*Why* don't people sell washers and driers as one unit? Not one unit for washing, one unit for drying, stacked on top of each other, but one unit the size of one that does both?
That would *so* save resources, to say nothing of money and space.
It's not that difficult. You just need a mechanism for pumping hot air into that drum that is impervious to the water when it's washing. Here are some ways to do that:
(this is the style of washing machine that opens from the side.)
have two layers of drum. washing machines must have this anyway so that all the water can escape out the holes and be captured by the larger drum during the spin cycle.
Have a tube that delivers hot air come up and back down right over the top of the inner drum. hot air can push in through the holes in the drum. this can be optimized by making very loong (but thin) holes at that distance around the whole cylinder. the tube can have a valve on the end of it, which for simplicity's sake automatically opens whenever hot air coming out pushes it open; when it's closed it's water-tight. that way no water will get into the tube.
have this tube come into a back part that doesn't spin (like with a dryer), but right near the top. equal anti-water advantage, no inner drum to block it. as the back part would be part of the outer drum, the inner drum is free to turn without having a back to it. just put the inner drum pretty close to the back so clothes don't fall through.
for the outgoing air, just have a large intake portal in the top of the larger drum. water shouldn't get that high, but if it does, the whole passage from there to the outside can be water-compatible, so it would just end up coming out the outside vent. we don't really want water constantly splashing into it and drizzling out, though, so have that passageway go a few inches up over the top of the drum, before it goes back down. or, just put something under it like what goes over chimneys to keep the water out.
just have an electrically opened/closed flap for the outtake passage that's relatively water-tight.
this could also apply to the hot air opening, of course.
btw, the hot air pipe should also go up a little after the drum before coming back down, in case the valve fails so water doesn't splash into it.
don't worry about water overflowing into it because there can be protections for that
1) electronic sensing to stop water from filling if it overflows
2) a drain near the top of the drum, just like in a bathroom sink. the drain leads into the pipe that drains used wash water.
You don't really need a flap for the air outtake vent because it can be put above the level of the overflow drain and then also rise further up before it goes back down. the same pretty much applies to the hot air input passageway.
although you don't really want your outtake vent getting the hot air back that was just put in there because it's next to the hot air input. so you could
1) have the hot air input deflect the hot air to the right or the left like a jet--the same direction the clothes spin in, and have the air outtake vent receive air from the opposite direction
2) have the hot air input in the back of the drum and the hot air outtake vent near the front (by the door). also should try 1) in conjunction with this.
that sounded complex, it's all really very simple.
now you can program the washing/drying cycle as one sequence, controllable from one single interface.
-clothes don't get left in the washer undried
-users don't have to take the clothes out of one drum and put them into another
-obviously, saves double the space, money, resources, manufacturing pollution, transportation pollution, time and effort.