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"Bun is such a sad word, is it not?" -- Watt, "Waiting for Godot"
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Imagine a large piston engine. Single cylinder. Now invert
it. At the bottom, we have the intake valves, above that a
wire mesh which suspends wet laundry in the "combustion
chamber" of the device. The piston is above that. When
we start the device the piston moves up, pulling a partial
The vacuum should get the pressure down to
about 1/5th of atmospheric. This will encourage some of
the water in the laundry into the air as the boiling point is
now around 40-50C.
When the piston reaches bottom dead center (BDC, which
is now at the top, because of the whole inverted thing) the
intake valves open. Behind those, in the intake manifold
some pre-warmed air now rushes upward toward the
clothes, hopefully lofting them into the air for a bit of a
tumble... maybe that happens a bit later when they're less
heavy.... anyhow. Shortly after the intake valve opens, the
exhaust valves open. These are in the piston itself, and
behind them is another modest vacuum. Think vacuum
cleaner level. Just as the piston starts to move down again,
the exhaust valves remain open. The scavanged exhaust
will remove the water laden air and help remove the air
displaced by the descending piston to ensure no
compression. The exhaust valves close when the piston
reaches the bottom (which is the inverted TDC, stay with
Then the whole thing starts again. Lovely. Should be
needlessly expensive and complex, but a touch more
Highly relevant [8th of 7, Jul 18 2016]
||When the intake valves open, the water vapour will condense instantaneously into fog.
||No need for vacuum on the exhaust; better just to maintain positive pressure on the inlet as the piston moves down.
||//When the intake valves open, the water vapour will
condense instantaneously into fog.//
||But the intake air is hotter than the boiling point by a fair
way... well, sort of, and I want to whoosh that fog out of
the exhaust anyhow.
||//No need for vacuum on the exhaust; better just to
maintain positive pressure on the inlet as the piston
||My thinking is that the exhaust vacuum would pre-
depressurize the cylinder a little. Adding to the
decompression ratio. Or subtracting from the
compression ratio. Which is a minus number. It gets
||The thermodynamics are easier if you just invert the signs on everything.
||[+] Interesting. In order to actually //save energy// though, the piston would have to be allowed to spring back on its own. Water would evaporate from the clothes on the upstroke then, mostly above the clothing, condense in the chamber on the freewheeling downstroke, after which some air could be blown through to clear the mist (which is where you'd lose piston momentum).
||Vacuum drying, if carefully designed, tends to be more thermodynamically efficient than simply heating things above ambient.