How can we get clothes out of a vacuum chamber without condensing water on them?

[OmCheeto]

Temperature is a huge problem. Let's take @OmCheeto's dirty towel (I found a towel the same mass, and could reproduce the numbers):
We have 130 grams of water. It will start to freeze as soon as we lower the temperature by 20 K. That is sufficient to evaporate ~1/25 of the total mass, or about 10 grams. We get roughly 20-30 grams more to evaporate before all the water is frozen. Doesn't work.

We can start by heating the towel, but not enough to get rid of the whole water by evaporation without freezing some. Multiple vacuum cycles with hot air in between could work, but that doesn't sound efficient.
A pressure below atmospheric pressure, but sufficient to still deliver heat all the time?

I'm not even sure if evaporation at room temperature and vacuum can beat evaporation at dryer temperatures and atmospheric pressure.

I have completed my calculations, and came up with a similar conclusion.
The amount of energy to dry clothes in a vacuum is roughly the same as at atmospheric pressure.
For 4.6 kg of water, I had to add 2.9 kwh of energy to maintain the temperature.

Fun project!

I think I'll look into dryer heat exchangers now.
This guy seems to have only one serious design flaw:

http://www.builditsolar.com/Experimental/DryerHX/DryerHXTest1.htm

Dryer into HX, 136°F
Dryer out of HX, 88°F
Room into HX, 79°F
Room out of HX, 95°F​

But even with the flaw, I think it's pretty impressive; "The heat exchanger, as it is, saves about 32% on the total energy used"​

 

[Buckleymanor]

I have completed my calculations, and came up with a similar conclusion.
The amount of energy to dry clothes in a vacuum is roughly the same as at atmospheric pressure.
For 4.6 kg of water, I had to add 2.9 kwh of energy to maintain the temperature.

Would you not dry the clothes in a vacuum shortly after the hot cycle used in the wash.
Eliminating some of the 2.9 kwh to maintain the temperature.
Most wash cycles have a hot portion to remove stains etc, this will be wasted as heat at the end of the cycle anyway or do your figures take this into account.

 

[sophiecentaur]

Would you not dry the clothes in a vacuum shortly after the hot cycle used in the wash.

They would be all soapy! There are several rinses with cold water after the hot wash. You would need to store the drained hot water and pump out the heat later.

 

[eltodesukane]

I've had the thought for years that it should be possible to dry damp clothes instantly (or almost instantly): You place them into an airtight container, then pump all the air out. In the vacuum, the boiling point of water drops to room temperature, so the water would all boil away, leaving perfectly dry clothes.
The only problem is that now the vacuum chamber is full of water vapor. If you open the door to the chamber to get your clothes out, the vapor would condense again, getting your clothes wet all over.

-- If you pump the air out, would that not also pump the water vapor out?
-- Also, I often dream of a microwave with a cooling switch, so that it could boil or freeze food.

 

[OmCheeto]

Would you not dry the clothes in a vacuum shortly after the hot cycle used in the wash.
Eliminating some of the 2.9 kwh to maintain the temperature.
Most wash cycles have a hot portion to remove stains etc, this will be wasted as heat at the end of the cycle anyway or do your figures take this into account.

I see at least one problem with this idea; "You see, back in 1988, the Consumer Product Safety Commission recommended that water heater manufacturers preset the maximum temperature at 120°F [ = 48.9 °C = 322 K] to prevent burns. And the manufacturers voluntarily followed those recommendations — supposedly". [ref: NPR]

But this did give me another idea. Just use the dryer to initially heat the clothes, just as we do now.
In a dryer experiment I did on Oct 5, 2008, I came up with the following information:

The dryer initially heated up to 145°F in all 3 heat settings. From there, the temperature was maintained at a lower range:

High 122-134 °F __ 50.0-56.7 °C __ 323-330 K
Med 115-125 °F __ 46.1-51.7 °C __ 319-325 K
Low 100-110 °F __ 37.8-43.3 °C __ 311-316 K​

From this, I'm guessing that some scientist determined that clothing has been shown to have an effective maximum, non-disintegrating temperature of 330 K.
Replacing 311 K with 330 K shows no appreciable change in energy requirement.

So I've decided that I will next look at Russ's idea, of using a microwave.
Somewhat of a minor inconvenience, as I assume I will have to separate my clothes: whites, darks, included metal.
A quick google search indicates someone made one 15 years ago:

Tech Notes; Using Microwaves to Dry Clothes
By CLIFFORD J. LEVY
Published: September 15, 1991​

And although wiki claims; "Japanese manufacturers^{[citation needed]} have developed highly efficient clothes dryers that use microwave radiation...", I can find no evidence of a production model.

Though this recent article; http://www.2450mhz.com/PDF/TechRef/Mw%20Clothes%20Drying.pdf
lists a possible solution; "...Noting how consumers have accepted and adapted to the microwave oven, the most likely scenario will be an evolution in consumer laundry habits and the birth of an entirely new industry of clothing and laundry products developed expressly for the microwave clothes dryer."

ie, microwavable safe undies!

But current technical problems aside, no one mentioned drawing a vacuum on the system, and the goal was "instant drying", so I'll continue with my research.
...

Still not instant. I come out with 35 minutes to dry clothing with 4.6 kg of water, @ 5000 watts.

double check on maths:
4.6 kg water * 2,264,800 J/kg latent heat of vaporization = 10,418,080 Joules to maintain constant temperature
= 10,418,080 watt seconds
10,418,080 watt seconds / 5,000 watt microwave = 2,084 seconds

=34.7 minutes



ps. I spent several hours on the beach yesterday with an 80 year old retired chemist, trying to get him to explain "partial pressure" to me, but all I got was an hour long "Oh my god, it was so cool when programmable calculators came out. It made figuring that out so much easier. .........." speech.

I learned nothing.
But it was a nice day.

 

[gjonesy]

You may put a chilled wall inside of chamber, at -50C. At 10 Pa, this value of temperature is below of the triple point of water,
so you can stop the vacuum pump and the water vapors will be collected by the chilled wall.

If that's the winning Idea I'll accept the royalty check...lol

 

[Scott C.]

I've had the thought for years that it should be possible to dry damp clothes instantly (or almost instantly): You place them into an airtight container, then pump all the air out. In the vacuum, the boiling point of water drops to room temperature, so the water would all boil away, leaving perfectly dry clothes.

The only problem is that now the vacuum chamber is full of water vapor. If you open the door to the chamber to get your clothes out, the vapor would condense again, getting your clothes wet all over.

It's a kooky idea, but I'm just wondering if anybody has a bright idea for getting the clothes out of the vacuum chamber without water condensing on them? If the idea makes a million dollars, I'll split the royalties with you.

The real problem is getting "perma-press" to work and the "spring fresh" scent of dryer type fabric softeners (both of which which require heat).

 

[A N Madhavan]

I've had the thought for years that it should be possible to dry damp clothes instantly (or almost instantly): You place them into an airtight container, then pump all the air out. In the vacuum, the boiling point of water drops to room temperature, so the water would all boil away, leaving perfectly dry clothes.

The only problem is that now the vacuum chamber is full of water vapor. If you open the door to the chamber to get your clothes out, the vapor would condense again, getting your clothes wet all over.

It's a kooky idea, but I'm just wondering if anybody has a bright idea for getting the clothes out of the vacuum chamber without water condensing on them? If the idea makes a million dollars, I'll split the royalties with you.

When you create vacuum in chamber, water vapour too comes-out along with air so purpose would be served.

 

[jim hardy]

When you create vacuum in chamber, water vapour too comes-out along with air so purpose would be served.

and

Still not instant. I come out with 35 minutes to dry clothing with 4.6 kg of water, @ 5000 watts.

double check on maths:
4.6 kg water * 2,264,800 J/kg latent heat of vaporization = 10,418,080 Joules to maintain constant temperature
= 10,418,080 watt seconds
10,418,080 watt seconds / 5,000 watt microwave = 2,084 seconds

=34.7 minutes

Has anyone figured the mechanical work to pump 4.7 kilograms of this air-water vapor mix from , just say 0.1bar to 1bar ?

 

[gjonesy]

Could you install a condenser coil in the vacuum chamber, perhaps separated by a thick perforated metal baffle with the condenser at the opposite end. Keeping the coolant circulating through the camber totally isolated from the vacuum process, perhaps pulling the moisture away from the clothing?

Did a little rethink on this, what if on the cloths side of the vacuum chamber you could pump in hot air say175F , keeping an open vent until the ambient temperature of the pressure vessel were 175F then pulled the vacuum from the condenser side quickly?

 

[Jeff Rosenbury]

My understanding of partial pressures is that the amount of water vapor is nearly independent of the amount of other gasses. So raising the temperature raises the amount of water vapor as well as providing the heat of vaporization. Reducing pressure only forces boiling when it drops below the partial pressure. The other gasses play one role, they provide extra heat for vaporization. So pumping them out slows the process of heat transfer.

To dry quickly, add extra air, of a form that carries lots of heat (molar heat capacity?). Increase the gas flow so water vapor is cycled out of the drying chamber quickly.

To save energy, use a condenser/heat exchanger. Have a large, low pressure (equal to the working pressure) gas storage area.

A nearly ideal situation would be to hang the clothes in a large, dry area and blow warm air over them. Perhaps use solar energy to add heat, and wind power to provide gas exchange.

I wonder if I can patent a clothesline?

But to speed the process, increase pressure and/or use a gas with higher heat capacity. As the OP pointed out, sucking the vapor out is fast. As others have pointed out, heat transfer is what slows the process.

I don't see a reason to reduce pressure. I see good arguments for increasing pressure and airflow.

In short, more hot dry airflow dries faster.

 

[OmCheeto]

...
Has anyone figured the mechanical work to pump 4.7 kilograms of this air-water vapor mix from , just say 0.1bar to 1bar ?

I could have sworn I was done with this...
The mechanical work was actually one of the very first things I calculated.
But given that I've learned a little bit since last Thursday, I'm pretty certain the answer is wrong.

I used the wiki value for water vapor density:

wiki; "The density (mass/volume) of water vapor is 0.804 g/L, which is significantly less than that of dry air at 1.27 g/L at STP".​

I determined that the volume of my spherical dry was 70 liters, and the volume of 4.6 kg of water vapor was 5800 liters, at STP.
The energy to pump that out is 592,000 joules. (=0.16 kwh = $0.016 electricity)
What's that phrase? "Not even wrong"?

But STP is 0°C at 1 bar, and we were trying to stay away from frozen water.
Anyone know how to calculate the density of water vapor at 0.1 bar @ 100°F? ( = 10,000 pascals @ 311 K)

hmmmm... Maybe Russ's link...

Yippie!
0.069067 kg/m³ with a saturation temperature of 46.1°C ( = 319 K = 115°F )

From that I get 1.73 kwh, as an answer.
But it's going to take my $500, 23 liter/min pump, more than 2 days to evacuate the system. (49.4 hours)

hmmm... Not very instantaneous.

 

[anorlunda]

I think I'll look into dryer heat exchangers now.

I looked into that many years ago. I had a house in a cold climate and with electric baseboard resistance heating. I thought that I could just vent the dryer into the house, achieving 100% efficiency for the drying energy because (during winter months) it would subtract from baseboard electric power demand. Ditto for any other interior energy wasting devices; in winter they caused zero waste.

I was also running an electric humidifier in winter because electric heat made internal humidity uncomfortably low. Eliminating the humidifier made the efficiency for internal dryer venting more than 100%.

But the down side was dust. A dust filter is only partially effective. Worse, if the filter came loose or had a leak, the accumulated dust would create a serious fire hazard. In the end, I reluctantly rejected the internal venting idea, and looked into heat exchangers instead. I had to reject the heat exchanger too because it would force me to move the dryer away from the wall, and there wasn't enough room for that.

 

[jim hardy]

From that I get 1.73 kwh, as an answer.
But it's going to take my $500, 23 liter/min pump, more than 2 days to evacuate the system. (49.4 hours)

You sir, are just amazing !

Bravo -I like the thought of a counterflow heat exchanger to recover whatever latent heat of vaporization could be recovered to warm incoming outside air. As simple as incoming and outgoing tube ducts being large concentric tubes.
But i suppose if one lived nearer Arctic Circle there'd be an upside to venting that warm moist air right into the living space.

old jim

 

[Jeff Rosenbury]

But the down side was dust. A dust filter is only partially effective. Worse, if the filter came loose or had a leak, the accumulated dust would create a serious fire hazard. In the end, I reluctantly rejected the internal venting idea, and looked into heat exchangers instead. I had to reject the heat exchanger too because it would force me to move the dryer away from the wall, and there wasn't enough room for that.

I've seen this done with nylon hose (panty hose) as a filter. It still produces lots of dust, but perhaps not dangerous levels. It also adds lots of humidity, but so does hanging clothes on a drying rack (my current solution).

 

[OmCheeto]

I looked into that many years ago. I had a house in a cold climate and with electric baseboard resistance heating. I thought that I could just vent the dryer into the house, achieving 100% efficiency for the drying energy because (during winter months) it would subtract from baseboard electric power demand. Ditto for any other interior energy wasting devices; in winter they caused zero waste.

I was also running an electric humidifier in winter because electric heat made internal humidity uncomfortably low. Eliminating the humidifier made the efficiency for internal dryer venting more than 100%.

But the down side was dust. A dust filter is only partially effective. Worse, if the filter came loose or had a leak, the accumulated dust would create a serious fire hazard. In the end, I reluctantly rejected the internal venting idea, and looked into heat exchangers instead. I had to reject the heat exchanger too because it would force me to move the dryer away from the wall, and there wasn't enough room for that.

Well, I live in Oregon, so what you've described is exactly what I do.
Though I ignore the dust and fire hazard(?). I have a 10ft dryer hose so it's pretty far away from the dryer inlet.
In the winter season only, of course.

In the summer I hang my clothes on my covered back porch for a day. It takes about 15 minutes to get rid of the residual moisture, and "crunchiness".
I'd put them in the sun, but there are so many birds around here, I'd just have to rewash them when they were done drying.

You sir, are just amazing !

Bravo -

I like the thought of a counterflow heat exchanger to recover whatever latent heat of vaporization could be recovered to warm incoming outside air. As simple as incoming and outgoing tube ducts being large concentric tubes.
But i suppose if one lived nearer Arctic Circle there'd be an upside to venting that warm moist air right into the living space.

old jim

Amazing? I wouldn't call "only 2 days to dry your clothes" amazing.
It strikes me as very peculiar, that pumping out all that vapor, has no effect, other than to waste energy.
No matter what I do, I always end up having to add 2265 kJ, for every kg of evaporated water.

Anyways, yes, I think a counterflow heat exchanger would work admirably. And people who live in warm humid environments should probably look into condensing dryers. But then again. I lived in Florida for a year, circa 1979, and 3 minutes after putting on dry clothes, they were soaking wet.

 

[jim hardy]

It strikes me as very peculiar, that pumping out all that vapor, has no effect, other than to waste energy.
No matter what I do, I always end up having to add 2265 kJ, for every kg of evaporated water.

My get rich schemes usually end similarly.

https://en.wikipedia.org/wiki/Critical_point_(thermodynamics)
At the critical point, only one phase exists. The heat of vaporization is zero.

but it's hard to conceive of a supercritical clothes dryer , ~3200psi & 700F .

 

[OmCheeto]

...
but it's hard to conceive of a supercritical clothes dryer , ~3200psi & 700F .

Good grief!
hmmmm... (google google google)
Well, there's a solution.
Send stevendaryl to Venus. Not quite supercritical, but the pressure and temperature are way above the liquid line: 872°F & 1350 psi
No water, no dryer.

ps. I had another idea. Heat up about 50 kg of those little desiccant bags. This place says you can heat them to 347°F.
Engineering toolbox says;

Silica gel - SiO2
blah blah blah... and adsorbs water up to 40% of its own mass. The bulk density of silica gel is 480 - 720 kg/m3. The specific heat is 1.13 kJ/kgK.​

They absorb best when at 70°F, so 347-70°F = 448-294 K = 154 K
154 K * 50 kg * 1.13 kJ/kgK = 8,700,000 joules. (heat energy provided by silica gel desiccant packs)

What did I come up with earlier?

4.6 kg water * 2,264,800 J/kg latent heat of vaporization = 10,418,080 Joules to maintain constant temperature​

Pretty close. Just reduce dryer load by 20%.

Benefits:
No additional heat needed.
No vacuum needed. (A vacuum might make it worse. I have no idea how desiccants work.)

Drying time: unknown. Might be several hours.

 

[mfb]

The silica will release heat when it absorbs water vapor. Probably more than vaporizing the water in the clothes needs.
The downside: you have to dry the silica afterwards.

 

[Nidum]

An efficient fast drying system would be one that needs little or no evaporation of the water in order to function . A system where water is more nearly displaced / transported rather than evaporated .

A spin dryer works this way . Common designs though seem to be effective in removing the bulk of the water but not in achieving total dryness .

Possibly arranging for a reasonably high velocity positive pressure air flow through the clothes in one direction would work better . Cold air initially changing to warm air for final stages of drying .

Essentially the same drying action as on a clothes line but achieved in a smaller space and much faster .

 

[LakeGazer]

A boss once told me that when one works with processes that use vacuum, the entire world is one big leak.

I thought of a variation on freeze-drying. Flash freeze the clothes and them bring them to a temperature where the ice undergoes sublimation. The process would be energy-intensive, but if you start out with the clothes on a form, they would come out as if pressed. That would save time.

 

[Ralph Dratman]

How about prepping the clothes by rinsing in ethanol? That would displace a good portion of the water and evaporate more readily. Crystal formation is the enemy of quick drying. Ethanol would resist crystalization under the temperature loss that would tend to accompany evaporation.

One could dry clothes entirely in ethanol as long as the concentration of water in the ethanol mixture did not get too high. But then how would you recycle the ethanol once it got too wet? You could distill it, but if the point was to save time, energy and complexity, that is not progress.

 

[Sherwood Botsford]

Hmm.

Chamber is not that difficult. Consider that the water tank in your basement is designed for 300 psi, has a safety valve set at 150 psi, and is routinely used at 60 psi. 15 psi is a walk in the park.

Because a vacuum would tend to close the door, all you need for a good seal is a loose hinge and a gasket. The rim will have to be re-enforced. The hazard of a collapsing vacuum chamber is small. No one is in the way of moving bits.

The gotcha is the the amount of energy. The lower the pressure the more litres of vapour you have to pump. At STP water vapour has a molecular volume of 22.4 liters per 18 grams. Water at 10 C (about 50 F -- cold water wash) has a vapour pressure of about 1.2 kN/m2 or about 1/80 of an atmosphere. So you have to pump 80 times the volume to get rid of the water vapour. A kg of water as liquid has a volume of a litre. As a gas it's some 1000g / (18g/mole) * 22.4 l/mole is about 1200 litres at 15 psi. At 1/80 atm it's about 100,000 litres. Even with good pump design this is starting to get non-trivial.

Worse: as the water evaporates, it chills the remaining water, dropping the vapour pressure, increasing the volume that needs to be pumped.

This is how water damaged books are dried, however. The process is not quick, but the lack of air coupled with the low temps means they don't go mouldy while drying.

***
In our arctic temperatures in winter, we can hang wet jeans outside at -40C Hang them in the sun, and they will dry. Air at that temperature has very little water vapour in it. The sun warming the jeans to -30 or so increases the relative water vapour pressure enough that the ice sublimes. It takes several days to be effective, and even so, when you bring them in they are still slightly damp.

***

Story time: At one of the big accelerators they were having a problem pulling the very high vacuum needed for the particles to run their laps on the track without running into stray gas molecules. They pumped down to some small fraction of a torr and there it stayed. Finally they opened it up, and did an internal inspection. One of the workmen had left a bologna sandwich inside the machine. One sandwich was able to outgas enough to keep the hundreds of cubic meters of nothing impure enough to matter.

 

[mfb]

Chamber is not that difficult. Consider that the water tank in your basement is designed for 300 psi, has a safety valve set at 150 psi, and is routinely used at 60 psi. 15 psi is a walk in the park.

Higher pressure is much easier than lower pressure. Every plastic bottle will tolerate a significant overpressure, but crumple easily if you squeeze it a bit.

Worse: as the water evaporates, it chills the remaining water, dropping the vapour pressure, increasing the volume that needs to be pumped.

And the remaining water (most of it) will freeze. This has been discussed multiple times already.

 

[sophiecentaur]

Higher pressure is much easier than lower pressure. Every plastic bottle will tolerate a significant overpressure, but crumple easily if you squeeze it a bit..

That doesn't aways apply. Many materials are far stronger in compression than in tension. An egg, for example, can withstand a lot of compression or even impacts from outside but a chick can knock its way out with relative ease.

 

[mfb]

The failure mode for vacuum vessels is rarely a uniform compression, it is a deformation.

The chick applies a huge amount of pressure to a very small place, you can break the egg in the same way from the outside (on that scale the egg is quite flat anyway, direction does not matter).
"Relative ease" - well, it still takes some time.

 

[sophiecentaur]

Ok. How about a drystone arch?

 

[Keith_McClary]

The hazard of a collapsing vacuum chamber is small. No one is in the way of moving bits.

CRT TV tubes were dangerous because the glass bits would bounce back.

 

[sophiecentaur]

CRT TV tubes were dangerous because the glass bits would bounce back.

Chambers don't need to be glass. Strength wouldn't be a problem so much as leaky seals and the required good fast pump.

 

[Buckleymanor]

Chambers don't need to be glass. Strength wouldn't be a problem so much as leaky seals and the required good fast pump.

Strength seals fast pumps and leaky seals are not a problem as all of these have been produced and used in manufacturing for many years.Vac packed ground coffee is a good example where all these problems have been overcome.

 

[sophiecentaur]

None of the mechanics is a problem but this chamber needs a large diameter door and the seal would need to be rugged, cheap and fairly good.
The thread has strayed far away from anything useful in the home. Any added chemicals would have to be recovered and reused. All fabrics and dyes would need to be suitable. You could not dry items that went brittle at low temperature.
It strikes me that the condensing dryer with heat pump is already a good improvement. Also, a design with a large drum (really large) would be a huge advantage, as with commercial dryers. Not instant drying though.

 

[Derek Potter]

I've had the thought for years that it should be possible to dry damp clothes instantly (or almost instantly): You place them into an airtight container, then pump all the air out. In the vacuum, the boiling point of water drops to room temperature, so the water would all boil away, leaving perfectly dry clothes.

The only problem is that now the vacuum chamber is full of water vapor. If you open the door to the chamber to get your clothes out, the vapor would condense again, getting your clothes wet all over.

It's a kooky idea, but I'm just wondering if anybody has a bright idea for getting the clothes out of the vacuum chamber without water condensing on them? If the idea makes a million dollars, I'll split the royalties with you.

Well the heat of vapourization of water is about 2200 J/g but the SH is only 4.2 J/g K^-1. So by the time 10% of the water has evaporated it will have cooled the rest down by up to 50 degrees so it will have started to freeze. The heat of freezing (around 300 J/g) will keep the temperature from falling any further until about 15% more has sublimed and the remaining 80% approx is ice. Ice has about half the specifc heat of liquid water so the temperature will fall even faster for a given amount pulled off. This process obviously runs out of steam, if you'll pardon the expression, long before absolute zero is reached, but you can see that a lot less than half of the water can be pulled off this way unless a source of heat is present. The clothes themselves will increase the thermal capacity and allow a bit more evaporation but polymers have only about 10% of the specific heat of water and the clothes would have to be almost dry for this to work. I'm not sure what say 5% dampness is like but I'm pretty sure my jeans come out of the spinner a lot wetter than that.

Not too many problems with opening the door. Charles's Law will push the temperature up as the vapour is compressed. AFAIK you never get compression condensation, you have to remove the heat of compression if you want to liquify a gas by compressing it. Mind you, if the clothes have cooled down significantly, there will be condensation on their surfaces, even if not in the air around them.

 

[SupramanTT]

After years, it looks like someone created this. :)

I was so curious if someone else had made a vacuum dryer before. I have found so many pages saying it was not possible, but it seems that it has been done. I am curious what the true reviews will be after they produce them.

 

[mfb]

It is still drying the clothes with hot air, just at a lower pressure.

 

[SupramanTT]

It is still drying the clothes with hot air, just at a lower pressure.

Yup, it looks like they are bringing it down enough so water boils around 102 degrees f. Definitely not full vacuum but the principle is functional. I am guessing the reason they kept it this simple is that it keeps the energy usage lower.

I just calculated it out and it looks like they are bringing it down to 8.5kpa to induce the 102 degrees f boiling.
https://www.omnicalculator.com/chemistry/Boliling-point