaged hairdryer
was originally intended to be satisfying. However, it turned out that the airflow is too high (enough to damage more
sensitive components or paint layers). More seriously, the noise of the running hairdryer was unbelievably L-O-U-D,
resembling a jet engine during takeoff. (Who can ever be comfortable running that next to one's head?) Therefore it was
decided to design a modified hot air source.
The motor with the fan blades was quickly determined to be the root cause of the noise. (Well, duh.) So it was removed and another airflow source was located. A 120x120 mm 220V AC cooling fan was picked up as the best cost/performance option. While still not entirely quiet, it was MANY decibels less and it was actually possible to withstand longer periods of working right next to it without turning mad.
First, to test if the airflow is sufficient for cooling of the heating spirals, a mockup was made, using aluminium foil and sellotape to channel the airflow from the fan through the heater block. The test results were satisfying.
An old ATX power supply case was determined to be the best and easiest option, due to being metal (and therefore
fireproof), easily available, and already with a precut hole for the 120 mm fan. The storage compatibility with
the Spot Welder was another plus. A hole was cut in its side (by drilling a lot of
adjanced holes, then cutting out the disc and filing off the ragged edges, ufff) and the heating element from the
hairdryer was affixed in it.
The airflow in the new assembly was a bit lower than expected from the mockup tests. The box was apparently creating more air drag than the aluminium foil structure, increasing the pressure gradient inside and lowering the efficiency of the fan. The impact was however less than critical.
The resistive wire turned out to be more fragile than expected. It was damaged on two more places during handling, and a repair had to be performed. The same approach, brazing with brass and borax, was used as with the original hairdryer repair.
The holes in the casing were covered with sticky tape, in order to channel all the airflow through the heater. The mesh on one of the sides was covered with aluminium sheet.
The electrical wiring was simple. The main switch powers up the fan and enables the heaters. Two other switches power up (if enabled) the two heater segments. Originally there was a plan to add diodes in series with the heaters, with a switch to allow reducing the energy by half, however at the end it was decided to hardwire the diodes there without the switch, as the airflow was too low and the heaters were quickly overheating at full power. (Fortunately there is a bimetal-based thermal fuse in the heater assembly, which cuts the power to the heater when it overheats, preventing lasting damage.)
The heater assembly was secured in place using silicone caulking.
It was measured that the air temperature at the outlet at full power is around 250 °C, which is just about right ceiling to not damage most things. The temperature drops with distance from the vent.
The unit proven to be useful for gentle drying of paints, melting of some compounds (e.g. wax), and softening adhesives and plastics. Its relatively low noise is a definitive plus in comparison with the original hairdryer; that goddamned thing was AWFULLY loud.
 Fan test |  Fan test |  Inside view, testing |  Inside view, testing |
 Preassembled unit |  Preassembled unit, slightly overheating |  Inside view, electronics |  Inside view, heater mount |
 Hot air blower, control panel |
 Broken wire |  Broken wire, twisted together |  Broken wire, added brass and borax, foamed up |  Broken wire, soaked with molten flux |
 Broken wire, brazed together, before flux removal |  Broken wire, twisted together |  Broken wire, added brass |  Broken wire, brazed together, before flux removal |