Today I aimed a normal household fan at the drying rack, purely out of "what could possibly go wrong", and the wash was dry in a fraction of the usual time.

My instinct was to say no AI would have come up with that. So I asked one. Of course it had: boundary layer, vapour pressure, temperature-humidity-airflow, the whole lecture, instantly, for free.

The answer had been sitting there the entire time. I never asked, because I didn't know there was a question.

That's the bit nobody has automated. The model will answer anything you put to it — it cannot be standing in your Waschküche at nine in the evening wondering what happens if you turn the fan around. Curiosity is still BYO.

Anyway. Since I had the lecture, here it is, short.

The whole thing is one equation

Water leaves your laundry at roughly

ṁ = A · k · (c_wet − c_air)

• A — wet surface actually exposed to air
• k — how fast moving air hauls vapour off that surface
• c_wet − c_air — how much thirstier the room is than the fabric

Three knobs. Every dryer ever built, and every Kellerabteil full of dripping shirts, sits somewhere on that formula.

A is free and everybody wastes it. A towel overlapping itself has half the area it looks like it has. Spread everything, hang shirts open, turn pockets out. Cheapest term in the equation, costs nothing but rack discipline. Same reason a hard spin cycle beats everything else here — water thrown out mechanically never has to evaporate at all.

k is the fan. Wet fabric wraps itself in a thin skin of saturated air and then suffocates in it. The fan strips that skin off. But the payoff is sublinear, and that's the part that surprised me: in the textbook flat-plate case k grows with something like the square root of air speed, creeping toward U^0.8 once the flow turns turbulent. Take those as illustration, not as a measurement of your cellar — the point is only that doubling the wind does not double the drying. So: low or medium, aimed across the rack rather than blasting one unlucky towel. Evaporation is fastest where the air first meets the fabric, which makes rearranging things halfway through worth more than another 30 watts.

c_wet − c_air is the ceiling, and the term everyone forgets. Warm air holds more water, so a warm room drinks harder. But nothing described so far removes a single gram of water from your flat — the fan only relocates it out of your T-shirt and into the room. When c_air catches up with c_wet, the bracket goes to zero and your expensive wind is just noise. Opening a window helps only when the air outside is genuinely drier than the air inside; on a raw, wet November evening it isn't, and you've traded drying for heating.

And the room does not start empty. The Energy Saving Trust reckons a household can put up to 100 litres of moisture a week into its own air just by cooking, showering and breathing — and if that moisture isn't removed by ventilation, it settles on surfaces or stays in the air. A wet wash is more of the same, on top. That's the actual risk, and it isn't the fan's fault: it's water with nowhere to go, finding the coldest wall in the flat. Which is why the fan needs a partner that takes water out — extraction to the outside, or a dehumidifier.

So: spin it hard, spread it wide, a cheap fan across it on low, and give the water somewhere to go.

A tumble dryer does those same four things in a box. Tumbles the load to expose surface, blows air over it, heats the air to widen the gap, and either vents the vapour or condenses it into a tank. Nothing magic — just all four knobs turned at once, which is exactly the part I got wrong by only turning one.